OPA Alice Circuit

[newmod]

Hello everyone,

I have been hearing a lot about this circuit and it seems like a very interesting project to undertake.

Through my research, I cannot figure out who actually created this circuit and the history of it?

I also noticed there is a single channel and dual channel.............. what's the difference?

https://www.jlielectronics.com/search.php?search_query=opa alice circuit &section=product
Thanks

 

[kingkorg]

Jules Ryckebusch is responsible for this one, however it's not like he invented it. There has been many takes on the circuit, one of which @rogs is responsible for. I believe opa Alice started in micbuilders group.

The version for LDC with polarizing voltage for the capsule is inspired by Røde's mics.

It is just a simplest possible impedance converter using an opamp. You can come up with it just using spec sheet of an opamp and basic knowledge. "Tricky" part is to find low current, quiet fet input opamp.

Dual is for dual output mics like TL Sphere, or AA 818.

 

[JohnRoberts]

Perhaps look for Ted Fletcher's balanced input circa Alice Mixers.

JR

 

[mhelin]

It started here:
https://groupdiy.com/threads/all-opamp-mic-design-no-fet-at-first-stage.62636/https://groups.io/g/MicBuilders/topic/55090883
There's also this closely related TI application note :
https://www.ti.com/lit/pdf/sboa320

 

[rogs]

The reference to 'Alice' came from Jules' acknowledgement of the contribution that Scott Helmke's original 'Alice' mic project had made as a starter project for mic builders, some time back (See here: Alice Microphone )
I would have originally purchased one of the Alice OPA PCBs from JLI, but their shipping costs outside of the US are too high for my pocket, so I took the OPA164* op-amp that Jules had used, and created a series of simple stripboard mic projects of my own.
Simpler than Jules' Alice OPA realisations, by using single sided audio output (Output is still impedance balanced though )
I've been quite pleased with the results... ( Some notes here: OPIC Impedance Converter )

 

[mhelin]

The single OPA circuit would be good enough. You could also use a little bit gain if needed to compensate, guess that wouldn't add too much noise (or lower the input impedance). An output transformer could also be added, there are many options.
I got few PCB's from PCBWay including the hex inverter power supply:
https://www.pcbway.com/project/shareproject/Dual_OPA__Alice__Microphone_Circuit.htmlhttps://www.pcbway.com/project/shar...age_Multiplier_for_Condenser_Microphones.html
For some reason my microphone starts producing noise from the multiplier quite easily, maybe a startup issue with the power supply (P48).

 

[Ricardus]

Interestingly, a friend just texted me a pic of some commercial product called the "Alice Mic Amp Pak 2" and I had no idea what it was. Then 20 mins later I saw this thread up here and I'm thinking the circuit must be related.

 

[mhelin]

So there is now everything Alice - mics, mic preamps and mixers. I don't think the two latter are related to microphones anyway.

I don't understand this (what Scott Helmke wrote): - Somehow it's hard to convey the actual size in photos... that's one reason why this mic is named "Alice".

 

[Khron]

So there is now everything Alice - mics, mic preamps and mixers. I don't think the two latter are related to microphones anyway.

I don't understand this (what Scott Helmke wrote): - Somehow it's hard to convey the actual size in photos... that's one reason why this mic is named "Alice".

Likely some sort of Alice in Wonderland reference?

https://en.wikipedia.org/wiki/Alice's_Adventures_in_Wonderland#Plot

 

[mhelin]

Yeah, that was when Alice went down the rabbit hole. IMO, "Rabbit Hole Microphone Series" would be better name, sounds like:
https://intshop.jzmic.com/collections/black-hole-series

 

[AUDIO FREQ]

The reference to 'Alice' came from Jules' acknowledgement of the contribution that Scott Helmke's original 'Alice' mic project had made as a starter project for mic builders, some time back (See here: Alice Microphone )
I would have originally purchased one of the Alice OPA PCBs from JLI, but their shipping costs outside of the US are too high for my pocket, so I took the OPA164* op-amp that Jules had used, and created a series of simple stripboard mic projects of my own.
Simpler than Jules' Alice OPA realisations, by using single sided audio output (Output is still impedance balanced though )
I've been quite pleased with the results... ( Some notes here: OPIC Impedance Converter )

I am ordering parts for your LDC OPIC guide. I am stuck on the 1 Giga Ohm resistor. Is there a particular kind? Low. noise? Or will any brand and type do?

 

[Khron]

Is there a particular kind? Low. noise? Or will any brand and type do?

TE Connectivity RGP0207 series. Absolutely no point in going for any 1/2W or bigger (and nevermind more expensive) ones.

 

[AUDIO FREQ]

TE Connectivity RGP0207 series. Absolutely no point in going for any 1/2W or bigger (and nevermind more expensive) ones.

Thank you kind sir!

 

[AUDIO FREQ]

Would this work for ZD1?

 

[rogs]

As Khron says, the RGP0207 is ideal for this task. Mouser an Digikey - for example - usually have stock

If you're looking a doing a stripboard version, do take a look at the OPIC43 VERSION

Same performance, slightly simpler construction - and with an adjustable voltage multiplier output.

I'm always trying out little evolutionary 'tweaks'

 

[rogs]

Would this work for ZD1?

It would be fine. If you decide to go for the OPIC43 version, that will ideally need to be a 15v Zener instead.

The original OPIC project works fine. As I find ways to improve the assembly slightly, I've added them as options

 

[Fenton Bresler]

Greetings all,

I'm trying to wrap my head around the voltage multiplier circuit. Could someone please explain how series resistance on the input varies the output voltage? As far as I can tell the circuit draws next to no current, how is the voltage dropping across said resistance?

Referring to R4 and VR1 here:
https://www.jp137.com/lts/OPIC.43.pdf

Many thanks!

 

[Khron]

As far as I can tell the circuit draws next to no current,

... But not zero. I think i measured something in the region of 1-2mA in one of my incarnations, but don't quote me on that.

Could someone please explain how series resistance on the input varies the output voltage?

https://en.wikipedia.org/wiki/Ohm's_law

Current through a resistor drops a certain voltage across said resistor, thus reducing the input voltage to the charge pump / voltage multiplier. Lower input - > lower output.

 

[jp8]

For some reason my microphone starts producing noise from the multiplier quite easily, maybe a startup issue with the power supply (P48).

Not sure, but could be the result of HF electromagnetic fields generated by the multiplier circuit. For some reason that completely escapes me, the designer thought it would be smart to spread the components out over the entire available PCB surface area. That maximizes EM field emissions.

Jan

 

[jp8]

... But not zero. I think i measured something in the region of 1-2mA in one of my incarnations, but don't quote me on that.

Actual current consumption depends on switching frequency, IC supplier and most notably the supply voltage. It rises with voltage squared. But 1-2 mA is also what I measured with several incarnations of this circuit. If current is on a budget, consider applying a lower Vdd on the IC and add more multiplier stages to obtain the desired polarization voltage. But that is not feasible with this particular circuit implementation.

Jan