Hi,
I'm building a class a amplifier similar to the design in this wesite:
https://sound-au.com/tcaas/jlhupdate.htm
This is my first class a build, and I have never do biasing before. I've searched online on how to bias a class a, and this is what I found:
Please correct me if I'm wrong. I've seen some videos where they biasing with load (speakers) connected, and I don't know which one is correct
Thanks
I'm building a class a amplifier similar to the design in this wesite:
https://sound-au.com/tcaas/jlhupdate.htm
This is my first class a build, and I have never do biasing before. I've searched online on how to bias a class a, and this is what I found:
- to set the bias voltage: no input, no load, connect the multimeter to the output and V+, using the vr set the voltage to 18v (I'm using 18v symmetrical power supply)
- to set the quiescent current: no input, no load, connect the multimeter between the (+) input voltage and power supply, using the vr to set the current to 2.8 amps as stated in the website
Please correct me if I'm wrong. I've seen some videos where they biasing with load (speakers) connected, and I don't know which one is correct
Thanks
You can adjust the quiescent current aka bias by measuring voltage across R10 and adjusting VR2.
You can calculate the current by applying Ohm's law.
Optimal quiescent current values depend on the PSU voltage, see table at the bottom.
No need to attach a speaker during measurement (leave the output open) but you might want to short the input.
VR1 allows to adjust the DC offset across output terminals which should be as close to 0V as possible, usually a few mV is OK.
Have fun with your build.
You can calculate the current by applying Ohm's law.
Optimal quiescent current values depend on the PSU voltage, see table at the bottom.
No need to attach a speaker during measurement (leave the output open) but you might want to short the input.
VR1 allows to adjust the DC offset across output terminals which should be as close to 0V as possible, usually a few mV is OK.
Have fun with your build.
Thanks for the reply. The one I build no longer has the R10. The schematic is almost at the bottom of the page
In that circuit it is measured across R13 or R15.
In any circuit, yoiu want to set the current through the output stage, and that is measured by the voltage across the emitter resistor.
Ohm's law applied gives you that.
jan
In any circuit, yoiu want to set the current through the output stage, and that is measured by the voltage across the emitter resistor.
Ohm's law applied gives you that.
jan
VR2 trim pot sets the bias. Adjust it so that you can see a 200mV (0.2V) across each of those 0.1-ohm resistors. You'll get beautiful sound and a LOT of heat dissipation. Something like a 5U Delux case sold here in the store would do the trick only just... but the heatsinks will be very hot.
Go for it, you'll be blown away by the sound.
The 200mA on the photo should say 200mV...
Go for it, you'll be blown away by the sound.
The 200mA on the photo should say 200mV...
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Thanks for the reply. I'm using 18v power supply. Should I lower the voltage or stick to 200mV?
After eximining your notes I realized that I can't set the bias voltage to 200mV because it makes the amp pulls 3.8 amps. My transformator is only 3 amps. So the maximum voltage that I can set is 150mV, which makes the amp pulls 3 amps. I think for safety I should set the voltage at 100-125mV. Am I correct?
If your transformer is only 3A, you can safely pull 75% max.... around 2A max from both amp PCBs. Even that will run the transformer quite hot, especially if it is potted. If not potted, it will buzz/hum... no free lunch with class A.
You need 2 transformers, 300VA each for a dual mono construction if you want decent power output, reliable performance and good sound.
You'd also want around 25W of total dissipation across each output transistor to ensure no failures in a long run. Note: the Class-A Amplifier Site article states double that... which is a lot of heat...and very large heatsinks, but a lot of output power as well! As I said - no free lunch.
You need 2 transformers, 300VA each for a dual mono construction if you want decent power output, reliable performance and good sound.
You'd also want around 25W of total dissipation across each output transistor to ensure no failures in a long run. Note: the Class-A Amplifier Site article states double that... which is a lot of heat...and very large heatsinks, but a lot of output power as well! As I said - no free lunch.
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Yes I'm using 2 x 3A transformator, one for each channel. So on the safe side I should set the current 2-2.5 amps for each channel, right?
I hope so. For each channel, the final transistors are attached to a 20cm x 10cm, 2mm thick heatsink with fins (i think this one is too thin). On this heatsink I attached 2 more L shaped 20cm x 5 cm, 5mm thick heatsinks. In total I'm using 6 pieces of heatsinks for both channels. Also there are 2 fans, one blowing the heatsink, one pulling air out.
I'm still working on the cabling, so not yet turned on
I would start with a smaller bias current. Check the temperatures at each bias setting.
That design does not have good control over its output bias current. It is a poor design, IMO. Be careful.
Ed
That design does not have good control over its output bias current. It is a poor design, IMO. Be careful.
Ed
Thanks for the reply. You guys have been very helpful. Now I have a much better understanding on what to do.
It's 90% done but I only have time this sunday to finish it up and start biasing
It is an excellent class A design according to the opinion of many. A true classic.
Sorry. The bias current depends on the HFE of Q1/Q1A. That will vary a lot with temperature. There is no protection from reverse Vbe on Q2/Q2A. Class A can be done without such mistakes.
Ed
In praxis: This circuit would run stable even without the emitter resistances;-)
... which further improved the sound, because mostly bad sounding resistors are used;-)