Zen current amplifier

[Kashmire]

After reading Nelson's most recent AudioXpress article, I've been thinking about a "zen" current amplifier, or transconductance amplifier, if you will.

I have a pair of Fostex, and have never been satisfied. I am building my third set of cabinets this weekend. I've tried horn, ported, and now I'm trying a dual-ported design. I wonder if my problems have been amp related? I've tried the Zen v.4, McIntosh MC-2100, and various other amps with disappointing results with my previous cabinets.

Nelson's article may have been very timely for me. Any ideas for a single-ended transconductance amp? The Zen v.4 delivers output voltage based on input voltage. It clips when it either runs out of voltage, or if it can't sustain current into a low-impedance load.

A current amplifier would output current based on input voltage. Such an amp would clip when it either runs out of current, or it can't sustain voltage into a high-impedance load...

With infinite output impedance, such an amplifier seems appealing for my Fostex projects.

[Kashmire]

Not to look naive, I’ve built current amplifiers before. Pictures are posted here.

I’m looking for the “zen” approach, such as the Lewis amplifier mentioned in the AES publications. I suppose that is what Nelson has adopted for his First Watt amplifiers.

[tschrama]

The first ZEN amplifier is very easy to modify into a current output amplifier. Just remove the voltage feedback by removing the series input resistor. But now the gain of the amplifier might be too much and linearity might suffer. Therefore insert a source-resistor (... emittor resistor) of about 1 or 1.5 Ohms.. Now the gain is more practical and the amplifier is more linear. The DC output value must be re-adjusted now, but it allready is a current output amplifier. It probberbly has about the same distortion figure as the original ZEN amplifier, just 3 volts or so less voltage output..


Regards,
Thijs

[Nelson Pass]

You will also want to remove the feedback resistor from
the output to the input.

[tschrama]

Thx Nelson, allthough I was refering to the very first ZEN version, without the feedback around the output cap.

Something similar can be done with ZEN4 amplifier.. resulting in a new unique amplifier?


Offtopic.. I was very pleasantly suprrised that my simulations THD were so close to the THD values you published. I measured at 0.1, 1 and 8Watt, using the ZEN, ZEN revised and the Aleph ZEN, with IRF044 or IRF140.. everything was really close the the published graphs...

Kind regards,
Thijs

[Nelson Pass]

There has always been a feedback resistor of some sort
from Drain back to Gate, if only to set the DC bias value.

[tschrama]

Offcourse... but shouldn't that stay there to set the DC bias point..?

[stefanobilliani]

Originally posted by tschrama
Offcourse... but shouldn't that stay there to set the DC bias point..?

Yes , but it is feedback .

The setting of the DC point should not use it
in this case .

[tschrama]

Hmmmm maybe I wasn't clear... the series-input resistor should be removed..that detemmines the amount of feedback.... do not remove the bias setting resistors. ..


I never said there was no feedback...

[stefanobilliani]

Originally posted by tschrama
Hmmmm maybe I wasn't clear... the series-input resistor should be removed..that detemmines the amount of feedback.... do not remove the bias setting resistors. ..


I never said there was no feedback...

I was thinking that the input series resistor was a consequence of the out-to-in feedbak resistor(sum point);
so that if you want remove the ser.input resistor removing the out-to-in feedback res. is a must.