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Old 10th June 2013, 01:07 PM   #1
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Default Cross-channel overtalk with strange effects

I am making a amplifying stage for I/V, with ECC83 (5751) as first stage, White Cathode Follower as output. I do have a mu-stage. But nothing really spectacular here.
And then noted there is a lot of overtalk (In Dutch: overspraak; I do not know the good word): the sound also appearing in the other channel.

It is very low at low frequencies but above 2 kHz (at about -45 dB) it starts to rise and above 60 KHz it is almost -20 dB!

And above that: it looks strange, with a typical cross-over bump like a transistor output. And there is 90 degrees phase difference . . . making it even more mysterious.

Audio L1020615.jpg
  • It could very well come from the heater, but I tested, with a 150K resistor instead of the mu-stage, and though it differed a bit by 6 dB, it did not disappear. The heater is at + 50 V; connected to ground via 1 uF. So this cannot be the source (effective short circuit of HF signals.
  • The cathodes are bypassed. So that cannot be the source. Not bypassing inceases it. This is also true for the output stage . . .
  • The two channels have their own grond wire (but in my test setting, the HV is inserted at a point furthest away from the common reference). But that misallocation of wire can't be the source of cross-channel interference.

My creativity ends here. Who has a suggestion?
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Old 10th June 2013, 01:16 PM   #2
impuls is offline impuls  Yugoslavia
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Quote:
Originally Posted by triode_al View Post
[*]The two channels have their own grond wire (but in my test setting, the HV is inserted at a point furthest away from the common reference).
Did You connect ground from both channell to ground D/A? And do you have resistor for i/v?
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Old 10th June 2013, 01:48 PM   #3
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Default The preamp

Just for further information.

Page-01 kopie 3.jpeg

The CCS is made from a 2SK30, to get 1,0 mA (Rs=1k)
Bandwidth is somewhere between 400-500 KHz.
I have a low-pass at 80 kHz between the stages.
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Last edited by triode_al; 10th June 2013 at 02:08 PM. Reason: added details
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Old 10th June 2013, 01:58 PM   #4
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Originally Posted by impuls View Post
Did You connect ground from both channell to ground D/A? And do you have resistor for i/v?
  • Yes, there is one point in which all grounds come together (save that in my testing, the HV is connected to both channels at the same time.)
  • And the input is connected to the signal generator (R-out = <50 ohm); the second inputs is not shorted to earth with a wire, just 47k.
  • I have a 39-51 ohm I/V resistor for my TDA1541.
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Last edited by triode_al; 10th June 2013 at 02:07 PM.
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Old 10th June 2013, 02:18 PM   #5
DF96 is offline DF96  England
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Quote:
Originally Posted by triode_al
(In Dutch: overspraak; I do not know the good word)
In English this is called crosstalk.

The 90 degree phase shift strongly suggests that the coupling is via inductance or capacitance. The increase in level with increasing frequency is consistent with this.

Could it be stray capacitance between the two lower 5751 anodes? This will be a high impedance point, especially when the cathode is not decoupled. How have you arranged the two 5751: one (upper and lower) per channel, or one upper and one lower (for both channels)?

Another possibility is decoupling caps injecting signal currents (from the supply rail?) into ground wires.

The crossover blip is a bit of a mystery. It suggests you have some positive feedback somewhere. This can happen with any follower.
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Old 10th June 2013, 02:20 PM   #6
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In the picture in the first thread, the primary signal is 75V peak-peak output, the other channel (crosstalk) is 60 mV pp. (so at -62 dB)

It is below 10 kHz (I can only partly see the signal generator..)

Audio L1020613.jpg
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Old 10th June 2013, 03:32 PM   #7
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Quote:
Originally Posted by DF96 View Post
In English this is called crosstalk.

The 90 degree phase shift strongly suggests that the coupling is via inductance or capacitance. The increase in level with increasing frequency is consistent with this.
  1. Could it be stray capacitance between the two lower 5751 anodes? This will be a high impedance point, especially when the cathode is not decoupled. How have you arranged the two 5751: one (upper and lower) per channel, or one upper and one lower (for both channels)?
  2. Another possibility is decoupling caps injecting signal currents (from the supply rail?) into ground wires.

The crossover blip is a bit of a mystery. It suggests you have some positive feedback somewhere. This can happen with any follower.
ad 1: yes I have thought that too. The bottom tube serves both channels; how to test it though? - otherwise I can use a ECC85 with its center shield. However the 5751 is known for a nicer signature.

ad 2: this is a good possibility indeed.
Look at the following bounce from inductance on the ground plane (30V pp square output; other channel probably 20 mV:

Audio L1020618.jpg
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Old 10th June 2013, 05:56 PM   #8
DF96 is offline DF96  England
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I think you need to improve the grounding - it is clear that signal currents from one channel are getting into the other channel. Simply moving the ground connections of the supply rail decouplers may be sufficient.
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Old 10th June 2013, 06:15 PM   #9
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Originally Posted by DF96 View Post
I think you need to improve the grounding - it is clear that signal currents from one channel are getting into the other channel. Simply moving the ground connections of the supply rail decouplers may be sufficient.
Thanks for the support.

I will make a true star ground then, to which both stages go separately (now it is a 'bus'), this star being the point to which the PS is grounded too.

Im afraid it is also the typical problem of a test bench

Note:
[When I made the chassis I am re-using, I had a separate PS with windings etc for each stage, and grounded at each stage too, even separate from both channels: I had two transformers with 2 separate HV windings, four 6.3V windings each..- and hence saw no problems.]
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Old 10th June 2013, 07:09 PM   #10
hpeter is offline hpeter  Europe
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Quote:
Originally Posted by triode_al View Post
ad 1: yes I have thought that too. The bottom tube serves both channels; how to test it though? - otherwise I can use a ECC85 with its center shield. However the 5751 is known for a nicer signature.

ad 2: this is a good possibility indeed.
Look at the following bounce from inductance on the ground plane (30V pp square output; other channel probably 20 mV:

Attachment 353606
such spikes i saw when had g1 (screen driven tetrode) grounded trough high resistance to gnd
(low resistance fixed it)
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