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Old 18th January 2006, 06:59 PM   #1
Stew is offline Stew  United States
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Question OpAmp Resonance?

I installed AD826 opamps in my Yamaha CD Player (CDC-745) a few months ago. All of my CDís sound much better from top to bottom Ö. until I played Pink Floyd's Dark Side of the Moon with the volume up a bit, that is. Thereís a resonating sound (synthesizer?) on this CD, e.g. after the intro on ďTimeĒ. It's a lower midrange/upper bass sound. It never caused a problem before but itís very pronounced with the new opamps and at approximately 9:00 on the volume dial, it sounded like the drivers were coming out of their frames and the roof was falling in. I could actually see the speakers vibrating. Fortunately, I turned it down quickly and donít think my speakers are damaged. Has anyone else had a problem like this after replacing opamps? Is this the dreaded opamp resonance Iíve read about?
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Old 18th January 2006, 07:18 PM   #2
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Sounds to me like their going into spuratic occilation, I replace some in a Pioneer cd player once, they did the same thing, very dangerous for your power amp and speaker, do'nt use them, try something a bit more stable, the way you can see it happening is if you hook up a scope to the output and play that same cd.
For you can try the very cheap but nice sounding OP275, they are stable, or AD825, but they are single smd and you need brown dog coverter boards for them but great sounding if you want the AD826 sound.

Cheers George
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Old 18th January 2006, 07:52 PM   #3
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Default Re: OpAmp Resonance?

Quote:
Originally posted by Stew
I installed AD826 opamps in my Yamaha CD Player (CDC-745) a few months ago. All of my CDís sound much better from top to bottom Ö. until I played Pink Floyd's Dark Side of the Moon with the volume up a bit, that is. Thereís a resonating sound (synthesizer?) on this CD, e.g. after the intro on ďTimeĒ. It's a lower midrange/upper bass sound. It never caused a problem before but itís very pronounced with the new opamps and at approximately 9:00 on the volume dial, it sounded like the drivers were coming out of their frames and the roof was falling in. I could actually see the speakers vibrating. Fortunately, I turned it down quickly and donít think my speakers are damaged. Has anyone else had a problem like this after replacing opamps? Is this the dreaded opamp resonance Iíve read about?
Some tunes on Dark Side of the Moon has extremely low bass! I think they have recorded EKG signals even! Have you tested this with the original opamp?

BTW: Have you heard the cover of the Dark Side of the Moon? Extremely good sound quality, very much alike the original. The recording is amzing!

The Moon Revisited by Magna Carta

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Old 18th January 2006, 08:01 PM   #4
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I've had Burr Brown 2604 Opamps oscillate on me. The're a real pain sometimes.

I was told to put a small capacitor, 12pf or so, across the inputs of the opamp. This done the trick for me. Not sure if you have the same problem but you could try it and monitor the opamp output with a scope. Probably a good idea to use a dummy load instead of a speaker and maybe keep the volume down.
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Old 18th January 2006, 09:17 PM   #5
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Default Value for Bypass Capacitor

Thanks for the replies. This is all new to me and I appreciate the help. The datasheet for the AD826 suggests using two 0.1 uF caps in parallel from each power supply pin to ground. From the above replies and searching this site, I gather that itís common to put a capacitor between pins 4 and 8 instead. Would a single 0.1 uF capacitor between pins 4 and 8 be a good choice? Are metalized polypropylene caps a good choice?
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Old 18th January 2006, 10:00 PM   #6
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Default Re: Value for Bypass Capacitor

Quote:
Originally posted by Stew
Thanks for the replies. This is all new to me and I appreciate the help. The datasheet for the AD826 suggests using two 0.1 uF caps in parallel from each power supply pin to ground. From the above replies and searching this site, I gather that itís common to put a capacitor between pins 4 and 8 instead. Would a single 0.1 uF capacitor between pins 4 and 8 be a good choice? Are metalized polypropylene caps a good choice?
the purpose of the 100nF bypass capacitors is to produce a low impedance ground return for high frequency garbage which makes its ways on the power supply rails -- in this case, you are best off with a ceramic surface mount capacitor -- if you are going to parallel it with anything a 1 uF tantalum.

i am going to hazard a guess that perhaps one of the solder joints is high resistance but not completely intermittent.
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Old 18th January 2006, 11:15 PM   #7
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Default Re: Re: Value for Bypass Capacitor

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Originally posted by jackinnj
the purpose of the 100nF bypass capacitors is to produce a low impedance ground return for high frequency garbage which makes its ways on the power supply rails -- in this case, you are best off with a ceramic surface mount capacitor -- if you are going to parallel it with anything a 1 uF tantalum.

i am going to hazard a guess that perhaps one of the solder joints is high resistance but not completely intermittent.
Thanks for the advice on caps. Wouldn't a bad solder joint (or socket connection) cause problems on other CD's as well? Every other CD I've listened to sounds great.
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Old 18th January 2006, 11:52 PM   #8
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The power supply by-pass caps won't help much if it is an occillating opamp, it's the high speed of the opamp itself being excited to go into occillation by high frequency input noise from the previous stage (dac noise maybe?).
But then you could try the caps doesn't cost much and will lower power supply noise.

Cheers George
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Old 19th January 2006, 12:24 AM   #9
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Putting a small (eg 12pF) capacitor between the opamp inputs will likely cause a great peak in hf response in many configurations. A small capacitor from output to inverting input is more likely to help (only for unity gain stable)
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Old 19th January 2006, 02:03 AM   #10
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Default Re: OpAmp Resonance?

Quote:
Originally posted by Stew
I installed AD826 opamps in my Yamaha CD Player (CDC-745) a few months ago. All of my CDís sound much better from top to bottom Ö. until I played Pink Floyd's Dark Side of the Moon with the volume up a bit, that is. Thereís a resonating sound (synthesizer?) on this CD, e.g. after the intro on ďTimeĒ. It's a lower midrange/upper bass sound. It never caused a problem before but itís very pronounced with the new opamps and at approximately 9:00 on the volume dial, it sounded like the drivers were coming out of their frames and the roof was falling in. I could actually see the speakers vibrating. Fortunately, I turned it down quickly and donít think my speakers are damaged. Has anyone else had a problem like this after replacing opamps? Is this the dreaded opamp resonance Iíve read about?
if its the part of the song after the 'heartbeat' bassline, theres a strong bass line that sounds like its resonating...or 'bellowing'
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