Transistor comparo

[xyrium]

Hello All,

I have two low end 2 channel amps. Both are claimed 80W/ch. One has a pair of A1941s and a pair of C5198 per channel, they're 70W parts (that's 4 per channel).

The other has one C4029 and A1553 per channel. That's two per channel, and they're 120W parts.

The rest of the boards seem very similar if not identical.

Is one transistor layout/design better than the other? I know this is asking to simplify a circuit tremendously, but let's just look at the part with the transistors if possible.

I was thinking that the two transistors per channel would be faster, while the 4 per channel would most likely be more stable since it has a slightly higher power rating across more pieces.

Thanks,
Paul

[Christer]

There are many things to consider, so let's just take a few, simplified:

It is not only the power that matters, but also current capacity. If loudspeakers were purely resistive this would be correlated. However, loudspeakers are reactive, so you can have a lot of current flowing with very little voltage, that is, a lot of current but not so much power. Using several devices in parallel is a common way to increase the current capability even if one device would seem sufficient if just looking at the power. I recently looked inside one amp rated at 120 W that had three 15 A devices in parallel. That is 45 A. For a purely resistive 8 Ohm load that would correspond to around 16 kW!!! Obviously they both wanted an excellent current capacity and generous safety margins. In reality it is more complex since you have to look at the so called SOA curve, telling which combinations of voltage and current over a transistor is safe.

Usually you will find resistors between emitter and output also for single devices, but for parallel devices it is necessary. If one transistor gets a little bit hotter, it will start to take a larger share of the current, making it even hotter, taking an even larger share of the current. Eventually it may take the larger part of the load and overheat. The resistors provide a local feedback that chokes a transistor that tries to take over too much of the load, thus evening out the load between them.

The speed of a transistor will mostly depend on its capacitances (which is partly related to power but also to design) and on the base width which is (I think) mostly a design issue, or at least not related to the power. While higher power transistors will on the average tend to be slower than lower power transistor, one can never assume any such relationship between two particular transistors. One has to consult the datasheets.

Hope this makes things a little bit clearer.

[xyrium]

Thanks Christer. I read something about the feedback circuit you mentioned by doing a search on those transistor part numbers on this board, so that makes sense.

I guess since one amp is a few years older than the other, and thus the transistors are actually no longer current, the newer parts probably have better electrical insulation and isolation, along with other factors that _should_ make them superior.

I'll check the datasheets to see if anything that I might understand jumps out at me. Strangely enough, the unit with the single pair sounds harsher than the dual pairs in the highs and mids, though it seems to have better low frequency reproduction. Maybe I could replace them with a newer version...

[anatech]

Hi xyrium,

Maybe I could replace them with a newer version...

Please, don't do that unless the originals are blown and you have no other choice.

Strangely enough, the unit with the single pair sounds harsher than the dual pairs in the highs and mids, though it seems to have better low frequency reproduction.

That may have more to do with circuit and layout than anything else. Even bypassing the filter caps may help there.

Since these are both inexpensive units, don't expect the finest design work, the best components or execution of design.

-Chris

[xyrium]

Thanks Chris. Yeah, I bought on unit new, and one older unit used. They are Audiosource Amp One and Amp One-A. Oddly enough, i believe they claim short term 2 ohm capability as pro amps do (alluding to another discussion we're all in).

Doing the bypassing sounds like the cheapest fix for a small improvement, even if psychoacoustical.

I may still pick up the 2SA1553/2SC4029 pairs to have them on hand and do a quick swap.

[anatech]

Hi xyrium,

I may still pick up the 2SA1553/2SC4029 pairs to have them on hand and do a quick swap.

If you're going to do that, measure the bias current now and record the value. You have a 'scope and variac, right?

-Chris

[xyrium]

Jeez, you guys don't make this stuff easy. I forgot about adjusting bias. Nope, no o-scope or variac. I have an iso xformer, but it won't vary the output, just save me from killing myself generally...

[anatech]

Hi xyrium,

Jeez, you guys don't make this stuff easy.

Whatdyamean??!
We're trying to make it easy so you don't get jammed up!

You need the o-scope for checking for oscillation and distorted waveforms. The variac will greatly reduce your parts consumption (as will your 'scope and meter).

These things are so cheap these days. Nothing like when I was starting out (5 MHz was above hobbits grade). If you are interested in working with electronics, pick these things up when you can.

At least you remembered about the bias current.

-Chris

[xyrium]

Yeah, I need to bounce around ebay for some deals this week. Thanks again!