ZV9 as transconductance amp

[we416b]

Hello,
Except for a brief fling building ic phono stages around 1980, I'm basically a newbie to solid state. My system from line stage on is all DHT's. But all this current source amp and jfet talk has got me curious for use with full range drivers. In ZV9 Nelson, whose generosity I deeply appreciate, suggests that the fig.4 amp may be sufficient for someone like me. But that Aleph current source in fig.9 is awfully tempting. If I cut out the FB network (R16, 17, and C8) and use fig.4's input circuit, have I still got a current source amp? Or does the Aleph circuit itself control output impedance in ways that prevent current source operation? Paul Ebert posed this question in the Zv9 thread, but I don't think it got answered.
Thanks,
Fred

[Nelson Pass]

Yes, it will still work as a transconductance amp, but with no
feedback, the stability becomes more dependent on the
load, so you probably want to think in terms of loading
the amplifier with some resistive value to ensure stability
regardless of speaker impedance.

[we416b]

Thanks, I'll try it.
Fred

[Diogenio]

Nelson:

To further the topic, I am following the same path as Fred. My speakers are full-range driver types, and I want to try a current-source amplifier with freq compensation networks per your article.

You mentioned potential stability problems with ZV9 in the absence of feedback. Questions:

1) Are all Zen amp topologies (less feedback) subject to this same potential instability, or are some versions better suited to current-source operation? I see you chose a differential circuit for the F1...

2) Same question as above, but taken from a sonic standpoint. Does any one Zen version seem better at delivering high quality sonics when running in current source mode?

I am thinking to start off by making the ZV9, as I am intrigued with the triode-like FET curve, and the fact that one can run it with or without feedback. Might be a good platform for me to experiment with...

Thanks,
Dennis

[SMathews]

I too am curious about the performance of the ZV9 as a transconductance amp. I have a bakc loaded horn with a fullrange driver, an AER MD2. Refering to Nelson's "Current Source Amps And Full Range Drivers" article, the AER I believe will be a good match, though the stability without the feedback is of concern.

SMathews

[Diogenio]

Southbury...? Hey, I'm originally from Bristol, been out here in CA for 20 years. I usually get back to CT 3 - 4 times/year...

[Nelson Pass]

If you have any stability concerns, use a CCS as shown
in the article.

[SMathews]

Miss the Winters ? What kinda of FR driver are you using?

SMathews

[SMathews]

If you have any stability concerns, use a CCS as shown

Thanks Nelson. Need to figure out how this works.

SMathews

[Diogenio]

Nelson...now I'm confused. Fred's original post in this thread refered to a ZV9, but using the Aleph current source and no neg FB. Your response to him has been interpreted as the described circuit might have stability problems. In your last post, you have prescribed a current source as a means to stabilize the amplifier.

I guess we are all just wondering if a ZV9 w/ Aleph CCS and no neg FB will be stable straight into the speaker - without any additional compensation or resistive loading?

Thanks,
Dennis