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Author Topic: Joe Davisson's new Guitar amp  (Read 68762 times)

joecool85

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Re: Joe Davisson's new Guitar amp
« Reply #15 on: October 16, 2006, 08:49:21 PM »
Thats great, looks like we should be able to do some cool stuff with this.
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Joe

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Re: Joe Davisson's new Guitar amp
« Reply #16 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).


joecool85

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Re: Joe Davisson's new Guitar amp
« Reply #17 on: October 17, 2006, 07:45:40 AM »
I still wanna see pics :-)
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Joe

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Re: Joe Davisson's new Guitar amp
« Reply #18 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.

Joe

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Re: Joe Davisson's new Guitar amp
« Reply #19 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).


joecool85

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Re: Joe Davisson's new Guitar amp
« Reply #20 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.
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Stompin_Tom

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Re: Joe Davisson's new Guitar amp
« Reply #21 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!
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Joe

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Re: Joe Davisson's new Guitar amp
« Reply #22 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 :)

joecool85

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Re: Joe Davisson's new Guitar amp
« Reply #23 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.
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Joe

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Re: Joe Davisson's new Guitar amp
« Reply #24 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.






joecool85

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Re: Joe Davisson's new Guitar amp
« Reply #25 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.
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Joe

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Re: Joe Davisson's new Guitar amp
« Reply #26 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?




teemuk

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Re: Joe Davisson's new Guitar amp
« Reply #27 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.
« Last Edit: January 06, 2007, 08:29:31 PM by teemuk »

Joe

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Re: Joe Davisson's new Guitar amp
« Reply #28 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.





joecool85

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Re: Joe Davisson's new Guitar amp
« Reply #29 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.
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