Egmont/Tubecad with CCS

[Ulfar4]

Hi, I know there are many schematics like this but this aims to be a very cheap amplifier for the estats and it should be better than the SRD boxes.

The power supply for tube has +-300 V and there will be four windings on transformer for heaters. I know I can not trust the simulation so I think it will have a maximum output voltage around 500 V peak to peak with some luck, probably less.

I took the CSS from SRX+, so credit goes to JimL. Output current is set to 6mA per one output. So 12mA per channel.

I added small feedback to stabilize it with 470k resistors.

My goal is to make a small cheap amplifier. Voltage output is rather small than other amps but I will not use it with something like SR-009 or simmilar.

THD is approximately 0.4% at 460Vp-p. But I do not know how much are those models reliable, so it can be very different in the real world.

Fr of course drops around 20kHz like all these amplifiers (Triology .....).

Tubes I intend to use are:   ECC83

                                                6SN7

Thanks for any comments and sorry for my english if there is some mistakes.

[PretentiousFood]

Neat!

I'm not sure the feedback works there; the two outputs are summed through R22 and R23 and terminate to a common-mode node. But the two stages are cap-coupled, so common-mode feedback won't do much. Perhaps add some resistance between the two cathodes? Or connect the feedback resistor to the plates rather than cathodes for some trendy schade feedback?

What's C2 for?

[spritzer]

Yeah that feedback is troublesome.  I for one like this circuit without feedback but you could run it through caps. 

C2 isn't needed and neither is C1.... 

[kevin gilmore]

feedback is absolutely not going to work, need resistors between cathodes and R3. But then the gain goes down And then you need C2. And there is not enough voltage gain to begin with.

[Ulfar4]

Thanks, so with feedback it should look like this. I also changed R3/C2 fo CCS. Or in this case it is better to run it without feedback?

[PretentiousFood]

I would probably skip the feedback and cathode Rs (which reduce gain and increase rp), and use a lower mu, lower rp tube like a 12AT7. If you do want to stick with the 12AX7 and nfb, they really do benefit from a plate load several times greater than rp, which shouldn't be too hard to pull off with an lnd150-based gyrator.

The DN2540 only really shines at quiescent currents above 10mA or so. Something like a BSS126/DN2540 cascode or LND150 cascode may be better for the tail of the LTP, but note that the LND150 usually cuts off around 700uA if you try to cascode it.

[JimL]

The CCS should definitely improve the Egmont as congo5 has reported previously, but you are still limited by the basic circuit - yes it's OK if you want to build the simplest, cheapest possible amp, but the basic SRX circuit is a significant step up and not that much more complex or expensive.