How quiet should the Mainline be?

[MikeSattler]

Just need a sanity check here and was curious as to others experiences with the amp. I've never really been able to get mine totally dead silent. It probably has 1000+ hours on it at this point and I can still hear the faintest 60 hz hum with HD800's if my room is totally dead silent. It has to be DEAD silent though. Like, if a I turn on a ceiling fan on low I can't hear it anymore. However, much more noticeable with IEM's which is why it's kinda been bugging me. My IE 600's sound awesome out of the amp but the hum is a bit louder with those.

Should I expect complete silence or is what I'm hearing a normal noise floor?

 

[Paul Birkeland]

Is this with the impedance set to high or low?  The noise floor will drop a bit if you set that to low.

 

[MikeSattler]

Hey PB

So it does drop very slightly if I switch between high and low but it's still audible. Obviously I use high for the HD800's and low for IEM's but regardless of the impedance setting it's just kinda always there in the background.

I guess I'm just looking for confirmation either way that what I'm hearing is just normal noise floor for the amp, or if there's something out of wack. Of course, impossible to describe the sound accurately over text but was just wondering other's experiences.

 

[Paul Birkeland]

I would suggest trying this:
https://bloomaudio.com/products/ifi-iematch-plus
To a large degree IEMs are made to play very loudly off a very low output voltage/power level, so plugging them into an amp doesn't always get the best results.  Using a pad like this between the IEMs and amp will allow you to turn the volume up a bit on the amp and will increase the SNR.

 

[MikeSattler]

Thanks I'll look into that. Any suggestions for how I could reduce the hum when using higher impedance headphones like HD800's?

 

[Paul Birkeland]

Any suggestions for how I could reduce the hum when using higher impedance headphones like HD800's?

Set the impedance switch to low.

I also sometimes see where loose hardware or paint on the back of the chassis plate doesn't quite allow things to touch the chassis that need to, and that can cause some noise problems. 

 

[Doc B]

A really low level, almost indiscernable hum like you describe could be the noise floor of the particular tubes you have in the amp. You might try another set of tubes. 6C45pis are very quiet, in fact we did extensive tests using them as low output moving coil step up preamp tubes because of that. But tubes is tubes and they can vary a bit.

And as Peebs says, IEMs are designed for the low output level of cell phones. They typically don't match up too well impedance-wise or sensitivity-wise with amps designed for over ear cans.

 

[MikeSattler]

Thanks for the suggestions guys. I've tried new tubes as well as made sure all the ground points on the chassis are connected well.

I had another thread about this same issue late last year but was never able to resolve it. I figured I'd let the amp get a bunch of hours on it and see if it stopped but it never did. Not sure what else to try at this point.

 

[MikeSattler]

Okay, so in an effort to quantify this hum I picked up a halfway decent USB scope and started playing around with it this weekend.

I made a quick screen recording showing the hum that I'm getting measuring off the XLR output: https://photos.app.goo.gl/HH9S51yRhFPm7W7k7

In the recording, I have the amp switched off, then after the initial warmup you can see a steady oscillation (which I assume is the hum I'm hearing).

Took quite a bit of trial and error to get it to show up on the scope since the hum is pretty quiet. I ended up using 2 probes in a differential setup to reject any common noise. It's still not a pretty waveform but it's the best I could get so far.

First time playing around with a scope, but where would be the next place to measure to start tracking down where this is coming from?

 

[Paul Birkeland]

Use the picoscope to measure the magnitude and frequency.

 

[MikeSattler]

There's a fair amount of noise so it's hard to get an exact reading but it looks like based on the minimum and maximum measurements I'm getting ~116Hz at around 200 µV.

 

[MikeSattler]

Sorry scratch that, it's a much cleaner waveform at 10ms/div: https://photos.app.goo.gl/DgGT4eQQDxsXWYAh9

Based on the averages it's 180uv at 68hz

 

[Paul Birkeland]

Is this on high or low impedance?

 

[MikeSattler]

Thats on high impedance. Low looks pretty similar, just with slightly less magnitude, though not by much.

 

[Paul Birkeland]

That's 75dB SNR relative to 1V of output. For super sensitive headphones, you can try something like this with the amp set to low impedance:
https://www.amazon.com/iFi-iEMatch-Headphone-Ear-Monitor-Attenuator/dp/B08S55MPH5/ref=asc_df_B08S55MPH5/?tag=hyprod-20&linkCode=df0&hvadid=692875362841&hvpos=&hvnetw=g&hvrand=16021748331066331575&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9051868&hvtargid=pla-2281435179058&psc=1&mcid=021e51f29690356fb7bf6b98fe4fa840&hvocijid=16021748331066331575-B08S55MPH5-&hvexpln=73&gad_source=1

 

[MikeSattler]

Okay thanks for the info, so it seems like that noise level is just normal then?

Is there anything I can do to reduce it a bit with high ohm headphones like the HD800? They sound the best with the impedance set to high but the noise floor is definitely noticeable in a quiet room.

 

[MikeSattler]

So just out of curiosity, I measured for AC at the +275 connection on the PSU. I'm getting 20-24 mv of AC 120hz ripple. Is that within spec? Just wondering if something is out of wack with my power supply and it's not totally filtering out the AC.

 

[Paul Birkeland]

Power supply noise is 120Hz and you'd want to measure at the OA terminals on the C4S board to see residual power supply noise.

 

[Paul Joppa]

The output transformers could (with some difficulty) be re-wired for 8 or 16 ohms, which would buy you another 6 or 3dB respectively.

 

[MikeSattler]

Oh interesting! Okay well, when measuring at the OA terminals on either C4S board I'm only getting around 100 microvolts of ripple which seems pretty good. I just wanted to make sure the filtering caps in the power supply were working properly since they sat around for a few years before I built the kit. Thanks for the help!