Counterpoint - A Fully Symmetrical I/V stage

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Hello Folks,

Some of you may already know I have had a new I/V stage in the works for a while now. Some of you have probably been a bit frustrated it took me so long, but this is has been a labor of love and I have poured a lot of thought and care into the design.

The initial design was a voltage feedback design similar to a THS4131. This design is more like (If I am not mistaken) a current feedback design. It is not Super Symmetrical in that the feedback in not taken at the bases of the input transistors, but rather at the emitters, but it still is fully symmetrical in operation. It is not really an opamp per se either, but a true transimpedance stage. While it does employ feedback, it is not subject (at least to a great degree) to the same negative issues that plague most opamp based I/V designs.

Here were some of my basic design goals:

1) The design should not just be simply balanced, but fully symmetrical for use with balanced current output DACs. Good candidate DACs include PCM1794A, ES9008 (ESS Sabre), and many others.

2) The design should not be subject (at least less subject) to the usual slewing limitations of a inverting or conventional VFB fully symmetrical opamps.

3) The design should be approachable for DIY use. What I mean by this is it should use commonly available parts and not be expensive to make while still offering excellent results. No exotic or expensive parts are required.

4) It should be very low distortion with a high dynamic range. This it is, but I do not have any solid numbers yet. I have a volunteer who will measure it for me once I provide him the PCB. What I do know is the test circuit had THD too low for EasyLab to measure, what I mean is it was at the floor which is about 110db. So I have to have someone with better test gear test it out for me.

5) It should be unique! I have not seen anything exactly like it, but it does share similarities with other designs.

6) It should allow input voltage bias adjustment so the the input can be at AVCC/2 for chips like the ES9008 or at 0V for chips like PCM1794A.

7) It should have no DC at the outputs either between the outputs or from the outputs to GND. The integrator handles this and it also allows for single ended output without any significant THD penalty. There is a potentiometer to trim the output DC to 0V. This is important as without it you would need very very good parts matching to obtain good results. As it is, while good part matching helps it is actually not actually required.

8) It should have effective HF filtering. It does :)

OK enough about the design goals. Here is my cct.

I have tested the cct (on protoboard) , but I have not received the final PCB yet, when I do I will have a better idea how it sounds.

The power supply should be +/- 9-12V rails with the parts shown. I would not go above 12V without adjustments.

Cheers!
Russ
 

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PCB

Here is the PCB:
 

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Very neat.

As I understand it, it has (near) unity current gain, and a high open loop output impedance - it basically turns the DAC into a near perfect current source. The open loop bandwidth will be super wide; so it should tolerate nasty switching edges pretty well.

You then can do the filtering at the output with C4.

If you wanted, you could also do some filtering at the input, shunting the virtual earth points (X1.1 and X1.7) to ground via a cap, so that when the feedback loop eventually runs out of puff in the HF, the impedance falls, rather than rising.

This would doubtless mean ages fiddling to get it stable again!
 
PigletsDad said:
Very neat.

If you wanted, you could also do some filtering at the input, shunting the virtual earth points (X1.1 and X1.7) to ground via a cap, so that when the feedback loop eventually runs out of puff in the HF, the impedance falls, rather than rising.

This would doubtless mean ages fiddling to get it stable again!

Thanks PD, I actually thought of this too, but I found that adding the caps at the inputs alone seems to produce the opposite of the intended effect. It actually caused more gain at high frequency, and not less... at least in simulation.

But perhaps the use of both C4 and those caps would have some positive effect, I am not sure. It does appear to remain stable in this setup. At least with 1nf at the inputs to GND.

Cheers!
Russ
 
mmmmm,

I see 2 points labelled "out"

I see no points labelled "in"

perhaps X1.6 & X1.2 are input's but above those u have a cap labelled "out"

X1.1 & X1.7 could be inputs but they are just connected with a resistor to the points labelled "out" . . .

your right Russ I didn't see a circuit like this before !

or is it just too early in the morning here . . .

mmmmm . . . ??????
 
coloradosound said:
nice work Russ!


rjbaldwin said:
As ever Russ, it is the care and thought you put in that most impresses me. Mind you I'm looking forward to hearing it too : )

Russ


Knorfski said:
Good job Russ!

Can't wait when the Counterpoint will be available. I just obtained a Buffalo today but I don't have a I/V Stage and they are not available at the moment.

This one looks very promising! When do you think when it will be available ?

Thanks Folks,

As soon as I get a chance to test the circuit on the PCBs which should be here in a week or two. So I would guess 2-3 weeks. I ordered plenty so that early adopters can try it out with me. :)

I will likely try the indicated transistors plus BC550/BC560. Using transistors from the same batch is likely going to be sufficient matching. There are quite a few transistors that will work very well.

Cheers!
Russ
 
korben69 said:
Hi Russ,

Could you give the values in order to make it works with Opus...
Maybe I'm wrong and it doesn't fit with it.
Thanks ;)


It is not really designed to work with Opus, as Opus a voltage output DAC that works better into a high impedance. The counterpoint is an extremely low impedance. About 65 milliohms. Current sources like a low impedance, and voltage sources a high impedance. :)

Counterpoint will work great with COD or Buffalo.

I will put the details to make it work with COD in the Manual. It pretty simple just connect the AVCC input to GND, and adjust R20-R21 to get the desired voltage swing. I am guessing something around 680 ohms will get you close.

:EDIT: OK, just simulated for COD(PCM1794A) and it looks like R20/21 at 887 ohms will get you close to 2VRMS.:/EDIT:

Cheers!
Russ
 
Hi Russ, George from Australia, I've bought a few things from you in the past. I'm looking for a new discrete I/V supplier.
I been using the LC Audio Zapfilter MkII in current input mode (transimpedence) for a while now with all the current output dacs that I've moded for customers, they have an class A output as well on their board, and overall it has about 6db of gain which can be good for passive pre's but a bit much when using an active pre, how would you compare yours against their's as because of the Rohs thing in Europe now I cannot get it anymore, as they are having problems with the lead free solder.

LC Zap in current imput Transimpedence mode attach (dc servos not shown)

Cheers George
 

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HI George,

The gain can be adjusted pretty easily. The output impedance will be somewhat higher with the counterpoint, but not a lot. The counterpoint will have lower distortion.

The counterpoint is truly fully symmetrical, but you can take the outputs either balanced or SE.

If you need a very low output impedance then a buffer would be desirable after the counterpoint.

Cheers!
Russ
 
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