18v and JFETs

[shawnee]

Quick question- If you usually bias a J201 or 2N5457 at 4.5v, would you still do so if you run the circuit at 18volts? Would there be any benefit to running at 18v if you do bias at 4.5v?

[madbean]

Your bias point should be around half the supply voltage. However, the higher the voltage supply, the more range you have on bias voltage. IOW, you can probably set it anywhere from 8v-12v and it will sound good.

I have a thing I'm working one where the circuit is run at 18v via a charge pump. Under load, the supply is actually about 17.6v and I set my bias on the JFETs to about 8.3v. Just as an example.

[midwayfair]

Doubling the supply voltage will not necessarily double the bias voltage, because the current drawn by the FETs is unlikely to change linearly with the voltage increase. You will sometimes need to rebias.

Some circuits are designed to distort on a 9V supply with a specific ratio of bias voltage to supply. 4.5V is hardly a rule -- many overdrives are biased to 2/3 supply, and some preamp type effects are biased even colder (the Fatpants, for instance, is variable between about 16V down to 9V as the boost pot is turned). A lower or higher supply voltage might not sound "right" with the bias ratio indicated for the design, so a 9V circuit run on 18V might not distort as much as it's designed to.

If you just want the most possible headroom, then increasing the supply voltage is good.

The other reason to consider a larger supply voltage with FETs is that fixed biasing becomes more reliable. For that, 18V is good, 24V is better, and 27 or 30V is outstanding.

[madbean]

Doubling the supply voltage will not necessarily double the bias voltage, because the current drawn by the FETs is unlikely to change linearly with the voltage increase. You will sometimes need to rebias.

Oh, that's a good point Jon!

[shawnee]

Thanks guys!