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Old 30th September 2011, 03:42 AM   #11
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Yes, certainly lower power (or bridged) are possibilities. Would you be able to use your advanced test equipment to find out what phase shift the TDA7293 used as buffer introduces? Its certainly cheaper than the LME or BUF634 power buffers. If its wideband enough it could be a lower cost (and higher power) solution.
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Old 30th September 2011, 04:20 AM   #12
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I just made a test confirming that TDA7293-slave can be used solely as an output stage. This is done by not connecting the slave output and bootstrap pins to the master's pins. I still use the master buffer output to driver the standalone slave output stage. The diagram is shown below.

THD vs power for the master and the standalone slave is shown below. The slave-output stage is essentially operating in open-loop (no feedback) mode. Hence, we see very high distortion.

Regarding phase shift of the buffer output stage, I used a 200 kHz tone. Scope connected to master's driver output pin11 and the standalone slave output. The delay due to the buffer output stage is about 0.15 us.
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File Type: jpg THD vs power slave-output.jpg (146.0 KB, 768 views)
File Type: jpg Slave-output Master driver.jpg (43.7 KB, 724 views)
File Type: jpg slave-output schematic.jpg (139.5 KB, 757 views)
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Old 30th September 2011, 04:29 AM   #13
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Good work - the 0.15uS delay corresponds to a phase shift around 60degrees at 1MHz so its not going to be stable inside an opamp feedback loop. The open loop distortion going up to about 5% looks seriously worse than the other buffers I mentioned but then they don't normally drive 8R. That high distortion is being measured into 8R load which case its understandable I think. What does the distortion look like with 32R load (say as might be from headphones)? I see the unloaded distortion is pretty good
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Old 4th October 2011, 09:12 AM   #14
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My previous time delay might be not accurate. I used a square wave today. The slave input and output waveforms are shown below. The time difference between them is definitely less than 0.1 usec as reported earlier.
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Old 5th October 2011, 04:34 PM   #15
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Quote:
Originally Posted by panson_hk View Post
I just made a test confirming that TDA7293-slave can be used solely as an output stage. This is done by not connecting the slave output and bootstrap pins to the master's pins. I still use the master buffer output to driver the standalone slave output stage.
...
I guess this is the approach used in some amps being sold on "e-Buy"...
7* TDA7293 In parallel 555W Mono Power Amplifier board | eBay
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Old 6th October 2011, 02:41 AM   #16
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I believe the board uses master-slave as described in the data sheet. The slave's bootstrap is connected to the master.

How come the pencil got burned? Is its equivalent to an low impedance load? The driving voltage is just +/- 20 V.
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Old 6th October 2011, 04:40 AM   #17
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Originally Posted by panson_hk View Post
I believe the board uses master-slave as described in the data sheet. The slave's bootstrap is connected to the master.

How come the pencil got burned? Is its equivalent to an low impedance load? The driving voltage is just +/- 20 V.
Black magic?
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Old 6th October 2011, 04:45 AM   #18
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Quote:
Originally Posted by panson_hk View Post
I used a square wave today.
Yes that does look a lot more promising. So the acid test is to put the buffer inside the feedback loop of say a 5534 then play the squarewave through it again. The amount of ringing compared to with no buffer in the loop will give a good indication of how much the delay impacts the phase margin.

<edit> Yeah pencil leads are rather like carbon composition resistors. I think the resistance might depend on the softness (B, HB, H etc) but I haven't verified that. Looks like that 555W amp certainly put lead in that pencil
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Old 7th October 2011, 11:23 AM   #19
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I doubt whether the higher distortion at 20kHz frequency is audible. Given than the graph of power distribution versus frequency is logarithmic in nature, at 100W the power upwards of 15kHz is going to be under 1W.

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The higher levels of distortion that you observe at 20k is typical for this chip amp. It's not a great full range amp because the distortion above 1k rises too rapidly. It should be very nice in a multi-amped system for lower frequency use, however.
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Old 7th October 2011, 06:54 PM   #20
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Originally Posted by sudhir81 View Post
I doubt whether the higher distortion at 20kHz frequency is audible. Given than the graph of power distribution versus frequency is logarithmic in nature, at 100W the power upwards of 15kHz is going to be under 1W.
That is not the correct way to interpret the amplifier distortion versus power output graph. Anyway, my point is about the rising distortion above 1k Hz, not just at 20k Hz.

I am not sure where you get your info about power distribution - music does not follow a logarithmic power relationship, it's more like a band concentrated between 100Hz and 2kHz, with power dropping off on each end. This has been documented in published literature and on at least one DIY site I know of...

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