Maybe because all the clipping/distortion comes from FET before output pair?
Getting capsule polarization voltage from phantom power in a Schoeps-style circuit
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thor.zmt
Well-known member
No, it's the P12 12V Phantom power thing.
If you take the design and optimise for 48V (without consideration of polarisation voltage by using a classic step-up oscillator for that) you get reduced distortion at higher (non-clipping) levels vs the original and an onset of clipping at much higher levels.
That was the first step that led in steps to my all FET design with capsule bias from phantom power and P68 option with 60V capsule bias.
Incidentally, we have tiny modern switchers that operate at 1Mhz+ that can be made to work for the capsule bias. I'm surprised nobody uses them. 1MHz is very easy to filter out and the whole circuit using SMD parts is tiny. Only reason I didn't do this was the unwillingness to design and spin a PCB for a few mikes when instead a few minor mods on the desk give me P68.
Thor
Matador
Well-known member
I have only designed circuits with MOSFETs as switches; my first thought when looking at that FET variant of the Schoeps style circuit is that the variability in FET threshold voltage would make DC coupled circuits like that difficult to match side to side. How does that work out in practice?
thor.zmt
Well-known member
Modern parts have very good consistency. Mosfet's and J-Fet's. Parts from the same reel commonly are essentially identical. Panta Rhei, Tempus Fugit.
I never observed material offset, given the inputs are usually AC coupled anyway it matters zip.
Thor
thor.zmt
Well-known member
I was more thinking something like this:
Axelite AX5511
This tops out at ~ 24V so we drive a greinacher cascade, capsule bias draws zero actual current once stable, so 1N4148 and 100n X7R ceramic cap's will be plenty. With regulation set to 66V and a tripler cascade we get 66V Bias with 22V at the switching node, all well within safe parameters.
The rest is still about an optimised Schoeps topology.
We only need 5V input for the switcher (note, there is a trick to use more than 5.5V input, drive the inductor from a higher voltage, the chip's Vin pin from a divider with zener clamp), so we can get (say) 32V across our 6.8k Resistors (9.5mA total current), 11V across the output followers for around +26dBu at hard clipping, giving some headroom.
We use the main 22V from the switcher to drive our J-Fet frontend at the usual 0.5mA or so (we have up to ~1mA available.
This incidentally places the J-Fet gate at ~ 5.5V, hence the 66V bias to get actually 60V bias (66V - 5.5V). The input resistance/diode is bootstrapped in this case, no gate resistor, capsule straight to gate.
Our frontend can handle around +20dBu output (bal) before hard clipping. With ~ -32dBu/Pa (20mV @ 94dB) we can theoretically handle 146dB SPL at the point where electronics clip.
Thor
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Matador
Well-known member
I had breadboarded up a test circuit, using a OPA210 (lower noise than your choice, although at higher current), and without phantom power (just using a test supply that provided +- 15V, and also +60V for the capsule). The chip was in SOT23-5 which allowed the non-inverting pin to be bent upward into the air for connecting to the 1G / capsule point.
The main different to your implementation is that it used a second op-amp to add a LPF to tailor the HF response. If I recall I preferred a 7kHz pole, however that was using a K67 copy that was quite shrill at HF.
But other than that, I thought it sounded great.
rogs
Well-known member
I hadn't realised that the OPA210 had an input impedance high enough for this task?... Although the OPA164* series is a little noisier, it does have a JFET input, making it ideal for condenser capsule interfacing.
I've not found the noise floor a problem -- the ambient noise tends to swamp the op amp noise in most 'real world' applications.
I know that some folk like to experiment with EQ within the mic, but I have found that adding resistors into passive filters in the signal path at mic level can tend to get a little bit noisy..... But if you're trying to copy a specific mic schematic, for example, it is of course entirely appropriate.
I prefer to try and just use a single op-amp unity gain buffer as an impedance converter, with no extra resistors in the signal path, and then add eq etc outside the mic.
At the moment I'm doing a dual polarity DC voltage multiplier, so I can get rid of the sum and difference amps in my multi-pattern OPIC mic.
Even simple functions like adding and subtracting place extra resistors into signal path -- and they add noise (Not a lot -- but it's still extra noise!)
I'm currently experimenting with a Dickson charge pump ... Nice simple reliable concept that should allow a simple multiplier with an adjustable range of ± 60 to 80V.... And no 'inductor coupling' orientation problems to worry about!
Matador
Well-known member
You are correct! The part number was OPA1656, I just looked at the wrong part in my stash. It looks largely the same as what you tested. I think it even worked with a TL072.
I'm also interested in placing the SOT-23 op amp and 1G resistor on a tiny PCB located up in the head basket to completely remove the high impedance parts of the amp from the shell to the head basket.
TLC072 is used in Townsend Labs Sphere. Note it's TLC not TL.You are correct! The part number was OPA1656, I just looked at the wrong part in my stash. It looks largely the same as what you tested. I think it even worked with a TL072.
I'm also interested in placing the SOT-23 op amp and 1G resistor on a tiny PCB located up in the head basket to completely remove the high impedance parts of the amp from the shell to the head basket.
Very different animals in this application. TLC has 15dB less noise current, 8dB less noise voltage.
maosante
Active member
Hi, I've just finished the Mic Scharf board, but I'm only getting 10v at every diode output and capsule is getting also 10v, when I connect the mic it makes a low frecuency pulse sound, 24v is still the measurement on each XLR input (Pin 2 and 3). Any ideas? I've configured at 3.3k at the output for 60v.Using a DC- DC converter uses exactly the same principle as Schoeps - although I tend to prefer voltage muliplier circuits that don't require inductors.
EQ is a bit more tricky. One intention of using circuits like the one I posted is to keep them very simple.
That means simply using an opamp as a high impedance non-inverting buffer, with no other active electronics. There really isn't anywhere to add EQ easily into this type of simple circuit.
Personally, I prefer the option of adding EQ at line level - externally.... I've made some notes HERE on that idea.
I've found adding passive EQ internally in the mic itself tends to add noise, and can be a bit brutal in its function.
At line level, circuit sophistication can be much more comprehensive -- and noise is much less of an issue.
If you're recording then adding EQ from within a DAW is another option, again more flexible than adding internal passive EQ
I've made notes on several types of Op-amp mic circuitry in my pages here: OPIC Impedance Converter Just click on any of the schematics to reveal circuit descriptions - and even stripboard layouts - which are cheap and easy for experimentation!
Mic Scharf has even done some PCBs for those who don't like using stripboard (Links on the mic pages).
Using op-amps for this task allows for them to be used in the simplest of circuits.
The technology that goes into making these excellent modern op-amps perform as well as they do is a different matter. Companies like TI produce excellent documentation for those who want to know more of what goes on inside the op-amps themselves.
That's a whole different subject of course!
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How are you measuring it? Even a DMM loads too much the circuit.
Have ypou checked correct operation using a generator instead of a capsule?
maosante
Active member
Hey, thanks for your quick response. Yeah with a DMM, I don't have any other measurement tools available.
maosante
Active member
By the way the only thing that I couln't find to place in the circuit is the ceramic MLCC 1nf for the chip. I place a regular ceramic 1nf, not MLCC. Could this be the problem?
Which "Mic Sharf board"?
That works out to about 7mA current consumption, which is relatively huge.
maosante
Active member
It takes special equipment to measure accurately. You have to resort to alternative methods.
You probably don't have an oscilloscope, so I would suggest you measure the voltages at the different inverter's outputs I.E. pins 2, 4, 6, 8, 10 &12. You should measure approximately half the voltage at pin 14.
It shouldn't but you might want to substitute another. The value is not critical, 820p or 1.2n should work. I would suggest a replacement with a CG0/NP0 type.
rogs
Well-known member
As you seem to be getting no voltage multiplication at all, it sounds as if the oscillator may not be not running?
And - as Khron has pointed out - you should not be drawing 7mA from this circuit..... More like 2mA.
As you only have a DMM for testing, it can be difficult to know for sure whether the oscillator is running or not.
You can get a rough idea by measuring the voltages on pins 1 and 2 of the IC with your DMM. They should both measure around half the IC supply voltage. Around 5 volts with the values you've described.
In fact all the IC pins - with the exception of pins 14 and 7 (c. +10v and 0v respectively) - should measure between 5v and 6v, using your DMM.
If the oscillator is running you should be able to measure c. 17v at the cathode of D1. That will confirm the muliplier is working.
The subsequent diode cahodes should measure with increasing DC voltages. (Not sure of Mic Scharf's component idents, which are different from my stripboard original ).
Hand soldering SMT devices can be tricky. Double check for any shorts between IC pins.
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