Member chuckd wrote a wonderful bias procedure, here it is so it's easy to reference.
Bias Procedure for F5 Turbo Version 2
After you have completed your build you need to bias the output (measured in mV) in order to generate enough power to activate the MOSFETS. You need to do this while keeping the DC Offset (the potential across the speaker terminals) at 0V. This is achieved by increasing gradually, in an alternating pattern, the resistance of P1 and P2. You are not directly measuring the resistance but the voltage across P1 and P2.
Things you’ll need:
3 x DMM
1 x completed F5 Turbo v2
Set P1 and P2 to 0Ω
Setting these to 0 prevents current flow through the FE stage while you power up the amplifier. To make sure you are starting at zero bias at the beginning.
- Attach leads to TP1 and TP2, and TP3 and TP4 on the front end board.
- Turn P1 and P2 whichever way is required to set the resistance across the test points to 0.
- Make an arrow on each pot to indicate what way to turn the pot to increase resistance. Clockwise may not be “up”.
Powerup the amp, check the PSU voltages.
- Make sure the Variac and the power entry module (PEM) (the Schurter switch) are correctly and properly fused.
- Connect the PEM to a Variac or light bulb tester with appropriately current rated power cord (AWG12 and above should suffice).
- Turn Variac to 0V.
- Switch Variac and amp on.
- Check voltage between positive and negative (neutral in the U.S.) with the DMM (set to check VAC) across the barrier block connection. Ensure it reads 0V.
- With the leads still connected to the terminals, slowly increase the voltage on the Variac. If smoke appears turn off the power and troubleshoot the problem. If the fuse blows, the there is a short in the system. Locate and troubleshoot the problem.
- If there is no smoke, continue slowly increasing until you reach mains voltage.
- Once at mains voltage, you can check the step down AV voltage across the secondaries to ensure it matches the value of the transformer.
- Switch DMM to measure DC volts.
- Check the voltage out of the rectifier and before the capacitor bank to ensure it is DC.
- Finally, check the DC voltage at the terminals of the PSU. This is the power that will go to the Gain Stage and Output boards.
- Connect the positive lead of the DMM to the V+ and the negative lead to GND. You should have a positive reading.
- Now connect the positive lead from the DMM to GND and the negative to V-. You should have the same potential but a negative signal.
- If you don’t you need to check your wiring.
- If you do then proceed to setting the bias.
Setting the bias
- Make sure the mains are at max voltage. Remove a light bulb tester if you are using one.
- Set your three DMMs to check DC voltage. If the DMM is manual, set to 2V or 20V.
- Using alligator clips, attach one DMM to TP2 and TP3 on the output boards: One on the P-Channel and one on the N-Channel. Ensure it reads 0V. These will measure the bias.
- Attach the third DMM across the speaker output. Ensure it reads 0V. This will measure the DC offset.
- Grab an insulated screw driver and turn P1 (or P2 – it doesn’t matter) 360o. There may be no change on the DMM. This is normal.
- Turn the other potentiometer 360o. Again, there may be no change in the voltage. This is normal.
- Continue alternating increasing P1 and P2 with 360o turns until you start to read a voltage across the potentiometers.
- While increasing the resistance, keep a watch on the DC offset. It will increase or decrease depending on which potentiometers you turn. It should remain around 0V.
- Continue alternating turning P1 and P2 and bring them up to 300 mV (0.3V). Ensure that the DC offset remains at 0V.
- As you get closer to 300mV, you may have to adjust the turns to a smaller amount since a full turn can have a dramatic change in voltage.
- The heat sinks should be warming up.
- Once you get the bias on each output board to around 300mV (there may be a slight difference in order to keep the DC offset at 0V. It is ok if the bias voltage difference is within 50mV, and the offset zeroed.
- Set the lid on and let the amp rest (or ‘cook’) for 20 minutes.
- Keep a watch on the DMMs. As the thermistors heat up they will adjust to keep the DC offset at 0V.
- The bias may change as the MOSFETs heat up. This is normal and should settle after about 20 minutes.
- After 20 minutes, and if the bias readings have settled, and there is no smoke, increase the bias to 350mV by alternating turning P1 and P2.
- You may not get exactly 350mV bias on both boards at the same time as keeping the DC offset at 0V. This is normal. Get the bias between P1 and P2 as close as you can to each other within the 350mV range all the while with DC offset at 0V.
- Let the amp cook for another 20 minutes.
- The bias should not change too much, if at all, since the heat sinks have already warmed up.
- If this is a stereo amp, repeat this process for the other channel.
- Once the other channel is done, recheck the bias and DC offset for the first channel, and make any adjustments to the bias to bring it up/down to around 350mV and 0V DC offset.
- That’s it! Congratulations, you have just biased your amp.
- Turn off the amp and let it cool a little.
- Attach your test speakers (if you have them) and turn on the power. If nothing happens then you should be able to integrate the amplifier to your system.
- If you don’t have test speakers, and you attach them to your system speakers, you run the risk of damaging them. So proceed down this path at your own risk.
my jfets were purchased from the diy audio storeMatched JFETs - Toshiba 2SK170/2SJ74 Quad 6-8mAgive details aboyt JFets are using (rang, measured Idss) and point us to reference schematic
pretty much - all you need is to increase resistors in parallel to trimpots in drains of input JFets