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The Dynaco Tube Audio Forum » Basket » Dynaco PAS-3 RIAA phono equalization upgrade - photo

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Bob Latino


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If you have a PAS-2 or PAS-3 preamp and also use the phono section you may be interested in upgrading the RIAA equalization which (with stock parts) departs from the "ideal" equalization by + or - 2 dB. Also consider that the small value stock picofarad caps have probably drifted from their original values by as much as 50% in the past 40+ years which could now result in as much as a + or -4 dB drift from the ideal equalization curve. The steps are outlined below and the schematic shown at the bottom has red dots where the changes are to be made.

1. Remove the 750pf caps from the selector switch on both channels.
2. Remove the 27K resistors from the selector switch on both channels.
3. Replace the 68pf cap on the PC-6 pc board with a 820pf silver mica cap.
4. Parallel the 100K resistor with a 2 Meg metal film resistor
5. Replace the 2750pf cap with a parallel combination of 2200pf and 470pf.
6. Replace the 4.7Meg resistor with a 2 Meg metal film resistor
7. Replace the two .1uF caps connected between teminals 1 & 3 (left channel) and 7 & 9 (right channel) with a larger .33 or .47 cap at 400 volts or higher.

If you do the changes above your RIAA equalization should now be off no more than .3dB (3/10 of a dB) of the ideal equalization.

Bob

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Flyquail56


Bob,

I posted a link to this procedure yesterday in the Going Back To Vinyl thread here. Other than one step that has been left out, isn't this Joe Curcio's design as posted in Audio Asylum? Or did he "borrow" it from someone else? Just want to see credit given where it is due.

Best regards,
Mike

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Bob Latino


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Flyquail56 wrote:Bob,

I posted a link to this procedure yesterday in the Going Back To Vinyl thread here. Other than one step that has been left out, isn't this Joe Curcio's design as posted in Audio Asylum? Or did he "borrow" it from someone else? Just want to see credit given where it is due.

Best regards,
Mike


Mike,

I have has this info lying around here for a couple of years at least. I do not know the source as it was sent to me by Email at least 2 or maybe 3 years ago. I copied and pasted it into a file I have at home here with a bunch of other Dynaco mods. It could very well have come from one of Curcio's or someone else's post. It is also possible that this could have been one of Val Alstine's mods for the PAS-3 from the 1980's? Not sure ?

Bob

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Flyquail56


Thanks Bob,
Here's Curcio's post from 2005, http://tinyurl.com/bedhrz
It looks like it must be the source of the information, but Joe does not say whether or not this is his design or that of someone else.

Either way, I would agree that it is a worthwhile improvement. I have done this upgrade in a PAS and also adapted it to an SCA-35 phono stage (which really needs the help). I have another PAS with the VanAlstine Super PAS Three circuit. It has a number of differences from the above upgrade. But it's still possible that this could be an earlier effort from AVA or someone else. There certainly have been a number of PAS mods out there in the last 40 years, from VanAlstine to ARC's William Z. Johnson to Pro Musica to Audio Dimensions, and probably a bunch I don't know about!

Best,
Mike

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GP49


It is not the same as any of the AVA Super PAS versions. They use a different means of deriving the low frequency pole in the RIAA curve.

The procedure given is a very basic one. It removes the switchable equalization for NAB tape from the selector switch and puts all the equalization components on the circuit board. This removes a set of contacts from the feedback loop. The resistance and capacitance values are mathematically derived for the frequency poles defined by the RIAA equalization standard and don't really "belong" to anybody.

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