ZV9 as transconductance amp

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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
 
The one and only
Joined 2001
Paid Member
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.
 
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
 
AER driver

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
 
CCS for stability?

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
 
FR Driver, CT Winters

SM:

Sometimes I do miss the winters; believe it or not, living in this constant sunshine and 70 degrees can get pretty boring, as does the skin cancer. I drive up to a nearby mountain village every time they have snow up there, just to see evergreens and get a feel "like home..." Plus, we don't have Frank Pepe's Pizza out here.

My present setup is not very exotic...Fostex FE103 in a small vented cabinet. Amazingly good little speaker, driving it with a SE 2A3 of my own design/construction. I plan to branch out and get more seriously into the FR driver thing, maybe build a back-loaded horn or a Voigt pipe with a larger driver.

I LOVE that FR sound!

Dennis
 
The one and only
Joined 2001
Paid Member
ZV9 contains the schematic to a perfectly fine transconductance
amp using a CCS. From there, we added feedback and then
we modified the current source for AC gain.

The CCS version has been properly evaluated as a real amplifier,
and has about 20 dB of gain into an 8 ohm load with an output
impedance of about 80 ohms (with the 100 ohm output resistor).

The Aleph version without feedback will have about 6 dB more
gain, and I have not evaluated it for stability with all loads -
if you want to use it, I suggest you hang some resistance
off the output and take a look at the square wave.

:cool:
 
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