SIGNALS & NOISE (Letters to Editor) Feb. 1983

Improving Our Analyzer

Dear Editor:

Robert R. Cordell's THD Analyzer (Audio, July-Sept. 1981) is an amazingly capable device, especially considering the modest parts cost. In building mine, I noted a number of useful design changes that I would like to pass along.

The Signetics 5534s that I used for most of the op-amps are of more re cent manufacture than those in Cordell's prototype and appear to be more conservatively phase compensated. To get proper operation of the analyzer section up to 200 kHz, I re-compensated as follows: C26 and C28 were changed to 18 pF, C25 and C27 to 4.7 pF, and C24 to 2.2 pF. Cordell has found similar values necessary when using T.I.'s 5534s. I also changed the oscillator compensation C2 and C3 to 18 pF, but similar performance can be achieved by fine tuning the lead compensation resistors in series with the 200-kHz range C(KM) and C(LN).

The dominant distortion mechanism in the mid-band of the analyzer is due to incomplete rejection of the second harmonics produced in the FET multi pliers. I found that replacing R10, R11, R13, and R14 in the oscillator-and corresponding parts in the analyzer with 1% tolerance parts (267a 10k, 750k, and 10k, respectively) improved my distortion by 5 to 6 dB. To get this same improvement, 5% resistors can be selected for the proper ratio.

I have been unable to obtain proper operation of the oscillator at 200 kHz without replacing C12 with 0.0022 µF.

The a.g.c. circuit is not adequately stable with the set of parts I used, due to the propensity of the circuit to go into slewing oscillations and the very marginal stability of the a.g.c. loop when changing frequency (D2 or D3 turns on and increases the loop gain by 100:1 at frequency change times).

Bob Cordell has been unable to reproduce this failure on his prototype.

The dominant low-frequency problem is hum. I found that most of the injected hum was because of a floating top cover on the Ten-Tec enclosure both Bob and I used-removing some of the plastic covering material and screwing the case down to ground helped considerably, reducing the 50 Hz/1 V reading from 0.0018% to 0.00035%. Twisting cable E9-10 with E4-5 brings that down to 0.00023%.

The suggested trimmers, though economical, are probably of inadequate quality for so sensitive an instrument as this. I found a substantial improvement in the stability (lack of jumpiness) of the analyzer by replacing R59 and R62 with fixed resistors, and I would recommend using sealed trimmers everywhere.

The parts list is missing R159-1.5k1, C75-2 pF and IC21-LM318. With all these modifications, the analyzer reads no more than 0.0005% THD at 3 V from 20 Hz to 20 kHz; this is remark able performance at any price. The highest distortion and greatest noise both occur at 20 kHz, each being about 109 dB below the fundamental (0.00035%).

-Joe Gorin; Symmetric Sound Systems; Santa Rosa, Cal.

A Word from the Designer

Dear Editor:

My thanks to Joe for sharing the considerable improvements he's made to the THD Analyzer. I've confirmed all of his points, except the oscillator stability problem. In particular, ICs 1, 10, 11, and 13 are somewhat overcompensated due to a design error on my part (101 was not mentioned by Joe but can be compensated with 4.7 pF).

Correcting the compensation will re duce the need for Q-enhancement compensation. A small capacitance in the range of 0.5 to 2 pF across R5 and R60 can also provide this improvement. A simple way to realize these capacitors is to attach a 1 1/2-inch piece of solid insulated #22 or #24 AWG hookup wire to each end of the resistor and twist as necessary.

Joe and I have been unable to uncover the difference between our analyzers as far as high-frequency oscillator stability. However, I should point out that the oscillator was designed to be stable even with D2 and D3 permanently shorted; only higher quiescent distortion will result.

Several people have written with questions about substituting parts, particularly for the 5534 op-amps, the National 2N4091 J-FETs and the co axial cable. I don't recommend op-amp or J-FET substitution under any circumstances. The coaxial cable is not critical and the capacitance per foot designated in the article is only a guideline. Sets of printed wiring boards are still available from Circuit Works, 1118 7th Ave., Neptune, N.J. 07753, for $35.20, postpaid. About 120 sets have been delivered to date and the feedback I've received indicates there are many highly competent Audio readers out there.

-Bob Cordell; Tinton Falls, N.J.

"Harmonizer" Is a Trademark

Dear Editor:

In the November 1981 issue of Audio, page 31, column 3, in an inter view, it is stated, "... if someone sings a note slightly out of tune, you can use the harmonizer to make it right."

"Harmonizer" is a registered trade mark of Eventide Clockworks, Inc., U.S. No. 1,147,075. It has been the trademark for this pitch change product of Eventide ever since its introduction in the trade many years ago. We recognize the writers, Jon and Sally Tiven, may not have been aware of the trademark status of "Harmonizer," and, if terms of description are needed, we and our competition use "pitch change," "pitch shift," "pitch alteration," and "pitch transposer."

"Harmonizer" is a valuable right of Eventide, and it is our obligation to protect this property right and to pre vent confusion in the trade.

-Orville N. Greene, President, Eventide Clockworks, New York, N.Y.

Company Pride

Dear Editor:

I recently shipped for repair a four year-old expander to dbx, Inc. I in formed them that the unit was not covered by warranty. Dbx promptly re paired the unit and shipped it back to me at no charge for repair or shipping.

In this day and age, when so many companies are ripping off consumers, a company such as dbx, who obviously stands behind their product with pride, should be publicly recognized and commended.

-Frank M. Wolfe; Cocoa Beach, Fla.

Blank Cartridges

Dear Editor:

I greatly applaud Peter Milton's clarification in your March 1982 issue of some of the functional differences be tween the various types of phono cartridges, as well as some of the structural differences. However, it seems to me that the enclosed quotes make claims contrary to a major thrust of Mr. Milton's article (i.e., that "as cartridges get better and better in measured re suits, the tonality differences become vanishingly small"). To quote Matti Otala, "With the present embodiment of these principles, I don't think that there is anyone who would prefer moving magnet." Or, quoting Mitch Cotter, ".... it would seem as though nature made it simpler to get us the orthogonality that we want with the moving coil, and to get more power from a given mechanical impedance." A few further comments: The cartridges used in the National Research Council (of Canada) tests were not, to the best of my knowledge, the state-of the-art contenders in their respective fields of moving iron and moving coil; I wonder to what extent the cartridges were aligned for lateral and vertical tracking, and I also wonder what types of ancillary equipment (speakers, electronics, etc.) were used. Finally, I ob serve that Mr. Milton mentions, but does not deal with, the issue of improved phase response.

-Douglas Weinfield; Seattle, Wash.

Alive and Well

Dear Editor:

In your recent article, "Get the Most Sound from 78s" (June 1982), the author indicated that Lenco variable speed turntables, which effectively play 78s, are "becoming rare." Quite the contrary, Lenco turntables are alive and well and being distributed by Benjamin Electroproducts, Inc., 75 Austin Blvd., Commack, N.Y. 11725. Three moderately priced units, B55, L75 and L78, can be purchased from many audio retail stores.

-Tony Vespoli; Benjamin Electroproducts, Inc.; Commack, N.Y.

Frustrated Builder

Dear Editor:

Over the years your magazine has given me insights, thought-provoking entertainment, humor, dependable product and record reviews and build-it-yourself projects. This last item is something I have looked forward to most eagerly. The Nelson Pass design Class-A amp drives my Klipsch corner horns quite nicely. The Marshall Leach pre-preamp has done justice to my Denon MC cartridge that the Denon AU 320 could not approach.

However, your unexplained and undesired discontinuation of this particular feature has definitely reduced its value to me.

-Kenneth E. Gunnoe, Jr.; Los Angeles, Cal.

(Source: Audio magazine, Feb. 1983)

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