I replaced the selector switch on a PAS 3 using the Curcio (like) switch and rewired it to the RCA jacks. The instructions are silent about the 27,000 olm resistors and the 750 PF capacitor. So, if you replace the selector switch where would you re-place the 2 27K resistors and 2 750 PF capacitors?
3 posters
Replacing Selector Switch on PAS 3
jerdobi- Posts : 6
Join date : 2010-05-30
Bob Latino- Admin
- Posts : 3276
Join date : 2008-11-26
Location : Massachusetts
jerdobi wrote:I replaced the selector switch on a PAS 3 using the Curcio (like) switch and rewired it to the RCA jacks. The instructions are silent about the 27,000 olm resistors and the 750 PF capacitor. So, if you replace the selector switch where would you re-place the 2 27K resistors and 2 750 PF capacitors?
Hi,
This is a question that you should put to Mr. Curcio in an Email directly to him.
Bob
GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
Using Curcio's recommended rework of the RIAA equalization in the PAS, those capacitors would not be used at all. They were there to provide for switchable EQ, usable with NAB tape head and with a microphone. Curcio recommends eliminating them (and thus eliminating switch contacts in the feedback line) and putting all the EQ frequency-shaping capacitors on the circuit board.
Curcio's recommendations are on his forum, at:
http://db.audioasylum.com/cgi/m.mpl?forum=dynaco&n=7492&highlight=curcio+riaa&r=
There is one minor error that affects his description, not the circuit: Where he writes "DO NOT remove the 47K feedback resistors between the cathodes of each tube", those are feedFORWARD resistors, not feedback.
Another option would be to rebuild the phono stage as per the Van Alstine Super PAS, a more involved but still relatively simple task (as DIY audio goes) which I think has a better result; however Van Alstine has removed the schematic and instructions from his Audio Basics page, so it can't be accessed any more. Someone may have a copy of it that can be provided privately (since it's copyright material it can't be posted publicly). I am very happy with my homebrewed Van Alstine Super PAS Three.
And remember...both the Curcio and Van Alstine phono circuits REQUIRE a load of no less than 100K at the phono stage output. This applies to the linestage of the PAS, no problem there; and to the TAPE OUT jack of the PAS. If this caveat is not observed you will get bass roll-off. This is true of the stock PAS phono, too. This eliminates most solid-stage tape and cassette recorders, wireless headphone transmitters, whole-house distribution amplifiers, line inputs to computers for digital recording...in fact most solid-state equipment of any kind, unless a buffer stage is used. This problem was solved in the Panor "Dynaco" preamps by the addition of a cathode follower stage, though all-tube purists would consider the additional stage in line with all signals to be less desirable than an output buffer which would not be in line to the power amps. The only thing I have connected to the TAPE OUT jack of my PAS is an Ampex F-44 tape recorder, a tube unit with a 500K input impedance. Putting a typical cassette deck on there DOES result in a huge loss of bass in phono.
Curcio's recommendations are on his forum, at:
http://db.audioasylum.com/cgi/m.mpl?forum=dynaco&n=7492&highlight=curcio+riaa&r=
There is one minor error that affects his description, not the circuit: Where he writes "DO NOT remove the 47K feedback resistors between the cathodes of each tube", those are feedFORWARD resistors, not feedback.
Another option would be to rebuild the phono stage as per the Van Alstine Super PAS, a more involved but still relatively simple task (as DIY audio goes) which I think has a better result; however Van Alstine has removed the schematic and instructions from his Audio Basics page, so it can't be accessed any more. Someone may have a copy of it that can be provided privately (since it's copyright material it can't be posted publicly). I am very happy with my homebrewed Van Alstine Super PAS Three.
And remember...both the Curcio and Van Alstine phono circuits REQUIRE a load of no less than 100K at the phono stage output. This applies to the linestage of the PAS, no problem there; and to the TAPE OUT jack of the PAS. If this caveat is not observed you will get bass roll-off. This is true of the stock PAS phono, too. This eliminates most solid-stage tape and cassette recorders, wireless headphone transmitters, whole-house distribution amplifiers, line inputs to computers for digital recording...in fact most solid-state equipment of any kind, unless a buffer stage is used. This problem was solved in the Panor "Dynaco" preamps by the addition of a cathode follower stage, though all-tube purists would consider the additional stage in line with all signals to be less desirable than an output buffer which would not be in line to the power amps. The only thing I have connected to the TAPE OUT jack of my PAS is an Ampex F-44 tape recorder, a tube unit with a 500K input impedance. Putting a typical cassette deck on there DOES result in a huge loss of bass in phono.