Lots of Questions

[jteef]

What I am looking for here is any helpful articles or advice you guys might be able to give me to help fill in the practical knowledge my college education has left out about audio.

#1: mosfets vs BJT's Here I am looking for the pros and cons of each related to audio. What specs are the most important, general design considerations etc. I know a lot more about bjt's than fets, so a good mosfet article would be good. What are JFET's good for?

#2: Ported vs Sealed. Sound quality approach here. other than the transiet response, are there any compelling reasons to give up the lowest octave. Comments on the linkwitz transform circuit found on Rod Elliots page?

#3: Capacitors. I know that ceramics are bad and why, and that polystyrene's are good, but not why. What about electrolytics, mica's, etc.

#4: I'd like to learn some more about Class D amps. I have read the links in the Class D amp post below and they helped a lot. A lot of 12V folks claim they are the best thing since sliced bread and they do look very effective for what they are designed for, but I would like to hear why you would or would not use one in your system.

and #5: Regulated and Switch mode power supplies. The only power supply i've built was just the toroid, bridge rectfier, and filter caps. How/why do the others work. I have a brief understanding of how the switch mode supplies work, but not in an audio application.


Thanks a lot!

jt

[Edited by jteef on 09-24-2001 at 12:05 AM]

[paulb]

Hmmm, I just posted this link in another thread. About capacitors:
http://www.capacitors.com/pickcap/pickcap.htm

[Thoth]

Originally posted by jteef
#1: mosfets vs BJT's Here I am looking for the pros and cons of each related to audio. What specs are the most important, general design considerations etc. I know a lot more about bjt's than fets, so a good mosfet article would be good. What are JFET's good for?

The complete answer for this would fill a book (try to read the Self and Sloane books). The short answer is that a properly designed MOSFET amp is usually more rugged (electrically) than a properly designed BJT amp. Also, the MOSFET amp often sounds smoother. On the other hand, (for those who believe in numbers) MOSFETs usually put out more distortion, and more heat.

JFETs are low power devices that are sometimes used in the input circuitry of an amp. They are very sensitive to the balance of the circuit, and are rarely used in DIY circuits.

#2: Ported vs Sealed. Sound quality approach here. other than the transiet response, are there any compelling reasons to give up the lowest octave. Comments on the linkwitz transform circuit found on Rod Elliots page?

In general, a ported speaker (or a PR speaker) will keep moving longer after the signal stops then a sealed speaker. WRT subwoofers, the ported speaker normally works better for movies, and the sealed speaker normally works better for critical listening (music).

Also, if you try to run a ported speaker into that lowest octave at a high sound level, you'll often hit physical problems (Xmax). This is much easier to do in a ported speaker than a sealed speaker.

#4: I'd like to learn some more about Class D amps. I have read the links in the Class D amp post below and they helped a lot. A lot of 12V folks claim they are the best thing since sliced bread and they do look very effective for what they are designed for, but I would like to hear why you would or would not use one in your system.

Class-D amps and switching power supplies are normally used in cars (12V folks), as there are no other good options.

When designing a Class-D amp, the rule-of-thumb is that the switching frequency should be at least 20X the highest frequency being amplified. I distrust this rule-of-thumb. While it may be OK for a car, where there is lots of external noise, I feel that the number should be more like 50X or 100X for home listening. This would make a good Class-D amp OK for use up to about 2.5KHz (sub and woofer). I certainly wouldn't try using one to drive a ribbon speaker (i.e. Raven-2).

#5: Regulated and Switch mode power supplies. The only power supply i've built was just the toroid, bridge rectfier, and filter caps. How/why do the others work. I have a brief understanding of how the switch mode supplies work, but not in an audio application.

The logic behind regulating a power supply is that it removes much of the noise that comes from the power supply, and gives a more consistant voltage to the amp.

The problems are as follows:

Without large output capacitors, the regulator will need to respond VERY fast to current draw of the amplifier. Often, this is not possible, or commercially feasible.

The regulator complicates the circuit, and creates additional heat. Isn't this just what everyone wants in their amp.

Regulators add their own noise to the power. A properly designed regulator should be cleaner than a simple capacitor filter. The problem is that a properly designed regulator gets expensive. This is even harder when using a switching regulator (which also put out RF noise).

Good luck.

[jteef]

Thanks for the reply, I will certainly try to get a copy of the books.

I fully understand the high frequency faults of the Class D design, but what are some of the faults (if any) in the low end. Distortion relative to AB Designs, clipping behavior etc. I've only had a very brief chance to examine one in the lab.

The reason I am interested in ported vs sealed is that I had considered using just a 2 way setup but to get the LF sound I am looking for I would have to port the woofer. So, I thought by adding the 3rd driver, I could pick up the lower octave+ and then seal the midrange for the better transients in areas where distortion will be more apparent. Almost all of my simulations have been done using car audio subwoofers like the alumapro alchemy and audiomobile mass, but they are quite pricey and inefficient.

Thanks again.

jt

[Thoth]

Originally posted by jteef
Almost all of my simulations have been done using car audio subwoofers like the alumapro alchemy and audiomobile mass, but they are quite pricey and inefficient.

They also drop out the lowest octave or two. They get away with this, as the cabin in a car is usually small enough to give a lot of gain at low frequencies.

Try running sims with a high quality driver. Focal, Peerless, Human and Hi-Vi are probably good starting points. You probably want either an 8" or a 10". A pair of 6" might also work.

If you're looking to go with a two-way, you'll probably need a steep crossover (3rd or 4th order) to get adequate safety margin for the tweeter, while not forcing the woofer to get high enough to 'beam' (NOTE: Beaming in a woofer is almost completely determined by the driver diameter, and the frequency).

Good luck.