Possibly a silly question, but given say for instance a Williamson amplifier or an amplifier with balanced input, is there any real disadvantage to running feedback off of each side of the transformer secondary?
Like so-
Ignoring the parts values, zeners, etc, what would be the disadvantages to running it this way? It seems like it would avoid the phase shift inherent with looping an additional stage inside, with maybe less THD reduction than the traditional way, assuming that you could keep phase shift in line? Might be better for lesser quality transformers?
Like so-
Ignoring the parts values, zeners, etc, what would be the disadvantages to running it this way? It seems like it would avoid the phase shift inherent with looping an additional stage inside, with maybe less THD reduction than the traditional way, assuming that you could keep phase shift in line? Might be better for lesser quality transformers?
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Apart from this you can connect the cathodes of the drivers with those of the power tubes with 68k resistors making the Ck's of the power tubes superfluous. Also the zeners should be left out. The 3k feedback resistors are way too small. The ratio of feedback resistor and Rk should be about 20:1 so use 15k there (or more if you want less feedback).
With the polarities shown (input to output) you have positive feedback. So cross coupling would be necessary assuming the OPT was wound as pictured. But now you cannot ground the secondary for safety concerns. I would rather see a grounded four ohm tap with the common and sixteen ohm used for balanced feedback. I have seen this balanced feedback scheme used with a tertiary winding to the driver cathodes. It was also center tapped to ground.
BTW, those zeners serve no real purpose.
BTW, those zeners serve no real purpose.
You need to use a 4-8-16 OHM transformer to make that work.
Usually the 4 ohm tap gets grounded and you use the 0 and 16 ohm taps for feedback.
Usually the 4 ohm tap gets grounded and you use the 0 and 16 ohm taps for feedback.
(Schematic is from an old tubecad article, not from a finished amplifier, just fyi, not my design)
Yeah, 50/50 chance of getting the feedback polarity correct 🙂
I'm planning on using a center-tapped secondary or a 0-4-8-16 transformer anyway, but wouldn't the secondary still have a ground reference to the driver cathodes?
Yeah, 50/50 chance of getting the feedback polarity correct 🙂
I'm planning on using a center-tapped secondary or a 0-4-8-16 transformer anyway, but wouldn't the secondary still have a ground reference to the driver cathodes?
If the OPT has a Zero, 4, 8, 16 secondary the 4R tap should go to the amp common.
No tap at all on a PP OPT is unusual, never seen one like that. 🙂
No tap at all on a PP OPT is unusual, never seen one like that. 🙂
When I use a balanced circuit the p-p OT trafo custom has single 5 ohm with center tap
This is the best way to do the job
To make the like easier you can use a input signal trafo to have a conversion bal-unbal
This is the best way to do the job
To make the like easier you can use a input signal trafo to have a conversion bal-unbal
Intention was propably to get a transition from automatic bias into fixed bias as soon as the tube enters class B operation, could work, sort off
Yes. That's what is tipycally done on most Audio Research amps, namely D40, D75, D76, D76A, D79, D150... Below the D40 :
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I am not sure about the transformer winding art, but based on the overall schematic and the +/- labels at I/O, I take the intention to be that the transformer windings are in phase. In that case, the driver is CG and non-inverting to feedback, followed by inversion at the output tube.
To ground the secondary or not . . .
Some have never witnessed a [high voltage] primary to secondary short.
Safety First!
Prevent the "Surviving Spouse Syndrome".
And ground the transformer frame.
I once had a primary to frame short.
Ouch!
Some have never witnessed a [high voltage] primary to secondary short.
Safety First!
Prevent the "Surviving Spouse Syndrome".
And ground the transformer frame.
I once had a primary to frame short.
Ouch!
Loading effects from the speaker.
I use this scheme off and on for line level, but it could easily applied with an output amp. Having the secondary in the cathode circuit of the driver tubes increases the dampening factor dramatically. I do remember someone doing that with an amp section before..