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Old 28th October 2012, 09:10 AM   #1
Baldin is online now Baldin  Denmark
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Default Preamp output relay - or other device

Hi All

I have some problems with by subwoofer pre-amp, at especially start up, where it makes a very large pop which is no good going into a 500W amp

My question goes in general to preamps, based on op-amps (here TL074).
Is there a way to tame the opamps at start up, so they don't makes such noise.

Is the only way to go for a relay on the output of the pre-amp, or alternatively using a fet.

In my experience the only reliable way to use relays is to control these from the AC line, so that they turn off immediately when power is turned off, and of course use a delay at turn on ... like this:
Audio Amplifier Circuit Output Relay Delay Audio | Audio Circuits

But it would be nice to have a more simple way ..... any best practice her?

Thanks in advance Baldin
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Old 28th October 2012, 11:20 AM   #2
AndrewT is offline AndrewT  Scotland
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Muting is essential if there is a powered amplifier to follow.
That muting must, as you have observed, be on start up and shut down.
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Old 28th October 2012, 07:46 PM   #3
Mooly is online now Mooly  United Kingdom
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Best results are when the relay is normally closed and so shorts the output to ground. When the relay is open there is no signal degradation since the signal doesn't pass through the relay. Most opamps are short circuit proof. The only practical point is to add a small series resistor (say 100 ohm) from opamp output to the preamp amp output sockets with the relay muting the socket side of things. The resistor ensures the opamp can stabilise properly.

Instant off isn't quite such an issue because any rail capacitance keeps the circuit alive for a few moments.
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Old 17th April 2014, 08:43 AM   #4
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Default delayed start

Quote:
Originally Posted by Mooly View Post
Best results are when the relay is normally closed and so shorts the output to ground. When the relay is open there is no signal degradation since the signal doesn't pass through the relay. Most opamps are short circuit proof. The only practical point is to add a small series resistor (say 100 ohm) from opamp output to the preamp amp output sockets with the relay muting the socket side of things. The resistor ensures the opamp can stabilise properly.

Instant off isn't quite such an issue because any rail capacitance keeps the circuit alive for a few moments.
Hi, I have a valve pre-amp which I implemented this 'muting' method on. It works very well for start-up, no more squeek and thump on start-up while the tubes are powering up but I have a plop when switching off. How can I reduce this effect. (I have a 100E resistor in series after the output cap from pre-amp which I pull to ground with the relay)

Any suggestions?
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Old 17th April 2014, 11:18 AM   #5
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by valvesound View Post
Hi, I have a valve pre-amp which I implemented this 'muting' method on. It works very well for start-up, no more squeek and thump on start-up while the tubes are powering up but I have a plop when switching off. How can I reduce this effect. (I have a 100E resistor in series after the output cap from pre-amp which I pull to ground with the relay)

Any suggestions?
You mean the circuiy in the link in post #1 ?

Could be a couple of reasons for this. Firstly you need to check that there is zero volts DC (that means 0.0000 volts) on the point the relay pulls to ground. If you have a series coupling cap and there is voltage on the relay then you are going to need another cap (so two in series, observe polarity) together with a high value resistor (1meg ?) to ground to give a point where there is no DC voltage that you can then ground with the relay.

You can easily test for this by simply shorting the transistor C to E to fire the relay. It should mute and unmute silently.

The other possibility is simply that the relay isn't dropping out fast enough but I suspect that the least likely tbh.
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Old 17th April 2014, 12:11 PM   #6
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Thanks Mooly, I will thry the 1M to ground between 2 caps idea. I think that might be the answer.

BTW mine is not like the cct in post #1, it is a 555 timer driving a signal relay. The original cct switched the signal line through but that was even more troublesome so I modified it to switch the signal to ground through a low value resistor.
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Old 17th April 2014, 02:06 PM   #7
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Use this muting circuit from Rod:
Project 104 - Preamp/ crossover muting circuit
This works well and I have been using it .
.Dinesh
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Old 17th April 2014, 02:30 PM   #8
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Hi Dinesh, That is what I have essentially. It works well for the startup sequence, just the power-off doesn't work so well for me. I have a large capacitor on the output of a SE valve pre and it must be that the slightest DC offset causes the plop sound when it dumps that energy to ground.
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Old 19th April 2014, 02:00 AM   #9
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it should work since it detects ac from the transformer. The circuit detects the loss of ac once the power is switch off and the relay mutes within milliseconds. I also use it on tube preamp and never had issues at shutting off.
Dinesh
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