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Old 27th May 2005, 06:18 PM   #1
Nuuk is offline Nuuk  United Kingdom
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Default Suitable relay for delay cicuit

I want to make up the following circuit to protect my speakers while powering up my pre-amp.

Click the image to open in full size.

Would this relay be a suitable candidate (to work both channels)? (I intend to use a 12 volt supply)


Click the image to open in full size.
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Old 27th May 2005, 06:29 PM   #2
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Would the relay be in series with the output of the preamp or with the output of the amp? If it is the preamp I don't see any major problems. If it is to be in series with the amp output then they are waaaayyy too small, you will need something a lot beefier.

One thing I would add is a diode to prevent the IC from being damaged by the kick back of the relay coil.

Hope this helps!
Sébastien
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Old 27th May 2005, 08:31 PM   #3
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Quote:
Would the relay be in series with the output of the preamp or with the output of the amp? If it is the preamp I don't see any major problems.
Sorry Sébastien,

I didn't explain fully. The idea is to short (to ground) the output of the pre-amp/buffer until the power supply has 'settled'. So the relay would be normally closed, then after a short delay the relay is activated and opened, breaking the short.

So the relay does not have to handle much power.

Thanks for the tip about the diode.
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Old 27th May 2005, 10:16 PM   #4
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Quote:
The idea is to short (to ground) the output of the pre-amp/buffer until the power supply has 'settled'
The idea is excellent but don't forget to use a resistor in series with the ground when you clamp your output to ground otherwise you will overload your output buffer and burn it. (magic smoke ensues)

Hope this helps!
Sébastien
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Old 27th May 2005, 10:19 PM   #5
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Quote:
he idea is excellent but don't forget to use a resistor in series with the ground when you clamp your output to ground otherwise you will overload your output buffer and burn it. (magic smoke ensues)
Thanks again. Is there a formula for working out the resistor value?
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Old 27th May 2005, 11:01 PM   #6
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Now that I think of it, the resistor would be better in series right after the output of your buffer. No matter what load you will apply to your buffer, your buffer would always be protected.

As for the value, it greatly depends on the device you are going to use as a buffer and what voltage your supply is going to be.

For something with high drive capabilities (i.e. BUF634) I'd say anything between 75 and 100 ohm would do alright (I personally used 100 ohm because that is what I had on hand with excellent results). With a power supply of 15+15V that would limit the output to 200mA.

For something of a little less powerful, (i.e. OPA2132) you need to limit the output to about 40mA, the resistor should then be (for the same supply) of 375ohm.

All those calculations are made assuming you already have some signal applied to your preamp and that your gain makes it so that the output of the buffer is close to the rails. In practice this will rarely happen, especially at DC where the current would be hard to supply.

So my best advice would be to go for a value low enough to not make your damping factor too low, but high enough to protect your buffer in most cases. 50 to 100 ohm is what I've used in many occasions with good results.

These values allow your buffer output to be limited to a few hundred mA, more than enough to drive even hard loads (which shouldn't happen considering most amps have input impedance in the tens of kilo ohms). And when shorted to ground, the buffer would still be limited thus protected even with quite heavy signal applied.

I hope all of this is clear and will actually help you instead of confusing you more than anything
Sébastien
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Old 28th May 2005, 04:17 AM   #7
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you know a distributer of those in the US?

also, i know what the relay does, but do you have a schematic of how its implemented after the pre amp output. Does it just wait for that voltage on the cap to reach a certain point then connect the outputs?
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Old 28th May 2005, 09:32 AM   #8
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Thanks again Sébastien,

This is one of the buffers that I use.

Click the image to open in full size.

It already has the 68R in series with the output so would it be OK if the relay shorts the 100K to ground?

G4ME,

The relay contacts are closed to begin with. This shorts the output of the buffer/pre-amp. The circuit 'waits' for a set time and then opens the relay contacts, the short is broken and the signal can travel on to the power amp.

You can order through Farnell internationally and I believe Newarkare in the USA and are part of the same organization. If you can't find that exact relay, one with a simialar spec will do.
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Old 28th May 2005, 12:40 PM   #9
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What happens on "power off"? Would a circuit like this help prevent the nasty sounds that the opamp makes when the power supply drains towards zero?
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Old 28th May 2005, 12:53 PM   #10
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What happens on "power off"? Would a circuit like this help prevent the nasty sounds that the opamp makes when the power supply drains towards zero?
If you are talking about the opamp in the pre-amp then I would gusess it would depending on how quickly the relay reverts to closed mode.

If you are talking about the chip in the amp, then no it wouldn't make any difference and you would need to use something that disconnected the speakers when the power is removed.
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