Dynaco PAT-4 preamplifier

This review of one of the first solid-state preamplifiers was published in 1968. It includes some of JGH's first thoughts on the ongoing subject of "Accuracy vs. Musicality."

Preview, from July 1968 (Vol.2 No.6): Overall sound extremely good, but phono sounds slightly lacking in deep bass, despite impeccable measurements. Scratch filter judged very highly effective, but tone controls felt to be less than ideal because of excessively coarse action and marked tendency to affect midrange output. Spring-return Tape Monitor switch probably will not appeal to serious tapesters. This preamp is slated for a full report in the next issue.

The Review
Through long experience, we've come to expect a lot from Dynaco equipment. None of it has ever enjoyed the status value of the Marantzes and the Mclntoshes, perhaps because Dynakits are inexpensive and don't have that look of chromium-plated Class that impresses status-seekers. The strong points of Dyna's components usually aren't visible, they're audible. They just sound very good, some in fact being the sonic equals of the highest-priced Status models. And although Dyna's preamps have lacked some of the flexibility of the costlier ones, and recent models have had less-than-ideal tone control character: there were always enough inputs, outputs, and controls to do a job of preamping. This is why, though our exposure to other state preamps to date had left us unimpressed, we undertook the tests with considerable curiosity.

At first glance—and every second and third glances plus perusal of the specifications, the PAT-4 looks like nothing more than a solid-state version of Dyna's type PAS-3x preamplifier. The differences, though, and some them are even visible.

The PAS-3x had provision for selecting full stereo separation, partial blending (to reduce ping-ponging of hyperstereo recordings) and full blending (A+B mono reproduction of two-channel sources, as well as the usual arrangement for piping either A or B input to both channels. The is the same except that the PAS-3x's blend mode has been eliminated. Instead, the A+B mono mode retains 6dB of separation, which is low enough to suppress vertical-tracking distortion when reproducing mono discs, but also enables the system to use the Hafler (Dynaco) center-speaker hookup arrangement without further system modification.

Also new in the PAT-4 are a front-panel input jack for a high-level stereo signal source, and a highly effective sharp-cutoff treble filter, for getting rid of noise and distortion from imperfect program material.

The PAT-4 has two extra outputs per channel, too. One stereo output jack is on the front panel, and there are two pairs of output receptacles at the rear of the preamp. One pair of these is operative at all times, but the other pair is controlled by the front-panel output jack, so that inserting a plug in the jack kills the signal from the controlled rear-panel receptacles. Thus, if the controlled receptacles are used to feed the power amplifier, the speakers will cut off automatically when a headset is plugged into the front-panel jack.

The front-panel outputs are in parallel with the main outputs, and deliver the same 600 ohms impedance. Dynaco advises the use of any head set having an impedance of 600 ohms or higher, but since high-impedance headphones aren't all that common, you'11 probably end up using a pair of small matching transformers to drive 8-ohm phones. Most headphone makers can supply suitable transformers at moderate cost.

That controversial switch
The PAT-4 differs from previous Dyna preamps in one other minor respect, too, but this is one change that we question. As usual, this preamp has a TAPE MONITOR switch that allows a three-headed tape recorder to be inserted in series with the signal path, to allow monitoring from the playback head while recording. But unlike any other preamp we have seen, the PAT-4 has a spring-return MONITOR switch that selects the playback mode only as long as the switch is held down.

It has been our observation that most tape recordists who have three headed tape machines prefer to do most of their taping while listening to the playback from the tape. To do this with the PAT-4 would result in a very tired pinkie. On this preamp, you use the MONITOR switch to sample the tape's playback, or you jam a toothpick under the switch to keep it in the MONITOR position.

Our first question about this was Why the heck? To which Dynaco replied: Stupid customers, that's why! It seems that, ever since Dyna started including a tape monitor switch on their preamps, they have been plagued by customers who would flip the switch the wrong way, get no sound from any input, and bounce their preamps back to Dynaco for "servicing." Usually under warranty, at no cost (to said customer). The spring-loaded arrangement on the PAT-4 was Dyna's way of ensuring that the switch would stay in the Normal position except when the user decided that he wanted to monitor from tape.

We find it hard to believe that Dynaco is the only preamp manufacturer that has had this trouble. Yet no other manufacturer has, to our knowledge, felt it necessary to dilute the usefulness of the MONITOR function as a service to those customers who don't care to use it. Frankly, we don't think Dynaco has any right to blame those unnecessary "service" kickbacks on the stupidity of customers when there has been practically no effort made to warn the customer about this possible source of "trouble." A short paragraph in bold-face type in the "In Case of Trouble" section of the instruction manual would probably forestall 90% of the user problems with the MONITOR switch, and a little red tag attached to the back of the preamp might take care of another 5% of them. Instead, previous instruction manuals have had a single, eight-word sentence on the subject, buried in a paragraph about wiring errors, bad connections and short circuits. If the problem was severe enough to warrant the design change in the MONITOR switch, it certainly deserved more attention in the instruction manual than it got.

There's a standing gag in the manufacturing business that goes "If all else fails, read the instructions." And it is true that most people don't, if they can avoid doing so. But when all else fails, people will read the instructions, and they'll usually follow them if given the chance. We don't really think Dynaco gave them the chance.

As we noted, if you're not a serious tape recordist, the PAT-4's spring-return switch won't bother you. If you are, you can order a regular-type switch from Dynaco before you build the kit and use it instead. You can even make the necessary modification in a factory- built PAT-4, but it's a rather nasty job.

Sound Quality
So much for the features. How does the PAT-4 sound?

One extremely critical test of any preamplifier is the "bypass test," whereby the sound coming out of it is compared with the same sound fed directly from the signal source into the power amplifier. We have never yet found any preamp that did not add at least a subtle coloration—a little extra bass or a little added high-end roughness—to the sound, although the PAS-3x and the Marantz 7C (not the 7T) came as close to perfection as anything we had ever tested. Now, both of these have been bettered—sonically, at least.

With all of its tone controls and filters set to Flat, and feeding any high-level input, we were simply unable to tell whether we were listening to the original "raw" signal or the output from the PAT-4. In this respect, we cannot see how any preamp, present or future, could surpass the PAT-4.

Accuracy?
This is not to say that everyone will like what he hears from it; some systems, and some ears, need the colorations introduced by other preamps. But if you like what you hear when you pipe your tuner or your tape machine directly into your power amp, with no intervening preamp, you'll like the same lack of coloration in the sound that passes through the PAT-4.

We feel a little less positive about the PAT-4's performance on Phono inputs. As compared with other top preamplifiers, the PAT-4 sounded rather thin at the low end when reproducing discs. Equalization measurements were not at all enlightening; there was no more than 1dB of deviation from the RIAA curve down to 30Hz, below which there's little more than rumble on any disc

COMPANY INFO
Dynaco/Radial Engineering, Ltd.
ARTICLE CONTENTS

COMMENTS
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

Hi

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

Hi

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.

Hi

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

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