Yes, used 2N3906, BD139, TIP3055 power transistors instead
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I'm sorry, I don't know. I presume it's what happens when there's no load.
I'm sure Mooly know's his stuff so there's nothing to worry about
The output capacitor has 12 volts on one end which is the voltage at the output transistors.
It also has the same voltage at the other end (initially) and relies on a load (the speaker or your 100 ohm test resistor) to charge the cap. As the cap charges the voltage across the 100 ohm falls to zero.
Your meter is also a 'load' but it draws far to little current to charge the cap in a reasonable time.
So its all normal behaviour.
I have a Meanwell 27 Volts capable of 7 amps. I would have to open up to see the model number, but there is adjustment on it between 24 and 30V. I will add a mains input filter on it. I also have 20 000uF on the amp board though, since it was originally meant for transformer supply.
I have done a JLH for a friend and used an HP laptop transformer with great results. Power out would be lower though.
Thanks Mooly, I guessed the same, but wasn't very sure...this is a very touchy circuit and the max you can do is set the DC offset using the singleton trimmer...had tested the voltages as per the table from original JLH doc and they were all good for the 4 transistors and the PCB did not show any faulty connection, so doubted the caps...now I have shorted the output with 10ohm + 0.1uF cap combo to ground and going to leave it as it is...
Only observation so far, this amp too needs a preamp...sound is not great without it, but way better than Amp Camp Amp...sorry Papa Pass, JLH win's this round and it doesn't need any rare JFET's too...
Sorry, can you explain this a bit more, do you mean the zobel before the output capacitor, or after in series or parallel?
Unfortunately with this update my design becomes JLH 1984 ... [emoji8]
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Good thing is that I am running this thing with the Laptop power brick 19V 3.4A, believe this should be sufficient for both channels....the current draw is merely 0.648 A with this supply
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If you want more gain then you try reducing the 200 (220) ohm feedback resistor to say 82 or 100 ohm. If you do that then increase the value of the 100uf cap in series with it to at least 470uf.
Also make sure that the current drawn by the amp agrees with JLH's original values. If not then alter the 560 ohm until it does.
Also make sure that the current drawn by the amp agrees with JLH's original values. If not then alter the 560 ohm until it does.
If your supply is just 19 volts then you should aim for something around 900ma per channel for 8 ohm speakers.
I don't think though that it's vastly different, just a few modifications for better stability as far as I can see?
I am learning Mooly, with baby steps....don't have much testing equipment, can you suggest few steps of achieving this...
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The safest way would be to temporarily add a 1 ohm or lower resistor in series with the positive supply to each channel.
The voltage across the resistor would then correspond to the current draw of the whole amplifier and can be used to accurately set the current. 900ma through 1 ohm would generate a 0.9 volt drop across the resistor. Although that is the total current draw of the amp, its close enough for setting the bias. You would alter the 560 ohm up or down to achieve that. Lowering the resistor would increase the current.
Remember to calculate the required resistor volt drop for the actual value you use. 1 ohm would be 900 millivolts, a 0.22 ohm would be 198 millivolts. Just ohms law.
Remove the resistor when finished.
The upper 1nf cap shouldn't be needed and is actually harming the high frequency performance of the amp. If you find you really do need the cap because of stability problems then try something much smaller such as a 220pf. Built correctly and the JLH doesn't need it though.
The voltage across the resistor would then correspond to the current draw of the whole amplifier and can be used to accurately set the current. 900ma through 1 ohm would generate a 0.9 volt drop across the resistor. Although that is the total current draw of the amp, its close enough for setting the bias. You would alter the 560 ohm up or down to achieve that. Lowering the resistor would increase the current.
Remember to calculate the required resistor volt drop for the actual value you use. 1 ohm would be 900 millivolts, a 0.22 ohm would be 198 millivolts. Just ohms law.
Remove the resistor when finished.
The upper 1nf cap shouldn't be needed and is actually harming the high frequency performance of the amp. If you find you really do need the cap because of stability problems then try something much smaller such as a 220pf. Built correctly and the JLH doesn't need it though.
The upper one is damaging the HF response. The lower 1nf shouldn't be needed either and although it probably does no harm in practice, it is actually tending to make the amplifier less stable. You don't need either cap 🙂