Roender's FC-100 prototype and builder's thread

[Rudi_Ratlos]

Andrew,

the current status is:

Input short, 4 diodes activated, VR adjusted to 18mV emitter-resistor voltage drop, no load on the output.

I waited 10 minutes to get the temperatures "settled".
Then I measured the following voltages:

Pre-Driver 2SC3423 (NPN)
Emitter: 595mV
Collector: 26.5V
Base: 1.14V

Driver MJE15035 (PNP)
Base: 27.2V
Collector: 600mV
Emitter: 27.7V

Pre-Driver 2SA1360 (PNP)
Emitter: -600mV
Collector: -27V
Base: -1.15V

Driver MJE15034 (NPN)
Base: -27V
Collector: -590mV
Emitter: -27.4V

Best regards - Rudi

[Rudi_Ratlos]

I have already been told that my voltage measurements are not "reliable".

For example: the voltage measured at the collector of Q19 (pre-driver SC3423) must be the same as the voltage at the base of Q3 (driver MJE15035).

I have just switched on the AMP again and let it settle within the next 30 minutes.

I will then measure again.

Best regards - Rudi_Ratlos

[Rudi_Ratlos]

Andrew,

I measured once more after 45 minutes:

MJE15034: Base: -26.29V, Collector: -583mV, Emitter: -26.8V
2SA1360: Emitter: -583mV, Collector: -26.24V, Base: -1.113V

MJE15035: Base: 26.08V, Collector: 587mV, Emitter: 26.58V
2SC3423: Emitter: 587mV, Collector: 25.92V, Base: 1.114V

There is some measuring inaccuracy; this is inevitable.

Bes regards - Rudi

[AndrewT]

Originally Posted by Rudi_Ratlos

Pre-Driver 2SC3423 (NPN)
Emitter: 595mV
Collector: 26.5V
Base: 1.14V

Pre-Driver 2SA1360 (PNP)
Emitter: -600mV
Collector: -27V
Base: -1.15V

Originally Posted by Rudi_Ratlos

2SA1360: Emitter: -583mV, Collector: -26.24V, Base: -1.113V
2SC3423: Emitter: 587mV, Collector: 25.92V, Base: 1.114V

the base to base voltage of the pre-drivers is 2.29V in the first measurements.
The voltage across the bases of the pre-drivers has changed to 2.227V

This shows that the diodes are changing the bias voltage as the outputs heat up.

If the 2.3mA of bias current stays constant then we see that the diode string has changed from 2.29-0.38V to 2.227-0.38 in the two sets of measurements.
You can measure this voltage directly on the PCB. Find locations so that a probe slip does not blow up the amplifier.

BTW,
2.227-0.38 gives ~ 462mVf across each diode. This seems about right for a diode passing only 2.3mA
Are you using the same NJLs as Roender?

[Rudi_Ratlos]

Yes, Andrew, I use NJL0281 / NJL0302 with integrated ThermalTrack diodes.
Maybe my ones are not as perfectly matched as Mihai's.

The following questions remain:

Why am I not able to activate the 5th diode?
Will a 4-diode string be safe enough for temperature compensation?

Remember: I am currently using only 4 integrated diodes; I shorted the 5th one.

I will wait for the results of ZSAUDIO's prototype.

Best regards - Rudi_Ratlos

[roender]

Originally Posted by Rudi_Ratlos

Why am I not able to activate the 5th diode?
Will a 4-diode string be safe enough for temperature compensation?

Best regards - Rudi_Ratlos

This is a very strange issue! Everything it's looking just fine and I have no idea why you are not able to insert the 5th diode.
With only 4 diodes string the output stage will be lightly under compensated. Please test how will performs in cold and hot situations from Vre drop point of view ... otherwise everything looks good.

[zdr]

I think I remember someone else having the same problem in the original thread. The solution was to chose another diode from the string to be shorted.

[Rudi_Ratlos]

ZDR,

I (the layout) bypassed the last TT-diode (of transistor Q4) in the string.

I will change the final layout of the PCB to include jumpers on the diodes.
It will then be possible to bypass another diode, but this is impossible with the current layout.

Best regards - Rudi

[AndrewT]

I over-drilled the pads of one diode location on the original PCB.

Then insulated the two diode leads to pass through these over-drilled holes.
Left the diodes leads long for a future use, eg temperature monitor to activate a warning light or klaxon or over-temp power off.
Shorted across the remains of the two pads with a 0.4mm diam tinned copper lead, to complete the diode string route.

[pooge]

Maybe it's time to experiment with different value emitter resistors, now that there are no glaring errors found. Mihai designed the amp with .1 ohm resistors. This would create a larger standing current at idle, thus creating a higher idle temperature in the output transistors, and thus a different voltage across the diodes at idle. If you drop to .1 ohm, make sure that you have adequate heat sinking, and don't forget to remove the shunt you added across one of the diodes. Perhaps at this higher idle temperature, the fifth diode would result in a similar bias spread as you have now.

But before doing this, why not give the amp a listen?