preamp dc servo

[udip]

hi everyone!!

i have a problem with dc servos..i don't know how to design them
i've searched the net but couldn't find technical papers about them..

i'm designing a mic/di preamp based on the opa627 and don't wanna use capacitors in the audio chain...

does anyone know a good link about designing dc servos???

thx...

 

[soultek]

Check out the white papers and schematics on:

www.jensentransformers.com

 

[peranders]

If you check the datasheet of LT1028 and LT1115, there you have a mic preamp with DC-servo. I have made simulation files for a non-inverting amp and for an inverting amp. Just download LTSpice from Linear Tech.

 

[peranders]

Here are the files. If you present a schematic I can help you to tune the servo.

 

[udip]

thx everybody,

but still i can't find a good white paper on designing them...

soultek, i searched the jensen white papers and couldn't find
one about dc-servos...

i'v checked the lt1115 datasheet, and this is the circuit my preapm
is based on... can't i use the dc-servo with other opamps?
(let say if i replace the lt1115 with opa627, and the lt servo opamp with some other opamp?)

thx

 

[udip]

hi peranders and thanx...

here is my schematics....

can i use the opa2604 as the dc-servo opamp, or the op27?

and how can i combine the DI section into the preamp??
i want to use it either as a DI or MIC preamp with a selector
switch.
i don't know exactly because of the 10r resistor shounted to
ground....

any suggestions?

thanx...

 

[peranders]

Fist of all you have drawn the schematic wrong. You have swpped the inputs. I'll guess you are aming at a non-inverting amp.

I suggest also that you take a look at the jensen site.

Have you checked my simulations files? This is a good way not to calculate too much.

When you design the servo you should aim at 0.5-3 Hz in cut frequency and the range of the servo must also be determined.

Check also the AN-1192

Notice also that OPA627 can't be used with 60 dB gain. The speed is too slow. You need at least 20 dB feedback at 20 kHz.

 

[udip]

peranders,

there is a mistake in the drawing. it's should be non-inverting.
about u'r simulation files...what is the format, i can't open them..

can u please explain how the servo works? and why is the servo
connected to ground via 10 ohm resistor?

thx for the links and files...

 

[udip]

btw... about the opa627 gain... do u think if i put a buffer like
the buf634 inside the feedback loop it will be ok with
52db of gain? (the 60db gain is total, including transformer)

and is biasing the opa627 into class a operation will make a different??

thx..

 

[peranders]

The OPA627 is the wrong device for 60 dB gain. I wouldn't go higher than 40 dB. OPA637 is better. If you have more current demands than your opamp can deliver you must have a buffer. This has to do with the load and the gain is not affected particulary much if you have feedback.

 

[Sze]

Udip

Here is a pdf file of how to calculate the values for a dc servo.

I used this to put servos in a number of amps and preamps. This was from an article way back in the Audio Amateur magazines.

Hope this helps

 

[Sze]

oops

here is that file for my late post

 

[udip]

thank u Sze ,

That's what i was looking for....thx again

 

[arthur]

Please take a look:
http://home20.inet.tele.dk/kou/dcservo.html

 

[peranders]

oops

here is that file for my late post

A good thumb rule is that R4 = 10 x R6

 

[udip]

In the lt1115 datasheet the servo configuration is
different, and the output is connected to ground via 10r resistor.

can u tell which one is better??

 

[udip]

can i use simple opamp like the tlv072 for the servo??
or maybe something like the opa2604 or op27??

 

[ble0t]

I also found this link/program a couple months back...it's old, but it works pretty well. Hope it helps

[EDIT] Of course, right after I post, I see the link a couple posts back...woops

 

[markp]

There is a great book by Walt Jung about audio op-amps that has a large section on servo design for all sorts of audio uses.

 

[peranders]

can i use simple opamp like the tlv072 for the servo??
or maybe something like the opa2604 or op27??

The servo will feed back audio up to 100-1000 Hz so the opamp must have good audio properties to approx. 1-10 kHz. It is also wise to use an opamp with small input bias currents so you can use 100k-1M resistors. The opamps must also be unity gain stable. The opamps you have mentioned above will probably work.

Have you played with my simulation files? Since the DC servo is so slow it's very easy to simulate and you can use general opamps.

One thing which is important is that the servo must not be too fast. Maximum speed is at 20 Hz and full output signal (this is just an example). This output signal will create max output from the servo. If you have a too fast servo a satureated servo will induce distortion som therefore you set a frequency and a output level where the servo will clip.