Valve buffered IGC - volume pot causes DC offset, crackling - diyAudio
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Old 13th June 2009, 06:13 AM   #1
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Default Valve buffered IGC - volume pot causes DC offset, crackling

I've just finished building a valve buffered IGC based very closely on this design here http://www.customanalogue.com/diytub...lone/index.htm .

It is all up and running, but unlike my previous GainClone (unbuffered , non-inverting), when I turn the volume pot, I get crackling and, according to my multimeter, DC offset of up to about 200 mV, which then quickly goes away (presumably cancelled out by the amp's DC blocking capacitors).

Is this normal? Or is something wrong? Is this safe for my speakers?

Also just wondering, how much DC offset is it safe for an amp to generally put out? One channel of mine constantly puts out 90 mV. Is this okay?

Thanks in advance!
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Old 13th June 2009, 10:00 AM   #2
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Quote:
It is all up and running, but unlike my previous GainClone (unbuffered , non-inverting), when I turn the volume pot,
There must be some DC on the pot. Could be, the +/- voltages for the tube are not symetrically?

Is the tubes filament voltage correct, e.g. does the tube work?

Please measure the DC voltage at the potis center pin (full volume and turned downwards). It should be always 0V.

It makes sense to do the measurements with and without attached input device.

Franz

P.S.
200mV bias is on the very high side, for some speakers a problem. 90mV for a non inverted gainclone: lets say in the max range. You could adjust by inserting a trimmer between +input pin and ground.

/Edit
Be carefull with your speakers during startup of the tube: you will have full DC (high current) voltage on the outputs during some seconds! This can overheat the voice coil of the speaker! I recommend to switch on the chip by a delay circuit some 20 seconds after powering up the filament or by grounding the chips input pin.
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Old 13th June 2009, 10:35 AM   #3
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Yikes - I measured the voltage on the middle pins of the pot - i.e. the input pins. When the pot is all the way down, voltage is effectively zero, but when the pot is all the way up, the voltage on the middle pins is around 8 V! Lucky that didn't fry my source. Lesson learned.

The voltage rails for each side of the valve are balanced to within 0.1 V (+-35.1 V), so I don't think that's the cause of the problem.

The problem is the same for both channels, so I don't think it's likely to be a stray wire or anything like that.

One thing I did slightly against recommendation was to use a single transformer for both the valve and chip amp power supplies (+-25 V ac), but I do have a separate rectifier bridge for each section, and for each channel (four rectifier bridges total). Grounding was done according to the diagrams on this page http://www.customanalogue.com/diytubegainclone/ps.htm

I even tried two different valves. And yup, the amp works.

Thanks so much for your help, I really appreciate it! And thanks also for the warning about powering on the amp. Fortunately I did already know this, but thanks for taking the time to make sure

Any ideas about what could be causing this problem?
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Old 13th June 2009, 04:03 PM   #4
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Quote:
Any ideas about what could be causing this problem?
Must be some problem with this valve.

A "proper" circuit would consist of the poti, followed by a coupling cap. From the other side of the coupling cap to ground a so called grid resistor. This circuit is also a high pass filter.

This would also protect the input device in case of a valve fault.

But it is not very attractive, when your input device already has some output caps.

Franz
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Old 14th June 2009, 03:25 AM   #5
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Thanks again Franz. So there should be a coupling capacitor/high pass filter between the pot and the valve, as well as between the valve and the chip amp?

Do you think that in addition to this I also have a faulty valve?
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Old 14th June 2009, 08:25 AM   #6
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Quote:
So there should be a coupling capacitor/high pass filter between the pot and the valve
Indeed, but maybe the sound quality is better without. Soundwise it is still true: the best cap is no cap...

Quote:
Do you think that in addition to this I also have a faulty valve?
When I understand you right: one is working, another not, then I guess one has a fault.

What tube type do you use?

Franz
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Old 14th June 2009, 08:40 AM   #7
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There is absolutely no need for additional caps. You have wired something wrongly and that's why you get the strange behaviour. What is the voltage at the cathode? If the tube is correctly biased the grid voltage is definitely zero, irrespective of pot position. Tube failure at such low voltage is extremely unlikely.
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Old 15th June 2009, 12:21 AM   #8
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Cathode voltage (with valve plugged in and everything warmed up) is -6.6 V on one channel and -4.6 V on the other. Grid voltage is -2.7 V and -2.1 V. That is with the volume pot all the way down.

With the volume pot all the way up, cathode voltage is -10.7 V and -12.4 V, and grid voltage is -9.0 V and -10.8 V.

This is all with no source plugged into the input.

I may not have been clear about the valves - I tried two, and both exhibited the same behaviour. I assume neither one is faulty.

The valves I am using are 6922's made by Shuguang.

Thanks again!
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Old 15th June 2009, 06:50 AM   #9
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You have a wiring error, probably something simple as a PS ground not connected. The tubes are running with positive bias which is completely impossible if your wiring was correct.
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Old 15th June 2009, 08:43 AM   #10
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Thanks heaps! I'll check my wiring.
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