Dynaco PAT-4 preamplifier Reviewer's Addendum

Reviewer's Addendum
Possibly, our sample PAT-4 did have "a noisy front- end transistor" (see "Manufacturer's Comment") but since both channels were equally noisy when using the high-level phono inputs, and since noise was extremely low when using the low-level inputs, we did not get the impression that we were testing a defective sample.

Dynaco offered us the schematic for an input equalizer like the one we tried to put together, except that this one works. We accept the offer with many thanks.

Further Thoughts, from June 1970 (Vol.2 No.10)
In our report on the Dyna PAT-4 preamplifier, we stated that we were unable to tell whether the unit was in or out of circuit when we fed a signal to one of its high-level inputs and then bypassed it entirely. This is no longer the case.

We are not sure whether our hearing acuity has improved or whether our PAT-4 has changed in some subtle way, but we now find a very slight loss of transparency when the PAT-4 is inserted in the signal path. It is still, however, better in this respect than anything we have tested to date, so the gist of that test report remains essentially unchanged.—J. Gordon Holt

A letter about the PAT-4 was published in March 1971 (Vol.2 No.12)

Editor: After having put up, grudgingly, with the very poor volume-control tracking on my Dyna PAS-3 preamp, I was disappointed and frustrated to see that you made no comment at all about this aspect of the PAT-4's performance.

Can you tell me how they compare, or was the PAT-4's tracking too bad to mention?—H. A. Lewis, St. Louis, MO

It was neither bad enough to criticize nor good enough to praise, but you're right, we should have said something about it.

It was, in fact, much better than that of previous Dyna preamps. In our sample, the maximum balance change through the volume control's entire rotational range was a shade under 2½dB, and was no more than 2dB through the normal operating range of 9 to 2 o'clock.—J. Gordon Holt

Dynaco/Radial Engineering, Ltd.

dc_bruce's picture

The successor -- the PAT5 -- was Dyna's first effort to use an integrated circuit. The original IC resulted in a not very musical sound; a replacement was offered that sounded much nicer. Interestingly, the PAT5 did away with the spring loaded monitor switch.

My PAS3X was probably a bit more "musical" but seemed more noisy. That could have been from the tubes (which used an AC filament supply) or it could have been from the fact that all of the wiring was point-to-point, increasing the potential for cold solder joints or suboptimally routed wires.

jmsent's picture

...was DC not AC. The PAT-4 was crap, plain and simple. It didn't have a regulated power supply, so anytime someone switched on a high wattage appliance, you'd get a big dc level shift at the output of the preamp. The whole circuit was only 4 transistors/channel, basically, a transistorized version of a PAS 3X. But the reality was, the sound quality wasn't near as good.. The PAT5 did use a "second generation" op amp in the line stage and a 2 transistor phono stage. And at least they.used a regulated supply. The Bi-Fet versions were a small improvement. Still, by this time, Dynaco stuff was no longer the great sounding audiophile bargains that they were famous for in the vacuum tube era.

Michael Fremer's picture

I agree! I had a PAS-3X and a stereo 120 combo. I loved it. The 120 was kind of hard but the PAS-3X was a perfect compliment. As soon as the PAT-4 came out I bought and built one. I hated it! So much so that I called Dynaco to ask if something might be wrong...... the whole thing was wrong....

dc_bruce's picture

If memory serves, now that I think about it, the B+ was supplied by a tube rectifier and a selenium diode handled the filaments supplies. I had the PAS-3x paired with a Stereo 70, which sounded quite nice until the output tubes aged a bit. I sold them as a pair, got an integrated; and sometime later got the PAT-5 (with a Stereo 400). By that time, I had changed speakers several times and really couldn't compare it with the PAS-3x.

Jack L's picture


For sure transistors never sound as good as triode tubes, IMO.

Why? Transistor, or technically a bipolar junction device is NOT as linear as triode tubes, used in PAS2 & 3, which get a full set of linear signal transfer curves vs all bipolar junction devices get nonlinear transfer curves which are each bent down by a kink or 'knee' ! This is physics.

That's why all my design/built photostated/linestages & power amps only installed with triodes, period. 'Cause they are MUSICALLY friendly.

My question is: what was the problem of using only 4 transistors for the PAS-4 ?? Circuit design simplicity is MUSICALLY correct though it may not be so "polically correct" to many in marketing & sales of the products.

J Gordon Holt already commented PAT-4 was one of the best sounding preamp with minimum tonal coloration vs a straight line bypass. I wholeheartedly agree to his comment !! He knew what he was talking.

PAS-4 was built of 4 transistors per channel consuming so little power. Voltage regulation was indeed not necessary considering it was it was low power era half a century ago.

To many designers who design audio amps with op-amps being considered high-tech & 'fashionable'. I would never like their sound let alone installing them in my audio amps.

An op-amp is built up with many bipolar junction devices, e.g. transistors, & FETs with tons of global loop feedbacks.
Yes, they may be measured great great, but IMO, are not musically correct at all as the music complex harmonics got to pass through the many many capacitive bi-polar junctions & feedback loops. It is 'pain' for music signal to go through. This is physics.

Listening is believing

Jack L

RH's picture

Interesting read.

I've always liked reading Gordon Holt's old reviews. He's a very good writer: very crisp, and never holds any punches. In contrast audio reviews these days seem to do a tip-toe dance around negatives about a piece of gear. It's usually "wonderful, wonderful, wonderful..and then saved for the end 'well there's this teeny little negative thing, but I don't think it will bother most people.."

Holt just jumps in and I never get that feeling of reticence or holding back on criticism.

Jack L's picture


Yup. So many commercial journals bank on their sponsors & advertisers & do not want to publish anything deemed offending.

That's why Gordon earned my profound respect in holding himself accountable to his readers in his audio reviews - honest & straightforward !!

He might be deemed 'politically incorrect', but so what ? Apparentlly he did not give a rat ass.

Jack L

a.wayne's picture

They work best in homes with more than 2 breakers .. :)

Jack L's picture

.............. when reproducing phono inputs" quoted J Gordon Holt.


The bass boost was intended so in the PAS-3x phonostage circuitry design.

When you read carefully the schematic of the phonestage, there was a 47KR resistor bridging the cathodes of first stage (1/2 12AX7) & 2nd stage (1/2 12AX7), forming a POSITIVE loop feedback as both cathodes were IN phase.

I have not yet seen similar positive feedback design in tube phonostage ever since till todate. I recall reading this bass boost design for PAS-2 & -3 phonostage published in some journals.

Jack L