[DESIGN] Low power DC-DC converter for capsule charging

Look at the end of the thread for schematics.

Original message:
I have been looking all around and I have not been able to find schematics for a really low power DC-DC converter to make +-60 Volts for multipattern mic capsule charging.
It needs to draw 1ma or less and be powered from P48. Anyone has something that works?

In the mean time I am dorking with two circuits. One is a CMOS inverter based charge pump (Jakob has a good document on his site on this). The other one uses an oscillator and a pulse transformer.

Thanks,
Tamas

Take one from MXL603, add a couple diodes and caps, and here we go.

Usually I am greatful for any reply, helpful or not. However, this one about cannibalizing an MXL603 lacks merit altogether.

[quote author="tk@halmi"]Usually I am greatful for any reply, helpful or not. However, this one about cannibalizing an MXL603 lacks merit altogether.[/quote]

Sorry Tk@halmi, I meant MXL603 schematics . In fact, to cannibalize 603 will take more time just for tracking the board, and will be much more expensive then just make it on a perfoboard. The schematics is very simple, and was posted here a few times, and I think you can find it on Stewart's website, as well.

These very-low-power DC/DC converters are not simple builds at all.

And you need to be extremely carefull with layout, or they will interfere with audio parts of the mic.

Look at the U87Ai schematic for a schematic example.

Jakob E.

[quote author="Marik"]The schematics is very simple, and was posted here a few times, and I think you can find it on Stewart's website, as well.[/quote]

Thanks, I get it now. I did not mean to be nasty, and now it makes sense. I will search the forum for the schematics.

Tamas

[quote author="gyraf"]These very-low-power DC/DC converters are not simple builds at all.

And you need to be extremely carefull with layout, or they will interfere with audio parts of the mic.

Look at the U87Ai schematic for a schematic example.

Jakob E.[/quote]

I am just finding out this myself. These things, especially the one with the pulse transformer, are like little radio stations on their own! I may have to pot them in their own metal cases.

Tamas

Hi,
I don't have the 603 schem on my site, just the 2001. Isn't it based on one of the schoeps mics?
z50

I am looking at the Rodent Two here. It uses a 98% accurate replica of the Schoeps circuit. I guess this is why it has cardioid and omni modes only. For figure eight it would need an opposite and equal charge of the supplies.

[quote author="zebra50"]...Isn't it based on one of the schoeps mics?...[/quote]...with a switcher to increase phantom voltage. It runs at around 2MHz so as not to interfere with the audio. (Edit: Just checked the Schoeps schematic and it has the switcher too. Looks like it makes 60V.)

Come to think of it, I was supposed to send zebra50 a schematic!

zeb - pm me your email address and I'll send it to you.

Here is some update on the tests.
The chain of CMOS inverter solution works great even at several hundred Khz.
I made a four gate chain and obtained 36 volts form a 9V battery that actually put out 8.8V. The current consumption of the inverter chain was at 5.6mA, and that is a bit high because the mic amplifier needs about 1-2mA as well. So that adds up to over 7mA for the mic and many preamps will not provide that much current.
The output can provide about 1mA that is way too much for this application anyhow.
Next I will try to drive a Cockroft-Walton multiplier from a single gate isntead of the chain of gates. My goal is less than 1.5mA consumption to provide both +-60V.

Good going. Let us know what the circuits you try look like if you get the chance.

The tlm103 and nt1000 use a cmos chip with a x string of diodes and caps for the capsule.

Thanks Gus. That is what I was going to try next.

I am using a CD4069B hex inverter. So far it appears that smaller (10nF or less) caps in the Diode-Capacitor chain means less current consumption.

Do you know how much current is required for proper polarization?
I speculate that we are talking about uA values.

Only as much as you need to overcome the leakage current.

Very very low if the leakage paths around the capsule are low leakage(very clean non hydrosopic and isolated)


Have to think about this a bit to come up with a number.

Check out the mic meta - there was a thread a while ago where myself and Dale where discussing the U87ai's DC-DC converter. Dale had one up and running, and with his help I got one going as well. I haven't tested mine in a mic yet (working on building a capsule for it right now). I belive Dale had though and as far as I know it worked fine.

tim

Thanks I found the thread here:
http://www.groupdiy.com/index.php?topic=1651&postdays=0&postorder=asc&start=0

There is an excellent pointer in there about the DMM loading the output too much. I will use the scope to check voltages from now on.

I like the Cockroft-Walton multipliers because they require no inductors and the capacitors can be very small. I am hoping to make the entire circuit on a 1"x2" board and shield it.
If I cannot get this to work at or below 1.5mA then I will try the U87ai circuit.

Thanks,
Tamas

http://www.gyraf.dk/schematics/Voltage_multipliers_with_CMOS_gates.pdf

I am using the above document on Jakob's site. I made a 4x mulitplier using six inverters.
The first two make an oscillator and the others do the pumping. Very oddly the oscillator eats up most of the current. It consumes about 4.75mA and only about 300uA is used up in the charge pump chain when using 1nF capacitors. How odd!
I isolated this by disconnecting the pump gates and tying their inputs to Vcc. It is possible that I started out with the wrong part, the CD4069B that has buffered outputs for strong current drive.
Maybe I should decrease the capacitor value and increase the resistor value to get an RC constant with less current feedback.
I will try the 74C14 when it comes in next week. With the Schmitt triggers it takes just a single gate to make an oscillator.

I just prototyped a DC-DC converter using a single gate of the 74C14 and a diode-capacitor charge pump circuit. When powered from 15V it can make +60V measured with a run of the mill DMM so it puts out some current as well. The oscillator is running a bit over 200Khz. Under these conditions it consumes just 1mA.

It is very important to ground the input of ALL UNUSED gates when experimenting with this stuff. If you leave the inputs of the gates floating you will get very high current draw.

Next I will make an inverter charge pump to make -60V using another gate of the six.

Schematics will be posted this weekend.

Tamas

http://mysite.verizon.net/res75okq/sitebuildercontent/sitebuilderfiles/unipolarcapsulepolarizer.jpg

It is this simple. All capacitors are 100V miniature ceramic types. The 100n cap should be located closer to the capsule. With the values shown it operates around 300Khz.

It is optimized for the least current draw ~1.1ma and highest voltage 57V, with a 10M ohm load, and +15V supply. I think the capsule presents a lesser load than 10M so it will be higher in use, but I have no way of measuring it. My oscope probe presents 10M ohm at the x10 setting.
If you crank the supply up to 18V you can get 65V with about 1.3ma draw at 10M load..

The 74C14 chip will take a lot of abuse. I have run it on 24V for hours and it did not even blink. All Hail the CMOS process!
The DC-DC inverter for negative charge still needs to be prototyped.

Cheers,
Tamas