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In Reply to: RE: Phase Shift posted by Triode_Kingdom on August 02, 2017 at 07:22:22
Isn't this simply a SRPP driving a differential splitter? Granted the 15K in the tail doesn't make it the best one but that is the designers fault not the topologies fault.
dave
Follow Ups:
Unless... Are you speaking of the 1M / 47µ combo being bad practice rather than simply grounding the undriven grid.
Being a SE guy, I never really looked into what that actually does.
dave
Yes, that's the problem. The grid must be at AC ground. I suppose the reason this doesn't immediately grab everyone's attention is that the same RC network creates significant attenuation of the signal at the grid. After all, that's its purpose (and to bias the grid to the same DC level as the upper section). The problem is, the phase shift is nearly constant at 90 degrees across the audio band, whereas attenuation varies as a function of frequency. Use of this circuit demands that we assume the shift to be inaudible across the entire range of attenuation, whatever it might be, and I don't think it's reasonable to do that. I also think this is the cause of the phenomenon the OP is describing.
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Buy Chinese. Bury freedom.
...the section of the PI under discussion is cathode drive so under ideal conditions the output at the anode is in phase with the input at the cathode. Under ideal conditions, g1 of this section should be at AC ground at all frequencies and phase shift would not be an issue. In the practical implementation with a finite value grid bias resistor and a finite value decoupling cap, there is phase shift approaching -90 deg at g1 of the cathode coupled section. But at all but the very lowest frequencies, say <10 hz, the amplitude of the phase shifted signal applied to g1 is minuscule such that the output at the anode is shifted by just a few degrees and not the -90 deg at g1. IOW, the signal applied to the cathode dominates the output at the anode.
The "long tailed pair" PI certainly isn't perfect but gross phase shift at the non-inverting output is typically not an issue. As you know, this style PI was used successfully in many classic designs including those from Marantz and McIntosh.
The problem is, the phase shift is nearly constant at 90 degrees across the audio band, whereas attenuation varies as a function of frequency. Use of this circuit demands that we assume the shift to be inaudible across the entire range of attenuation, whatever it might be, and I don't think it's reasonable to do that.
I will agree with you on this. I wonder what the result would be if the 1M resistor was replaced with a grid choke. The resultant LC filter would be a 2nd order filter with a 180 instead of a 90-degree phase shift across the frequency band. Given a sufficiently large decoupling cap to ground, the same signal would appear (same relative phase angle) at the grid as it would at the cathode input. Of course this would be a feed-forward signal, but it might not be too detrimental given that it'd be very small.
Could it be a way to attempt to compensate for LF phase lag caused by finite inductance or is it simply to deal with the direct coupling as CB says below?
I get all turned around and my head starts to hurt when I start attempting to think of what is going on. You are feeding a frequency attenuated 90º shifted signal that is 180º out of phase with the cathode to the grid. I do see that cutting the cap value in half will give a 6dB increase of whatever effect this does.
dave
I've been bothered by that structure when I last saw it (in some Dynaco monoblock driver boards from somewhere) and I wondered why you'd bother with the direct coupling only to have a coupling cap stuck on the other grid.
In these fixed bias amps with negative rails, why in the world wouldn't they put a CCS between the negative rail and the LTP, then ground one grid and have a single coupling cap to the other? You'd get the added bonus of being able to drastically raise the impedance of the plate load resistors on the LTP which doesn't hurt anything either.
I totally agree with your premise and I've been thinking the same thing for a long time now, at least on the few occasions that I've contemplated building an amp with an LTP splitter - haven't built one yet. Here's the basic idea of the circuit taken from tubecad.com with the only difference being that in the case of the cathode coupled inverter, we would need a plate load at the first tube.
Edits: 08/02/17 08/02/17
That's not a phase inverter, but could be turned into one with some parts added.
As I wrote in my post, the circuit is referenced to show the general idea.
Edits: 08/02/17
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