Noise and osci in audio amp

No, I looked at many schematics and then I modified one a bit and now I'm trying to build it, so no warranty :)

Dominik

Uh good point.. Because the 66kHz signal I have on the output is not at full output level at all, only 6V.. But on the other hand it is also there if I don't apply any input. ..that's gonna be a long debugging process.

Regards, Dominik

Hello!

While playing around to find the problem, I doubled the capacitor between collector and base of the BJTs that drive the output stage BJTs and this helps a lot, sound is much better, but I have to wait until I can look at the output with an oscilloscope. Besides that I still have no clue where this problem originates...

Regards, Dominik

almost

I'd recommend 100-330n plastic film caps for good measure. 10n is near useless for power circuits at audio frequencies.

unlilely

should

Be very aware of common current paths. This is how the some proportion of the output can easily couple back to the input.

Are you familiar with 'star-point' grounding ? If so, use this technique to establish a separate ground with no significant current flowing through it to the input annd feedback side of the circuitry.

Graahm

Quite typical for this kind of oscillation.

See what happens with the input shorted. You may be getting some capacitive coupling to it.

Do you have a schematic you could post somewhere suitable ?

Graham

capacitor),

I try and avoid it actually but I can tell you I've seen plenty of *leaded* 0.1uF

Graham

I didn't realise it was a discrete design. That indicates a problem with the feedback stability - actually the entire transfer function of the gain block .

Whose design is this ?

Graham

It sounds like its time for you to change vendors. :(

for

uh ok, I admit that I didn't calculate it, so I have to buy some of these first (I'm not a professional as you know)..

output

establish

annd

No, didn't know of star grounding yet, but I think I've found some good material to read on the web :) grounding seems even more important than I thought, but fortunately it seems that most of my wireing already follows this star grounding philosophy..

bye! Dominik

I decided to try this:

I liked it because it is all built from discrete components, and I'm doing it for fun and learning :)

bye! Dominik

They were perfectly good ones. I recall some were AVX for example ! I forget what the others were but 'nice blue ones' -lol. Not cheapies at all.

It caused us considerable puzzlement. It seems to be a switch-on event caused by high dV/dt.

Graham

useless for

the output

establish

input annd

Good. That's very important. I can barely exaggerate just how important in fact.

Graham

Are you using the exact same components ( esp semiconductors ? )

I *would not* use TIP41&42 for drivers myself btw ( very slow devices exactly prone to this kind of trouble ).

I could be mildly critical of the design generally btw.

Graham

On Sun, 16 Jul 2006 23:29:50 +0200, in message , Dominik Werder scribed:

Most people don't know proper grounding, and I'm not excluding many of those actually working in the industry. It's poor technical education, I think, maybe because it's too easy to just toss the concept of "ground is zero volts" and forget about teaching return currents.

Just remember that all the current that goes in has to go back to where it came from, and it will go back any way it can find. If you take care to think about each bit of the circuit's current return, you'll be doing good design. It sounds like maybe intuitively you've already done that.

what the

by high

So, were you "induced" to change the design?

Yes.

Is there any other device that comes to your mind for that usage?

I've looked a bit around the web by myself, but.. is there any website where transistors are listed by there characteristics or do I have to check every datasheet to find semiconductor that fits my needs?

I'm very interested in any critique!

btw, no news with the amp because I have so few spare time to spend on the project the last days..

thanks for all your posting!!

Dominik

what the

caused by high

We substitued polyester film ( mylar ) types in those positions. Very rarely even a wound mylar type would burn up too ! Haven't seen the problem with 'box polyester' types though.

Graham

I too am interested in power transistor pairs for power amping. ;-)

-phaeton

exactly

Here's my advice then...... !

Typically, the larger devices used as the main output transistors are relatively slow. The driver devices are typically somewhat faster.

This is sightly less true of modern devices and especially Japanese devices where they've had durable fast power devices since the 70s but I'll continue.....

There is typically a tradeoff between device speed and durabilty ( notably second breakdown characterisitcs - an interesting subject in its own right ) although once again the Japanese parts seem to obey different trules ( presumably due to smarter device design ).

By slow I mean an fT of say 800kHz of the 2N3055 of old to the 2-4MHz of more recent Motorola/On-Semi parts like the MJ150xx series.

By fast I mean an fT of > 10 MHz and today more like 20MHz.

Slow devices cause a lag in the transfer function which can give rise to problems with loop stability which may exhibit itself as low or high level oscillation which is typically above 20kHz. ( i.e inaudible ).

Such oscillation will cause possible failure due to overheating of the output devices due to charge ( carrier ) storage issues. Note that mosfets don't suffer in the same way.

So, the trick is to ensure that the output stage ( normally emitter follower ) is preferably as fast as possible. But you want the ruggedness of what may be slower devices too.

This is acheived by using fast drivers coupled to slower main output devices but ensuring that that the drivers carry a significant proportion of the quiescent bias current. Indeed in my own designs and also many others in pro gear especially - the main output devices are in fact non-conducting in the quiescent condition. This also help thermal stability !

I'm quite fond of the MJs for output devices ( esp metal can TO-3 ) and Toshiba for drivers. ( select according to your voltage, current and dissipation needs ). Toshiba also make some good fast devices for the main output with respectable dissipation too

- but not in metal can.

Graham