I've been wanting to both get back in the game of building things that glow in the dark (its been years) and a preamp for my Shure SM7B (that I could use as an alternative to the FetHead that I like) so after the first impulse to simply copy a preamp topolology I saw in a TapeOp article I decided to scratch my head a bit and came up with my own thing. One of the design constraints was a low-ish 240-250V plate supply which I already have in a "brick" form. The choice of a 12AU7 for the WCF was mainly due to the hefty 200V h-k rating which reduces the need to elevate the heaters supply.
I've looked at different topologies, including Aikido cascode with no NFB but settled on 2-stage RC coupled amplfier with "gentle" -20db nfb.
My first desire was to use something fairly uncommon like 6EJ7s strapped in triode for the gain stages, but 6N1P so far has given far better results in simulation (I've tried 6N3P, and other medium-mu triodes in this circuit) plus with 6N1P and other double triodes you only need 2 and not 3 tube sockets.
Looking for critique and hope to proceed to prototype this little "mic warmer".
Regards, Alex
I've looked at different topologies, including Aikido cascode with no NFB but settled on 2-stage RC coupled amplfier with "gentle" -20db nfb.
My first desire was to use something fairly uncommon like 6EJ7s strapped in triode for the gain stages, but 6N1P so far has given far better results in simulation (I've tried 6N3P, and other medium-mu triodes in this circuit) plus with 6N1P and other double triodes you only need 2 and not 3 tube sockets.
Looking for critique and hope to proceed to prototype this little "mic warmer".
Regards, Alex
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The amplifier following the mic amp is Behringer audio interface. The input impedance is not given by the manufacturer but generally mix preamps are 1.5-2K.
I wanted to preserve the 1:10 impedance bridging principle, and besides, have a balanced signal for the XLR input on the interface. You would need a differential output to drive 600 ohm balanced line if no transformer.
Improvements to the SNR and CMRRR and ground loops at the cost of -12db gain in the 4:1 transformer.
Can you please further elaborate on this idea? Does that imply following a differential topology all the way through including differential output?
In the old days . . .
They used microphone transformers at the preamp input to reduce interference from:
Ground loops, RF, and other noise sources, from the microphone and microphone cable.
And they used preamp output transformers to reduce interference from:
Ground loops, RF, and other noise sources . . . which could come from the preamp output cable to the next device (sound board, etc.)
Nobody knew what they were doing back then, Right? . . . Wrong!
Go visit some old recording studios, radio stations, TV stations (if you can find any).
Quality Costs.
Cheap is not Good; Good is not Cheap. Frank Reps; Baby Ongaku; in Sound Practices magazine.
Nowadays, everything has been improved.
Just answer why the old Ma Bell phone service could instantly give you a dial tone after ending a call . . . if you poked the button and instantly let it back up.
The only time delay was current through the phone lines at 1/2 the speed of sound to the phone relay building, the first relay dropping out, and then the new relay connecting the dial tone.
Try to get that kind of performance on the time share phone building of the Great Telephone Experiment (GTE).
Bell Labs, Western Electric anybody?
Phones that never broke, transatlantic wire cable (not fiber cable), and 300B tubes, etc.
Simetimes history has a great way of Not repeating itself.
They used microphone transformers at the preamp input to reduce interference from:
Ground loops, RF, and other noise sources, from the microphone and microphone cable.
And they used preamp output transformers to reduce interference from:
Ground loops, RF, and other noise sources . . . which could come from the preamp output cable to the next device (sound board, etc.)
Nobody knew what they were doing back then, Right? . . . Wrong!
Go visit some old recording studios, radio stations, TV stations (if you can find any).
Quality Costs.
Cheap is not Good; Good is not Cheap. Frank Reps; Baby Ongaku; in Sound Practices magazine.
Nowadays, everything has been improved.
Just answer why the old Ma Bell phone service could instantly give you a dial tone after ending a call . . . if you poked the button and instantly let it back up.
The only time delay was current through the phone lines at 1/2 the speed of sound to the phone relay building, the first relay dropping out, and then the new relay connecting the dial tone.
Try to get that kind of performance on the time share phone building of the Great Telephone Experiment (GTE).
Bell Labs, Western Electric anybody?
Phones that never broke, transatlantic wire cable (not fiber cable), and 300B tubes, etc.
Simetimes history has a great way of Not repeating itself.
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A step-up input transformer also helps for noise matching to low-impedance microphones. Valves have very little noise current, but the noise voltage is usually a bit on the high side if you want to support 200 ohm dynamic microphones. A good step-up input transformer can solve that.
One important consideration is the power supply that can be very accurate with a perfect decoupling between the stages.
It will have a great impact on sound and residual noise
Walter
It needs the first twin triode cathodes tied together & that tied to some kind of
cathode tail resister for improved balance, just as any Diff Amp.
No, just make a good selection of tubes with a good testerIt needs the first twin triode cathodes tied together & that tied to some kind of
cathode tail resister for improved balance, just as any Diff Amp.
Amplitrex is one good choice
Or with an good audio card and Arta or REW software is possible to check the gain and noise
Walter
Thank you! E88CC is the choice for WCF for sure, but I wanted to shoot for grounded DC heater supply vs. elevated.
Well, I have also gave some thought again and reimagined my older idea of using triode-strapped 6EJ7/EF184 here. This probably deserves its own thread but will post it here for now.
I recall some old threads on this site about that tube. What an interesting possibility, hi gm, hi mu and low rp. Seems that Miller capacitance (that is on the order of 200 pf) is a problem considering the high mu.
And it is!
One stage choke loaded has plenty enough gain. And the distortion is pretty low with choke load and unbypassed cathode, no need for global NFB. The distortion spectra are nice, triode 2nd harmonic milk and honey and less high-order products. No need to worry about NFB loop stability either.
But if the 1:10 mic transformer is connected directly to the grid of 6EJ7 "triode" and is driven by a 150 ohm source, the -3db rollow due to Miller C is around 36 khz. One could live with that, maybe. The actual impedance of, say SM57 is around 300 ohms and that, reflected, brings the -3db rolloff down into the 19-20 khz. And if the source is 1.5K then its even lower.
But what if we are not afraid of going a little hybrid and just drive the tube with a low Zout unity gain buffer, like an opamp? That will shunt the Miller C and voila! Flat out to 300 khz and beyond. I've used a lowly TL072 because it was an easy cut and paste from another project, but I can come up with a OPA2134 in my box of junk parts easily enough. Maybe there are even newer and better devices out there that approach an ideal unity gain follower. Will need a dead quiet dual polarity +/-12VDC supply though.
I recall some old threads on this site about that tube. What an interesting possibility, hi gm, hi mu and low rp. Seems that Miller capacitance (that is on the order of 200 pf) is a problem considering the high mu.
And it is!
One stage choke loaded has plenty enough gain. And the distortion is pretty low with choke load and unbypassed cathode, no need for global NFB. The distortion spectra are nice, triode 2nd harmonic milk and honey and less high-order products. No need to worry about NFB loop stability either.
But if the 1:10 mic transformer is connected directly to the grid of 6EJ7 "triode" and is driven by a 150 ohm source, the -3db rollow due to Miller C is around 36 khz. One could live with that, maybe. The actual impedance of, say SM57 is around 300 ohms and that, reflected, brings the -3db rolloff down into the 19-20 khz. And if the source is 1.5K then its even lower.
But what if we are not afraid of going a little hybrid and just drive the tube with a low Zout unity gain buffer, like an opamp? That will shunt the Miller C and voila! Flat out to 300 khz and beyond. I've used a lowly TL072 because it was an easy cut and paste from another project, but I can come up with a OPA2134 in my box of junk parts easily enough. Maybe there are even newer and better devices out there that approach an ideal unity gain follower. Will need a dead quiet dual polarity +/-12VDC supply though.
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This is one way to have the life more complicated.
The SM57 is declared to have -3dB at 15 kHz and the cost of it is around 99$.
The question is:
if I would spent time and money for a complicated circuit ( this only in the virtual world, it is a simulation) or is better to adjust a more simply one.
Considering that the chocke must be a good stuff , so not cheap.
Then, I will amplify the signal then I reduce it in output by 4:1 ratio (12 dB!!!) using a trafo that is not so cheap.
For which reason?
Walter