NewYorkDave
Well-known member
RCA published a nice little RIAA preamp schematic in the back of the later editions of their receiving tube manual. Add a cathode follower and go 
Schematic
Description
Schematic
Description
tube noise?
- Thread starter jhaible
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[quote author="NewYorkDave"]RCA published a nice little RIAA preamp schematic in the back of the later editions of their receiving tube manual. Add a cathode follower and go
Schematic
Description[/quote]
Interesting circuit.
I wonder what C3 is for. It's not part of the RIAA EQ courve, is it?
(I already see two time constants after the 1st stage.)
JH.
Schematic
Description[/quote]
Interesting circuit.
I wonder what C3 is for. It's not part of the RIAA EQ courve, is it?
(I already see two time constants after the 1st stage.)
JH.
pstamler
Well-known member
[quote author="PRR"]> If I'm not mistaken, the signal level at high frequencies is *lower* after the first stage + passive filter than on the input!
Probably not a real problem. The output from a dynamic phono needle is VERY hot in the highs. You are really more likely to run into overload than tube noise, even if the first stage and EQ shows a "loss" and the electronic noise level is set in the second stage.
People whine about "low noise" phono stages, but it is mostly bogus. You can use quite rude amplifiers and still recover the surface noise of the vinyl. Not a lot of point in reducing tube noise lower than that. Not a lot of fun listening to a needle or dummy-source when it isn't in a groove.[/quote]
You're not really likely to run into overload on that first stage, or the second stage either. The worst peak output level I was able to measure on a record was the equivalent of ~98cm/sec, on a jumping scratch which was on a 78 by Pope's Arkansas Mountaineers. On a typical magnetic catridge, with 5mV output at 5cm/sec (standard level), that's 98mV. If your input stage gain is 30dB, that's ~3.1V peak output, which a respectable tube circuit should handle without breaking a sweat. If you then insert a passive RIAA or similar EQ network, the same superpeak measures 26dB over nominal level, so if you assume that the preamp should boost nominal level to -10dBV, the standard consumer level (-7.8dBu), or +18.2dBu, which is about 6.3Vrms, presumably 8.9V peak. No sweat there either for a well-designed tube circuit.
As for noise, as long as you're not trying to amplify a low-output moving-coil cartridge, a tube like the 6SL7, 12AX7, 5751 or the like can do an adequately quiet job. (Cough.) I wrote an article for audioXpress last year outlining noise and overload considerations for phono preamps, and the upshot of it is that a good tube circuit will do fine, way below the noise of any record I've ever owned.
Peace,
Paul
Probably not a real problem. The output from a dynamic phono needle is VERY hot in the highs. You are really more likely to run into overload than tube noise, even if the first stage and EQ shows a "loss" and the electronic noise level is set in the second stage.
People whine about "low noise" phono stages, but it is mostly bogus. You can use quite rude amplifiers and still recover the surface noise of the vinyl. Not a lot of point in reducing tube noise lower than that. Not a lot of fun listening to a needle or dummy-source when it isn't in a groove.[/quote]
You're not really likely to run into overload on that first stage, or the second stage either. The worst peak output level I was able to measure on a record was the equivalent of ~98cm/sec, on a jumping scratch which was on a 78 by Pope's Arkansas Mountaineers. On a typical magnetic catridge, with 5mV output at 5cm/sec (standard level), that's 98mV. If your input stage gain is 30dB, that's ~3.1V peak output, which a respectable tube circuit should handle without breaking a sweat. If you then insert a passive RIAA or similar EQ network, the same superpeak measures 26dB over nominal level, so if you assume that the preamp should boost nominal level to -10dBV, the standard consumer level (-7.8dBu), or +18.2dBu, which is about 6.3Vrms, presumably 8.9V peak. No sweat there either for a well-designed tube circuit.
As for noise, as long as you're not trying to amplify a low-output moving-coil cartridge, a tube like the 6SL7, 12AX7, 5751 or the like can do an adequately quiet job. (Cough.) I wrote an article for audioXpress last year outlining noise and overload considerations for phono preamps, and the upshot of it is that a good tube circuit will do fine, way below the noise of any record I've ever owned.
Peace,
Paul
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