April 1936 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
I tried to find some information
on "direct-impedance amplification," but alas none could be located. At first, I
suspected that it being in the April edition of Radio-Craft magazine might have
meant it was a ruse on readers, especially since author L. Mitchell Barcus
focused on the nearly sub-audible portion of the sound spectrum down to around 16 Hz.
Careful reading of the article reveals, however, that his pièce de résistance was
designing the circuit with a minimum of biasing resistors, capacitors, and inductors
that might otherwise attenuate low frequencies. Success was in the form of direct
coupling of stages sans Rs, Ls, and Cs. I'm guessing the technique was not adopted
by the wider radio industry, except maybe for sound systems installed in aquariums
where whales and dolphins could appreciate faithful reproduction of 16 Hz tones.
Part II of this Direct-Impedance Amplification article appeared in the September
1936 issue of Radio-Craft, but unfortunately I have not acquired that edition
Fig. A - (in heading) the appearance of the three chassis
of the set. Fig. 1, below. The circuit of the set with its three-chassis construction.
This is the first of a series of high-fidelity receivers which will interest
the set builder.
Part I - A 10-Tube Set
L. Mitchell Barcus
It is with considerable pride that this radio receiver is presented to the public
for the first time. Designed especially in response to many requests for a receiver
of moderate price, this circuit is the smallest and "easiest to build" of the series
to follow. Here is an easily-constructed, low-priced radio receiver which will reproduce
a 16 cycle note at a much higher volume level than the balance of the audio range!
In the design of this series of radio sets, all the problems of present day reception
were taken into consideration. Primarily, the serious dropping off of frequencies
below 70 cycles presents the greatest handicap to perfect reproduction. (The author
discussed this point in a preceding issue of Radio-Craft. - Editor) It may be safely
considered that no broadcast station of today transmits a 16-cycle note with even
a near-approach to linearity compared with the balance of the A.F. range. The only
satisfactory manner in which these losses may be retrieved is by employing additional
amplification. The use of boosters or resonant tuned circuits, as in transformer
coupled amplifiers, fails to achieve the ultimate corrective factors. Further, all
other amplifying systems themselves face the same attenuation of the low frequencies
and these resonating circuits at best barely make up for their own inherent overall
In the set described here, the exclusive "direct-impedance" circuit offers amazingly
level response over the entire A.F. spectrum. Thus, given a practically flat 16-cycle
output, the additional pentode amplifier stage brings even weak low notes up to
a satisfactory level. It is this almost unbelievable performance in the lowest frequencies
which gives the set its characteristic mellow depth heretofore unobtainable.
"Tuning Out" Individual Musical Instruments!
The importance of the low-frequency response, unbelievable as it may appear,
is often not so much in the reproduction of the two bottom musical octaves, although
these determine the entire character of an organ, as in the bringing out of the
background noise to which we are accustomed in life. As an example of this we may,
on the larger models which incorporate a low-frequency volume control, cut out the
low response on a xylophone solo. While we have thus attenuated the lows below the
effective range of the instrument, we still have the normal high-fidelity performance
of contemporary radio receivers, a lifeless though accurate reproduction. By restoring
the background, which may sound only like a series of soft thumpings and indefinable
noises, the solo assumes a solid and convincing reality completely unlike the bare
To many, having never heard a low note on a radio set, the effect of the two
bottom musical octaves is almost inconceivable. The entire depth usually associated
with an organ is dependent on these vibrant notes and their absence alters the rich,
soft music to the thin, piping effect normally heard on even the best of radio sets.
Simplicity of Construction
Should it seem that the construction of this receiver is difficult, it may be
well to bear in mind that actually it offers fewer problems than many conventional
sets. Both power units are extremely easy to build, being little more than 2 speaker-field
The receiver itself is most conventional and is not designed to offer 'round-the-world
reception 'and other features of no consequence to high-fidelity reproduction. The
incorporation of some method of visual tuning indication is of value to builders
who do not object to the slight additional expense. The usual 2nd I.F. stage has
been omitted to cut down background noises and to offer less side-band cutting.
While the variable-coupled type of I.F. coils have not been specified in this receiver,
their use is not objectionable.
It is by no means fatal to vary the layout given in the heading illustration.
As shown, however, there are several basic principles to which the builder must
adhere. In the receiver unit, for example, it may be noticed that the power transformer
(L) occupies the left corner with the 2 filter chokes (J and K) ranging along the
left end. This placement segregates the power supply from the balance of the chassis,
especially the two A.F. chokes (G and H) which may be seen behind the tuning condenser.
The units in the heading photo correspond as follows with the symbols in Fig. 1:
1-V1; 2-V2; 3-V3; 4-V4; 5-V5; 6-V8; 7-V6; 8-V9; 9-V7; 10-V10; A - aerial coil; B
- R.F. coil; C - C2; D - oscillator coil; E - first I.F. transformer; G - Ch.1;
H - Ch.2; J, K - Ch.3 and Ch.4; L - Pt.1; M - Pt.2; N - Ch.5; P - Pt.3; R - Ch.6.
The mention of induced hum naturally calls for added discussion of that phase
of construction. In view of the tremendous low-frequency gain of the amplifier,
it might seem that the elimination of hum would be a serious and difficult obstacle,
but such is not the case.
It is unnecessary to purchase heavily-shielded A.F. chokes to arrive at completely
hum-free performance. The fact that considerable current is flowing through the
windings limits the inductive pick-up.
The uninitiated constructor would very likely connect units 1 and 2 with a 4-wire
cable. Nothing could be more disastrous to humless performance. It is always essential
that the plate-to-grid lead be separate from the rest of the cable, but may be wired
to the same plug and allowed to hang loosely from the cable.
The Low-Frequency Unit
The low-pass filter which precedes the final 2A5 is so adjusted as to pass only
notes below 70 cycles, to any extent. As shown in the diagram, it is composed of
(a) a small A.F. transformer with the primary shunted and (b) a bypass condenser
of unknown value. This transformer is usually one of the low-priced universal replacement
units of about 3-to-1 ratio and the condenser varies in value depending on the set-up
used. It frequently has a value of about 0.25-mf., but may vary from this value
to as high as 0.5-mf. or down to 0.1-mf.
In addition, a condenser is placed across the output transformer to resonate
it to these low frequencies. This, too, depends on the speaker. The correct value
for maximum performance is found by experimentation. It usually runs from 0.5- to
List of Parts
One 3-gang tuning condenser, 350 mmf., C;
Four Cornell-Dubilier condensers, 0.1-mf., C1;
Eight Aerovox condensers, type GM, 8 mf., 525 V., C2;
One Aerovox mica condenser, type 1467, 370 mmf., with Hammarlund EC-35, C3;
One Aerovox mica condenser, type 1467, 0.001-mf., C4, C8;
Two Aerovox mica condensers, type 1467, 100 mmf., C5;
One Aerovox mica condenser, type 1467, 250 mmf., C6;
One Aerovox mica condenser, type 1467, 500 mmf., C7;
One Aerovox condenser, type P-5, 8 mf., 525 V., C9;
Two Cornell-Dubilier condensers, 0.5-mf., C10;
One Cornell-Dubilier condenser, 0.25-mf. C11;
Two Cornell-Dubilier condensers (see text), C12. C13;
Two Centra lab carbon resistors, 500 ohms, 1 W., R1, R5;
One Centralab carbon resistor, 400 ohms, 1 W., R2;
One Centralab carbon resistor, 25,000 ohms, W., R3;
One Centralab carbon resistor, 30,000 ohms, W., R4;
Two Centralab carbon resistors, 1 meg., 1 W., R6;
One Centralab carbon resistor, 50,000 ohms, 1 W., R7;
One Centralab potentiometer, 1/2-meg., R8;
One Electrad voltage-divider, 15,000 ohms. R9;
Two Electrad resistors, 25,000 ohms, 10 W., R10, R11;
*Two A.F. chokes, type T-3736, Ch.1, Ch.2;
*Three filter chokes, type T-1607, Ch.3, Ch.4, Ch.5 ;
*One A.F. transformer, type T-2999, Ch. 6;
*One power transformer, type T-7021 supplying 700 V. C.-T., 6.3 V., 5 V., 2 1/2
*Two power transformers, type T-4900 supplying 700 V., C.-T., 2'h V., 5 V., P.T.2,
One Wright-DeCoster speaker, model 790, 2,500-ohm field, single 2A5, reproducer
One Wright-DeCoster speaker, model 1,980, 2,500-ohm field, single 2A5, reproducer
Sockets, speaker plugs, chassis, etc.
Two Sylvania type 78 tubes, V1, V3;
One Sylvania type 6A7 tube, V2;
One Sylvania type 85 tube, V4 ;
One Sylvania type 56 tube, V5;
Two Sylvania type 2A5 tubes, V6, V7;
Three Sylvania type 80 tubes, V8, V9, V10.
*Name of manufacturer will be sent upon receipt of stamped and self-addressed
Posted September 18, 2023
(updated from original
post on 1/23/2018)