35W MOSFET Power Amp - Your Opinion

[traderbam]

What is the reliability issue with tantalums? I have never had any problems with them when used within spec. Tantalums fail if they are reverse-biased alright. In terms of capacitance they have the benefit of being very small and having very good HF performance, sometimes better than some ceramics. I quote from National Semi's LM317 datasheet:

In general, the best type of capacitor to use is solid tantalum. Solid tantalum capacitors have low impedance even at high frequencies...Ceramic capacitors are also good at high frequencies; but some types have a large decrease in capacitance at frequencies around 0.5MHz. For this reason, a 0.01uF disc may seem to work better than a 0.1uF disc as a bypass.

[darkfenriz]

This amp deinitely needs driver stage

MikeB
Your calculations about temperature assume no heatsinK????
I thought IRF540/9540 is better choice than IFRP240/9240 because of lower on resistance...

[nuvistor]

I've had to replace lots of solid tantalums in '70s vintage test gear, they fail short. They also can fail short with excessive dV/dt but this isn't an issue with audio frequency applications. I use them only when low leakage current is needed and only for DC bypassing. Usually Al electrolytic is OK for DC bypassing, if you need extended frequency response, parallel with a low-inductance film or ceramic. Certainly film is preferable to electrolytics for AC coupling but for low impedance at 20Hz the choices are limited.

[ilimzn]

Originally posted by traderbam
What is the reliability issue with tantalums? ...In terms of capacitance they have the benefit of being very small and having very good HF performance...

Nuvistor actually answered this for me, and keep in mind that caps in test equipment are usually of high quality.
Until the advent of reasonably priced higher capacitance multilayer ceramics, tantalums were the only way to go for bulk bypasses on high speed digital stuff, and I used tons of them by necessity. I have had some explode on first application of voltage even though they were brand new and of course properly polarized (35A power supplies do not tolerate shorted caps well ). Some tantalums are partly self-healing which can actually be the origin of untold anguish, with equipment that occasionally fails. More than any other component, with tantalums you never know when they are going to turn from a decent cap into a short circuit.
Of course, the better the quality, the less chance of that (but I've had even the best fail), but the price, which is high to begin with, tends to end up on the order of several low ESR electrolytics bypassed with ceramic caps, which give better results anyway.
Tantalums have one characteristic which is rare in high capacitances, low leakage. Unfortunately, the leakage current that there is, tends to be very noisy.
Also, while they have negligible series inductance (ecxept the inevitable leads, if present), tantalums do tend to have highish series resistance, especially if they are very small.

Originally posted by darkfenriz
I thought IRF540/9540 is better choice than IFRP240/9240 because of lower on resistance...

Lower on resistance will hardly help in a linear application - especially if your case has high thermal resistance and the temperature rise increases the said resistance (people who design switchmode PSUs know that MOSFETs can exhibit a kind of thermal runaway effect because of this). What counts is the ability to shed heat (IRFP240/9240 are far better at that), and good gain, the more linera the better. IRFP240/9240 are much better in both respects, but also more expensive. And, granted, they may be a bit of overkill for 35W.

Originally posted by nuvistor
Certainly film is preferable to electrolytics for AC coupling but for low impedance at 20Hz the choices are limited.

I can only agree. Also, there are ceramics that are almost ideal, but unfortunately, the largest one you can get of that type tends to be about 2.2nF
The typical ceramic cap tends to have large changes in capacitance with respect to the voltage applied, which is why you never want to be caught using those in filters or coupling caps that will have any appreciable voltage swing across them - the distortion gets quite high.

[nuvistor]

All of the failed tantalums I have seen were dipped radial lead, these are the cheapest type but are used in a lot of DVMs, signal generators, etc. I have not seen any hermetically sealed tantalums fail, but these are not as common. There are mil-spec tantalums with established reliability levels. Personally I just prefer to avoid components that fail short, unfortunately all semiconductors fail this way (and tubes usually do not).

[jleaman]

Originally posted by darkfenriz
This amp deinitely needs driver stage

MikeB
Your calculations about temperature assume no heatsinK????
I thought IRF540/9540 is better choice than IFRP240/9240 because of lower on resistance...

I chose IRFP240's because of current and the voltages that im using.

[MikeB]

Originally posted by darkfenriz
This amp deinitely needs driver stage

MikeB
Your calculations about temperature assume no heatsinK????
I thought IRF540/9540 is better choice than IFRP240/9240 because of lower on resistance...

The temps are for the to92 transistors in vas, so no heatsink !
But these calculations seem not to match real world, maybe the
to92 transistors get a lot more cooling through their legs ?

Mike

[binary]

[QUOTE]
Originally posted by darkfenriz
This amp deinitely needs driver stage

MikeB
Your calculations about temperature assume no heatsinK????
I thought IRF540/9540 is better choice than IFRP240/9240 because of lower on resistance...

I' d prefer the IRF540 / 9540 because they are easily found in Brazil and are cheap (about US$1.5 each one).
Of cose, the output transistors will be assembled in a large aluminium heat sink.

[MikeB]

Yes, for the sa970 and sc2240 !

[lineup]

Originally posted by binary
Hi,
Attached is the pdf for the amp I designed using IRF540 / IRF9540 Mosfets complementary out.
I would like to read your comments in order to improve this project where needed, as I am planning to assemble it soon.
Thank you.

It is a very simple amp.
Maybe too simple to really work, without some adjustments.
Well, it is only 35 HIFI WATTS output ......

Have you come around to build it, by now?

I like that you are using two different supplies, 40 and 33 Volt for output.
This also will make better use of output power.