CHAMP ELECTRONICS -" THE VALVE AMP
HOSPITAL"
NOTTINGHAM, ENGLAND
SERVICING OF A MUSICIAN OTL
Hi-Fi
AMPLIFIER
This
pair (only one shown) of Musician OTL (output-transformer-less) amps
were brought to me by a gentleman Mike, Mike deals in Hi-Fi gear and is
a bit of a buff himself too. He had just sold them on EBay, and whilst
the buyer was very pleased with the sound, he couldn’t cope with
the background hum!
Personally
I am not a big fan of OTL amps, and hum is an inherent problem with
them if little thought has gone into the design of the power supply. In
amplifiers that do use an output transformer, some hum is generally
cancelled out within the transformer but, with the OTL’s, the hum
is harder to suppress.
This
mono model uses four twin-triode 6N5P tubes for the output, arranged in
a parallel push-pull half-bridge/by-phase configuration. As the speaker
is in the centre rail to ground of the twin “plus and
minus” power supplies, this is why they can be a bit "hummy" if
not designed correctly.
I fitted two chokes and another small mod which reduced the hum issue by about 90%!
The
two chokes can be clearly seen here, which are actually two 9 – 0
– 9 volt transformers @ 12v/a. The problem trying to smooth the
ripple on this kind of circuit arrangement is the fact the HT voltage
is fairly low (150 – 0 – 150)……..Fair enough,
but the current is quite high. We need chokes that can easily carry
about 1 amp, but with a very small DC resistance as we can not afford
much of a voltage drop! The two small power transformers, with their
low resistance secondary windings, are used for the purpose of a choke
and the primary windings left un-used. These did the job perfectly with
only two volts drop on both the plus and minus HT rails!
Just
changing the subject here, all these Chinese amps seem to come through
with no ground connection on the power input sockets (IEC connector or
“Kettle plug” as we call them here in the UK). I know the
best way to ground any system is with the main pre-amp being grounded
and the rest of the system (power amps etc) taking their grounds from
the main pre-amp. However, if the ground breaks down on the
interconnecting cable(s) for any reason then we have a power amp
ungrounded…..not a good, sensible thing! By simply soldering a
10 ohm resistor, usually about 2 watts, from the input connector ground
to chassis ground, this makes things safer and, as the grounds are now
10 ohms apart, we have no “earth-loops” (eddie-current).
Going
back to the original hum problems, this photo shows one of the amps as
received….no chokes. I would bet my reputation on the following
statement:
The
two large caps, about in the middle of the board, are the two main
smoothing caps for the “plus and minus” rails. They are
1000uf @ 160 volts. It is obvious that these were drawn-up to be on the
main board as they are. However, on test, I would suspect the designers
were not happy with the amount of hum present and had decided to add
two more 1000uf @ 160 volt caps, in parallel with the original ones on
the board (look at the two caps, side mounted on the chassis, with
sticky pads and cable-ties) in an effort to suppress the hum somewhat!
Yes it does…..a bit…..but still not enough! Nevertheless,
instead of paralleling these extra caps, simply adding them, with two
chokes, and a cap each side of the said chokes does the trick. These
are a “very budget” amp and not so greatly designed/made.
Apparently they sound pretty good now after my work on them!
Carefully
marking out and drilling the mounting holes for the chokes.
In November 2009, I received the following
email from Martin, the person who purchased the amps off dealer Mike:
"Hello John
As the new owner of the Musician OTL amps you worked on a while ago (the ones featured on your website), I feel bound to correct an error in your feature on them. You say at the end of the article "Apparently they sound pretty good now after my work on them".
What you should have said is that they now sound utterly stunning! Seriously, they are among the best amps I have ever heard, which is quite remarkable considering their humble origins. Some of that is surely due to the work you did on them, so this message is just to say........wow, great work!
I know who to contact if they ever need work on them again. That's all really.
All the best, Martin."