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!
Thanks!
Oh. Can I stop the clicks by putting a 1 meg resistor on each of the inputs (like on a guitar amp)?
Henry
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?
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?
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.
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.
Why?AndrewT said:
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.
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
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
Last edited:
To get a silent changeover you need one or more of the following:AndrewT said:
1. proper DC blocking on all switch ports (best option)
2. extremely low input bias current (possible)
3. high capacitance at the switch output (unlikely)
4. make before break switching (but this shorts source outputs together briefly)
If the first device in the amp is a valve or FET with a relatively low value ground leak resistor then you might get away without a coupling cap there. I would not recommend it, as you are then relying on all sources to ground their outputs even under fault conditions. If the first device is a BJT then you need a cap.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.