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Dynaco PAS-3 RIAA phono equalization upgrade - photo

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Bob
Latino

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Bob
Latino

Admin

Posts: 2196
Join date: 2008-11-26
Location: Massachusetts

Post n°1

Dynaco
PAS-3 RIAA phono equalization upgrade - photo

by Bob Latino on Sun May
24, 2009 2:18 pm

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

Flyquail56

Flyquail56

Posts: 22
Join date: 2009-05-04

Post n°2

Re:
Dynaco PAS-3 RIAA phono equalization upgrade - photo

by Flyquail56
on Sun May 24, 2009 8:45 pm

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

Bob
Latino

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Bob
Latino

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Posts: 2196
Join date: 2008-11-26
Location: Massachusetts

Post n°3

Re:
Dynaco PAS-3 RIAA phono equalization upgrade - photo

by Bob Latino on Sun May
24, 2009 9:39 pm

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

Flyquail56

Flyquail56

Posts: 22
Join date: 2009-05-04

Post n°4

Re:
Dynaco PAS-3 RIAA phono equalization upgrade - photo

by Flyquail56
on Sun May 24, 2009 11:09 pm

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

GP49

GP49

Posts: 588
Join date: 2009-04-30

Post n°5

Re:
Dynaco PAS-3 RIAA phono equalization upgrade - photo

by GP49
on Sun May 24, 2009 11:46 pm

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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