[dcrane]

I just finished my 1st PIMETA and it sounds good except under certain circumstances. After I connect my DAP(iAudio x5L) for a while, I hear a high pitched buzz. It takes about 5 sec and goes away turning it off then on again. However, this does not happen when I connect it to my cdp (Rotel). Any ideas?
The gain is high at almost 10.5. It is biased into class A too.

 

[dcrane]

More info:
The buzz does not change with the volume. The sound continues when the music is stopped. The sound stops when the dap is disconnected from the amp.

Also, I am using 1M for R6. Tangent mentions that using above 1M in the multiloop can introduce noise. Should I put something around 500k instead?

 

[dcrane]

Another strange thing: This only happens with my Senn 650s(300 ohm) and not my Sony CD3000(32 ohm). I thought that if R6 was introducing noise that the CD3000 would be more susceptible.

 

[dcrane]

Also, using 1M for R2. Tangent states the R2 should be 10x volume. But schematic shows 50k volume with 1M R2. I'll poke around some more and let you guys know how I do - I'm sure somebody else will/has encountered this problem.

 

[dcrane]

Another conflict I noticed:
"The simplest path is to just use the values given on the schematic"
"As a rough rule of thumb, make R3 five to ten times lower than R5"
schematic shows R3=1k and r5=3.32k

 

[joneeboi]

The buzz might just be coming from your player.

 

[Pars]

If you have a meter (DMM), and I would hope if you built a Pimeta that you wouldn't leave home without one (DMM), I would hook the meter up in series with the power + lead and in ammeter mode look at the current draw. When you can get it to buzz, does the current draw jump? I am getting at oscillation here... that would be my initial suspicion.

You should calculate the acceptable current draw for your amp by looking at the datasheets for the opamps and buffers and add up the quiescent draw for all of these. Plus a couple of mA for LED... that should get you quite close to what you should be seeing. Also, do you have the coupling caps (C1 IIRC) in? if not, what is the DC offset of your DAP?

 

[dcrane]

It does seem to jump (it fluctuates) about 2 or 3 ma, about 78 to 81ma. I am using 3 OPA627s and 6 buffers. Also, if I wait after stopping the music, it will stop after about 10 sec. My current draw seems to be within reason:
opa627 - 11.7-16, 14 typical
buf634 - 8?
LED 2
should be at least 83 but the buf634 are stacked 2 each.

 

[Pars]

2-3mA isn't enough to indicate oscillation... that would normally be double, or jump alot. An amp that is oscillating typically runs hot also. THis doesn't mean that it isn't low level oscillation however.

Do you hear this buzz thru the headphones, or is it coming from the amp? You also didn't indicate whether you have input caps, and what the DC offset on your DAP looks like?

Also, there is a small mica or ceramic cap (C6 from memory) on the ground channel... did you use this?

 

[dcrane]

no input caps. the buzz/hum is through the headphones. didn't check the dc offset yet. c6 is 10pF 100V

 

[dcrane]

If I measured correctly, the dc offset is .1 to .2mv. With the source on plus with.without headphones, I measure left to ground and right to ground at the headphone jack

 

[tangent]

R2 and R6 are not the problem. The extra noise you get from using values that are higher than necessary here would just be hiss, and it would be constant. It would literally be physically impossible for it to change, because the resistor values would have to change.

I don't see that you've ruled out noise from the DAP. What with the high gain and higher fidelity of the new amp, you might be hearing noise that was already there, but inaudible. You could be hearing power supply noise from the hard drive, for example, with the 10 second delay being the time it takes for the player to power down the drive.

If you retry the current draw test, don't do it with music playing. If you have to play music to get the noise to start, go ahead and make it start, but then stop the music and wait for the noise to stop and see if the current draw falls on the noise stopping. What I'm getting at is that this 3mA jump you're seeing may be due to the varying power requirement of driving music through the headphones. We need to be sure, though.

 

[dcrane]

You might be right about the hard disk noise - I used to have a rio karma and I could hear the hard drive with my shures. Assuming it's just noise from the DAP, what can I do to reduce this effect? Other than replacing the DAP.

 

[dcrane]

actually the rio karma problem was the backlight. I checked the hard drive access and the backlight of the iAudio X5 - they are not causing it. It seems random. I have to put some tape or something on my tests leads so I can stop the player - I'll have to get some leads with clips....or grow another pair of arms.

 

[dcrane]

I should mention that I'm using 3 627s with 2 9v batteries. It sounds good to me when there is no hum - which it has decided not to do for a while. I read other people suggesting 3 or 4 9v. I did use sockets and have 3 8610s as backup. Is this combo doomed? Or just use easier to drive headphones? Maybe I'll try out the 8610s and see how they are. The problem is that I love my sennheisers.