[AKSA]

Here's a question from Possum (Greg) from NJ:

I finished 2 channels of the AKSA 100W kit and went through the probing and adjusting procedures. Both channels started out with about 8.2V across R25, so I changed R12B (150 ohms) to 215 ohms, as instructed.

On one channel, the voltage across R25 became 2.2V or so, and I was able to use the trimmer pot to set it to 5V.

On the other channel, the voltage across R25 became 6.4V. I tried turning the trimpot to make sure it was fulling counter-clockwise (I had spun it probably 40 turns before) and it was. Then I turned it clockwise to see if it would change the voltage, and the voltage increased as expected. I minimized the trimmer again and rechecked the voltage, but this time it was 8.4V. Turning the trimmer clockwise only increases the voltage.

What could be wrong in the 2nd channel? I built both channels at the same time, step for step. Voltage between R5/R6 and Earth on the good channel was about 29V, and on the bad channel 25.5V.

Any ideas? I'd like to get the 2nd channel finalized before I hook up a speaker to listen for the first time, which I'm eagerly awaiting.

It is impossible to predict exactly what this voltage across the fuse resistors will be when you first bias up an AKSA.  It is controlled by the tolerances of no less than five semiconductors, so all are different.

However, when you increase R12B to reduce the initial lead bias voltage across the fuse resistor, and it does not reduce the voltage, there is a problem elsewhere in the circuit.  Increasing R12B from 150R to 215R should reduce this voltage by around three, from 8.2V to 5V or less.  If it does not, it is a symptom of a malfunction.

Greg, you must now remove one terminal of R12B.  This makes its resistance effectively infinite, and this should close down the output stage altogether.  The voltage across the fuse resistors should drop to around 1-1.5 volts.  If it does not, the problem lies not in the output stage but the first two stages.

First measure the voltage across R10.  It should be around 0.7 volts.  If that is so, then remove C4, the boostrap cap.  If the voltage is now 0.7 volts across R10, you've found the problem.  Replace C4 and you are away.....

Why does C4 fail?  Because it is possibly overheated during soldering.  Happens all the time....

Cheers,

Hugh

[Possum]

Greg, you must now remove one terminal of R12B.  This makes its resistance effectively infinite, and this should close down the output stage altogether.  The voltage across the fuse resistors should drop to around 1-1.5 volts.  If it does not, the problem lies not in the output stage but the first two stages.

I lifted one end of R12B and the voltage across R25 became about 0.94V.  So I guess the problem is in the output stage?

First measure the voltage across R10.  It should be around 0.7 volts.  If that is so, then remove C4, the boostrap cap.  If the voltage is now 0.7 volts across R10, you've found the problem.  Replace C4 and you are away.....

The voltage measured across R10 is correct at about 0.7V.


What in the output stage could cause the problem?

Thanks,
Greg

[AKSA]

Hi Greg,

You have identified the problem, which is inappropriate value of R12B on the bias generator.  With this fixed, the amp will bias up fine.

Replace R12B with 270R.  This should fix all your problems (unless P1, the trimmer, is defective).

Hope this helps,

Cheers,

Hugh

[Possum]

I replaced R12B with a 270 ohm resistor, and the voltage after plugging in the power was first 5.5V, then was slowly climbing (at the same rate as discharging a capacitor with a resistor).

This climbing was noticed before on this bad channel, but did not happen on the good channel.  The good channel's voltage was stable, which leads me to believe that the resistors and trimpot (which was measured to be fine) was not at fault, but something else.

What kind of fault in the circuit would cause the voltage to change like the charging/discharging of a capacitor rather than remain stable?  I hope this helps to narrow down possible problems.

Thanks,
Greg

[AKSA]

Greg,

The most likely cause is improper thermal bonding between T4, the Vbe multiplier, and T7, the transistor upon which it sits.

Tight them down, one atop the other, put the fuses in, and bias it up.

The rising voltage across the fuse resistor is not too important as long as it stabilizes.

Switch off, put in the fuses, do NOT connect a speaker, switch on, bias up according to the instructions, and watch the bias voltage as measured from emitter to emitter of T9 and T10.

You should be able to achieve a reasonable range of adjustment.  Get it first to 30-40mV, measure output voltage offset with respect to ground, then if less than 30mV, complete the adjustment of bias to 55mV.

Come back to it in 15 minutes, after idling, measure the offset is OK, below 30mV, then connect your speaker and start listening!

Cheers,

Hugh

[Possum]

The T4-T7 bond was very tight before, but I rechecked it just in case.  The R25 fuse voltage still gradually increased after power up regardless of whether T4 was mounted tightly or even left floating.

I'd like to be able to solve this, even if it is not a critical problem, as I know the other channel does not behave anything like this.

[AKSA]

Greg,

Be absolutely sure that all semis are tightened down by roughly the same amount of torque (this is the head of the amplifier!) so that no one semiconductor heats more than another.  The usual reason this happens is because the thermal attachment of one device or more to the sink is impaired;  this heats the junction rather more, and results in a lower Vbe.  Since the bias hardly changes, this in turn puts more voltage across the emitter resistors, which naturally pass more current. Ergo, the slowly rising figures you are getting.

So, check all transistors are tight (also the drivers on the copper sinks) and it should be fine.

Cheers,

Hugh