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reliability and replacement of Mag ribbon tweeters?
wrote:
chung wrote in message news:Ob4Ic.31985$WX.5763@attbi_s51...
Many preamps and receivers do not have separate buffers for the
tape loop, and the loads on that loop are in parallel with the main load
which is the power amp input.
Actually that's not true exactly, since the volume control is usually
not in the tape-recorder output path. But the same buffer (if there is
one) is driving that path and the volume control pot most likely. That
buffer (or the output of the upstream device) will still clip because of
the diodes to ground at the input of your recorder when it's off.
Yes you are right. I've seen at least a couple different preamps that
could allow a device in the tape loop to record a source, without the
preamp being turned on. This would mean the source is directly exposed
to the recording device's inputs just like they are in my passive
attenuator, right?
That probably means that there are mechanical switches or relays in the
signal path that allows a connection from source to record-out when the
preamp is turned off.
The difference is that there may be little or no resistance in that
path, whereas in your case there is the attenuator in the path.
I called Marantz tech and they said they never heard of anyone using a
unit in a passive setup. He said something about CMOS switches being
in the analog input stage and hypothesized that a big input voltage
might trigger them when there is no power to the input stage. I didn't
understand what he was saying. But overall he said as long as it works
fine when turned on why worry about it? I agree.
As I said earlier, the likely culprit is the analog switch or op-amps,
or protection diodes on the PCB, in your recorder. The analog switch is
typically a pair of CMOS transistors. When no power is applied to the
unit, these switches act like diodes to the supply voltages, which are
at ground level. Therefore you get the clamping action, meaning gross
distortion starting at a few tenths of a volt. That was why when you
were listening via headphones, you did not hear the distortion, since
the level is much lower.
The fact that you have a passive attenuator makes the distortion fairly
obvious, but even if you have an active preamp, you will still have the
problem, since those diodes clamp the voltage going to the power amp.
You need to leave the recorder powered up, or switch it out so that the
preamp does not see it, using a mechanical switch when the recorder is
powered off. RS and Sony both sell simple mechanical switches for very
little money. Or you can build your own, using a pair of mechanical
switches from RS.
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