Len Feldman
Audio Magazine, 1985

    If you are nostalgic about the early days of audio and high fidelity, you will love this little product designed and produced by Mike Stosich of Esoteric Sound. If you are a newcomer to good audio, you may well wonder what a Re-Equalizer is, and why anyone would need one. Perhaps a few words about my own history will help to answer those last questions.

    Way back in the dark ages (around 1950 and earlier), there was no standard equalization within the recording industry. By then, pretty much everyone realized that it was a good idea to cut master records with bass tones diminished in amplitude (to prevent running one record groove right into the
next) and with treble tones accentuated (to get up and over the record-surface noise). The problem was that no two record companies agreed on just how much to cut the bass and boost the treble. The result was pretty chaotic. Of course, if you played records on a cheap, low-fi phonograph it really didn't matter much, since most of them were so far off in frequency response to begin with. But if you sought more accurate sound reproduction, it was important that your phono preamp stages incorporated the reciprocal of the bass and treble recording equalization curves used in making the discs you played.

    When I worked as a design engineer at Fisher Radio back in the 1950's, we and such other companies as H. H. Scott and McIntosh tried to provide as many of these playback curves as possible. One of the pre-amps that I helped to design had a total of 36 possible playback equalization settings, six for the bass end and six for the treble. And that's exactly how many playback curves you
can select from with Esoteric Sound's Re-Equalizer.

    There are two major differences between the old pre-amps I worked on and the Esoteric Re-Equalizer. First, the Re-Equalizer is not a phono pre-amp. It is meant to connect at a high-level point in the signal chain between your pre-amp outputs and your main power amplifier inputs or in a tape out/in loop. Second, instead of providing a total playback curve, it provides the difference between standard RIAA equalization and those older equalization curves used to make early-vintage records. In other words, if I wanted to play a modern record while the Re-Equalizer was in my system, I would set its two selector switches to RIAA. Thus set, the Re-Equalizer would deliver perfectly flat response from input to output, just as it would if I had thrown its Bypass switch. The Re-Equalizer works this way because it is presumed that it will be used with a pre-amplifier that has RIAA equalization built in.

    The Esoteric Re-Equalizer actually goes beyond what we did in the early days of hi-fi i. It even allows you to play back early acoustic records and cylinders, most of which used no equalization at all! When you set the two selectors to their Flat positions, the unit introduces the exact reciprocal of
an RIAA playback curve. Bass frequencies are attenuated while treble is emphasized, so that the net response (your pre-amp stages plus the Re-Equalizer in series with the signal) is flat.

Control Layout:

    The Esoteric Re-Equalizer's front panel fits into a standard 19-inch rack, though the actual chassis behind the panel is barely half that width. The power transformer is mounted on the outside of the chassis housing, for best signal-to-noise ratio and minimum hum pickup. There is no on/off switch; the unit consumes only 2 or 3 watts and can be left on continuously. You can also, of course, connect its power cord to a switched a.c. outlet on your amplifier or receiver. A pair of selector switches one for the bass Turnover frequency settings, the other for the treble Rolloff settings are located on either side of a Bypass toggle switch. The six positions of the Turnover switch are labeled Flat 300, 400, RIAA, LP and700. The Rolloff switch positions are identified as Flat, 5, 10, 12, RIAA, and NAB.

Circuit Description:

    The Re-Equalizer employs a total of two type NE5532 ICs for its four op-amp stages. Separate bass and treble equalization networks are used, with an isolating stage between them. As shown in the schematic of Fig. 1, total gain of the system is supposed to be unity. Precision-tolerance components are used throughout the single, neatly laid-out PCB.


    The manufacturer did not specify what this unit's rated signal input level should be, so I measured distortion and signal-to-noise ratio with respect to 1 V input. Harmonic distortion measured a very low 0.006% at 1 kHz, 0.0055% at 10 kHz, and 0.0065% at 100 Hz; SMPTE-IM distortion was 0.01% for the same input level. For my signal-to-noise ratio measurements, I reduced the input level to 0.5 V in order to conform with the IHF/EIA Amplifier Measurement Standards. Unweighted S/N measured 71.3 dB and weighted S/N was 84.8 dB. If the input level were 1.0 V those results would be 6 dB higher.

    Having finished these basics, it was time to check out the Re-Equalizer's EQ curves. Before I did that, however, I wanted to check Out the accuracy of the device's own RIAA settings. As I explained earlier, since this device introduces the difference between RIAA equalization and other playback curves, setting both switches to RIAA should result in flat response if I feed my test signals into the Re-Equalizer without using a phono preamplifier ahead of the device. This plot comes very close to being perfectly flat. It had a 0-dB deviation at 100 Hz and was up +2 dB at 10 kHz.

    I introduced an accurately calibrated phono lab preamplifier into the signal path so that the reference RIAA settings would produce the familiar RIAA playback curve when I fed in signals of constant amplitude. Since the basic principles of phono equalization have always been the same (at least when equalization was used), all the vintage record EQ curves obtainable with the Re-Equalizer resemble the RIAA curve enough so that I had to label it, so you can tell which curve is which. Though I ran similar curve comparisons on other settings of the Esoteric Sound unit, the differences remained similarly subtle to the eye some closer to RIAA, others showing greater differences yet easily audible.

    The one exception is the curve obtained by setting both of the Re-Equalizer's selector switches to the Flat position. These settings would be used for playing old acoustic disc records or even older cylinder recordings (which were made with no equalization) through a modern preamplifier. The resulting curve is almost perfectly flat. That means that the Re-Equalizer effectively canceled the + 17 dB of boost at 50 Hz and the 13.1 dB of cut at 10 kHz that the RIAA characteristic of the preamplifier would normally have supplied.


    Obviously, the Esoteric Sound Re-Equalizer is not for everyone. If your record collection consists entirely of LPs that were mastered and pressed after about 1960, you won't find any use for this add-on. On the other hand, if you own many records issued from before that date, old 78-rpm records or even older LPs which do not comply with RIAA characteristics, the money you spend for one of these Re-Equalizers may well be one of the best investments you can make to
further your enjoyment of those vintage recordings.

    Rather than leaving you to guess what settings to use for your archival records, Mike Stosich has gone to a great deal of trouble to research just about every old record label that I know of plus a great many that I had never heard of. (Have you ever heard of Supra phone Records, for example? Or Hit of the Week?) He has tabulated and listed the correct settings for no fewer than 90 different record labels, in four pages of the carefully prepared owners pamphlet that's supplied with the unit. (Did you know that the 78 rpm records pressed by RCA around 1935 required a different equalization from those pressed in 1938, which in turn required a still different equalization from those made after 1948?)

    In using the product experimentally in my own system, I found that none of the settings materially increased distortion readings. I did note, however, that when using the Flat or 5 settings of the Rolloff switch, the amount of boost introduced by the unit is substantial. (It has to be, to offset the
substantial treble roll-off of the standard RIAA playback curve.) If you are playing an old record that really requires that setting, there will be no problem, since there is very little high-frequency content in such records to begin with. If, however, you simply experiment with these settings and use them with records that don't really require them, the added treble boost might well overload the device, which can't deliver much more than 3.5 V output before high distortion levels occur. Esoteric Sound isn't going to make a fortune marketing this product. The audience is limited, and decreasing with time. But in my opinion, they have provided a very valuable product for those individuals and institutions that
have rare and valuable record collections dating back to before RIM became the world standard. Even if you knew what the differences were between RIAA and the settings you need for certain old records, you would have a hard time arriving at those required curves with any degree of accuracy using typical bass and treble controls or even graphic equalizers.

    Esoteric Sound's Re-Equalizer performs its task very accurately without the introduction of distortion or other undesired effects. Its designers deserve credit for coming up with an unusual and very original idea and for carefully translating it into a real product.

For more information call 630-933-9801.

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