Cello P603

[chrissugar]

Hi all

I'm looking for any documentation (schematics, service notes, detailed pics, anything ) for the very rare Cello Audio Suite, P603 tape replay module.
Anyone can help, please?
Thanks in advance

chrissugar

[ironbut]

Hi Chris,
I don't know if you've done a search under "cello" here yet but there's been a few threads regarding it. I think that Stellavox (aka Charles King) had said something about documentation in one of the thread (just going by memory here though). He'd be a good person to contact regarding this unit too.

[chrissugar]

Thank you Steve

I did a search for Cello but nothing relevant came up. Maybe Charles will see my post and chime in if he has any info.
He is a great guy, already offered his help with my Stellavox request.

chrissugar

[stellavox]

Chris, et al,

Yeah I have everything related to the P603 module but for a number of reasons can't release anything beyond a few pictures - email me if you'd like something.

Key to the operation of all the Cello preamps (and power amp input stages) was their development of what were effectively, op-amp modules constructed with discreet parts.  They had a series of OTA modules which were VERY low noise / high speed voltage amplifiers, and a series of VF modules which were current amps configured for various ouptut drive / load requirements. 

The Audio Suite used these in their modules - phono, line level and tape input modules as well as their output modules, and their later preamps incorporated subsequent versions as well.  The P603 tape preamp used the same amplifying circuit as their P100 phono preamp with the exception of the (active) equalization network.  The 603 had a bunch of 10 turn pots for "tweaking" the tape response at 30hz, 1kHz and 10Khz (Similar to what is in the tubed Manley tape repro electronics).

As an aside, I "grafted" the 603 circuit onto the P100 cards I have and found that some of the pots didn't do much of anything (as I also found playing with the controls on the Manley).  What I've found matters most is the ablilty to control head damping (to minimize head bumps without rolling off the lows) and some method of easily varying the HF pole response (up around 10Khz).  Can only do this well if you have the ability to sweep the entire audio spectrum with a tape / analyzer setup like what the ST 1500 provides.  Also have tried to "recreate" Cellos op amp modules, but found (as they did) that parts like critical semiconductors get obsoleted and finding suitable replacements may be next to impossible.  Cello never obsoleted any of their products and they were selling Audio Suites (first introduced in 1985) right up until they went out of business in early 2001!  The "speed" of their op amps is what I believe gives them their superior sound - superior to everything else I've listened to.  They get the transients right (leading / falling edges with minimal overshoot) - am working with square waves coupled to the head via a flux loop.

Charles

[Studer Fool]

Charles,
This is a very interesting statement:  "What I've found matters most is the ability to control head damping (to minimize head bumps without rolling off the lows)"
Can you elaborate?  Is head damping a matter of adjusting the impedance as seen across the head?  Or is it playing with the "Q" of the playback EQ RC time constants or what?  Is a simple variable R involved or do C's need to be adjusted etc.?????  This is a very interesting subject to me. 

I've also noticed that the Studer A80 electronics have jumpers that slightly shift the EQ turnover frequencies slightly up or down.

Thanks!
Chris

[stellavox]

Chris,

First of all I've had a lot of opportunity to run frequency response tests with a variety of machines and most lately various heads (Nortronics, JRF Magnetics, Nagra, Technics) into my prototype Cello electronics preamp.  It's probably moot to say that most of the heads are pretty flat over the audio spectrum  - so I have been trying to "tweak" the playback EQ circuitry to arrive at the FEWEST number of controls to accommodate the "maximum" number of heads.  To get "flattest" response - typically within 2dB, I've found that a simple way to adjust the HF pole say +- 3 dB at 10khz, combined with some simple way to damp the LF response/headbumps seems to be all I need.  I've been able to tame the LF response by simply "tuning" it via a resistor across the input which could also be done in the feedback network.  As you increase damping the headbump excursions are reduced  - go too far and the overall LF response falls off. 

Interesting that the A80 allows for various HF time constants - I assume this may have to do with  correcting/allowing for mazimum HF extension as the heads wear.  Luckily we'll never have to worry about that; LOL!   

And as posted elsewhere - I'm hearing significant differences in the "sound" of heads - most noticeable in soundstage "smearing" (combined with loss of "air") or what I generically call "reduction of information".

Charles

[Studer Fool]

Thank-you Charles,

Very interesting stuff you're doing, thanks for posting! 

Of course the next question is which heads do you like best?  I'll go out on a limb and guess Nortronics based on your prior post indicating their usage in Stellavox machines???  (if I got that right)  Or maybe you'd rather not say, lest you get caught-up in controversy?  Or cause a run-up in market value etc.  I'd understand that.  Sorry, I just can't resist asking!

Chris

[ironbut]

I agree with Chris, wonderful stuff Charles.

To add to our data base here, would you mind posting the spec's of the different heads you've tried ( in the case of Nortronics, with model numbers).

[newmedia]

Charles:

Good to find you and Rich on this forum -- looking forward to some interesting exchanges.

I have some CAD files that document some aspects of the Cello effort, including OTA-3 schematic, but I'm wondering about releasing it into the "wild" -- perhaps you'd like offer to advice?

Mark Stahlman
New York City

[stellavox]

Hi Mark,

Good to "see" you also.

I'm making up a limited number of preamps based on the P603 electronics but without all the (somewhat unnecessary) pots.  has switchable NAB/IEC tape EQ and also includes RIAA.  Those who have heard them love them.  Colangelo was a genius.

Charles