Help in Troubleshooting noise in audio Amplifier
Help in Troubleshooting noise in audio Amplifier
(OP)
Hi all, I just recently built my SE Tube amp with parafeed output transformer. Everything looked fine and as expected when I tested it with a 8 ohm resistor dummy load. However, as soon as I hook up to a speaker and crank up my signal generator, I heard a loud high pitch noise and the noise vary in pitch as I vary the input frequency. The outputs apprear normal on scope. I did not see any oscillation. I tested it on a different speaker and I have the same result.
I ran out of thought what would be wrong. Do you have any suggestion?
I ran out of thought what would be wrong. Do you have any suggestion?





RE: Help in Troubleshooting noise in audio Amplifier
Does the noise add to the exepcted sound?
Does it only appear when there is another sound produced?
Does it appear at any volume?
Is it proportional to the volume of the input signal?
RE: Help in Troubleshooting noise in audio Amplifier
RE: Help in Troubleshooting noise in audio Amplifier
Everyone obviously hear it when hook up to the speaker. It's darn..loud. As long as I crank up the volume on the signal generator, then you can hear it and the pitch varies with the frequency. It appears with any volume level. I have not actually tested it with a CD player yet because of this. I might try it to see if it makes any difference.
RE: Help in Troubleshooting noise in audio Amplifier
Can it be magnetostriction noise from the tranformer?
RE: Help in Troubleshooting noise in audio Amplifier
The plate choke mentioned is the load of the 300B tube and couple to the output transformer with an output cap.
I subsituted the plate choke with an identical one last night, but I still have the same problem. Changing different speaker would not help either. So I suspected that the noise came from somewhere within the amp. But why I could not see on the scope? and the sound appears in the entire audio frequency range. I could hear it up to 10Khz.
RE: Help in Troubleshooting noise in audio Amplifier
It involves that there must be an oscillation somewhere in the circuit, or a ringing versus the signal input. But you should be able to see something on the scope.
RE: Help in Troubleshooting noise in audio Amplifier
I still do not understand why I have problems mentioned if the input came from the signal generator. I have tried two different signal generators with different brand and have the same results. It's still a mystery to me.
RE: Help in Troubleshooting noise in audio Amplifier
Your portable CD player is isolated from AC power. Your signal generator most likely is AC powered through it's own power transformer.
Finally, your amp has several transformers and a plate choke. Make sure they are located far enough apart and oriented so that you avoid magnetic coupling between them.
RE: Help in Troubleshooting noise in audio Amplifier
Is it possible that you are over-driving the output transformer, i.e. the transformer is on the edge of saturation? This would give you a magnetising current comprising all sorts of harmonics, the lower ones falling within the audio band where you can hear them if they cause vibration of chokes or within the valve. If it is a single ended design there may well be even-order harmonics as well as odd. Similar wierd results may well occur if the transformer is carrying a DC bias, as this will also affect the magnetising curve and will generate harmonics. The closed feedback loop would act to minimise output distortion, so this problem would not necessarily manifest itself at the output.
I'm not suprised that you can't hear it with a musical passage - the effect might still be there but there is no pure tone against which you could identify any abnormal sound.
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If we learn from our mistakes,
I'm getting a great education!
RE: Help in Troubleshooting noise in audio Amplifier
I am not really sure if the power tranny have internal electrostactic shield (I will check on that it's been awhile I have not checked on that spec. I checked on it before though), but it's a military potted transformer. Since this is a prototype one, may be the wiring/grounding induced the noice into the system. I might try to put into the new chassis to see if it help.
RE: Help in Troubleshooting noise in audio Amplifier
You're probably right the problem's probably still there, but I can't hear it.
RE: Help in Troubleshooting noise in audio Amplifier
Could you expand a little on the term 'parafeed' w.r.t transformer windings? It's not a term I'm familiar with in the UK.
The above not withstanding, the only sure-fire ways I can think of avoiding DC through the winding would seem to be 1) capacitively couple it; 2) use a DC servo loop to drive the DC to zero. I think the latter would be tricky with a valve amp (not impossible though).
If the signal generator output is not floating, can you make it so by using a coupling transformer between it and the amplifier input before repeating your test.
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
RE: Help in Troubleshooting noise in audio Amplifier
I did try to feed the signal generator via my tube preamp, but got the same result.
RE: Help in Troubleshooting noise in audio Amplifier
Is there a switcher power supply in the signal generator, or a linear one?
RE: Help in Troubleshooting noise in audio Amplifier
Parafeed is apparently an abbreviation of "parallel feed". Parafeed is not indexed in Langford-Smith, but parallel feed is. The interstage coupling transformer in this valve based system is capacitively coupled so that no DC passes through the high inductance transformer.
Hlong,
I suppose your scope is earthed. Applying the scope therefore changes the conditions. Did I understand you correctly, did you say that you applied the scope to the loudspeaker terminals (not just the dummy load) and that no noise was visible with a pure sine input signal form the signal generator?
Since the output is clean with a CD input, you would think the problem was feedback from the loudspeaker to the signal generator. The fact that you have tried two entirely different types of signal generator negates that theory. Is there anything else different between the CD set-up and the signal generator set-up? (eg a signal cable routed over the output cable in both signal generator configurations but not with the CD system.).
RE: Help in Troubleshooting noise in audio Amplifier
RE: Help in Troubleshooting noise in audio Amplifier
Thomas
RE: Help in Troubleshooting noise in audio Amplifier
Thomas
RE: Help in Troubleshooting noise in audio Amplifier
Felixc, can you explain when/what case magnetostriction noise happen? Does it happen when you pass too much DC current through it? I am very sure that this loading choke can handle much more DC current than the current condition. I am passing from 60 to 75mA DC through it. And it rated at 50H @100 mA.
Logbook, yes scope is earth. I don't want to mess with this amp unless everything is reference back to earth since I am dealing with 800VDC power supply here. The routing cables are exactly the same as the function generator.
I did suspect some type of feedback, but why can't see on the scope??
BigTom, the output impedance of this tranny is specially designed for 300B output tube with a reflected impendance of 3K with 8ohm loaded. I do not think loading is the problem. The output from the function generator is 50 ohms and it feeds into the 6SN7 driver tube with 150K input loaded.
Thanks for all suggestions.
Just a note: I have been burning this amp for 8 hrs continously yesterday with my portable CD player. Sometime I did hear some distortion, but not really sure if it's from the actual burned CD or from the amp itself. I might have to repeat the CD to see if I hear the same thing. I play many different CDs though when sitting in my lab room doing other thing.
RE: Help in Troubleshooting noise in audio Amplifier
If you use voltage divider resistors at the input to reduce the output from the signal generator, but you drive the signal generator at a higher voltage, is the noise the same?
If say you add an RC filter at the input to filter out whatever exceeds say 8KHz, is the noise still there?
You have to find if the noise comes from your generators, from a ground loop somewhere, or from within your amp.
There is virtually nothing that you can do about magnetostriction noise. It is caused by the current modulation in the magnetics. Some manufacturers pot the parts in epoxy to reduce the noise. All you can do is try a similar part from another manufacturer.
Is there a high-end audio store in your neighborhood? They may have a closed-loop analyser that can tell about the performance of your amp.