[wab]

I built an Apheared 47 amp today. I used 4 OPA134's and run it with a 24V powersupply. At lower volumes it sounds really good, at higher volumes I get lots of distortion. I checked for solder-bridges and found none. What could be the problem here?

 

[source direct]

Sorry about this being irrelevant, but props to your Aphex Twin.

 

[wab]

hehe it's pretty cool eh?

thanks for bumping this thread!

 

[Apheared]

Must be all that Cornish Acid. Since no one else will see it anyway, I'll put this here: http://www.andrew.cmu.edu/~paw/music/afx.htm Although giving Cornish Acid only a B+ means this guy's a nutbag.

wab, lemme apologize for myself and all the other morons here that let a brother slip thru the cracks... 11/03 and no reply? damn musta been a busy day that day.

Anyway.

What kind of distortion? How loud is loud? Does the amp have a pot or are you using a source with it's own volume control? What's the total current drain at idle? What headphones, and have you tried different headphones and what was the result?

 

[wab]

Apheared, the distortion comes in at low to moderate volume. The distortion is a fuzzy recorded-too-loud kind of distortion. I used an Alps 10K volume-pot. I tried it with my Sennheiser HD495 (32 ohm, low efficiency) and cheapo Philips (32 ohm, high efficiency), both distort heavily at higher volume levels. Measuring the current drain is kinda hard, I would have to desolder the power-jack and don't have acces to my soldering tools right now.

I checked my circuit and there's no errors in it and no cold joints or anything.

 

[Apheared]

I would have to desolder the power-jack and don't have acces to my soldering tools right now

That is a bad thing. Why'd you put em away when it didn't work in the first place? Hmm. Buy another $10 iron and keep it in your backpack!

It could be lots of things. Do the ICs get warm, at all? Any different than ambient air temp? (unless you're sitting on a radiator or working in the snow, that is)

If you futz with the jacks, both input and output, can you change the distortion? Do you use an input cap? is there a resistor to virtual ground after it? What value load share resistors did you go with? What is your source? Can you use a source with it's own attenuator like a portable CDP and see if less input clears it up?

Usually you simplify; work methodically forward or backwards until you find the spot(s) of trouble. This requires jumpers and (de)soldering.

 

[wab]

Quote:

That is a bad thing. Why'd you put em away when it didn't work in the first place? Hmm. Buy another $10 iron and keep it in your backpack!

I know, I know! It's just that not everything is mine to begin with.
Quote:

It could be lots of things. Do the ICs get warm, at all? Any different than ambient air temp? (unless you're sitting on a radiator or working in the snow, that is) If you futz with the jacks, both input and output, can you change the distortion? Do you use an input cap? is there a resistor to virtual ground after it? What value load share resistors did you go with? What is your source? Can you use a source with it's own attenuator like a portable CDP and see if less input clears it up?

The IC's don't get warm. I can't change the deistortion. I followed the exact schematic from the article. I tried with my Marantz CDP and portables, less input volume still gives distortion.
Quote:

Usually you simplify; work methodically forward or backwards until you find the spot(s) of trouble. This requires jumpers and (de)soldering.

I will as soon as I have access to my tools again.

I think it's kinda weird I have equal distortion in both channels, that rules out a lot of possible causes right?

 

[paulb]

I wonder if the power supply voltage is too high. With low impedance phones, you need less voltage and more current, so more voltage gets dropped across the IC. Although you did say they don't get warm.
Have you got access to higher-impedance headphones, or a way to reduce the supply voltage to see if that makes a difference? Or try putting a 100 ohm or so resistor in series with the headphones.
Another suggestion for troubleshooting: disconnect all the other stages and drive the headphones from a single op-amp. As Apheared says, simplify.

 

[Apheared]

Ok, is it a choppy crackly kind of distortion that's (for lack of a better word) edgy? Or more a smoother warmer "fuzzy" distortion, like what an overdrive for a guitar sounds like? (the former is probably oscillation, the latter is probably a short on the output, although I haven't fully tested these)

Here's an idea; take two 9V batteries and run them in series for 18V and use this as your power supply... see if it is power supply related distortion.