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Old 24th May 2008, 05:05 PM   #1
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Default need help with TDA7293

Hello

The other day i noticed that my guitar amplifier enetrs distortion at sound levels much lwoer than i expected. So i conencted it to an oscilloscope and noticed that clipping started at around +-24V. I am using a 28V 300VA toroid transformer with 4700 uF caps for each suplly rail and measured the dc voltage as 40.7V at idle. On datasheets this lm3886 has a typical power of 50W into 8 ohms at +- 35V suplly. I am using the standart typical application circuit that is listed in lm3886 datasheet except the output inductor, capacitor, etc. (im very disappointed with lm3886, its expensive, its inefficent, it has audible backgroud noise!)

I checked the LM7290s datasheets the otehr day and noticed that they are much more efficient than lm3886, they are cheaper and they probably sound as good as lm3886. Problem is they dont have a simple circuit as the lm. I read a few articles in forum and in the datasheets it was written that the tda7293 could suplly around 100W of power at +-40V which is my suplly at hand. Problem is it says it cannot handle this power due to dissipation limits. I also saw people blowing this chip instant they put the power on. This chip also has soem mute, standybye functions (i dotn want to use them and i want to completely get rid of all components associated with them) and 2 seperate suplly rails.

I want to simplify the typical application circuit (at least no mute-standbye stuff) and make the bypass caps closer to the ic. What modifications can i make over the typical application circuit? If i simply not conenct the mute-standby pins to anywhere, will it work fine or burn etc? Also i will suplly this unit with around 40V dc with 4700uF caps at each real. the gain of my preamplifier is large enough to put the lm3886 into distortion, which emans this chip may run at 70-80W constantly at full power. can a heatsink - fan etc handle all this? Or there is no way that i can use a single 7293 to run at 80W power continuoisly.

Thank you for your help.
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Old 24th May 2008, 07:37 PM   #2
Nordic is offline Nordic  South Africa
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I would up the supply to 2 x 4700uf per rail as a minimum.

For your application I recommend the trusty old LM3875... no mute pins to deal with, can live at 40VDC provided you have good heatsinking... can deliver adequate power into 8 ohms...

I would make the high pass filter quite high to save the amp from dealing with power wasted on low frequency harmonics...

I do not think any chip can live at a sustained 80W output...
There is no need for that amount of power anyway unless it is for large or outdoor venues... 1 Watt can drive a speaker to full output, WHAT that output is depends on speaker efficiency....

There are many high efficiency, low power guitar speakers available, even the famous Celestion 30 is.. well, a 30W driver... way loud though.
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Old 24th May 2008, 08:20 PM   #3
Minion is offline Minion  Canada
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I recently Built a couple amps based on the TDA7293, I also couldn"t find a PCB design so i finally just broke down and used the PCB design in the datasheet but modified it a bit so that the Mute/standby was disabled...It does make a little pop when you start it up but it isn"t bad...I don"t notice any real hum or noise useing the Datasheet PCB design....

I used them with a Bi-amped bass guitar amp, I have a 12db/oct Low pass filter on one channel for the Sub set at 400hz and a high pass filter on the other for the driver and a Volume controll on each so I can tune it how much of the sub and/Or driver I want....

I allready have the TDA7293 PCB done up in MS Paint so if you want to try it let me know and I"ll post it for you.....

Cheers
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Old 24th May 2008, 09:41 PM   #4
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thank you, a pcb without mute standbye stuff will be very helpful
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Old 24th May 2008, 09:41 PM   #5
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will also try lm3875, which i just checked teh datasheet and surprisingly it is more powerful than 3886 (40w min vs 30w min, 56w average vs 50). i will immeaditly swap the 3886 with 3875 and put 2 more 4700 uF in parallel to psu. Thank you for the advices.

also the amp gets in distortion at around frequencies 100-150w which i believe the guitar produces the strongest output at those frequencies and the total amplifier sounds as if it has much base indeed. shall i decerase the input cap (1uF with a 470n or 220n one) or i used a 10uF at negative feedback path in junction with a 330 ohm resistor (22k between output and inverting input pins). shall i reduce the 10u F electrolytic to a 1uF polyfilm etc?
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Old 25th May 2008, 08:06 AM   #6
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Quote:
Originally posted by weatherlight
shall i decerase the input cap (1uF with a 470n or 220n one)
That's a good start on getting rid of some clipping. The 470n is the one I use for popular music with my LM1875. Although it doesn't clip, it does have a 4 amper limiter. The speakers I used for its projects are 8" and 6" and they are 4 ohms. So, I simply had no need of sending signals that didn't come out of the speakers.

Its now able for more than twice the output, without setting off its limiter. The limiter on LM1875 makes abrupt "blanks" of silence. In my usage, that's more polite than the alternatives. . .

On the larger NatSemi chip amps, lm3875 and lm3886, the spike system can make some terribly loud clipping; however, the combination of sufficient power and a "right sized" input filter cap, should help considerably.

On the very powerful ST chips, TDA7293 and TDA7294, they're prone to overamplifying even order harmonics, especially at around 70hz, and can sound much like a jukebox. The simple cure is to "right size" both its NFB cap and "right size" its input filter cap. However, their interesting character could make for a nice "warm" guitar amp, and there's no shortage of power output.
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Old 25th May 2008, 09:55 AM   #7
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I just checked the modular application of the tda7293 in the datasheets. It seems simple (much simple than the high efficiency one). Will the modular application be suitable for the +-40V and the gain capable of reaching 80-100W? Also i noticed many people dont like the sound of electrolytic caps (22uF etc) in amplifiers. I should use a 47u at bootstrap and probably something around 5-10u at NFB. Is there a non electrolytic cap at that size or a good quality one? (the ones i can find around here are mostly standart cheap caps)
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Old 25th May 2008, 10:03 AM   #8
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Quote:
Originally posted by weatherlight
I just checked the modular application of the tda7293 in the datasheets. It seems simple (much simple than the high efficiency one). Will the modular application be suitable for the +-40V and the gain capable of reaching 80-100W? Also i noticed many people dont like the sound of electrolytic caps (22uF etc) in amplifiers. I should use a 47u at bootstrap and probably something around 5-10u at NFB. Is there a non electrolytic cap at that size or a good quality one? (the ones i can find around here are mostly standart cheap caps)

Bootstrap, 47uF is right.

NFB, 10uF to 22uF, but not larger

Electrolytic caps do come in "made for audio" varieties.

Power? Do you have 4, 6, or 8 ohm speakers? What is the lowest desired frequency response? What is the LF rolloff of the speaker?
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Old 25th May 2008, 10:17 AM   #9
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I need a 3db cutoff around at most 70Hz or so. I believe the low e string gives 82 Hz at 0 fret position. I have a 12" 8 ohm 100w guitar speaker. Problem is, i plotted out the mid band frequency range and the preamp-equalizer-poweramp total 3db response was already something like 60Hz-4kHz. i can degrease the input 1uF cap with 470nF and the 10uF cap with something a bit smaller but if i go beyind that the low e string will fade abit
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Old 25th May 2008, 10:25 AM   #10
Nordic is offline Nordic  South Africa
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The experiment is cheap, the lessons priceless.. give it a try.
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