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The moving-coil pre-preamplifier which I described in the February, 1978, issue of Audio has a gain that seems to be dependent not only on the manufacturer of transistors Q1 and Q2, but also on the particular batch of transistors from a given manufacturer.
The parts list and specifications for the article were based on a set of 25 transistors each for Q1 and Q2 from a particular batch manufactured by Motorola.
It is a simple matter to re-bias the circuit for any pair of transistors for Q1 and Q2. The value of R2 should be selected to give a battery current of 150 to 160 microamperes with a fresh battery for 131. Once the proper value for R2 has been selected for each channel, it will not be necessary to change it again unless the transistors are changed. My experience has shown that R2 should be between 330 kilohm and 560 kilohm, depending on the parameters of the transistors. It is a simple matter to determine the correct value for R2 by substituting a 1 Megohm potentiometer for this resistor. After adjusting the potentiometer for the correct battery current, it can be removed and measured with an ohmmeter. A standard 5 percent carbon-film resistor is sufficient for R2.
The battery recommended is an Eveready No. 522 alkaline battery which measures exactly 9 volts when new.
The kit supplier for the pre preamplifier will supply the correct value for R2 with each kit. Matched NP-N/P-N-P transistor pairs will be supplied with the correct value of R2 specified for each pair.
-W. Marshall Leach; Georgia Institute of Technology School of Electrical Engineering Atlanta, Georgia 30332
Audio Magazine is to be complimented for publishing the W. Marshall Leach construction articles on the audio preamplifiers and pre preamplifiers. The preamplifier is a particular success. I have constructed several copies of this preamplifier and found it to be as good as, or better than, any commercial preamplifier that I have had the opportunity to use and test ... this includes units costing $1000.00 and more.
This circuit can be built for under $50.00 and works excellently. I have made a version to replace the preamplifier circuit board of the older Marantz 7T, as well as using the unit as an independent preamp to feed other control and power amplifiers.
The circuit has low noise, matches the RIAA curve very well, will put out over 35 volts peak-to-peak at all frequencies up to 120 kHz, and overloads in a symmetrical and well-controlled manner. The overload at the input is as close to perfect as can be made with the current state of the art.
I would not like to leave the impression that a $500.00 preamplifier is not worth the money ... but one should realize that he is not paying for some secretor magical engineering breakthrough (that defies the laws of physics and electricity) but rather for the quality of the parts, the labor to assemble them, the packaging and the profit for the manufacturer, the wholesaler, and the retailer, in addition to the warranty and repair services.
Essentially the same technology is available to everyone. This includes parts, active devices, and the rest.
Thus, the good construction articles. Audio magazine has provided enable the serious constructor of circuits to take advantage of the engineering work done by the author of those articles. This is a service I would like to see Audio magazine expand.
-R.A. Greiner; Professor of Electrical Engineering, Univ. of Wisconsin Madison, Wisc.
Editor's Reply: Thank you for your kind words about Mr. Leach's construction articles. We would like to publish more of these quality construction pieces, but this has been hampered by the increasing sophistication of designs offered commercially, as well as the inability of the home constructor to match the sophisticated construction techniques of the large manufacturers.
In any case, we do intend to continue to offer such articles, and we welcome inquiries from potential authors and suggestions from those interested in particular projects.
(Source: Audio magazine, May 1978)
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