Valve Itch phono

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If not common mode it will buzz..

I hope the SUT will appear decently balanced then. Will try that first chance. Is it explainable it loses gain from expected x10 (20dB) in this configuration? I.e. floating cart and primary, grounded secondary feeding single ended phono. Lost 6 dB within experimental error figure. The number is fishy there is a plausible electrical explanation.
 
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I hope the SUT will appear decently balanced then. Will try that first chance. Is it explainable it loses gain from expected x10 (20dB) in this configuration? I.e. floating cart and primary, grounded secondary feeding single ended phono. Lost 6 dB within experimental error figure. The number is fishy there is a plausible electrical explanation.

Hi Salas,
Yes something is very fishy, grounded primary or not should make no difference, just be sure that the cartridge returns (-) are not common somewhere, but again this would just tend to unbalance things from the standpoint of CMRR and should not result in any change in gain.

I would be very suspicious of any SU transformer whose maker insists must have a ground on the primary side as it is almost impossible to guarantee sufficient symmetry for balanced operation without substantially degrading CMRR. A transformer that was not specifically designed for balanced operation might have substantially more capacitance from winding to core at one end than the other, but even this should not be a significant issue at 50Hz.

A pair of resistors creating a pseudo center tap matched to 0.1% will in an otherwise perfect set up degrade your CMRR to just 60dB, a correctly designed transformer in a "perfectly" balanced set up can do much better than this at 50Hz. (Maybe as much as 30 - 50dB better..)

A lack of capacitive symmetry between the phases may start to degrade CMRR above few kHz or so, but in this application that is not really so important as long as HF response isn't impacted.

Regardless of connection scheme as long as the same taps are used gain should not change.

Are you certain that the cold ends of both windings are completely isolated from each other?
 
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You will not find more info than from those two builds that my friend Michael made and I presented here. Nobody else made another one till now as far as I know. About the losing some SUT ratio VS nominal, we saw (thanking jackinnj for feedback on his HMN experience and measurements) that a much less secondary resistance Sowter loses some dB (half than mine) but still does, so it must be impedance matching and reflection to secondary/division thing. Wired the other way around it just hummed, so it wasn't wired sub optimal.
 
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I meant that it wasn't wired in wrong polarity or something. If it was wrong polarity and the primary was earthed then hum was apparent. It also plays best and louder when seeing 47k secondary load no matter the particular cart can work down to 120 Ohm reflected load. The specific SUT feels better with 47k on its secondary. It has 1.7k secondary resistance and I haven't looked in its pulse behaviour with RC compensation.
 
Went to Mikvous's place today to examine another Itch he made. With my Rev1 changes included.

Salas

Is this the latest version or are there any other mods to this you would care to share? Being a relative novice compared to the wise and esteemed members of this forum I have lots of questions most of which I can solve with a bit of research others I may have to ask if anyone can stomach answering them.
 
Went to Mikvous's place today to examine another Itch he made. With my Rev1 changes included.

Salas
A couple of probably dumb questions from an inexperienced enthusiast. This additional circuitry just replaces the Maida 300v, nothing else changes?

Secondly can you tell me what PP is on PSU circuitry? I am sure it is incredibly obvious but has got me stumped.
 
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PP is polypropylene. In place of the Maida, an HV Mosfet shunt reg was used in Rev1. Also the 470k-220k divider has changed to two equal resistors (whatever handy above 200K). Its needed to lift higher because in Rev1 the upper tier tubes cathodes sit even higher for potential from their heaters. What kind of cartridge are you planning to use?
 
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Hello

The 'cooking' of an LM317 based reg has some peculiarities and some preferences. That one in the schematic was for a split. I.e. the two heaters regs remained in the PSU box and a final big electrolytic cap was decoupling the heaters in the phono box. It is depicted on the schematic. So the real termination were not just the polypropylenes but a combination of the poly, the 1m connecting wire and the final electrolytic. Yes those Cadj capacitors were not recommended as special low ESR capacitors but as plain spec. Say a Nippon SME or a BC Component standard quality will do fine. If you will have space to host all the regs and sinks in the phono box and can have the transformers, bridges and main filter caps in the PSU box, that will be the best, as it was done in the second build. Normally you will use a roomier box than that wooden one of the first build example. In this case you must have 0.1uF capacitors for each LM317 from input to ground so to decouple the incoming long connecting wire, and only use 100uF plain quality for Cadj and Cout. No poly caps, no final big electrolytic.
 
Many thanks for that. I like the two box approach and am planning on building the boxes once I have done a rough build of the system first so I can see how much space I want. I quite fancy the look of wood on the box sides and ends and have some nice oak that would look great but am thinking about lining the boxes with something to provide electrostatic shielding. Whats the best way to create the umbilical chord between psu and phono. Looks like you used a cable gland at one end rather than two plugs. To get the right type of cable between the boxes is it necessary to make your own using good wires, sleeving and say a 5 pin plug? Thanks for your help. I am enjoying this thoroughly, it's keeping me sane!
 
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If you will consider electrostatic shielding its best you will keep the PSU and the phono circuit including the valves inside grounded ferromagnetic metal enclosures. You can attach wooden face plates or side panels of choice by all means. If fully wooden, you can line with copper sheets. In those two builds by Michael (mikvous) the input valves although exposed did not prove susceptible to hum and buzz even without metal caps as it happened with the higher internal impedance Steve Bench cascode circuit for instance. Provided that the main unit is not in proximity to a transformer AC field, its not especially troublesome in the Itch design. But it can always measure better harmonic noise, so the former mentioned approach is the optimal one, although you can go by nicely with exposed valves as in the examples. As for the umbilical, no special thought went into it. It should be rated for handling 300V DC and be flexible enough. The cable gland just saved even more wiring breaks and attachments for yet another multi-pin connector PSU side, being captive to one box did not bother the builder, but following the usual approach of having connectors on both ends won't be noticeably detrimental either I suppose. You could use one separate heavier gauge 4 wire chord for the rectified DC PSUs of the filaments, and one HV umbilical if you fancy. Detached routes and will look more business like even.
 
Thanks for that. Copper sheeting sounds good. I used to have tons of copper sheet obtained from a friend for doing some welding for him. It was for making circuit boards but had a fault. I used it to build a roof on my garage, it looked great and lasted really well - at the time copper was not that expensive. Unfortunately I no longer have the garage or the sheeting. I guess I will have to buy some more...