November 1957 Radio & TV News
[Table
of Contents]
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
acknowledged.
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"The term, positive current feedback, is disturbing to some because,
as is well known, positive feedback increases the distortion of
an amplifier to which it is applied. This is true in this application
also, but it must be noted that the net feedback applied to the
amplifier is never positive but simply less negative in the region
where the positive current feedback is effective." Wow, that is
a lot like what politicians refer to a 'baseline
budgeting.' When one political party says it is going to 'cut
the budget,' while the other complains that doing so will 'starve
children' and 'hurt women,' what it really means is that spending
will not be increased this time as much as what had originally been
planned, although it will actually be higher than the last time.
We know the government never actually spends less money one year
than it did the year before, but there are a lot of 'low information'
people who never suspect a thing - just keep the welfare checks
and food stamps flowing.
A New Look at Positive Current Feedback
By H. D. Zink and L. R. Sanford
Tests show that positive current feedback improves hi-fi systems
which already have good loudspeakers providing the correct feedback
circuit is employed.
The
use of current feedback to provide improved bass response in a high-fidelity
speaker system has caused a lot of discussion pro and con. It has
been argued that it cannot greatly improve speaker damping because
the mechanical parts of the speaker are not coupled closely enough
to the electrical parts.1 It has also been argued that
it might help on an inadequate speaker system but that it was worse
than useless on a truly high-fidelity speaker.2 On the
other side, curves have been presented which give dramatic proof
of the improved damping obtained with a particular kind of current
feedback,3 but few details are given about the speaker
system used and, therefore, no adequate conclusions can be drawn.
This difference of opinion is understandable since the most desirable
mode of loudspeaker operation for the best listening is not agreed
upon even by experts in the field. The only way an individual can
determine for himself the merits of such feedback is by the use
of his ears.
Listening tests in commercial demonstration rooms are not necessarily
conclusive for two reasons; (1) many of the current feedback circuits
are so arranged that a common ground between the amplifier input
and a speaker lead destroys the feedback network and common grounds
are frequently used in demonstration rooms, (2) many amplifiers
provide only for negative current feedback which can only decrease
the damping on a speaker and thereby accentuate its undesirable
characteristics. Therefore, opinions formed by a brief listening
test in a demonstration room may not be valid about current feedback.
The question in the minds of the authors was whether positive
current feedback (that which increases speaker damping) could add
anything to a truly high-fidelity system which already had good
speakers. The results of tests showed the answer to be conclusively
yes, if the correct kind of feedback circuit were used. However,
the feedback configuration most suitable was different from those
heretofore used and for best results different speaker enclosures
required somewhat different configurations.
What is actually accomplished with current feedback can be best
understood by forgetting the ideas of damping, negative impedance,
etc. for the moment and concentrating only on the frequency response.
Anyone who has heard an audio oscillator through any speaker system
has probably observed that while the response may be poor below
a certain frequency, frequencies much lower than this can usually
be reproduced if the power to the speaker at these frequencies is
increased relative to the higher frequencies. Often, when this is
done, appreciable harmonic distortion is present and the speaker
cone rattles. Usually for music system use, if the bass output from
the speaker is increased by conventional bass-boosting techniques,
then such distortions are objectionable. In the optimum use of positive
current feedback these objections to low-frequency boosting are
overcome by using a rising bass characteristic as part of the feedback
network. This compensates for the loss of low-frequency acoustic
output without the harmful effect noted, since the positive current
feedback keeps the speaker cone under control and, thereby, significantly
reduces the distortion which would otherwise result.
In some cases, the frequency below which no acoustic output at
all is obtained is actually lowered.

Fig. 1. Effect of positive current feedback on the frequency
response of amplifier.
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The term, positive current feedback, is disturbing to some because,
as is well known, positive feedback increases the distortion of
an amplifier to which it is applied. This is true in this application
also, but it must be noted that the net feedback applied to the
amplifier is never positive but simply less negative in the region
where the positive current feedback is effective. See Fig. 1. The
slight increase in distortion, which results from the decrease in
the amount of negative feedback applied in the bass region, is more
than offset by the decrease in speaker distortion in the same region.
In the high-frequency region, where amplifier distortion is more
disturbing, no positive feedback is applied and the amplifier characteristics
remain unchanged. The important point to note is that positive current
feedback applied to the amplifier is effectively negative feedback
as far as the speaker cone is concerned. This point is not obvious,
so the following experiment will be described to suggest why this
is actually the case.
Arrange a speaker, battery, multi-range ammeter, and a switch
as shown in Fig. 2. With the switch on the "A" contacts so that
the battery is out of the circuit, push the speaker cone in the
minus direction and note the direction of the current generated
by the movement of the voice coil through the speaker field. Assume
this current flows in the direction of the arrow I. Now connect
the battery through the "B" contacts so that the current which it
causes to flow is also in the direction of the arrow, and note the
direction in which the speaker cone moves. It is found that the
speaker cone moves in the plus direction, that is, in the opposite
direction from which it was moved in the first case. The current
which acts on the speaker results in a plus motion of the cone whereas
a minus motion of the cone produces the same direction of current
when the cone reacts on the circuit. This means that when the cone
oscillates after a driving signal has ceased, the current generated
by the erroneous motion can be fed back through the amplifier to
produce a driving current which will be in the same direction as
the error current and that this current will drive the cone in the
opposite direction. The net effect will be that the cone moves very
little after the original driving force ceases. It can thus be seen
that in order for the forces on the cone to cancel out (negative
feedback) the error signal must be fed back without a change of
phase (positive feedback).

Fig. 2. Experimental setup that is used to determine
how positive feedback works.
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When these facts are realized, the correct application of positive
current feedback to any speaker system then becomes merely a matter
of cut and try until the right boost characteristic is found. Since
no electrical measurements can indicate the total effect, the final
results must be reached by listening tests. The correct results
are achieved when the speaker has a deeper bass than it has ever
reproduced before - without any trace of boominess. A very good
test is when low-level, low-frequency bass notes, such as the light
tap of a tympani, bass drum, or soft organ pedal, are clearly evident
without being boomy or muffled. An excellent demonstration of the
effect of positive current feedback was given when the low-frequency
response of a Klipschorn was extended from 27 cps to below 20 cps
with a clean fundamental response. The difference in reproduction
of a complex 16 cps organ tone before and after was quite impressive
and easily noticed even by the untrained ear.
The block diagram of the current feedback network used by the authors
is shown in Fig. 3A. The essential difference between this circuit
and similar ones used on commercial amplifiers is that no provision
is made for negative current feedback, and an LC circuit is used
in the frequency discriminating section of the feedback network
instead of a single capacitor. It is necessary to use an LC circuit
because the single capacitor gives too much bass boost in a region
where no boost is needed when used with some speaker systems (especially
the Klipschorn and "Rebel" series). This results in an unpleasant
over-accentuated bass sound and is probably the reason some have
rejected the use of current feedback with high-quality speakers.
The 25-ohm potentiometer shunted across the 1-ohm resistor provides
a means of varying the feedback from zero to full positive. Its
use, except tor comparison purposes, is questionable since usually
full positive feedback is the most desirable condition. It could
be omitted with no harmful effects in which case the 240-ohm resistor
is tied to the ungrounded end of the 1-ohm resistor.

Fig. 3. (A) Location of positive current feedback network.
(B) shows Klipschorn network. (C) shows Rebel 4 LC network.
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The amount of feedback and therefore the degree of bass boost may
be varied in several ways aside from the use of the potentiometer.
The principal way is by changing the value of the 1-ohm resistor.
It will be noted that a dividing network is formed between the speaker
impedance and the current feedback resistor, such that if the speaker
is high impedance (16 ohms) less feedback voltage will be developed
across the resistor than if the speaker impedance is low (4 ohms).
That is, for a given resistor more bass boost would be obtained
when feeding a 4-ohm speaker than when feeding a 16-ohm speaker.
The 1-ohm resistor has been found satisfactory when used with a
speaker system having a net impedance of 4 ohms and, therefore,
in some instances a 4-ohm resistor might be desirable for a 16-ohm
speaker system.
The amount of feedback and, therefore, the amount of boost can
also be changed by changing the "Q" of the circuit elements used
in the feedback network. The values called for usually require electrolytic
capacitors and if these units are leaky or are used singly instead
of in series pairs back-to-back, then less feedback will be obtained
than would be expected. If the inductor used is variable, its "Q"
will vary as it is tuned and this will also change the feedback.
It should be noted that since the resistance in series with the
speaker absorbs power it represents a loss in peak output, therefore,
it is desirable to keep it as small as possible while still obtaining
the required feedback voltage. Since high "Q" elements in the feedback
network represent more voltage feedback than do low "Q" ones, they
are to be preferred unless they give a boost characteristic that
rises too sharply. This is an unlikely occurrence. It should be
noted that the characteristics of the network, when not connected
in the feedback loop, are not a good indication of the overall amplifier
response when the network is in the loop since a "Q" multiplication
effect is obtained and the amplifier response is sharper than the
network response.
To determine the constants of the LC network shown in Fig. 3A,
procure an audio oscillator or frequency test record whose range
is slightly lower than the lowest range of interest and listen to
the performance of the speaker system using a conventional negative
feedback amplifier. Note: (1) the frequency at which the bass response
just begins to roll off and (2) the frequency at which no more acoustic
output is obtained irrespective of how much power is used to drive
the speaker. An LC network having a low-pass or bandpass filter
configuration is then designed so that the upper turnover frequency
occurs slightly above the frequency at which the response starts
to roll off and the peak response occurs slightly below the frequency
at which no output is normally heard. (The hypothetical termination
resistance necessary for calculating the filter sections can be
assumed to be about 600 ohms since it has been found experimentally
that this value gives networks that are satisfactory.) This network
will serve as a starting point and by varying the parameters while
listening to the system using an audio oscillator or tone record
the best sounding arrangement can be determined. For those not technically
able to perform such calculations, the networks to be discussed
will give moderately good results on any speaker system and will
serve as a starting point for more experimentation. It is not advisable
to use music for the first tests since low-frequency tones occur
rather infrequently and are of rather short duration so it is difficult
to notice the effect of circuit changes.
The specific LC circuit configuration used with a Klipschorn
is shown in Fig. 3B. This type of enclosure normally falls off below
27 cps so the feedback network is designed to become effective in
this region and to provide 10 db of boost at 20 cps as measured
across a resistive load. The response curve of the amplifier, when
this circuit is used, is shown in Fig. 4A. It must be remembered
that this curve was taken with a 16-ohm resistive load substituted
for the speaker and does not necessarily represent the actual boost
curve obtained with the speaker connected. In this case, sufficient
feedback is obtained from a 1-ohm resistor even though the speaker
is 16 ohms. Listening tests with an audio oscillator indicate that
this amplifier-speaker system appears to be acoustically flat to
below 20 cps.

Fig. 4. (A) Amplifier response with networks shown in
Fig. 3B and (B) Fig. 3C.
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Fig. 3C shows the network found best for use with the Klipsch "Rebel
4" enclosure. In the specific case considered here a G-E A1-400
speaker is used in the "Rebel 4," but the same circuit is also used
on a "Rebel 4" with a much cheaper speaker and gives excellent results.
The configuration is different from that used with the Klipschorn
because more boost is required and it was found that a network that
gave a steadily rising bass characteristic, such as used with the
Klipschorn, caused the amplifier to motorboat when the feedback
was increased to the correct point. This was because, when enough
positive feedback was provided in the required region, all of the
negative feedback was cancelled out at some lower frequency and
the net amplifier feedback became positive in this region and caused
the oscillation (see Fig. 1). This condition is avoided with the
configuration shown since it is arranged to peak at the lowest usable
frequency and then fall off below this point so that the amplifier
has almost full negative feedback in the critical motorboating frequency
range. This configuration also largely eliminates thumps that occur
when tuning through FM stations. Curves obtainable with this configuration
are shown in Fig. 4B and it must be noted that these curves also
were taken with a resistive load in place of the speaker. The solid
curve was found most suitable in this case and the low-frequency
response of the "Rebel 4" enclosure with the G-E A1-400 speaker
was extended from 40 cps to slightly below 30 cps. It seemed as
if several slight peaks in the response were also eliminated.
An attempt has been made to explain positive current feedback
from a different viewpoint than is normally used and it is hoped
that this article will clear up some of the controversy surrounding
the subject. Three high-fidelity systems have been in use for almost
one year with the networks described and at no time have any unpleasant
results been observed nor has any listening fatigue been felt. Using
these circuits on the Klipschorn and "Rebel" series enclosures gives
a life-like bass that is only equaled by the most elaborate systems
that are available.
References
1. Crowhurst, N. H.: "What's All This About
Damping," Audio, September, 1955
2. Klipsch, Paul: "Variable Damping," Radio-Electronics,
October, 1956
3. Wilkins, Charles A.: "Variable Damping
Factor Control," Audio, September, 1954
Posted September 26, 2014 |