What is Single Ended
From reaike–(at)–x.netcom.com Thu Nov 9 15:55:37 CST 1995
From: reaike–(at)–x.netcom.com (Randall Aiken )
Newsgroups: rec.audio.tubes
Subject: Re: Please explain “single ended”
Date: 9 Nov 1995 02:58:53 GMT
X-NETCOM-Date: Wed Nov 08 6:58:53 PM PST 1995
In <475knc$3k--(at)--ewsreader.wustl.edu> wilhel–(at)–akima.wustl.edu (Wilhelm
Reich) writes:
>
>What does this mean in amplifier terminology? Thanks
Single-ended refers to the output stage topology. The two primary types
of output stages for audio are single-ended and push-pull. The main
difference is the output transformer. Other differences are explained
below.
In a push-pull amplifier the power supply is connected to the
center-tap of the transformer and a tube is connected to both the upper
and lower end of the center-tapped primary. This allows the tubes to
conduct on alternate cycles of the input waveform. A push-pull stage
can be biased class A, where current flows in both tubes for the entire
input cycle, or class AB, where current flows alternately in both
halves, but less than a full cycle in each, or class B, where current
flows only half the time in each tube. Most designs are biased class
AB for best efficiency and power output with minimal crossover
distortion (but not necessarily best “tone”). A push-pull stage
requires at least two tubes to operate, but can have more connected in
parallel with each side, resulting in an amp with four, six, or even
eight output tubes for higher-power amps. Other advantages of this
circuit are: there is little or no DC current in the output transformer
if the tubes are matched and the stage is balanced, since current flows
in opposite directions to each tube, allowing a smaller transformer
with less iron (translation: cheaper). Also, even order harmonics and
distortion products generated in the output stage are cancelled out
(translation: good for hi-fi buffs, bad for guitar players!) In
addition, power supply hum is cancelled out, allowing less filtering of
the supply. This stage generally clips symetrically, resulting in more
odd harmonic distortion. One disadvantage is that the DC supply current
changes dramatically between off and full signal, requiring heavier
filtering to prevent supply “sag”, unless that is what you are looking
for, as in guitar amplification.
A single-ended output stage is always run class A (unless you are
talking radio transmitters or a “SEPP” – see below). It’s output
transformer primary is not center-tapped, having only two connections.
One connection goes to the power supply, the other to the plate of the
power tube or tubes. Tubes can also be paralleled for more power as in
a push-pull stage. This is the type of output stage used in the
venerable Fender champ guitar amplifier and countless millions of early
radios and tv’s. It is making a comeback in high-end vacuum tube audio
as a single-ended triode stage, which supposedly is the ultimate in
sound reproduction. As inefficient as it is good sounding. Another
problem with this type of stage is the transformer must handle a
continuous DC current. This results in a physically larger and more
costly output transformer, which must be gapped or made of special
material such as MPP to prevent saturation of the core. Disadvantages
include: no rejection of power supply hum; no rejection of even order
harmonics (a great advantage to guitar players!); generally asymetrical
limiting on overloads which further emphasizes even order harmonics
(which are more pleasing to the ear than odd order harmonics).
There is another type of single-ended stage called a “single-ended
push-pull” or SEPP. This is similar to a totem-pole MOSFET or
non-complementary transistor output stage. One tube is run from a
positive supply on its plate and is configured as a cathode follower,
the other tube has a negative supply on it’s cathode. The transformer
(if used) connects to the between the center of the two tubes and
ground. Heavy negative feedback is used to assure linearity and
minimal crossover distortion. This type of configuration is also used
in the OTL or “output-transformerless” tube stages which are scarce as
hen’s teeth, probably due to the difficulty of matching the tube
outputs to low-impedance speakers, and the danger of DC on the
speakers. Output impedance of tubes (even in the cathode follower
configuration) is high compared to solid-state devices. The heavy
negative feedback reduces this output impedance to usuable levels, as
well as maintaining the DC output at zero.
Hope this answers your questions and didn’t put you to sleep!
