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Old 14th April 2014, 02:07 AM   #1
kevinkr is offline kevinkr  United States
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Default The Muscovite Mini II 6N14P Phono Stage

As promised I have created another thread to discuss my latest foray into the land of phono stage design. I will admit to taking a short cut and rebuilding the Mini into the Mini II.
The major tradeoff is a reduction of gain on the order of 6dB perhaps more, the major benefit is the elimination of a critical capacitor in the original mini design.

I confess that I have not yet traced the 6N14Ps in my possession and in fact designed the new front end referring to the various 6N14P parametric charts found on the net. I then tweaked things on the bench, but it is a relatively simple front end design.

Note that the ECC84, PCC84 and 6CW7 should all work in this front end as well. No current substitute for the 6S3P however.

I have just now simulated the circuit performance in spice using a commonly available model for the ECC84 which on the gain front might be just a bit optimistic.

The predicted gain is 48dB which I expect is not realized in practice (I need to measure it).

RIAA conformance in this particular pre measured as +0.2dB, -0.0dB.

Distortion as in the Mini is primarily determined by the second stage and due the reduction in overall front end gain is better than -70dBr @ 5mVrms input @ 1kHz.

The cascode is bias at ~10mA, with fixed bias derived from an IR led having a VF of 1.2V and dynamic impedance of a couple of ohms. The upper cascode is referenced to a resistive voltage divider, one per channel and is ~87V.

I have a lot of catching up, I will post traces and models to the spice model thread when I have them. Another member has mailed me a number of ECC84 and PCC84 which I will measure and compare to the 6N14P I am using.

Noise performance turned out to not be a concern despite my previous worries about this issue.

More measurements to follow as I have time.

I'm listening to it as I write this post.
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Old 14th April 2014, 02:16 AM   #2
kevinkr is offline kevinkr  United States
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A few details about the 6N14P; I am particularly concerned about the placement of the grid pin (6) of triode 2 next to the filament pin (5) as this the point with lowest signal level in the entire tube. Probably not a bad idea to choose a low leakage socket material like ceramic or teflon for this application..

Special care is required for the filament supply if using this tube - I recommend extremely well filtered and regulated DC to heat the filaments for this reason.

Tie both cathode pins (7,8) together on triode 2 and connect directly to the IR led..

The tube does not seem particularly prone to oscillation if some minimal precautions are observed as noted in the schematic.
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Old 14th April 2014, 10:40 AM   #3
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Kevin, why do you want to use the valve this way round with the shield at an elevated potential? Is that normal? (I suspect this is a stupendously stupid question, but better to ask it!)
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Old 14th April 2014, 03:15 PM   #4
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Hi,

@Piano3: The valve is supposed to be used like that for cascode service.

Ciao,
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Old 14th April 2014, 03:42 PM   #5
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Thanks Frank; what stops the shield then interfering with the operation of the lower triode?
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Old 14th April 2014, 04:04 PM   #6
kevinkr is offline kevinkr  United States
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Hi Piano3,
This is actually a great question... Note that the grid and shield are both very close to AC ground in my implementation of the cascode configuration, and from a DC perspective the shield is at close to the same potential as the plate in the lower half of the cascode, and roughly half the plate potential of the upper half of the cascode - from a DC leakage standpoint this is as close to ideal as it gets. The shield actually prevents the plates of the two sections from coupling capacitively which is not a big deal for an audio cascode, but is a problem with RF ones, in any event it cannot hurt.

This tube of course was not designed for audio applications, and the location of the most sensitive node - the grid at pin 6 is far from optimum as coupling from filament pin 5 can occur both by leakage across the socket and through capacitance.

In the distortion and noise FFT I did yesterday I observed a considerable peak at 120Hz, and while I do not know for sure I strongly suspect the filament supply is the culprit. I purchased a regulator off of eBay using a fairly high performance LT regulator (5 pin device I currently can't remember) and have measured the ripple at the output > 6mV which is fairly poor performance. (The seller of course claims ridiculously low output noise, which in practice is not realized) I suspect the PCB layout is to blame (dremel time?) and I probably need to build something from scratch to do the job - if that proves to be the case I will post it here.
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Old 14th April 2014, 04:20 PM   #7
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Thanks Kevin, very clear explanation.
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Old 14th April 2014, 04:46 PM   #8
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Have you ever considered using what everyone is calling a capacitance multiplier for the heater supply? Digikey stocks a very high hfe transistor, 2SC4495, (50V, 3A) which would make a excellent emitter follower without having to use a Darlington. You could even supply the Zener from a CCS. I would think that noise could be made arbitrarily low.
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Old 14th April 2014, 11:31 PM   #9
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Default Of Cap Multipliers and Noisy Computers

Funny you ask about the capacitance multiplier - I'm actually not a big fan of the approach, spent a good deal of my professional life working on, fixing and designing VIs for semiconductor ATE so I like an active solution, cap multipliers seem like a brute force solution to me.

Now I have had yet another epiphany relating to my computer based measurement hardware, and if I said a dead computer is a good computer when it comes to coexisting with analog hardware I would not be understating the situation. Even turned off the evil thing is radiating across the RF spectrum so I finally just unplugged it.

It seems I need to investigate getting a lower EMI computer as it is really messing up everything I try to do whether it is in use or not.

So there was no issue with 120Hz ripple on the filament supply, it is indeed down in the uV region as claimed, and the high voltage regulators broadband are under 400uVpp as well. (Ripple is much lower)
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Old 14th April 2014, 11:36 PM   #10
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Default Some statistics

Ok given that I have given up on the FFT I did a few measurements the old fashioned way..

I measured a gain of +46dB relative to the input @ 1kHz which as I surmised proved to be a couple of dB lower than the prediction.

The gain of the cascode was measured as 63.4 or almost exactly 36dB (that should sound like a familiar number )

Channel to channel match at 1kHz was about as close to perfect as I have ever seen at better than 0.05dB in fact.

Overall gain compared to the pentode version is approximately 6dB lower, FFT based second harmonic measured is dominant but is better than -70dBr relative to the fundamental @ 1Vrms out. Third harmonic is down in the noise.
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