Chinse DX amp compensation capacitor

[sreten]

Hi,

I bought some boards which I plan to use as small mostly class A amplifiers,
I also plan to use them for mid/treble with a separate AB amplifier for bass.

But further research revealed them to be the DX amplifier from this forum.

The major variation with the schematic is a Ccomp of 100pF compared
to the the DX amplifiers 22/27pF used on a BD139 VAS transistor.

This is a change of two octaves and around 12dB of open loop gain
in the mid treble regions. It also implies an extra 12dB of gain margin.

But is this needed ? Or is it being overly slugged with a baseball bat
for stability ? How stable is the 22/27pF variant ? Don't know myself.

Any informed opinions would be appreciated.

rgds, sreten.

(somebody please fix my crap title spelling and change Dx->DX)

[Tekko]

This is sadly how it works nowdays, they take a design, change a component value or two and call it theirs.

[lineup]

THe rule is to use as small value compensation cap as possible.
To keep the amplifier stable.
This can be determined exactly only through an oscilloscope.

Using 100p when such value is not needed
is called overcompensation.

[CBS240]

Yup. They say 'your idea no good, can't be productive' and the next thing you see the exact product appear out of China, or somewhere else in the world.....and people wonder why I don't disclose all of my IP here.




I suppose imitation could be looked upon as flattery.

[Ian Finch]

It could also have been a necessary step, now we know that BD139s ain't BD139s any more. I've also had to replace specified 47pF and 68pF comp. capacitors in some cheap amps with typically 82 pF. Usually, there had been substitutions in the small signal transistors too. The amps. could only just have been marginally stable when sold new.

DX himself claimed that marginally stable amps were "the good ones" so it may not be all bad to have an occasional fried amp in some people's quest for "sonics".
With kit supplies though, any risk of oscillation is probably much worse than overcompensation. Caveat emptor!

[sreten]

Originally Posted by Ian Finch

It could also have been a necessary step, now
we know that BD139s ain't BD139s any more. !

Hi,

What do you mean ? Hfe is not up to specification ? So the miller effect
is less ? And therefore the capacitor needs to go up ? But I thought
the miller effect in the VAS compensated automatically Hfe variation .....

Guess I might need to get a proper circuit emulator out ....

rgds, sreten.

[Bigun]

I have used two methods to pick Cdom, one using my ears, the other using a 'scope. The use of a 'scope I have mixed feelings about - I used the square wave test signal from the 'scope (used for tuning up scope probes) which has extremely steep fall-rise times. I'm not convinced that this is a good way to do it unless you are really just looking for a bomb proof system - the steep edges are extreme for an audio amp.

I was much happier choosing Cdom by ear. My amp wasn't that different from the DX so my experience may be relevant. I found a value of Cdom that was too low and a value that was too high was quite audible. The too-low value (22pF) was thin sounding and in one of my amps I also had some sibiliance with low values. The too-high value (139pF) was dead sounding. The middle value (in the range of 47pF), the Goldilocks value, sounded just right.

I used BD139 for the VAS and I've read some people who find a better sound can be achieved with a device that has higher performance (in particular, lower Cob).

[Boscoe]

Good old jims audio! They don't sell these kits anymore...

I find it funny that people slag ebay shops like this off. Okay it's not their design. But I have no way of making PCBs that are that good myself. Jims audio PCBs are double sided thick PCB thick copper, solder masked and silk screened for essentially very little price! they are fantastic quality as well. Where can I buy a DX amp PCB right now and get it in less than a weeks time for a tenner? I personally think they fantastic it's just a shame that their change the schematics, if they just sold the PCBs and did not call them their own which jim's audio doesn't what's the problem? I think it's a great resource to the community.

[CBS240]

Originally Posted by Ian Finch

It could also have been a necessary step, now we know that BD139s ain't BD139s any more. I've also had to replace specified 47pF and 68pF comp. capacitors in some cheap amps with typically 82 pF. Usually, there had been substitutions in the small signal transistors too. The amps. could only just have been marginally stable when sold new.

DX himself claimed that marginally stable amps were "the good ones" so it may not be all bad to have an occasional fried amp in some people's quest for "sonics".
With kit supplies though, any risk of oscillation is probably much worse than overcompensation. Caveat emptor!

Hi

I disagree that borderline stability improves sonics, as in specifically intending to operate near instability. If the GBW of the amplifier is high enough, you can cap it well below that instability point and still have plenty to work with. This way you have a high 'sonics' amplifier and an unconditionally stable one to boot. Design/PCB layout becomes much more important and specific tho.

[ostripper]

Some think to bring a amp to the "edge" (undercompensate) will improve sonics.
Wrong , and it will not always improve slew - which WILL have a impact on sonics.
I seen a slow ft amp with impressive slew and a "fast amp" with mediocre slew. More to it than just changing a cap. Devices and topology are bigger factors.

OS