Zen current amplifier

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K

Kashmire

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Joined 2003
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.
 
T

tschrama

Member
Joined 2001
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
 
T

tschrama

Member
Joined 2001
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
 
S

stefanobilliani

Member
Joined 2002
tschrama said:
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...
Click to expand...


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.


:bigeyes:
 
T

tschrama

Member
Joined 2001
Due to the limited 'openloop' gain, the input impedance stays fairly high, even if the series-input resistor is removed. So the bias-resistors can stay, while the series-input resistor can be removed... this is a very easy modification for people wanting to experiment with current drive...
 
Nelson Pass

Nelson Pass

The one and only
Joined 2001
Paid Member
The way this discussion is going, I think you're better
off using the Source resistor (1 to 2 ohms) on the
Mosfet and biasing the input Gate with an adjustable
voltage source (4-6 volts) through a high value resistance.
Keep the 221 ohm Gate resistor and input capacitor.
 
K

Kashmire

Member
Joined 2003
I’ve drawn up a schematic of the Zen v.4 in “current mode”. I’ve studied it a lot over the past couple days. When running a MOSFET in common-source configuration, it will operate as a current amplifier – right? Nelson forces the thing to operate in voltage mode with a feedback loop. Remove the feedback loop, and you’ve got a current amplifier.

However, there is local feedback, which is defined by the AC component of Vgs (gate to source voltage). As the AC signal is being amplified, any resistance between the source and “ground” is in the local feedback path. The bigger the resistance, the more local feedback, resulting in less gain and less distortion. A value of about 1 ohm, as mentioned by tsc, is a very good starting point.

The DC component of Vgd is irrelevant to being either current-amp or voltage-amp. The DC component (otherwise known as the bias point) should keep the drain at ½ of the power supply voltage.

Thanks for the replies. It made me go back and study the common-source configuration.
 
Vix

Vix

Member
Joined 2003
Hi Kashmire

I have a ZEN V3 amp and Fostex FE 206E speakers. While ZEN was wonderful with some other speakers, with Fostex it was disapointing. In fact, Fostex sounded bad. I've also been thinking about modifying ZEN V3 into current amp.

So far, what are your results? Does it sound better? Could you post the schematics of the modified ZEN?

thanks

Vix
 
K

Kashmire

Member
Joined 2003
I noticed the Aleph current source can't be used in current-amplifier mode. The feedback up to the current source acts as positive feedback, which increases gain and distortion. With some loads, it may go unstable.

The feedback to the voltage regulator can still be used.

I have a Fostex FE207E. I'm on my third enclosure. I can't make it sound good. This was my last attempt, until Nelson published his latest article. Now I'm interested again, and I'll give the Fostex one more try, this time with a current amp.
 
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