cello OTA-1

[bcarso]

[quote author="mikep"]I don't know Brad, I just don't want to be "that guy". Ill share the schematic with anyone else who wants to try to learn from it (like me), or who wants to try it in their own noncomercial DIY projects.

Tom C. is still making equipment
http://www.violalabs.com/index.html

and this?
http://www.cello-audio.net/intro.cfm

there is also some people (in washing state I believe) doing system design/integration under the Cello name. I think they had something to do with the original company as well but I'm not sure.[/quote]

Cool! Best of luck to them.

 

[mikep]

[quote author="bcarso"][is it really a 3819? [/quote]

I picked that one for no reason, I hadn't really run a sim yet when I posted that. Ive got it on a breadboard with 2sk170 for the input FET and the CCS (with a 330 ohm resistor in the source for 1mA). This is what I have on hand. seems to work. Do you think this is an ok FET for this circuit?

sounds good as a headphone amp by the way. Im using 600ohm K240s in mono.

mike

 

[bcarso]

[quote author="mikep"][quote author="bcarso"][is it really a 3819? [/quote]

I picked that one for no reason, I hadn't really run a sim yet when I posted that. Ive got it on a breadboard with 2sk170 for the input FET and the CCS (with a 330 ohm resistor in the source for 1mA). This is what I have on hand. seems to work. Do you think this is an ok FET for this circuit?

sounds good as a headphone amp by the way. Im using 600ohm K240s in mono.

mike[/quote]

So I guess we don't know what the other transistors are in the picture (that is, besides the ones we can read off the tops of the TO5 cans)?

I'm sure a number of FETs will work with suitable adjustment. In the case of the 2SK170 the pinchoff voltage is so low that the input offset with the PNP grounded will likely be fairly positive (|Vgs| < Vbe + Is*R2). But nothing wrong with that, unless you are d.c.-coupling to a sensitive load. You can always run the Q1 base to a suitable voltage divider and trim out the offset.

 

[bcarso]

Having tons of other critical things to do, naturally I felt compelled to spend some time looking at this circuit a bit more.

It would definitely be interesting to know the FET type. But, assuming a 2SK170, and MPSW06 and MPSW56 parts for the others, I find that my rough analysis is borne out by simulation.

The gain in the FET source path is quite low compared to the FET drain path. So when you are swinging current into a load, the upper output Q does most of the work. I suspect that partly due to this asymmetry, the block has more second harmonic distortion than it could with a somewhat different configuration. However, it also has quite a lot of higher harmonic products, and at the levels mentioned below they are not falling off with order very fast. So I would expect this to sound a little transistor-ey in the pejorative sense.

To put this in perspective though, the overall THD for 1V peak in, 4V peak out with the values shown in the original schematic is still only 16 ppm (0.0016%), which is pretty decent.

I will try to bias it just right so I can look at open-loop distortion.

 

[mikep]

Brad, the FET is PF5102, the PNP's are 2N6730 the output pair is 2n5323/5321.

[quote author="bcarso"]However, it also has quite a lot of higher harmonic products, and at the levels mentioned below they are not falling off with order very fast. So I would expect this to sound a little transistor-ey in the pejorative sense.[/quote]

Im no spice master but my sims show 2nd at -90, nothing else visable. my breadboard circuit with K170, 2N4403 and BD139/140 seems very clean. I can hear a *slight* 2nd harmonic in the residual when driving +18 into 500 ohms. but it is below the noise floor. I can't measure any THD at lower levels.

 

[bcarso]

[quote author="mikep"]Brad, the FET is PF5102, the PNP's are 2N6730 the output pair is 2n5323/5321.
.[/quote]

Was that PF5102 in your original post pre-deletion? I remember reading it and then couldn't find it.

The 5102 is indeed a process 51, and selected for low pinchoff voltage. It may even be that Cello tried to select additionally for a Vgs at the operating point to have a temperature coefficient complementary to the PNP.

In any event, your sim results are consistent with mine, except that I may see down further into the higher order products with my program. They will be low enough if they follow the model to be difficult to measure, to be sure.

I set about trying to modify the design without changing things too much so that the lower section would work about as hard as the upper. I haven't been entirely successful yet, but along the way reduced distortion significantly. The trick is to replace R3 with a current source, which for a practical one needs a bit more voltage compliance room than a simple current regulator diode, so that's a little less practical right there. Then, I got another major improvement by cascading common-base stages in series with the output transistors.

Although the results will be spoiled by self-heating distortions, I'm now seeing, for the same output swing into 2k, second at -138dB.

Almost worth a build :razz:

 

[Nrik]

Hi
...New to this forum, and found this thread. (i know it is old)

I am very interested in the Audio Palette, so are there any-one that could mail me the shcematics of the OTA and the VF ?
Thanks a bunch.
Henrik

 

[okgb]

Hi mike , just saw this as someone else replied
would love to see the schematic , not likely to get to building
even ,  how did you make out  ?
was the amp the special thing and the rest of the eq average ?

tia regards Greg  gboboski at shaw.ca

 

[thermionic]

Unfortunately, Tom Colangelo was killed in a car accident just over a year ago: http://www.stereophile.com/news/091707colangelo/

Interestingly enough, I spent some time last week with his 1978 ML-1 preamp. I found it to be quiet (particularly considering its age) and felt it was ahead of its time in many ways. I'm not sure about those connectors though - they didn't seem to catch on.

I'm totally going OT here, but along these lines:

You can see some internal shots of the later Cello 'Pallette' preamp, complete with the Devil's own op-amps and pots here: 

http://www.diyaudio.com/forums/attachment.php?s=16efb95384898c13b676d7a103d61ea6&postid=1246367&stamp=1183349410

http://www.diyaudio.com/forums/attachment.php?s=16efb95384898c13b676d7a103d61ea6&postid=1246383&stamp=1183352125

http://www.diyaudio.com/forums/attachment.php?s=16efb95384898c13b676d7a103d61ea6&postid=1246387&stamp=1183352605

http://www.diyaudio.com/forums/attachment.php?s=16efb95384898c13b676d7a103d61ea6&postid=1248251&stamp=1183544843

http://www.diyaudio.com/forums/attachment.php?s=16efb95384898c13b676d7a103d61ea6&postid=1248264&stamp=1183545803

(I think you might need to join the DIYAudio group though - sorry for the inconvenience).

Going off at even more of a tangent, you can see schematics for the ML1's successor, the JC1, designed I believe by Colangelo and Curl, here:  http://www.marklev.com/JC2/jc2schematics.jpg



Justin

 

[mikep]

Thanks for the post, justin. I missed that announcement, sad.  What do you guys think, should I post the old tom c. Cello schematics? I never did share the main opamp block "ota-2"

 

[Nrik]

mikep
At www.diyaudio.com people usually just email schematics to each other, to avoid the risk of violating copyrights.

 

[Nrik]

What do you guys think, should I post the old tom c. Cello schematics? I never did share the main opamp block "ota-2"

MikeP  yes definitely, all three 
Pretty-please with sugar on top

superduper thankyou in advance

 

[clintrubber]

Please post them to the gmail-account of this group.

Thanks

 

[enymion]

Time to revive this subject!
Did any one ever get these? Or did anyone save the pics from the original posting?
I'm extremely interested in this and would really appreciate any help on the subject.

Also, being relatively new to this group, could any one enlighten me to what is meant in the previous post by the mention of a group g-mail account?

All the best

 

[clintrubber]

Also, being relatively new to this group, could any one enlighten me to what is meant in the previous post by the mention of a group g-mail account?

All the best

I sent you a PM with the info.

In the light of the recent real or not real deletions at the account
I guess it's best to have this info not displayed at too many places...


Bye,

  Peter

 

[enymion]

Thanks Peter,

very useful,found some other interesting stuff there. Rogue deletions seem to be the bane of all shared applications!

All the best

 

[michele.buzzi]

Hi all,

I'm trying to send a PM to mikep but I always get the message of a full mailbox.
Am I having some problem with the PM system?

Thanks,
Michele Buzzi

 

[elsokwak]

Hi Mike, I did send you a private message but your mail-box here is full.
Could you or anyone else send me the schematic of the Cello OTA discrete opamp please?

Please send to elsokwak at home.nl
at=@ (no spaces)
Many thanks in advance!

 

[elsokwak]

Anyone can help me, please?
Anybody???