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Old 18th March 2017, 06:08 PM   #1
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Talking Darlington Amp

Hi, I own some TIP147-142 's and this is the schematic I build some time ago but it had some noise-hiss that i didn't like.
Today I was messing around in LTSpice and "modified " the schematic ( used NPN Long tail pair, I heard it's better than PNP and i have 20x 2n5551 without complementary so I can use them )
and in the end it looks very simmilar to mr Rod elliot's p3a.
Would it be worth making it ? at least for fun / to see how it performs, take a look.
- Cheers, Bruno.
Attached Images
File Type: png 190w-rms-darlington-transistor-amplifier-car-amp.png (18.8 KB, 367 views)
File Type: png TIP142-147-2n5551.png (59.8 KB, 367 views)
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Old 18th March 2017, 06:13 PM   #2
Mooly is offline Mooly  United Kingdom
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Sure You learn much by building stuff like this.

Are you using reasonable models in the simulation for the TIP's or just standard transistors models ?
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Old 18th March 2017, 06:39 PM   #3
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i have made a " darlington " out of BD139 and 2sc5200 and their complemetary to test , now i used standard , I dont have ltspice models for TIP142-TIP147 and i didnt searched the web for them , but I will ..or , have you models for them ? Thanks Moly
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Old 18th March 2017, 06:40 PM   #4
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Also , is it necessary or would help if I had two small 50-100p caps on the TIP's B-C ? . In lt spice there are no osscillations .
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Old 18th March 2017, 06:47 PM   #5
Mooly is offline Mooly  United Kingdom
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Not sure on the caps.

There are Darlington models already in LTIV If you look in the examples there is a PA amp which you can resave under another name and then modify.

Try this.
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File Type: jpg Capture.JPG (290.6 KB, 332 views)
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File Type: zip Darlington Power Amp.zip (5.9 KB, 21 views)
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Old 18th March 2017, 06:53 PM   #6
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Thank you.
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Old 18th March 2017, 07:44 PM   #7
Mooly is offline Mooly  United Kingdom
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You're welcome
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Old 19th March 2017, 06:24 PM   #8
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What is the " optimum / recommanded " current across the LTP ?
In simulations almost 1mA per transistor works good , 5mV dc offset ( I dont think in reality i can obtain that low ?! )
Also the VAS at 3.6 mA.

The 240 ohm resistor ( R2 ) is a pot in the actual PCB , Should i connect the multimeter on the output and just adjust for lowest DC offset possible and that would be ? or I need to connect on the LTP emmiter's and measure mA ?
Thanks.
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Old 19th March 2017, 07:04 PM   #9
Mooly is offline Mooly  United Kingdom
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Your VAS current is a major limiting factor with your design, particularly with that 100pf cap slugging the stage. Reducing the 4k7's to say 1k5 would give you around 6ma (and you could go a little higher still with those low supply voltages.

Also you will find in practice that the amp is much better behaved if you reduce the feedback factor by reducing the 1k.

The LTP currents are fine for what it is. The 240 ohm can be tweaked for minimum offset and you measure this directly across the speaker output.

Another possible issue is bias stability... it really needs a vbe multiplier in place of those two diodes.

This shows the squarewave response at 25khz with 6ma VAS current and the 1k now at 220 ohm. Second picture is the original values.
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File Type: jpg Capture.JPG (284.0 KB, 218 views)
File Type: jpg Capture2.JPG (283.7 KB, 205 views)
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Old 19th March 2017, 07:42 PM   #10
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First time i made the schematic in LTspice there were 3.3k resistors but the DC offset was to high and the quiss current too i think ( can't remember why i increased them )
Here are some FFT's with the 4.7k resistors ( in square wave they looked weird - the ones you posted , but why should i care about square wave if the signal is sine wave ? )

It was a bad connection ! now it works fine even with 1.5k ,
Soo , I use 1.5k instead of 4.7k ? for the was to be 6mA or so.
Attached Images
File Type: png FFT-1Khz-1K load-2V out.png (125.2 KB, 48 views)
File Type: png FFT-1Khz-4Ohm load-2V out.png (127.1 KB, 31 views)
File Type: png FFT-1Khz-8Ohm load-2V out.png (119.5 KB, 19 views)
File Type: png FFT-4Ohm load-1khz-15v out.png (116.4 KB, 17 views)
File Type: png FFT-8Ohm load-1khz-15v out.png (111.4 KB, 17 views)
File Type: png Screenshot (169).png (73.7 KB, 38 views)

Last edited by xXBrunoXx; 19th March 2017 at 08:05 PM.
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