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offcenter virtual ground, opamp mic with transformer
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On my opamp mic with a transformer, my virtual ground is unbalanced, it is closer to actual ground than to V+
I thought it might be DC running through the transformer primary, I've tried with the transformer DC decoupled with a cap (on the virtual ground side), and I think that helped somewhat but I'm still getting an unbalanced virtual ground. here's the schem: (possibly irrelevant aside - in my latest version I'm using a 7:1 transformer and appropriate value changes ie R3/R4 are 6.8k, D1 is 27V, R6 is 6.8k and R7 is 1k, but I have had this issue with a 1:1 transformer as well.) what am I missing here? From memory the version without the transformer doesn't have this issue, but I don't see how the transformer could be causing it. thanks in advance! |
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and here's the link direct to the schematic, which is what I meant to post:
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On 30/04/2025 20:53, thet via groups.io wrote:
On my opamp mic with a transformer, my virtual ground is unbalanced, it is closer to actual ground than to V+ |
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How far off-center is the virtual ground voltage?
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I would try disconnecting stuff:
1) remove the opamp and measure voltages (all voltages)
2) restore the opamp, measure again?
3) disconnect the transformer, measure again,
4) disconnect the mic element, measure again
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etc. The idea is to try to find what's creating the imbalance |
开云体育If anyone sees a theoretical issue with my schem I'd be grateful for any enlightenment. If the schem is OK then I guess I'll find it through regular troubleshooting procedures. Actually I'm getting readings all over the place. Part of the problem seems to be that I'm testing the board out of context, as a bare board with a 100pF cap in place of the capsule. Anything disturbing the input seems to result in massive changes to the virtual ground voltage. I'm not sure why this would so massively affect the DC operating point. Removing the opamp is not an option as it's a surface mounted one and not socketed. The transformer isn't much easier. I now think it probably isn't the transformer at issue. On 30/04/2025 21:18, kennjava wrote:
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On Wed, Apr 30, 2025 at 09:53 PM, thet wrote:
On my opamp mic with a transformer, my virtual ground is unbalanced, it is closer to actual ground than to V+How are you measuring it? What value are you measuring? What voltage are you measuring on pins 2, 3 and 6 of the 1641? I thought it might be DC running through the transformer primary,Wrong assumption. I've tried with the transformer DC decoupled with a cap (on the virtual ground side), and I think that helped somewhat but I'm still getting an unbalanced virtual ground.That confirms it's not the origin of the problem. |
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I have put the board in a body for a more stable test situation. Though the cover is off of course.
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With a 100pF cap across the 1G resistor to simulate a capsule, I now have a very high virtual ground. With a 10M resistor across the 1G resistor all the voltages come right. Since I'm running the opamp with a gain of 7 and a 7:1 transformer I have my V+ at 24V With just the 100pf cap I get a virtual ground of about 20V. Drop across R5 is just over 2V (~26-24v) so current draw is nominal. Pins 2 and 6 are approximately at virtual ground. ie 20v or so. With the 10M resistor in place those pins drop to ~ 12v Pin 3 is the input at 1G resistance so hard to measure - but measured with my 10M DMM is 0.2v so from the opamps POV all the way negative. That seems wrong, but also the DMM is dragging it down, so that measurement is almost meaningless. Measured across the 1G resistor is near enough to 0v, but again that's equivalent to measuring with the 10M resistor in place. If the board is leaky ie imperfect insulation across the input from dirt would it cause this? It is definitely related to the high input resistance. I will try cleaning it again. I used different solder on the smd opamp on this board, a low temp bismuth based paste, maybe the flux is harder to clean? On 01/05/2025 09:44, Jerry Lee Marcel via groups.io wrote:
How are you measuring it? What value are you measuring? |
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Hello all, I don't normally post here much but @thet is there a specific reason why the transformer primary needs to use virtual ground versus "original recipe" ground at pin 1? I can guess we're able to skip a DC coupling cap with your current schematic. Some transformers have gapped lamination or insulation between the windings to help with DC voltages like a HIPOT transformer. This may be a clue as to what's happening when you tried various transformers previously.
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My tinker fingers suggest using a DC coupling cap after the opamp U1-A output to the TX and using original ground at the primary from XLR pin 1 to see if you get virtual ground to behave again. Worth a possible lashing lol
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As well, transformers present inductance, capacitance and most importantly a resistance (DCR). No two transformer companies truly approach ratio vs DCR the same, my own company included. A Jensen 1:1 vs a Cinemag vs a Sowter will show different statistics and approaches to the same concept. This is what makes them all special.? That's the mystical art of designing iron. But despite these differences, we? still follow the laws of physics. So in simple terms imagine having a 240 ohm resistor to ground vs a 35 ohm resistor to ground on a load line that's sensitive to those resistances.? That's part of what we experience when using varuous 1:1 transformers. I would be highly interested in the DCR numbers on the transformers that behaved vs ones that didn't. DMM set to proper resistance range, between the input pin on the primary and the "ground" pin you're using, may be worth checking the secondary side as well.
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Glad to help throw spitballs at the board with you.?
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Thanks
-L.? |
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On Thu, May 1, 2025 at 01:40 PM, thet wrote:
With a 100pF cap across the 1G resistor to simulate a capsule, I now have a very high virtual ground.This is not normal, with a 15V zener the voltage should be less than 16V. With just the 100pf cap I get a virtual ground of about 20V.The VG voltage should be pretty close to half of V+. Drop across R5 is just over 2V (~26-24v) so current draw is nominal.OK Pins 2 and 6 are approximately at virtual ground. ie 20v or so.OK. With the 10M resistor in place those pins drop to ~ 12vThis shouldn't happen. If the board is leaky ie imperfect insulation across the input from dirt would it cause this?It is a possibilit |
开云体育It's not a 15v zener any more I increased it to 27v to give me
headroom for the opamp gain. from my original post: "(possibly irrelevant aside - in my latest version I'm using a 7:1 transformer and appropriate value changes ie R3/R4 are 6.8k, D1 is 27V, R6 is 6.8k and R7 is 1k, but I have had this issue with a 1:1 transformer as well.)" Agreed it shouldn't drop with a lower input resistor. I cleaned the board better, paying special attention to the gap between pins 3 and 4 which are +In and V- and they are very close together on this smd opamp. that helped! there must have been some flux still right under the edge of the opamp - the virtual ground is now at 13.5V with the 1G input resistor, so only a volt or so too high. so if V- leaks to In+ how exactly does that make the virtual ground go high? the opamp must(?) be amplifying DC from the leakage. presumably it's worse because I have the opamp set up for gain of 7 rather than just unity? On 01/05/2025 13:00, Jerry Lee Marcel
via groups.io wrote:
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On Thu, May 1, 2025 at 02:20 PM, thet wrote:
Didn't notice that. I thought the schemo was the latest issue. This is good. Remember that, even with FET opamps, there is still some input bias current, which may alter the ideal calculated value. Th evoltage at the non-inverting input is transmitted to the output and to the xfmr's primary, which in turns is connected to VG. It sure does Likely. |
开云体育if the virtual ground is high through the route you describe that suggests that: * isolating the transformer with a cap should help. It didn’t noticeably earlier but maybe now things are close and stable it would. * any leakage to the +ve non-inverting input must be coming from the +ve rail not the negative since the VG is too high not too low. This is despite the -V pin being much closer to the input. Which way would the bias current alter the VG? Would isolating the transformer help with that too? If the route to unbalancing the VG is through the transformer that would explain why I never had this problem with the transformerless version, but not why it was unbalanced earlier even with the transformer isolated. I will try isolating the tx again and see what happens now the board is cleaner
On 01/05/2025 18:33, Jerry Lee Marcel
via groups.io wrote:
This is good. Remember that, even with FET opamps, there is still some input bias current, which may alter the ideal calculated value.Th evoltage at the non-inverting input is transmitted to the output and to the xfmr's primary, which in turns is connected to VG. |
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On Thu, May 1, 2025 at 02:20 PM, thet wrote:
A friend of mine recently had exactly the same problem you described here and also solved it by thoroughly cleaning the PCBA with IPA. I gave him the advice to add a guardring. The guardring connects to the inverting input of the OPA and encircles the high-Z node on the non-inverting input. Ideally, the guardring should be placed on both sides of the PCB and, connected with vias and should be left free from solder mask, also on the area inside the guardring. This prevents the DC drift when there are flux residues on the board or when the board is used in a high humidity environment. But you still want to thoroughly clean the board as the resistive flux will decrease SNR.
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The close spacing of the conductors that is inherant to SMT OPAs makes it hard to design PCBs that are free of noise and drift issues in high-Z circuits. TI should have given it another package, leaving out some pins as in some High Voltage ICs. This would allow the use of isolating slots, in combination with a guardring and prevent noise and drift issues.
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The low-melt solder that you use could be part of the problem, as you already reasoned. The flux has to do its work at lower temperatures, so I assume it has to be of a more aggressive type which possibly lowers the Surface Insulation Resistance. You could check its datasheet or just use regular solder from a name brand like Kester.
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Jan |
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The guardring seems like a good idea, but I don't see how it could be done with one of these tiny opamps. it would have to be very fine, and if it were left free of solder mask, it would certainly result in shorting the + and - inputs together when soldering.
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I am trying to go completely lead free. It's not easy for some of the things I solder. The bismuth solder paste is something I'm trying instead of lead solder when temperature is an issue, and I'm hoping it will be easier to use for smd than regular solder. I use SnAgCu for most regular soldering, but there are some things where I haven't found a good substitute for lead. However so far the bismuth paste has been tricky to use and the lead solder just works better. If there's a opamp at all equivalent to the 1641 in a DIP8 package I would love to hear about it. On 02/05/2025 07:01, j.postma8 via groups.io wrote:
A friend of mine recently had exactly the same problem you described here and also solved it by thoroughly cleaning the PCBA with IPA. I gave him the advice to add a guardring. The guardring connects to the inverting input of the OPA and encircles the high-Z node on the non-inverting input. Ideally, the guardring should be placed on both sides of the PCB and, connected with vias and should be left free from solder mask, also on the area inside the guardring. This prevents the DC drift when there are flux residues on the board or when the board is used in a high humidity environment. But you still want to thoroughly clean the board as the resistive flux will decrease SNR. |