Adjusting DC bias for Zen amplifier - diyAudio
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Old 27th September 2001, 08:52 AM   #1
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Hi there,

I am a new starter and interested in building the Zen amp.
I have several questions regarding the building of the Zen amp:

1) Once the amp is built, according to Mr Pass's original Zen amp article, under section "V.Operation":
"Once built, turn the amp on. Then input signal and 8ohm load, adjust P1 for symetrical clipping. Readjust P1 after the amplifier has warmed up."

What I don't understand about this part is the meaning of symetrical clipping.What am I expect to see? Does it matter when P1 is turned to an extreme, eg 0 ohm or 10 Kohm when you turn the amplifier on?

2) My speakers are nominal 6ohms but can dip to 4 at some regions. It is 95dB efficient. Will the Zen sound ok with my speakers?

3) The supply voltage for the Zen amp is 35VDC. Why does all capacitors in the Zen part list specified as rated 35V? Shouldn't these capacitors be rated higher for safety reason?

Looking forward to your helpful comments.
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Old 27th September 2001, 03:56 PM   #2
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Symmetrical clipping means that you'll need to use an oscilloscope to watch the output. When a sine wave clips, the rounded top (or bottom) flattens, making it appear more like a square wave. The upper and lower peaks will need to start flattening at the same time and at the same rate as you turn up the input signal.
Set the bias pot for the lowest voltage at turn-on. Since the MOSFET is an enhancement device, it will then be closer to the 'off' state, hence passing the least current. You can turn it up from there.
Whether the Zen will sound good with a given pair of speakers depends on a thousand factors, none of which can be predicted from where I sit (or anyone else, for that matter). How loud you listen. What kind of music you listen to. Etc. The fact that you have fairly efficient speakers is in your favor. I wouldn't get too worried about an impedance dip--most speakers have them, and frequently far worse than 4 ohms.
Cap voltage ratings have been done to death in at least three or four recent threads. I regularly run caps within 5% or less of their rated voltage. Others want 20% or more headroom. In part, it will depend on the stability of the AC where you live, again something that can't be predicted from here. Go read the other threads and make a choice that you feel comfortable with.

Grey
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Old 4th November 2001, 05:09 AM   #3
fcel is offline fcel  United States
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Grey,
I'm assuming a signal generator is needed for the sine wave - which I don't have. Is there any other way and where can I buy them?
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Old 4th November 2001, 10:29 AM   #4
djk is offline djk
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You don't need a generator.Just use a normal audio signal.It can yield an increase in output power on the order of 1~2dB to adjust the clipping with an audio signal vs a signal generator.Likewise using a real speaker as the load vs an 8 ohm resistor." Fig.4 also suggests that if the .... output stage were dimensioned differently and optimized for these transient conditions instead of the usual steady-state sinewave condition, the heavy positive clipping could have been avoided. This deserves investigation, but that means a whole new project... " http://www.stereophile.com/showarchives.cgi?357


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Old 4th November 2001, 10:07 PM   #5
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fcel,
There's no real substitute for a signal generator, although I suppose you could contrive something out of a small transformer and a voltage divider to give a 60 Hz signal stepped down from wall voltage. Like a good meter, a signal generator and an oscilloscope are fundamental; there's no way to do audio work without them.
djk's link suggests using music to look at clipping, but assumes that you have a sampling scope. The article raises more questions than it answers, but is interesting regardless.
These days there are programs that work with sound cards in your computer. I have no opinion as to what's good in that area, as I have done no research. I still use a separate signal generator that I bought years ago. One way or another, you'll need to get or build something if you intend to do much DIY.

Grey
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Old 5th November 2001, 05:45 AM   #6
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A great substitution of signal generator is a reference TEST CD with 1k sine wave track in 0db, And a CD player. You can find it in Music store.
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Old 5th November 2001, 07:43 PM   #7
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If you have a DC voltmeter, I suggest setting the
bias so that the Drains of both Mosfets are at
1/2 the supply voltage. This is very close to
optimal.
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