Dynaco PAT-4 preamplifier Page 2

And to compound the puzzlement, the other two preamps we checked (the PAS-3x and the Marantz 7) gave almost exactly the same measurements. The difference we were hearing could not be explained in terms of equalization error. What, then, could it be? And in fact, which is the more accurate reproduction of the discs: the fuller bottom of the tube-type preamps, or the somewhat sparser but tighter sound of the PAT-4?

To try and settle the matter once and for all, we spent a couple of days trying to devise an equalizing network which could be connected to a tape machine's outputs and would yield low-level signals with a response that was exactly complementary to the RIAA curve. Ye came up with what looked like just the thing, trimmed it to within 0.5dB from 20Hz to 20kHz, tried it on a PAS-3x, and it worked beautifully.

It also showed that the PAS-3x had a very slight tendency to exaggerate lows when reproducing phono inputs. Then we tried it on the PAT-4, and it proved to be an utter flop. It seems that the input loading supplied by transistors is not uniform over the audioband. And since our equalizer was designed to use the preamp's 47k-ohm input as an integral part of the circuit, the transistor loading threw everything askew. So, the equalizer was shelved for the time being—we'll work on it some more at a later date. It did however show a slight bass-boosting tendency on the part of the PAS-3x (and would probably have done the same with the Marantz, had we had one on hand for comparison), but although it suggested that the PAT-4 differs in the correct direction, it couldn't show whether the PAT-4 was actually thinning out the low end or was reproducing it as it is on the discs themselves.

In view of the equalization measurements we did, we suspect that the PAT-4 is reproducing pretty accurately. And our past investigations into disc-equalization practices showed that nearly all discs (and probably many tapes, too) have slight to severe attenuation of extreme lows, which would tend to confirm our feeling about the PAT-4.

Tone Controls
All of which raises a dilemma. There is no doubt but that the vast majority of discs sound better with a spot of exaggeration in the extreme low-bass range. Their built-in bass cut may not correspond to any industry standard like the RIAA reproducing curve, but if it is virtually universal practice in the recording industry, it does not strike us as any great violation of audio ideals to at least have provision for correcting the deficiency in the preamp. True, there may be increased problems with rumble as a result, but the PAS-3x has had few such problems when used with good turntables, and good turntables are becoming increasingly common. And a well-matched arm-and-cartridge combination can serve as an effective filter for subsonic rumble without impairing the response in the deep-bass range.

One obvious answer might be the judicious use of the bass tone controls to add the necessary low-end correction. But it is obvious from the curves (fig.1) that you won't be able to do this with the PAT-4; you have to crank its bass control nearly all the way up before it gives more boost in the low-bass range than in the upper parts of the range. The Marantz 7C is the only top-quality preamp whose bass control will provide the necessary correction.


Fig.1 Dynaco PAT-4, tone control curves, showing midband overlap and level changes (5dB/vertical div.).

As a matter of fact, we found the PAT-4's tone control action to be its least attractive feature. It seems that the extremely clean sound of the PAT-4 (and of the PAS-3x) is due at least in part to the use of special potentiometers which automatically bypass the tone control circuits when set to their Flat positions. In both preamps, though, this advantage has been bought at the cost of degraded tone-control characteristics. The out-of-circuit position of each control is a very narrow arc of rotation, and the entire rotational range is only about 200°, as compared to the 300° of a typical potentiometer. Both controls provide about the same amount of maximum boost or cut as other control systems, but the fact that it is squeezed into a smaller rotational range (fig.2) and controlled by a rather small knob makes it a ticklish business to set a control for Flat response and makes it virtually impossible to return to a previously established setting of a control.


Fig.2 Dynaco PAT-4, this is all the rotation of the bass control that is needed to produce a 6dB change in apparent volume.

In addition, instead of restricting their action to the bass and treble ranges, both controls have some effect on the volume of the musical middle range, too, so that rotating a control causes an apparent change in the volume of the entire sound.

The tone controls were not of much practical value in the PAS-3x; they are no better in the PAT-4.

A goof?
We found one other shortcoming in the PAT-4 that may but probably won't concern you. Like the earlier-model Dyna preamps, this one has inputs for high- and low-level pickups. But unlike the earlier models, the PAT-4's high-level phono input is useless. We tried it with an Ortofon SPE—certainly one of the highest-output stereo magnetics that have ever been made, and got a horrendous hiss along with the signal. The preamp was not defective; perusal of the schematic showed that the trouble was due to a design goof—the first real blooper we've ever known Dynaco to make. Fortunately, the "low-level" phono input will handle up to about 80mV of input signal without overloading, so most pickups can be plugged into that. We would not, however, recommend using any pickup with the PAT-4 whose output is too high for the low-level input to cope with.

It's easy enough to tell how much signal a pickup will put out if you remember that most pickup output specifications cite millivolts of output for a groove modulation velocity of 5cm/s, and that the maximum velocity on most discs is around 25cm/s. Thus, a pickup rated at 6mv output would deliver peaks of about 5 times that figure from loudly-recorded musical discs, which would amount to a mere 30mv. A pickup rated at 7mv/cm .like the Ortofon SPE) will put out a whopping 175mv on the same peaks, which is a shade over the permissible 80mV maximum of the PAT-4.

It should be quite clear by now that the PAT-4 is not The Ultimate preamplifier. But where does it stand with relation to the competition? Well, to, put it bluntly, there's no Ultimate preamp there, either. Of all the ones we've heard, the Marantz 7C and the Dyna PAS-3x are the only ones that can even compete with the PAT-4 for sheer sonic accuracy. The Marantz has by far the most useful tone controls, but has a bit more of the typical preamp coloration than either of the Dyna units. The PAS-3x sounds better than the Marantz (that is, on program material that doesn't need certain kinds of tonal correction) and has much the same tone control characteristics as the PAT-4, and although it introduces slightly more coloration than the PAT-4, its phono preamp section yields somewhat more satisfying disc sound than the PAT-4. On the other hand, the PAT-4's high-end sweetness makes discs sound less like discs than does the PAS-3x.

In other words, there's no clearcut winner here. Of the three, we have found the PAT-4 the easiest to live with, mainly because we find we are less bothered by slight deficiencies in reproduction than we are by the addition of even small amounts of audible distortion or by inadequacies in tone control facilities. There is something very relaxing about the easy lucidity of the PAT-4's sound, and it is something one cannot really appreciate until, after living with it for a while, one goes back to a less uncolored preamp.

So, for anyone who wants the most musically listenable sound, and is willing to put up with the PAT-4's shortcomings in exchange therefor, we recommend this unit above any other we have tested to date.

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