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Old 11th March 2012, 12:58 PM   #11
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Originally Posted by tictac View Post
Do you have a drawing of your power supply and the voltages needed for the preamp stage?
At work again, I'll try to get to it later. SS rectification, CLCRCRC filtering. 280V for the input stage and 320 for the CF.

I tried the circuit first without the cathode decoupling, but had some hum problems. Works better this way. The CF CCS could be better, now that I think of it. At least the circuit seems to be reliable, the first one I made identical to this has worked without problems for three years now, and is in constant use. And the sound isn't bad either! At least one person preferred it over his Quad QC24 and Audio Research preamps, but this of course is subjective and a matter of taste. I measured a distortion figure of 0.19% at 2Vrms in (the volume pot is in the output side), but that includes the distortion of the generator I was using. I did'nt know it then, but only later measured that the 2nd harmonic of the Rigol generator was at -50dB...
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Old 11th March 2012, 01:40 PM   #12
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I'm showing some 39dB of gain with 0.001% distortion @ 3volts output, 1kHz input. The current sources in the simulation have an internal impedance of 1M ohm. I tried it with your CCS arrangement and I think there's a problem with them. On the CF, I didn't have any bias. What bias do you measure on your CF and on the first stage? In the simulation, bias on the first tube is 1.8V, on the second it's about 1V. Obviously, the circuit inverts the signal phase also.
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Last edited by dirkwright; 11th March 2012 at 01:43 PM.
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Old 11th March 2012, 01:58 PM   #13
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Originally Posted by dirkwright View Post
I'm showing some 39dB of gain with 0.001% distortion @ 3volts output, 1kHz input. The current sources in the simulation have an internal impedance of 1M ohm. I tried it with your CCS arrangement and I think there's a problem with them. On the CF, I didn't have any bias. What bias do you measure on your CF and on the first stage? In the simulation, bias on the first tube is 1.8V, on the second it's about 1V. Obviously, the circuit inverts the signal phase also.
That sounds very, very wrong. But in your schema it shows an AX7 on the input? Wrong tube. The DC connected, CCS loaded CF biases itsself. It's cathode sits a couple of volts higher than the anode of the first stage, governed by the current set by the CCS. The first stage is biased at around 6V (1k2 * 5mA=6V)at the cathode. Both stages have ~5mA Iq.
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Old 11th March 2012, 02:06 PM   #14
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Sorry, didn't read your first post. I don't have a model for the 12BH7. I substituted a 12AU7 for the first tube. Gain is now around 24dB. Distortion is really low.
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Old 11th March 2012, 03:09 PM   #15
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That's better. Much closer to what I actually measured, aside from the distortion. I measured a gain of ~25dB with the ECC99 in place. It has a slightly higher mu than the 12AU7/ECC82.
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Old 11th March 2012, 03:11 PM   #16
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Originally Posted by Adolf Corkscrew View Post
That's better. Much closer to what I actually measured, aside from the distortion. I measured a gain of ~25dB with the ECC99 in place. It has a slightly higher mu than the 12AU7/ECC82.
I don't know if you asking for advice on this circuit, so I hesitate to make suggestions. It's already built anyway so there's no going back now. It's really nice looking. Glad you enjoy it.
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Old 11th March 2012, 05:32 PM   #17
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I don't know if you asking for advice on this circuit, so I hesitate to make suggestions. It's already built anyway so there's no going back now. It's really nice looking. Glad you enjoy it.
Built and the circuitry is on PCB's, so little room for modifications. As I stated earlier the first one of these has already served for three years or so, but now that I built another one, I see room for improvement. Doesn't matter really, though, since this is what I've been asked to do. Maybe a retrofit MKII PCB kit in the future... This one is part of a set of a preamp and two KT88 monoblocks I've been building for a friend; copies of what I already had.
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Old 11th March 2012, 07:31 PM   #18
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I agree with dirkwright, that you should be able to eliminate C2 without significant performance degradation. Whether doing so subjectively improves the sound or not, only empirical evaluation would reveal. Assuming that the 'hum' you heard is not due to field coupling, I can only imagine that it is due to power supply coupling via the CCS. Short of reducing the ripple on the HV supply rail itself, you could improve the CCS by replacing that 100k bias resistor with a JFET or, better yet, a cascoded JFET. Perhaps, that might reduce the hum to in audible levels without dramatically redesigning the HV supply. Even though your CCS is cascoded, the biasing scheme is likely limiting the maximum impedance it might otherwise deliver.
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Last edited by Ken Newton; 11th March 2012 at 07:34 PM.
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Old 11th March 2012, 07:55 PM   #19
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What J-FET would take the voltage at that point? Good idea though, hadn't thought about CCS'ing the CCS references... But I don't think the hum is from the PSU. It's been so long since I debugged this circuit, but the PCB's in the first version didn't have the decoupling caps since I thought as well that they were unnecessary, but apparently the heater couples to the cathode capacitively or otherwise. Anyhow having that cap on the cathode made the hum lessen. I might try taking it out again, but having already soldered the boards in, I'm not enthusiastic about going back to do that. Currently I have zero problems with hum, the amp is sufficiently quiet.

If I redesigned the circuit now, I'd concentrate on improving the CF's CCS, not the first stage. It could have a higher dynamic impedance. It works OK, but could be better. Then again I just had the idea of applying my DAC output stage as a preamp, so I'm not likely to revisit this schematic anytime soon, since I'm already quite happy with it, and so is the "customer" who'll be getting the one on the opening post's picture soon. He's been waiting for a while already...
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Old 11th March 2012, 08:11 PM   #20
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I suggest either a JFET cascoded by a depletion MOSFET, or a depletion MOSFET cascoded by another depletion MOSFET - using a high-voltage depletion mode MOSFET as the cascode device is the main constraint. Below, is a link to some such devices from Supertex, there are others, such as from IXYS. As you said, perhaps, for a future project.

http://www.supertex.com/pdf/misc/d_m..._SG_device.pdf
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