Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Schematics and Layouts => Topic started by: syndromet on August 10, 2006, 02:25:45 AM

Title: Joe Davisson's new Guitar amp
Post by: syndromet on August 10, 2006, 02:25:45 AM
Joe just posted this over at DIYstompboxes.com.
if it's anything like his other circuits, it's deffinitly worth checking out!

http://www.diystompboxes.com/analogalchemy/amps/guitaramp.html
Title: Re: Joe Davisson's new Guitar amp
Post by: teemuk on August 10, 2006, 04:02:04 AM
I built something very similar recently: Single 60V rail, turned a simple TIP142/147 output to quasicomp and doubled the amount of output devices. It's perhaps 65-70 watts.

Some small points I feel the author should have mentioned: Not that I really care about the effect, but the output capacitor really will add some distortion. Check D. Self's book as a reference. The author mentions the use of plastic jacks - I'm sure he refers to insulated jacks (uninsulated can be plastic too). Use them. After all the trouble you might go through with curing oscillations you'd surely hate to bother with hum too. Make sure your grounding scheme is top quality as well.

Do the first power on always with a current limiter (i.e. light bulp in series): A short may not show in DMM. The DMM will show shorts from supply to ground but hardly ever shorts in transistors. Even figuring out a short based on the resistance readings can be troublesome. The only way to measure some shorts in transistors is to lift them off the circuit. You do not want to toast all the power transistors at once, do you?

VBE-multiplier instead of diode-based thermal compensation should be a must with darlington output transistors - unless you use a very large heatsink. The thermal coefficiency of a darlington is not the sum of the transistors, it is their multiplier and hard to track with ordinary diodes. I don't know if the author matched the diodes to output devices but an easy way to mount diodes to heatsink is to use TO-126 case transistors connected as diodes. At the same trouble I would convert to VBE-multiplier though.

I know from experience that something like this can indeed be a very nice sounding amplifier but also b*t** to tweak till fully operational: I spent more than two weeks sorting out all oscillations and component failures and in the end I had to add a hefty amount of parts to stabilize the thing. Besides the Zobel network and 10k base resistor I see no stabilization in this thing. Start with the fact that unless you use exactly the same layout and circuit board the schematic only contains the parts the amp requires to work on paper.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 11, 2006, 10:46:28 PM
The docs now recommend a thermal shutdown switch on the heatsink. I mistakenly assumed that the diodes would behave as they do in a non-darlington output stage. I did find a thermal shutdown circuit consisting of a couple of transistors and a zener diode, which I'll try to figure out values for sometime. Not sure how to go about testing such a thing, though.

I've also added a diode clamp to the input to prevent overloading. (Obviously this will add distortion if the input goes past about a volt, but also limits the power output to a safe level.)

My goal was an easy build, not great quality. Hence the diode biasing (less to adjust) and output capacitor, etc. Thanks for the insights.

Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on August 12, 2006, 07:43:41 AM
Sounds like a good simple build, not a stage rig, but definetly something cool to play with.  Thanks for signing up too btw!
Title: Re: Joe Davisson's new Guitar amp
Post by: Crystallas on August 12, 2006, 12:56:16 PM
Welcome aboard Joe!
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 13, 2006, 04:34:59 PM
Thanks for the invite. Looks like a real nice board.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 13, 2006, 08:55:39 PM
I tested the clamping diodes. The downside is that you have to trim the gain/volume on pedals. Actually, the added distortion didn't sound bad. Shouldn't be TOO big of a deal, and protects from overloading. And it's still pretty loud before you get to that point.

Also attached the thermal switch, so hopefully it's safe enough now. :)

There's another design I've been stewing over, based on this:
http://depalma.pair.com/Analog/analog.html

It's been on the net for years. The simulated circuit:
http://www.diystompboxes.com/analogalchemy/temp/depalma1.gif
(subject to removal, don't link.)

I altered the feedback structure slightly, and used a Vbe multiplier. The interesting part is that it appears to soft-clip in a very rounded, ideal way, especially with little or no negative-feedback.




Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 29, 2006, 05:46:13 PM
The bass seems to be too high, didn't really notice it until plugging in pedals. I'm guessing the 1uF feedback cap is too high, but who knows.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 15, 2006, 03:05:47 PM
I redesigned/built/tested this thing:
http://www.diystompboxes.com/analogalchemy/sch/guitaramp.html

It should fire up pretty close to the right bias level with the trimpots centered. Preamp coming soon!






Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on October 15, 2006, 03:16:47 PM
Wow, way cool!  Any guess on how many watts this thing puts out?  I'm thinking this might be a good winter break project for me :-)
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 15, 2006, 03:58:13 PM
About 30 RMS @ 4-Ohms, guesstimated from Spice simulation. The 56k limits it somewhat, but also prevents the input transistor from being saturated, making overdrive more pleasent if it occurs. The accompanying JFET preamp will work similarly, and uses the same 40v supply.


Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on October 15, 2006, 04:09:05 PM
WOW!  This is great!  I can't wait to get some people building a few of these so we can really test 'em out.  It would be cool to have a non-chip amp SS 30watter.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 15, 2006, 09:56:22 PM
Here's the preamp:
http://www.diystompboxes.com/analogalchemy/sch/jfetpreamp.html

The completed amp has some volume, but should be louder. Guess it needs a buffer after the tone stack.









Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on October 15, 2006, 10:06:19 PM
This is looking really great!  Correct me if I'm wrong, but wouldn't this work with other preamps pretty ok as well?  I have a marshall preamp I'm using on my 50w LM3886 amp right now, and I'm working on a preamp based around the TL071 which is probably what I would use on this.  Is it possible to get any pics of the amp guts all put together?

**edit**
Also, any ideas on what the minimum/maximum would be for the voltage on the transformer?  I can get a 36v (18v+18v) transformer with 160VA pretty easily, but I don't want it to be too much, and the next one down that I have is a 25VCT @ 2amp.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 16, 2006, 07:54:19 PM
The biasing will change with the voltage somewhat, although 25-50 seems reasonable. The transistors themselves can handle as much as 80 I think, but not the resistors.



Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on October 16, 2006, 08:49:21 PM
Thats great, looks like we should be able to do some cool stuff with this.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 16, 2006, 11:43:47 PM
Did some more work on this. The power amp has a buffered input, and tons more volume. It sounds like 25-30W is about right after hearing it maxed:
http://www.diystompboxes.com/analogalchemy/sch/guitaramp.html

The preamp sounded slightly honky so I upped the bypass caps to 3.3uF:
http://www.diystompboxes.com/analogalchemy/sch/jfetpreamp.html

Overall it's real nice except my build quality (perfboard with stuff tacked on everywhere).

Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on October 17, 2006, 07:45:40 AM
I still wanna see pics :-)
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 17, 2006, 11:23:03 PM
http://www.diystompboxes.com/analogalchemy/tmp/

Pix of inside and front panel, and dirty socks. It's a recycled Fender combo.

This is not how to build an amp, it's a rough test built from junk. I'm not particularly thrilled with the power amp, it works alright but seems it could be better.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 18, 2006, 12:29:53 AM
another idea:
http://www.diystompboxes.com/analogalchemy/tmp/fetamp1.gif

This is kinda neat because the biasing only varies a little with different supply voltages. So you can pick your wattage pretty easily. (Shows 80V, but should be 25-50 range).

Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on October 18, 2006, 07:33:57 AM
http://www.diystompboxes.com/analogalchemy/tmp/

Pix of inside and front panel, and dirty socks. It's a recycled Fender combo.

This is not how to build an amp, it's a rough test built from junk. I'm not particularly thrilled with the power amp, it works alright but seems it could be better.


I think it looks good, except the large caps sitting on the chassis on the right.  That looks sketchy.  So whats the matter with the power amp?  Doesn't sound good?  That's the part I'm really interested in as I already own a preamp box (marshall DRP-1) and I'm working on a TL071 preamp.
Title: Re: Joe Davisson's new Guitar amp
Post by: Stompin_Tom on October 18, 2006, 12:18:32 PM
Joe- I like the looks of your preamp. How do you think it would sound run at a lower voltage? say 20 or 30v? What's the (-30v) notation in the upper left about?

I think your schematic will work well as a rough perf layout, so the build should be pretty straight foward. Thanks.

Oh, and your build looks pretty good to me!
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 19, 2006, 12:04:03 AM
The caps aren't sitting on the chassis :) They are on a terminal strip. The 1k decouples the 40v supply and lowers the current. (Most JFETs can't handle much current at all.) The actual voltage powering the fets is around 30-32.

I'm redesigning everything. The preamp is good for high gain, but that's about it. I think my original power amp design was better/cleaner/louder, so I'll probably work on that some more. I'm very indecisive :)
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on October 19, 2006, 07:30:13 AM
That makes more sense (about the caps).  And as far as being indecisive, no problem, I think it's great that you are designing these.  There isn't really any sort of good SS amp to build right now that isn't a chipamp.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 25, 2006, 07:18:08 PM
I'm almost done redesigning the power amp to use more appropriate parts for a 30-watter. Parts are radio shack-available too. Will check back here after I've tested the build, hopefully sometime this weekend.





Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on October 25, 2006, 08:43:38 PM
Wow, a radio shack 30 watter, that's sweet.  Although its too bad, the radio shack right near me closed.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on November 13, 2006, 10:50:40 PM
Possibly a candidate for the radio-shackable amp:
http://www.diystompboxes.com/analogalchemy/tmp/amptest.gif

The 400k is supposed to be a 1M trim. The amp only requires slight biasing changes for different supply voltages. Of course going beyond 40V would require heftier transistors/resistors/100V caps in some places. Will report back when it's built. RS doesn't sell insulator kits for the 2N3055 which it does sell. Is that stupid or what?



Title: Re: Joe Davisson's new Guitar amp
Post by: teemuk on January 06, 2007, 05:44:41 PM
I was playing a bit with this topology again and remembered why I do not really like it... No offense Joe; you have done nice work but I just hate this kind of topology in general. Anyway, I decided to bring this thread back to life.

Placing a potentiometer in front of the circuit will cause an "unnatural" variation in gain while volume is adjusted since it effectively creates a voltage divider together with the base resistance. Ideally the potentiometer should have a small resistance to keep the effect of the voltage divider as small as possible. A resistance at least 10 times smaller than the resistance of the resistor from input transistor's base to common is required to ensure even mildly accurate volume adjustment. If input stage's impedance is high it's good but far from ideal.

Basically, this topology will require a very low value potetiometer so that the adjustment will happen logarithmically. Or better: The volume potentiometer is separated completely with another gain stage or buffer. Since the transistor count before voltage amplifier stage effectively becomes two it makes one to think of using differential input instead.

Couple of mods could do good for this design: Dividing the feedback resistance to an AC bypassed trimmer and plain resistor will allow to adjust the DC level at the "output" node (before capacitor coupling) to half supply.

Edit: Missed the point of implementing the trimmer... Well, let's just call the method I described another way to do it.

I see that the output is quasi-complementary. One should change the place of "emitter" resistor on the PNP side so that it will actually be an emitter resistor. This is important at least if more output transistors are used in parallel. The former place of emitter resistor should house a diode that has its anode connected to feedback loop/output node and cathode to driver transistor's emitter. it should have a parallel resistor. The collector of the power transistor connects straight to the feedback loop/output node. Without these components the quasi-complementary EF is prone to oscillation bursts near clipping region. By the output node I mean the node before AC coupling.

The 1k resistors used in the driver section are "at the edge". If their resistance is too high it might be that the amplifier will not get pulled away from saturation state. Suggested value is between 100 ohms - 1kohm. An inclusion of Zobel network would possibly be beneficial as well.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on January 24, 2007, 08:46:48 AM
I made some changes you suggested, this is gonna be it...
http://www.diystompboxes.com/analogalchemy/sch/guitaramp.html

Thanks for all the great advice.




Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on January 24, 2007, 01:24:16 PM
How's it sound Joe?  I'd love to hear some clips and maybe see some pics of the final revision.

And maybe you could add this to the wiki, or if you want, I could add it for you.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on January 28, 2007, 10:01:50 PM
Pictures of the build, which is the result of a complete lack of planning:

http://www.diystompboxes.com/analogalchemy/tmp/amp3.jpg
http://www.diystompboxes.com/analogalchemy/tmp/amp4.jpg

I didn't use a feedback loop. A guitar is enough to make it distort just slightly when playing hard, but does so in a crunchy kinda way (the point of using that particular input stage, it can't really hard-saturate).

I estimated 28W, and it sounds about that volume when maxed. The preamp is going to be a 4-stage Vulcan type circuit with 3-band + presence.

Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on February 01, 2007, 11:25:28 PM
I settled on a 10k volume, but I agree it's not the best situation. The preamp will have a buffer before the tone stack, hopefully that will help.

Owell, it's a cheapskate amp but was a good learning experience. :)

Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 02, 2007, 07:15:25 AM
I still think it's a really cool thing.  As much as I like chip amps, discrete circuits are just so damn cool.
Title: Re: Joe Davisson's new Guitar amp
Post by: teemuk on February 02, 2007, 09:13:32 AM
Getting the thing to work with such long wiring tells something about the stability of the design. I like the way you mounted the emitter resistors.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on February 09, 2007, 11:10:51 PM
Went back to the drawing board (not complete):
http://www.diystompboxes.com/analogalchemy/tmp/amp5.gif

Basically a giant opamp. Probably needs some capacitors here and there. Automatically adjusts for rail voltage, just need to adjust the Vbe multiplier. Thoughts and ideas welcome.





Title: Re: Joe Davisson's new Guitar amp
Post by: teemuk on February 10, 2007, 07:13:28 AM
Current mirror to LTP (depends on how well balanced the LTP is already), Zobel network to output, VI limiter? I don't see a limit for DC gain in the feedback path?

No high frequency compensation? Dividing this for both VAS (Miller cap) and the feedback loop yields better results than using the Miller cap only. Do not use too high value in the loop or you get oscillation.

Well that's about it. This reminds me some Rickenbacker amps - they also used an inverting LTP configuration.

I see you run this on a simulator: Replace those constant DC supplies with a sim of a real supply (at least for the part of rectifying, filtering and DC resistance of the source). Replace the constant load resistance with a Thiele-Small model of a speaker. You will get a lot more realistic results by doing that.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on February 11, 2007, 10:30:25 PM
few changes, but not done:
http://www.diystompboxes.com/analogalchemy/tmp/amp6.jpg

The fuse is there as a reminder that it needs output protection of some sort. Will probably put this on the back burner for awhile.




Title: Re: Joe Davisson's new Guitar amp
Post by: teemuk on February 12, 2007, 10:57:26 AM
I assume the correct link is:
http://www.diystompboxes.com/analogalchemy/tmp/amp6.gif

Edit: Looking good. C8 seems like an unneccessary component as it's in CCS. You can't utilize Miller effect there so the value should be quite large to have any effect on high frequency roll off. I'd leave slots for C5 and C6 on PCB but for starters would try to get rid of them. As the rail voltage is pretty high you're really pushing the limits with just one pair of output devices.

For anyone interested, a very good information source concerning VI limiter design is found from US patent 7,061,740, System and method for protecting an audio amplifier output stage power transistor, Eric Mendenhall.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on February 13, 2007, 08:45:05 AM
http://www.diystompboxes.com/analogalchemy/tmp/amp7.gif

I planned on doubling up the outputs but it's shown now. I raised the 100 ohms back to 470 ohms because of the higher rails. The output centers up better (at least on the simulator) this way.

I decided to build it (nicely this time, heh). Should get about 100W, give or take depending on what values are used for the 470 ohm resistors.

Thanks for the correction and other information. Will check into adding a limiter to it.





Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on February 25, 2007, 09:45:18 PM
Built and tested this:

http://www.diystompboxes.com/analogalchemy/tmp/newamp.gif
http://www.diystompboxes.com/analogalchemy/tmp/clip.gif

I have to rebuild it for several reasons, but the initial test was a thumbs up. This amp sounds great (check out the soft-clipping on the simulation).

The 2nd stage is prone to thermal runaway so requires good heatsinking. The output is centered with the 10k trimmer, and the quiescent current with the 1k trimmer, like most other amps. The part running off the bottom is the short-circuit protection circuit. Negative feedback is through the 470k resistors.

Edit:
I don't have a scope so I can't do much more testing than that. 4 ohms didn't really work in my proto (hum/oscillation), but I mounted the transistors/board pretty close to the fuse/power transformer. The heatsinking was probably inadequate anyhow. I also think better transistor choices can be made than the relics I was using.


Title: Re: Joe Davisson's new Guitar amp
Post by: XinTX on June 15, 2007, 09:50:22 AM
Is it just me?  None of the links work. 
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on June 15, 2007, 10:22:48 AM
Nope, its not just you, it seems all the links are down.
Title: Re: Joe Davisson's new Guitar amp
Post by: J M Fahey on May 12, 2009, 12:14:55 PM
Hi friends. I got interested/curious about this amp, after seeing that Joe is back at the drawing board, but none of the 15 or 20 links works. Can anybody re-post or re-link them?
Thanks.
Title: Re: Joe Davisson's new Guitar amp
Post by: sergscar on May 12, 2009, 06:44:58 PM
Hi guys !,
I've browsed the entire site, and seems to be the directory (and the files) have been removed from the server.

At the present stage, the only possible solution should be that some of our friends post the files/schema in some place, and link it here. Obviously, if someone copied/downloaded them !

Rgds,
Sergscar
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on May 12, 2009, 07:14:25 PM
Sorry guys, my computer was ruined in a flood, so I can't replace the images. The good news is that they probably weren't worth looking at anyway :)

Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on May 19, 2009, 10:59:19 PM
A little update on the amp project:

Have been doing a lot of simulation work, and it looks like I can make the "diode compression" trick work with the Depalma amplifier. This will be a very easy build (~40 parts), but I'm pretty sure it needs to be capacitor-coupled because of possible DC offset problems.

If the extra peak compression is worth bothering with (the Depalma amp already soft-clips) then it's very easy to make this switchable, or even variable.

I've made a cheap 12" combo test rig & chassis for the project already, just need to build the thing when I get some parts in.

Sorry again about the files, will keep them available in the future.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on May 20, 2009, 06:57:18 PM
Keep us updated, I'm pretty stoked about this little guy.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on May 22, 2009, 12:05:25 AM
This is the latest circuit, which "probably" will bias itself alright. Went back to the basic circuit, which is probably good enough for government work.

It looks like a strange amplifier, but in reality it's not that much different than anything else. The 1uF/4.7K is a positive-feedback network which increases gain.

Different supply voltages are easily accomodated by changing both 22K resistors to be around the value of the supply voltage. So a 50-0-50 volt transformer would use 47K, etc.
Title: Re: Joe Davisson's new Guitar amp
Post by: J M Fahey on May 22, 2009, 11:46:52 AM
Hi Joe.
What would the "diode compression trick" be?
Thanks.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on May 22, 2009, 09:18:04 PM
This is how "diode compression" is added to the amp. It does a similar thing to this pedal:
http://www.diystompboxes.com/analogalchemy/sch/vulcan.html

The first waveform shows the amp with the diodes disabled (shorted with a jumper) being heavily overdriven at the input. The second picture shows the effect of the diodes. The amount of peak compression depends on the value of the 680K resistor, which in this situation can be from 470K - 1M.

The diodes are biased so that they aren't quite on, but not quite off either. In this situation, the diodes resist signal peaks as they rise, producing the more curved output.


Title: Re: Joe Davisson's new Guitar amp
Post by: J M Fahey on May 22, 2009, 11:44:14 PM
OK Joe, thanks.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on June 11, 2009, 11:47:04 PM
Posted a better copy:
http://www.diystompboxes.com/analogalchemy/sch/diodepwr.html
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 06, 2009, 11:02:52 PM
Ok, I'm getting ready to make this, but don't really know where to get a 50VCT transformer. Would a 50VA transformer be enough for 25 audio watts? Thanks.



Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on August 07, 2009, 06:42:29 AM
Ok, I'm getting ready to make this, but don't really know where to get a 50VCT transformer. Would a 50VA transformer be enough for 25 audio watts? Thanks.





50VA would be enough, but it's always better to have more than not enough.  I'd try this one:

48VCT, 3amp (144VA) - $28
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=120-225
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 07, 2009, 08:47:28 PM
Thats great, thanks!
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on August 07, 2009, 08:52:15 PM
Thats great, thanks!


Any time, Joe.  Keep us posted, I love this little amp of yours.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 16, 2009, 02:12:35 PM
Did some work on the chassis today:
http://www.diystompboxes.com/analogalchemy/sch/dc_wip.html

I found some huge extrusions awhile back, which I cut the heatsink out of. That gets bolted to the chassis with heatsink compound between, sorta making the whole chassis part of the heatsink.
Title: Re: Joe Davisson's new Guitar amp
Post by: Brymus on August 16, 2009, 04:08:22 PM
Hey Joe looking good!
Thanks for sharing ,I am looking forward to seeing the finished build.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on August 16, 2009, 09:39:05 PM
Did some work on the chassis today:
http://www.diystompboxes.com/analogalchemy/sch/dc_wip.html

I found some huge extrusions awhile back, which I cut the heatsink out of. That gets bolted to the chassis with heatsink compound between, sorta making the whole chassis part of the heatsink.


Looking awesome, as always!  Decided against the 48VCT I posted?
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 16, 2009, 09:45:36 PM
Thought I would try it this way, since the transformers are from RS, and only $10 a piece. Also, I think it will bump the output some.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on August 18, 2009, 11:35:14 AM
Thought I would try it this way, since the transformers are from RS, and only $10 a piece. Also, I think it will bump the output some.


Let me know how it works out.  I've thought about using the same setup with those transformers for a LM3886 project.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 22, 2009, 12:55:22 AM
Put another pic up of the board/heatsink. Just a layout, will solder the parts this weekend. You can see the protection circuit on the right-hand side.

http://www.diystompboxes.com/analogalchemy/sch/dc_wip.html
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on August 22, 2009, 08:05:47 AM
Put another pic up of the board/heatsink. Just a layout, will solder the parts this weekend. You can see the protection circuit on the right-hand side.

http://www.diystompboxes.com/analogalchemy/sch/dc_wip.html


That looks great!  Good job, Joe!
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 24, 2009, 03:55:02 PM
I tested the amp today, although it's sorta tacked together at the moment and not at all pretty. I designed it to be compatible with a pedal circuit, so that's what I used for a preamp. Sounded pretty good distorted, but I still need to run a clean tone and see how that works out.

My NPN and PNP transistors were mismatched, so there was about 2V offset. I adjusted one of the emitter resistors on the first stage to compensate, but I really think all complementary pairs need to be matched. (Not all the transistors, just each NPN/PNP pair.)

The power supply uses the two radio shack transformers, two bridge rectifiers wired in series, and two 4700uF filter caps & some bleeder resistors.

Edit: Tested the short circuit protection with a screwdriver across the speaker leads, and it works good. Did a little more play testing and am happy with the sound. So it's a matter of wiring some things nicer and tweaking the bias a little.




Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on August 25, 2009, 06:50:03 AM
I tested the amp today, although it's sorta tacked together at the moment and not at all pretty. I designed it to be compatible with a pedal circuit, so that's what I used for a preamp. Sounded pretty good distorted, but I still need to run a clean tone and see how that works out.

My NPN and PNP transistors were mismatched, so there was about 2V offset. I adjusted one of the emitter resistors on the first stage to compensate, but I really think all complementary pairs need to be matched. (Not all the transistors, just each NPN/PNP pair.)

The power supply uses the two radio shack transformers, two bridge rectifiers wired in series, and two 4700uF filter caps & some bleeder resistors.

Edit: Tested the short circuit protection with a screwdriver across the speaker leads, and it works good. Did a little more play testing and am happy with the sound. So it's a matter of wiring some things nicer and tweaking the bias a little.






You should enter this one in the contest!  Also, once you get it a little more finished I would love to hear a clip or two.
Title: Re: Joe Davisson's new Guitar amp
Post by: Brymus on August 25, 2009, 05:26:59 PM
Radio Shack is gonna wonder why they are selling so many of those trannys soon  :tu:
Again excellent Joe- I imagine alot people will try this one.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on August 28, 2009, 10:52:28 PM
Updated the schematic to match my build, and added a few notes.
http://www.diystompboxes.com/analogalchemy/sch/diodepwr.html



Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on September 14, 2009, 05:31:32 PM
Another update:
http://www.diystompboxes.com/analogalchemy/sch/diodepwr.html
(might have to reload the image)

Did some better parts selection, otherwise not too much different.
Title: Re: Joe Davisson's new Guitar amp
Post by: Brymus on September 14, 2009, 06:21:14 PM
Hi Joe really cool.
Are those all Radio Shack components?
That would make this really easy for anyone to build.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on September 14, 2009, 09:15:53 PM
I tried to design a "radio-shack" amp before. It's almost possible, but would be more expensive than jusr ordering better stuff.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on September 16, 2009, 10:23:54 AM
I tried to design a "radio-shack" amp before. It's almost possible, but would be more expensive than jusr ordering better stuff.


How well are those transformers working out for you?
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on September 16, 2009, 09:03:20 PM
The transformers are working great for this, I'll be posting the complete "radio shack" power supply soon!
Title: Re: Joe Davisson's new Guitar amp
Post by: Brymus on September 16, 2009, 09:33:21 PM
I was eyeing them at RS this afternoon.
Thats really sweet idea you had using two of them at 10$ each hard to beat that price.

I am dying to hear what this amp sounds like.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on September 17, 2009, 08:34:16 PM
Look what the cat dragged in :)
Title: Re: Joe Davisson's new Guitar amp
Post by: Brymus on September 18, 2009, 12:22:26 AM
Oh thats quite a coincidence.
I just picked up those RS trannies today.I was gonna try em with my lm3886 but...
Any chance I could get one?
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on September 18, 2009, 06:24:01 AM
Look what the cat dragged in :)


Sweet  8)

Did you order those or etch them yourself?
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on September 29, 2009, 01:10:10 AM
I ordered the boards. Rebuilt the amp using the new design and it works, except I think the balancing control is too touchy (easy to fix). The rest seems okay, so I'll run it full blast tomorrow and see what happens.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on September 29, 2009, 06:48:27 AM
I ordered the boards. Rebuilt the amp using the new design and it works, except I think the balancing control is too touchy (easy to fix). The rest seems okay, so I'll run it full blast tomorrow and see what happens.


Sweet.  And we want pics of the build - as usual  ;D
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on September 29, 2009, 09:10:36 PM
Checked this out some more.

With the output shorted, and no input signal, one of the output transistors gets warm, and one of the emitter resistors reads .650v across it. It's either one transistor or the other, but never both, depending on how the balance/bias controls are set. I can hold it for 20-30 second, and it's fine except for the warming. Does a VI limiter always throw to one side or the other like that?

The amp is pretty quiet if the balance/bias is tweaked slightly after plugging in the speaker. (plugging in the speaker throws the balance off very slightly). Raising the quiescent from 20mA to 75mA gets rid of most of the hum.

So I guess it's working alright. Sounds nice cranked up, which is what counts. :) Some build pics:
Title: Re: Joe Davisson's new Guitar amp
Post by: rowdy_riemer on September 30, 2009, 10:51:21 AM
Are you going to post any sound clips? :)
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on September 30, 2009, 11:29:19 AM
It looks excellent, Joe.  Are you going to be selling those PCBs?
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 01, 2009, 10:00:51 PM
I'd get the boards out there if people wanted them, but there are some caveats: Because the output drifts slight with temperature, exact output centering is not guaranteed. I'll explain the biasing procedure to see what you all think:

1. With speaker/input sources removed, adjust dc balance and vbe multipler back-and-forth until the output is zero and voltage between output emitters is around 75mV.

2. Connect speaker (or load resistor) and adjust controls again. Check again after a warming period. In the future, the amplifier will begin with a very slight offset (and accompanying hum) which disappears after warming up.

3. Glue trimmers into place.

So the balancing/biasing is a little tricky. I've made changes to the balancing network, which now limits the possible output deviation to around +/-2V, making adjustment easier and somewhat safer.

The amp also requires choosing positive-feedback values appropriate for the application. (This can also program the level where power amp distortion begins.

I have a nice preamp going, using some leftover MPSA06's running off the +35V supply. Gets about a "Marshall" amount of gain, and seems to be a perfect match. Will try to post a sample.
Title: Re: Joe Davisson's new Guitar amp
Post by: teemuk on October 02, 2009, 09:57:21 AM
This is an interesting design but I have to say that on my part I can't really verify this "diode compression" effect taking place. Unfortunately, my experience of this circuit only limits to few SPICE simulations so I don't doubt if the effect actually happens on a real circuit and with the VAS/output transistors in question.

Anyway, here's what the SPICE sim I ran seems to show. Hopefully it provides some alternative food for thought and some further insight to this design.

- The output indeed compresses rather softly but it does so without the diodes as well. With the diodes bypassed, the 470K resistor between the VAS bases can be removed as well.
Where does the compression come from then: The DC bias injected to emitters (instead of a more typical base-bias scheme) is also a resistive divider, that is an alternative shunt path for the feedback. Because of it, the corrective feedback is reduced and this provides the more gradual saturation characteristics. The symmetric topology is also much more immune to "rail sticking" than the generic bootstrapped/CCS-loaded circuits.

- The diode's capacitance introduces less resistance at HF, which in turn gives the amp more gain at high frequencies. If the diodes are bypassed the gain becomes more linear and consequently the high frequency response of the amplifier gets narrower. The effect is only few dBs though and really starts to show above 7 kHz. Basically, aside phase shifting, it's insignificant. Nevertheless I found it interesting.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 04, 2009, 02:18:07 PM
You're right, the diodes really don't do a whole lot. They round out the waveform a bit more, especially if the 470K is increased (but too high and the transistors won't bias). The hi-fi and headphone versions I've made don't use diodes and sound great.

On the preamp circuits the diode helps a lot, because it keeps the transistors out of saturation, but I doubt it's more than a subtle effect here, except perhaps at full blast.

Still need to do an extended full-power test through a load resistor, but other then that seems to be a nice little amp that gets plenty of volume.

Thanks for the comments and help guys!

Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 04, 2009, 03:59:30 PM
I took the diodes/resistor out and couldn't tell the difference, so I guess I'll forget about that idea.

The higher voltage preamp didn't pan out either, went back to a 4-stage vulcan circuit at 9V which sounded much better.

So at this point everything is sounding good and even got some tone controls in there. When I get it all boxed up I'll try to record some samples.

Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 05, 2009, 06:49:26 PM
Made some minor changes to the balance control and lowered the two 10K emitter resistors to 1K:
http://www.diystompboxes.com/analogalchemy/sch/diodepwr.html

Power supply schematic (using the Radio Shack transformers):
http://www.diystompboxes.com/analogalchemy/sch/rs_pwr.html

I think it does sound a little better to use the diodes, so I went back to that on my build.
Title: Re: Joe Davisson's new Guitar amp
Post by: Brymus on October 05, 2009, 07:49:48 PM
Thanx for the update Joe.
Wouldnt be OK to use 4amp diodes X4 instead of those 25amp bridges?
I'm curious cause thats what I was planning on using with those trannies along with snubbers or is that too low a rating?
And are snubbers unnecessary ? I noticed you didnt use any.

Edit OK I went in the garage and looked and those are only 2amp trannies so I think 4amp diodes will be fine.
But what do you think about adding snubbers?
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 07, 2009, 10:46:08 PM
I don't know anything about snubbers, so I can't help you there. I'm sure diodes of proper rating are fine.

Edit: sorry I yanked the preamp circuit, needs more work.

Edit: Preamp circuit is back, seems to work pretty well this way:
http://www.diystompboxes.com/analogalchemy/sch/v4preamp.html



Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on October 26, 2009, 10:31:31 PM
Working on a transplant. Sort of a Recto knock-off, but with an effects loop for each channel, and less features. (Nothing is really mounted yet, just doing a test-fit.) Still working on the channel-switching and fx-loop circuits.


Title: Re: Joe Davisson's new Guitar amp
Post by: Brymus on November 14, 2009, 06:39:18 PM
Wow again it looks really nice Joe!
Cant wait to see more.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on February 01, 2011, 03:11:37 PM
Been a while since I've been on...

Never finished my build, mainly couldn't get the preamp right. Still working on the power amp design though (fixed the VI limiter for starters).

Might explore using an output cap, only to prevent the slight bias shifts (which can occur with different loads or possibly extreme cold/hot) from putting DC onto the speaker. I don't believe it would ever be enough to matter, but cabinets are expensive, and maybe it's "just not right."

:)

Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 01, 2011, 03:38:03 PM
Good to see you back Joe.  I was just wondering today if I should email you to see where you were at with this.

Maybe you should try a different preamp, may I suggest the K20-X?

http://www.ssguitar.com/index.php?topic=1885.msg12904#msg12904

I'm working on making it a 9v battery based pedal but have stalled on the project.  As it is right now it works perfectly on +/- 6v and up.
Title: Re: Joe Davisson's new Guitar amp
Post by: Davelectro on February 01, 2011, 06:14:54 PM
Been a while since I've been on...

Never finished my build, mainly couldn't get the preamp right.

Hi Joe,

What was the problem with your preamp?
Title: Re: Joe Davisson's new Guitar amp
Post by: askwho69 on February 05, 2011, 10:37:08 PM
HELLO ALL!

is there a layout on this? can can i ask quick question? :D how much watts is this sorry cant really thorough my child keeps roaming around need to watch and type  :trouble
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 07, 2011, 09:22:40 AM
Last I remember, something like 30watts were expected from this amp.
Title: Re: Joe Davisson's new Guitar amp
Post by: askwho69 on February 08, 2011, 12:12:02 AM
ohhthis is nice :D but i need something like 70 - 100 watts :D any ideas for chip tda7294?
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 08, 2011, 11:11:35 AM
ohhthis is nice :D but i need something like 70 - 100 watts :D any ideas for chip tda7294?

Any reason you need so much wattage?  Also, are you looking for 70-100w RMS or 70-100w PEAK (aka "Musical")?

A LM3886 will give you 50w RMS/100w PEAK at 8ohm.  They are also rated for 64w RMS/128w PEAK at 4ohm but many have complained of the chip being thermally unstable at that point.
Title: Re: Joe Davisson's new Guitar amp
Post by: askwho69 on February 08, 2011, 09:30:31 PM
well the reason isit has different tone for me and also gain . . .  i dont know really but higher wattage for me is somewhat good to my ear :D LM3886? how about the TDA7293-94? is not good?

:D

A2
Title: Re: Joe Davisson's new Guitar amp
Post by: bry melvin on February 08, 2011, 11:47:41 PM
I Don't think most of the forum is fond of TDA7294 it has an awkward pin out for perf board and a nuisance for designing circuit board. I use some but you need good heat sink designs.

I also get circuit boards from hong kong. I don;t try and make them. I use them for repairs of hopelessly fried power amp pcb's when I repair others' stuff.

As for watts there is minimal difference of sound level from the 30 watts you can get out of a TDA2050 ( you need a LOT of heat sink to do this at 4ohms) the 60 or so from a 3886 or 729X. They are all just fine for home or studio/small gigs... From there I jump to 300-600 wats for gigs.

to double the sound level of a 60 watt amp you really need about 600 watts. (x10)

My stage rig...to keep clean at big gig levels I use two Carvin 200SX plus extensions powered by Peavey 160 watt ss power amps 2 vintage JCM800s for blues/dirty a peavey heritage (130 watts) and an Ampeg g212 (125) for steel and a Guild thunderbird 40 Watt (tube  1964) for some vintage sounds miked to the 1200 Watt PA system.

At home and studio I use a modded Epi valve jr ( about 3-5 watts) Fender FM15 and 25R (modded in the gain channel)(and through an old sound city 2X10 with eminence alnico speakers) for miked sounds and a Peavey (tube)TGRaxx and KMD SS preamps for DI.

There is a BIG difference btween gig and studio amps. TOO much power amp at home and studio at low level and your speakers are not putting out the sound they should be. OR you could have  low power speakers in your amps and you'll blow them if the amp is cranked when you play out. You CAN use something like a pair of eminence alnico 10s with the high powered amp in the home setting and get the 30-60watts worth of speakers in their sweet spot...BUT crank the power too high and you just got a 200 dollar junk pile.

My experience tells me a 100 watt amp is of minimal use at home and in most studio settings. Most lower power amps have crappy speakers. Put a low power amp into a godd speaker and you'll get a surprise.

Here's some examples May with little 10 watt amps  Clapton with a Champ (electric version Layla) With all the amps I have around I spend mopost of my work day playing through a Frontman 15 and a valve jr!
Title: Re: Joe Davisson's new Guitar amp
Post by: askwho69 on February 09, 2011, 03:29:01 AM
Wow bry melvin you really stack me some info there! i thought "before" in recording 100 watts is the best! wow now i know! speaker  - 30 watts - good preamp and tonemender will be good home and studio :D Lm3886 is my goal :D

by the way i play metal so 10 watts still good for heavy riffs in recording? :D
Title: Re: Joe Davisson's new Guitar amp
Post by: bry melvin on February 09, 2011, 08:32:39 AM
To get that kind of gain here in our studio...or in between that and bluesy I often use: guitar active pickups---->pedal of choice---->peavey TJraxx preamp or a KMD SS recording preamp----->Modded Valve JR to mike OR  Crate GX130C preamp ----DI to mixer board or to a homade TDA2050 power amp.    We have better luck using DI with heavy distortion...If not and miked we run a 20 second or so leader (quiet instruments...just hum and  hiss) at the beginning of the track to sample and then filter out later. I have used high powered amps in the studio....but in an "isolation" booth. To get mikes to tolerate high volume is very difficult. I almost never record with a high powered amp unless it's live and outdoors.

 
Title: Re: Joe Davisson's new Guitar amp
Post by: J M Fahey on February 09, 2011, 08:38:54 AM
Quote
LM3886? how about the TDA7293-94? is not good?
Agree and add: LM3886 is powerful enough for pubs and such, has good pinout and you can find usable layouts on the Net, or draw your own, or buy ready made PCBs (Chipamp.com)
TDA729x has *stupid* pinout, must have been designed by a drunk on a Friday night, needs to have pins connected, which have 3 or 4 *other* pins in the middle connected to other components, its heatsink must be *over* the PCB and can't be on one side, etc.
Why didn't they join said pins *inside* the chip?
Stupid.
It forces you to use a double-sided board, and even then the design is klunky, or go the Marshall route: mount it on its own miniboard (still double face) and connect it to the real world through a very unreliable 15 pin connector and ribbon cable.
Have I already said it?: stupid.
And you can only pull 70W out of it (in the Guitar world); use them for 100W (you can) and you'll replace them every 4/6 months.
EDIT:
Quote
by the way i play metal so 10 watts still good for heavy riffs in recording?
Your microphone does not know about watts, its output level will be adjusted at the recording console to the level needed in the mix.
Mor than power, go for a *good* speaker cabinet, because its response *does* get recorded.
A "big" cabinet will give "big" sound.;)
Title: Re: Joe Davisson's new Guitar amp
Post by: askwho69 on February 10, 2011, 10:47:28 PM
Wow again! thanks for very informative and critics about 729X:D i  <3) this forum :D
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 11, 2011, 05:57:10 AM
I didn't know about the TDA729x series chips either, always learning something new here.
Title: Re: Joe Davisson's new Guitar amp
Post by: J M Fahey on February 11, 2011, 07:51:12 AM
Just as an exercise on a long cold boring Winter night, draw the TDA7294 pinout on a sheet of paper, put needed components around and try to solve the maze, penciling lines from pin to pin without crossing them.
After getting tired, redraw using a blue and a red pencil meaning top and bottom layer, it won't be enough either.
Don't worry about drawing a "nice" PCB, just solve the connectivity paths.
You'll soon say: "why oh why they didn't set such and such pins side by side?" or even worse: "why didn't they connect those points *inside* and use only 1 external pin for them?"
And when you solve it, you'll see that inevitably some tracks must pass on the flange side of the chip, making it impossible to bolt it to a regular heatsink or the backpanel.
Or even worse, you'll need to mount your heatsink *on* the board.
Not a good idea to mount heavy parts on the board, specially on a guitar amp.
Google TDA7294 images and see for yourself.
None of that happens with older but well designed LM3886.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 11, 2011, 08:07:49 AM
Juan, this one doesn't look too bad:

(http://i19.photobucket.com/albums/b170/echoloft/Parts%20For%20Sale/tda7294Chipamp.jpg)
Title: Re: Joe Davisson's new Guitar amp
Post by: askwho69 on February 12, 2011, 08:06:18 AM
Wow that is nice!  :-X
Title: Re: Joe Davisson's new Guitar amp
Post by: J M Fahey on February 12, 2011, 08:55:49 AM
HEY !! That's GOOD!
Now there seems to be some problems.
The "readable" pages (from a French site) are labelled as malware.
The actual www.diyzone.net page does not open ¿¿¿???.
They seem to have addressed the problem of joining non-contiguous pins using the very narrow space between the IC legs and the back panel.
I couldn't, (at least not with "safe" width tracks), and had to cheat by using external parallel links on top of the board, between the legs, turning it into a virtual "3 layer" board.
I also needed to cheat by joining standby and mute pins , driving both from the same RC network, to free some precious real estate.
I *hate* running tracks between pins and in general avoid using tracks less than 30 mil wide in Guitar amps.
10 mil ones were invented for PC boards, not for us.
If you can get another pictures from that board, I would love to see them, to check how they did that.
Thanks.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 12, 2011, 04:59:34 PM
I found it doing an image search on yahoo.  Here is the photobucket site it came from though: http://s19.photobucket.com/albums/b170/echoloft/Parts%20For%20Sale/?action=view&current=tda7294Chipamp.jpg
Title: Re: Joe Davisson's new Guitar amp
Post by: morgoth2006 on February 12, 2011, 06:52:52 PM
Not very helpful but here goes
http://web.archive.org/web/*/http://www.diyzone.net (http://web.archive.org/web/*/http://www.diyzone.net)
Title: Re: Joe Davisson's new Guitar amp
Post by: J M Fahey on February 12, 2011, 10:45:28 PM
Hi Morgoth, thanks.
Good links.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 13, 2011, 02:22:58 PM
I fixed your link for you Morgoth.
Title: Re: Joe Davisson's new Guitar amp
Post by: morgoth2006 on February 14, 2011, 06:12:47 AM
Hey JM and Joe. Sorry about the ninjapost of the other day, but free time is something I don't seem to have much lately.
I just searched the Wayback Machine for the server, saw that it was in chinese  xP and posted it, never checked if it was working  :-[
I'll post some doubts soon and some updates on my projects. Here's a sample of what I've been cooking up (literally  ;D)
(http://r-moreira.dynip.sapo.pt/questionario/PIC110214105153.jpg)
Title: Re: Joe Davisson's new Guitar amp
Post by: J M Fahey on February 14, 2011, 08:20:20 AM
WHAT'S THAT??????????  xP
A knob-shaped cookie?
A knob (half) carved in stone by the Flintstones ?
PS: don't worry about the Chinese, I'm already studying it.
Well, still struggling with Lesson 1, but that's the beginning .
Nín Hao !! <--- (hello)
Xiè xie ! <--- (thanks)
and so on, so in about 1000 years I'll be able to read those pages  :lmao:
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 14, 2011, 08:42:49 AM
WHAT'S THAT??????????  xP
A knob-shaped cookie?
A knob (half) carved in stone by the Flintstones ?

I think it's knob shaped fudge.  If that's the case, YUM!
Title: Re: Joe Davisson's new Guitar amp
Post by: J M Fahey on February 14, 2011, 10:26:47 AM
Believe it or not, I once tried to clone a chickenhead knob using polyester resin and that pink or green elastic material Dentists use to take tooth casts (you know I love to experiment with weird materials).
Obviously some chemical there must have reacted with polyester in  a *bad* way, because dental gypsum casts came out perfect, although unusable, and "definitive" plastic ones came out looking like that or worse.
Oh well.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 14, 2011, 10:43:29 AM
Believe it or not, I once tried to clone a chickenhead knob using polyester resin and that pink or green elastic material Dentists use to take tooth casts (you know I love to experiment with weird materials).
Obviously some chemical there must have reacted with polyester in  a *bad* way, because dental gypsum casts came out perfect, although unusable, and "definitive" plastic ones came out looking like that or worse.
Oh well.

You could make a clone if you made a mold then used JB Weld (http://jbweld.net/index.php) as a substrate.  Plenty strong and easy to work with.  Not horribly cheap, but for a knob you wouldn't use a whole lot.
Title: Re: Joe Davisson's new Guitar amp
Post by: morgoth2006 on February 14, 2011, 07:02:57 PM
Thank God I have a pretty good self esteem. I showed that to my wife today and she said "Oh, how nice, you made cookies..." now I get here I and it's flintstones era.... oh well...  ::)
Well, jokes apart, I tried making some moulds, this one was made with gypsum plaster, that's why the "cookie look"  :D
I'll start a new thread to share my experiences.
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on February 14, 2011, 10:43:17 PM
Fixed the preamp, apparently the direct-coupled buffer before the tone stack was screwing up the sound. So I cap-coupled the buffer, and altered it to include an input diode as in the other stages. Don't have time to fix the schematic(s) right now, unfortunately.

Has anyone tried FIMA clay for making knobs, etc?




Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on February 15, 2011, 08:28:35 AM
I look forward to seeing the new schematic.
Title: Re: Joe Davisson's new Guitar amp
Post by: yaff on June 23, 2011, 01:18:54 AM
So, is there anybody made this new JD diode compression power amp? What about it's overdrive?(only power amp)
Title: Re: Joe Davisson's new Guitar amp
Post by: Joe on September 15, 2011, 11:00:11 PM
Finally finished this project. Ended up using a Dr. Boogey style preamp, but added channel switching and fx loops. It's all pretty basic but works. Turned out to be a reasonable fascimile of the original, but a little harsher sounding.

Decided against using the diodes in the power amp section, like Teemuk said the amp soft-clips anyway and sounds great cranked on its own. Seems more stable this way too.

Overall not bad for a SS amp, sometime I would like to redesign the power amp board and use a pcb for the preamp instead of rat-shack perfboards (and the rat's nest wiring job I did) :)

Edit: Drew up the power amp in its final state.
Title: Re: Joe Davisson's new Guitar amp
Post by: joecool85 on October 17, 2011, 02:24:24 PM
Any sounds clips or pics of the final version?