Gainclone with buffer & DC servo

[Dxvideo]

Hi all,

I'm going to make a gainclone with dc servo and buffer stages..
This is what I'm planning to make;

As you can see in the schematic, I am using OPA2134 for both dc servo and buffer stages.. DC servo is directly from National BPA200 application and just made a T filter added to the output..
And the second modification is; a class A bias circuit added to the buffer output..
Thats all...
Do you have anything to say about the project?
Thanks in advance..

[AndrewT]

Add HP and LP filters to the input.
Add all the optional components to the chipamp.
Add DC detection and speaker protection, crowbar and fuses, and/or relay

[Dxvideo]

Thanks Andrew,
I was already planning to add a speaker protection to the outputs.. Thats ok.
And, I guess it will be enough to add a little value (say 100p) ML ceramics after 680R for HF filter.. (Do I also need it for the NFB line?) So thats ok too..
Concerning the LF filter............. I am not sure about that, because as my experiences the input capacitor affects the sound too much. I have tried various values and quality of input caps and heard a lot of bad affects (espacially for the bass stage) on the sound. I know, it looks a bit dangerous but if I will add a dc protection to the outputs then what will I loose?

Anyway,
In fact I was asking for especially about the biasing circuit on the buffers output.. Do you think its placed truely? (its sinking 2mA)

[AndrewT]

Hi,
does the 2134 have a quasi output stage and does it use NPN devices there?
If so, then your 2mA sink goes to the correct rail.
2mA does not drive significant capacitance particularly fast. Allow some current into your expected resistive load and also allow current to charge up all the parasitic capacitances @ your chosen slew rate. Your total should then determine what current sink you add on.
2mA seems a bit low.

Have you tried the 2134 as a buffer with and without the ClassA sink? does it sound better with a resistor, or BJT CCS, or a Jfet CCS or a cascoded BJT/FET CCS or a cascoded all FET CCS.

I would be very tempted to sort out that, before incorporating it into a buffered chipamp.

I only see a buffer as a fix for a high source impedance, not as a fix for the receiver.

[Dxvideo]

Dear Andrew,

I dont have information about the OPA2134 output stage! And there is no data on the datasheet also..So it seems a problem about this issue...
And yes, I have tried OPA134 as input buffer for LM3875. It sounds a bit "bright" compared to 2SK170 buffer (Pedja's buffer) and LM4562 preamp (3X gain). However I think it has better PSR and THD ratios (not against LM4562, its exactly better than OPA) also. And if you consider I am making an "all on-board" tiny amplifier then it looks a better alternative to use just a double opamp for all.. I dont use LM4562, because it is not suitable for both dc servo (because it has BJT inputs) and unity gain applications..
Concerning the bias current for buffer stage; I will replace the fixed resistor with a trimpot, so I can find the best value by listening I guess..

[AndrewT]

why on earth are you trying to fix the receiver when you already know that your proposed fix sounds a bit bright?

[Dxvideo]

May be I like sounds brighter Jokeing anyway,
In my experience with OPA134 there was no LPF.. So I am expecting to have more smooth sound with some frequency compensation!
Or do you advice a better opamp for this position that sounds more musical?

[AndrewT]

you don't need an opamp in front of the chipamp.
It is an opamp with a beefy back end.

Don't try to fix the receiver, when there's nothing wrong with it's input.

Use the two channels of the 2134 for two DC servos.

[Dxvideo]

This is a mono project and there are a lot of alternatives for just DC servo against OPA, may be a LF411 will do that job like BPA 200 project.. Or a LF412 for both dc servo and buffer!
By the way,
I found a nude LM3875's sound enoughly "bright".. So there will be no difference on the sound. I was expecting to have a lower input bias current receiver with an OPA on the input. Because I feel some lack of treble detail with nude gainclone on some sources.
By the way; what can you say about AD797 as an input buffer?

[AndrewT]

Originally posted by Dxvideo
I feel some lack of treble detail with nude gainclone on some sources.

this could be a source impedance problem.

As I said, don't try to fix the receiver. Fix the source.

I found a nude LM3875's sound enoughly "bright"..

and you now want to fit a bright sounding opamp in front of it????