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Old 3rd May 2015, 12:21 AM   #1
kevinkr is offline kevinkr  United States
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Default The Muscovite Mini III (6N23P) Phono Stage

This is a spin off thread from the Muscovite Mini II phono stage here: The Muscovite Mini II 6N14P Phono Stage

It is essentially very similar to the previous version with a few minor changes and the substitution of a 6N23P (or ECC88/6DJ8/7308) for the 6N14P used in the previous design.

With twice the transconductance, frame grid construction and slightly better linearity the 6N23P gives me about 6dB of additional voltage gain. I think in some ways it sounds better too.

In addition the recommended 6S3P-EV used in the second stage may be replaced with a 5842/417A with the only changes required being wiring with the correct pin out for the 5842.

Refer to the referenced thread for power supply and other details.

Note that the picture shows diodes for bias but I am in fact using LEDs now as shown in the schematic.
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Old 3rd May 2015, 11:43 PM   #2
kevinkr is offline kevinkr  United States
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A couple of days in I have concluded that this is a significant improvement in performance over the 6N14P based design..

I would recommend selecting tubes for the cascode stage that operate in the region of 10mA with a bias LED having a VF of 1.75V or thereabouts.

The inexpensive 6N23P are quite good although I will admit that I have switched to NOS Telefunken E88CC, just a bit nicer overall.
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Old 4th May 2015, 05:04 PM   #3
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Nice circuit. Output impedance may be a little high, though. Perhaps a white-cathode-follower for the output if you want to use all tubes. Or wrap a little feedback around?

Also why apply the negative bias to the grid of V2 by a battery? Does it run out quickly? Is there any difference from using a 1.6V LED on the cathode of V2?
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Old Yesterday, 01:08 AM   #4
kevinkr is offline kevinkr  United States
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Good questions all..

The output impedance is actually pretty low - in the vicinity of 150 ohms including the build out resistor, it will drive quite capacitive cables without any difficulty. It could be reduced further by adding a resistor or CCS to ground from the source of the mosfet to get the current up - currently it is around 10mA and that results in a transconductance of about 50,000umhos - it increases substantially at higher currents at the price of considerably higher dissipation.

An LED would give good service biasing the second stage, but I actually prefer the battery based grid bias in the second stage; I guess it's a choice as much as anything, and not necessarily a rational one, but I'm having fun and batteries work quite nicely as bias sources. Some claim some DC voltage across the EQ caps is not a bad thing but since I generally use polystyrene or teflon I am doubtful if there is an audible benefit. The AA battery will last many years as there is no discharge path other than the leakage in the EQ caps, the bigger problem is the A23 - I've encountered some with internal discharge paths that were dying after a year, but the Duracell A23 I am currently using seem to last at least a couple of years.
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Old Yesterday, 01:12 AM   #5
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Battery is only providing potential, not current-it might outlast the valves. Output impedance is not much over 100 ohms.

Sorry Kevin, you answered while I was considering my reply
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Old Yesterday, 07:21 AM   #6
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Hmm. If those batteries run down a little faster than usual or fall out of their holders then it will not be doing V2 any good.

Also, why the need for R5? It seems you are just increasing noise, the output impedance from the plate of V1A will be a steady 11-12K once the circuit is powered up, why not use that as the first resistor in your EQ network? You'll be able to remove a component and improve your noise performance.
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Old Yesterday, 10:38 AM   #7
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Well, what happens as the valves age?- the assumption of infinite output in a cascode is only an approximation. Losing the series resistor to decrease noise also means a steep AC loadline for the input stage.
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Old Yesterday, 01:49 PM   #8
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Q1 is a constant current source in addition to its function as a follower, with adequate heatsinking the fet will survive a bias failure- the tube will not see a significant increase in current, and dissipation will decrease with the reduction in plate voltage as the tube saturates..

Note in 10 yrs I have yet to have a bias battery failure.

R5 is chosen as a reasonable compromise between minimizing sensitivity to parametric variations in the first stage, noise, and cap size/quality considerations. Available current and distortion are a consideration too. The overall SNR is dominated by Johnson noise from cartridge and transformer resistance.
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Old Yesterday, 10:43 PM   #9
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Hi,

Quote:
The output impedance is actually pretty low - in the vicinity of 150 ohms including the build out resistor, it will drive quite capacitive cables without any difficulty.
That's OK if you need the phono pre to drive an amp with a vol ctrl sitting close to the speakers but if you use an active line preamp it's less of a problem, right?

Ah, I'm already dreaming up some circuits using high transconductance valves in parallel to get rid of the step up xfrormer and so on.
Maybe a maxi muscovite, he....

Cheers,
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Old Yesterday, 10:57 PM   #10
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I'm sure you'll have lots of fun getting it hum-free Frank!
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