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PDG ACTIVE CARTRIDGE STABILIZER (ACS): Head amp/interface for moving coil and moving flux pickups. Price: $395 (includes one set-up kit for a specific pickup; extra kits $20). Source: manufacturer's loan.
by Bill S.
DYNAVECTOR PE-2: Phono enhancer for moving coil pickups. Loads pickup with a dead short (for maximum damping), and amplifies resulting cur rent; gain adjusts automatically to cartridge's source impedance. Price: $375. Source: manufacturer's loan.
MANUFACTURER: Dynavector Systems, 2217 South Grand, Santa Ana, CA 92705. (714) 549-7204
We're starting to see a new approach to phono preamp and head amp design, as exemplified by the Meitner preamp, the Electrocompaniet MC-2 (reviewed in Vol. 8, No. 5), the PDG ACS, and the Dynavector PE-2 Phono Enhancer for moving coils.
It was discovered about 10 years ago that many phono preamps were mis-designed.
(Since there are no perfect preamps, all are, in a sense, mis-designed, but that's another matter.) The EQ feedback loop could inter act with the pickup's inductance to produce unexpected changes in frequency response.
Designers scurried to correct this problem, and the "all-preamps-sound-alike" brigade breathed a sigh of relief as they realized the last possible obstacle to preamp perfection had been surmounted.
But the products mentioned above are intentionally designed to interact with the pickup, some in ways that are relatively gentle, others more vigorously. Both the MC-2 and PE-2 are current-sensing head amps.
They short the pickup, and the resulting current is transformed into a voltage. This kind of amp has a nice side benefit, in that its gain is inversely proportional to the source impedance. Roughly speaking, higher-output moving coils have higher coil resistances, since the higher output is due to use of more wire. Therefore, the higher resistance of a high-output pickup translates into lower gain, giving us a kind of pickup automatic gain control.
The idea isn't just to be technically clever, but rather to apply maximum electro-dynamic damping to the pickup, by shorting it, while still getting an output signal.
Whether or not this works in practice is something we shall examine shortly (groan). The PDG Active Cartridge Stabilizer, as its name suggests, attempts to go further. Be sides loading the pickup (either MC or MF) with a dead short, it supposedly monitors the cartridge's output, and applies a counter voltage to compensate for transient errors.
Looking at it another way, the ACS is a dynamic damping device, optimized for the electrical characteristics of a particular pickup. The resistors and capacitors which set these characteristics may be changed by the user to adapt the ACS to a particular pickup. This design is also claimed to be effective at raising the threshold of mis tracking.
It seems to me that neither of these designs goes far enough. Take the PE-2: Is it necessarily true that a dead short is optimum damping for all pickups? I don't think so. A cartridge with little or no mechanical damping may still be underdamped, while one whose internal damping has been carefully adjusted could well become overdamped.
It might even be argued that a sluggish pickup could stand some goosing. The PDG is supposed to provide this transient correction, as well as correcting the pickup when it is on the verge of mistracking. PDG says the ACS injects current into the pickup; the greater the pickup's output, the greater the current returned. This would supply the most damping during the loudest passages, precisely the time when the pickup might have trouble coping.
Besides active damping, the ACS also pro vides the EQ required when a high-impedance cartridge feeds a dead short. The EQ is needed because, at the frequency where the pickup's inductive reactance equals its resistance, the output current starts falling at 6 dB/octave. MC pickups, with their vanishingly-small impedances, push this frequency well into the ultrasonic (for a Dynavector the corner is above 56 kHz). But for the Signet TK10ML/II moving flux (also referred to as moving magnet), it's at 450 Hz! This requires 33 dB of EQ to maintain flat response at 20 kHz.
PDG says the ACS' internal adjustments only set the corner frequency and the over all gain; there is no specific compensation for the transient behavior of the cartridge.
It would be difficult to obtain samples of every decent pickup on the market, and design compensation for each one, but, jeez, guys, that's what I was hoping for.
I think that both the PE-2 and ACS have their hearts in the right place, but their feet haven't carried them far enough. This doesn't, however, mean that you shouldn't consider them.
My reference Acoustat TNP preamp has a single phono section, with switchable gain for MC and MF pickups. The two gain structures sound almost alike, which makes it possible to judge the absolute sound character of a head amp, rather than just com paring it with other head amps.
The Dynavector PE-2 is specifically to be used with MC pickups, and there's a PE-1, not tested, that's designed for MFs. The compact case has a handsome brushed aluminum finish, the connectors are gold plated, and the power supply is in a separate box.
But the sound is a major disappointment.
It is extremely flat and two-dimensional, with an almost total loss of air and space. It is uncomfortably forward-sounding, with out any compensating gain in immediacy or aliveness. Piano takes on a slightly nasal quality, instrumental color turns into a drab grayness, and there is a noticeable loss of vocal and instrumental character.
To top it off, the PE-2 is unacceptably noisy. With the volume set to 9:00 (a reasonable playback level in my system), the PE-2's noise was plainly audible. With my preamp set for MCs, the gain has to be advanced past 1:00 for noise to be heard. In short, the PE-2 alters the signal too much for me to recommend it.
The PDG ACS, which can interface with both MC and MF pickups, is a more complex product to evaluate. Let's consider it first as an MF interface, its primary use.' In this mode it is set for unity gain, plus compensation for the roll-off described earlier.
In the case of my Signet TK10ML/II cartridge, the more than 30 dB of gain needed
[ 1. It was in this mode that contributor AL used it for the report preceding this one. –LA]
to compensate for the roll-off raises the noise level so much that the added hiss is clearly audible in the quieter passages of recordings with wide dynamics.
Despite the noise, the ACS did effect one improvement in the TK10ML/II; it removed about 50% of the upper-midrange suckout, which was audible as slightly increased definition and openness. This change does not come from the EQ, but rather from running the pickup into a short, thus removing the effects of cable capacitance and the inter action of the cartridge's inductance with the preamp's standard 47-kOhm load resistor. Unfortunately, if the cartridge designer has carefully juggled these factors to get flat response, use of the ACS may produce an elevated upper midrange and top end.
After discussing this noise problem with PDG, it was decided to use a lower-inductance pickup requiring less EQ. I was sent a Grado 8M and the needed EQ components, which produced a 6-dB-high shelf above 2 kHz. PDG blamed inaccurate cartridge specs (which I believe), and sent another 8M with an ACS tweaked for it. This time, the response was flat and there was no noise problem.
Unfortunately, the sound was a bit disappointing. As with the PE-2, there was a flattening of perspective and loss of spacious ness, though not nearly as severe. Stage width was marginally narrower. There was no added hardness, grit, grain, or change in frequency balance, but there was a loss of vitality and character to the sound. Both the 8M and the TK10ML/II showed a slight added grunginess in the upper midrange and highs, especially with massed strings. I preferred listening to the 8M straight.
The ACS is also usable with MCs. Mine was set up for the Dynavector 17Ds, but the exact set-up is not as critical as with moving flux cartridges since, as mentioned earlier, the HF rolloff frequency is well beyond audibility, and the self-adjusting gain tends to compensate for differences in output.
Although the ACS added a bit of noise, you had to turn the volume well past any reasonable listening level to hear it; the ACS should be noise-free with most MCs. The sound showed the same kind of changes (loss of spaciousness, tonal drabness), though, that were heard with MF pickups, but the problems were not as noticeable; you had to listen a bit before you were certain they were there.
It's easy to blame these changes on the electronics, but I think another effect is at work. It is believed by many that phonograph recordings tend toward overdamped transient response. This isn't unbelievable; the cutting stylus has significant inertia, and the acetate mastering surface suffers from "spring-back" effects that blunt transients. (Perfectionist recording engineers try to plate the master as quickly as possible, for precisely this reason.) Since the better pickups are designed in part by listening, their designers would tend, even unconsciously, to compensate by underdamping the cartridge.
Further, I know what master tapes sound like; compared to live sound, there is an excessive liquidity and sweetness from tapes, combined with a backing-off of perspective and an exaggerated sense of spaciousness and depth. Using the ACS moves the sound in the other direction, towards increased dryness and reduced spaciousness. I suggest that this occurs because shorting out the pickup results in overdamping.
The fact that less of the effect occurs with MC pickups is strong evidence in favor of my hypothesis. The average MF pickup has a resistance of about 400 ohms, but is loaded with the much higher resistance of 47K; this is hardly any electrical damping at all! But a 30-ohm MC works into a "nearly equal" 10 to 100 ohm load, where it is more heavily damped. Therefore, if the designer adjusts a pickup's mechanical damping to make it sound right, an MF pickup will be more heavily damped than an MC, since it doesn't get as much help from electrical damping. Loaded with a dead short, both MF and MC pickups are overdamped, but the MFs more so than the MCs. And that's exactly what I observed; the ACS had more effect on the sound of the former than the latter. 1/4 In case anyone doubts this, there is no question that the ACS is damping the pick up. This is plainly audible on low-pitched surface defects. Into a regular preamp, the "pop" shows noticeable overhang and ringing; with the ACS, it is more tightly con trolled and ring-free.
I was not to observe any evidence of in creased damping improving tracking ability.
The Dynavector 17Ds just starts to mistrack on the highest levels of Shure's Era III test disc; with the ACS, there was no reduction of mistracking. I also suspect that the in crease in distortion heard from the Signet and Grado is not the sound of the electronics in the ACS, but is due to their not "liking" to work into a short.
In short (more groans), the ACS technique of turning an MF pickup into a current source seems to generate more problems than it solves, particularly if the pickup is well designed. I can't really recommend the ACS as an MF interface. On the other hand, its low noise, adaptability to a wide range of pickups, mostly-subtractive errors, and low price (as such things go) make it worth considering as an MC head amp. Sales are factory-direct, and PDG won't cash your check until you've had a month to audition.
Don't like it? Just send it back. Under those circumstances I can recommend that you at least try the ACS.
--
[based on a Feb. 1986, Stereophile review article]
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