abechap024

Re: Preamp cuts out at high gain
« Reply #20 on: July 05, 2011, 10:48:30 AM »
Hmm I know I mentioned this before and hard to kinda guess what is going on without seeing anything, but sounds like something that could easily be caused by grounding, or lack enough of power supply decoupling or or or...
AC Sound - some DIY circuitboards


Ptownkid

Re: Preamp cuts out at high gain
« Reply #21 on: July 05, 2011, 10:54:17 AM »
Yeah...I'm a tool, TOTALLY forgot about supply pin caps...everything seems to be working properly now.

abechap024

Re: Preamp cuts out at high gain
« Reply #22 on: July 05, 2011, 11:04:47 AM »
Sweet! :)
AC Sound - some DIY circuitboards

MikeClev

Re: Preamp cuts out at high gain
« Reply #23 on: July 05, 2011, 11:39:55 AM »
I was going to suggest checking to see if c107 (1000uf) had gone bad / leaky as this would mean unwanted dc gain. Also a good cap to think of replacing if its a cheapo one and uprating would also extend the bass end. Low impedance is vital in that application. I replaced similar caps in a 8 year old mixer and the sound was vastly improved. The old dried up caps had been clouding the sound. Worth checking I say!
Soldering burns: No pain, no gain.

If my opamps can't be discrete, I at least try to keep them discreet.

Ptownkid

Re: Preamp cuts out at high gain
« Reply #24 on: July 05, 2011, 04:57:36 PM »
Ok, so adding the caps to the supply pins has made a huuuuge difference, 5 of 8 channels no longer exhibit any odd behaviour all the way up to full gain, but 3 of them still crap out at about 7/8 gain....

Ptownkid

Re: Preamp cuts out at high gain
« Reply #25 on: July 05, 2011, 06:46:20 PM »
I think I may have finally got it...I used 0.1uF ceramics on the supply pins of the TL072, and while the problem was much better, it still was not gone. I read that some of the faster opamps may require 0.01uF instead and that seems to have done the trick.

Can someone enlighten me a little more as to why?

JohnRoberts

Re: Preamp cuts out at high gain
« Reply #26 on: July 05, 2011, 07:54:32 PM »
Capacitors, especially when used for HF decoupling need to be looked at in terms of C, R, and L...

If you plot the impedance curve vs frequency across a ceramic cap you will see a falling slope starting at LF dominated by the real C, at some high frequency it stops falling and flattens out that region often small is dominated by the series R, above that the impedance rises again dominated by inductance.

The trade off with caps at HF is to get low impedance where you need it. The .1uF may start at a lower impedance and fall lower than a .01uF because it has more C, but it also has more R and L, so while a .01uF may start at a higher impedance, it will often stop dropping at a higher frequency then the larger cap so actually be more effective at very HF.. This is why we often use both larger and small value caps in parallel.

 

http://www.signalintegrity.com/Pubs/straight/resonance.htm

Go to the link for an actual article by a real person...  Imagine overlaying a plot for a .01 and  .1, with the .01 shifted to the right and higher. If it is still falling while the .1 is already rising, the .01 can be better for HJF decoupling. 



JR
Cancel the "cancel culture", do not participate in mob hatred.

Ptownkid

Re: Preamp cuts out at high gain
« Reply #27 on: July 06, 2011, 07:52:53 AM »
Thanks John! A little over my head, but I will re-read it several times. I have the desire to understand this stuff so I keep trying.

Cheers

bruno2000

Re: Preamp cuts out at high gain
« Reply #28 on: July 06, 2011, 01:21:12 PM »
+1
Thanks John!
Best,
Bruno2000

abechap024

Re: Preamp cuts out at high gain
« Reply #29 on: July 06, 2011, 01:34:20 PM »
Yes! I never really understood why that is either. Thanks!
AC Sound - some DIY circuitboards


hobiesound

Re: Preamp cuts out at high gain
« Reply #30 on: July 06, 2011, 02:39:03 PM »
+4 i guess! very nice and simple explanation. Makes total sense.

greetings,

Thomas

Ptownkid

Re: Preamp cuts out at high gain
« Reply #31 on: August 29, 2011, 04:55:52 PM »
So I came back to this again...as I was getting inconsistent results. While I was playing with various cap values, I accidentally connected a 10nF cap between one of the power pins and pin 7 of the TL072 and the problem went away...

Can someone enlighten me as to why? It works when there's a cap between either pin 4 and 7, or pin 8 and 7...

JohnRoberts

Re: Preamp cuts out at high gain
« Reply #32 on: August 29, 2011, 07:39:56 PM »
This is speculation but marginally stable/unstable circuits have direct and indirect interactions with power supply rails.

Speaking hypothetically power supply caps to ground, can pump current into or out of ground nodes changing the voltage at that exact ground node in a way that looks like positive feedback to the overall opamp circuit.

Connecting a cap from the opamp output to one power supply pin alters the power rail-cap-ground loop current, but this is mostly a symptom for the problem and not suggestive of a strategy to make it more stable.

The real solution is to make it generally more stable and less sensitive to power supply loop currents.

Sorry if this sounds like a non-answer, but there is a capacitive connection between power supply and grounds and the opamp inputs. I'd look at where PS caps connect to ground and things like that. 

JR


Cancel the "cancel culture", do not participate in mob hatred.

Ptownkid

Re: Preamp cuts out at high gain
« Reply #33 on: August 29, 2011, 08:58:12 PM »
Not a non-response at all...good food for thought.

I ended up putting 10uF electros in parallel with the 10nF films from supply pins to ground and it seems to have cured everything....BUT I want to do far more testing to be sure. I breadboarded the circuit and got much better performance which leads me to believe that it's my pcb layout that it the problem...

keefaz

Re: Preamp cuts out at high gain
« Reply #34 on: August 30, 2011, 07:22:04 AM »
Maybe add a small capacitor in the IC1B feedback (parallel to the 33k resistor)

Ptownkid

Re: Preamp cuts out at high gain
« Reply #35 on: August 30, 2011, 08:03:57 AM »
Another good point, i have read that in several op-amp design papers, definitely worth some experimentation.

Who knew that such a simple (or so i thought) circuit would lead to so much quality learning. I am both frustrated and thankful for the knowledge all at the same time, lol.


 

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