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Old 1st June 2002, 12:22 AM   #1
rljones is offline rljones  United States
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Default Tda7293v

I started playing around with this IC and its smaller brother, the TDA7294 about one year ago. The spec sheets can be downloaded from http://us.st.com/stonline/profiles/p...ns/index.shtml and doing a part number search for "TDA729*" (without the quotes). These devices are power op amps using a bipolar input stage and mosfet outputs.

I found the basic 7294 to have a very pleasant sound, not as open or detailed as the Tripath 104, but reasonably good. It was built according to the plans in the spec sheets. Before building this unit, I searched the Internet and found no variations aside from the spec sheets.

After my experiences with the STK modules (all bipolar stages) described in another thread, I've decided to re-visit this IC power amp using the TDA7293V. This differs from the others (except the TDA7294S is now similar) in that one can create a parallel slave using only the output devices of the amp. In other words, unlike using LM3886 or 3875s in paralllel, where the input stages are duplicated for each device, paralleled TDA7293V use the input stage from one device and run all of the output mosfets from all devices in parallel. This should result in a cleaner setup. Additionally, it may be bridged. Although the spec sheets recommend the abysmal feedback method that is guaranteed meager sound quality. So far I've built up one paralled board and run tests on it. It seems very stable with no tendency to oscillate.

After looking at, I almost had to slap myself on the head. The engineers who designed it had also allowed one to separate the input stage power supply from the output stage power supply. The spec sheets never commented on this. I cut up my board today to decouple the input power supply (pins 7 and 8) using a 100 ohm resistor with 470 microfarads going to ground for each and the distortion was dramatically lowered. This unit has one master and one paralleled slave, to use ST's descriptions.

One thing that was terribly obvious was that prior to decoupling, whenever I changed the signal level while running tests, the distortion tracing would go crazy. This bothered me since music is an ever changing AC signal. After decoupling the input stage, this effect with varying input signal almost completely disappeared. I have to say, I've not listened to this amp yet.

So now I'm reconfiguring the pcbs to account for this, plus a few more changes from the spec sheets (I'll post the pcb once it seems finalized), then I'll build up a balanced bridged amp to compare with the STK. The STK also allows (and their app note encourages) input stage decoupling. The LM3886 series does not permit this. So on paper, the TDA7293V seems intriguing and is in production. Anyone else have much experience with the TDA7293V or decoupling the supplies?

Regards, Robert
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Old 2nd June 2002, 05:44 AM   #2
rljones is offline rljones  United States
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Today I powered up two monoblocks in a pair of previously used amps with +/-36VDC power supplies as described above and there was a big problem.

The +V decoupling resistor self destructed and took out both 7293s in the first channel I tested at full power. I un-did the mods in the other boards, and re-installed devices in the damaged unit. All is now working fine, but one must not use my previous suggestion for decoupling. I had in the circuit 0.4W 100 ohm resistors but obviously the + leg is doing something else with some of the current. This wasn't seen when I was testing with 40W bulb, so I'm not sure what went on.

As for paralleling two 7293s, the power difference is as follows (36V rails): with one 7294 85W into 8 ohms, 125W into 4 ohms and 30W into 2 ohms. With the paralleled 7293, 85W into 8 ohms, 150W into 4 ohms, 230W into 2 ohms and 215W into 1.6 ohms. No signs of oscillation. Parallel listening tests in a days. I'll tried some bridged versions in near future.

Again, don't decouple the input stage unless you take other measures like maybe a resistor with a much greater power rating or a much lower reistance.

Regards, Robert
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Old 26th June 2002, 07:05 AM   #3
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Default TDA7294

I have some TDA7294 chips (no suffix like V) and am planning to build an active system, mainly for classical music.

I was a little discouraged to read in one of the thread in this forum that this chip does not sound very good. While I know that such remarks are subjective ("warm", "bright" etc.), what I want to know now is, whether there are different 7294s, with and without suffixes. If there are, are there any differences in their equivalent circuitry?

I had automatically assumed that a DMOS output stage should sound warmer. You think so?

Thanks. Please reply.
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Old 26th June 2002, 08:43 PM   #4
rljones is offline rljones  United States
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ST makes a TDA7294S. This 'S' version can be run in parallel like the TDA7293V, but runs at a lower voltage.

I would not say that amps with these chips sound bad. The sound of the parallel/single ended is slightly less open or airy than the bridged STK4048XI or the Rowland model 10 (LM3886) amps--both bipolar--that I described in other threads.

In parallel/bridged mode, the TDA7293V sounds very different. Transients seemed to be accentuated more than with the other amps. Details at the rear of the soundstage seemed more defined, but there also seemed to be more grain or a harder quality to the sound than with the STK4048XI bridged amp.

Since I'm having trouble with this amp at the moment, and since I did not complete my listening tests, I cannot fully comment on the parallel/bridged TDA amp as it sounds in my system. But this amp is definitely not bad sounding. Two TDA7293V in parallel make a wonderfully robust, single ended amp (as I said elsewhere, it easily drove a 1.6 ohm load at full power!) that costs relatively little to put together.

regards, robert
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Old 26th June 2002, 10:27 PM   #5
e96mlo is offline e96mlo  Sweden
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rljones, when you do the parallel/bridge setup, do you invert the signal for one of the pairs with an OP amp or is there a simpler way?

/Marcus
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Old 26th June 2002, 10:28 PM   #6
e96mlo is offline e96mlo  Sweden
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Forgot to mension; It's the TDA7293 I'm wondering about.
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Old 2nd December 2002, 05:35 AM   #7
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Default so, where can one buy a couple TDA7293 chips

it seems that everyone but me can find these chips in small quantities. I need a couple for a repair job on a sub woofer and all I can find is wholesale houses with a 250 chip minimum. There must be some source in the USA that sells these at a reasonable price and in quantities of 2 to 10. But, what is that magic source??

thanks in advance
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Old 2nd December 2002, 07:17 PM   #8
rljones is offline rljones  United States
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I've gotten them from Pioneer: http://mypioneer.com/default.asp?visit=yes

Their supplies vary, but they usually have some in stock.

Regards, Robert
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Old 5th December 2002, 06:36 PM   #9
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I built an aux supply board for my 7293 project, taking advantage of the extra 50VAC center-tapped secondary on my transformer. Hooked it to the signal-stage power pins on the 7293 after rectifying and smoothing the AC to suitable DC. The loaded output was 30VDC above and below ground, whereas my driver-stage power is 44VDC. Grounds for both otherwise isolated supply sections were tied together so that the voltages will reference each other properly.

It didn't work. All the 7293s did was try to dump raw DC into the speakers, tripping my MOSFET-based speaker protector circuit, which immediately pulled the plug on the outputs.

Apparently the reason the 7293s' specs don't mention isolated supplies for signal and final state is that the ICs do not like it AT all if you're not feeding both off the same supply, even if you do common-ground them.


Looks like other methods of dealing with distortion are going to be needed...

oO
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Old 5th December 2002, 06:51 PM   #10
vuki is offline vuki  Croatia
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I tried to isolate TDA7293's driver supplies with 12R & 100uF and the IC exploded very nice immediately after power up
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