Silicon chip studio 350: HELP NEEDED (please!)

[81gl]

Hey guys,

New to the forum but things on here look pretty damn cool.

Okay with out blabbing to much here is the problem:
Recently built my self a pair of studio 350's, how ever these were not in kit form, all has been done from scratch.

The problem im having is that the sound coming out is bassically all treble, no lows no mids.

The other thing i have noted is that the main transistors are getting hot relatively quickly, and the amplifers arent even going overly loud.

Im guessing that there might be a problem with the inductor in the circuit knocking off all the lower end of the music, as this would explain the lack of low end, and also the heating of the transistors.

If anyone has had similar resulst please pass it on.... starting to be pushed by this project being so big....

Thanks all in advance!

Nick.

[Arius]

If your output inductor is like everybody else's (i.e. wound with 14awg - 20awg wire), then I strongly doubt it is the output inductor ruining your low freq response.

For the (over)heating transistors, I'd check:
1) Output stage bias setting
2) Mounting of Vbias transistor
3) Compensation caps (high freq instability?)
4) Connection to base of output devices (high freq instability?)
5) Inadequate heatsinks
6) Decoupling (or lack of)

For the lack of bass, I'd check:
1) Value of input dc blocking cap (if any)
2) Value of feedback dc cap (if any)
3) Main PSU caps (too small?)

Anytime you need to troubleshoot a discrete power amp, a good starting point is to cover the basics, then take the schematic diagram, measure the voltages at key points and see if they make sense. Good luck.

[ilimzn]

A series inductor would possibly ruin your treble, not your bass - so much for the basics. I am sure more help would be forthcoming if you posted the schematic.

[quasi]

I have the schematic here.

First make sure that you are indeed getting clear treble and not something else that is a bit trebly but more distorted.

Are you getting the same result on both channels?

If it is clear treble then first check the input capacitor, it should be 1uF. Then check the feedback capacitor, this should be 47uF (maybe replace it with a different brand).

The only other thing I can think of at this stage is the output network. Make sure you haven't transposed the coil and the filter capacitor.

Can you post a photo of your boards, I can then compare these with the layout I have here.

Does your output go through a DC protection circuit? Somethimes these run a capacitor across relay contacts and if there is a fault some treble gets through to the speakers.


Cheers
Quasi

[mikeks]

oscillation?

[81gl]

Thanks for the help guys...

Ive spent the day looking all confused as i havnt been able to fault the circuit at all... (not being full of my self either...), and all voltages are reading text book...

Ended-up changing the source from my computer (as i usually use on all my amps and they are fine) to my cd player (dare i say... mini hifi), and long behold things sound bloody nice...

So thats my stupid fault there i spose....

How ever... i am finding that one of the modules runs abit hotter than the other. Im guessing this could also be due to the voltage being set different via VR2 giving a different current through that main output transistors? then causing more heat?

Heat sink isnt a problem... using the fan assist job from jaycar with twin 80mm fans.... (over kill i know.... lol)

Hmm... well sus the heat thing out tomorow.

THanks so much for the help guys! was getting the feeling i was alone...

Cheers again!

Nick.

PS: here is the link; http://www.siliconchip.com.au/cms/A_100503/article.html
&
http://www.siliconchip.com.au/cms/A_100618/article.html

[81gl]

Hey guys,

Have spent this morning re-setting the voltages on the amp, re adjusting VR1 & VR2.

How ever, there is still one module running alot hotter than the other. Like alot!

when i say running hot, meaning that both banks of output transistors are running pretty hot, especially compared to the other module.

Both modules have the same situation, driving the same speakers, same input etc.

the sound coming from the speakers is the same, and both are sounding great =P!

Anyone got some suggestions? (still looking my self)

Cheers in advance!

Nick.

[ocaukrell]

Are they fed from the same powersupply ?
If not check the voltages of that.

check the voltages of the output trannies, might piont to one getting more supply voltage or bias drive than the other.

[quasi]

If everything is exactly the same...then you may need to refer to post number 5. Do you hav access to a CRO?

Cheers
Q

[81gl]

Nar, they are fed Via totally seperate power supplies (toroidals/caps/rec.s etc). Both power supplys are putting out +/-70VDC varrying by about 2V =].

Checked over all voltages given by silicon chip on the schematic, and still cant find a fault.

guess ill keep looking....

Cheers all.

Nick.