DC Servo question

[Dxvideo]

Hi all,

I am planning to make another LM4702 amp with SAP15 devices (which is published in another topic in this forum). The amp will be in non inverted configuration with 33/1K (34X gain) and will be feeded by a 2 x 30V AC trafo and 20,000uF per rail PSU. That gives me ~42v per rail.
And in this amp there will be a DC servo circuit to prevent DC offset on output and for DC blocking.
My question is;
What opamp should I use for DC servo?
Is this configuration ok for me?;

An externally hosted image should be here but it was not working when we last tested it.

Of course some calculations will be made. For U2 I will use LF412 as National guys recommended.
May you say something for this circuit?
Thx in advance..

 

[artquake]

My question is; What opamp should I use for DC servo?

LF412 in my thoughts sounds ok ... since the circuit run's about @ 7Hz i think it does not matter if you get ones with higher slewrate ...
In my setup (same as in schematic) with a pair of LM4780 they do their job very well.

User "eva" sugested me a time ago, to add also a LowPass-Filter in that circuit as it's in the feedback ...

artQuake

 

[Dxvideo]

Thx again.
In fact my real question is,
If it (the opamp) runs under 7Hz and just for DC error correction, do you think it really affects the sound quality?

 

[artquake]

Theoretically it will affect as things will be amplified thru the amount of gain you'l set, but in my application i practically could not hear any difference with or without servo. Only that the amp's run's really cool, even when pushed harder by using servos.

I found, read, respectively got good informations from eva, tiltedhalo, peranders, andrewT gootee and many other i do not remember at the moment to this topic ... search for DC-Servo in the forum ... i remember that peranders has written something about the frequency and if i remember right he suggested even to go lower (3 ... 5 Hz) ...
thats all i can say

artQuake


Addendum:
Found peranders' statement: LINK

 

[AndrewT]

Yes,
split R5 into 100k & 100k.
add a cap to form a T filter.
Otherwise all the distortion and noise from the servo opamp gets injected into the inverting input of the power amp. The T filter also reduces the level of audio signal passing to the inverting input.

412 or 411 both perform well.
The important parameters are:
FET input for ultra low bias current.
Low offset voltage
Low drift of offset voltage (with time and temperature).

Many modern opamps meet this spec. They do not need speed.

Consider adding a DC detect to the output and what to do do with the detect DC signal. This is necessary since the range of DC error that this circuit can correct is quite low.

 

[Dxvideo]

Ok.. Is this what you mean?

An externally hosted image should be here but it was not working when we last tested it.

If so,
What do you think about the LPF components?
I put there Rf X 100 resistors because National guys say that.
Do you agree?
And what about the T filter cap? Does 470nF enough?
Thx...

 

[artquake]

If I’m right 470nF equals to fc ~ 3.3 Hz … enough ? i think yes. 1uF will take more space i guess

Is C2 in the right position ? Would it not be better to place it between R3 and R4 // to D1 ... electrically it equals the same but in views of flow ... maybe i'm wrong, would be also nice to know ...

artQuake

 

[AndrewT]

post6, remove R4.
Try 470nF in the T filter.

Have you worked out yet what range of output offset can be corrected by your circuit?

 

[Dxvideo]

In this case, inverted input goes float situation ?????
What do you say about the resistor coefficients?
And the circuit is not tested yet. But simulated.
I will feed the LF412 with +/- 18v so I expect at least 13v swings on output. Is that mean it can companse +/-13v DC offsets?

 

[AndrewT]

Hi Dx,
go back and read your own thread.
http://www.diyaudio.com/forums/showthread.php?postid=1283350#post1283350

 

[Dxvideo]

Ok I will..
I dont like flashbacks but sometimes theyre usefull.

 

[artquake]

Hi,

Andrew you wrote that dx should omit R4 in his schematic ...

here's about the same on mine ...

An externally hosted image should be here but it was not working when we last tested it.

Concerning R3, and R13 ... but i do not understand why it should be left out ... could you please explain the reason ?

Thanks in advance
artQuake

 

[AndrewT]

put R4 back in

Hi Art & Dx,
that R4 is correct.
I don't know what I was thinking about, Sorry.

 

[Dxvideo]

So the coefficients are correct.
If artquake's circuit is for 20K/1K gain resistors.

 

[Rob_S]

Hi,

Greg has done a DC servo with relays, you could take a look at this for details
http://www.ska-audio.com/Forum/YaBB.pl?num=1211785192

Cheers
Rob

 

[Dxvideo]

I think its not a DC servo, just a speaker protector..

 

[AndrewT]

No, it's a DC servo with added DC detection and cut off when beyond the range that the servo can correct.

 

[Dxvideo]

So where is the NFB return?
I can only see a speaker in and speaker out connections!

 

[artquake]

So the coefficients are correct. If artquake's circuit is for 20K/1K gain resistors.

That's true dx

Thanks for clarification andrew ...

Isn't it enough adding a servo ? Is there really need for dc detection circuit when servo is running permanently ... would it get somehow more safe when adding dc detection in addition to servo for the worst case when the servo would change polarity ?

best regards
artQuake

 

[Dxvideo]

I think a DC detection circuit is for fault conditions. Because in some case there may be very high DC offset levels on outputs. And if its a fault condition (like output transistors fail) you cannot control the DC offset with a DC servo..