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[syndromet]

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

[teemuk]

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.

[Joe]

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.

[joecool85]

Sounds like a good simple build, not a stage rig, but definetly something cool to play with.  Thanks for signing up too btw!

[Crystallas]

Welcome aboard Joe!

[Joe]

Thanks for the invite. Looks like a real nice board.

[Joe]

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.

[Joe]

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.

[Joe]

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!

[joecool85]

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 :-)

[Joe]

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.

[joecool85]

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.

[Joe]

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.

[joecool85]

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.

[Joe]

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.

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