Optimizing TDA7294 Output

[JMFahey]

Yes, TDA7294 pinout is stupid crazy.
Why do you need to join pins which have 2 or 3 different others in the middle?
Why aren'y they joined *inside*, at the chip level?
Crazy .

[danielwritesbac]

Bob? What is your power supply voltage?

I ask because there's 3 TDA7294's.
TDA7294S = TDA7293 Cull (all pins are active)
TDA7294V = What you expect (pins 5, 11, 12 do nothing)
TDA7296/clone = Bose table radio or pod amplifier

They've all got TDA7294 printed on the chip!
But, all three have different needs for operating voltage.

The clone, actually pretty little radio/pod amp, is designed to be run from a DC power supply that has an inexpensive 12+12vac (14+14vac max) transformer, latfets at low voltage are extremely efficient and the gigantic 15 pin package is for reducing heatsink expense (rather dramatically). Unfortunately the label is misleading, sounds like junk on TDA7294's operating voltage and overvoltage/overcurrent accidents that destroy speakers, is frequency reported. This is one of the long list of reasons that I mention output caps. The clone/radio/pod chip is hard to identify, but look at Mouser's 250 unit price of $4 each, in comparison to the radio/pod amplifier's typical street price of $1.50. Well, if the fine tuning efforts mentioned earlier, just don't work, then try a much lower voltage transformer at the supply and find out if the amp sings well at low voltage, possibly indicating that you accidentally bought a wholesale priced Bose.

Meanwhile, back to output caps and non-fried speakers. I'd advise to create low loss 6600u caps with either 3x2200u or 2x3300u (typical paralleling of caps for low loss). That's for an 8 ohm speaker. And then put this 6600u cap in series to the Speaker + at the speaker jack. This is highly effective speaker protection for blocking chip amp DC accidents. That's my excuse, but there's great fun to be had by choosing a just right value for bass extension enhancement, customized to a given speaker--a reliable and easy way to get deep power tube bass from a chip amplifier. For treble fine tuning, small caps and/or RC's can be used in parallel to the output cap. Whatever you do, please consider speaker protection any/every time you're using a Chinese amplifier kit. My preference is obviously output caps, but if your preference is different, then consider the Velleman speaker protector kit. Speaker protection costs less than speakers.

[gootee]

Originally Posted by bcmbob

Stanton was fine with posting the info on the board I'm using. It is the same standard design as was posted earlier but with english comments and little larger. I'm adding a picture of the board to be a visual aid for folks like me to understand which segments are being discussed.

Do you have an image of the other side of that PCB?

[ashok]

Originally Posted by danielwritesbac

Do you have a link or some examples?

Hi Daniel,
Offhand I don't have a link but I plan to find my old board and measure it again. I had looked at the FFT when I used it. I seem to recall seeing a FFT on the Net. Will check that out also. I will post it if I find it or post my own measurements a while later. I need to find the board as I don't have time to build a new board.
Cheers
Ashok

[danielwritesbac]

Originally Posted by ashok

Hi Daniel, Offhand I don't have a link but I plan to find my old board and measure it again. I had looked at the FFT when I used it. I seem to recall seeing a FFT on the Net. Will check that out also. I will post it if I find it or post my own measurements a while later. I need to find the board as I don't have time to build a new board. Cheers Ashok

Thank you very much. I'm so curious to see what you were talking about in post #24.

[ashok]

About the sound aspect in that post , you will have to listen to a 7294 as explanations will not do justice to the subject.

Regarding the supply pin/ voltage issues, this was covered long ago , maybe 2006 (?) . I'll try to recollect the details. There are two supply pins ( 4 pins if you consider both the + and - rails) on the chip. The input stage and the output stage supplies. The differential between the two pins like +Vs and +Vp and -Vs/-Vp must not exceed 15 volts I think and the -Vs pin must not be more positive than the -Vp rail by about 1V or so. If it does you get a mini bomb !

So ( typically ) a Schottky diode and a zener diode are connected between +Vs/+Vp and -Vs/-Vp pins to avoid such a situation. This is done only if your two supplies ( Vs and Vp ) come from different supply circuits. All circuits that use a common supply don't need these diodes.

Check this link: Tda7293v

Lots on info including FFT's of performance of the 7294 and the 7293 . Don't have time right now to go through it and put up a summary.
Cheers.

[danielwritesbac]

It seems that some of this effect could be had with just 2 parts, a cap and a diode. So, a fast silicon diode from pin 13, to pin 7, stripe at pin 7, power connects to pin 13. Pin 7 has a diode drop, and then re-stiffen pin 7 with a cap to ground. At pin 7, some fluctuations/noise has been removed, Right? Well, I'm not sure how to make that symmetrical.

For a more symmetrical approach that works with any chip amp, especially for a stereo build, using 4 diodes to convert to dual mono, just by sticking 2 fast silicon diodes into the V+ output of the power board (stripe away from power board) and 2 fast silicon diodes into the V- output of the power board (stripe towards power board). Viola! Dual mono! Half of the jolts from the left amp aren't seen by the right amp and vice-versa. None of the power circuit of the left amp can be utilized by the right amp, and vice-versa, so although they can sag the power board, they can't directly sag each other's amplifier board power caps. Shields up Scotty! Wow, really cheap dual mono, with just one transformer. Also, that won't be dull no matter how much capacitance is piled onto the power board.

[danielwritesbac]

Originally Posted by bcmbob

Thanks, all useful information. I did install two LEDs on the PS boards when they were used with the BPA150s. Nowhere as sophisticated as what you suggest - just a straight connection.
Since I hadn't used them in months I forgot about the retained energy, and got a quick reminder when a short happened. Luckily, my body wasn't included in the circuit, but that shock hazard has to be constantly in mind.

Supplies:
8 of 10k 1/4w resistors
2 of red or amber LED's

Install simple drainers:
Put 3 of 10k resistors under the board, parallel with C1
Put 3 of 10k resistors under the board, parallel with C2

Install safety indicators:
Red LED series to 10k for V+ indicator
Red LED series to 10k for V- indicator

[bcmbob]

Sorry folks, I got caught up in some other projects and threads since my last response here. So much has been posted it will take me a little time to read and digest all the comments. Thanks so much for all the interest and involvement on the topic.

I B Back

[bcmbob]

Daniel - That's a lot of options to try. I'll sort through my stash for what I have and add some things to my next Mouser order. Thanks for the definitions and examples. May need further assistance later so I don't do the square plug/round hole thing

The power supplies I'm using now are 34.5 VDC. It is not a mono-block as both supplies are fed with one 200VA transformer. Havn't had a chance to try the discrete 125 VA setup yet, hopefully today. As stated earlier, I do have two of Peter Daniels bare Universal PS boards that I can put to use if that's a better option.

Jay - My speakers are the Sunflowers by Paul Carmody. I haven't used a pre yet, but my go to piece is a JC-2 clone from the same vendor. My other good pre is a Carver C-1 and I have a Mackie Satellite that is super clean. I have been using JRiver Media Center through a Mini2496 DAC with Dario Inserra's upgrades to feed the amps. The available SPL is more than suficient, though I think I'm hearing that more gain in a pre and less on the amp may benifit the sound quality. Is that correct?

gootee - attached a photo of the bottom.