DOGC-H by Dr. Jagodic

[EUVL]

Have you tried a reactive load , say 8R // 200n ?


Patrick

 

[kacernator]

No, I haven´t. Amp is sounding very good, running for almost 4 years without any problem.

 

[mgottwa]

I do agree. Mine is running for about 2 years without any issues.

 

[viki_v2]

HI Guys.
LF411 is not available online. Can you suggest and equivalent.

 

[andrewlebon]

DOGC AMPLIFIER

Incomplete project

warm regards
Andrew

 

[PRR]

LF411 is not available online. Can you suggest and equivalent.

TL071; or TL072(dual).

 

[andrewlebon]

TLO71 +VCC pin is 7 TLO72 +VCC pin is 8. LF411 +VCC PIN IS 7

 

[mgottwa]

I am Running an OPA1611. It Shows a DC Offset of about 6mV which I think is fine.

 

[andrewlebon]

OPA1611 is a good choice in terms of low noise and distortion slew rate almost
the same as LF411.

 

[viki_v2]

There is one 960 ohm resistor on DOGC board. Cant find anf 960 ohm resistor. Will the closest work. Say 976 ohm, 953 ohm or 1k.

 

[mgottwa]

Hi viki_v2,

that value is critical, because it forms the "bridge" together with the coil and the 120pF Capacitor. I got mine from mouser: 71-RS02B960R0FE12

 

[viki_v2]

Thats the problem there. Shipping is very costly.

 

[overall feedback]

There is one 960 ohm resistor on DOGC board. Cant find anf 960 ohm resistor. Will the closest work. Say 976 ohm, 953 ohm or 1k.

I would chose 976R, with a very little bit higher gain and distortions, but also little better stability.

 

[EUVL]

LF411, like most opamps used for DC servos, has a JFET inputs and very low input bias current.
OPA1611, on the other hand, is bipolar input.
Input bias current can be up to 250nA, compared to 5ßpA of the former.
I would suggest the likes of OPA140, LT1792, etc.

976R // 56k will give you 959R. Close enough.


Patrick

 

[mgottwa]

Is input bios current a problem for a DC servo?

 

[PRR]

Yes.

 

[mgottwa]

Why that? Could you be so kind and give a brief explanation?
Thank you

 

[EUVL]

A DC servo usually deploys high impedance RC network, e.g. 1µF 1M-ohm.
A high input bias current of the opamp will create a much larger DC offset than otherwise.
A FET input opamp is usually preferred with such high impedances.
Also they results in lower noise.

Opamp?? FET vs Bipolar
DC Servos
opamp for DC servo


Patrick

 

[mgottwa]

Hi Patrick,

thank you very much for your explanation. So I might try a FET or JFET input opamp. In my DOGC the OPA1611 does a good job with a DC-offset of about 5...10mV

P.S. I still have some JFETs to match... did not forget it ;-)

Michael

 

[viki_v2]

LF411, like most opamps used for DC servos, has a JFET inputs and very low input bias current.
OPA1611, on the other hand, is bipolar input.
Input bias current can be up to 250nA, compared to 5ßpA of the former.
I would suggest the likes of OPA140, LT1792, etc.

976R // 56k will give you 959R. Close enough.


Patrick

Looks good. Will give it a try.