Hi All,
this is for the audio gurus:
I recently got an SC480 kit from Jaycar, as I needed to attach a power amplifier to the output stage of my network analyser. Its a nice little kit, very easy to make, and works well, although I can hear the zobel choke singing as I sweep thru the audio range whilst sticking out a couple of amps.
I added a DC offset and DC on/off switch, so now I can have a +/-30V DC offset in addition to my ac-coupled input, and clamp diodes across both sets of BJTs so I can easily drive inductive loads.
It works great, and allowed me to measure the transfer function of a DC line filter driving a SMPS *while it was running* with the amp supplying all 50W. cool!
For my next trick I want to increase the upper corner frequency. at present its just on 100kHz (I have a pretty plot of it), and I'd like to push it as high as I possibly can.
I havent got much time right now, and specifically dont want to break it, cos then I'd have to rebuild it. and I dont have the time to measure the open-loop transfer function repeatedly as I mess with the various compensation caps.
So what limits the upper corner frequency? the input network goes thru a (well it used to be...) 1uF BP cap, 22k to 0V (my DC network is coupled in here) then a 2k2 into the base of Q2 with a 330pF shunt.
it looks like the 330pF shunt is responsible for some of the HF rolloff, with the 2k2 alone its a corner of ~ 220kHz, ignoring the BJT - which, of course, I cant, and which will lower the corner further.
there is a 10pF miller cap across the feedback input transistor Q3, and a 68pF miller cap across the B-C junction of Q5. Im guessing the 68pF does the bulk of the work here.
Any suggestions as to what I can get away with?
Cheers Terry