Futterman H-3A power amplifier Manufacturer's Comment

Manufacturer's Comment
We did not design the H-3A specifically for use with the KLH Nine. In fact, monophonic versions of the basic design were in use 15 years ago, mainly with the RCA LC-IA loudspeaker, with which it gave superb results.

There is one coupling capacitor in each channel of the H-3A; computer-grade 800µF capacitors are used to isolate each loudspeaker from the output tube circuitry. In our opinion, direct-coupling a power amplifier to a loudspeaker (especially with transistor amplifiers) is inherently dangerous to the speaker system. In fact, some of the latest high-powered transistor amplifiers use elaborate and expensive circuitry to protect the loudspeaker from damage due to the direct coupling should some "unlikely" defect occur in one of the output stages. We emphasize this point because the subjective listening results that Stereophile obtained in the low-bass region may well be due to the size of the coupling capacitors used in the H-3A that was tested. We ore currently experimenting with much larger values of capacitance, as this is the only part of the H-3A's entire circuit that could affect the very low bass response.

We have found, and eliminated, the ground loop that was causing hum from three-terminal pickups.

Reviewer's Addendum
We, too, had wondered if the output coupling capacitors might not be responsible for the slight rolloff at the extreme bottom, but could find no conclusive objective evidence to support this, There was some tilt on bass squarewaves, but no more so than from other top-grade amplifiers, and the measured frequency response was within 0.5dB of flat down to a bit above 25Hz. On the other hand, since there isn't anything else in the circuit that could affect low end, it is quite likely that increasing the size of the coupling capacitors will indeed improve the low-bass response.

COMPANY INFO
Harvard Electronics Co.
New York, NY 10025 (1967)
Company no longer in existence (2019)
ARTICLE CONTENTS

COMMENTS
John Atkinson's picture
I chose this review to republish on the website because this was the very first example of that extraordinary beast, the output-transformer-less tube amplifier, that I encountered!

John Atkinson
Technical Editor, Stereophile

Ortofan's picture

... an output transformer or a coupling capacitor?

As an experiment, buy a pair of "computer-grade" 800µF aluminum electrolytic capacitors, connect one in series with each channel's output of your amplifier and determine what effect, if any, they might have on the sound quality.

scottsol's picture

You should have realized, and I have verified this from the Acoustat .comments, that publishing old material is a colossal waste and that nobody could possibly be interested.

John Atkinson's picture
scottsol wrote:
You should have realized, and I have verified this from the Acoustat .comments, that publishing old material is a colossal waste and that nobody could possibly be interested.

With all due respect, the page views stats show that you are wrong.

John Atkinson
Technical Editor, Stereophile

scottsol's picture

My post was intended as a satirical play on that ridiculous comment in the Magnepan 20 thread. The Acoustat reference was a mistake as I had just posted about the 2+2.

michelesurdi's picture

how time does fly.nowadays it is quite common to suggest that tube amps apparently deli ver more than their rated power.

Bogolu Haranath's picture

Manufacturer's specs say ......... IM distortion <0.05% at 90W continuous into 16 Ohms ....... S/N ratio better than 93 db below 90 Watts ....... Damping factor 200 ........ SNR of 93 db is approx. 15 to 16 Bit resolution ....... Can an OTL tube amp achieve a damping factor of 200? ....... Beast indeed :-) ..........

Bogolu Haranath's picture

Certainly, Futterman was ahead of his time, prepared for future-proofing ......... He was thinking about hi-res audio in the 60s, with that kind of SNR :-) ........

T-NYC's picture

The amp was ideal with QUADs if you told Julius that's what you planned to use. He adjusted the output to lower wattage running pure class A. He also strongly recommended Rogers LS35A (15 ohm) for use with this amplifier.

Herb Reichert's picture

they were broken I fixed them (new caps and tubes) tested them and hooked them to both Quad 57s and LS3/5a. For about 48 hours I was completely taken with their midrange. Then I sold them quick while they were still working. lol

hr

Ortofan's picture

... Hoovie's Garage approach to Hi-Fi.

Bogolu Haranath's picture

Wonder whether HR also has a photo-album diary? :-) ........

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