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Old 21st March 2008, 06:30 AM   #1
decky is offline decky  Australia
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Default Muting relay problem

I built recently a DoZ preamp based on Rod Elliott kit (P37A) powered by his P-05-rev B power supply.

http://sound.westhost.com/project05b.htm

The PSU is a standard LM317/397 dual supply but with a twist - it has a AUX voltage output to drive a muting relay.

I had a turn ON thump so the relay was a must. However I did not have any turn OFF noise. I included an output relay where the output is normally shorted and the relay has a delay of around 0.5 sec, just enough to avoid switching noises.

The turn ON thump is gone but I have a sharp thump after switching preamp OFF - looks like from the relay itself. CAn I tweak my relay setup somehow to get rid of this? MAybe a capacitor across the relay supply to slow down its reaction.
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Old 21st March 2008, 07:21 AM   #2
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Did you put a diode across the relay's coil to suppress the inductive "kickback"?
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Old 21st March 2008, 09:04 AM   #3
decky is offline decky  Australia
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That diode is on the relay supply board already, together with a 10uF cap in parallel with the diode.

It seams to me that the shorting of the preamp output causes that - like there is significant level of signal at the output even after the relay delay time. Should I increase the delay? However I am not sure how to do it.
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Old 21st March 2008, 10:09 AM   #4
AndrewT is offline AndrewT  Scotland
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the relay should be delayed ON and instant OFF.
That capacitor across the coil is keeping the relay powered and preventing instant OFF.
Post the circuit in case there are other delays built into your relay operation.
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Old 21st March 2008, 10:26 AM   #5
decky is offline decky  Australia
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Here is the PSU - AUX out is the relay supply
Attached Images
File Type: gif p05b-f1.gif (9.5 KB, 258 views)
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Old 21st March 2008, 10:37 AM   #6
AndrewT is offline AndrewT  Scotland
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take your relay power from d2. That will discharge c11 more quickly.
Is c13 & r10 necessary?
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Old 21st March 2008, 12:08 PM   #7
decky is offline decky  Australia
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I am not the designer - its Rod Elliott's kit - here is his explanation:

"When power is first applied, Q3 is turned on by the charging current of C13. This shorts the base of Q2 and prevents it from turning on, so there is no output. After C13 is charged, Q3 turns off, Q1 and Q2 turn on, and DC is available at the aux output. As soon as AC is removed, C11 discharges rapidly, removing base current from Q1, so Q1 and Q2 turn off, removing the DC. An attached relay will promptly drop out, activating the mute function."

I hope it makes things clearer.

But why there is that noise on power off? CAn it be the relay itself?
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Old 21st March 2008, 12:35 PM   #8
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I am assuming that you have a seperate power amplifier.

The correct turn on and turn off sequence is as follows:

Turn the preamp on first and after it is stable then turn on the power amplifier and there will be no noise.

Turn off the power amplifier first and allow a few seconds for the power supply to die down and then turn off the preamplifier and no noise will result.

Not being familiar with what you have there I can only make a guess. No need for a relay when the units are powered on and off as I mentioned.
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Old 21st March 2008, 01:12 PM   #9
decky is offline decky  Australia
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I know about the powering sequence and you are right I have a power amp (gainclone) after the preamp. I just wanted to do things "properly"
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Old 21st March 2008, 01:24 PM   #10
Mooly is online now Mooly  United Kingdom
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All sounds a bit weird this, just to recap then, you say you have a relay connected across the output of preamp that shorts this output to ground and the relay is in this state with no power supplied (the output reads s/c to ground with all off). You switch on and after delay relay powers up removing short on output. No noise there ? On switch off if the relay instantly shorts output I can't see it's possible to get noise.Possibilties include, 1. A conflict off time constants, does the relay drop out and then pull back in again perhaps for a fraction of second. 2. Relays often have significant contact resistance, is it directly across a low impedance output, try a 100 ohm resistor in series with output before the relay. As Andrew points out it has to be "instant disconnect" which in this case means contacts to close. 3. There is a D.C. offset somewhere and the sudden short perhaps discharges a coupling cap somewhere causing thump.
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