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Salas 17th March 2009 08:09 PM

Valve Itch phono
After my NJFET phono, the shunt Mosfet LV and HV psus, I felt too deeply buried in sand. Got an itch. The valve itch. :clown: Had to mess with glass again.
Having the Simplistic NJFET and the Steve Bench 12AY7 & 6DJ8 cascode already, both on Mosfet shunts of mine, I knew it would be a tough call. Comparisons would come inevitably. So I thought to compare along testing and tweaking phases so to make something that has some sonic merit to stand there. To have something compelling to tell in its own tone way.
The design had to use preferably cheap and available but very silent tubes. Had 6N2PEV Russian tubes that I wanted to test. I liked their characteristics. Also 6N8 NOS Soviet black anode 6SN7 equivalents. I want the most gain there is available on first stage, I don't want extra resistors there, want good PSRR, and lower gain at second stage. Also low impedance Riaa I prefer, so some good drive is needed. Two mu follower stages with passive Riaa between, seemed appealing for my concepts. I have seen the Phono Dude doing that too, have never had one, but it was good to know it already works as phono with some acclaim. Put it together on the simulator, tweaked this and that, seemed nice. I decided not to look for how low THD, as long as it would be predominantly second. Wanted some triode like tone this time since the NJFET and Bench do the monitor style already.
Good DIYer mate Mikvous liked the idea and decided to make one in some spare boxes that used to house a Thorsten phono, so we can test and tweak. It took us about 2 weeks so to decide its what we wanted.
The noise floor is amazingly low for 61dB gain of step up and valve phono. Pitch black background. The THD is 0.2% 2nd, and the Riaa curve is identical to the NJFET Riaa. No metal shield caps on the input valves for such a low noise floor, and we can happily knock em on their glass while the volume is set loud and hear nothing. The sound is great. Lush but has it all. Best valve phono we ever had bar none. And we had tested about 7 DIY ones.:cool:

Salas 17th March 2009 08:11 PM

2 Attachment(s)
The ''Valve Itch'' circuit.

Edit: Later updated schematic Rev1.1 Also see Rev1.2

Salas 17th March 2009 08:17 PM

2 Attachment(s)
The PSU circuit.

This one is regulated with a Mosfet Maida reg.

A Mosfet HV shunt can be used, but we did the quicker, cheaper Maida for testing, since mu follower has nice PSRR too. Normally there is more to listen with the HV shunt. Delete the 0.1uF parallel to the B+ connector if with the SSHV shunt reg.

Salas 17th March 2009 08:40 PM

2 Attachment(s)
The Riaa components values have been tested both by measurement and subjective tone. Silver Mica bypassed PP is used for the 100nF, K72 33nF Teflon + 2200pF Siver Mica for the 35.2nF. Resistors are a mix of Kiwame and cheap carbon 2W. Buss bar grounding has been used, silver content solder, and Mogami thin shielded coax for signal. Lundahl 9206 step up is included in Riaa box, set at 10X. If someone wants to use it for high MC or MM, the gain without the step up is adequate, but has to block the bare input with some 2.2uF capacitor since there always be some mV offset due to grid leak that may fry some cartridge.

Salas 18th March 2009 12:06 AM

Some details.

To check for channel balance, the voltage drop must be measured on one of the series resistors of the first mu stage. Either on the 560R or the 47k. If it is the same, then the channels have the same gain. This is important so to check for sections gain of 6N2PEV used. They seem like following well though. For example, by randomly putting one couple, I was able to see if the channel RIAA curves met on the FFT screen for level, when the generator was set for pink noise and 1/12 octave scaling. By just swapping those valves between sockets, the two curves matched, as well as the voltage drops on the respective resistors. I did not need to go through a batch. Maybe a bit lucky, but non the less its a hint to good manufacturing tolerances. Pin 9 is screen between sections on 6N2PEV and must be grounded near on the ground copper buss bar. The gain of first section is about 85X and this gives 200pF input capacitance. No weird interactions with the Amorphous Lundahl 9206's and it looks it can still do OK with many MM, and Grado does not care. Someone must measure the input for mV DC before attempting to use the Valve Itch without a transformer as MM or High MC stage (44dB gain) because there can be grid current leak, although the 6N2PEV is made to produce low grid current. A coupling capacitor is safety in any case. Of course its best use is with MC due to the really low noise floor and beguiling sound quality. The step up won't pass DC. With the standard 61dB measured gain (Lundahl set at 10X) even a Denon 103 can be used if there is normal gain in the rest of the system. Still there is 5X and 20X option in the Lundahl 9206. I measured 30% less than claimed step up at 1X10. I would fearlessly try even the most low output MC cartridge with this phono, since it is the quietest valve phono I have ever had. Don't forget, the cart load is the phono input resistor, divided by step up ratio squared. This phono needs about a week of enough everyday play so to break in, as it proved in practice.

Grid current alternative solution to AC coupled input

RCruz 18th March 2009 05:54 PM

Hi Salas

I noticed the Riaa values are quite different from the jfet version.


Must sound beautifully.

Salas 18th March 2009 06:18 PM

Did that side by side with the other phonos. The interesting thing is that it has the same balance with the NJFET. You don't listen to some fictional thermionic rendering of the recording. Also has the noise floor of well executed solid state. So there is a fair platform to compare. I will run a session of exchanging the Itch, the Simplistic, and the Bench. I am waiting for it to fully break in. Mikvous got perplexed when initially exchanging with the Simplistic. He told me, ''maybe now I hear what the different devices do, not the designs''. I don't think it has the depth resolution of the Simplistic, but has different breath. Its much less linear (0.2% THD mainly 2nd, ouch!), but benign. On the other hand, it can swing much more dynamically of course. Careful designs of different nature must not sound strongly wrong or correct in my view, but in the long run must provide a different aftertaste for suitability choice to different TTs, speakers etc.
The Riaa values are different to the Simplistic due to different source impedance. Their curves just overlap on FFT. I started it with the trendy 50kHz fix included, but the classic Riaa just sounded better, as it happened some other times before. Will tell more about the ''Valve Itch'' after a while. For now, I am sure its a worthy vacuum state device.

RCruz 24th May 2010 05:02 PM

I Salas

Is there a thread on the bench ?

Allen Wright 24th May 2010 05:21 PM

A zero biased 6N2 has only 1.4mA current?

And if the grid current under zero bias condidtions doesn't affect the sound of the input traffo, I doubt very much if it will hurt a MM cart. But an input cap will certainly hurt the sound.

Regards, Allen

Salas 25th May 2010 02:10 AM

Hi Allen,

Those 1.4 & 4mA on the schematic are actually measured as far as I recall but could have made a typo as well.

I would just measure its safe if I had an MM to hook up and exchange some tubes if leaky, so to avoid the input cap. But you know how it goes with safety recommendations when publishing ccts. Tubes age etc.


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