Article
below by Graham Slee:
Graham, I have hum with your phono stage.
That's odd, it didn't before I shipped it but I can replicate your
problem, and if you try my suggestions above under Graham Slee Basics,
you'll get rid of the hum.
Whenever I read an enquiry like this it always gets
qualified with "but my XYZ stage I was previously using does not" - you
just know that's coming.
And why? It's because of a little dodge I'd never
quite picked up on before - the XYZ stage has a thing called a "Notch
Filter". Set between 50 and 60Hz to cover both USA and European
markets, it's a well kept secret. It's also an easily kept secret as not
one hi-fi magazine I know spec-tests anything anymore. If it hadn't got
this, then the customer would have experienced hum before and would have
learned previously that careful positioning cures the problem.
So why don't we fit a Notch Filter? Why? Because
it screws the low frequency harmonic structure and that's just one of
the reasons our products sound better. Not all but enough other
manufacturers use this dodge so that the customer, having little or no
record playing experience, can just throw a vinyl front end into his
system like you'd do a CD player, and hey-presto! No comebacks.
Add to this, the common practice of bandwidth
limiting, and you have a nice phasey sound reminiscent of 70's AM radio
reception. So that's where you heard that nostalgic sound before - not
at all the nostalgia of vinyl records.
I didn't twig to this when I tested a rather
expensive phono stage a few years ago sent to me by The Cartridge Man.
I'd been told not to dissect the thing because he wanted to try and sell
it, but he felt it didn't sound quite right. I measured the performance
and was amazed how well it rejected hum. This was not a balanced input
stage, it had virtually no shielding and was housed in a plastic case.
How could this be? It also had coiled input wires which makes its input
wiring inductive and should therefore be even more susceptible to hum.
I suspected something was out of the ordinary when I
tried to measure the input impedance. This is something you can't do
with a resistance meter because impedance is an AC thing. What you do is
put a resistor in series with the input and sweep through the
frequencies and then you can work out the impedance from the ratio of
the two output voltages and the value of the resistor.
What threw me was that at a particular low frequency
the output disappeared then came back again further on. Like I said, it
didn't register - it just bugged me that I couldn't take the thing to
bits any further or I'd end up having to buy it. Eventually, time the
great healer made me forget, even though I was working on a Notch Filter
as part of a design for some American Medical equipment a few months
later.
It was only the other day, when re-investigating the
old hum problem for a customer, that I tripped right over it. How else
could a plastic cased, unbalanced, unscreened amplifier, having a gain
of about 11,000 at 50/60Hz, get such a fantastic hum rejection?
Like I say, we don't do such a thing because the
phase gets screwed up, not just at the notch filter point, but it
extends throughout the audio band meeting up with the phase error from
the high frequency end, that being down to bandwidth limiting.
What happens with screwed-up phase is the timing
sounds all wrong. It can sound spacey and out of this world, and I guess
that's what sells such stages. But when I listened to this stage I felt
that half the orchestra had decided to go home. A call to The Cartridge
Man confirmed my thoughts as he said exactly the same. Great minds think
alike maybe?
So where does that leave you? You have the
chance of listening to a revelation in record reproduction but you can't
because of the hum. Well, that's because high gain equipment, which
phono stages are, will be influenced by the surrounding hum field. The
stage itself doesn't produce hum. Not mine, not his, not transistors,
resistors or capacitors.
Transformers produce hum, wiring with mains
power in it produces hum, unscreened valve amps produce hum (modern
valve amps are usually supplied without a cover these days to look
trendy and macho - the cover that used to be fitted in the
pre-macho-image age afforded some screening).
The modern practice of showing off equipment
on "the hi-fi rack" is not conducive to good vinyl replay. I'm sure you
know (or can imagine) what an earth tremor feels like at the top
of a skyscraper? You put the big amp on a lower shelf. It contains a
transformer. Ever picked up a working transformer? It vibrates at
50/60Hz and you can feel it (yes, even a torroid). The hi-fi rack
vibrates at 50/60Hz and by the time these vibrations reach the turntable
placed on the top shelf, it's been amplified by the structure - and the
cartridge vibrates - and the cartridge produces an output - and the
phono stage amplifies it - unless that is, it has a notch filter.
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