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Old 24th March 2014, 11:16 AM   #1
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Default Mystery noise has got me baffled

I've designed and built a fully-balanced line stage for a preamplifier based on a long-tailed pair of E88CCs / 6922s with a cascade constant current source in the tail and cascaded current mirror active loads. I've used a similar configuration in the past, which has worked very well, but this particular circuit generates about 150mV of common-mode noise on its outputs! I have tried numerous things to work out the cause of the problem but have so far failed to fix it.

Circuit.jpg

The tail current is set at about 11mA and the active loads are configured to set the anode/plate voltage at around 80V. The +200V and -15V supplies are regulated and completely quiet, and there are 100nF decoupling capacitors to the ground plane where the supply rails connect to the circuit.

The fact that the noise is common mode, first suggested to me that the problem lay in the tail current source. However, The voltage across the 220R resistor is completely stable and quiet. I then turned my attention to the current mirror, but the voltage on the emitters of the upper transistors is also completely stable.

This has me completely baffled. Can any of you brainier guys tell me where I'm going wrong?
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Old 24th March 2014, 12:39 PM   #2
DF96 is online now DF96  England
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I can't see a current mirror in the anode circuit, just a pair of current sources (or gyrators?) with a combined bias network.
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Old 24th March 2014, 12:51 PM   #3
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Yes, in effect the anode loads are separate current sources with a common reference voltage. This is a handy circuit that I came up with a while ago that enables the gain for the stage to be maximised and accurate current balance to be maintained. It's worked very nicely in other designs that I have done, which is another reason why I'm baffled.
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Old 24th March 2014, 12:52 PM   #4
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Have you always used a very low current and high resistor value for the reference?

Also, what sort of noise? White? 1/f?
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Old 24th March 2014, 01:06 PM   #5
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Yes, the component values are much the same as I have used before. When you refer to a high resistor value, are you talking about in the tail current source or in the active loads?

As for the type of noise, I'm not entirely sure. I guess I may need to feed it into a spectrum analyser to find out - or is there an easier way?

The 10uF capacitor in the active load reference voltage network keeps the programmed current substantially constant across the audio frequency range and also decouples noise from the bases of the upper transistors.
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Old 24th March 2014, 01:27 PM   #6
DF96 is online now DF96  England
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Precise details of your measurement method might help. Inputs shorted? Outputs buffered how? Differential-mode removed how?
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Old 24th March 2014, 01:53 PM   #7
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I shorted both grids to ground and attached scope probes to each anode. With the scope inputs AC coupled I was able to assess the peak-to-peak noise signal to be around 150mV. Using the maths feature on the scope, I can subtract one input from the other, and the result is noise-free. If I feed a balanced sine wave signal into the grids, I get very noisy sine wave outputs on the anodes. Again, if I use the maths feature on the scope, the result is a clean sine wave (okay, there's a bit on noise on it as one might expect because of CMRR).
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Old 24th March 2014, 02:19 PM   #8
DF96 is online now DF96  England
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What do you see at the cathodes? The noise can only come from three sources: cathode circuit, anode circuit, or the valve itself.
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Old 24th March 2014, 02:39 PM   #9
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There is noise on the cathode that mirrors the noise on the anodes, albeit at a much lower level as one might expect. What I'm not clear about is whether this indicates that the noise is originating in the current source or elsewhere - after all, I would expect to see the noise signal on the cathode given that it is common mode. I suppose I could try replacing the current source with the fixed resistor and see if the noise is still there. I can't see why my current source should be producing noise, though...
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Old 24th March 2014, 04:40 PM   #10
DF96 is online now DF96  England
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Can you be absolutely certain that the valve is not suffering from parasitic oscillation? Unexplained noises from RF valves used in audio often turn out to be due to parasitics.
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