CCS is your friend!

[jane]

This is a very simple circuit -- of course there are some compromises here and there.
One of them is the diff stage or input stage/long tail splitter, that it’s not very well balanced. Which will cause some distortion, especially 2nd harmonic (one phase is not equivalent in amplitude too the other).
The sad thing is that the global feedback will be feed through the other side of that stage and produce even higher order distortion. So, 2nd and 3rd order distortion will become 4th, 5th, and 6th order distortion at the output.



The main cure for that is to fix the problem where it's generated: In the diff stage.
Here are some spectrum analyzes of the output signal at 5V rms, before and after putting in a CCS to balance the driver stage:

Output with a 10k resistor as tail (input is 400Hz):


Output with a CCS as tail (input is 400Hz):


It is magic, isn't it?

Jan E Veiset
NO-6600

[rdf]

Part of my standard toolkit now are IXYS CCSs pre-mounted on heatsinks with 10-turn pot adjusts. Incidentally, you may already know but with an adjustable CCS I've found it's possible to null the 3rd and higher deeply if you ignore the 2nd. So far all my projects have been SE though it sounds very promising for PP.

[ddd]

Eliminating 2nd and oth. even harmonics does't lead to better sound by all means. The ratio 2nd/3rd/... is more important imho.

[ray_moth]

Agreed. The odd harmonics are not pleasant, even in small quantities, and they get worse the higher the order.

[gingertube]

Jan,
Thanks for posting the spectrum analyser outputs.

I have found that in using current sources you can in fact make the sound too "dry" - that is too much 3H or not enough 2H.

On solid state stuff I generally match the devices - then use a high value resistive tail OR a single transistor current source - occasionally deliberately compromised by a resistor added between collector and emitter. I use a maximum of 10% emitter degeneration on the diff pair.

On tube stuff I have generally found that I need a cascode transistor current source and rely on increasing 2H as I turn up the volume to balance the 2H and 3H. BUT its still possible to overdo the performance of the CCS. Once again its possible to compromise the CCS performance to "tune" the harmonic content by wiring a resistor from collector to emitter in the bottom transistor.

Cheers,
Ian

[ddd]

Originally posted by gingertube
...Once again its possible to compromise the CCS performance to "tune" the harmonic content by wiring a resistor from collector to emitter in the bottom transistor.

Ian

It's a good method to combine the advantiges - lower |-v| ("C-" on the circuit) and acceptable 2nd/3rd/... ratio. Nice idea, Ian.
The posting of Jan is wery useful anyway! The CCS makes the LTP more symmetric. Every triod of LPT becomes better voltage follower.

[SHiFTY]

Wow that's an impressive result.

Does anyone have any links to schematics for CCS, or where they can be bought?

Thanks

[arnoldc]

CCS? diyAudio sells CCS boards

There's a thread for that and a Beta of the manual.

[VT67]

This is the one: CCS for tubes/valves PCB

Regards

[jane]

The basis of my first posting was pure technical. What sounds better, more sweet, what ever, is of course another discussion.
The CCS that I use is a very simple one, just one single transistor (MJE340) and the reflected source impedance is about ~100k. I'll suppose the measurements would have been even better with a proper CCS...

Jan E Veiset
NO-6600