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In Reply to: Adding Headphone capability to a C-J PV-12A? posted by tvr2500m on March 11, 2014 at 15:45:19:
BUT...a cathode follower reduces such output impedance to 200 - 300 Ohms*. (This compares with a SS amp's [NOT preamp's] output impedance of a tenth or hundredth of an Ohm.) My understanding of such a mismatch, recognizing that the nominal goal is to have the load's input impedance be at least 10 times the source's output impedance, is that at some relatively low level, the circuit's operating Voltage will sag dynamically--that is, with music pulses--and the system will NOT sound very good. So while the linestage will drive your 300-Ohm headphones to some low level, it won't sound very good and will clip quickly.I think you need a headphone amp.
I vaguely recall that some headphones have MUCH-higher impedances. If that's correct, your preamp probably will drive one of those more easily.
* If someone knows better than that, PLS correct me--I'm one of those guys with no technical training and with that little bit of knowledge that makes me dangerous. :-)
Off-topic for-whatever-they're-worths:
1. All coupling caps, a 0.15uF and 2uF in the fono stage and a 2uF in the linestage, are polystyrenes. That's very good.
2. Powersupply-regulator outputs are 'propylenes bypassed with 'styrenes.
3. The balance control uses series-connected resistors, surely a better-sounding solution than another pot.
4. The fono stage uses paralleled sections of the first 12AX7 for lower noise and lower output impedance (and no cathode follower, as the, for instance, PV5 did) into the 2nd stage.
5. The line stage has only one gainstage, so the preamp inverts signal polarity at all outputs but the Record Out (if there is one).
6. A relatively inexpensive sonic improvement could be gotten from bypassing the linestage output-coupling cap, a 2uF 'styrene, with something higher in quality such as a 0.1-to-maybe-0.47uF SoniCap Platinum (Teflon-alloy) cap. If you like the increase in clarity (AFTER breaking-in the Platinum for at least 400 - 500 hours) and you use the fono section much, do the same bypass to the fono section's 2uF output-coupling caps.
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Follow Ups
- The PV12 linestage uses a 12AU7 cathode follower to reduce its output impedance. - jeffreybehr 03/12/1411:08:22 03/12/14 (3)
- RE: The PV12 linestage uses a 12AU7 cathode follower to reduce its output impedance. - tvr2500m 11:51:21 03/12/14 (2)
- Have never had a PV12. Had 3 PV5s and rather-substantially improved one. - jeffreybehr 15:19:58 03/12/14 (1)
- RE: Have never had a PV12. Had 3 PV5s and rather-substantially improved one. - tvr2500m 17:16:15 03/12/14 (0)
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