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In Reply to: RE: non-inverting preamp w/schematic ... posted by deathtube 667 on June 15, 2017 at 16:57:41
I appreciate the concept that what comes out of a preamp is exactly what goes in. IMO, that should include the input-output phase relationship. The two-tube circuit you posted is a variation of the "Blue Velvet" preamp, and it's a design I've been considering and analyzing for a long time. However, if additional system gain isn't needed, a cathode follower will provide much greater bandwidth, lower distortion and more substantial driving ability. A follower done right will also be sonically transparent, a characteristic that's difficult to achieve using common cathode stages. A zero-gain preamp has another potential advantage. The less gain the preamp exhibits, the more clockwise the input potentiometer will be set in normal operation. That means less carbon track in series with the signal path and a higher resistance (less signal current) to ground. All this adds up to less invasive control and a greater likelihood signal integrity won't be compromised.
Buy Chinese. Bury freedom.
Can you post a schematic of a proper cathode follower in this application with 6sn7.
I have been working one for while but it has a couple issues and it may be my implementation as I am but beginner.
idlers and tubes....life is good
"I appreciate the concept that what comes out of a preamp is exactly what goes in. IMO, that should include the input-output phase relationship."
OK, I'll take the bait on this one, and ask another question or two.
I used to be very skeptical about the notion of absolute phase mattering, until I read up a bit about the waveforms of musical instruments. And in fact some can really be quite asymmetric between the positive portion and the negative portion of the waveform. This can mean that if there is some other amplifier or transducer in the chain that has an asymmetric response, then it can matter what the absolute phase of the signal that is driving it has. That is to say, the perceived sound could be different depending on whether the asymmetry in the sound source is in phase or out of phase with the asymmetry in the amplifier or transducer.
I suppose one example of a potentially asymmetric transducer is the human ear. Thus it can, I suppose, matter whether the phase-sense of the original sound source is the same, or the opposite, of what the human in the room where the sound is being produced would experience.
Another potentially asymmetric transducer is the loudspeaker. And apparently loudspeakers can really introduce a very significant amount of distortion, which is very likely asymmetric.
Normally these days, an amplifier would not be an appreciable source of asymmetric distortion. But perhaps an SET amplifier could introduce enough (largely second harmonic, and so asymmetric) distortion to matter.
On top of all that, from what I have read, there is no attempt made by most recording studios to preserve any definite absolute phase of the audio signal picked up by the studio microphones.
In the light of all that, I wonder where this leaves someone who cares about absolute phase? If the home stereo amplifier is of low distortion I would have thought that everything that could be possibly be under the listener's control, as far as absolute phase were concerned, could surely be achievable by choosing the polarity with which the speaker leads are connected to the amplifier. (Of course, obviously, left and right channels would be flipped, or not, in unison.)
If there is concern about asymmetric distortion in the amplifier too, then one might want to be able to flip the phase going into the distorting amplifier as well as the phasing of the speaker leads.
But I don't quite understand why one would want a hard-and-fast rule that the preamp should not be phase inverting. If absolute phase is important to the listener, and if there is concern that the power amplifier amplifier has enough asymmetric distortion that it needs to be taken into account, then it seems to me the listener would want to have the option to be able to flip the phase going into the power amplifier.
If, on the other hand, the power amplifier is not considered to be a significant source of asymmetric distortion then everything that the listener could control, as far as absolute phase were concerned, could be done by flipping the wires on the speaker leads.
But I don't really understand, given the recording studios' alleged cavalier attitude towards maintaining absolute phase, why one would consider it important to have one's preamp preserve a fixed, unswitchable phase choice of +1 rather than -1. If absolute phase going into the power amplifier mattered, then you would want to be able to switch it to fit with the recording studio's random choice. If absolute phase into the amplifier didn't matter, then it wouldn't matter whether the preamp was inverting or not; any desired switch of absolute phase could be achieved by flipping the speaker polarity.
Sorry if this went on a bit. I'm just trying to understand the issues here.
I made some adjustments to the schematic that resulted in more gain, lower THD% but slightly higher Zout.
Without the SubOut and 4.7K isolation resistors, Zout @ 20Hz = 5.8K & Zout at 1KHz = 4.9K.
I needed 23dB of Gain to drive my SE 6CB5 triode amp into clipping, so this circuit works OK for that.
Didn't have room on my preamp chassis to add a follower, but with short cable runs in my system, it does not sound like I needed it.
I set my volume pots at 8-9 o'clock for average listening.
Subjectively, I find no issues with sound quality so far.
Thanks to all for comments and ideas.
Does LT Spice calculate impedance for you?
If so, whats the method?
The formula is just ohms law.
You need to set an AC current source at the output, do a small signal AC analysis from the simulate command.
Use a linear BODE plot for the left vertical axis ohms magnitude.
Look at the schematic for the Spice commands.
Hope that helps.
The easiest method is to set the main excitation voltage to zero and drive the output with a one amp AC current. Then you plot the voltage at the output and view it on a linear scale and the output in volts will be the output Z in ohms.
If it makes you feel better you can enter an expression to divide the voltage output by the Current you will get the impedance listed in ohms on the vertical scale.
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