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Old 8th November 2005, 09:51 AM   #1
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Default A very stupid question about input caps

Does input capacitors on power amps do anything if the output of the preamp already has a cap on it? What about a DC servo? Will the signal wire actually pick up DC on its way to the power amp?
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Old 8th November 2005, 10:03 AM   #2
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Default Re: A very stupid question about input caps

Quote:
Originally posted by eVITAERC
Does input capacitors on power amps do anything if the output of the preamp already has a cap on it? What about a DC servo? Will the signal wire actually pick up DC on its way to the power amp?

No, if the preamp has an output cap, the extra one in the power amp doesn't do anything as far as offset from the preamp is concerned. Same with DC servo: assuming you mean a servo in the pre, that means the pre has 0 V DC, and you can connect that to the power amp with or without a power amp input cap (but why pay twice?).

The wire doesn't pick up DC out of thin air. However, but that's another story, it can (if not propoerly screened) pick up magnetic and/or electric fields (acting as antenna) and in that way end up with AC on the input of the poweramp that doesn't come from the preamp.

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Old 8th November 2005, 10:22 AM   #3
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Default Re: A very stupid question about input caps

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Originally posted by eVITAERC
Does input capacitors on power amps do anything if the output of the preamp already has a cap on it?
Actually a bad thing may happen, two caps in series give smaller capacitance, so those two may attenuate or phase shitf some bass.
The only reason for a cap being here and there too is that both amp and pre should be idiot audio chain proof. If all the power amps in the world have had a cap on input, preamps wouldn't have one on output.
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Old 8th November 2005, 02:46 PM   #4
anatech is offline anatech  Canada
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Hi eVITAERC,
Some units come without. I highly recommend the amplifier have the cap. I did get a pair where the amp cooked the speakers (Krell) because the servo in the preamp (Krell) failed and the amp passed the DC to the ex-speaker.

Generally the LF cutoff from both caps in series will not hurt the sound in most systems. Heck, even the room cutoff is higher. The speakers may be higher still. 0.2 Hz, 5 Hz, 18 Hz. For most, this is completely inaudible except the 18 Hz cutoff may sound cleaner.

-Chris
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Old 9th November 2005, 07:25 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
input caps on power amp and output cap on preamp will, as suggested earlier, add together.
If both were the same value then the bass roll off frequency would move up one octave. Decide if this is a problem. You can alleviate this by ensuring one of the caps is at least twice the size of the other, choose the poorest one to replace if you need to go that route. One of my preamps has a pair 10uF PP, that will never need replacing.

If you intend to run multiple parallel power amps from the same preamp source then all the power amps except one MUST have a dc blocking cap on the input. This prevents interaction of the input offset voltage between the paralleled input stages.

Both sides of the input and output caps should be connected to signal ground. You can add 1M0 to 2M2 at the RCA connectors to achieve this since it is very often omitted from amps and preamps. Croft are one company that are now recommending removal of these extra resistors from their preamps to improve sound quality (by removing excess loading from their direct coupled high impedance valve output stage).
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Old 9th November 2005, 06:31 PM   #6
anatech is offline anatech  Canada
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Hi Andrew,
Wow, I can imagine that Croft's amplifiers sound different with various interconnects too then.

I would still recommend coupling caps on all amplifiers. One never knows what can happen in the outside world.

-Chris
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Old 9th November 2005, 11:47 PM   #7
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Allright, input caps are useful and they can in fact contribute to our general safety and such, we should learn to love it.

Or should we? I know of many many people that have goine to great lengths just so they can "mod out" the output cap of their preamps. Beit DC servos, transistor matching, buffers, etc. Isn't that all ruined by a input cap on the power amp?

I was wondering if it is fairly standard practice to take out the input cap on the power amps if purist "no caps in the signal path" approaches were used in the preamp. I guess this applies to the likes of people that use their Gilmore Dynalo's as preamps.
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Old 10th November 2005, 12:05 AM   #8
anatech is offline anatech  Canada
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Hi eVITAERC,
I think removing all caps in the signal path is misguided and just plain silly. That's like wishing there was no money and we'd get whatever we wanted. Pure Communism. (I don't believe in pure Capitalism either)

The small signal caps in the input circuit will have the least effect on sound quality. There are far worse things than these small caps. Speakers for one, circuit design for another.

As I posted earlier, a matched Krell pre and power needlessly killed a woofer (from a new PSB Stratus Gold). The customer was none too pleased. Why did the amp not have a coupling cap? I doubt you could hear it if it was there as long as you used a good film cap.

So everything in life is a balance and compromise to some extent.

-Chris
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Old 10th November 2005, 01:18 AM   #9
Jay is offline Jay  Indonesia
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Quote:
Originally posted by AndrewT
input caps on power amp and output cap on preamp will, as suggested earlier, add together.
If both were the same value then the bass roll off frequency would move up one octave.
I'm glad you brought this issue. But I noticed that they usually use bigger capacitance than necessary so the chance for this problem should be minimized. But then again, it seems that the theoritical value for -3dB at 10Hz (also in feedback decoupling) is never sufficient (when the system allows for lows reproduction)!? Is this somekind of "headroom" issue? And with shunt resistor between pre output cap and amp input cap, is the Ct=1/(1/C1+1/C2) still apply?

Quote:
Originally posted by AndrewT
Both sides of the input and output caps should be connected to signal ground. You can add 1M0 to 2M2 at the RCA connectors to achieve this since it is very often omitted from amps and preamps.
So what is the purpose of connecting both sides of the caps to ground? (I have been wondering about this mega-ohm resistors. It doesn't seem like an impedance issue). If the Croft thinks that removing the resistors is better soundwise, why shouldn't it be?

My stochino amp (under construction) in the input is using 47K (shunt), 48uF (series), 1K5 (series), 4K7 (shunt), 820p (shunt). Will the 1K5 be useless?
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Old 10th November 2005, 01:32 AM   #10
anatech is offline anatech  Canada
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Hi Jay,
I think I can shed some light on this. Andrew can kick me if I'm wrong here.

It's not the frequency cutoff but rather the phase shift that might worry some people. Depends on the speakers and program material. A true perfectionist would want to keep the cutoff points 10X from the band of interest at each end of the spectrum.

Referencing the other sides of the capacitors to ground through 1M0 or higher resistors will hopefully prevent pops and bangs as the capacitors are suddenly charged or discharged. This may happen if the impedance at the input suddenly changes if the capacitor is allowed to float. Some muting circuits do this.

-Chris
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