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Mike Rivers[_2_] Mike Rivers[_2_] is offline
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Default Pultec EQP clone problem

On 2/2/2021 6:35 PM, gareth magennis wrote:
How can I force this motorboating to happen on my bench, so I can deal with it?
I've not seen it yet.


If it only happens when connected to a patchbay, it's likely due to the
cable capacitance hanging across the output.

You should ask the owner with the problem a couple of questions:

1. How long are the cables between the EQ and the patchbay? If it's
going to a normalled jack, add the length of the cable between the
normalled jack of the pair to wherever it goes.

2. If it's going to a normalled jack, what's that jack going to?

or

Does it motorboat if the normalled connection is broken (stick a cable
in the "destination" patchbay jack)

You could experiment with loading the output with a capacitor. Start
with 500 pF and go up in value to about 0.01 uF to see if you can make
it take off.

I agree than 100 ohms sounds like a pretty hefty load, but you can see
what the source impedance really is by putting in a tone and measuring
the open circuit voltage at the output. Then hang resistance across it
until the voltage drops in half (6 dB if you're using a meter calibrated
in dB). The resistance value that gives you the 6 dB drop will be
representative of the output impedance of the unit.



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