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don't know if it's the right forum to post this but...
I use a DIY Kaneda preamplifier (not built by me, I'm totally unable to build anything at this point) and a Technics SE-A5 power amp. Both circuits are pure DC and that leaves me a little worried because I don't really trust the preamp's reliability... The power amp is supposed to cut if DC is detected but I don't really trust that either (amp is from 1981).
I've been reading advices on the internet to put some caps at the preamp's output. Granted, I could use the amplifier "normal" input (as opposed to the DC input, there's a choice of both) but I'd prefer to chose a great quality output cap rather than use the amp's probably rather mundane entrance caps.
Question is: I have no idea wich type and wich value I'm supposed to use...
I'd flip the normal switch and see if that makes you feel better.
As far as reliability goes, well, since you've come to distrust both your preamp and power amp for no apparent reason, how long do you suppose your confidence in the capacitor will hold up? And it's only provides partial protection anyway...
The only way you can really add another layer of protection for your woofers is to fuse them or put a big bipolar cap in series with them. Just adding a cap would produce little reduction in risk since it would take a compound failure for it to make any difference and half of that would still be downstream. And you risk possible sonic problems and power-on transient thumps...
Maybe my first question should be: what do I risk exactly in case of DC? My woofers are fused -thank god, the amp's left channel went "poof" once already, the fuse in the speaker filter melted but the woofer was fine.
Isn't it bad for the power amp if a sudden burst of DC occurs (or a long, small leak of DC)? If it's only the woofers that are in danger, considering mine are fused -and it looks like it's a reliable protection- am i safe to go?
I may seem overly "untrusty" but since I'm using vintage and / or DIY equipment and the amp repair was so much more money than I expected, I'm feeling much more cautious now. Not that the amp's failure had anything to do with the preamp leaking DC (could it be??) but it made me think...
"If it's only the woofers that are in danger, considering mine are fused -and it looks like it's a reliable protection- am i safe to go?"
Yes you are! The cool thing is that you have already experienced a worst case failure with little consequence so that should boost your peace of mind no end...
"sudden burst of DC"
"DC" doesn't come in sudden bursts. You can just look with a voltmeter at you woofer terminals with no music playing and see what sort of offset you have. Or look at your woofer cones, are they in the same place with the Amp. on and off? Your amplifier may still have some offset correction built into it even though it's DC coupled. That scheme uses a path outside of the signal to keep the bias point stable. Systemically it does form a zero at DC for very small signals but it's usually thought of as a servo system with very low loop gain that is way way out of the signal band. If your Amp. doesn't use that sort of scheme you will probably see a small offset that will vary as the amplifier warms up.
I used a power amp for many years that deliberately had a small DC offset at it's output as part of the biasing scheme, so the woofer ran a little off center, never caused a problem.
But those are trivial details, having a woofer fuse provides far more protection than a capacitor ever could from any upstream failure. I say forget the capacitor and just enjoy your system!
Might be easier just to replace the amp caps since there is already space for them.
To figure the value you need to know the input impedance of the amp. The minimum cap value would be 1 divided by 2πFR where R = input impedance of the amp and F is the lower cutoff frequency. Larger is OK and lowers the cutoff frequency.
The voltage can be low since it normally only sees DC.
They type is open for debate but definitely not electrolytic.. I like oil/film motor run but they can be physically very big. However, since you are talking DC they could be mounted between the 2 pieces since there is no danger of getting shocked.
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