Cello VF-1

[mikep]

voltage follower.  I think this was being used in a gyrator but I didn't trace the whole circuit so no promises.

[pstamler]

Very elegant circuit! How much current are those CCS's supplying?

Peace,
Paul

[SSLtech]

Cunning!

Keith

[JohnRoberts]

The cross-coupling just jumped off the page at me. Would anyone care to comment on that? It kinda reminds me of an old National buffer -  I think an LH0002...

Damn! I have a better memory than I thought:

But the LH0002 doesn't cross-couple to the emitters but to the rails in a more conventional-looking topology. So what's with the emitter-tied toplogy?

There's only so many ways to connect those devices to get a first order cancellation of Vb-e drops. The current sources and extra resistors improve drive capability with simple single +/- rails, and help set output stage class A bias which the LH0002 manages with device geometry and relative PS voltages at V1 and V2. IIRC they could be current hogs if biased up.

I too am a little uncomfortable with the collectors of the first stage connected to the emitters of the latter. Operating on the cusp of saturation like that can somewhat impact device parameters but apparently not enough to concern them in the middle of a larger circuit driving known well behaved loads.

The 0002 IIRC was designed to be embedded inside an overall feedback loop of another device for analog applications so it's nonlinearities would be corrected for. If the VF-1 introduces a desirable coloration, it might be useful to understand precisely how it deviates from linear and perhaps use that if/when appropriate.


JR

[mikep]

Very elegant circuit! How much current are those CCS's supplying?

J505's are 1mA (+/- 20%)

that sets up roughly 6ma in the output stage.

John Roberts, which device parameters do you think would be impacted and how?  If you don't mind explaining.

[mikep]

If the VF-1 introduces a desirable coloration, it might be useful to understand precisely how it deviates from linear and perhaps use that if/when appropriate.

acording to my tests it is very clean, no noticable coloration at all.  it sims flat to 50MHz w/ 2n4401/4403.  It can swing within .7v of either rail and has well behaived overload characteristics.  there is a DC offset though.  it doesn't seem like they did any device matching to minimize offset, either. there are six of these in the unit and the offsets are spread over a range of maybe 60mV IIRC.

mike

[mikep]

mikep: So these VF-1 blocks drive the filters somehow?

Out of respect for the designer I don't want to put the whole circuit out there.  Im not sure myself, I haven't traced it yet.  but yes, they are in the filter sections.   Each band (low-Q peaking response) has an OTA-1 inverting hi-gain amp and one of these followers.  the maximum boost and cut on the high and low bands is something like 25 dB in 1dB steps. in the midrange it is much less and smaller steps.  the VF-1 block is not used to drive an output.

mike

[bcarso]

What the first Q's' collector connections do is bootstrap out the Ccb's, making the input C very low at midfrequencies.  It will as well produce a negative input Z at sufficiently high frequencies though, so beware.

A more brutal direct connection of collectors, putting zero volts C-B across the input devices, is shown in Feucht's book, Handbook of Analog Circuit Design.

Another advantage: reduced power dissipation in the input devices, and opne which is roughly invariant with signal level.  However, this still leaves the output devices' thermal distortion to contend with.

[JohnRoberts]

John Roberts, which device parameters do you think would be impacted and how?  If you don't mind explaining.

I believe I couched my comment with words like “could” and “somewhat”.

For one I believe the Miller capacitance (Cob?) may increase as collector voltage drops which could impact GBW or HF coupling from output to input. I'm not sure if beta changes around saturation are cause or effect, but input impedance will change as the collector diode transitions from reverse to forward conduction. Presumably this circuit is intended to be driven from a high source impedance so input effects could be important.

Not to weasel too much, but my comments are speculative and will surely be different for a heavily loaded circuit compared to a lightly loaded one. If it sounds good used as designed it is good, but wider application may need to explore how it acts under different loads.

JR

PS: as a vaguely connected dot,  I attended the Mark Levinson bankruptcy auction (in New Haven as I recall). I won my bid for a few Sound Tech distortion analyzers, but at the end the individual items auction total didn't exceed an entirety bid (Madrigal?) so I pretty much wasted my day and left empty handed.

[mikep]

If the VF-1 introduces a desirable coloration, it might be useful to understand precisely how it deviates from linear and perhaps use that if/when appropriate.

acording to my tests it is very clean, no noticable coloration at all.  with low-z drive, it sims flat to 50MHz w/ 2n4401/4403. higher input-Z causes peaking.  It can swing within .7v of either rail and has well behaived overload characteristics.  there is a DC offset though.  it doesn't seem like they did any device matching to minimize offset, either. there are six of these in the unit and the offsets are spread over a range of maybe 60mV IIRC.

mike

[bcarso]

If the VF-1 introduces a desirable coloration, it might be useful to understand precisely how it deviates from linear and perhaps use that if/when appropriate.

acording to my tests it is very clean, no noticable coloration at all.  with low-z drive, it sims flat to 50MHz w/ 2n4401/4403. higher input-Z causes peaking.  It can swing within .7v of either rail and has well behaived overload characteristics.  there is a DC offset though.  it doesn't seem like they did any device matching to minimize offset, either. there are six of these in the unit and the offsets are spread over a range of maybe 60mV IIRC.

mike

That peaking at higher input Z I believe is a consequence of the negative input impedance due to the bootstrapping, mostly.  Putting a small C to ground at the input (few pF) should correct it.

[elsokwak]

Without seeing the schematic I am not able to follow this thread

[Gordy]

Hello elsokwak, the original post with the circuit diagram was in December 2006, so I guess that it has dissapeared from the server since then. Maybe you can contact mikep with a private message...

[elsokwak]

Hello elsokwak, the original post with the circuit diagram was in December 2006, so I guess that it has dissapeared from the server since then. Maybe you can contact mikep with a private message...

Hi Gordy I did send Mikep a PM on the forum system but it did not get through as his "mailbox" here is full. Perhaps you have his email? All the Cello schematics he posted are gone (404 error). Thanks!

[marginwalker]

Hi guys, i have the same problem. Mike's PM box is full and i cannot find the schematic.
If someone wish to share, please send me a PM.
THANKS!

[okgb]

maybe he took it down , if it is still
a comercial selling item ?
we have had some problems with cloning

[tsvisser]

If anyone can PM or email me schematics for the VF-1, or any of the OTA's for that matter, I'd greatly appreciate it.  Sorry my first post is grubbing for info.  Otherwise I'll try to PM mikep again when his inbox is not full.

t s v i s s e r !AT} y a h o o :DOT| c o m