opamp weirdness

puretube · July 14, 2005, 05:15:43 PM

i still believe in the "floating mid-point" theory...
(maybe a lower resistor between +/gnd, and -/gnd? 22k? and a little cap from + to - 10Âµ...)?

puretube · July 14, 2005, 05:59:44 PM

do you guys have a physical (offboard) connection between "IN GND"/"OUT GND", and the "GND" next to "12VAC" ("power section", looking at Paul`s layout page)... ?

Paul Marossy · July 14, 2005, 06:25:12 PM

Quote from: puretubedo you guys have a physical (offboard) connection between "IN GND"/"OUT GND", and the "GND" next to "12VAC" ("power section", looking at Paul`s layout page)... ?

I do. I never thought to ask that question, I guess it was too obvious in my eyes...

markr04 · July 14, 2005, 08:10:26 PM

I made a direct connection between the two. Now my sound is consistent, but brittle sounding. I'm going to try a resistor in series to emulate what my DMM was actually doing.

Paul: Never underestimate the ignorance of others

markr04 · July 14, 2005, 09:15:44 PM

I meant me... don't underestimate my ignorance.

Okey-doke... I'm still playing with values but:

Connecting the input ground to the AC "ground" solved the problem. I have an old Park amp with an 8" speaker as my bench amp and I just now hooked the circuit up to my stack. The tone is like an icepick with a direct connection, where the tone with my DMM trick was much warmer. 10K in series between the grounds is better. I'm going to try some much higher values next... I suspect my DMM had a heckuva lot of resistance.

I'll post my findings soon.

Paul Marossy · July 14, 2005, 09:55:33 PM

Well, that sounds hopeful.

I added a PCB layout based on a TL072 to my Shaka Tube page to give people a choice between which one they want to use. I still need to confirm that my single opamp layout works OK, though...

markr04 · July 14, 2005, 10:12:28 PM

Don't worry about that, Paul. I'm going to de-mod mine and go back to the TL071 right now. You'll hear back from me soon.

markr04 · July 14, 2005, 10:32:04 PM

Well, that took all of three minutes. The original layout with the TL071 works, Paul. Connecting the gounds was the trick... with some added resistance there in my case.

Paul Marossy · July 15, 2005, 01:03:05 AM

Thanks for clearing that up markr04.

puretube · July 15, 2005, 02:48:26 AM

Paul: I never doubted your PCB layout, nor that you would have missed out that GND interconnection...
but from the symptoms and measurings of others, it only could have been that circuit-ground not being referred to the PS...

missing out what`s obvious to the one, can turn out to become a not so obvious problem to the others...

Paul Marossy · July 15, 2005, 08:46:38 AM

No worries puretube. I was assuming that since I didn't actually build mine using a single opamp that there was a problem with the layout. Half a dozen people that have used my layout have had this same gating problem, so I was beginning to think that it must have been a mistake on my part. :wink:

Paul Marossy · July 15, 2005, 08:47:50 AM

QuotePaul: I never doubted your PCB layout, nor that you would have missed out that GND interconnection...
but from the symptoms and measurings of others, it only could have been that circuit-ground not being referred to the PS...

missing out what`s obvious to the one, can turn out to become a not so obvious problem to the others...

No worries puretube. I was assuming that since I didn't actually build mine using a single opamp that there was a problem with the layout. Half a dozen people that have used my layout have had this same gating problem, so I was beginning to think that it must have been a mistake on my part. :wink:

I'm going to be out of town for three days, but I will try to get to a computer somewhere to keep up on this.

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opamp weirdness