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 Forum index » DIY Hardware and Software
Troubleshooting OTA based circuits
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elmegil



Joined: Mar 20, 2012
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PostPosted: Sun Oct 13, 2013 9:07 pm    Post subject: Troubleshooting OTA based circuits
Subject description: In particular the Analog Metropolis 4023 VCF
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I'm struggling with the ARP4023 filter, in the version by Analog Metropolis:
http://www.amsynths.co.uk/Filter_Bank/AM8023/am8023.html
Schematic on the last page:
http://www.amsynths.co.uk/wp-content/uploads/2012/09/AM8023-Project-Notes-V1.3.pdf

I have 4 PCBs to troubleshoot that don't have any output. They appear to be correctly built, and my understanding is that these same PCBs have been used successfully, so I have no reason to believe it's a PCB error.

In an attempt to confirm the circuit/measure the effects of component changes, etc, I also built one on a breadboard, except that I have substituted two op amps from a TLC274 in place of the OPA134's, and a couple of resistor substitutions--because I don't have 134's or those particular resistors. In particular: for the 475K R20 I have substituted a 470K, for the 196K R25 I have substituted resistors summing to 198K (a 180K + an 18K). I was also able to sub in two resistors that get me the 61.9K R4 and R10, as I have 12K and 49.9K resistors available. I have also subbed MPF102 for the 2N5459 FETs.

I expect that the op amp sub is reasonable since the way I read the circuit, their primary function is to act as buffers (when combined with the FETs) to the basic OTA-C filter poles.

And I have seen folks suggest MPF102 as a sub for the 2N5459 so I expect those should be ok as well.

Regardless, those subs are only on my breadboard version, the PCBs have all the listed components.

On the breadboard circuit I have only implemented one input, without the attenuator, and only the coarse frequency adjustment. I have been able to observe that the control current ranges from roughly .4uA to 140uA as i adjust the Frequency pot, so I expect that the expo converter is working reasonably, although not trimmed...but I did adjust the trimmer so the wiper was at roughly 0V.

I thought I might measure a voltage across R6, the 15K between the two filter stages, but no luck there, all I see is background noise at 50mV.



SO where I'm struggling has to do with the nature of OTA circuits. My scope's lowest V/div setting is 50mV.... As I read the schematic, a 10Vpp signal would be cut down to .9mVpp before it gets to the negative input of the OTA, so I can't really observe it directly. I can amplify that point far enough to hear it, barely, but not visually observe it.

I've watched Aaron Lanternman's lecture on OTA-C circuits, and I barely grasp the math, but it doesn't really help me with the buffer, nor with what I should expect to see at those points, voltage wise. Additionally, his lecture strongly suggests that to be a successful OTA-C filter, the feedback needs to come in on the negative terminal--this circuit comes in on the positive, and I'm not at all sure what that does to any of my understanding (feeble as it is) of this circuit.


I'm curious if anyone has any general advice for dealing with the tiny voltages (I do expect at least one chorus of "get a better scope" Smile but that's not yet feasible....), or other ideas for checking out OTAs or figuring out where things are breaking down in this circuit.

Thanks....
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analogmonster



Joined: May 30, 2011
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PostPosted: Sun Oct 13, 2013 11:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

First, you could concentrate on the first stage. The output of IC4 should deliver a measurable amplitude. Unless you don't see anything here, the rest of the circuit is not interesting.

Second, a (small) output of IC1 should be readable by a scope. If not, the control voltage mixer could have a problem.

Third, if all boards show the same misbehaviour, I would suspect the boards or the documentation have a general problem.

If nothing helps I can highly recommend to simulate (parts of) the circuit in LTSpice, starting with the CV mixer and the first OTA stage (I found an error in the Moog modular Service Manual circuits by doing this).

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andrewF



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PostPosted: Mon Oct 14, 2013 3:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think TLC274 are designed to run on a single sided supply, maximum 18V.
So if you are using +/-12V or +/-15V, these are probably getting fried, as they are getting a total of 24V or 30V across the power pins.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Mon Oct 14, 2013 7:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

andrewF wrote:
I think TLC274 are designed to run on a single sided supply, maximum 18V.
So if you are using +/-12V or +/-15V, these are probably getting fried, as they are getting a total of 24V or 30V across the power pins.



AAARG. Yes, just confirmed it in the datasheet, recommended 3V to 16V supply, 18V max.

I didn't expect the TL07X series that I have scads of would be a good choice because they already have JFET inputs, not sure what cascading them would do. Same for LF44X.

I have RC4558, MC1458, --CA3140-- that might work, I guess I need to pay more attention to datasheets Embarassed

Edit: CA3140 has MOSFET inputs, so no go there either.
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andrewF



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PostPosted: Mon Oct 14, 2013 8:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

just use TL074, they will be fine
Thats what my Electronotes/Arp4023 boards use.

The jfet input just means they have a high input impedance (about a billion ohm) whereas the 741 is quite low at about a million ohm. In most situations, the higher the better. An ideal op amp has infinite input impedance, of course it doesn't exist.
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elmegil



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PostPosted: Mon Oct 14, 2013 10:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

Which issue of Electronotes?? I have a few, not the whole set, and would love to go over that info.

In other news, RC4558 has no love for me.

I'll have to try the TL074 another night.
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andrewF



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PostPosted: Mon Oct 14, 2013 11:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

EN#71
VCF2
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elmegil



Joined: Mar 20, 2012
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PostPosted: Tue Oct 15, 2013 5:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks. Not one I have, so I'll have to go see about getting another batch....
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elmegil



Joined: Mar 20, 2012
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PostPosted: Thu Oct 24, 2013 11:54 am    Post subject: Reply with quote  Mark this post and the followings unread

good news and good news. I went ahead and ordered the earlier Electronotes and they arrived today.

Last night I cracked the problem, turns out some of the integrating caps have heat damage on each PCB, and 3 out of 4 where I replaced the damaged caps I have a working filter. The 4th is not yet repaired, but I expect the same result, it has the worst looking caps of the bunch.


My breadboard problems were bad build, btw. Getting that working and comparing to the bad ones was what led me to the caps, at which point it seemed obvious Smile
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