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Mark Osborn 15th March 2001 05:36 PM

The Zen amps are sure popular. I would like to take mine and turn it into a mono block. Can someone tell me if its just as simple as tying the outputs together or do I need something else inbetween the outputs. Thanks much

Joe Berry 15th March 2001 06:46 PM

Hi Mark,

For best results with monoblock operation, I would jumper the inputs and outputs together as well as the feedback return points (junction of R5 and R8). Then readjust P1 in each channel as needed for symmetrical clip at each mosfet's drain. Tying the feedback loops together equalizes any gain difference between the two channels, so they are more in sync when driving the load. Hope this helps.

Mark Osborn 15th March 2001 07:25 PM

Thanks much, wasn't sure if i'd get a feedback loop back through things or not.

peted 15th March 2001 11:27 PM

Joe, are you sure on this - jumpering R5/R8 (the virtual earth points) together? I'm about to (not quite completed yet) run strapped Zen Revisited amps for 4ohm loads, but the Master said nothing about this in response to my queries to him, except literally paralleling the channels at i/p & o/p (and then adjusting both channels iteratively for symetrical clipping when strapped)


Mark Osborn 15th March 2001 11:57 PM

Thanks for the reply. I'll try with and without and see what happens. I'm a few days away from firing this thing up but when I do I'll post the results.

Joe Berry 16th March 2001 02:11 AM

Mark and Peted,

I'm sure that the Master would have said so if there were a practical need for three-point strapping in the Zen. I have identified this need in other designs, so I habitually recommend it unless the amp has no global feedback. Having not played with the Zen myself, I readily defer to the opinion of the designer and others with relevant hands-on experience.

The background on this issue is that mono-strapping of stereo amps has sometimes resulted in the amps blowing up. My own analysis of this problem led me to conclude that the two channels had slightly mismatched feedback network components. Thus, while they saw the same input signal, their view of the output signal was sufficiently different that one channel tried to take the output positive while the other tried to take it negative, with the result that the output stage exceeded its SOA and blew up.

This really can be a serious problem for op-amp type designs, where the input diff pair might have a 20mV linear window and see a net drive signal on the order of 1mV or less. On reflection, I recognize that this probably is not a real concern with the Zen, as it operates under much more relaxed conditions. With a net drive signal on the order of half a volt, small mismatches in feedback components are not likely to make much difference.

But I will be much the richer if Mark will try it both ways and report his findings.

GRollins 16th March 2001 03:27 AM

Well, hell, if it's a question of settling questions, then I'll add one, should anyone have the time and energy (and a sufficient number of Zens on hand):
--To wit--try the two Zens with one input inverted, thus converting the whole contraption into a push-pull, class A amp. (Connect grounds and run the speaker from the two hot output connections.) Yes, by definition, you lose your 'single-endedness'...or do you? A nice philosophical point upon which to split hairs.
Anyway, depending on your perspective, you could easily come to look at this as a mid-point between a Zen and a Son of Zen. Call it an Illegitimate Son of Zen, so to speak. Consider the possibilities--an active current source (I)SOZ, with the efficiency increase inherent in having replaced a resistor or two.
Such a thing wouldn't be all that difficult to build from the ground up, but even so, having complete Zens on hand reduces it to a simple question of building blocks.
And speaking of feedback, if you were to connect a resistor between the two halves of the circuit, you'd have this really neat way of cancelling distortion. Oh, never mind...I'm sure if it worked, someone would have done it by now...(ahem)


peted 16th March 2001 09:23 AM

Joe, you mention amps blowing when strapped - agreed - I reckon this could be a problem when there's a) lots of gain, b) very low o/p impedance, therefore the conflict at the output due to gain mismatches is serious => channels fight each other. Neither of these conditions exist in a Zen, so I would think any mismatch (minimal with 1% resistors, even with out tweaking) will just be 'soaked' up by the high o/p impedance. I do wonder though what the effects are of the o/p signal from one amp being fed back (shared) to the other amp - we'll see. I'm about a week away from 'fire up' myself.


promitheus 16th March 2001 09:41 AM

When one input is inverted and you connect the speakers on the two plus outputs you have bridged operation. It has nothing to do with pushpull. It is still single ended but it works totally different. It´s like the X-amps from Pass Labs but it doesn´t have the super symmetry connection.
To parallel 2 Zen amps you just connect inputs together and outputs together.

Joe Berry 16th March 2001 04:20 PM

Peted and Mark,

Thinking more about this, I now see that you may actually be better off *without* strapped feedback in this case. In the Zen, such a strap will force each IRFP140 to see the same DC bias voltage. And since the Zen does not use source degeneration, if there is any mismatch between these MOSFETs they will not share the quiescent current equally.

So, I'm going to backpedal shamelessly and suggest that you follow Nelson's guidelines to the letter. Sorry about this unecessary detour - hope it was interesting anyway. ;-) And again, I do ordinarily recommend three-point strapping for circuits in which a 1% mismatch in feedback could cause the two amps to head off in opposite directions.

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