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jsalk 17th March 2003 04:31 PM

Tripath RB-TA3020-2 question
I am considering designing a speaker system that is self-powered. It will be a modular system. The satellite (2-way MT) will have a passive xover and can be used with a standard amp.

In a separate cabinet I would have a woofer to extend the bass response. The satellite would sit on top of this cabinet.

In this configuration, I would have a two-channel amp for each speaker. One channel would drive the woofer and the other would feed the satellite.

I am looking at the Tripath RB-TA3020-2 for the amp. I would need to design an active xover at about 150 - 200 Hz.

If this is done outboard, I would end up with a single 4-channel amp (two RB-TA3020-2's) with a front-end active xover.

Is this workable? How difficult will it be to design the xover portion?

Any comments will be greatly apprecaited.

- Jim

nw_avphile 21st March 2003 06:14 AM

I've worked with the Tripath parts and they're a can of worms. If you haven't already, download all the Tripath application notes and read them carefully. It's really hard to get their designs to meet some of the specs they publish. Even commercial Tripath designs I've tested perform poorly on the test bench with respect to noise and distortion.

A flaw in their design seems to be the deadtime selection for the output MOSFETs. You have 3 fixed choices. If the deadtime is too small, you run the risks of the MOSFETs failing from shoot-thru currents (which tends to get worse when they get hot so you have to account for that). If you choose the next option up, you get a lot of high frequency distortion that's actually audible in critical listening. I'm betting their published specs are with the deadtime set lower than is practical or safe.

Another flaw of the 3020 is the chip itself runs HOT! If you read the fine print, you'll find burried somewhere that it dissipates about 2.4 watts as I recall. That's A LOT for a 48 pin DIP package. Mine, in free 20 degree C air, ran a 60 degree C package temp just idling. If you package it up inside of an amp in a poorly ventilated speaker, I would worry the junction temp could easily get hot enough to greatly shorten its life.

Grounding and PCB layout are SUPER critical. The switching output devices make so much high current high frequency hash that it finds its way into everything if you don't keep loop areas, trace lengths and ground points perfect.

Finally, they talk a lot about the output inductor selection, and it really is critical. The output inductors tend to run very hot with the amp just idling. The quiescent power of the 3020 design I played with was about 15 watts which is far HIGHER than a typically biased class A-B design. It will approach 90% efficiency at full power (with the right output inductor) but it's efficiency idling is actually pretty poor.

I'd possibly consider using one as a subwoofer amp, but I haven't had much luck getting them to work well enough (by my standards at least) to drive sattellites.

jsalk 21st March 2003 10:39 AM

Thanks. What might you recommend as an alternative?

nw_avphile 21st March 2003 03:36 PM

That's a good question! Frankly, the only digital switching amp I've actually been able to spend some time with that I really liked was a Crown K1 BCA amp. There's a post here from last year that goes into the BCA technology, but it's patented by Harmon International:

I've heard the new Sharp one-bit amps and the Tact, but only in a show environment where it's hard to tell much (unfamiliar system, nothing to compare them to, etc.).

I'm also fairly pleased with what I know about BASH/Indigo technology but am just starting to get more into it. The modules are sold through distribution, so in theory, they should be at least as easy to get as Triphase parts (which are tough to get for a DIY person).

I've also looked at the National digital amps and wasn't impressed.

If there's any way you can use a digital amp for your bass and a conventional AB amp for the rest I'd think about that even if it means a bigger external heatsink.

I'd guess, in a few more years, we'll have some practical digital amps (most likely modules) for DIY folks that perform well. Right now, however, I'm still searching. Does anyone else have any suggestions?

jsalk 21st March 2003 06:19 PM

While an AB amp design may not be mountable internally, I could consider a freestanding active xover/amp module.

I am not that familiar with AB DIY designs, are there a few you could recommend looking at?

Thanks, for your thoughtful help.

- Jim

nw_avphile 22nd March 2003 06:54 PM

Class AB is what 95% of the amplifiers in use today are. There are ltos of DIY AB amplifiers available in kit form or as construction projects for DIY folks. The links section of this website has several links to additional information and vendors who sell kits.

My favorite DIY design is probably the Doug Self "Blameless Amplifier". It's a very refined (yet still relatively simple) version of a classic AB push-pull amplifier with very low levels of all kinds of distortion (hence the term "blameless"). You might want to buy or see if your library system has the "Audio Power Amplifier Design Handbook" by Douglas Self. But buying a kit might be the easiest way to go and would probably save you money and lots of time in the long run.

The downside to AB amplifiers is they put off more heat than a digital switching amp does (at least when they're working hard). AB amplifiers are typically about 50% efficient while digital amps can approach 90% efficient at full power. So you'll need a bigger heatsink.

As for space, the power transformer and capacitors usually take up the most space in terms of volume if you don't count the external heatsinks. So I'm not sure a digital amp would be much smaller (in terms of cubic inches) than a conventional one as both would require a similar power supply, PC board, etc.

Like I said, a happy compromise might be to bi or tri amp your system with a digital amp on the bass where you need to most power and conventional amps for the mids/highs. That gives you the best of both worlds.

ceros 19th July 2004 05:27 PM

I want to build RB TA3020-2 board ,I've got the PCB, TA3020 and the mosfets STW34NB20. I wonder if you can help me aquire the remaining parts for the RB TA3020-2 ( SMD capacitors ,diodes, inductors).Thanks .
Best regards.

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