Getting capsule polarization voltage from phantom power in a Schoeps-style circuit

[skyy7]

Why would you want to do that?

Since it derived its polarization voltage from P48, they had to deal with about 40V instead of 60. There was no other motivation than simplicity. Apparently, the outcome was not entirely positive, since Neumann designers used DC/DC converters in many subsequent models.

I'm currently building in MXL990s with perfboard, and fitting a pimped alice with a DC-DC converter proved to be a little too difficult.

 

[Khron]

I'm currently building in MXL990s with perfboard, and fitting a pimped alice with a DC-DC converter proved to be a little too difficult.

You're not trying hard enough

https://khronscave.blogspot.com/2017/05/se-electronics-se-x1-rebuild.html
Yeah, perfboard has its uses, but it's at the same time also quite restrictive...

 

[skyy7]

Here's a tweaked version which gives (in simulation) just over 42V bias from a 48V supply:

View attachment 120612

(I've just included the components relevant to the DC operating point here)

I don't know what bias current you want to run the FET at, but 1mA (modelled by source I1) should be plenty. The changes are:

1) Adding (on my diagram) R8/R9. This is now a 'Vbe multiplier' circuit giving about 10V between the emitters and collectors of Q1/Q2. This will allow a much larger output voltage swing before the transistors saturate.
2) Increase R1 (R7 on the original schematic) to 12K. This just allows the Vcoll voltage to be higher (about 32V in simulation).

So, in simulation, the capsule voltage (Vbias1/Vbias2) is 42.5V, Vcoll is 32.0V, Vfet is 12.6V.

While I'm here...
- The C2/C4 values (47nF) give a bass rolloff -3dB point of 34Hz (with the 100K resistors). No problem if you're happy with that, personally I'd make them at least double that.
- (Little-publicised fact about this type of output) the 'RFI suppression' caps C8 and C9 impose a slew rate limit on the positive-going edges of the output signals, determined by the capacitor value and the supply current. In this case it's 0.8mA each side, giving a decidedly poor 36V/ms. IMO these could be significantly smaller (e.g. 2.2nF).

Oh, wow, I did not know about the caps at the output... I'll need to fix that on my other mic as well.

How did you come up with the value for the Vbe multiplier circuit? What would be the reason for having a higher Vceo and having the voltage at the collector be higher?

And thank you I did not realize that C2 and C4 were 0.047u I generally have it at 0.47u, but I made a typo.

 

[Voyager10]

That's what I did, but doesn't work. It seems it rearranges text as a spreadsheet.
Do you use LTspice' Mac version or under emulated Windows?

Yes, I'm using the Mac native version (LTspice for MacOS, v17.1.4.).

Does the version below work for you? I had a look at the older one in a hex editor and it's all 16-bit characters, how very weird.

 

[abbey road d enfer]

I'm currently building in MXL990s with perfboard, and fitting a pimped alice with a DC-DC converter proved to be a little too difficult.

My question was why 30V when 40 is easily achievable?

 

[abbey road d enfer]

Yes, I'm using the Mac native version (LTspice for MacOS, v17.1.4.).

Does the version below work for you? I had a look at the older one in a hex editor and it's all 16-bit characters, how very weird.

OK, got it. Default coding is UTF8; had to change it to ANSI.

Found the reason for discrepancy; I had neglected the 12k resistor.

 

[skyy7]

My question was why 30V when 40 is easily achievable?

Not sure.... that was the voltage I was getting in Spice and wasnt sure how else I could implement something like this although it seems like people here reccomended better solutions

 

[skyy7]

It is a testament to the simplicity and elegance of the Schoeps CMC5 schematic that it is still so highly regarded, some 50 years after it appeared.
There are probably hundreds of threads online discussing the various aspects that can be 'tweaked' to 'improve' it .

The fact remains though it utitlises components that were easily available 50 years ago. Some - like suitable discrete 'thru hole' JFETs for example - are becoming increasingly hard to find these days.
Following hobbyist Jules Ryckebusch's suggestion to try OPA164* series opamps in place of a separate JFET input a couple of years back, it will be interesting to see whether that kind of topology starts to replace 'Schoeps' style circuits in the DIY world, going forward?

I've been using the OPA1641 for a while now, and find it a very useful alternative to Schoeps style.....
• No need to regulate the DC, and thus limit headroom.
• No need to include any FET bias adjustment
• Very low distortion
• Rail to rail output swing
• Acceptably low noise.
• No noise creating resistors in the signal path
I have used the attached schematic as a simple way to interface with a conventional 34mm LDC condenser capsule, using phantom power for the capsule polarisation supply and find it seems to work at least as well as the Schoeps circuit......
View attachment 120619
Although the audio output is single sided, the line is passively impedance balanced, to help maintain a reasonable CMRR.

So far, there seem to be relatively few hobby folk who have tried this type of circuit, instead of using the ubiquitous 'Schoeps'...
As suitable JFETS get more and more difficult to find, maybe more DIY hobbyists will venture over to this 'dark side' alternative?

Oh, Ive always wanted to mess around with opamp circuits although im not sure if I understand them at all... well I'm not sure if I even understand a schoeps circuit...

Would implementing an eq and getting capsule voltage using a dc-dc board be the same as a schoeps?

 

[Voyager10]

Oh, wow, I did not know about the caps at the output... I'll need to fix that on my other mic as well.

How did you come up with the value for the Vbe multiplier circuit? What would be the reason for having a higher Vceo and having the voltage at the collector be higher?

And thank you I did not realize that C2 and C4 were 0.047u I generally have it at 0.47u, but I made a typo.

I didn't want to start a fashion for taking off output caps on the Schoeps circuit in general! They do valuable work in reducing any RF from the oscillator getting through to the output (in circuits with an oscillator), and the slew rate only becomes an issue if other parameters change.

The 10V on the Vbe multiplier circuit is slightly arbitrary. The main factor here is how big we want the negative voltage swing on the output to be. The positive voltage swing can't exceed 48V - 42.5V (the steady-state output voltage) so 10 V is plenty. In fact, the limiting factor for output voltage will be the FET stage, which won't exceed +/- 3V.

 

[rogs]

Oh, Ive always wanted to mess around with opamp circuits although im not sure if I understand them at all... well I'm not sure if I even understand a schoeps circuit...

Would implementing an eq and getting capsule voltage using a dc-dc board be the same as a schoeps?

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!

 

[thor.zmt]

So far, there seem to be relatively few hobby folk who have tried this type of circuit, instead of using the ubiquitous 'Schoeps'...
As suitable JFETS get more and more difficult to find, maybe more DIY hobbyists will venture over to this 'dark side' alternative?

Tried it. Didn't really like the results.

If I want those results, I'd just take a 16 core X-Mos and an array of 64 PDM output MEMS mic's in two 32 Mic PCB's spaced apart (mechanical adjustment screw with encoder?) costing with the right pcb much less in volume than a decent condenser capsule and do a 384k/32 Bit digital output Mic. Wireless using WiFi, just because I can.

Include full emulation of classic mic's as well as any number of crazy new stuff. Any pattern, continuously variable across the whole frequency range you desire with maximally flat response and ~ -83 dB self noise @ 94dB SPL.

As it happens I like classic vintage Mic sound and just like a bit more headroom, less noise and LF roll-off etc, as this 2024 and not 1964. And I don't feel like faking it in software.

Thor

 

[rogs]

Tried it. Didn't really like the results.

If I want those results, I'd just take a 16 core X-Mos an array of 64 PDM output MEMS mic's (costing with the right pcb much less in volume than a decent condenser capsule) and do a 384k/32 Bit digital output Mic with full emulation of classic mic's as well as any number of crazy new stuff.

As it happens I like classic vintage Mic sound and just like a bit more headroom, less noise and LF roll-off etc, as this 2024 and not 1964.

Thor

That's the beauty of this hobby... we can all try out different things, and see what we like.
With much of the character of a mic often attributed to the capsule, I'm not sure a 'MEMs array' would really appeal to me personally?

When it comes to non-linearity in mic circuitry, people will make all sorts of claims about the benefits of circuit 'a' or JFET 'b'.
This of course is all manna from heaven for the 'snake oil' brigade!

This being the internet, the only way to really know what is real - and what is 'snake oil' - is to try things out for yourself ......
It's called 'hobby mic building'

 

[thor.zmt]

With much of the character of a mic often attributed to the capsule, I'm not sure a 'MEMs array' would really appeal to me personally?

Well, you can "fake" at least the objective "not snake oil" in software, to your heart's content.

The Mic will take > 140dB SPL linearly have 11dB noise floor and any directional and frequency response, plus HD profile you care to program.

One such mic should cost a fraction of a "boutique mic" today and be able to sound like anything, unless the snake oil wasn't snake oil.

Hell, you can throw in a compressor with dynamically controlled timing, autotune and reverb/chorus for free, add emulation for vocal cabs and tube vocal mixer and amp and recording microphones, so you can sound like a 1940's live recording of some of greatest jazz vocalists, with minimal talent, given the DSP on board.

We see emulators for other instruments, why not mic's, preamps, compressor and EQ baked in too? And fully wireless, digital from the capacitive MEMS modulator to the mixer.

Thor

 

[rogs]

Well, you can "fake" at least the objective "not snake oil" in software, to your heart's content.

The Mic will take > 140dB SPL linearly have 11dB noise floor and any directional and frequency response, plus HD profile you care to program.

One such mic should cost a fraction of a "boutique mic" today and be able to sound like anything, unless the snake oil wasn't snake oil.

Hell, you can throw in a compressor with dynamically controlled timing, autotune and reverb/chorus for free, add emulation for vocal cabs and tube vocal mixer and amp and recording microphones, so you can sound like a 1940's live recording of some of greatest jazz vocalists, with minimal talent, given the DSP on board.

We see emulators for other instruments, why not mic's, preamps, compressor and EQ baked in too? And fully wireless, digital from the capacitive MEMS modulator to the mixer.

Thor

Or -- you could take a high quality 'flat' capsule - like Ari's 'flat K47' perhaps? - and couple it to a simple, low distortion, low noise op-amp impedance converter....
That should give a high quality, reasonably priced 'neutral' mic, which could then be used with whatever audio processing software you like, to create the exact mic characteristics of your choice.....

 

[mhelin]

To start with you could use any capsule, K87 type also, without the de-emphasis EQ, and then flatten the response in software.

 

[kingkorg]

Ideally, because of 180° and off axis response you'd want to use k67, k47, ck12, and SDC capsules and have appropriate calibration files for each in order to emulate diferent mics based on specific capsules.

Another solution would be to use a dual capsule with superior 180° rejection, and then introduce additional leakage from rear at certain frequencies for diferent capsule models. Typically k67 has better rear rejection than k47. Well tuned CK12 would have better rear rejection than both k67, and k47. A good SDC end adress mic would have even better rejection.

I am afraid MEMS array wouldn't work, as multiple capsules cause null cancelations throughout the spectrum, depending on placement of course. This is shown in most mic array documents i've seen. In theory this could be avoided with some serious programming, research, true anechoic measurements, tons of trial and error.

Something i call 2nd, or 3rd order dual sided capsule cardioid would be ideal. A capsule that has 20db+ rear rejection throughout the spectrum. Low end rejection would still be an issue because of the wavelength, but all the capsules have this "problem", most become omni approaching 50hz.

I don't think this will ever happen, it would be too expensive mic, and no one would believe it really does good job of emulating their favorite u47.

 

[thor.zmt]

I am afraid MEMS array wouldn't work, as multiple capsules cause null cancelations throughout the spectrum,

SMD MEMS microphones are TINY.

Here an all analogue design (long obsolete) with rather large size analogue MEMS Mic's and discussion of beamforming and the various implications.

The wavelength at 20kHz is 17mm.

Current generation MEMS Mic's are 3.5 X 2.5 mm appx.

So cancellations are at very high frequencies. Imagine 32 pcs in a star shape on a 32mm diameter PCB's - two identical PCB's at a distance, which can be similar to that of a normal capsule distance between diaphragms.

Something i call 2nd, or 3rd order dual sided capsule cardioid would be ideal. A capsule that has 20db+ rear rejection throughout the spectrum. Low end rejection would still be an issue because of the wavelength, but all the capsules have this "problem", most become omni approaching 50hz.

Using enough digital processing and my MEMS arrays it's quite possible.

I don't think this will ever happen, it would be too expensive mic,

My version, using (say) 64 pcs Knowles SPH0641L (0.5 USD in volume) and a 4 USD (in volume) XMOS Chip would come with a BOM of around 50 USD including case. That means 399 USD retail.

Note, this is a Microphone with USB digital out, it plugs directly into a USB Hub. So no Mic-Pre and AD needed. Making it wireless via wifi makes it a 449...499 USD Mic depending on precise solution employed.

and no one would believe it really does good job of emulating their favorite u47.

Yup, that is the main reason.

And the great unwashed rather spend 1,000 Bux on a bad replica made in far east, instead of half that on a device that is on a whole 'nother level. Which is why no potential customer was ever the least interested in the project.

Thor

 

[kingkorg]

SMD MEMS microphones are TINY.

Here an all analogue design (long obsolete) with rather large size analogue MEMS Mic's and discussion of beamforming and the various implications.

The wavelength at 20kHz is 17mm.

Current generation MEMS Mic's are 3.5 X 2.5 mm appx.

So cancellations are at very high frequencies. Imagine 32 pcs in a star shape on a 32mm diameter PCB's - two identical PCB's at a distance, which can be similar to that of a normal capsule distance between diaphragms.



Using enough digital processing and my MEMS arrays it's quite possible.



My version, using (say) 64 pcs Knowles SPH0641L (0.5 USD in volume) and a 4 USD (in volume) XMOS Chip would come with a BOM of around 50 USD including case. That means 399 USD retail.

Note, this is a Microphone with USB digital out, it plugs directly into a USB Hub. So no Mic-Pre and AD needed. Making it wireless via wifi makes it a 449...499 USD Mic depending on precise solution employed.



Yup, that is the main reason.

And the great unwashed rather spend 1,000 Bux on a bad replica made in far east, instead of half that on a device that is on a whole 'nother level. Which is why no potential customer was ever the least interested in the project.

Thor

Remarkable concept to say the least!

 

[skyy7]

Here's a tweaked version which gives (in simulation) just over 42V bias from a 48V supply:

View attachment 120612

(I've just included the components relevant to the DC operating point here)

I don't know what bias current you want to run the FET at, but 1mA (modelled by source I1) should be plenty. The changes are:

1) Adding (on my diagram) R8/R9. This is now a 'Vbe multiplier' circuit giving about 10V between the emitters and collectors of Q1/Q2. This will allow a much larger output voltage swing before the transistors saturate.
2) Increase R1 (R7 on the original schematic) to 12K. This just allows the Vcoll voltage to be higher (about 32V in simulation).

So, in simulation, the capsule voltage (Vbias1/Vbias2) is 42.5V, Vcoll is 32.0V, Vfet is 12.6V.

While I'm here...
- The C2/C4 values (47nF) give a bass rolloff -3dB point of 34Hz (with the 100K resistors). No problem if you're happy with that, personally I'd make them at least double that.
- (Little-publicised fact about this type of output) the 'RFI suppression' caps C8 and C9 impose a slew rate limit on the positive-going edges of the output signals, determined by the capacitor value and the supply current. In this case it's 0.8mA each side, giving a decidedly poor 36V/ms. IMO these could be significantly smaller (e.g. 2.2nF).

Hey, sorry for the late reply. I was messing around with the VBE multiplier circuit in LTSpice, and it seems to decrease distortion and headroom with seemingly no downside? If there aren't any downsides, why don't all variations of the Schoeps circuit use this?

 

[Voyager10]

The most likely reason is to allow the circuit to work from 12V phantom power.

(The IEC 61938 standard allows 12V and 24V power as well as 48V - it made battery-powered mixers easier to design, in the days before simple IC switching regulators).