Hello,
I have an issue with an oscillating line stage, depending on which opamp to use in the DC-servo circuit.
Some details:
Line stage (PCB + circuit is attached)
Issue
Trials to find the root cause:
Does anyone has some experience with this kind of issue? All advice is appreciated.
I have an issue with an oscillating line stage, depending on which opamp to use in the DC-servo circuit.
Some details:
Line stage (PCB + circuit is attached)
- clone of the famous CTC Blowtorch preamp
- power supply: CRCRC + LM317 + shunt regulator + Lateral Fet cap multiplier
- inverting type. Both opamps inside 1 DIL08 used for balanced line stage.
- small PCB connected to line stage PCB with headers.
- Power supply: R + Zener solution derived from line stage PSU. 2x15V zeners bypassed with 100n Wima FKS.
Issue
- Preamp oscillates easily when using AD712 + switching on/off main amp or source.
- Preamp does not oscillates when using LF412. Tried many things to get is oscillating, it remains stable.
Trials to find the root cause:
- 110pF RF cap at input. Between GND and signal. ==> No difference noticed. It still oscillates.
- 100µF elektrolytic caps // to 100nF WIMA’s, close to opamp + and - connection ==> No difference.
- Other AD712 ==> No difference
Does anyone has some experience with this kind of issue? All advice is appreciated.
Attachments
Last edited:
On other thing, could be important maybe.
This line stage is fed via a Muses 72323 volume control IC, without any buffer in between.
https://www.diyaudio.com/community/threads/muses-volume.322983/page-37#post-7023606
This line stage is fed via a Muses 72323 volume control IC, without any buffer in between.
https://www.diyaudio.com/community/threads/muses-volume.322983/page-37#post-7023606
Hi,
I have two ideas for your issue:
1. The diodes on the input of your servo can be problematic
2. Inverting Servo Type can be unstable with "modern" opamps
I recommend, remove these diodes and try again.
If this does not help, alter your design to non inverting topology.
There is a good article on servos on the net here:
https://sound-au.com/articles/dc-servo.htm
Good luck 🙂
Michael
I have two ideas for your issue:
1. The diodes on the input of your servo can be problematic
2. Inverting Servo Type can be unstable with "modern" opamps
I recommend, remove these diodes and try again.
If this does not help, alter your design to non inverting topology.
There is a good article on servos on the net here:
https://sound-au.com/articles/dc-servo.htm
Good luck 🙂
Michael
These are worst issues to fix. Some opamps tend to oscillate after supply turn-on or when there is a fast transient arriving through mains net. Try change of bypassing. Me, I would use OPA2134 instead of AD712 and I believe the issue would be fixed. I do not like AD711/12.
Just removed the diodes….yes, no oscillation anymore. Thanks for the advice and link.
Attachments
I was talking specifically about AD711,12. It is nothing but a bit improved TL071,2. You may search for old posts with Scott Wurcer. And you do not need to use it in the servo just because JC did use it. There is no special advantage of this opamp. But if you want to copy everything to the smallest detail, it is your project. In any case you will not use original PCB.
