Class A Power Up Procedures

[jleaman]

Class A Power Up Procedures,
When poewring up a normal class a/b amplifier you use a variac and 2 multi meters. One meter for the rail voltage ( positive ) and one for the output on the speaker terminal. The reason for the one on the output speaker terminal is to check for voltage to see if you have shorted output transistors or another problem. if you see voltage normally you would power down the amp and check other components and other things maybe re checking over things that you might have misses. With class *A* amplifiers this is a totally different story.

After spending a few days working on a class a amp ( mini a ) i have found out that when you see full rail voltage on a class a amp it is not bad. I have found that any rail voltage positive or negative below 10V is normal. Maybe some other class a amps are different but with these mini a’s i have found with any voltage below 10V it will be seen on the speaker terminal. Last-night after learning this going over 10V it actually goes down on the speaker output. i got to about 12V and it was at .024mv on the output. It’s weird why this happens. can any one confirm this or explain this why this happens ?

[jleaman]

Any one ?

[lineup]

I think it is not clear
what is 10 Volt and 12 Volt.
And how you exactly do it.

When you explain so we can understand
what procedure you are using to first power up class A amp,
then I am sure you will get some comments.

You use variac both for A/B and class A amps?

[JCM]

>When powering up a normal class a/b amplifier you use a variac >and 2 multi meters. One meter for the rail voltage ( positive ) >and one for the output on the speaker terminal.

I don't have a variac so I just use fuses. It works for me.

>i have found out that when you see full rail voltage on a class a >amp it is not bad.

Full rail voltage where ? On the output ?

>going over 10V it actually goes down on the speaker output. i >got to about 12V and it was at .024mv on the output. It’s weird >why this happens. can any one confirm this or explain this why >this happens ?

It's probably the output capacitor charging itself a little, through your voltmeter's resistance to ground.

[jleaman]

When powering up a class a amp with a variac it will show you the full rail voltage into the speaker output terminals untill you get to about 10V on the dc rail's. After about 10V the voltage on the speaker output goes down. it does it on all my amplifiers. Even the ones that are working. Shal i make a movie of this for all to see ?>

[wuffwaff]

Hi,

below 10V the current source for the input dif pair is not working (9v1 zener).
This has nothing to do with class A or AB.

William

[AudioFreak]

Yes but not a huge worry given how quickly the rail voltage will ramp up under normal conditions (without the variac). If you're really concerned you can always put a relay with a couple of seconds delay on the speaker outputs.

[jleaman]

i'm not worried its the fact that this happens when powering up a class a amp. ill remember it for the next class a amp that i power up.

[Nelson Pass]

It's not an issue of Class A versus some other class. On the
Aleph X type amps, if you power them up slowly you will see
an absolute (not differential output) DC which goes away once
the current source on the input diff pair has enough voltage to
work properly. The speaker doesn't see it, so nobody particularly
cares. When the amp is simply turned on without a Variac, the
charge time of the output stage current sources mitigates this
effect.

[needtubes]

When up and running, my Class A power amp shows -24mV and +9mV DC offset at outputs, which is fine with me. However, when I turn the amp off, immediately afterward, the voltage starts to climb slowly. I am not sure how it does this with no power to it, but it grows by 20mV or so increments until it reaches a peak of 3V (yes, I did say 3V) DC offset at the outputs. Anyone know why this is? For reference, it is a push-pull design instead of single-ended.