Hum in PIMETA

[robzy]

[this is a repost because a lot of circumstances have changed

Hey guys,

I have a pretty much stock-standard PIMETA running with an OPA2132 l/r, OPA227 G, 2 x buf634 l/r, 1 x buf634 g and with a gain of 11. It is running off an off-the-datasheet lm317t configured to ~24v. I have removed it from a metal enclosure, so i know its not me shorting grounds or something.

The problem is that with or without source plugged in, when the volume is turned up to max, there is a hum in both channels. This hum changes as i move my hand next to C1s, as i touch the pot, as i touch ground on the output jacks, as i touch the top of the power caps and if i short out PG/IG/IR/IL with my finger.

My noob opinion is that this is a grounding problem. But i dont really have any idea what that means. I checked all the ground connections and everything appears tip top.

How can i go about removing this hum?

(I have two output jacks hooked up using wires with y splits in them, could that be the issue?)

Thanks,
Rob.

 

[MisterX]

I don't see a problem.
A) you are never going to listen to the amp without a source connected to it.
B) You are never going to be able to listen to that amp at that volume level.

Connect the amp to the source and adjust the volume to your normal listening level.
Mute the source..... now touch the volume knob.
Did it just go bzzzzzzzzzt?
If it did you need to ground the pot's shaft.

 

[Garbz]

The hum comes from the floating inputs. Input shorting plugs solve that but why bother. When a source is connected the inputs no longer float. My amp and I mean my real amp not my DIY one does the same thing when no source is connected, and the balanced lines too when ground lift is flicked on.

 

[robzy]

*slaps forehead*

Hum problem fixed by connecting it to a source. Thanks MisterX & Garbz.

Now to fix the hiss i get at full volume with a source

Rob.

 

[tangent]

Quote:

Originally Posted by robzy Now to fix the hiss i get at full volume with a source

Do you really need a gain of 11?

 

[n_maher]

Quote:

Originally Posted by tangent Do you really need a gain of 11?

And to add to what Tangent said, do you really think you'll ever listen to anything at full volume? The hiss could be from your source so you've got a couple of options, either leave it as is and see if it's audible during regular listening or lower the gain. You could also add R8 if don't already have it in.

HTH,

Nate

 

[robzy]

"R8: If you are getting audible hiss at normal listening volumes with the source disconnected, put a 10 to 100 Ω resistor in R8. Alternatively, you could put the same amount inline with the line from the output pad to the jack. (This would be like R9 in the META42.) The difference is between output resistance inside the feedback loop or outside it, which have different effects but both can fix the problem. The side effects are different, so investigate it if you want to try this." - Tangents website.

Unfortunetly it sounds as if R8 is not going to be able to help me out here.

Logically, lowering the gain sounds like it is a good way to go. Might be the only way to go

A quick question - would using resistors tying the inputs to ground be an appropriate way to find out if it is the source creating the hiss or the amp? If so, what resistors should i be using?

Thanks a lot,
Rob.

 

[tangent]

Quote:

would using resistors tying the inputs to ground be an appropriate way to find out if it is the source creating the hiss or the amp?

First off, I just realized that you're using C1. This is the reason floating inputs cause a problem. If you short C1, the inputs will be tied to ground through the pot. If you don't need C1, you should think about shorting it. It causes problems, so why add it if it isn't providing benefits to exceed those problems?

Second, the way to test if the hiss is from the source is simply to unplug it and/or turn it off. If the hiss goes away, you've got your culprit.

 

[robzy]

Quote:

Originally Posted by tangent First off, I just realized that you're using C1. This is the reason floating inputs cause a problem. If you short C1, the inputs will be tied to ground through the pot. If you don't need C1, you should think about shorting it. It causes problems, so why add it if it isn't providing benefits to exceed those problems?

Thanks, ill try shorting out C1 and seeing if that can narrow down the source of the hiss to the source. The reason i have C1 in it at the moment is because i change sources all the time, and dont always have the state of mind to check their offset. (Also, this amp will be sold in a month or two, and its probably a smart thing to keep on an amp that will be going to a headphone newbie).
Quote:

Originally Posted by tangent Second, the way to test if the hiss is from the source is simply to unplug it and/or turn it off. If the hiss goes away, you've got your culprit.

The problem with this was that the floating inputs caused a terrible hum that wouldnt allow me to hear whether there was hissing or not.

[edit]: I just tried my PIMETA with another source (ipod). Hiss still exists. Damn, narrowed down to the amp. Ill turn down the gain and possibly install pin headers at r8 to try out different resistors there.

[edit2]: No point in headers, ill just throw in a resistor

Thanks a lot,
Rob.