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Radford SC22 Preamp PSU

Discussion in 'd.i.y.' started by JemHayward, Apr 3, 2019.

  1. JemHayward

    JemHayward pfm Member

    A bit of a long story... but...

    I've moved my turntable into the barn, where my Quad 57s live. The Quads are part of my TV system, but I want to use them with my vinyl system as well. I bought a used Cyrus AV5 preamp which is very good, but for some reason lacks gain, but is usable for TV via the optical input. It has three analogue inputs too, but they seem to have stopped working, as has one of the optical digital inputs. Otherwise it's fine, but I can't now route the turntable through it, though I could last week! The issue then was lack of gain, but now the issue is no gain at all.
    I have a Naim based phono stage, which works well, but doesn't have a lot of gain either. I have a Bruno Putzeys balanced gain stage, but when connect it between the phono stage and the gainclones currently driving the Quads, it sounds distorted at higher volumes.
    The long term plan is to use my Radford STA25 to drive the Quads, but at the moment its driving the Martin Logans in the digital system in the house, and will stay there until I get my VFET amplifiers from John Westlake (that may be some time...) but I have the matching SC22 preamp, which I hadn't intended to use, even long term, but I'm thinking it may solve my gain issues in the barn, but I then need a power supply for it. I'm assuming I'll need about 300v DC at a few mA and a 6.3v AC heater supply.
    I'm guessing I need a relatively small transformer with 220v and 6.3v windings, a bridge, a capacitor and the inevitable case and wires. Has anyone done this and am I thinking along the right lines?
  2. Marchbanks

    Marchbanks Golly, do I ever have a lot of soul!

    There is an SC22 with external PSU currently on eBay. The listing states the PSU was made by Croft Acoustics. Might it be worth asking Glenn to make you one?

  3. JemHayward

    JemHayward pfm Member

    Just spotted that and have just emailed glenn@croftacoustics.co.uk...

    may be the easy option and may be better then powering it from the STA25 in the long term, though in the long term may want a better preamp, as these older valve preamps were never that great...
  4. Marchbanks

    Marchbanks Golly, do I ever have a lot of soul!

    I really like the SC22 with my STA15, but don’t use it much nowadays as it has too much gain for my Tannoys and I end up with the volume control around 1, where the stereo tracking is useless. Great with BC1s though.
  5. JemHayward

    JemHayward pfm Member

    My current issue is lack of gain! How 'transparent' is the SC22 - I've not yet used mine. I've had Leak and Quad valve preamps in the past and have found them, err, interesting. I only bought the SC22 as I didn't feel that the Radford kit I was buying should be split up, and was never intending to use it originally.
  6. Marchbanks

    Marchbanks Golly, do I ever have a lot of soul!

    I can’t really comment on transparency - I don’t think my ears are good enough to deal in absolutes, if you see what I mean. But my experience is detailed in the first post here


    ...and I went on to buy Tannoys, although the SC22 had to be sidelined because of the gain problems mentioned previously (although I now have a passive pre that I might daisychain in front of it for a lark) and was replaced by a 25R.

    I agree with you, the Radfords should never be split up!
  7. ian r

    ian r 401's Nakman

    i gave my sc22 away to audioflyer 2014 as he had the same interest as you but i felt spending the time and money wouldnt get me the quality i sought for my williamson monobloks ( World design version ).

    check in on Sharif (audioflyer ) as he may have sorted the psu by now.
  8. Marchbanks

    Marchbanks Golly, do I ever have a lot of soul!

    I found these diagrams amongst my Radford documentation - they might help.

    [​IMG] [​IMG]

    In case it isn't clear the first is the PSU built into the SC22P by Radford, the second shows the way the standard SC22 is powered via the umbilical.

    The docs also say the required HT is 'nominally 300V 9mA DC' (although the schematic shows the HT rail at 350V) and 'a heater supply of 6.3V 1.5A.'
  9. Chops54

    Chops54 pfm Member

    Is there a choke and another cap hiding somewhere? can someone explain how the hum balance works. I've had this on a few bits of old kit and it never seems to do much.

    If you're building your own psu I'd aim for 15mA at 300 volts to give the preamp some headroom.
  10. JemHayward

    JemHayward pfm Member

    Looks like I'm thinking along the right lines. I can't find an off the shelf transformer giving 200v and 6.3v nowadays but I assume that two separate transformers would be fine. The 6.3 V would need to be about 10W and the 200v about 50W if my guestimaths is correct. A bridge and a 32uF 350v cap and I'm up and running?
  11. Marchbanks

    Marchbanks Golly, do I ever have a lot of soul!

    I think you are 10x too high with the HT VA rating.
  12. JemHayward

    JemHayward pfm Member

  13. JemHayward

    JemHayward pfm Member

  14. Marchbanks

    Marchbanks Golly, do I ever have a lot of soul!

    That looks good. If you want to stick to the original cap value there is this -


    Looking at the SC22 schematic I notice that decoupling cap C23 has a value of 64uf on the SC22 but 32uf on the SC22P, which I don’t claim to understand.

    I’d also think about adding a second switch in the HT line.
  15. Marchbanks

    Marchbanks Golly, do I ever have a lot of soul!

    Not according to the schematic, no. But if the pre is powered from an STA15/25 the HT is smoothed by CLC before passing down the umbilical.

    I think the hum balance pot is supposed to enable equal but opposite polarity AC on either filament pin.
  16. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    I build such things to order, at any voltage, for any amplifier, regulated or unregulated and with DC or AC heater supplies again regulated or unregulated.
  17. JemHayward

    JemHayward pfm Member

    What sort of cost would something like this be? I'm capable of DIY, but have a lot of other things to do at the moment!
  18. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    If you PM me with your specific requirements I'll get a price for you. Obviously as it will be designed to suit and built as a hand made one off don't expect ebay/"Maplin" prices!
  19. JemHayward

    JemHayward pfm Member

    Just won a suitable transformer on eBay so I'll give the diy a go first...
  20. Barrymagrec

    Barrymagrec pfm Member

    Bear in mind that the caps need to be rated at about 1.7 - 1.8 times the transformer voltage to allow for transformer regulation and mains variation particularly since their will be no loading until the valves warm up and only a few milliamps when they do. A 350 volt cap is not enough on a 230 volt nominal transformer.

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