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Old 22nd June 2001, 04:17 AM   #1
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Can someone explain the need for capacitor C6 (47uF) in the ZEN schematic. I understand the use of the DC coupling capacitors on the output to act as a high pass filter thus blocking DC going to the speakers. But what role does the input capacitor have? The Son of Zen does not require such a cap, is this because it has a balanced input? Would it require coupling caps if driven from an unbalanced input?

Thanks

Dan
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Old 22nd June 2001, 04:31 AM   #2
jam is offline jam  United States
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Default Input coupling cap

Dan,

The Zen uses a single supply and not a split supply as in the son of Zen. Therefore the input has to sit at a potential higher than ground hence we require an input coupling capacitor.

Jam
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Old 22nd June 2001, 04:42 AM   #3
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Default Split supply

First of all when you say split supply do you mean the inner pin of the RCA (+ve) goes to one input and the outer ring (-ve) goes to the other input?
If this is so then wouldn't both of these have a (+ve/-ve)potential with respect to ground?

Thanks

Dan
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Old 22nd June 2001, 04:59 AM   #4
jam is offline jam  United States
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Dan,

If you look at the power supply you will that there are two supply rails +ve and -ve. Ground sits between these two rails. This allows the input to sit close to ground potential.
The Zen on the other hand has only one supply +ve and therefore the input has to sit at higher than ground potential to bias the input mosfet on. This is also true for bipolars the exception being a jfet or depletion mode device which is another story...

This is true for single ended and balanced inputs.

Jam
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Old 22nd June 2001, 04:59 AM   #5
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Dan,
Split rails refers to +/- rail voltages. The way Nelson has the SOZ laid out, it runs off of positive & negative power supply rails. The Zen, however, has only the one rail at (I think) +34V.
The SOZ gates are at ground potential (we're assuming no signal). The Zen gate will be something on the order of +4V in order to get the MOSFET to wake up. If you didn't have the DC blocking cap, the 4V would short back through the output of the preamp to ground, shutting down the amp (unless, of course, you're using a tube preamp, which will already have a cap at the back end--read between the lines here, and you've got the potential to eliminate a cap...)

Grey
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Old 25th June 2001, 04:19 AM   #6
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Default I may be crazy but......

Would it be possible to bias a mosfet by using a (say 4V) battery with the negative terminal at the source pin and the positive terminal to ground? If this is possible could the same setup be used in a SOZ circuit to bias the mosfet without using a split supply? This would drastically improve the idle heat dissipation of this amplifier.

Another question regarding SOZ: This may be even stupider

How does an amplifier achieve balanced operation? How do you invert the phase of the input signal, do you just connect positive to earth and negative to the gate of the mosfet?

Can someone please have a go at explaining this to me in simple terms.

Thanks

Dan

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Old 25th June 2001, 02:02 PM   #7
jam is offline jam  United States
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Default Differential Amplifiers

Dan,

We seem to have some confusion here.
Yes we can make the SOZ run off a single ended supply, but then we have have to bias the inputs (there are two inputs because the SOZ is a differential amplifier) with a positive voltage with respect to ground to turn both mosfets on. This can be done with a voltage divider between the positive supply and ground or a battery. In either case you will still have to cap couple the inputs because both inputs sit at a higher potential than ground.

I would suggest you read a text on the workings of a differential amplifier , eg. Nelson Pass's article on the balanced Zen preamp is excellent and is available on the Pass Labs web site, because it would probably too difficult for me to explain without some diagrams.
To avoid any confusion you first have to seperate the inputs, outputs and supply rails. The SOZ has two inputs (+ and - phase), two outputs (+ and - phase) and two supply rails (+ve and -ve which are dc voltages). This might be part of your problem in understanding the circuit.
You will see that a differential amplifier can also be used as a phase inverter which is is intrinsic to its operation.

Jam

P.S. If you still have a problem with with the circuit please e-mail me and I will send you some photo-copied notes on the subject.
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