Hey there,
While a capsule replacement is being considered on a LDC mic (AT4040),, would anyone have an idea about inputting a signal into the inputs of a mic for analyzing the output? this is just to prove that the circuitry is performing the way it should. The input of a microphone somewhat eludes me due to knowing of the high impedance and phantom power to the capsule.
thank you,
Phil D.,
While a capsule replacement is being considered on a LDC mic (AT4040),, would anyone have an idea about inputting a signal into the inputs of a mic for analyzing the output? this is just to prove that the circuitry is performing the way it should. The input of a microphone somewhat eludes me due to knowing of the high impedance and phantom power to the capsule.
thank you,
Phil D.,
Good question, I'm curious to see what answers you get.
@wintermute has done some sims, but I don't remember what his source was.
@wintermute has done some sims, but I don't remember what his source was.
The At4040 is advertised as a true cardioid condenser but after this weekend I’ll be able to power it through my preamp and see if phantom power voltage shows u.
thank you,
Phil D
thank you,
Phil D
Do you mean simulating or emulating?
If you want to measure whether the preamplifier of a condensor microphone works, you could just capacitively couple a signal into it via a capacitor of a few hundreds of picofarads with a rated working voltage of 250 V or more.
I'm not sure what is the best way to test the electronics that bias the capsule. You could measure the DC voltage across the terminals that the capsule is normally connected to, but because of the 1 Gohm...10 Gohm bias resistor and the 1 Mohm...10 Mohm input resistance of a typical digital multimeter, you will then measure only about a thousandth of the actual voltage.
If you want to measure whether the preamplifier of a condensor microphone works, you could just capacitively couple a signal into it via a capacitor of a few hundreds of picofarads with a rated working voltage of 250 V or more.
I'm not sure what is the best way to test the electronics that bias the capsule. You could measure the DC voltage across the terminals that the capsule is normally connected to, but because of the 1 Gohm...10 Gohm bias resistor and the 1 Mohm...10 Mohm input resistance of a typical digital multimeter, you will then measure only about a thousandth of the actual voltage.
What i've done to check if preamp is working is to couple output of waveform generator through cap to input of circuit, then check out of circuit with oscilloscope.
I am mainly into tube circuit with LDC( k47/67 clone), so max 1v pk/pk voltage expected.
I am mainly into tube circuit with LDC( k47/67 clone), so max 1v pk/pk voltage expected.
you can try to capacitively couple signal into the mic. remove the popfilter and hold a coin soldered to the coax cable core close to the capsule.
set a signal with 10V dc offset and some 1khz 1V ac, with the gnd of the coax to the gnd of the mic.
set a signal with 10V dc offset and some 1khz 1V ac, with the gnd of the coax to the gnd of the mic.
Thank you everyone, some ideas emerging in this thread.
the capsule itself is shorted (reading 1.9 ohms) so I probably cant or don't know how to include it in a test. This test is really just to make sure that the electronic circuitry in the mic is functioning properly before I make a call to try a replacement capsule. And getting the theory behind how and how much of a signal is being driven is what eluded me. I imagine it is the 47 volts dc phantom signal across the cap creating the charge across the cap, and then the distance between the plate changing according to the amplitude/frequency that is generating the input voltage to the mics own preamp. It was just impossible for me to tell just what that input fluxuation range might be.
In line with some of the suggestions I could try using a signal generator in series with a small cap and into the 2 mic pcb input connectors, and gradually turn the voltage up with a sine wave while looking at the pcb inputs and the mics output on a scope.
Does that seem reasonable? Have I over looked anything? I imagine that its not a very large voltage being seen by the mics first input section since its likely very small motions of the caps diaphragm with 47 volts across it. The cap would be there to block DC and provide some high pass filtering.
Sound like a reasonable plan?
Thanks everyone, I really had no idea at first how to go about this !
Best,
Phil D.
the capsule itself is shorted (reading 1.9 ohms) so I probably cant or don't know how to include it in a test. This test is really just to make sure that the electronic circuitry in the mic is functioning properly before I make a call to try a replacement capsule. And getting the theory behind how and how much of a signal is being driven is what eluded me. I imagine it is the 47 volts dc phantom signal across the cap creating the charge across the cap, and then the distance between the plate changing according to the amplitude/frequency that is generating the input voltage to the mics own preamp. It was just impossible for me to tell just what that input fluxuation range might be.
In line with some of the suggestions I could try using a signal generator in series with a small cap and into the 2 mic pcb input connectors, and gradually turn the voltage up with a sine wave while looking at the pcb inputs and the mics output on a scope.
Does that seem reasonable? Have I over looked anything? I imagine that its not a very large voltage being seen by the mics first input section since its likely very small motions of the caps diaphragm with 47 volts across it. The cap would be there to block DC and provide some high pass filtering.
Sound like a reasonable plan?
Thanks everyone, I really had no idea at first how to go about this !
Best,
Phil D.
I do have a question that is irking me. If a capsule became shorted with a phantom voltage across it, would you expect any damage to the phantom voltage circuitry? That will likely be easy to find out once I get back to my bench, I would expect to see that voltage across the two pcb connections that go to the capsule, just curious if those phantom voltage circuits are self protected in any way.
thanks!
Best,
Phil D.
thanks!
Best,
Phil D.
Chances are that connecting a scope to the input does no harm, but it might mess up the biasing (until you disconnect it again when DC coupled and only a couple of minutes when AC coupled). You could measure straight at the generator output instead, just to be sure.
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No, as there is usually at least 1 Gohm between the polarization voltage supply and the capsule. The polarization voltage need not be equal to the phantom voltage, by the way.
Thank you for that info MarcelvdG, it looks like the 1Gohm maybe in series with the cap and supply a load of sorts, or, voltage divider. And yes, based on what you've described I would want to ultimately look for a different voltage across the capsule than the phantom power voltage (42vdc I believe it is). (just to mention, I didn't find a schematic for this but am getting the notion that most of these true condenser input circuits are probably similar and operate under the same electrical dynamic.
Very helpful, thanks again,
Best,
Phil D.
Very helpful, thanks again,
Best,
Phil D.
Hi,
If you want to be sure the capsule is not shot, i would rather test for capacity rather than resistance. Should be in the pf range (from some tenth to hundred of them).
What are the symptoms you encounter with the mic?
Capsule bias voltage should be in the 40v range but it could be more than that: in some 'classic' transistor mic circuits there is sometimes a dc/dc converter used for capsule bias.
Have you asked AT for a schemo?
If you want to be sure the capsule is not shot, i would rather test for capacity rather than resistance. Should be in the pf range (from some tenth to hundred of them).
What are the symptoms you encounter with the mic?
Capsule bias voltage should be in the 40v range but it could be more than that: in some 'classic' transistor mic circuits there is sometimes a dc/dc converter used for capsule bias.
Have you asked AT for a schemo?
Hi krivium,
I can bring the mic into work and put the capsule on the LCR bridge. The one we have will show quite a bit about the electrical status of the capsule. The mic was reported to be working one day, then put away in its case for a few weeks, and the produce no output the next time it was used. I do double shifts the whole month of august so need to wait a couple more weeks to really dive into testing it. I just took the time to open it up, visually inspect the capsule and throw a DMM across the capsule out of circuit. When it measured 2 ohms, I did make an assumption that it was no longer a “capacitor” and that would give every reason for it being dead in the water. If capacitance is a function of distance between the plates, then 2 ohms told me that there was no distance between the plates. But, I’ve merely scratched the surface and just accumulating info so to understand better what I’ll be working with when I get bench time back.
thank you!
best,
Phil
I can bring the mic into work and put the capsule on the LCR bridge. The one we have will show quite a bit about the electrical status of the capsule. The mic was reported to be working one day, then put away in its case for a few weeks, and the produce no output the next time it was used. I do double shifts the whole month of august so need to wait a couple more weeks to really dive into testing it. I just took the time to open it up, visually inspect the capsule and throw a DMM across the capsule out of circuit. When it measured 2 ohms, I did make an assumption that it was no longer a “capacitor” and that would give every reason for it being dead in the water. If capacitance is a function of distance between the plates, then 2 ohms told me that there was no distance between the plates. But, I’ve merely scratched the surface and just accumulating info so to understand better what I’ll be working with when I get bench time back.
thank you!
best,
Phil
This is not what I expected, but, very interested in your thoughts to this finding.
Finally tonight, had a little time and with disconnected capsule, powered the mic with the normal balanced phantom power preamp (N72).
Looking at the voltage between the 2 posts that feed the AT4040 capsule read -110vdc. I was expecting for one, a much lower voltage (expected around 60-70vdc) and also, I expected the polarity to be different, erroneously assuming that the backplate would be recieving a + voltage. (I hope I kept correct track of what wire went to what post on the pcb).
Just to see if I could determine better the polarity of what voltage from what post, I connected my black ohmmeter lead to earth ground, and measured the 2 pcb posts leaning on the theory that I at least could have some reliance on referencing the capsule voltage to a near zero volt reference.
Well, one of the capsule feeds read +125.3vdc, while the other post gave me 12.17vdc which might roughly explain the 110 or so volts differential I initially measured. Again, the problem with expectations - I expected one of the capsule feeds to be an electrical ground, darn near or at zero volts.
So, perplexed, I thought about this, and had to wonder if it was AT's attempt to shape the pickup pattern a bit with differently applied voltages to the backplate and diaphragm. But, I believe you need two diaphragms in order to achieve that kind of thing.
So, this bewilders me quite a bit - two differing positive voltages feed to a condenser capsule that obtains a 110 volt differential.
I was really thinking about getting a new capsule for the 2 ohm capsule it came with but, I'll need to go back to the drawing board on my limited understanding of capsule theory. I have successfully built 2 nice double pattern condenser mics but, things were much more simple then. Build it, apply 60-70 volts dc across the capsule. Or thats what I believed I was doing!
Thanks for your patience, I'd love to hear what you have to say about those findings.
Best,
Phil Dl
Finally tonight, had a little time and with disconnected capsule, powered the mic with the normal balanced phantom power preamp (N72).
Looking at the voltage between the 2 posts that feed the AT4040 capsule read -110vdc. I was expecting for one, a much lower voltage (expected around 60-70vdc) and also, I expected the polarity to be different, erroneously assuming that the backplate would be recieving a + voltage. (I hope I kept correct track of what wire went to what post on the pcb).
Just to see if I could determine better the polarity of what voltage from what post, I connected my black ohmmeter lead to earth ground, and measured the 2 pcb posts leaning on the theory that I at least could have some reliance on referencing the capsule voltage to a near zero volt reference.
Well, one of the capsule feeds read +125.3vdc, while the other post gave me 12.17vdc which might roughly explain the 110 or so volts differential I initially measured. Again, the problem with expectations - I expected one of the capsule feeds to be an electrical ground, darn near or at zero volts.
So, perplexed, I thought about this, and had to wonder if it was AT's attempt to shape the pickup pattern a bit with differently applied voltages to the backplate and diaphragm. But, I believe you need two diaphragms in order to achieve that kind of thing.
So, this bewilders me quite a bit - two differing positive voltages feed to a condenser capsule that obtains a 110 volt differential.
I was really thinking about getting a new capsule for the 2 ohm capsule it came with but, I'll need to go back to the drawing board on my limited understanding of capsule theory. I have successfully built 2 nice double pattern condenser mics but, things were much more simple then. Build it, apply 60-70 volts dc across the capsule. Or thats what I believed I was doing!
Thanks for your patience, I'd love to hear what you have to say about those findings.
Best,
Phil Dl
I'm amazed you can measure the voltage across the capsule without it collapsing due to the meter load. What kind of voltage meter did you use?
It is very well possible that one side is AC grounded but not DC grounded. It would be nice if you could obtain a schematic somehow, via Internet or by reverse engineering for example.
Wild guess:
Suppose one side is connected to a relatively low output resistance and well-decoupled 125.3 V supply. That side then acts as the AC ground and you can measure its voltage with a normal meter because of the relatively low output resistance.
The other side could then be connected to the gate of the input JFET and via a resistor of a couple of Gohm to a bias voltage that is suitable for the gate. Compared to circuits with AC coupling capacitors, this has several advantages:
-No very-low-leakage AC coupling capacitor needed
-Only one very high resistor required rather than two, one for the capsule and one for the FET
-Only the noise current of one large resistor rather than two
This does mean that my reasoning about it being unlikely that a shorted capsule damages anything was a bit too simple. There is still a good chance that everything survived, but I'm not sure about that anymore.
It is very well possible that one side is AC grounded but not DC grounded. It would be nice if you could obtain a schematic somehow, via Internet or by reverse engineering for example.
Wild guess:
Suppose one side is connected to a relatively low output resistance and well-decoupled 125.3 V supply. That side then acts as the AC ground and you can measure its voltage with a normal meter because of the relatively low output resistance.
The other side could then be connected to the gate of the input JFET and via a resistor of a couple of Gohm to a bias voltage that is suitable for the gate. Compared to circuits with AC coupling capacitors, this has several advantages:
-No very-low-leakage AC coupling capacitor needed
-Only one very high resistor required rather than two, one for the capsule and one for the FET
-Only the noise current of one large resistor rather than two
This does mean that my reasoning about it being unlikely that a shorted capsule damages anything was a bit too simple. There is still a good chance that everything survived, but I'm not sure about that anymore.
If my guess is correct, then connecting a long wire to the 12.17 V side should result in a very loud hum, while connecting a long wire to the 125.3 V side should not. (You could also use a signal generator with a small AC coupling capacitor, of course.)
Thanks for stopping by once again Marcel,
The voltage measurements were with the capsule out of circuit. Since I measure 2 ohms across the capsule with a normal DMM, it didn't make any sense to have it in the circuit for any reason. I just really, really can't imagine a capsule that has the conductivity of a wire in its resting state - unless there is some weird thing where the diaphragm and backplate become physically separated under voltage. I'm only under the understanding that they are separated in space, similar to a capacitor, and can only touch each other/make contact either with an over voltage or some kind of mechanical force or duress.
These findings make me think that AT made a decision to do a very unordinary approach to design with this mic. It seems that most of the well known ubiquitous capsules are seeing between 50-80 volts, give or take. Not a differential of 110 volts from two differing potentials. Huh. and I thought this was going to be easy. That's what I get for thinking I guess.
And there have been no schematics, diagrams or even discussions found on the circuitry or electrical specs of this AT4040 mic. Its like a phantom entity. Ha, even phantom power isn't usually phantom, but, with this mic, it is. I may have to tell my buddy to send his mic to an AT service center where they have schematics, specs, parts capsules - all those things that no one else appears to have.
Thank you Marcel for your input on this.
Phil D.
The voltage measurements were with the capsule out of circuit. Since I measure 2 ohms across the capsule with a normal DMM, it didn't make any sense to have it in the circuit for any reason. I just really, really can't imagine a capsule that has the conductivity of a wire in its resting state - unless there is some weird thing where the diaphragm and backplate become physically separated under voltage. I'm only under the understanding that they are separated in space, similar to a capacitor, and can only touch each other/make contact either with an over voltage or some kind of mechanical force or duress.
These findings make me think that AT made a decision to do a very unordinary approach to design with this mic. It seems that most of the well known ubiquitous capsules are seeing between 50-80 volts, give or take. Not a differential of 110 volts from two differing potentials. Huh. and I thought this was going to be easy. That's what I get for thinking I guess.
And there have been no schematics, diagrams or even discussions found on the circuitry or electrical specs of this AT4040 mic. Its like a phantom entity. Ha, even phantom power isn't usually phantom, but, with this mic, it is. I may have to tell my buddy to send his mic to an AT service center where they have schematics, specs, parts capsules - all those things that no one else appears to have.
Thank you Marcel for your input on this.
Phil D.
I understood the capsule was out of the circuit, but phrased my reply incorrectly. I meant I'm amazed the voltage doesn't collapse when you measure across the terminals of the electronics in the microphone that are normally connected to the capsule.
What type of meter did you use?
What type of meter did you use?