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-   -   What BIAS current is suitable for me? (http://www.diyaudio.com/forums/pass-labs/72917-bias-current-suitable.html)

 lscangus 30th January 2006 09:27 AM

What BIAS current is suitable for me?

I am planning to make a aleph-x, i ahve got the spreadsheet which calculates the coltage, the heat dissipation... etc etc. But what current do i need for the BIAS? i am using ProAc 1 SC, and also, how much power otuput do i need to drive rmy speakers?

Angus

 lscangus 30th January 2006 10:03 AM

as well as the supply voltage and the fets voltage as well..

Angus

 wuffwaff 30th January 2006 10:55 AM

Hi,

well you have to start somewhere. Normally you should know how much power you want for your speakers. How much do you have now? Is it enough? When not you should at least double it to have an effect.

If you know the power and the impedance of your speakers you can set the right combination of voltage and bias. After that you can set the amount of fets and heatsinking. Voltage to be set is that at the fets, the other voltage depends on your power supply (C or C-R-C or C-L-C etc.)

William

 lscangus 30th January 2006 11:38 AM

2 Attachment(s)
Quote:
 Originally posted by wuffwaff Hi, well you have to start somewhere. Normally you should know how much power you want for your speakers. How much do you have now? Is it enough? When not you should at least double it to have an effect. If you know the power and the impedance of your speakers you can set the right combination of voltage and bias. After that you can set the amount of fets and heatsinking. Voltage to be set is that at the fets, the other voltage depends on your power supply (C or C-R-C or C-L-C etc.) William

I think 70-80 W will be enough, i think i know my speaker impedance. SO what BIAS current and Voltage i should use? Thank you.

Angus

 wuffwaff 30th January 2006 03:26 PM

Hi,

looks not too difficult as a load.

You could try 17V at the fets and 6A of total bias.

At 50% ac-current-gain this would give 56 watts 8 Ohms and 90 at 5 Ohms (being also the max. power output)

You would need 8 fets per channel for 26 watts dissipation per fet and 208 watts / channel.

Transformer voltage depends on the used configuration. Try Duncans PSU designer (there´s a link in the spreadsheat).

Heatsink should be lower than 0,15 °K/W per channel

William

 lscangus 30th January 2006 05:51 PM

Quote:
 Originally posted by wuffwaff Hi, looks not too difficult as a load. You could try 17V at the fets and 6A of total bias. At 50% ac-current-gain this would give 56 watts 8 Ohms and 90 at 5 Ohms (being also the max. power output) You would need 8 fets per channel for 26 watts dissipation per fet and 208 watts / channel. Transformer voltage depends on the used configuration. Try Duncans PSU designer (there´s a link in the spreadsheat). Heatsink should be lower than 0,15 °K/W per channel William
I am going to use active cooling.

What BIAS current i need to have if my rail voltage is 21.7volt.

 BobEllis 30th January 2006 06:36 PM

 BobEllis 30th January 2006 08:56 PM

and bump up the bias and rail voltage until your power requirements are met. Then you can vary the thermal resistance of the sinks to get the mounting temperature expect. watch the junction temperatures - add more devices if they go over 100.

 lscangus 30th January 2006 09:26 PM

Quote:
 Originally posted by BobEllis I meant to add: and bump up the bias and rail voltage until your power requirements are met. Then you can vary the thermal resistance of the sinks to get the mounting temperature expect. watch the junction temperatures - add more devices if they go over 100.

but the shape of the graph will change with the BIAS current. What kind of shape i am aiming for?

Angus

 BobEllis 30th January 2006 09:51 PM

One that meets your power goals at your speaker's impedance - roughly 5 ohms minimum, so tending towards higher current will satisfy that. Wuffwaf gave you one operating point that will meet your goal of >80W across the board.

With 21.7 Volts AT THE FETS you'll get around 90W with 6A bias. at your impedance minimum, and you'll still be above 80W to 9.5 ohms. To the left of the peak you are current limited - to raise power increase the bias. to the right of the peak you are voltage limited, increase rail voltage to increase power.

If you can keep your heatsinks under 50C with active cooling you can use 12 FETs. That's >500W to get rid of, plus the active cooling system's waste heat.

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