Mute Circuit Connections for My PSU

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Hi,

my basic bipolar power supply uses a transformer with two secondaries like the one in the bottom attachment. Im using it to power some balanced drivers and some op amps.

In order to remove the turn on/off clicks/pops, I implemented the Rob Elliot Muting circuit. Basically, This circuit uses a schmitt trigger with some RC's for the timing and AC detection part. When engaged, it shorts the outputs of my preamp. See below and/or the link to Rob's muting circuit.

Project 104 - Preamp/ crossover muting circuit

The circuit has a problem when powering down. In this case it doesn't short the outputs immediately but rather delayed. I have tested the triggering of the circuit with a signal generator. In this case it works exactly as expected. When the triggering signal goes beyond the circuits off-threshold, it shorts or activates the relays after the programmed delay period. When the trigger signal is lowered below the circuits on-threshold, it immediately shorts or activates the relays again.

My theory is that the ground signal, which this circuit relies on for AC detection, is not optimal in my case, due to the topology of my power supply.
My supply is a bit more complicated then the one I posted here. It has some active filtering on both rails and some LM 317/337 regulators. My ground connection is defined at one single point after all that circuitry. In Rob's connection scheme, he's show's the AC connection being relative to a ground from a center tapped transformer. For the AC connection in my circuit, I have used one of the secondaries. My ground exits at one point at the "end" of the power supply circuit. There is no common center tap / ground connection from the transformer to the end of the power supply.

I added a resistor from the AC triggering input to the ground of the mute circuit, as suggested by Rob for testing purposes only. This did fix the turn off behavior for a while, but now its delayed and a bit undefined.

Would appreciate any ideas on how to connect this problem.

Chris

http://sound.westhost.com/p104-f1.gif
http://sound.westhost.com/p104-f2.gif
 

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check the waveform @ pin3.
This voltage is repeatedly driven to near zero on every alternate halfwave of the AC.
If the AC misses a cycle then the pin3 voltage stays high for long enough to trigger the instant off detection.
I suspect the instant off detection is not working.
 
Hi AndrewT,

yes the instant off is not working all the time reliably.
If i run it shortly, it works ok. After a couple of hours, the instant off no
longer works as expected but is delayed. Non of the parts are hot or warm by touch.

My trigger signal is about 11vac with a fluke meter. With my old scope, its read more, about
12.5 vac and the wave form is not so symmetric around 0v ac. It goes down to about -10 and peaks at about +15

After the resistor @pin1 I measure about 2vac. Seem's a bit weak to me. Also a little asymmetric, from -2v ac to +3v ac.

@pin2 i get around 1.8vac with a nice sqaure wave wave form 0v ac to +3v ac.
@pin3 i get barely any signal, e.g. about 171mv ac.


Regards,
Chris
 
Hi AJT,

regarding the capacitor, is this zero volt ref pin3?

As you inferred, this board is being powered by the main supply.
Maybe I will think about using a separate supply for it, if that
makes it more reliable.

Cheers,
Chris

that cap is just to bypass rf, it is optional...
a separate supply with "light filtering" is the way to go imho,
after all you are just driving a relay....
and at power down, it discharges quickly...
 
that cap is just to bypass rf, it is optional...
a separate supply with "light filtering" is the way to go imho,
after all you are just driving a relay....
and at power down, it discharges quickly...

I don't see how a separate supply will help the circuit engage quicker when powering down. Or do you mean using that supply also as the source for the triggering signal and operating both supply's by a single switch?


Regards,
Chris
 
RodE's design was based on the premise that the center tap
of the power traffo is connected to ground...
you changed the configuration of the the main psu..
therefore the muting circuit will not work,
you will need a separate psu...
but you can still use the secondary of the power traffo
for the separate psu...
a lightly filtered psu will discharge more quickly..
 
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