[poty]

HM... I'd not trust such techs. (IMHO) What did the tech check/measured/do? You mentioned a receiver the tech hooked your preamp to... Most receivers have line inputs (which have processor/volume control) and power amplifiers' inputs (which do not have any controls - just like in your power amp). Why the tech connected 2 line stages serially?
Such simple line stage should not show any audible noise from the normal listening distance. I absolutely sure about that.

[rtm00x]

My Clarinet and Cornet2 are dead quiet with 102dB/1W/1M horn speakers.  Audible hum is not normal, but if at normal listening volumes  the hum is audible, then there is an issue.  If the hum is only audible at abnormally high volumes and/or "ears to the speakers", then it may be just fine.

Cheers,
Geary

Yeah, I expect the same from the clarinet I have. I do expect a bit of tube hiss when turned near max, and I do hear that, but this is not that. This is present at all volume levels, comes on when the LED turns green.

The balance control is an attenuator.  Middle position has both channels attenuated 3dB.  All the way to one side is 0dB and 6dB (or something like that, I forget the exact numbers I used).  So it does sound like one channel does get louder. 

jh

Thanks for clarifying Jim, just wanted to be sure.

HM... I'd not trust such techs. (IMHO) What did the tech check/measured/do? You mentioned a receiver the tech hooked your preamp to... Most receivers have line inputs (which have processor/volume control) and power amplifiers' inputs (which do not have any controls - just like in your power amp). Why the tech connected 2 line stages serially?
Such simple line stage should not show any audible noise from the normal listening distance. I absolutely sure about that.

He just hooked it up to line inputs of an older receiver, and turned the receiver up. I could hear the hum as he turned it up. I might try the two prong adapter just to see if it does anything. If not, anyone here willing to offer their services to try and get the hum out?

[Speedskater]

Did the tech use the same pair interconnect cables as you normally do?

[rtm00x]

Did the tech use the same pair interconnect cables as you normally do?

No, he just had like a radio shack set of interconnects, cheap stuff.

[Speedskater]

Have you tried it at home using cheap or Radio Shack interconnects.

Some of my friends speedskate at the Edge ice rink on S. Ward Street.

[rtm00x]

Have you tried it at home using cheap or Radio Shack interconnects.

Some of my friends speedskate at the Edge ice rink on S. Ward Street.

I've tried using several different intercommects. Besides, the hum was present on his system as well. I just found it less acceptable than he did. He is a vintage audio guy, much more accepting of hums and buzzes.

[poty]

Sorry, I'm in Russia... Too far for the service...

[bregez]

For service I would give Audio by Van Alstine a call.

[rtm00x]

My Clarinet and Cornet2 are dead quiet with 102dB/1W/1M horn speakers.  Audible hum is not normal, but if at normal listening volumes  the hum is audible, then there is an issue.  If the hum is only audible at abnormally high volumes and/or "ears to the speakers", then it may be just fine.

Cheers,
Geary

I think I might just be a bit too picky. The hum/buzz is pretty much ear to speakers, you can't hear it when sitting at a couch or when music is playing. Though a part of me still thinks something is off, I think I have exhausted my abilities and rather not spend a bunch of money to have someone else tell me there is nothing wrong with it.

The preamp does sound great as is. Appreciate everyones help.


EDIT: That was short lived, I've posted in denver regional forum to see if there is anyone else locally with a clarinet that might help me compare mine to theirs.

[rtm00x]

So today I took the clarinet to user mgalusha's house. It was a great experience, Mike was very knowledgeable, very friendly, and we got my clarinet's noise levels well below specs!! I really can't thank him enough, he spent hours of his day working on this and offered constructive methods of improving the noise level. The audiophile community here locally in Denver and abroad never ceases to amaze me. In what other communities will someone invite you into their home, spend hours of their time trying to help you, and once you are all fixed up jam out to some dead can dance and floyd?! Truly amazing, thank you all for your help!!

[poty]

Congratulations! Great outcome! Just curious: what was the main thing that was wrong?

[rtm00x]

The thing that made the biggest difference was swapping the 47uf caps @ C300L & R to 220uf caps. Mike also swapped the resistors at R312 to value of 100k, this improved the dc offset, if I am correct. The only other thing that was done was a ground lift was installed. I think we got the SNR down to 89db. The only thing I need to do is replace the volume pot, it is starting to crap out, cuts one channel if overtightened.

But yeah, really happy with it

[poty]

Oh! Great! The C300s change seems to replace bad capacitors, but increasing the value also helps I think.
R312s are the other thing. Your measurings earlier showed some problems with the resistors (or their connections). The value here is not important at least for some degree.
But all above is not important - just my thoughts about the changes - the only thing that matters - the beautiful outcome!

[mgalusha]

We swapped the output to ground resistors from 330k to 100k, this kept it from drifting as much. Initially it had up to ~20mV of drift on the output, the 100k resistors dropped this to just a few mV.

Since this is tube rectified, the initial cap can't be very large or it kills the rectifier but since Jim used a CRC design and there is 20kOhm between the C's, we bumped up the second caps from 47uF to 220uF. This dropped the B+ ripple from about 30mV to just a couple of mV. The initial output noise was a couple of mV and engaging the 400Hz high pass made a big improvement, showing most of the noise was PS related. After the changes the output noise was about .2mV and engaging the 400Hz filter made very little difference. It did not hum in my system, ~97dB sensitive speakers and 28dB of gain in the power amps.

[poty]

We swapped the output to ground resistors from 330k to 100k, this kept it from drifting as much. Initially it had up to ~20mV of drift on the output, the 100k resistors dropped this to just a few mV.

Actually, I wonder where was the drift sourced? It means there was some very low frequency oscillation. Normally there should be no such problem. Certainly I can not be sure and completely right, but it seems the main problem was the power filter capacitors in the B+ circuit (dried? had high leakage?...). Second thought - changing the resistors just solved the problem of their connectivity. In earlier measurements there was a strange dependence of the output RCA resistance on the connection of the next device. Third thought - there might be very high input impedance of the next devices and nobody has tried such high resistors on the output for comparable in impedance next device input...
In any case - great job, mgalusha! And great attention to the person problem!

[mgalusha]

Actually, I wonder where was the drift sourced? It means there was some very low frequency oscillation. Normally there should be no such problem. Certainly I can not be sure and completely right, but it seems the main problem was the power filter capacitors in the B+ circuit (dried? had high leakage?...). Second thought - changing the resistors just solved the problem of their connectivity. In earlier measurements there was a strange dependence of the output RCA resistance on the connection of the next device. Third thought - there might be very high input impedance of the next devices and nobody has tried such high resistors on the output for comparable in impedance next device input...
In any case - great job, mgalusha! And great attention to the person problem!

Yes, I think the original caps were most of the problem. I have built a Clarinet before and it had no such issues, nor did my Cornet. I didn't have any 47uf caps on hand but did have a handful of the 220's and since they had the same pin spacing it was an easy swap. The 100k resistors are not in series, they shunt the output after the output cap to ground, so the ZOut should not change much since they are in parallel with the output of the tube. I didn't measure the ZOut before or after the change but I doubt it's very different.

[poty]

The 100k resistors are not in series, they shunt the output after the output cap to ground, so the ZOut should not change much since they are in parallel with the output of the tube. I didn't measure the ZOut before or after the change but I doubt it's very different.

I did not mean the serial resistors nor ZOut. My thoughts was about the source of the drift. I assumed that the oscillations was formed by output circuit due to very high RC values. In "normal" connection the input impedance of next device is connected in parallel to the output resistors and lowers the RC value. Having high input impedance of the power amplifier changes the overal picture. But it is only assumption, nothing more.

[sushimaster]

I'm late to the party. I had a similar ground hum with my clarinet. I finally resolved the issue by replacing my blue jeans interconnects with new interconnects that are directional. Not sure why this made a difference, but it did. Also had to use a cheater plug on my tube power amp to lift the ground wire. Fwiw, I have my clarinet and van alstine ultimate 70 plugged into a trip lite isolation transformer and it sounds wonderful.  Better than my Conrad Johnson 17ls, mv60 combo.

[hagtech]

Actually, I wonder where was the drift sourced?

It comes from the mains.  The "120Vac" wanders and is dependent on what's going on in your entire neighborhood.  With a simple B+ filtering design as in the CORNET2 this wander gets coupled to the output.  The high pass filter (output coupling cap and input resistance of the following amplifier) reduces the effect.  In the end it is a trade-off between bass performance and LF noise. 

Reducing the 330k to 100k only helps if measuring the CORNET2 by itself.  Once entered into an audio system (connected to an amplifier), the 100k change becomes relatively meaningless. 

jh