Can partial feedback be as effective as global fb for decent damping factor? - diyAudio
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Old 14th September 2004, 07:55 AM   #1
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Default Can partial feedback be as effective as global fb for decent damping factor?

Hi,

I've built a PP amp which sounds OK. It has a 6SL7 LTP splitter as the first stage, with a 6AU6 CCS in the tail. This is followed by 6SN7 differential driver and PP EL34 triode-strapped output stage.

There's no global feedback loop but I've used partial feedback (a) from the 6SN7 plates to the 6SL7 plates and (b) from the EL34 plates to the 6SN7 plates. My OPT is a locally made Indonesian product of unknown quality and doesn't have UL taps. This is why the EL34s are triode-strapped and why I'm not keen to use global feedback (because of the potential instability).

I'm thinking of trying the EL34s in class AB1 pentode connection, to try a different sound with more power. However, I'm aware that such a move will lower the damping factor considerably. In commercial designs, this is usually overcome by using global feedback but I'm wondering if partial fb can be sufficiently effective to give a reasonable damping factor?
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Old 14th September 2004, 08:00 AM   #2
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I don't believe local FB does anything but straighten out linearity. Global FB is what lowers output Z and hence, DF goes up.

Your option may be to change the output transformers. Can you get Hammond's where you are? They usually have the best overall bang-for-the-buck.

FB is not my strong point in amps though, so if more knowledgeable folks can help....
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Old 14th September 2004, 11:29 AM   #3
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an partial feedback be as effective as global fb for decent damping factor?
Theoretically yes but in practice no.

Voltage feedback over the output tubes i.e from anod to grid lowers the output impedance as much as the gain is decreased but the problem is that drive voltage increase the same amount, therefore it is not practical to use a high amount of local feedback. With global feedback mainly the gain of the first stage is affected and the input voltage increases but it doesn't matter as the first stage can handle higher input voltage due to the feedback.

The amount of local feedback is thus limited by the highest input voltage that can be allowed to that stage, (what the driver can give).

The only thing that limits global feedback is stability and that can be handled by phase correcting networks of different kinds.

I would advice you to learn about feedback stability and apply global feedback if you feel you need a high DF, I personally enjoy the maths involved and applying that in realkity, I measure phas and gain up to 20MHz and design my feedback networks after that.

Regards Hans
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Old 14th September 2004, 01:56 PM   #4
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If the transformer were perfect, shunt FB (plate-grid as you describe) would control things effectively. However, loss across the transformer, including phase change, frequency response and DCR, is independent of that. It's outside the loop.

If your transformer sounds good, no doubt it will be suitable for NFB. I've wrapped some crappy transformers with NFB before... just don't expect much below 50Hz if your transformer has low inductance.

BTW, I'd remove the local FB to provide more global gain, so you can correct the output stage errors without needing a massive input signal.

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Old 14th September 2004, 02:16 PM   #5
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It's a juggling act. You need quite a lot of feedback to lower output resistance to the levels required by most modern loudspeakers, and that usually means global feedback. However, cathode feedback at the output valves can be surprisingly effective at reducing distortion (more effective than theory would suggest) - but you need plenty of gain to begin with, so it works best with pentodes and beam tetrodes.

If you can go to an active crossover, that removes the inductor in series with the bass driver, typically removing an Ohm of series resistance, reducing the necessity to strive for really low output resistance in the amplifier...
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Old 14th September 2004, 10:41 PM   #6
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Quote:
However, cathode feedback at the output valves can be surprisingly effective at reducing distortion
Cathode feedback is indeed a good idea for reducing distortion but the drawback is that it increases output impedance so DF in this case would go down, quite the opposite of what was asked for.


Quote:
If the transformer were perfect, shunt FB (plate-grid as you describe) would control things effectively. However, loss across the transformer, including phase change, frequency response and DCR, is independent of that. It's outside the loop.
It must be a very bad transformer to affect the damping factor of the amp in any major way, (but maybe in the case with an output transformer with very high winding resistance but then you would have more serious problems than affected DF) in any case local feedback is not realistic as a method to increase DF, 20dB of global feedback is not that difficult to apply and would increase DF 10 times, that amount of local feedback is clearly unrealistic.

Regards Hans
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Old 15th September 2004, 08:57 AM   #7
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Quote:
Originally posted by tubetvr
Cathode feedback is indeed a good idea for reducing distortion but the drawback is that it increases output impedance so DF in this case would go down, quite the opposite of what was asked for.
Only if applied by an unbypassed cathode resistor. If applied by a winding from the output transformer (McIntosh or Quad style), it reduces output resistance.
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Old 15th September 2004, 01:39 PM   #8
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EC8010, You are quite right but I didn't realise that was what you meant by " cathode feedback at the output valves can be surprisingly effective at reducing distortion" Sorry if I misunderstood what you intended to describe, for me simply cathode feedback is cathode degeneration by unbypassed cathode resistors which still have a surprisingly good effect on linearity.

Regards Hans
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Old 15th September 2004, 02:06 PM   #9
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I think what EC8010 is referring to is called split load output stage, where a part of the transformer winding is connected between the cathode and ground and the rest between anode and B+.
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Old 15th September 2004, 03:20 PM   #10
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No need to apologise, I didn't make my original post sufficiently clear - I should have used the term "split loading" in the first place.
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