Using PAS3 as phono only preamp

I am interested in using my PAS3 as a phono only preamp.  I will be using my SP14 line pre to drive my ST70.

Is it possible to rewire the PAS to bypass the PAS line stage entirely and simply output the phono signal from the PC6 to the SP14 phono in terminal, perhaps using the PAS tape out as the vehicle?

Any thoughts are appreciated.

breading wrote:I am interested in using my PAS3 as a phono only preamp.  I will be using my SP14 line pre to drive my ST70.

Is it possible to rewire the PAS to bypass the PAS line stage entirely and simply output the phono signal from the PC6 to the SP14 phono in terminal, perhaps using the PAS tape out as the vehicle?

Any thoughts are appreciated.

Yes it's possible. And no, there is no rewiring ( or changes whatsoever needed),
connect tape out to the SP14 and be done with it.

The PAS phono stage is load sensitive. What is the input impedance of the SP14?

.

Peterh,

Thanks for the tip. Hooked SP14 up to tape out and selected phono on PAS3 and it worked like a charm.

Bruce

audioregenesis wrote:The PAS phono stage is load sensitive. What is the input impedance of the SP14?

This ^^^

The PAS phono is looking for a particular load (around 225K I believe). Pretty sure the SP14 input impedance is a lot higher than that. If you connent directly you will probably introduce a bass boost. Simple to fix using a single resistor on each channel to provide the proper load.

But if you like a little bass boost, it's fixed already

The response of the PAS phono stage, particularly the low end, is optimised for the load presented by the "PAS" line stage. Anything connected to the tape out jack will result in some additional load being placed in parallel with the existing line stage load. If the load connected to the tape out jack is fairly high impedance, say several hundred K or more, the resultant loss in bass response will be small. However, if the load approaches 100K or less, the loss of bass will become significant. Connecting anything to the tape out jack will always result in some loss of phono bass, never a gain, it's just a matter of how much. The reason for asking about the input impedance of the SP14 was to determine how much.

input impedance of the SP14 is 470K

tubes4hifi wrote:input impedance of the SP14 is 470K

Thanks for the info. Much higher than I would have guessed. A quick check on my PAS phono ltspice model shows that would drop the response at 20Hz by about -1dB while slightly taming the response peak that normally exists in the 30Hz range. So, not much to worry about running into the SP14.

A few more thoughts on this subject pertaining to the tape monitor switch. When set to the Input position, as would typically be the case for normal listening, the load presented at the tape out jacks appears in parallel with the PAS line stage. However, when set to the Tape position, the line stage is disconnected from the output of the phono stage, thus, the phono stage sees only the load at the tape out jacks. If that load is greater than about 230K there will be a a slight bass boost, a maximum of about 1.8dB at 20Hz with nothing connected to the tape out jacks. In this particular case, 470K will be present, resulting in a boost of about 1.4dB. If one was to tack an addtional 470K across the tape out jacks, with the monitor switch set to the Tape postion, the phono stage would be loaded almost exactly as intended. Either way, the loss or boost, in this case, may be small enough to be inaudible. Just though I'd mention it FYI.