[eric343]

When I power my Ety4S off of my Cmoy, I can hear white noise in the backround if the music is quiet. I do *not* have the 'optional load resistor' in place as described by Cmoy in the article, because the Etys have a 100 ohm impendance.

 

[Tomo]

Hello,

Your concern is not necessary. Load resistors should not interfere with the operation of your amplifier even if you use Etymotics. Considering how powerful OPA132/4 or anything you are recommended to use for CMoy amp, 100 ohm resistors will not prevent the opamp from doing its job. Only it can damp out the oscillation at output. In fact, I use mine with Etymotics and it has 100 ohm load resistors.

Your oscillation is probably from elsewhere. Have you checked the amp with different sources? Have you checked the board and make sure no parts have faulty connections? You could be picking up RF in that case you can test it by wraping the whole amp in a aluminum foil.

Tomo

 

[eric343]

Actually, I left out the load resistors because I wanted to make the amp as simple as possible and I didn't think I'd need them. However, I ordered some Hansen PCBs, and I think I'll build both with load resistors.

So, if 100ohms is good for Etys, what's good for Grados? 200? 400?

 

[Apheared]

Before yanking out your meter and iron, try turning the amp on with no source wired up... then try it with a cdp hooked up and on, but paused... see if the hiss goes away in either of those scenarios.

What's good for Grados? 0.00000000000000001 ohm - or less.

 

[Tomo]

Hey,

What I meant to say is that the load resistors do not interfere with the operation of amplifier. So you should not "choose" to remove it because you are using Etymotics.

I did not say you should have load resistor. But a fact is that I am using Etymotics with 100 ohm load resistor without problem. You should not have problems either.

I think load resistors are there more to prevent oscillation and not necessarily for sound quality. I drove 30 ohm headphone with 100 ohm load resistors without problems. I hear very little audible difference but I feel safer. (Note I say I "feel")

Extra: I got more difference from replacing my coupling caps with 1k Mil-Spec Carbon resistors. See what I mean? I have it because it is safer theoretically. But I have not confirmed this theoretical reasoning.

Tomo

 

[eric343]

Whoa, I think we have some MAJOR misunderstandings here, most of which are my fault and most of which could get comical

Apheared, the hiss IS there irregardless of the existence of a source.

What did you mean, .1E10 or less? I am talking about the 'optional load resistor', R5 in the original schematic. I was asking what a good R5 value for use with Grados (32Ohm) impendance is.

Tomo, the reason I even MENTIONED load resistors was that the original plans claim that "R5 is an optional load resistor, which reduces residual hiss when the amp is driving low impedance
headphones such as the Grados (32 ohms)." I was wondering if my Etys (100 ohms) would benefit from a load resistor to reduce hiss.
And by 'coupling capacitor', I assum you mean the .1uF C1 film caps?

 

[eric343]

<oops double post deleted>

 

[Apheared]

That's the Apheared sarcastic way of saying nothing at all for Grados... you want the amp's output to be incredibly low resistance... if you got it under 1/10th of an ohm you're doing good. (running a closed loop opamp as a buffer you're doing as good as you can with what you got anyway)

Hmm. Hiss in all cases, huh? Hmm. What gain? What resistors? I don't know what opamps you used, but I've rarely heard hiss from any of the BBs in low-ish gains (under x25). Some resistors are noiser than others, but even generic metal films are pretty doggone quiet in this use.

Is it volume independent? High freq oscillation can sound like hiss... well, kinda anyway. It doesn't sound like tape hiss; but it is "hissy"

 

[eric343]

Well, the hiss sounds like white noise at a very low volume. And when I turned the in-line volume control of my Koss TD70s down, the hiss went away and I could hear only the music-and it sounded amazing. And I'm using BB228s as opamps, with generic RadioShack metal film resistors. How do you figure out gain, anyway?

 

[Apheared]

For a non-inverting config where the R-in is smaller than Rfb, divide the resistor that goes from the -input to the output by the resistor that goes from the -input to ground. Then add 1.

If you went with 1k & 10k that would be gain x11.

The resistors themselves are fine. I doubt you're hearing them.

Using a resistor inline with the output is a cop-out, something's not right. While it might help reduce the hiss you hear, I think you'd rather fix it. And there is the problem!

When you have an amp made from 2228s you (and a few people can attest to this) can barely tell the thing is turned on, with any headphones... tiny tiny amounts of hiss or "air" might be heard with the volume maxxed... it's very clean and quiet.

 

[eric343]

Quote:

For a non-inverting config where the R-in is smaller than Rfb, divide the resistor that goes from the -input to the output by the resistor that goes from the -input to ground. Then add 1.

I don't quite understand. What's the -input, and to my knowedge there's no resitor from any input to the output. This would create a feedback loop.

 

[Apheared]

Why yes, it does!

heh. Two of them, even... and if you want low distortion and low resistance outputs, be GLAD for them. (not a law, but a strong rule) Don't believe every negative thing you hear re: feedback loops...

Sorry, I was being long-winded for clarity - guess it didn't work. For a hansen board, gain will be 1+(R4/R3)

Did you use a pot? If so, can you bypass it with some jumpers and use a source with it's own volume control, a portable cdp or a computer? Misconfigured pots are a high "whoops" factor with cmoys...

I wish I had a definate answer why it's hissy, but I don't. Luckily it's a small project with few parts; shouldn't take too long to completely troubleshoot from end to end.

 

[eric343]

Thanks for the clarification (although it's not a hansen board, it's on some copper-pad protoboard). I'm not using any pots, BTW.

However, I did some Tier 1 ("Do you need stamps to send e-mail"-type) troubleshooting, and found that most of the hiss vanishes when I connect it to my McIntosh instead of my computer (because I have a MAJOR jumble of cables behind my computer, and the sum total of all the electrical noise is probably totally random). However, there IS some *very* quiet, backround white noise which is barely audible when I move the (unshielded) amp away from the source of AC hum known as my audio setup.

However, it seems to suffer from some hum when connected properly (wired into the amp with its grounded phono plug), which I assume is due to the wires that connect some components, and not a soldering mishap.

So, in the end, it turns out the problem is probably not with my amp anyway. Well, I sure learned a lot

!