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Old 7th April 2011, 04:38 PM   #11
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Digits - I think you misread my entry - the builder was suggesting using a polyprop cap, not an electrolytic. I actually have a pair of 10 uF infinity wondercaps I could give him if they would work well.
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Old 7th April 2011, 04:59 PM   #12
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by digits View Post
I don't think you realy need 10uF though, and that wil be quite big and expensive. 3.3uF should be more than adequate, even lower, especialy if your speakers don't go all that low.
my very cheap end of the range Conrad Johnson Pre-amp has a pair of 10uF PP output caps.
Yet the audio press criticised the Pre, for excessively restricting the bass end extension due to adopting an output cap that was too small.

No Electrolytics anywhere inside, not even the PSU.
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Old 8th April 2011, 08:54 PM   #13
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My one amp has 100uf np electrolytic input caps (nothing else fitted)... you should see the poor speaker cones dance trying to reproduce subaudiable frequencies when I put a warped LP on.

I don't realy subscribe to the gainclones with the least amounts of best parts school, that is so common. Sure I started there. But I find these chipamps perform much more gracioulsy when given fixed parameters in how high or low they should extend themselves.

If I get a chance over the weekend I'll upload an example schematic.
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Old 8th April 2011, 10:28 PM   #14
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Peter,
I would say that a 2 or 3 F polyprop, PIO or teflon capacitor would be fine, a normal electrolytic (no bypass) would be about the worst of all.

There is an old saying in hifi, ''the wider you open the window (bandwidth) , the more muck flies in''.
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Old 9th April 2011, 12:08 AM   #15
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Thanks guys. You may be interested in a similar thread I put on audiogon, where Ralph at Atmasphere states that I probably don't really need an input cap to protect my speakers ("The only way most tube preamps are going to make DC is if the output coupling cap fails, not if a tube fails.")

AudiogoN Forums: Direct Coupled Amps and Appropriate Preamps


It's a bit confusing to me, whether I need a coupling cap or not!!! Thoughts?
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Old 9th April 2011, 12:52 AM   #16
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belt and braces?
No, if you have all sources capacitor coupled, and they are never going to fail, then you don't need two caps in series. However, if the DC does get through, then bang goes the 'speaker, and whats the use of one 'speaker, can you get another bass/mid range drive unit? The tweeter probably has a capacitor, too.

So my 2p's worth and my philosophy (and my practice) is ALWAYS put in a good quality cap.
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Old 9th April 2011, 06:23 AM   #17
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Peter, unless you know for sure your sources, like DVD players, etc. do not output any DC (sometimes this only happens when sources are powered down before the amp....), an input cap offers your speakers a line of protection...without it, it is DC in and DC amplified out... go ask your speakers how they feel about running off DC.
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Old 9th April 2011, 07:27 AM   #18
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Here is the filter arrangement I promised. The important parts sits between the input socket and the chip inputs.

I have to admit that it is not strictly my design, but learned from an old electronics magazine, that ran an article on the issue... I'm poor, it came from a library, so I don't remember which one... basicaly this arrangement will limit the chip to only trying to perform work on frequencies we can actualy hear... You could probably breadboard the required bits and insert it in your amp's input chain just to check it out.
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Old 9th April 2011, 10:02 AM   #19
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That is a standard input arrangement for any amplifier.
It contains an RF filter R4 + C4 and a sub-bass filter (=DC blocker) C8 + R2 and a grounding resistor on the input side of the DC blocker.
It is also drawn correctly to show the feedback R6 + R5 + C7 connected to the Signal Ground.
C7 + R5 sets the DC gain to ~1 and also rolls off the AC gain at very low frequency.

Power Ground (GND) is kept completely separate from the Signal Ground (J1-1).

The amplifier measures the voltage difference across J1-1 to J1-2 and sends a copy to the output.

So that the output voltage does not "Float", both the Signal Ground and the Power Ground must be referenced to Audio Ground (the Main Ground Star).

This is the way to connect every AC coupled amplifier.

There are exceptions. But save them for when you can decide why and how the exceptions can give an advantage, i.e. wait till you become an amplifier designer.
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Last edited by AndrewT; 9th April 2011 at 10:05 AM.
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Old 9th April 2011, 10:19 AM   #20
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Standard, but sadly way underused with chipamps. At best you sometimes see an RC highpass. Once again, as cointerintuitive as it might sound this limitation of the operating frequencies, make these chips sound better.

So, Andrew do you like it? I was not quite sure if your response was positive or negative, sure it just got lost in translation.
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