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Old 21st August 2013, 04:52 AM   #91
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Well, before making a new layout. I should evaluate the amp based on the existing setup. The TDA7293 board layout isn't visually bad to me (at least for TDA7293 used purely as buffer). I check distortion residual to see if there is any induction distortion (described in Self's amplifier book) due to layout or wiring issue.

BW filter 22 Hz to 22 kHz, 8 R dummy load, 1 kHz tone, supply = +/- 25 V

THD = 0.0033 %, 10 W
THD = 0.0065 %, 1 W

Output waveform and distortion residual are shown below. To my knowledge, no sign of induction distortion is seen. Agree?
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File Type: jpg 1kHz 1W 8R.JPG (536.0 KB, 129 views)
File Type: jpg 1kHz 10W 8R.JPG (545.2 KB, 121 views)
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Old 21st August 2013, 08:33 AM   #92
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Quote:
Originally Posted by abraxalito View Post
The second thing I noticed is the input impedance to the buffer (at pin 11) isn't high. I haven't explored this much further, but I wonder if its caused by the current source from the positive supply shown in the internal schematic. About 1.5mA seems to flow from the positive supply pin (pin7) to pin11. I plan to try an equal current source to the -ve supply to mitigate this.

My measurement of current coming out from pin 11 is about 3.0 mA. Two tests were used. One connected 7293 pin 11 to ground via a 1 k resistor. Second connected 7293 pin 11 to LME49811 output (SOURCE+SINK). Two tests gave very close results. As you said, a current source from pin 11 to -V can be used to balance the current instead of sourcing from the driver stage. Is it a good idea?
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Old 22nd August 2013, 04:27 PM   #93
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Quote:
Originally Posted by panson_hk View Post
Output waveform and distortion residual are shown below. To my knowledge, no sign of induction distortion is seen. Agree?
Yes, just the switching distortion from TDA7293 output...Could you load a TDA7293 output with an resistor to negative supply rail to force it a bit more to Class A? Start with ~10mA and up, until switching distortion hopefully disappear at 1W output. Watch the TDA7293 dissipation while doing it, and show as your results please.
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Old 23rd August 2013, 04:54 AM   #94
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Quote:
Originally Posted by aparatusonitus View Post
Yes, just the switching distortion from TDA7293 output...Could you load a TDA7293 output with an resistor to negative supply rail to force it a bit more to Class A? Start with ~10mA and up, until switching distortion hopefully disappear at 1W output. Watch the TDA7293 dissipation while doing it, and show as your results please.
Thank you for your suggestion. This is similar to crossover displacement described in Self's amp book. I can try it later.

I cleaned up the wiring between 49811 driver and 7293 buffer as shown below. Distortion residual of 1 kHz, 10 W, 8 R load was checked for two ground connections of dummy load. One used the 0 V of power supply board. The other used 0 V of the amp board. To fulfill start ground, 0 V at power supply should be used. My AP did not show difference in THD (0.0033 %) of the two grounding points. Distortion residual of the right grounding is closer to "ideal" waveform - purely crossover distortion.

Last edited by panson_hk; 23rd August 2013 at 04:58 AM.
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Old 23rd August 2013, 05:22 AM   #95
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Default LME49811 driver + TDA7293 buffer

Here is result for two 7293 buffer in parallel. 1kHz, 10 W 8R THD was 0.0033 % for single 7293. It reduced to 0.0029 % for two in parallel. Not much improvement under this test condition. Distortion residual is shown below.

Last edited by panson_hk; 23rd August 2013 at 05:41 AM.
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Old 24th August 2013, 12:09 AM   #96
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Quote:
Originally Posted by panson_hk View Post
Distortion residual of 1 kHz, 10 W, 8 R load was checked for two ground connections of dummy load. One used the 0 V of power supply board. The other used 0 V of the amp board. To fulfill start ground, 0 V at power supply should be used. My AP did not show difference in THD (0.0033 %) of the two grounding points. Distortion residual of the right grounding is closer to "ideal" waveform - purely crossover distortion.
As far as I can see from image, at the moment you are using one PSU with dual bridge rectification. If that is correct you should use power ground formed by local decoupling caps near TDA7293 power supply pins as your reference for signal and speaker ground reference.

I'am little suprised to see crossover distortion from TDA's even at low power level...they (invertors) claimed there will be no distortion
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Old 24th August 2013, 01:20 AM   #97
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Crossover distortion is inevitable given the substantial mis-match in gains between the positive and negative 'sides' of the output stage. I am curious how much of the XO distortion is even harmonics? Any chance to see how the distortion residual looks in bridged mode?

<edit> Incidentally I tried playing around with loading down the output stage - this does not appear to improve the distortion as it does nothing to reduce the inherent gain mis-match between the two halves.
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Last edited by abraxalito; 24th August 2013 at 01:34 AM.
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Old 24th August 2013, 02:18 AM   #98
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Richard,
Why is there an inherent mismatch between sides if you are working in bridged mode with neither side inverting, as in using a buffer before doing the inversion. Is the buffer itself causing the imbalance between sides?
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Old 24th August 2013, 02:33 AM   #99
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Default Output buffer mismatch

Here's the inner schematic from the DS - the top FET M1 is a source follower so its bound to have something less than unity gain. The M2 FET at the bottom is enhanced by the gain of the opamp.

It seems possible that a current source to the negative rail could help to raise the transconductance of the top FET but for reasonably small values of current (tens of mA) I didn't notice a significant difference. Perhaps the currents just need to be much higher.
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Old 24th August 2013, 02:44 AM   #100
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So is there no way to modify the buffer stage to match the other side so that both sides are equivalent?
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