Valve Phono Preamp [message #68420] |
Fri, 01 July 2011 21:58 |
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gofar99
Messages: 1951 Registered: May 2010 Location: Southern Arizona
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Illuminati (5th Degree) |
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Hi vinyl listeners. As promised I have the phono preamp at a stage where I feel comfortable in posting it. It has gone through many iterations and now sounds like I think it should. It is not a simple project but neither is it super complex. The write up and photos are fairly extensive and I am not sure how to get it out to you all. It is 1.8meg and should fit here. I will give it a try. It says it went. Comments are always welcome and I owe an Oddwatt "T" shirt to Shane for coming up with the name. If he will PM me his address I'll be happy to send it. I hope large is OK.
Good Listening
Bruce
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Re: Valve Phono Preamp [message #68441 is a reply to message #68420] |
Sat, 02 July 2011 15:49 |
FL152
Messages: 29 Registered: April 2011
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Chancellor |
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Hi, Bruce - I saw your preamp, and I must say that it is in some ways similar to mine. Interesting, it is also similar to RIAA preamp from Vanderveen`s book I wrote about. He used 12AU7/12AT7 tubes, both in SRPP with passive RC filter between stages. He wanted low Cin, and used low-mu first tube...but resultant amplification is also lower. He used anode out (not low-imp. out) in the first stage, similar to my approach.
Back to your preamp - you stated amplification like „60 dBv", and this is probably a mistake. We can expect maybe A~50 from 12AX7 SRPP stage, and about 10x attenuation after 1st stage through RC filter on 1 kHz. Then, total amplification can be about A ~ 5 x 50 = 250 times, or 48 dB maximum on 1 kHz. Still respectable.
What seems strange to me is a large value of series RIAA resistor, 113 k. Theoretically, relation between two RIAA resistors is 6.877, and that means value for 113k resistor about 75k, rough calculation without series Rout from SRPP and parallel 1 Meg resistor (about cancelled each other). Is 113 k value empirically found?
About noise - it is hard to do better without high-S tubes...an economical alternative can be EF184, but then we need four tubes/sockets and more current, plus two heater supplies, probably.
I like your shielding ideas.
P.S. speaking of „tube books" did you see this „ultimative" book about RIAA preamps and noise? Based on free sample pages, serious book but, the price is more then serious, too.
http://www.amazon.com/Sound-Silence-Lowest-Noise-Phono-Amps-Designers/dp/3540768831/ref=sr_1_14?ie=UTF8&qid=1309638744&sr=8-14
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Re: Valve Phono Preamp [message #68444 is a reply to message #68442] |
Sat, 02 July 2011 18:31 |
FL152
Messages: 29 Registered: April 2011
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Chancellor |
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Hm, I`d try half of that 113k value, say standard 56k. As you said, suprisingly, there are not too much RIAA deviation with some resistor values changes (tenths of dB, or 1 dB "over" 20 Hz in your case), but those deviations are in "wide" frequency spectrum and can be heard.
Quick calculation gives Rout~28k from SRPP (must be measured or at least calculated and/or simulated), and when we use about 56k for series resistor in parallel with 1 Meg, this gives:
R = (28+56)x1000 / (28+56+1000) = 77,5 k , about right.
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Re: Valve Phono Preamp [message #69237 is a reply to message #68581] |
Mon, 29 August 2011 17:31 |
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gofar99
Messages: 1951 Registered: May 2010 Location: Southern Arizona
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Illuminati (5th Degree) |
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Hi Everyone, Just a pair of photos of the latest incarnation of the preamp. Tube brands and types have mattered. The quietest ones so far are JJ gold pin ECC83S with the ECC803S close behind. Others (Tunsol, Mullard new production and EH) were not as good.
Good Listening
Bruce
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