Aleph30: Theory on C7

[swede]

Hi all,

This is probably a thread I shouldn't have started, because it could seem to be a bit silly for you.

I'm eager to know (learn) why there is a capacitor C7 in the design of Aleph 30. There is no equivalence in the Aleph 3 schematics ( see: Aleph 3 http://www.passlabs.com/pdf/aleph/a3srvr0.pdf , Aleph 30 http://www.volksamp.com/downloads/a30manr0.pdf )

Would it hurt to (for whatever reason) exchange the .001 uF cap with a .010 uF cap? What would happen to the circuit. Where do I learn what will happen? Can I omit the cap if I'm in a hurry to get to know if my board works? Should I do as the grown up DIY:ers do; Drink a case of beer and forget about it 'till tomorrow.

The reason for me to start thinking on the subject is that I lacked the cap in my big box of crap and wandered off to a local supplier. When I got home this evening I found out that the guy at the counter must have made a mistake. I got 10 nF (.010 uF) caps instead. And since I was forced into thinking, I could just go ahead and learn something.

Best regards,
//magnus

 

[Nelson Pass]

This cap has a nominal value and two positions:

.001 or so, Collector-Base on the NPN (Q5 aleph 30)
or .01 or so, Collector to Emitter.

Either is fine, but don't leave it out, or your amplifier will
probably see some parasitic oscillation.

 

[swede]

Collector to Emitter

Ah,

Thanks a lot!

So if I use my 0.01 cap, I just have to tweak my board a bit, so it goes between the collector and the emitter, right?

The word Miller Capacitance come to my head when talking about capacitance and collector-emitter. Am I totally wrong?

So the values are kind of rule of thumb, then? Something like a nice tool to have when building current sources?

Best regards,
//magnus

 

[GRollins]

Dunno why, but I seem to be the only guy around who can throw one of these critters together without that cap and have it behave. Maybe I'm just lucky.
Yes, Miller capacitance is internal capacitance between the grid/cathode or whatever little internals you happen to be working with.

Grey

 

[Peter Daniel]

My Aleph 5 is using only electrolytic caps and is quite happy.

 

[Koy]

When experimenting I omitted to solder back this caps - very nice oscillations - about 1V at 700kHz! Mr. Pass is right, he knows what he makes.