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Differences between the Dynaco PAS-2, PAS-3 and PAS-3X preamps

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Bob Latino

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Bob
Latino

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

Post n°1

Differences
between the Dynaco PAS-2, PAS-3 and PAS-3X preamps

by Bob Latino on Fri Dec
05, 2008 1:22 pm

Dynaco is
famous for making some of the best sounding tube equipment of the classic tube
era. The PAS preamps were among the best sounding preamplifiers one could buy up
until the time they went out of production. There are two circuit differences
between the earlier PAS-2 and PAS-3 units and the later PAS-3X.

1) The
"X" version had tone control potentiometers that were constructed to be
electrically removed from the circuit when set to their flat position.

2)
A 1uF electrolytic capacitor was placed in each channel’s tone control circuit
to block the DC from reaching the audio output.

Whereas the owners of
an earlier PAS-2 and PAS-3 preamps may not be able to find the specialized
potentiometers that were used in the "X" version, they can and should add the
1uF capacitor to eliminate the DC Voltage in the output of the preamp. This is
especially important if the amplifier that receives the signal from the preamp
has a direct coupled input such as the Dynaco ST-70. The DC Voltage from the
preamp will upset the bias point of the input stage of the amplifier and could
cause playing issues.

The original PAS-3X used a 1uF 25V electrolytic
capacitor but it might be better to use a polypropylene or polyester capacitor
with the same voltage rating or higher for the update. These capacitors may be
found at Mouser Electronics. To add the capacitors just replace the wires
connected between eyelet terminals #5 and #12 on the line amplifier circuit
board and the lower terminals of the bass controls with the 1uF capacitors. Just
remove each wire and replace it with a capacitor. Simple
!

Bob

PeterCapo

PeterCapo

Posts: 243
Join date: 2008-12-05

Post n°2

Re:
Differences between the Dynaco PAS-2, PAS-3 and PAS-3X preamps

by PeterCapo
on Fri Dec 05, 2008 4:31 pm

Hi Bob,

I believe the 1uF capacitors also served to suppress
transient pops from the x-mod pots. For example, the treble pot wiper lifts
near the center of its rotation, and the bass pot shorts near its
center.

Also, I believe the 1uF capacitors made it possible to use the
PAS with an amplifier having as low as 100K input impedance without snipping out
the 510K resistor, where the "regular" PAS could at best accommodate 250K
amplifier input impedance with the 510K resistor snipped out. I can say this
because I have early and late copies of the original manuals, and the late
PAS-3X manual specifies compatibility with 100K Z-in but doesn't say anything
about snipping out the 510K resistor.

The X-mod 1uF capacitors could be
especially helpful in the event anyone wants to use a PAS with some of the
modern tube amps. I have seen a couple of examples of these where the input
impedance is in the range of 100K – 150K. For example, the Audio Electronics
(Cary Audio) Six-Pacs have an input impedance of 150K, and my reissue McIntosh
MC275 Z-in is 100K.

Regards,
Peter

Bob Latino

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Bob
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Post n°3

Yes
- true

by Bob Latino on Fri Dec
05, 2008 5:02 pm

Hi Peter,
welcome to the forum ...

True - the wipers of the bass and treble
controls do lift near the center of rotation on a PAS-3X as the tone controls
are removed from the circuit. This could cause a transient pop and I am sure
Dynaco was concerned about that.

It is interesting that all the earlier
Dynaco amps - the Mark II, Mark III, ST-70 and Mark IV had no DC blocking caps
on their inputs but the later amps the ST-35 and the SCA-35 integrated do have
DC blocking caps. I bypassed the small .1 uF @16 volt input cap on my ST-35 in
hopes of "better sound" but in all honesty I can't hear any difference with this
small cap in or out of the circuit.

Bob

Luddite

Luddite

Posts: 235
Join date: 2009-02-04
Age: 65
Location: Texas

Post n°4

Dynaco
PAS-3X

by Luddite
on Mon Aug 10, 2009 11:19 am

The other night I was thumbing through a book by Bruce Rozenblit,
"Beginner's Tube Audio Design". In one section on classic circuitry, it was
mentioned that the PAS-3X, among the differences already discussed in this
thread, had one other change. The two resistors on the blend control were
changed from 560K/270K ohm to 68K/33K ohm. This was apparently done for greater
compatibility with the Hafler three channel system. I presume that the Hafler
three channel system utilized a derived center channel. I am familiar with the
Dynaquad system for deriving surround information, but I don't recall a derived
center channel system.

Charlie

GP49

GP49

Posts: 588
Join date: 2009-04-30

Post n°5

Re:
Differences between the Dynaco PAS-2, PAS-3 and PAS-3X preamps

by GP49
on Mon Aug 10, 2009 7:24 pm

Peter Capo:

> I believe the 1uF capacitors made it possible to
use the PAS with an
> amplifier having as low as 100K input impedance
without snipping out
> the 510K resistor, where the "regular" PAS could at
best accommodate
> 250K amplifier input impedance with the 510K resistor
snipped out. I
> can say this because I have early and late copies of the
original
> manuals, and the late PAS-3X manual specifies compatibility
with 100K
> Z-in but doesn't say anything about snipping out the 510K
resistor.

There is also a 62K resistor on the linestage circuit board
that can be removed if need be, to run any PAS into an amp with 100K input
impedance. Even the X-models benefit from this. The bandpass of the output
stage is limited by the 0.22uf coupling capacitor in the factory models; the 1uF
capacitors do nothing in this regard. If you increase the 0.22uf capacitor
sufficiently, generally in PAS modifications to 2uf or more, then the 1.0uf
capacitor becomes the limiting factor unless the tone controls and that
capacitor were removed.

In my shop I would ask PAS-owning customers to
bring in their power amplifiers. I would remove both the 62K and 510K
resistors, hook up a potentiometer across the output as a rheostat, and adjust
it to get flattest overall response in the bass (if you went to too high a load
resistance, you'd get a midbass hump!); then measure the resistance and install
a fixed resistor or paralleled fixed resistors to equal that
value.

bnc31:

> The other night I was thumbing through a book
by Bruce Rozenblit,
> "Beginner's Tube Audio Design". In one section on
classic circuitry,
> it was mentioned that the PAS-3X, among the
differences already
> discussed in this thread, had one other change. The
two resistors on
> the blend control were changed from 560K/270K ohm to
68K/33K ohm.
> This was apparently done for greater compatibility with the
Hafler
> three channel system. I presume that the Hafler three channel
system
> utilized a derived center channel. I am familiar with the
Dynaquad
> system for deriving surround information, but I don't recall
a
> derived center channel system.

The Hafler three channel
system used a single speaker connected between the two HOT terminals at the
stereo amplifier's output, deriving a "difference" channel. That third speaker
was supposed to be placed centered and BEHIND the listener. The later Dynaquad
system was a derivation of the three-channel setup, essentially two speakers
playing the "difference" signal.

Luddite

Luddite

Posts: 235
Join date: 2009-02-04
Age: 65
Location: Texas

Post n°6

Re:
Differences between the Dynaco PAS-2, PAS-3 and PAS-3X preamps

by Luddite
on Tue Aug 11, 2009 10:36 am

The Hafler three channel system used a single speaker connected between the
two HOT terminals at the stereo amplifier's output, deriving a "difference"
channel. That third speaker was supposed to be placed centered and BEHIND the
listener. The later Dynaquad system was a derivation of the three-channel
setup, essentially two speakers playing the "difference"
signal.[/quote]

Thanks GP49, that riddle is now solved.
Charlie

Rowbo1

Rowbo1

Posts: 6
Join date: 2011-01-27

Post n°7

Re:
Differences between the Dynaco PAS-2, PAS-3 and PAS-3X preamps

by Rowbo1
on Fri Jan 28, 2011 3:24 am

What are the diferences between pas2 and 3
Dave

GP49

GP49

Posts: 588
Join date: 2009-04-30

Post n°8

Re:
Differences between the Dynaco PAS-2, PAS-3 and PAS-3X preamps

by GP49
on Fri Jan 28, 2011 10:32 pm

Rowbo1 wrote:What are the diferences between pas2 and
3

Knobs and front panel

Rowbo1

Rowbo1

Posts: 6
Join date: 2011-01-27

Post n°9

Re:
Differences between the Dynaco PAS-2, PAS-3 and PAS-3X preamps

by Rowbo1
on Sat Jan 29, 2011 3:40 am

Thanks
Dave

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