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Old 3rd April 2002, 11:25 PM   #1
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Default Here's a high-power amp 4 u guys

This schematic is based loosely on some aspects of several well respected designs, along with some of my own additions. It's good for over 300W with proper heatsinking. The output devices are MJ or MJL21193/4 pairs, and 2SD669/2SD649 should be used for Q5 and Q6. It has been tested with 55V rails and gives an easy 300W into 4 ohms with 0.01% THD at 20KHz. The bus line in the middle is for later addition of multi-slope current limiting circuitry which I will post shortly.

Enjoy...
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Old 5th April 2002, 08:08 PM   #2
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I would suggest some Base resistance in series
with Q10 to prevent collapse of the LED voltage
reference when Q10 saturates on negative clip.
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Old 5th April 2002, 08:17 PM   #3
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Could this be why I had slightly non-symmetrical limiting with the VI limiting circuitry (not shown)? I was beating the schematic for hours trying to solve that one with no avail.
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Old 7th April 2002, 05:38 PM   #4
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650mW is a bit much for Q4 and Q10, I would have used the 669/649 pair here.The beta of the 669/649 is drooping at 0.5A in the drive stage and 2.4W is a bit much for them too.I would use the MJE15030/15031 .C1 should be a non-polar type with a film bypass and a pair of diodes, C2 needs a film bypass.No supply bypassing is shown.C3 and C6 would benefit fom being sacked film types.Some local feedback in the voltage gain stages would be nice too.
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Old 7th April 2002, 06:26 PM   #5
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Q7 and Q8 (pre-drivers) are not dissipating 2.4 watts... they are fine as 649/669's. There are two stages of power BJT following them to keep the dissipation low (see below). The power supply is not shown, you are right (along with supply bypassing and assorted fuses, ect). Q4 is ineed what you recommend. The model name somehow didn't make it into the BMP file =).

For example, if the predrivers are conductiong the suggested 0.5A, then the next stage in the CF section would conduct 0.5*20 or 10A, and the EF output stage would be roughly 10*20 or 200A. Highly unlikely -- this is also using worst case Hfe values. I don't personally agree with the use of film caps although it is obviously up to the reader to use whatever he/she pleases.



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Originally posted by djk
650mW is a bit much for Q4 and Q10, I would have used the 669/649 pair here.The beta of the 669/649 is drooping at 0.5A in the drive stage and 2.4W is a bit much for them too.I would use the MJE15030/15031 .C1 should be a non-polar type with a film bypass and a pair of diodes, C2 needs a film bypass.No supply bypassing is shown.C3 and C6 would benefit fom being sacked film types.Some local feedback in the voltage gain stages would be nice too.
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Old 8th April 2002, 05:37 AM   #6
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Maybe you could update the BMP for us.Q4, the second voltage amp is unmarked and thus assumed to be the complement of it's current source;Q10 identified as a 2N5551.Assuming a red LED the curent would be 1.2V/100R=12mA and 12mA@55V=660mW.The outputs were identified as MJ21193/94 and are rated for a DC beta of 25@8A, less at audio frequencies. 300W/4R=12.25A, or roughly 6A per device, 12.25A/beta of 25=0.489A driver current.Since the pre-drivers Q7 and Q0 are labeled 2N5551/5401 it is assumed the drivers are the mentioned "2SD669/2SD649 should be used for Q5 and Q6"(SIC, 2SB649).A class AB amp with typical bias levels is about 60% efficent (the often quoted 78.5% is for pure class B with a regulated supply.National Semiconductor has a good tutorial under the LM391N and suggests using the 60% figure ), so 300W*.4=120W in the outputs, and 120W/beta of 25=4.8W, and 4.8W/2=2.4W per driver.With a really good heatsink the D669/B649 will do it but the collector current is higher than the beta is specified at.
"I don't personally agree with the use of film caps " I'm hard of hearing and can still hear a HUGE difference between a film and an electrolytic cap in the signal path.
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Old 8th April 2002, 05:42 AM   #7
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ok im gonna post a new pic, the more i look at that one I see how crappy it is. some of the devices are labelled wrong or missing so hold the line im gonna fix it up.
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Old 8th April 2002, 06:18 AM   #8
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I was using 2n's for most of the simulation since it doesn't make much difference other than dissipation which I was ignoring during the design. Hopefully this image clears it up a bit... some of the reference designators were obscured by wires so this color image should help.

All black BJT's are MJ/MJL21193/4

Had to zip it since this forum doesn't allow large images. I made it bigger to show better detail and the rest such as the VI limiting circuit. For the VI circuit, if you compare the two busses, top goes to top, bottom goes to bottom, ect... I didn't want to include the circuit in the middle of the amp schematic. Hope it makes sense.

The multislope limiter reduces maximum output current to a few amps under short circuit conditions to prevent overheating and/or reduced amplifier life. Limiting starts around 2 ohms (purely resistive), although it may become evident with highly reactive 4 ohm loads, but only near full power.

I tried using film and didn't notice a thing - didn't see any difference on the AP either. What kind of difference do you notice?
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Old 15th April 2002, 09:16 AM   #9
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Good job McAnally. How about some simulation or real life measurements.

Any ideas, anyone on how to upgrade this design to deliver more power? I am looking for a hifi, high power amp that can deliver 500watts into 8 ohms.

Thanks for any input.
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Old 15th April 2002, 05:16 PM   #10
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more output devices is nearly all you have to add. Since there are two pre-driver stages it can readily handle the extra base drive for the added output devices (MJL21193/4). The limiting circuitry measures the current through only one pair only, so adding more pairs shouldn't affect its operation, it will only increase the maximum power output with lower impedances.

If you want to raise the supply voltage for more power at 8 ohms there shouldn't be a problem since all chosen BJT's are good for up to 100 volt rails or so..
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