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Old 8th November 2004, 06:36 AM   #1
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Default preamp dc servo

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...
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Old 8th November 2004, 07:00 AM   #2
soultek is offline soultek  United States
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Check out the white papers and schematics on:

www.jensentransformers.com
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Old 8th November 2004, 07:26 AM   #3
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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.
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Old 8th November 2004, 07:29 AM   #4
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Here are the files. If you present a schematic I can help you to tune the servo.
Attached Files
File Type: zip qrp01 dc-servo.zip (5.2 KB, 692 views)
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Old 8th November 2004, 10:35 AM   #5
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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
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Old 8th November 2004, 10:35 AM   #6
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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...
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File Type: jpg preamp.jpg (51.3 KB, 1830 views)
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Old 8th November 2004, 10:44 AM   #7
Electrons are yellow and more is better!
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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.
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Old 8th November 2004, 02:40 PM   #8
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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...
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Old 8th November 2004, 02:44 PM   #9
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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..
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Old 8th November 2004, 05:59 PM   #10
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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.
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