[crk]

Hi Expertz
I've just finished my Apheared 47 and managed to squeeze it into a Curiously Strong Peppermints tin. The circuit is built on Veroboard and exactly as described in Apheared's Headwize article, except the gain is 2. I also added 100pF in parallel with the gain feedback resistor on each gain stage. In fact, I had a lot of problems with stability. I don't have a scope but it looked as if both channels would start to oscillate and produce a DC offset at the o/p of ~80 mV (measured on a Fluke multimeter) and distort. AC coupling the inputs and the 100pF caps appeared to solve this problem. It is powered from a single 9V battery with 4.7K resistors setting the ground point.
Anyway, it sounds great except for a very slight crackle that is just noticable (from time to time) in certain passages (low volume mid-range). I've compared different amps and headphones so I'm sure it is the amp.
Any words from the wize (or fize) would be appreciated....
crk
90% of the time

 

[Daniel Pumphrey]

Hi,
The crackle could be from overloading the input or from a bad pot. What kind of pot are you using? The 134 and 132 should not need a cap to keep it stable, so there must be a problem elsewhere. Plus a gain of two is not very much.
Dan

 

[crk]

Dan
Thanks for the reply. I'm using the miniature Noble pot (from Carl Hansen). Good point about the caps, I haven't needed them in my CHA with X-feed amps. Gain of 2 is all I need but do you think the stability would improve with a higher gain? Also would a rail splitter be preferable to the resistive divider?
crk

 

[ppl]

Dose the Noise sound like a dirty Pot? if so the problem is oscilation. The 200Pf cap should not be added to stop oscilation. Fn fact in alot of cased addind the cap will not stop osc. just change the Frequency it oscilates at. Addind a coupling capacitor also is not a cure for Problems except in cases where the Source component has a DC offset. While the measures you taken may have made a nonworkable Amp into a workable one you have not fixed the Root of the Problem, just coverd up the symptoms. I would find out the Real cause and fix that rather than adding band ades.

 

[crk]

Thanks folks
Here's an update! The amp works fine with my Rat Shack Sporta Pro clones (60 ohms). With the Grado SR-80, the amp works fine if you don't change the volume control... (If you do, turning the amp off and on restores normally operation). With my RS-1's it won't work at all, or rather it sounds terrible.
When the amp is working, there is a dc offset of about 2mV at each channel, and the mid point of the voltage divider (ground) is within 0.1V of the mid value of the (single) 9V battery voltage. When the amp is distorting there is a dc offset of about 18 mV/channel and the mid point of the voltage divider is way off (over 2V!).
As it is in an Altoids tin, two 9V batteries won't fit. I'm going to try the TLE2426 rail splitter.
TIA for any suggestions
crk
ps PPl, with the SR-80's it does sound just like a dirty pot when you rotate it. (But no noise with the Sporta clones)

 

[tangent]

Have you checked your grounds? Specifically, are you absolutely sure your circuit and chassis grounds are separate? Measure from your circuit ground to the metal case to be sure. Also, make sure you aren't grounding out one of your signal paths. I did the latter once by melting a headphone jack so that it shorted out internally -- you couldn't tell it was bad, but replacing the jack fixed the problem.

I once had a very similar problem to what you're describing, and it turned out to be bad grounding. My hyper-efficient Porta-Pros worked fine, but any time I plugged something more demanding in, the opamp became unstable and either shut down completely or the volume dropped and the sound was horrible. Like in your case, power-cycling the amp fixed the problem temporarily.

ppl's right -- you're fixing the symptom and not the cause.

 

[crk]

Tangent
Thanks. The chassis and signal ground are connected togther through the input and output jacks ground connection. (All my other amps have been in plastic cases so this wouldn't be an issue). However, the circuit board is isolated from the chassis.
Thanks again
crk

 

[tangent]

Quote:

The chassis and signal ground are connected togther through the input and output jacks ground connection. (All my other amps have been in plastic cases so this wouldn't be an issue).

That's why your other amps work and this one doesn't! Again, the signal ground -- which includes your input and output jacks' grounds -- needs to be separate from the chassis ground. It's a wonder you're getting sound at all.

By the way, if the reason your input jacks are chassis-grounded is because they're uninsulated RCAs, you will need to add some insulation between the jacks and the chassis. Beware: the vulcanized rubber grommets you can find at your local hardware store are not the right solution for this; they actually conduct electricity weakly, so they're no better than no insulation at all. Use plain rubber or plastic insulation.

 

[Possum]

Quote:

needs to be separate from the chassis ground

In his case, this shouldn't be a problem. His chassis ground is his signal ground because nothing else using it as a ground other than his input/output jacks. He's using a candy tin for the case, and is powering it with a 9V battery.

Key clues from his posts:

"It is powered from a single 9V battery."
"However, the circuit board is isolated from the chassis."
"The chassis and signal ground are connected togther through the input and output jacks ground connection."

 

[tangent]

Quote:

His chassis ground is his signal ground because nothing else using it as a ground other than his input/output jacks.

I see my mistake. The amp I had fail because the RCA inputs were chassis-grounded also had a DC power input jack. Therefore, insulating the RCA jacks kept them from grounding out through the power jack, not because they actually had to be tied to the signal ground directly. I now see how crk's circuit grounding scheme works, and that it should be correct.

Thanks for the clue, Possum!

Now if I can only figure out what other similarities my troubled amp has with his -- the symptoms still are the same. Only my diagnosis was incorrect.

 

[crk]

Elementary my dear Watson....
Seriously folks, thanks for your thoughts.
Can anybody reassure me that the Apheared CH47 design (ganged opamps connected with 47 ohm resistors) works with a single 9V battery without the problems I've been having?
Thanks again
crk