And we really appreciate you sharing your ideas! What a boring place this would be without members posting whatever popped into their heads?
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Dumb idea for a mic circuit?
- Thread starter OneRoomStudio
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Yes, they are.
Of course, there may be some discrepancies due to the tolerance of the core parameters, but anything between 500-700H for the primary winding should be considered correct.
Matador
Well-known member
I did a few experiments with using a +-60V supply in a microphone with a discrete op-amp with FET devices (essentially one of Sam Groener's designs, with high(er) voltage NPN and PNP devices that could tolerate the 120V environment). It seemed to work well, with the only notable advantages I found being:
1) No need to do any voltage conversion in the microphone itself for the PSU rails.
2) Simple patterns as per the original poster.
3) I think perhaps noise was less, provided the two rails were clean (easy to do with separate PSU).
4) If trimmed for no DC offset, no need for coupling capacitor.
5) Since the op-amp could drive +-20mA without impacting the polarization voltage(s), no output transformer needed (just impedance balanced output).
While a "success", it's wasn't night and day different from the myriad other circuits that used phantom power. However I still use that microphone for testing new capsules, as the circuit is pretty ruler flat, with no emphasis or de-emphasis, so the differences between capsules are more obvious than with transformer coupled circuits.
1) No need to do any voltage conversion in the microphone itself for the PSU rails.
2) Simple patterns as per the original poster.
3) I think perhaps noise was less, provided the two rails were clean (easy to do with separate PSU).
4) If trimmed for no DC offset, no need for coupling capacitor.
5) Since the op-amp could drive +-20mA without impacting the polarization voltage(s), no output transformer needed (just impedance balanced output).
While a "success", it's wasn't night and day different from the myriad other circuits that used phantom power. However I still use that microphone for testing new capsules, as the circuit is pretty ruler flat, with no emphasis or de-emphasis, so the differences between capsules are more obvious than with transformer coupled circuits.
That seems like the most sensible thing if you are bothering with an external supply. The original post started out with the premise that you could avoid the compromises of using phantom power by using an external power supply, and then suggested a really compromised circuit which did not take advantage of the higher headroom available by using an external supply.
Very cool to see that you had a similar idea and tried it out.
The circuit I proposed was just one I happened to think of (hence, saying "One idea that occurred to me was to build a cascode circuit using bipolar 60V."). My main area of curiosity and the main point of my first post was to think about the benefits you laid out.
Matador
Well-known member
It's also possible to do it with a 120V supply, so you can reuse existing tube power supplies. The final iteration of my circuit used a single +120V supply, with the backplate at 60V, the front diaphragm at 0V, and the back diaphragm at 0V (omni), 60V (cardioid), and 120V (figure 8): this is standard C12 polarization.
Then the op-amp is floated to a virtual earth of 60V, which is then cap-coupled to the XLR. You can get really large signal swings with such a circuit, such as working non-inverting with x10 gain, meaning a 1V capsule can swing 20V peak to peak). However the real benefit is running at large open-loop gain, then linearizing the circuit with feedback to get a flat pass-band (and all the other benefits of using an op-amp). In fact, you can make it so that the signal from the microphone essentially never runs out of headroom, even with close miking loud sources with an LDC, however it can completely blow-up the input to most mic pres, even with hefty pads engaged.
Then the op-amp is floated to a virtual earth of 60V, which is then cap-coupled to the XLR. You can get really large signal swings with such a circuit, such as working non-inverting with x10 gain, meaning a 1V capsule can swing 20V peak to peak). However the real benefit is running at large open-loop gain, then linearizing the circuit with feedback to get a flat pass-band (and all the other benefits of using an op-amp). In fact, you can make it so that the signal from the microphone essentially never runs out of headroom, even with close miking loud sources with an LDC, however it can completely blow-up the input to most mic pres, even with hefty pads engaged.
The transformer winding is 3900 turns to 600 turns. It makes 6.5:1 ratio.
Seeker
Well-known member
I had an idea extremely similar to this, but using 2 fets, because why not… haven’t built it though. Pretty cool you’re thinking pretty much along the same lines!
Too many projects, too little time.
Too many projects, too little time.
That would be a basic FET cascode, as in AudioTechnica 4033. It wouldn't meet the same bias challenge as the hybrid FET+BJT cascode.
Seeker
Well-known member
I’m not familiar with the 4033 schematic…
The mic schematic looks pretty similar, so it reminded me of what I drew, though its functionality (if it worked) would be quite bit different.
The idea was to power it with 35v ish -+ and for polarization, a voltage doubler on both legs. Different power scheme, still one transformer and polarization would be essentialy the same as the OPs description… so it made me think the OP and I were thinking along the same lines…
The mic schematic looks pretty similar, so it reminded me of what I drew, though its functionality (if it worked) would be quite bit different.
The idea was to power it with 35v ish -+ and for polarization, a voltage doubler on both legs. Different power scheme, still one transformer and polarization would be essentialy the same as the OPs description… so it made me think the OP and I were thinking along the same lines…
The big difference is the OP had a flaw with the transistor bias. It could have been solved easily, as evidence by some AKG mics show, but the whole concept of high-voltage power is to be discussed.
If the main reason is to make the capsule bias easier, I think the head amp should have less gain, less distortion and more headroom, probably with a different structure.
Alternatively, if the reason is to increase input headroom, a voltage-follower circuit should be more appropriate.
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