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In Reply to: RE: Shure M65 is a great platform...& the others... posted by Interstage Tranny on August 20, 2015 at 17:32:13
The circuit topology is quite close to the Dynaco PAS-2 with a few changes in resistor values and different RIAA network feedback circuit values.
The other difference is the power supply voltage, about 100 V B+ specified for the Shure versus about 210 V for the Dynaco.
The higher voltage on the Dynaco should {theoretically} put the 12AX7's in a more linear state, with a higher overload margin compared to the Shure, albeit the Shure probably having a quieter noise floor.
The Mind has No Firewall~ U.S. Army War College.
Follow Ups:
Yes, the lower voltage B+ can also be found on the cute chrome phono preamps I enjoy; as well as many classic vintage phono stages. For example, the Eico HF-85 and Pilot 210 also use "lower voltage" for their phono tubes. The EQ parts and values are critical. With increased power supply capacitance, these "lower voltage" phono stages can sound very dynamic.
Lately, I have been arranging their power supplies for separate cap banks for each channel; simulating dual mono construction. Nevertheless, despite the very low current drain of the 12AX7s, actually using "dual mono" separate power trannies for each channel will sound the best. Furthermore, you would be able to hear the benefits of a dual mono phono preamp even if you use a stereo amp.
My penchant for building mono- bloc amps reflects your view, being more important in a preamp... makes even more sense. I dropped voltage in my own rebuilt Dynaco {via UTC potted Transformer that BARELY clears the cover being put back on the PAS 2}... Curcio mod for flat frequency response, over built power supply, 6X5 rectifier, etc.
I can't seem to leave anything alone... THAT circuit uses positive feedback and despite the negative view of positive feedback I have noticed that when it is employed ie. HK a500, Dynaco Pas2, the sound has an undefined {by me} organic realness. There are many pre-amps with more resolving power but the fact of that basic circuit being designed circa 56' by Ed Laurent, is impressive.
The Mind has No Firewall~ U.S. Army War College.
Those Dyna PAM-1 preamps can still be purchased one at a time for less than $75. each; sometimes with two Tellees hidden inside. The Dyna PAM actually gave the Marantz 1 mono preamp a run for the money. Simpler circuitry, less tubes, no cathode followers, but less versatile than the Marantz 1, Dyna sure knew how to satisfy on a budget.The PAM had three phono EQ positions, which the PAS should have kept IMHO, grid stopper resistors at the phono input tube's grid, decent EQ parts and values for the time, plus that cathode to cathode "positive feedback" resistor that many, many classic units and even more modern preamps have kept. Many CJ preamps after the PV-2AR, until they later decided to use passive EQ, also used that K-K feedback R. That is saying a lot !
The Shure M65 as well as the Shure SE-1, the transcription studio grade phono preamp also uses this technique. The SE-1 actually has Dynaco PAS type PC boards inside, but the EQ can be tweaked with air gap tuning caps, along with a simple 12AY7 line out stage using of all things; a UTC A series Plate to Line Output tranny. With this hobby, popularity is not important. Most important, to us, is the playback enjoyment factor. It is really all about playing the records and enjoying the listening...
Edits: 08/23/15
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