DC or not DC? That is the question.

triode17

Super Member
I've built a few tube preamps in my time now. Was wondering if anyone has considered a switch on the input signal line to switch in or out a coupling cap? All of my projects and commercial tube gear has an output cap. It has to , to block DC. So why have another series cap. at the preamp.
 
Because you never know where it will be connected. ..

An integrated amp doesn't have 2 caps in series between the pre and power amp section
 
Phase Linear gear had such a switch to bypass the input cap. If you know the preamp is well behaved you can certainly bypass the amp's input cap.
 
Because you never know where it will be connected. ..

An integrated amp doesn't have 2 caps in series between the pre and power amp section
Bingo. But integrated amps often DO have two caps in series, if they're equipped with pre-out and main-in jacks.

I have long harbored a prejudice against power amp input DC blocking caps, because I feel that a power amp should never be tasked with band limiting. This job is more properly delegated to the Control Amplifier, aka Preamp or Line Amp, which must never be guilty of emitting DC bias or any other signal pollution to downstream components.

On the other hand, some power amps need input DC blockers to prevent themselves sending DC bias upstream and possibly making the preamp volume control noisy or worse. Also, it has to be admitted that real-world power amp manufacturers will encounter customers with non-ideal signal sources and preamps. Without band limiting circuits, some of these customers will experience substandard performance and blame the wideband amplifier, resulting in lost sales. It's a bit too easy to criticize manufacturers who include band limiting circuits if you don't have a hungry family (or stockholders) to feed.
 
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Bingo. But integrated amps often DO have two caps in series, if they're equipped with pre-out and main-in jacks.

I have long harbored a prejudice against power amp input DC blocking caps, because I feel that a power amp should never be tasked with band limiting. This job is more properly delegated to the Control Amplifier, aka Preamp or Line Amp, which must never be guilty of emitting DC bias or any other signal pollution to downstream components.

On the other hand, some power amps need input DC blockers to prevent themselves sending DC bias upstream and possibly making the preamp volume control noisy or worse. Also, it has to be admitted that real-world power amp manufacturers will encounter customers with non-ideal signal sources and preamps. Without band limiting circuits, some of these customers will experience substandard performance and blame the wideband amplifier, resulting in lost sales. It's a bit too easy to criticize manufacturers who include band limiting circuits if you don't have a hungry family (or stockholders) to feed.

I forgot that "pre out > main in" connection. I was thinking in a closed unit without chance of splitting the signal.

But I think we all agree those caps are for protection, protecting the unit itself , and protecting other gear.
 
I have one power amp that came with a switch to bypass the caps. With integrated amps removing the caps from the circuit can cause some problems. A couple Marantz integrated amps I have used them. I could run them without the caps, however some DC current builds up on the filter board. If you push a filter button without the caps you hear a loud pop. As long as you didn't use the filters evetything was fine. I wound up putting the caps back in.
 
The circuit is simply a shorting switch across the input cap. Block layout is: Input cap/bypass to Selector Switch to Volume control to Voltage amp to EQ and so on and so forth.
 
The reason I asked for a schematic is that, in seeing that, we'd know the intended purpose of the input cap...
 
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