About
single-ended amplifiers
Single-ended
class A amplifiers belongs to the elite family of tube amplifiers.
Nowadays the famous Hi-End manufacturers like Jadis, Audio
Note, Cary...etc all have their top model based on 300B single-ended
amplifiers. The most famous vintage single-ended amplifier
belongs to Western Electric's WE 91A, which was used for theatre
and public PA application. They began in 1935 using a single
300B (early models were using 300A) with two 6C6 pentode and
an input transformer to accept balanced input. Since the 300B
has very good linearity, coupled with the simple class A single-ended
circuit the harmonic distortion generated at the rated output
is relatively small.
Usually
at the maximum output the total harmonic distortion is less
than 5% and they are mainly second harmonics. Matching with
greater than 90dB speakers it generates rich musical sound
and transparency. This is especially true when playing vocals
and small chamber classical music. Before 1946 most theatre
amplifiers made in the US were using triode as the power
tube. Standard frequency response was 80Hz to 10KHz and
the output power ranging from 10 to 16W. They were used
for driving horn speakers. In the late 60s, the development
of single end class A amplifier originated from Japan and
since then the popularity of using it with high efficiency
speakers increased.
The
tube used were mainly directly heated triode 300B, 2A3 and
845. Among them 300B has the best sound. This quickly spread
to Europe and then the whole world with this type of amplifier
being regarded as an all time classic. The top models in
Opera Audio's amplifier line up like the M500, M500S, J500SE
and J400 are all based on the 300B single-ended class A
design. Nowadays single-ended class A amplifiers are everywhere.
Not only the speaker sensitivity can now be made easily
greater than 90dB and the improvement of components quality,
the most important reason is the incredible sound reproduction.
Single-ended
amplifier's circuit is very simple. For a power amplifier
usually there is a voltage amplification stage, driver stage
and power amplification stage. The use of only one driver
tube and one power tube in a single-ended amplifier (like
the M500) is even more simple. The change in sound is almost
due entirely to the power tube and the output transformer.
Single-ended amplifier can also use two power tubes in parallel
to increase the output power (like the M500S and J400).
In that case the output power is doubled and the output
impedance is reduced by half. The driving voltage is the
same but the distortion is a bit bigger than using only
one power tube. Up to a certain degree, better linearity
of the power tube and high quality of the output transformer
has the most important effect on the sound of single-ended
amplifiers. Due to the output transformer of single-ended
amplifiers always has a large amount of DC flowing through,
in order to avoid distortion caused by magnetic saturation
of the iron core, there should be air gap in the iron core.
To obtain a good low frequency response, the size of the
iron core also needed to be increased. Usually the size
of a well designed output transformer of a single end amplifier
is physically 3 to 4 times bigger than a similar output
transformer designed for push pull..
Due
to this reason, the cost of single end output transformer
is therefore more expensive. The single-ended output transformer
used by Opera Audio are using high grade silicon steel and
reasonable design to enable good low frequency response.
At the same time optimized for high frequency distortion
and sound characteristic. Special fixation frame is used
and the whole assembly is dipped into special wax to maintain
a constant air gap which is vital for a good output transformer.
