fire up Aleph 30

[wijgers]

I wanted to measure some voltage values over resistors. To see if the soldering was good, of if the PCB was correct.

So I did apply a rails to the PCB, but without the mosfets. Because my heatsink are not there yet. I did measure some values, but the Diff pair transistors Q1 and Q2 were different in Temperature. Q1 got hot, and Q2 did not have increase in temp.

It was te same for the left and Right channel. Is this due to te fact that de power transistors were not installed? Or could there anything else be the problem. I don't think it's one of the IRFP9610 fets, beacause it's in both channels.

Please some comments, Tips, and suggestions.

Thanks,

Nils

[Brian Donaldson]

Without the feedback, I don't know what would happen. Try it with one output per rail and a large source resistor like 10 ohm. that would keep the current down to .05 amps. unlikely to blow anything but able to behave as a complete amp.

[BDP]

Differential pair inputs try to equalize to each other, so if one of your inputs is at 'some potential' then the other input will try to follow. Without the feedback from the output I suspect that the one input is trying to compensate for the high voltage on the other input and all the differential tail current is going through one of the differential transistors causing it to dissipate more heat.
I dont have the Aleph 30 print in front of me, but if you could provide a zero potential at the input to Q2 then the differential pair should operate with more equal currents. On the Aleph 3 you can short across C101, 220uF. This is of course if the output devices are still not in the circuit. I did something similar when I constructed my Aleph. I like to test as many parts as I can before bringing the whole thing up at full voltage.

BDP

[wijgers]

Do I really need an input? because I only want to test the circuit on de value's over some components. To see if they are the same as other people had.

So shorting the cap C101 from the Aleph 3 amp would be same bypasing the two diodes? (aleph 30)

Nils

[BDP]

Do I really need an input?

Q1 input is at ground potential through the parallel resistors of 68K and 100K.

So shorting the cap C101 from the Aleph 3 amp would be same bypasing the two diodes?

Yes, if you short across Z3, Z4 this would be the same as providing a ground to junction R10,R7.

[wijgers]

This weekend it's going to happen!

Testing the amp!

Does anyone have some values which I can check!

The rails is about 25 V according to Duncans program. The schematic is the Aleph 30 and I will ise the IRFP140 Mosfets.

What is the best way to Test the circuit? Just 1 channel at the time? Do I need to have an input, like an sinus or just any input?

Please give some hints.

Thanks

Nils

[wijgers]

Hello,

Just applied 20V rails and burned two Source resistors.

The two resistors are conected at the negative rails, and burned within 10 sec.

Voltage is +- 21.4 V
There was no input conected
There was a 30 W 8 ohm resistor at the output.

Please help what could be wrong???????

Nils

[BDP]

Nils,

Were you able to test the front end as per previous posts? What's the wattage on the source resistors? Sorry, have to ask. Obviously, if the resistors are of enough wattage, you are drawing too much current. Were you monitoring the voltage across the source resistors before they gave it up? This could be from to much voltage across 390 ohm differential drain resistor. What other measurements were you taking?

BDP

[wijgers]

Finally My first DIY (Aleph 30) project is up and running!

Post Pictures soon.

Rails both channels +- 24.4 V (right) 24.3 V (left)
6 x IRFP140 / Channel
DC at the output 50 mV Left, 80 mV Right
heatsinks get hot ( I think 55 ° C) Still have to measure.

The sound is Very nice, clear, and the staging is much greater than the NAD 317. There is much air in the sound, don't know how to explain this.

The only thing is that there is a plop when turnig the amp on en an even harder plop when I turn off the Amplifier. I Didn't install the Thermistors jet because they were not available at the time. Do those Thermistors (cl-60) prevent the output to plop? Or is there another way to avoid this plopping sound. I think it's not healthy for the speakers.

And there is a slight hum in the amp. audible just at the speakers. There is no resistor or choke used in the powersupply. I think the sound comes from a vibrating transformer which I can hear also, even if speakers are not conected.

Nils

[Andypairo]

Hello Nils,
first of all compliments for building this nice amp!

The vibration of the transformer can be caused by the saturation of the core for a fraction of the 50Hz cycle.
This saturation can be caused by a slight imbalance of the secondaries, when they are used with a single rectifier bridge.

Using two bridges (one per rail) helped me solving this problem.
A thermistor in series with the primary winding also helps, my amp has a "plop" too but is not that hard.

Cheers

Andrea