DC-DC CMOS (HEX INVERTER) REVERSE BIAS?

[tedsorvino]

This is not an option. You can't power the inverters with a negative supply. Unless you reference the whole circuit to the -12 rail.

By the word "reference" I may be lost in translation. But what I meant to do is building the WHOLE dc CMOS circuit around the converted -12V voltage.


Without quoting, the booster from 12v to 15v solution seems very straightforward and easy to me. Actually I can test it really easily by just supplying the circuit with 15v,, from my bench supply and see what I got on the out.

Good to know what other options would work for future reference.

Thanks again.

 

[Khron]

I believe i speak for everyone when i ask the following questions:

1) WHY do you absolutely NEED +/-80V (as opposed to the much more usual +/-60V or so)?

2) Why do you insist on choosing any and all but the simplest possible way to solve this issue, ie. switching the zener in the existing circuit (which we've yet to see a schematic of, btw) from 12V to 15V?

PS: What's that MC33761 powering? The audio circuitry itself?

Sure, it might involve cutting a trace, but surely the input to the bias circuit oscillator could be separated from the input going to the MC33761 (spec'd at 12V absolute max), and use a 15V for that alone. Of course, it sucks that you went ahead and possibly had PCB's made for this before ironing out issues like this in the design stage, but.....

 

[abbey road d enfer]

By the word "reference" I may be lost in translation. But what I meant to do is building the WHOLE dc CMOS circuit around the converted -12V voltage.

I'm not sure you understand all the implications. Can you post a schematic of what you're thinking, complete with how you connect it to the rest?

Without quoting, the booster from 12v to 15v solution seems very straightforward and easy to me.

What is the operating current of this booster? Are you sure you're within limits of phantom power?

Actually I can test it really easily by just supplying the circuit with 15v,, from my bench supply and see what I got on the out.

That would not tell you how it reacts with the rest of the circuit.

 

[tedsorvino]

Dear people ....it's this simple well known instructable True Condenser OPA Mics

All the schematics are there and how to order the pcbs you need. The rest is just modifications I'm thinking.

PS: What's that MC33761 powering? The audio circuitry itself?

In my initial post I wrote that we ommit the voltage regulator. MC33761 is voltage regulator.

And just a simple question. Since you give such an easy and good answer like the initial one, that the negative supply is one stage of 12v short of the positive, so it needs 12v more, then why are we trying to do things hard for other people to follow.....????

Anyways...thanks again.

I 've made up my mind. Haven't got the time to discuss the issue more. It's a cheap simple diy mic and the solution for a probable fig 8 function is a booster for me. If it doesn't work it's ok.
The rest is for more technical minded and diy microphone fans than me.

 

[rogs]

When I decided to try out the OPA164* series op-amp as an impedance converter I simply fitted a Zener as a regulator to the voltage multiplier circuitry, so as to be able to set the capsule polarisation voltage consistently - regardless of any actual differences in the applied phantom power supply.
As Abbey says, there's no need to regulate the supply to the OPA164* .... Simply draws more current, reduces headroom and potentially adds another noise source...
I've been quite pleased with the results I've obtained with THIS variation of my OPA1641 circuit....

 

[Drosselmeier]

DC\DC with current consumption less than 20uA

 

[mhelin]

When I decided to try out the OPA164* series op-amp as an impedance converter I simply fitted a Zener as a regulator to the voltage multiplier circuitry, so as to be able to set the capsule polarisation voltage consistently - regardless of any actual differences in the applied phantom power supply.
As Abbey says, there's no need to regulate the supply to the OPA164* .... Simply draws more current, reduces headroom and potentially adds another noise source...
I've been quite pleased with the results I've obtained with THIS variation of my OPA1641 circuit....

The capsule is floating between the DC polarization voltage (generate using the multiplier circuit) and half of the op-amp supply voltage (virtual ground). How do you know what will the exact polarization voltage be with different mic preamps if the op-amp supply is not regulated?

 

[rogs]

The capsule is floating between the DC polarization voltage (generate using the multiplier circuit) and half of the op-amp supply voltage (virtual ground). How do you know what will the exact polarization voltage be with different mic preamps if the op-amp supply is not regulated?

I don't -- but it's probably not actually that critical? The virtual earth might vary by a few volts - depending on the precise characteristics of the phantom power supply being applied - but not by enough to make any real difference to the performance of the mic, in my experience.
I am guessing one idea of deriving symmetrical positive and negative values is to assist with the performance in figure of 8 configurations...
In reality, many dual capsules probably don't actually generate identical output levels, so the precise accuracy of the polarisation voltage is probably not that critical...
When it comes to not regulating the multiplier circuitry supply, I think that can make quite a difference.
Any difference in the DC supply to the inverter IC is of course multiplied..... A 25% change from 12v to 15v in that supply, and the output from the final inverter would rise from around 80 v to around 100v ...

 

[AUDIO FREQ]

I used this dc dc hex board with the single channel alice opamp 1642 board from jli electronics, but it's got just a tad more noise (white noise) added to the signal than i would like. I've attached the schematic of the board.

I tried the dc-dc hex power board in another known fet mic, and it is indeed the dc-dc circuit board that adds small amount of white noise.

Could it be the diodes? I've read that diodes are known to be noisy. Would anyone have any ideas where I can start looking?

 

[abbey road d enfer]

I used this dc dc hex board with the single channel alice opamp 1642 board from jli electronics, but it's got just a tad more noise (white noise) added to the signal than i would like. I've attached the schematic of the board.

I tried the dc-dc hex power board in another known fet mic, and it is indeed the dc-dc circuit board that adds small amount of white noise.

Could it be the diodes?

In that instance, diodes cannot be incriminated.

I've read that diodes are known to be noisy.

Don't take everything you see on the internets at face value. It's a rather silly assertion. The noise mechanisms in diodes are well-known and well-controlled.
It's true that Zener diodes are noisy, though.

Would anyone have any ideas where I can start looking?

You don't show how this board is powered. I would look at that first.
Possibly, the voltage is regulated by a Zener (zeners ARE quite noisy), and quite often they are not properly bypassed. It takes a rather large capacitor (47-100uF) in parallels to decrease noise.

 

[AUDIO FREQ]

Hey Abbs! Its nice to get your brain on this. Thank you!

Here is a diagram of how I have the mic set up. It does seem like it gets it's power from the 12V zener via the op amp board. But it seems like there is a 47uF in there, maybe I can replace with 100uF?

 

[abbey road d enfer]

Here is a diagram of how I have the mic set up. It does seem like it gets it's power from the 12V zener via the op amp board. But it seems like there is a 47uF in there, maybe I can replace with 100uF?

Sometimes a single cap is not enough. Try inserting a 100-200r resistor instead of the link that's circled in red, and maybe increase C10.
There is another, very distant possibility of the CD4584 being unstable. Did you try another?

 

[AUDIO FREQ]

Sometimes a single cap is not enough. Try inserting a 100-200r resistor instead of the link that's circled in red, and maybe increase C10.
There is another, very distant possibility of the CD4584 being unstable. Did you try another?

I haven't. I will have to source one from mouser. But for now, I will try the other 2 suggestions you advised first. C10 being 4.7uf, how much should I increase it to? 47uF?

 

[abbey road d enfer]

C10 being 4.7uf, how much should I increase it to? 47uF?

Yes.

 

[rogs]

I used this dc dc hex board with the single channel alice opamp 1642 board from jli electronics, but it's got just a tad more noise (white noise) added to the signal than i would like. I've attached the schematic of the board.

I tried the dc-dc hex power board in another known fet mic, and it is indeed the dc-dc circuit board that adds small amount of white noise.

Could it be the diodes? I've read that diodes are known to be noisy. Would anyone have any ideas where I can start looking?

I notice from your attached schematic that you are using a 4584D, rather than the 40106 normally specified for this type of circuit.
Pin fpr pin compatible of course, but with a much lower hysterisis voltage. I suspect the frequency of your oscillator will be significantly higher than the c.125KHz normally encountered when using a CD40106 with those oscillator values. ( The HEF 40106 has slightly different hysterisis levels - and that causes the oscillator frequency to rise to around 180KHz ).
None of which should affect the noise figure significantly as far as I can see - ( unless the 4584D is unstable, as suggested by Abbey ? )
The output filters formed by R2/ C2 and R1/C1 should essentially remove all troublesome HF noise. There should only be some 5 to 10mV of measurable residual HF, which is not going to add any significant noise to the polarisation output voltage.

One other possibility for a noise source is less than ideal ground routing. ... If the current drawn by the inverter oscillator is routed via a signal gound route, that might create noise?

What will add some extra noise is the fully differential audio output employed in this OPA1642 configuration.
The 2 x 2k2 resistors included in the inverted audio ouput stage will add about 3dB to the overall noise figure.
I chose to use a single sided audio output for my OPIC versions of this same concept, to avoid generating that noise.
( The OPIC circuit is impedance balanced passively, to help maintian a reasonable CMRR figure.)

I would guess that any noise generated by the polarisation voltage oscillator would be lower than 3dB ?

 

[Murdock]

Hey Abbs! Its nice to get your brain on this. Thank you!

Here is a diagram of how I have the mic set up. It does seem like it gets it's power from the 12V zener via the op amp board. But it seems like there is a 47uF in there, maybe I can replace with 100uF?

Are your capsule wires directly soldered to the PCB?
That could also be a source of noise as the connection has a really high impedance and should be well isolated from the PCB either with teflon turrets or through air.

 

[AUDIO FREQ]

Yes the capsule wires are directly soldered to the pcb, where it says to solder them. Not sure where else I could solder them?

Rogs - The PCB came with the 4584D already soldered in place, should I replace it with a 74c14 or 40106 instead? I can always order one or two from mouser, they aren't pricey at all.

I was also wandering, instead of the OPA1642...could I use a OPA1612? I have a few of them lying around here in my parts bin. Just wandering.

Thanks guys!

 

[Murdock]

Yes the capsule wires are directly soldered to the pcb, where it says to solder them. Not sure where else I could solder them?

Rogs - The PCB came with the 4584D already soldered in place, should I replace it with a 74c14 or 40106 instead? I can always order one or two from mouser, they aren't pricey at all.

I was also wandering, instead of the OPA1642...could I use a OPA1612? I have a few of them lying around here in my parts bin. Just wandering.

Thanks guys!

You can use Teflon turrets. There are plenty of different sizes or you could drill the holes wider so they fit.
Don’t know how much different that makes but I always isolate these connections either with connections in the air, Acryl boards or Teflon turrets.

 

[AUDIO FREQ]

You can use Teflon turrets. There are plenty of different sizes or you could drill the holes wider so they fit.
Don’t know how much different that makes but I always isolate these connections either with connections in the air, Acryl boards or Teflon turrets.

Im very new to this. How does soldering to the pcb, differ from soldering to a turret thats soldered to the pcb?

 

[Khron]

Im very new to this. How does soldering to the pcb, differ from soldering to a turret thats soldered to the pcb?

https://en.wikipedia.org/wiki/Leakage_(electronics)
(Across the surface of the PCB, which may or may not have relatively conductive contaminants, or even the solder-resist itself; keep in mind, the JFET gate is connected to the half-supply through a 1Gohm resistor, and it doesn't take much to bypass that with a lower resistance. That's why, ideally, you'd want the opamp input pin NOT soldered to the PCB directly.)