[rtm00x]

Hello,

I just received a clarinet preamp that was built by someone else, I am the third owner I believe. I hooked it up, sounds great, really think I will enjoy this piece. There is one catch with this unit though, it has a hum/buzz noise.

It is definitely not tube hiss, I know what that sounds like, and I do get that if I turn the volume past 3 o clock. This is something else. I did notice that the hum stays there even if you unplug the unit. It's been unplugged for 5+ minutes and my speakers still have the hum. Any ideas on what to check to get rid of the hum?

Thanks in advance for any tips!!

[hagtech]

Sounds like induction into ground loops created by power cords and interconnects.  Try moving cables around / away from power transformers and other sources of magnetic fields.  Could just be a placement or orientation issue.

jh

[rtm00x]

Sounds like induction into ground loops created by power cords and interconnects.  Try moving cables around / away from power transformers and other sources of magnetic fields.  Could just be a placement or orientation issue.

jh

I tried moving the unit, facing it the adjacent direction, problem persisted. I also moved it to a completely different system in a different room, using different interconnects and a different power cord, again the hum persisted. The previous owner did mention the hum.

So I couldn't wait until the weekend, I took the thing apart. One thing I noticed right off the bat was one of bolts, out of the four, that is used to hold the transformer down is not present. Do you think this would cause the hum? I will go to hardware store this weekend to get a replacement for that anyway. The other thing I noticed was the socket for the rectifier tube was a bit loose, The solder joints seemed good, but the tube socket itself seemed to move a bit side to side, that is not normal correct?

Anything else I should look for? Let me know if a pictures of anything specific might be helpful in identifying problem.

Thank you Mr Jim Hagerman, your expertise is greatly appreciated!! I am really digging the sound of this preamp, now to just get rid of this hum >.<

[hagtech]

Is buzz or hum mechanical from phonostage?  Or via output signal into your amplifier?

jh

[rtm00x]

Is buzz or hum mechanical from phonostage?  Or via output signal into your amplifier?

jh

The clarinet itself makes no noise, I can hear the buzz/hum through the speakers. There is a low hum that comes through the woofer and a buzz that comes through the tweeter.

I did use it with a phonostage, a cambridge 640P, but the phonostage produced no hum. I only switched the interconnects going from it to preamp to the clarinet, the turntable is grounded and I had no hum on the other system. I also hooked it up to another system, the other system had a digital signal going to a DAC, which I connected to the preamp. The same hum/buzz persisted no matter what system I connected clarinet to.

[kylek]

Have you tried changing the tubes?

[rtm00x]

Have you tried changing the tubes?

Hi kylek. Yeah I have tried a whole other set of all three tubes. Problem persists.

[poty]

I'd try to disconnect all inputs and short out the active (selected) input. This eliminates any "device through" noise from any combination of sources and power amplifiers.
Second try (if the previous experiment reveals nothing) - not just switch off the device, but pull out the mains cable (from the mains outlet or from the device). If the noise is persisted - then it is not from the Clarinet definitely. Disconnect the output interconnects in this case - and you'll hear the same noise. If the noise in this experiment  is off - then IMHO it is a ground loop through the shielding of the output interconnects to the ground wire of the mains cable or the ground in your mains is noisy/improperly connected. Then you may try the ground breaker of some sort, but first - detect the source of the noise.

[rtm00x]

I'd try to disconnect all inputs and short out the active (selected) input. This eliminates any "device through" noise from any combination of sources and power amplifiers.
Second try (if the previous experiment reveals nothing) - not just switch off the device, but pull out the mains cable (from the mains outlet or from the device). If the noise is persisted - then it is not from the Clarinet definitely. Disconnect the output interconnects in this case - and you'll hear the same noise. If the noise in this experiment  is off - then IMHO it is a ground loop through the shielding of the output interconnects to the ground wire of the mains cable or the ground in your mains is noisy/improperly connected. Then you may try the ground breaker of some sort, but first - detect the source of the noise.

Hi poty. I tried disconnecting all the inputs and connected a rca cable from left input of one channel to the right input on the same channel. I connected the clarinet to my power amp. I turned on the amplifier, no hum. I then plugged in the two 12AU7 tubes, still no hum, the LED was RED at this point. I plugged in the rectifier tube, the hum creeped in just before the LED turned Green.

So I tried touching some of the components to see if that would make any difference. I touched the top of one of the taller caps, first the 47uf caps and then the 10,000uf caps, I did this as I was touching parts of the case, this made the hum go away completely. The locations circled in red in the pic are where I touched it. So... what does this mean?

[rtm00x]

Scratch my last post, I did not test it correctly. I put it all back together, the hum was still there. I took the bottom plate off, touched one of the taller caps, the hum was still there. I think when I had it all apart I was getting a different hum all together due to inproper grounding.

I did however find that C101 has a mechanical buzz, I am mostly certain the buzzing I felt through C101 is oscillating at the same frequency as the hum/buzz I am hearing through the speakers. Any ideas?

Poty, I tested with it all back together, and the hum does go away when I pull the power plug out. I also tried shorting the selected input, this had no effect on reducing the hum.

[poty]

First of all, I'd not touch any part when the device is on, especially with the second hand grounded through the case. The high voltage here is dangerous!!!
C101 itself is unlikely to buzz. It seems the mechanical force goes to the part of circuit from the transformer which is straight above the place. In normal situation there should be no problem about it, but if you use "microphonical" tubes it can be the source of faint noise.
The experiment with unplugging the mains is very interesting. If the device is on and you unplug the cord - the capacitors charge should support the power of the device for several seconds. I mean -after unplugging - the noise stops abruptly or gradually fainting? IF it is possible - could you (with GREAT precautions!!!) connect the device only to line and neutral, without ground connection and listen to the noise?

By the way... The book is cool!

[rtm00x]

First of all, I'd not touch any part when the device is on, especially with the second hand grounded through the case. The high voltage here is dangerous!!!
C101 itself is unlikely to buzz. It seems the mechanical force goes to the part of circuit from the transformer which is straight above the place. In normal situation there should be no problem about it, but if you use "microphonical" tubes it can be the source of faint noise.
The experiment with unplugging the mains is very interesting. If the device is on and you unplug the cord - the capacitors charge should support the power of the device for several seconds. I mean -after unplugging - the noise stops abruptly or gradually fainting? IF it is possible - could you (with GREAT precautions!!!) connect the device only to line and neutral, without ground connection and listen to the noise?

By the way... The book is cool!

Yeah I had read in one or two other hum themed posts here that it is "somewhat" safe to touch the components themselves as long as you are not touching the leads or traces on the board. I don't think it was C101 itself, it seems to catching the buzz from whatever source it may be coming from. I did listen to the transformer withe the power amp off, I thought I maybe heard something but not like a regular transformer buzz, which I have heard on other stereo equipment before.

So the buzz/hum cuts off as soon as I pull the power cord out of the clarinet, and the preamp abruptly shuts off, the LED almost instantly turns off. I did try to disconnect the internal wire which is connected to the power plugs ground and runs to the ground on the chassis, this had no effect.

One thing I noticed is the soldering job on the tube sockets isn't exactly "clean", am I correct to assume what I am seeing is rosin flux? If so I wouldn't think it would have that much of an effect, let me know if I am incorrect and need to reflow the tube sockets.

That was a book from my college days, I have been giving it another read, good stuff.

[avahifi]

Those huge Soncap capacitors certainly are not what the PC card was designed for!

I wonder if these or other so called good sounding parts are not the source of your bad sound (hum).

Frank Van Alstine

[rtm00x]

Those huge Soncap capacitors certainly are not what the PC card was designed for!

I wonder if these or other so called good sounding parts are not the source of your bad sound (hum).

Frank Van Alstine

Hi Frank. Yeah I suppose it is possilbe, the values seem to be right, they also seem to be connected correctly. Anyway I could test to see if they are causing problems?

[galyons]

Those huge Soncap capacitors certainly are not what the PC card was designed for!

I wonder if these or other so called good sounding parts are not the source of your bad sound (hum).

Frank Van Alstine

Certainly this is a jest.  I have a Cornet2 and a Clarinet full of "good sounding parts" without any hum or any other issues.  That is what DIY kit building is all about.  We all know that old Dyna kit could never be improved with "good sounding parts". Sad to see such a disingenuous post on this Circle.

Cheers,
Geary

[rtm00x]

Let's keep it friendly guys, I would rather this didn't turn into a flame thread until after I solved the hum problem  . I did measure the plate voltages with a volt-meter, they were pretty much there, some spots were off by about 15v (got 265v where it was supposed to be 280v), but I think this is acceptable, right?

Also, after listening to the preamp with sound all the way down, just my ear right by the preamp, I think there is a very slight mechanical buzz, and again I am convinced that its also coming through C101, I can't hear it, but can feel it when I put my finger on C101.

[poty]

Yeah I had read in one or two other hum themed posts here that it is "somewhat" safe to touch the components themselves as long as you are not touching the leads or traces on the board.

I strongly oppose this play with the danger. Use a wire or some isolated tools...

I don't think it was C101 itself, it seems to catching the buzz from whatever source it may be coming from. I did listen to the transformer withe the power amp off, I thought I maybe heard something but not like a regular transformer buzz, which I have heard on other stereo equipment before.

Sorry to made this confusing. Re-reading the first post I've remembered that your Clarinet makes noise even it is powered off. In this state neither the transformer nor microphonic tube can do any harm. On the other hand - removing the rectifier seems eliminates the noise. But the rectifier works only if the device is powered on too! Some inconsistencies in the experiments.

So the buzz/hum cuts off as soon as I pull the power cord out of the clarinet, and the preamp abruptly shuts off, the LED almost instantly turns off. I did try to disconnect the internal wire which is connected to the power plugs ground and runs to the ground on the chassis, this had no effect.

Lets make another experiment. While the preamp is running and producing some sound - could you:
1. turn it off and listen if the noise is off (immediately cuts off) or fainting gradually;
2. unplug the power cord from the mains and listen the same.

[galyons]

Use your DMM, set to frequency, and with the inputs shorted, measure the frequency at the outputs.  60Hz is ground, 120Hz is transformer, other is other.   If 60Hz, try a cheater plug, no ground prong, at the outlet.   Also, since you didn't build the unit, I would reflow the solder joints.  But first, measure the frequency of the noise and report back.

Cheers,
Geary

[rtm00x]

1. turn it off and listen if the noise is off (immediately cuts off) or fainting gradually;
2. unplug the power cord from the mains and listen the same.

Poty, in both instances the hum fades away gradually.

Use your DMM, set to frequency, and with the inputs shorted, measure the frequency at the outputs.  60Hz is ground, 120Hz is transformer, other is other.   If 60Hz, try a cheater plug, no ground prong, at the outlet.   Also, since you didn't build the unit, I would reflow the solder joints.  But first, measure the frequency of the noise and report back.

Cheers,
Geary

Geary,

I am not exactly sure how to measure this. My DMM does have a frequency function but am not sure how to use it, can you help me out here with step by step directions? I really appreciate it! When you guys say short the inputs you mean put for example use a single rca connecter with one end connected to left and the other to right for that input channel, correct?

[galyons]

Geary,

I am not exactly sure how to measure this. My DMM does have a frequency function but am not sure how to use it, can you help me out here with step by step directions? I really appreciate it! When you guys say short the inputs you mean put for example use a single rca connecter with one end connected to left and the other to right for that input channel, correct?

Simple shorting plugs....take a cheap RCA interconnect, (the ones that get shipped with most consumer products), cut the connectors off leaving about 1/2" of wire. Strip the wire and twist the center signal wire and ground wire together. That shorts the input to ground when you plug the sorting plug into the preamp's input RCA's.  You only need to short the pair of input RCA's that are switched into the circuit. 

Once the inputs are shorted and the preamp powered,  set your DMM to frequency.  Put the red lead into the output RCA to make contact as center pin and the black lead to the outside of the RCA to ground.  Make sure that the red lead does not contact and short against the ground.  Check both R & L outputs.  The DMM will display the frequency of the hum at the output.

Cheers,
Geary