Dear Editor (Jan. 1976)

Better AM Sound?

Dear Sir:

As Chief Engineer of the dominant AM radio station in our area, I take a great deal of pride in our sound. How that signal travels through miles of telephone lines, processing equipment, transmitter, out to the antenna, is important to me. According to the FCC this is where my responsibility ends. But does it really? I know we are broadcasting within 2 dB up to 8 kHz. How many AM receivers do you know of (reasonably priced) that "hear" much higher than 4 or 5 kHz? Since it's difficult to get wide bandwidth in receivers without sacrificing selectivity, how about a receiver with a two- or even three-position bandwidth switch? This would give audiophiles a choice between selectivity and fidelity. After all, selectivity is rarely a problem when listening to urban signals.

I want to do my job well, but it would be much more rewarding if someone else did too.

-Paul Swartzendruber; WTON Stauton, VA.

Any other broadcasters out there care to comment?-Editor Marantz 9 Modification Dear Sir: A number of people have written to Audio about modifications made to Marantz Model 9 tube-type power amplifiers, which have been mentioned from time to time in the amplifier reviews. The modification is a simple one, which amounts to a switch bypassing of the input tube that is used as an input level control buffer and phase inverter.

To make the modification, first remove the bottom screen of the amplifier. With the amp positioned upside down, locate the phase toggle switch.

Note that two wires come from the phase switch and terminate on the front-end board which is adjacent to the side of the amp and partitioned off by the shield. One of these wires is the plate signal of V1A and doesn't have a 24 kOhm, 1/2-watt resistor associated with it on the front-end board. Remove this wire from the unit.

Connect a 6.8 kOhm, 1/2-watt 100 ohm resistor from the now empty terminal of the phase switch to the top or wiper of the input volume control. The top of the volume control is an outside terminal with the inner conductor of a shielded wire connected to it. The wiper is the center terminal.

The preferred connection is to the top of the control or actual input in order to avoid a slight high frequency loss with the control set 6 dB down.

The action of the low-cut filter is retained with the phase switch set to Normal. When the switch is set to Reverse, V1A is bypassed, giving an audible improvement in the amplifier's sound. No difficulty has been encountered in modifying four amps thus far. Any other further improvement or new modification will be reported in the future.

-Bascom H. King; Geoffrey T. Cook

TIM Distortion--An Addendum

Dear Sir:

Since it has been impossible for me to answer in detail the many letters I have received in response to my recent article (February Audio) on TIM distortion in power amplifiers, I would like to make several statements which may clarify some questions which many readers have communicated to me. Although the slew rate, power bandwidth, and TIM distortion can all be related to the transient and high-frequency performance of an amplifier, it is impossible to predict one from the other in general. However, a poor slew rate or power bandwidth will usually always indicate TIM distortion. The complex relationships between these three parameters is determined by the specific design of the amplifier.

Let us consider two hypothetical amplifiers. Suppose each exhibits a nominal distortion level of 0.01%. Let the two amplifiers have a feedback ratio of 80 dB and 28 dB, respectively. It follows that the inter-loop distortion level is 100% for the first and 0.25% for the second. Which amplifier represents the best design effort? Now suppose the first amplifier is heavily lag-compensated and that it will produce full power at 20,000 Hz at 0.1% THD. Since the open-loop gain at 20,000 Hz is decreased markedly compared to its low-frequency value by the lag-compensation capacitors, it follows that the feedback factor has also markedly decreased. If the increase in distortion at 20,000 Hz compared to the low-frequency distortion does not match the decrease in feedback factor, it follows that the lag-compensation capacitors are filtering out the high-frequency distortion harmonics inside the loop. Now consider a high-level, high-frequency signal in conjunction with a lower frequency signal. The low-frequency intermodulation products between the two signals will not be filtered out by the lag compensation networks. This distortion mechanism will produce TIM. As a final consideration, let us look at the damping factor. Suppose it is 1000 for the first amplifier and 100 for the second amplifier, both measured at 50 Hz. Suppose the open-loop bandwidths of the two amplifiers are 100 Hz and 40,000 Hz, respectively.

At 20,000 Hz, the damping factor of the first amplifier will be 5, while it will be 100 for the second. This represents an increase in output impedance in the first amplifier by a factor of 200, while that for the second remains constant. Will this effect be audible with high frequency transient signals when electrostatic speakers are being used whose input impedance decreases with frequency? There are many more contrasts which we could make between amplifiers with narrow and wide open-loop bandwidths. I hope these examples have answered some questions and provoked further thought on the subject.

-W. Marshall Leach; Assistant Professor School of Electrical Engineering; Georgia Institute of Technology

Defective Albums

Dear Sir:

Several of my audiophile friends and I have over the years purchased records which were obviously re-sealed and had serious defects.

On visiting a number of large record stores in the Bay Area of California, I have noticed back-room heat sealers being used, ostensibly to seal records from companies or distributors which had not previously sealed their products in shrink-wrap; however, I suspected that a fair number of returns are unethically resealed and sold again. Since the manufacturers will give credit for defective discs, there is no reason for a store to argue with a customer about a defective album or to re-sell it.

Therefore I would suggest to the industry that it become a mandatory practice for the store to immediately and without argument accept a returned defective record and write "defective" in felt marker on the label and jacket, or to use some kind of punch to identify the album permanently.

I spend many hours each month in record dealers, and find very few customers who are returning their purchases; the small, critical minority who do demand quality should therefore be treated courteously and honestly.

-Stephen R. Waldee, Broadcasting Technical Consultant, Los Gatos, Calif.

(adapted from Audio magazine)

= = = =