Bias supply question

[audioquestM-40]

How much current does the bias supply need to be capable of for a typical DIY diaphram?

[RankineCycle]

Practically no current.

I've just begun the ESL adventure and it appears to be practically no current. Just enough to overcome the diaphragm's charge leakage...so we're talking microamps if not lower. Most supplies I've seen connect to the diaphragm through a 20 meg resistor - even at high voltage not much current is getting through that.

I read one account of a user of ES headphones powered by a supply with a 2 uF capacitor somewhere in his bias supply. When he unplugged the power the charge in this 2 uF cap was enough to keep the headphones charged for 30 minutes.

[audioquestM-40]

Thanks, I didn' think there was much current, but was curious on an actual approximate number. The reason is do I want to build the power supply or buy one of the many available out there, some being pretty inexpensive that put out a lot of voltage, but really low current, in the milliamps.

[electros]

200 milliamps will do - I think?

[audioquestM-40]

Looks like nearly zero amount of current is required from what I seem to find filtering through posts recorded here over the years, which would also make it a lot safer. There are some inexpensive wallwart dc based units that look interesting.

[I_Forgot]

Originally Posted by electros

200 milliamps will do - I think?

200 mA is 4 orders of magnitude more than required and will be extremely large, heavy, expensive, and most importantly, dangerous.

You only need a few uA - that's micro amps, NOT milliamps.

You don't need to connect a capacitor across the bias supply output either- the presence of a capacitor can make even a safe, low current supply into a dangerous one. Dangerous for you and for your speakers.

The speaker IS a capacitor. It is all the capacitance needed at the bias supply output.

[electros]

Originally Posted by I_Forgot

200 mA is 4 orders of magnitude more than required and will be extremely large, heavy, expensive, and most importantly, dangerous.

You only need a few uA - that's micro amps, NOT milliamps.

You don't need to connect a capacitor across the bias supply output either- the presence of a capacitor can make even a safe, low current supply into a dangerous one. Dangerous for you and for your speakers.

The speaker IS a capacitor. It is all the capacitance needed at the bias supply output.

My bad! - Quite right - mine takes between 2 & 8 uA. I was thinking of Audio transformers that day. - very sorry

[AcoustatAnswerMan]

A capacitor on the output of the bias supply is not necessary, and in the case of most ESL's, will actually harm the performance. Most ESL bias supplies use a very large resistance on the output (500 megohms for Acoustat), which allowed the VOLTAGE on the diaphragm to float, so that the CHARGE would remain constant, which is what you want to conform to Arthur Jansen's original constant-charge concept. Placing a capacitor there will cause the voltage to remain constant, while allowing the charge to vary (due to leakage), the OPPOSITE of what you want.

Funny anecdote - when Jim Strickland was deveoping Hafler's IRIS remote-control preamp, and was testing the static-electricity sensitivity of the remote (not easy to do in Florida's humid climate) he would seat himself on an insulated stool and touch the output of an Acoustat bias supply to charge up his body. He would then test the remote by discharging himself into its metal case. This gives you an idea, despite the high voltages, that the current furnished by a properly designed ESL bias supply is indeed miniscule, as clearly demonstrated by Jim's survival of the aforementioned technique!

[speedracer5]

Wonder if his body was at at the full 5Kv ? LOL
That a great story Andy!
I "snap" myself all the time on the bias supply screwing around while the interfaces are plugged in. Its a "shocking experience"! I do stay away from the wall power leads. Now that is dangerous.

[kavermei]

The trick is to actually place the current limiting resistor right next to the HV unit, so you only ever risk touching the current-limited part of the circuit.

Funny anecdote: myself I use a CCFL inverter (for computer "case modders", $5 for the lamp+inverter, discard the lamp). followed by a diode/cap ladder. I brushed against the ladder several times but, with the inverter running at about 35kHz, I never felt a thing!

Kenneth