Hi Guys, I would like to know if some of you knowledgeable guys could run your eyes over this attachment and see if anything obvious jumps out at you.
It is 14db gain stage of a Yamaha C85 Preamp which I am trying to upgrade.
In fact I upgraded it some time ago, but it had been put aside while I used some of my other preamps. The fault with it only came to light when I started using another Power Amplifier, a Harman Kardon Citation 22. After carrying out several trips back to the workshop(Overheating output stage), I discovered it was not the power amp at all.The Preamp was suffering from HF oscillation and I presume due to the wide bandwidth of the power amplifier, said HF was being amplified enough for heatsinks to get red hot.
I've since returned preamp to the bench and removed the ICs that I had upgraded to in this stage, an OPA1656, and replaced it with the only SIL opamps I had knocking about, JRC5532 and oscillations disappeared. I then still in search of improving performance , replaced these with opa2209, and checked and refined decoupling caps positioning and routing, 10uf + 0.1 sat directly on PSU pins and down to a chunky piece of local ground plane.I thought with the reduced speed of these opamps all would be well. On checking the output waveform, the obvious oscillating trace had all but disappeared, but I did however note that with the volume control at full max, there was still some sign of oscillation, but decided that as unlikely to have volume hard on, all would be well. After returning preamp to system and sitting down for a good listen, all seemed well, but on leaving room and returning 15 minutes later, found one channel on power amplifier had overheated and shut down again. Both heatsinks were uncomfortably hot, so looks like there is still enough HF being output from pre to upset the power amp.
I will shortly be returning preamp to the bench for further investigation, but thought I'd put it out there for any suggestions to address this problem?
It is 14db gain stage of a Yamaha C85 Preamp which I am trying to upgrade.
In fact I upgraded it some time ago, but it had been put aside while I used some of my other preamps. The fault with it only came to light when I started using another Power Amplifier, a Harman Kardon Citation 22. After carrying out several trips back to the workshop(Overheating output stage), I discovered it was not the power amp at all.The Preamp was suffering from HF oscillation and I presume due to the wide bandwidth of the power amplifier, said HF was being amplified enough for heatsinks to get red hot.
I've since returned preamp to the bench and removed the ICs that I had upgraded to in this stage, an OPA1656, and replaced it with the only SIL opamps I had knocking about, JRC5532 and oscillations disappeared. I then still in search of improving performance , replaced these with opa2209, and checked and refined decoupling caps positioning and routing, 10uf + 0.1 sat directly on PSU pins and down to a chunky piece of local ground plane.I thought with the reduced speed of these opamps all would be well. On checking the output waveform, the obvious oscillating trace had all but disappeared, but I did however note that with the volume control at full max, there was still some sign of oscillation, but decided that as unlikely to have volume hard on, all would be well. After returning preamp to system and sitting down for a good listen, all seemed well, but on leaving room and returning 15 minutes later, found one channel on power amplifier had overheated and shut down again. Both heatsinks were uncomfortably hot, so looks like there is still enough HF being output from pre to upset the power amp.
I will shortly be returning preamp to the bench for further investigation, but thought I'd put it out there for any suggestions to address this problem?
Attachments
There is a reason why the engineers designed around a slower op amp.
Fitting a superfast op amp will mess around with the frequency responses and cause instability. Exactly as you now have.
Fit the original parts!
Fitting a superfast op amp will mess around with the frequency responses and cause instability. Exactly as you now have.
Fit the original parts!
Hi Jon, thanks, I expected a few replies like yours.Lol., I may well take your advice, after having another poke around.
Thanks
Thanks
I endorse Jon's advice.
If it makes you feel any better, an ideal perfect opamp wouldn't have improved performance either. Performance is dominated by the differential pair.
If it makes you feel any better, an ideal perfect opamp wouldn't have improved performance either. Performance is dominated by the differential pair.
Hi BSST, I had another little play with it yesterday, but decided to abort that section.
It appears that anything other than the JRC5532S or the original 2401S which I still had knocking around causes stability/oscillation problems. Didn't really want to get into compensation, filtering scenario, so reinstalled the 2401s.
I haven't had a chance to listen to it yet with the downgraded, but stable opamps, so be interesting to hear the difference.
Many Thanks to both of you.
It appears that anything other than the JRC5532S or the original 2401S which I still had knocking around causes stability/oscillation problems. Didn't really want to get into compensation, filtering scenario, so reinstalled the 2401s.
I haven't had a chance to listen to it yet with the downgraded, but stable opamps, so be interesting to hear the difference.
Many Thanks to both of you.