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Old 19th May 2012, 05:52 AM   #1
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Default Doz Output capacitor

Hi all,

I'm in the last few stages of building a pair of Doz amp modules and I've a question about the output caps. Death of Zen - A new Class-A power amp

Firstly, how is the value of 4700uF for the output cap determined? And what difference would altering it make to the amp?

Secondly, I have a few 20uF MKT caps about; would they suffice?

Cheers to all that take the time to answer a newbie question!
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Old 19th May 2012, 06:53 AM   #2
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Location: Poole, England
Default DOZ capacitor

The size of the output capacitor (in conjunction with the speaker impedence) will affect the frequency bandwidth coming out of the amplifier. If you look in your speakers you will most likely see that there are already one or two low value capacitors in the crossover network there to filter the high frequencies over to the mid and/or high units - using a low value capacitor on the output of your amplifier will have much the same effect, hence the schematic specifying a much larger output capacitor value.

If you're only using the DOZ to drive a tweeter in an active crossover set up then you have less to worry about.
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Old 19th May 2012, 06:53 AM   #3
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See this calculator: Coupling Capacitor Calculator by V-Cap You can play with the values and see the effect on frequency response.

A coupling capacitor in combination with your load acts as a high pass filter. In the case of your 20uF caps, you would chop off a huge chunk of the frequency response, starting at around 900Hz. You may be able to halve the 4700uF cap or thereabouts if you only use 8 ohm speakers, but that would be about it if you wish to build a circuit with the same performance as Elliot's.

It's an AC-coupled design. If you want a DC-coupled JLH amp, you might look at the 1996 variant.
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Old 19th May 2012, 06:54 AM   #4
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Output capacitor value will form high pass circuit with the impedance of your speaker. If you have 8 Ohm speaker and 4700uF the roll off frequency will be 4.2 Hz. So 20uF MKT can be used as bypass for the 4700uF.
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Old 19th May 2012, 06:58 AM   #5
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I'll have a wee play with the mentioned Calculator and take it from there.

I've a couple of circuit boards for the dc coupled JLH 2005 to do later - it'll provide an interesting comparison in the long run.

Cheers for the clarification and for taking the time to answer.
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Old 19th May 2012, 07:06 AM   #6
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There's a fellow named "Shaan" who has built a bunch of JLH designs and the DoZ in both AC- and DC-coupled versions, IIRC. You might look at his (recent) thread on Death of Zen if you get a chance: Death of Zen 15 watt Power Amplifier
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Old 19th May 2012, 07:10 AM   #7
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I had a quick browse earlier in the week - the perfboard wiring he does is awesome

I'll do a bit more decent reading of that thread. cheers!
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