New to DIYAudio Help With New Build?

[buzz1167]

Hi guys, I've been reading around here for a while but just got around to trying it myself. I would like to pose a question if anyone could help that would be awesome.

Basically I'm starting with a Douglass Self Blameless Amp design. I've got it almost exactly as he does in the online pages on "Distortion in power amplifiers". I've read through and tried to understand everything and I think I've understood most of it however I've having some trouble.

I've built a Blameless amp with CFP output stage and an Emitter Follower VAS. I have changed the NFB HF cap to 33pf (cuz its what i had) and left the LF NFB stuff alone. The transistors aren't the same but I purchased them thinking they were better than what he has suggested.
KSA916Y and KSC2316Y for the higher power stuff (Vas + Output Drivers) and KSA992F and KSC1845F for the input stage. MJL4302 and MJL4281 for the final output.
Basically I was getting some nasty oscillation (or it sounded like oscillation - just constant noise, hum and hiss) that would quiet quite a bit when I probed with my multimeter from ground to the npn driver of the output stage or when I touched certain areas with my finger.

First thing I tried was replacing crap everywhere thinking I might have melted something soldering, then tried increasing the LocalFeedback Cap on the VAS which did nothing from 100pf to 1500pf except make it sound a bit worse.
Finally I found out that putting a 220pf cap from output driver base to collector on the main output (on both output transistors) made the noise stop.
Can anyone explain this to me? All I know is that the oscillation is not audible anymore and the drivers aren't overheating like they used to (like when I got the noise) which is good, but I don't know why what I did worked? I haven't tried anymore values. Does anyone have any other possible solutions that don't involve these caps? Did I buy crap transistors?

Thanks for any help...

[lineup]

I've built a Blameless amp with CFP output stage and an Emitter Follower VAS.

It is known that CFP pairs can oscillate sometimes.
In those cases it can help with one small value capacitor
from base to collector on the driving transistor of the CFP pair.

[jaycee]

Start with something simple - is your heatsink grounded? If not it can act as an antenna and make the circuit oscillate.

CFP's - nasty little buggers in my opinion. PCB layout is highly important here. Did you use a pre-designed one, or did you make it yourself? Can we see?

The problem with the "better" transistors is that they are faster, which in itself can make things unstable unless properly compensated for.

[buzz1167]

is there anything that says how I can properly compensate for the cfp stage? I'm looking for higher frew output transistors because I want more high freq NFB capability. Right now I have the main output transistors on separate heatsinks (npn and pnp) because I've seen transistors with continuity from the heatplate to the (emitter?) basically the goal was to have each heatsiink have it's own charge. I did't check that though, maybe grounding it is a good idea. I'll try that tonight.

to be honest I have looked at the layout almost none. I just went from a birdnest breadboard to a radioshack special solder board to get rid of wire loops.

[jaycee]

Forget using breadboard or those solder boards. Stripboard does not have the current capacity for a power amp, and the long parallel tracks introduce all kinds of parasitics which will just make for an oscillating mess.

[buzz1167]

What I've done is attach the power and output directly to the output transistors via dedicated heavy gauge wire and from them to the power and them to the feedback resistors to the speaker out. Thus the entire CFP setup attached directly to the leads of the output transistor. Now I only need 2 signal wires to attach between the CFP drivers and the signal (solder) board.

Separate +- power and ground are all pulled from a tri-star like setup at the power caps, thus attempting to keep high power currents out of the signal path. I don't think that the radioshack special should need to carry any 'real' current this way, just the signal currents... Correct me if this is wrong.

I plan to get (or make) a smaller version of the board, since I'm only using 15x15 pin area on the big one and everything is still a bit spread out. Basically I didn't want to prototype a board till I get something that is working, is that some sort of paradox? I can't get a good working one until I prototype, but I will obviously prototype it wrong based on what I learned before on the simpler one since it has these oscillating features built in?

Do you think it is ok to use a generic radioshack IC connection board for the signal path? What things would you add into prototype board that would make it better? Is that written anywhere?

Thanks so much for your input!

[Mooly]

Hmmm...

There's no quick fix for this... all your issues will be down to layout and wiring. Dougs design is a fully worked example and performs well... and it is fully stable.

Dare we ask to see a picture of your project

Amps like this must be wired correctly... you can breadboard it, but the wiring has to be right with separate returns for critical parts of the circuit.

[Mooly]

A thread of mine on layout,
Oscillograms/Testing, and why layout matters.

[buzz1167]

I can post a picture if that helps, but I don't see how. I'm more interested in why than how... Thanks for the link to layout I'll look at that.

I've been searching the internet and I ant find anyone describing the oscillation characteristics of cfps... Does anyone know about something like that?

[buzz1167]

[IMG]file:///C:/Documents%20and%20Settings/Administrator.GAMER/Desktop/Circuit.jpg[/IMG]Circuit picture attached...

Two wires on top right are power and ground.
Rca input is on top.
Ground and NFB are on the bottom of the circuit.

Top left section is the current source
Middle 4 are the differential and current mirror

Top middle is vas current source
Right side is bias and below it is EF VAS

Two wires in from the left are ground and NFB
Wire leaving down and right are CFP driver inputs