Don't knock the sim programs. I have no doubt that pentodes can be modeled as well as JFET's, MOSFET's, etc. The problem with simulated components is that real world components behave a little differently. While they are ultimately no replacement for testing using real components, simulations can be great for quickly running through different ideas. If it works in simulation, it MIGHT do well when you breadboard it. If it chokes in simulation, there's a good chance there's a problem with your design(but not always). Of course with digital applications, software simulation kicks lots of ass.

Thanks, JM.

The eq I picked up had a dead left channel, which would be fine since I only need one. But I just hated having a dead channel, so I pulled it apart and tried to diagnose the problem. None of the electrolytic caps looked bad. I couldn't see any problems with cracks in the pcb. But I did notice that each channel had a 4558 dual op-amp. On the left channel I could detect a signal on pin 6(+ input), but there was only a 60 Hz signal on pin 7 (- input) and pin 8 (output). Well, I knew that wasn't right, so I pulled that 4558 off the board, and soldered in another one. That did the trick. I know that's a fairly simple fix and not too big a deal, but being that that was the first factory-built audio appliance I successfully trouble shooted and fixed, I was pretty happy about it.

QuoteBE VERY CAREFUL BECAUSE OTHER PINS CARRY DEADLY VOLTAGES , ONLY A COUPLE MM AWAY, measuring probes and hands slip easily.

Yeah, don't forget the one-hand-in-your-pocket rule. xP

Cool, thx for the info. I'll definitely give this a shot. BTW, I just picked up a stereo eq yesterday at the thrift store. Only $5.

There's obviously a lot of convincing ss circuits out there. After building a dr. boogey, it's hard to imagine forking out $2000 for a similar sound. Of course that's all preamp distortion. And of course, there's good emulation of power stage distortion. After getting a tube amp, I really like the way it sounds clean, especially with single coil pickups. It's the clean triode sound that I think might be hard to emulate. But then again, my experience with different amps is limited.

Quote'm building the "Eighteen" Marshall 18W stompbox at runoffgroove.com http://runoffgroove.com/eighteen.html

I'm puzzled about the following :

-  In both the schematic and perfboard layout I don't see what the ground connector from the battery connects to on the perfboard.  It just shows the +9V connection in both instances.  Where does the negative terminal of the battery connect to the circuit?

On the lower left of the perf board layout you can see the GND(ground) connection point. On the schematic, anywhere you see the ground symbol is connected to the "-" terminal on the battery with negligible resistance in between(the resistance of wire/solder,etc.)

Quote

-  When connecting the circuit to a guitar there are two connections for the guitar cable.  I see the connection labeled IN on both schematic and layout but where does the other connector go?

"OUT" is the wiper terminal(middle lug) of the volume pot. The layout only shows where to connect lug 3. Lug 1 is connected to any grounded point. If you want to solder  a wire to any point on the perf board that's grounded, that's ok. Lug 2 will go to your output jack (or switch for bypassing).

Quote

-  Which connector on the guitar cable is the IN connection?  Is it the Tip or the Sleeve?

The tip is signal. The sleeve should be connected to ground.

Quote

-  I'd like to use some real POTs instead of the plastic little boxes with the small plastic screws inside so I can mount it in an enclosure and easily turn the parameters from the outside once it's built.  Any suggestions on POT brands and ideal impedance levels?  Any better than others?  Under the tweaking notes it suggests trying different spec POTs.  Just wondering if any of you have built this before and had recommendations.

The plastic little boxes with screws are trimpots used for biasing. Once you set the bias for each stage, you really want to leave it alone. These are not the pots for volume and tone control mentioned under "Possible Mods". I've not built this, so no suggestions on values. I like 16mm Alpha pots, but I haven't really tried any other brand. Alpha pots should be good to go, though.

Quote

- BOM is there a parts list for this thing anywhere?  I don't see it on their site.  From a beginners standpoint it's difficult to source the right parts.  Sometimes there are so many options I get lost trying to order parts because there are so many options and I'm not sure what some of them mean.

Thanks in advance.

It should be fairly easy to make your own bom, but send an email to their contact email address (holler@runoffgroove.com) and see if they can help you out with one. There might also be a bom somewhere out there on the net. 1/4 watt resistors should work for all resistors. Get metal film caps, except where on the schematic there are caps with a + sign. For those use electrolytics. As cheap as j201's are, just order several. That way, you've got plenty for future projects.

If you haven't ever built a stompbox, you might checkout the wiki at diystompboxes.com. I've honestly not built a circuit as a stompbox yet, though I've put some circuits in metal enclosures (basically a preamp without a stomp switch). Maybe if I ever get over being a cheap bastard(I really need to with this hobby), I'll order a proper stompbox enclosure and stop switch.

Good luck. Please post sound clips and stuff when you get it built. 

QuoteQuote from: rowdy_riemer on 16 May 2010, 22:27:13
Quote
Worth a mention is Speakers.
I'll tell you a little story;
My friend of many years came to me with his Teck21 Trademark60.
He simply asked if there was any way to make it sound like my gear.
I won't bore you with details but he was stunned at the result.

I simply replaced the speaker with a far *LESS Efficent* speaker.

I like the sound of my little 5W amp, but I sure wouldn't mind a less efficient speaker if for no other reason than to make it quieter.
BTW, next time I go to a thrift store, I think I'll pickup whatever old eq they have. Thumbs Up

Hi, Can I enquire if the little 5Watt Amp is a Valve unit?
If so you can do some tricks by ReAmplifying it and then run it through the cheap Graphic (you will no doubt find at a junk sale) then into a second amp.
I've done that trick to a simple Valve Amp, with great results.
No one wants those old HiFi type EQ's anymore so they tend to sell dirt cheap. Thumbs Up
Phil.

It is a valve unit. It's a Peavey Valveking Royal 8, with a few mods. I may do the re-amping thing. It sounds like a good idea. But I definitely would like to keep the setup as simple as possible for practice. I might try one of the various power attenuator projects out there, but I know they've got to have some effect on tone. I guess intentional impedance mismatching could also maybe help. Do you have any suggestions for a re-amping setup?

QuoteWorth a mention is Speakers.
I'll tell you a little story;
My friend of many years came to me with his Teck21 Trademark60.
He simply asked if there was any way to make it sound like my gear.
I won't bore you with details but he was stunned at the result.

I simply replaced the speaker with a far *LESS Efficent* speaker.
***Remember***
Modern Amp makers use very high SPL drivers simply because the louder it sounds in
the shop the better chance of a sale.
(apparently young minds only respond to loud sounds)

I like the sound of my little 5W amp, but I sure wouldn't mind a less efficient speaker if for no other reason than to make it quieter. When my wife is across the street at her grandmother's house, she can hear what I'm playing, and practicing with the amp after the kids go to bed is out of the question. I also do not like the way it makes my ears hurt when I've got the master volume cranked. I use a little sh#tty digitech RP-70 as a headphone amp for quiet practice, and it's just not the same as playing through a real amp. I think I might start looking for some old speakers to see how they sound. I added an external speaker jack to my amp, so experimenting with different speakers should be fairly easy.

BTW, next time I go to a thrift store, I think I'll pickup whatever old eq they have. :tu:

QuoteI probably won't have a chance to try anything for a few days because I'm bogged down with uni assignements

Does "uni assignments" mean university assignments?. If so, I sympathize. Sometimes I want to quit school so I can spend more time playing guitar and building stuff. I'm not going to, though.

QuoteIn close on 30Years of messing around with 100's of circuits I've only designed/built a couple of circuits that I'm seriously proud of.

That reminds me of something a professional photographer told me. He said he only liked about 2 or 3 pictures out of every hundred he took.

There are already many similar posts. You should look through them for ideas. I designed a simple (maybe too simple) preamp using fetzer valve biasing specs. It sounds pretty good to me for a clean tone. I don't know that it sounds as good as the clean tones I get through my little Valveking Royal 8. If you haven't already done so, check out the different projects at runoffgroove.com. If you're looking for a good heavy metal sound, the Dr. Boogie is great. Its bass response might be a little lacking, but through my GFX-212, it sounds pretty bad ass. One of the best sounding pedals I've heard(though not in person) is the Wampler Pinnacle II http://www.youtube.com/watch?v=7x1aq4ffmzM . I think it is based on the brown sound in a box 2 design, but I'm not sure. The brown sound in a box II might be a good choice too.

I suggest building a separate power amp, and try several different preamps until you find one you like. Put a solderless breadboard into a metal box with some holes drilled for jacks, pots, and switches. Then you can easily try different circuits without having to solder. By having the breadboard in a box, you can still shield the circuit from noise. When you get something you like, then you can go through the trouble of etching the pcb and all the other stuff you do with a permanent build already knowing you'll like how it sounds (barring any stray capacitance issues with pcb layout and stuff). Good luck. :tu:

Well, the .1 Ohm on resistance of a mosfet IS resistance. I'm saying that the output impedance of the amp is not all resistive. I'm also talking more about the output impedance of the output stage of the amp rather than the impedance of the load(the speaker). If you look at the instantaneous impedance of the power stage when it is on, it probably is damn small. It's also damn large when it is off. Averaging the current over time, during which it is switching on and off, will tell you what the average impedance is. Apparently, even at full output, there is enough off time to have an impedance somewhere around 16 Ohms or at least significantly more than 8 Ohms.

QuoteHi Rowdy,

Actually there's no "impedance match" problem for class D audio amplifier at all. The internal mosfet is working in on-off mode. When it's turned on the equivlant resistance is only about 0.1Ohm. The max power of class D amp is depend on the maximum current it can drive. When the current is fixed the bigger the load impandence, the bigger the power. That's why the output power is larger for 16Ohm than 8Ohm. Of course the max power is limited by some other factors such as supply voltage.

Ok, when switched on , the resistance of each MOSFET probably is about .1 Ohm. BUT, impedance is more than just resistance. The current limiting effect of the class D operation IS a type of impedance. ANYTHING that impedes current is impedance and can be measured in Ohms. Also, keep in mind that it takes time to switch from all the way off to all the way on. If you find the average resistance during this transition and multiply that by the switching frequency, you have that much more resistance to add to the .1 Ohm of on resistance. According to my calculations based on P=I^2 * R and what the data sheets shows, you have about 1.18 A of current with either 4 ohm or 8 ohm speakers, but only .968 mA of current with 16 Ohm. If it were as simple as multiplying some constant current times the speaker impedance, you would have 1.18 A ^ 2 * 16 Ohm to get about 22 W with a 16 Ohm speaker. You could also get 118W with a 100 Ohm speaker if this were the case.

Also, Class D operation is simply applying pulse width modulation to the signal and using the pulse width modulated signal to switch a CMOS or H-Bridge (or something similar) output stage. If there is some current sensing feedback to adjust the pulse width to achieve a desired output current, then this is a feature that is an addition to class D operation and not necessarily a part of class d operation any more than current or voltage feedback is a part of class a or class ab operation rather than an additional feature. Perhaps this ic is doing some sort of current sensing feedback. This does not change the fact that it has a significant output impedance. That impedance might not be absolutely fixed, but it remains higher than 8 Ohms.

The maximum output voltage at 16 Ohms would be .968 mA * 16 Ohms, or 15.5 V (Yeah, I know it wouldn't be precisely .968 mA). The output voltage at 8 ohms would be 1.18 A * 8 Ohms, or about 9.4 V. The output voltage for 4 Ohms would be 1.18 A * 4 ohms, or about 4.7 V. Regardless of class of operation or types of feedback or types of impedance, this tells me the output stage is getting loaded down more as you decrease speaker impedance, and this is not unlike what you would have with a Class A or Class AB amplifier with an output impedance significantly greater than 8 Ohm. What's great about Class D operation is that much of its output impedance is reactive rather than resistive, meaning less power wasted as dissipated heat.

QuoteThe power can be up to 15W with 24V supply on a couple of 16Ohm speaker. This is strange when comparing with traditional analog amplifiers.

It's just an issue of impedance matching. The output impedance must be closer to 16 Ohm than 8 or 4. Remember, with matching impedance, you get you're greatest transfer of power. With an 8 Ohm load, if I remember correctly, you get 10 W at that voltage. With the lower impedance, the power stage is loaded down and though you have more current, the voltage is lowered enough that you end up with less power.