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Chip Amps Amplifiers based on integrated circuits

LM3886 + output transistor
LM3886 + output transistor
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Old 17th February 2009, 12:06 PM   #1
luixssj3 is offline luixssj3  Italy
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Default LM3886 + output transistor

Hi to all!

I would ask you what you think about adding output transistor to a LM3886 to have more power, like the old TDA sch I've added....

What is the pro/cons of the topology?

Thanks to all!
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Old 17th February 2009, 12:59 PM   #2
AndrewT is offline AndrewT  Scotland
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you would be better using the chips that National has released to do just that.
See threads on lme498x0
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Old 18th February 2009, 07:34 PM   #3
luixssj3 is offline luixssj3  Italy
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Many thanks!

Very interesting IC!!! But a little difficult to find in Italy
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Old 18th February 2009, 10:18 PM   #4
Minion is offline Minion  Canada
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You can also make an amp with discrete output stage from a TDA7293 100w DMOS chip , if you need more than 100w...

check the TDA7293 data sheet, there is also a PCB design in the datasheet....


Cheers
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Old 15th March 2009, 08:46 PM   #5
djoffe is offline djoffe  United States
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Default boosting the output current

thanks for the info about the lme49811, etc....I'll have to play around with those.

If you're looking for more current drive, you could do something like this. Please note...it's a concept drawing...some components are omitted...the basic idea is that at low output currents, most output comes from the national part, as output current demand increases, the output transistors kick in above about 0.4 Amps. You'll need to experiment some for stability considerations...just consider this as a suggested starting point...
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Old 16th March 2009, 03:24 AM   #6
star882 is offline star882  United States
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It would be very impractical to use analog for high powers. The thermal dissipation would become a big problem (watercooling needed) and the power supply would be very expensive. A pure or hybrid digital would be a better solution.
TI has a pure digital chipset (TAS5261 power stage + TAS55xx digital driver) good for over 200W. Use a discrete power stage and it can be even higher. Only small heatsinks would be needed
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Old 27th July 2017, 01:37 PM   #7
penjoseph is offline penjoseph  India
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LM3886 + output transistor
I know I'm bit too late for this party from 2009. Anyway better late than never !

As an experiment I first tried to couple output transistors NJW0281 NJW0302 through 5W resistors to TDA7293
1) in the output stage
2) in the +ve & -ve rails

Used toroidal Transformer 25 0 25 600VA. Output DC at +38 0 -38 with 10A fuse on both rails

The circuit works fine only with TDA7293 i.e.: if the output transistors are not connected. As soon as the transistors are connected the circuit goes into oscillation with loud hum blowing the fuses. If the one fuse fails first the TDA7293 just explodes !

After blowing up 5x TDA7293s I decided its not my cup of tea.

Then I tried with LM3886 with output transistors in the +ve & -ve rails but the circuit was still oscillating sometimes when connected blowing the fuses. The good thing was nothing explodes because of the internal protection of LM3886 - which was a relief ! Then it dawned on me to replace the output transistors NJW0281 & NJW0302 with 2SC5200 & 2SA1943. Voila ! the circuit works however the issue was thermal dissipation due to higher quiescent current of LM3886.

Shifted the circuit to LM3876 which has lower quiescent current. Used LM3886 XY ebay amp board with output transistors in the +ve & -ve rails. At high bass levels the LM3876 dissipates much more heat w.r.t to output transistors.

But again for Subwoofer duty by altering the power resistors one can reduce the thermal dissipation at the cost of bandwidth /slew rate. Managed to even out heat by increasing the power rail resistors & adding a output resistor for LM3876

LM3876 / LM3886 with 2SC5200 & 2SA1943
Click the image to open in full size.

Left LM3876 on XY pcb - Right LM3886 on Wintek pcb (local)
Click the image to open in full size.

Last edited by penjoseph; 27th July 2017 at 02:05 PM.
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Old 28th July 2017, 08:03 AM   #8
penjoseph is offline penjoseph  India
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LM3886 + output transistor
Circuit of XY LM3886 -
Click the image to open in full size.

Modified LM3886 \ LM3876 with Transistors
Click the image to open in full size.

Last edited by penjoseph; 28th July 2017 at 08:18 AM.
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Old 28th July 2017, 09:03 AM   #9
jan.didden is offline jan.didden  Europe
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The crux is of course, what is the value of R2, R3.
What is the purpose of the 0.1ohms resistors? They don't seem to do anything?

Jan
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Old 28th July 2017, 09:16 AM   #10
penjoseph is offline penjoseph  India
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LM3886 + output transistor
0.1Ω resistors on the Collectors are fusable ones to partly protect the transistors/speakers from burning out.

If thermal dissipation is not a problem:
Use LM3886 / LM3876
R1 = 0.1Ω 5W
R2 & R3 = 2.2Ω 5W

I first had 0.1Ω 5W resistors on the Transistor collectors. With those resistors & LM3886 the Transistors conduct so much current that on heavy speaker excursion that both glows orange ! (only both no other resistors). That's why I changed it to 0.1Ω 10W in the circuit diagram.

If thermal dissipation is a problem:
Use LM3876.
R1 = 15Ω 5W
R2 & R3 = 10Ω 5W
However bandwidth will be restricted to subwoofer use.
In the XY board the feedback \ sync resistor of 22K is connected before the output shunt resistor. It should be altered to after.

The resistor combination is such that on idle the transistors should not dissipate any heat. Only the Chip-amp should.

Last edited by penjoseph; 28th July 2017 at 09:33 AM.
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