One pair of coupling caps on the input of my XO? - diyAudio
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Old 15th April 2015, 11:05 AM   #1
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Default One pair of coupling caps on the input of my XO?

My peripherals (tuner, cd and phono preamp) all go through an active line level crossover before going to the poweramps. The crossover input impedance is 25k and my peripherals all have low output caps (1uf). I want to raise these to 3.3uF. Rather than replace the output caps in all the peripherals, can I just pull out the output caps altogether and put one pair of 3.3uf in my crossover just after the input selection?

Thanks!
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Old 15th April 2015, 11:49 AM   #2
DF96 is offline DF96  England
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Only if you want loud thumps/clicks when switching between sources.
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Old 15th April 2015, 03:29 PM   #3
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Quote:
Originally Posted by DF96 View Post
Only if you want loud thumps/clicks when switching between sources.
Oh. Can I stop the clicks by putting a 1 meg resistor on each of the inputs (like on a guitar amp)?

Henry
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Old 15th April 2015, 03:37 PM   #4
DF96 is offline DF96  England
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You need a 1M resistor for each cap. That ensures that the switch is only switching AC not DC. So each input and each output from the switch needs a coupling cap and a ground leak resistor.
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Old 15th April 2015, 05:57 PM   #5
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Old 15th April 2015, 06:43 PM   #6
DF96 is offline DF96  England
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Yes. Repeat that for each input too.
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Old 15th April 2015, 09:10 PM   #7
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Thanks!!!!!
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Old 16th April 2015, 12:15 PM   #8
AndrewT is offline AndrewT  Scotland
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I had not thought about the DC effect across a switched multi-input pre-amp.

The idea of using dual DC blocking capacitors at the Power Amplifier is very much against what I recommend.

For the same reasons I would not recommend dual DC blocking capacitors between separate multi sources and a pre-amp.

But I can see from this Thread that DC switching is a problem that needs to be addressed. Although I have not noticed it in my system.

So here is some food to think about.

The Source has a DC blocking capacitor AND the leakage sink resistor after that cap.
Feed all these to a DC coupled selector switch and the output goes to a DC coupled Pre-amplifier, or to a DC coupled Buffer.
So far we have only single DC blocking cap in the audio stream.
Feed the DC coupled Pre-amp/Buffer to the Power Amplifier which is fitted with a DC blocking capacitor AND another leakage sinking resistor.
This again avoids dual DC blocking capacitors in the audio line.

Does this result in silent switching?
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Old 16th April 2015, 02:25 PM   #9
DF96 is offline DF96  England
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Only if your DC coupled preamp/buffer draws no input bias current, or otherwise can be arranged to have exactly zero DC input voltage at all times. That means either caps with ground leaks or transformer coupling.

Quote:
Originally Posted by AndrewT
The idea of using dual DC blocking capacitors at the Power Amplifier is very much against what I recommend.

For the same reasons I would not recommend dual DC blocking capacitors between separate multi sources and a pre-amp.
Why?

For click-free switching the requirement is quite simple: all switch inputs and the output must be referred to the same DC voltage. This need not be ground, but it usually is the most convenient option. Well-designed sources will already reference their outputs to ground.
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Old 17th April 2015, 06:12 AM   #10
AndrewT is offline AndrewT  Scotland
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Because both the Source and the Receiver need to have the capacitor values doubled to maintain the LF bandwidth.

If the source is DC blocked and the leakage sinking resistor is fitted, then the switch "ON" contact is near enough at zero volts DC. This holds both the Source output and the Receiver input at the SAME voltage.
As the contacts change over the Receiver will momentarily start to change it's DC voltage following some RC time constant. A quick change-over may well be silent.
I am using the Mezmerise at the moment. It is quiet. DC blocked inside the Sources and DC blocked inside the Power Amp. No noise !
Just fortuitous, or by design?

That's why I asked the question
Quote:
Does this result in silent switching?
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Last edited by AndrewT; 17th April 2015 at 06:15 AM.
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