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valve input stage and driver/phase inverter coupling
valve input stage and driver/phase inverter coupling
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Old 17th October 2019, 11:30 PM   #11
Wavebourn is offline Wavebourn  United States
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valve input stage and driver/phase inverter coupling
Quote:
Originally Posted by DF96 View Post
The capacitor grounding the grid has almost the same effect as a coupling capacitor, but most people do not realise this. A good LTP phase splitter will have high differential-mode gain and low common-mode gain. The LTP input signal appears between the anode of the previous stage and ground. To pick up this signal one LTP grid has to be coupled to the anode, and one has to be coupled to ground. At least one of these connections has to include a capacitor to block DC. It doesn't matter too much which grid you choose to have the capacitor.
Not the same effect. It works like an all pass filter, shifting phase between phase-splitting outputs, introducing common-mode output on lower frequencies. I did that in my Pyramid amp design, and had to take care of infra-sound frequency instability.
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Old 17th October 2019, 11:36 PM   #12
Matt Rowland is offline Matt Rowland  United Kingdom
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High Wavebourn. So how did you cure the instability at LF?
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Old 17th October 2019, 11:39 PM   #13
rayma is online now rayma  United States
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Quote:
Originally Posted by Chris Hornbeck View Post
Both are poor choices unless there's a very compelling reason to *not* have
normal drive to both output valves.
Of course push-pull drive to a diff amp is ideal, but these circuits both lack that.
Many tube amplifiers have had similar circuitry, including the Marantz Model 8B and 9,
so good performance is possible with such a topology.

Last edited by rayma; 17th October 2019 at 11:42 PM.
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Old 17th October 2019, 11:42 PM   #14
Wavebourn is offline Wavebourn  United States
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valve input stage and driver/phase inverter coupling
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Originally Posted by Matt Rowland View Post
High Wavebourn. So how did you cure the instability at LF?
As usual, playing with values of RC, this time in common mode servo feedback.
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Old 17th October 2019, 11:47 PM   #15
Matt Rowland is offline Matt Rowland  United Kingdom
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82uf, that's a big cap. M
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Old 18th October 2019, 12:15 PM   #16
DF96 is offline DF96  England
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Quote:
Originally Posted by Wavebourn
Not the same effect. It works like an all pass filter, shifting phase between phase-splitting outputs, introducing common-mode output on lower frequencies.
As I said, "almost the same effect". Not an all-pass filter, but a lead-lag filter. As you say, it can affect common-mode output at very low frequencies so may help or hinder subsonic loop stability. Within the normal audio frequency range it has a very similar effect to a coupling cap.
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Old 18th October 2019, 02:37 PM   #17
Matt Rowland is offline Matt Rowland  United Kingdom
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I don't see any compensation for this subsonic instability in amps like the Dynaco 60watt for example. How does it manifest, saturation of the o/p transformer core?
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Old 18th October 2019, 04:34 PM   #18
DF96 is offline DF96  England
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Whether there is subsonic instability depends on many things. As I said, this lead-lag filter may help or hinder stability. The ratio of differential-mode gain to common-mode gain in the LTP is an important factor, as is the exact response of the output stage to common-mode grid drive. The lead-lag filter does not in itself create instability as it is just one circuit feature among many others.
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Old 18th October 2019, 04:37 PM   #19
Matt Rowland is offline Matt Rowland  United Kingdom
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OK I see what you mean. I guess I'll just experiment and see what happens. Cheers.
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Old 18th October 2019, 08:42 PM   #20
Wavebourn is offline Wavebourn  United States
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valve input stage and driver/phase inverter coupling
The criterion of instability is, amplification factor greater than 1 on the frequency where phase shift is 180 degrees. Change values of coupling RC pairs, and you are safe.
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