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In Reply to: RE: MA-1 Coupling Cap upgrade posted by Barry on January 01, 2016 at 13:36:06
I did decide to stay with the 600V version.
0.47uF, 600V, 0.5% matched quad.
As far as that little yellow 1uF is concerned, I have a few Audience TO 1uF, 600V Teflon caps in my stash. I'll use those.
Time to open up my MP-1 pre-amp.
Just did an extensive search on coupling cap values.
It appears 0.22uF is the largest value we should go.
I'd like to thank Chris from VHAudio. He put up the red flag. That's service.
While you do go to 1Hz with 0.22, they don't sound as good otherwise, and with the 0.1 uf units, the squarewave tilt at 20Hz is unmeasurable. This means no phase shift at 20Hz, which is how you get impact.
Increasing the value will mess with the timing constants in the circuit which will increase the IMD.
Thanks for that insight. I once read about "slugging the bass" which amounted to using an excessively large value coupling cap. What's that all about?
Sometimes an amplifier could do with a larger coupling cap as the timing constants in the power supplies will allow it.
The issue is that the amp should not have bandwidth that goes below that of the power supply! No power supply is truly DC (batteries are) as they all have some sort of low frequency timing constant.
If the amp has bandwidth below that of the power supply, the result will be that it has the ability to modulate the supply. If it can do that, the noise in the supply will cause intermodulations to occur.
Now our amps don't have a particularly low THD figure as modern amps go, but our IMD figures are just fine in that regard. This is because we pay attention to this rule. There is a myth that if your THD is not low neither will be the IMD. In solid state amps that are direct-coupled input to output (meaning that they by default exceed the LF timing constant of their supplies), this does occur but it does not have to be that way if the amp has well-managed timing constants.
I've read too much. My tiny brain hurts.
Some people on other websites are using 10K on the HV caps.
I searched our archives, and can't find an answer.
Unless otherwise advised, I'll stay with the 1K.
What website? What amplifier was under discussion? Going from 1k to 10k will assuredly drop more volts across the resistor, thereby reducing the volts delivered to the circuit. (Ohm's Law). Moreover the 10k resistor will need to be re-rated to account for the increase in power across it. P= current x volts. The gain in filtering is not worth the problems it creates, imo.
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