Can a CMOS inverter chip invert an Audio signal?

[diydidi]

HI
I was wondering. Can I use a CMOS inverter such as a 4049 to invert an audio signal?
I usually use an opamp to do this.
My memories from my study days escape me. We never mixed audio with CMOS anyway...

[Mark Hammer]

The answer is Yes, though not always cleanly. For retaining the bandwidth and fidelity of the original signal, there are much better chips.

[diydidi]

Reason I'm asking, I recently bought a phase meter for checking speakers. It uses a little Electret microphone, but the manufacturer used the wrong mic. The unit shows me the incorrect phase. They said they will ship me the proper mic, but that it would take some time. The output phase of the mic is wrong.
I didnt know you can get electret mics with differing output phase. Maybe they use different internal JFETS? Anyways, i want to invert the phase of the mic. I think its output impedance is around 30k ohms.
Any ideas?

[olddawg]

Reverse the leads to the mic?

[diydidi]

Can't be done. Electret mics need a bias voltsge of around 6volts. If you swop the mic wires it doesn't work at all. They have an internal jfet.

[J M Fahey]

If you have the space for a CMOS inverter (usually 14/16 pins DIP) then you can fit an 8 pin DIP Op Amp.

If you dare, an SMT part.

It only needs one Op Amp (if it's a double neutralize the second one) and 2 resistors.

What kind of power rails do you have?
9V? .. +/- 12/15V?
What signal level are we talking about?
If all you measure is phase, extended frequency response or very low distortion is not a big deal, if it also measures speaker response you need a good Op Amp (TL07x or better).

Don't understand why would they need to switch Electret capsules, and if they did, not sure they will have the same sensitivity ... and that's an understatement.

2 terminal capsules exit from the FET drain, with a typically 2k2 (may be up to 10k) resistor to +V , which may range from 1.5 to 9V (although I used safely up to +15v) , have gain, and by definition invert the signal present at the FET gate; 3 terminal ones take signal from FET source (often there's an internal resistor to ground or you supply your own) ., do not invert phase and have unity gain.

My point is that they can not be freely exchanged , doubly so in a measuring instrument.

My gut feeling is that they should rather suggest inverting a couple rectifying diodes inside the main circuit, but I'm just guessing.

Just in case, ask them for the schematic and post it here.

Link some page about the device too.

[diydidi]

I managed to get a schematic for the unit. I'm thinking of adding an inverting opamp after U5. Something like a TL071 with two 10k resistors. I can just connect its non inverting (pin3) to the junction at C3 and pin 7 of U5. Any better ideas?

[J M Fahey]

I managed to get a schematic for the unit. I'm thinking of adding an inverting opamp after U5. Something like a TL071 with two 10k resistors. I can just connect its non inverting (pin3) to the junction at C3 and pin 7 of U5. Any better ideas?

Thanks.
Simple curiosity, please post some link to the product page and maybe a gut shot, just to see how it's done.

1) won't speak about the Logic side, hate Digital stuff, will analyze only the analog front end and the LM339 comparator.

2) the meter already has a phase reversing switch ... pity it works by selecting either the + or - inputs of what's presumably an XLR jack , Jk2 , does the same with what's presumably a balanced Line (or remote preamp) Line in, Jk1 .

Pity the Electret mic, which presumably will be the most used input device is an unbalanced one and they do not provide the function for it.
What would not be a problem if it were correctly set up at the Factory.

By the way, are you sure it's indicating the wrong phase ?

Big mystery: what kind of signal do they use to drive the speaker you are measuring?
Or the amp/mixer/processor/whatever under test?

3) so you should invert the signal just for the electret, in fact add a tiny switch so you can choose to invert or not.

4) it's even simpler because you already have the first 10k resistor, which is the electret load one, R22
It's a 10k impedance audio source because it's fed a constant current/*very* high impedance signal from the electret FET drain.

So you just connect the electret/R22 junction to Op Amp - in through a 10uF cap, we'll find proper orientation later, then Op Amp out to - in through a 10k resistor, + in to its own biasing net: 22k/22k from +Vcc to ground, plus a filter capacitor from midpoint to ground.

I'd add an extra tiny switch to choose between straight electret out and the inverted one.

As of coupling cap orientation, we know the Op Amp input stays at 4.5V or 1/2 Vcc ; electret DC voltage is not defined, measure it and orient copupling cap properly.

If (just by chance) it's very close to 4.5V , say within 100mV , cap can go either way, but anyway properly oriented is better.

Will draw and post a schematic, if needed, after lunch.

No, nothing yummy, just boiled chicken breast and boiled squash, only a little oil and no salt

[diydidi]

Thanks JM. Please post proposed solution schematic.
I will post the send unit schematic later. They use a 555 to generate a "blip" sound around 1HZ.
THANX AGAIN!

[diydidi]

Here is the schematic of the send unit.
JM in your proposed solution, instead of using the 22k/22k and cap for the 1/2Vcc bias, might I just connect the opamp + input to the current 1/2Vcc? I believe it is at pin 7 of U5?

[J M Fahey]

Here is the schematic of the send unit.
JM in your proposed solution, instead of using the 22k/22k and cap for the 1/2Vcc bias, might I just connect the opamp + input to the current 1/2Vcc? I believe it is at pin 7 of U5?

Ok, thanks for the sender schematic.

1) here's the suggested inverter schematic

a) earlier I thought a phase selecting switch could be added but no, not while keeping it simple that is, but anyway we are "correcting a wrong" and that's the main point.

2) don't know C4 polarity beforehand, because I don't know the actual microphone FET drain voltage, but you measure it and orient capacitor accordingly.

3) on my first mental idea I also thought to use 1/2Vcc voltage present at C5, but later decided against it because:

a) it's the reference point for a signal digital converter, not only biases the gain Op Amp but, more important, it provides 4 graded reference voltages for the LM339 comparator; to boot resistance values are relatively high, the full string resistance is 270k and is used all the time while converting audio ... I simply don't want to mess with it, much prefer to make an unrelated bias source I can trust.

To boot, I'm a firm believer in Murphy's Law, and I'm certain that this extra load, which "should" not matter, may create a couple unexplained gremlins messing things, so thanks but no

Famous last words: "what could go wrong"?

[diydidi]

Ok, thanks for the sender schematic.

1) here's the suggested inverter schematic

a) earlier I thought a phase selecting switch could be added but no, not while keeping it simple that is, but anyway we are "correcting a wrong" and that's the main point.

2) don't know C4 polarity beforehand, because I don't know the actual microphone FET drain voltage, but you measure it and orient capacitor accordingly.

3) on my first mental idea I also thought to use 1/2Vcc voltage present at C5, but later decided against it because:

a) it's the reference point for a signal digital converter, not only biases the gain Op Amp but, more important, it provides 4 graded reference voltages for the LM339 comparator; to boot resistance values are relatively high, the full string resistance is 270k and is used all the time while converting audio ... I simply don't want to mess with it, much prefer to make an unrelated bias source I can trust.

To boot, I'm a firm believer in Murphy's Law, and I'm certain that this extra load, which "should" not matter, may create a couple unexplained gremlins messing things, so thanks but no

Famous last words: "what could go wrong"?

Don't see it

[J M Fahey]

Don't see it

Oh !!! but I do .... nice and sharp !!!!!! in my screen of course

Mmmmmhhhhh, I SEE, you want it EASY, huh ?

Ok ok (spoiled modern kids if you ask me, you spare the rod, you spoil the child ) , now I'll UPLOAD it for you

[diydidi]

JM I see it now.
WOW you really did make great effort. thank you so much. Will implement and report back.
I think my browser (safari) is a bit wonky. Firefox did it.

[J M Fahey]

JM I see it now.
WOW you really did make great effort. thank you so much. Will implement and report back.
I think my browser (safari) is a bit wonky. Firefox did it.

NO NO NO !!!!!!!!
You are in South Africa, remember?

Go back to Ministry of Tourism mandated Safari or the friendly cops in ski masks will drop through your roof