current feedback amplifier org

428

05:23:59

The first current feedback amplifier I made! Hopefully I'll have the time to build it in real life and test it. It looks pretty legit :) I'll use1st 4 transistors 2N3904/3906 current mirrors BD139/140 pre-drivers A1046/D2058 drivers TIP/41c/42c Outputs TIP35c/36c don't know if it will work. UPDATE I builded the amp it works! How ever when you plug in the amp for a second it pulls 200-300mA and then suddenly drops and goes in class B mode. I'm assuming it needs biasing or other transistors that fit also the d2058 & A1046 pre-drivers do get worm so plan a heatsink for them. UPDATE I'm stupid, when the amp gets worn the distortion goes away. The amp doesn't get hot witch is weird considering I have a small heatsink. Btw if you are considering 4700uf filter caps on the power supply don't otherwise you get a bit of hum try With 6800/10000uf caps. I'm testing it with a 2x27.7V power supply and the transformer is in series with a 100W lightbulb. UPDATE NOPE the distortion comes when the volume is a bit above the pre driver region. At low volume barely hearable you get clean sound a bit upper and you get class B distortion and above to max it's gone. UPDATE again Screw 2n3904/6 they will overheat BD139/140 will be the deff amp as well. LAST UPDATE through the hole project I was an idiot I finally fixed it, It's far from perfect but at least it works! oh and sorry for bad grammar you know just in case if the grammar natzies come. Have a nice build cheers ;) Btw I added 4 output transistors for 180W 2 channels will run from a 550VA Transformer 2X36V AC filtering caps for 2 channels will be 3X 4700uf 63V or 6800uf per rail if you have hum. Biasing transistor on output transistors heatsink. I added it because there was thermal drifting.

published 8 years ago

thebugger

8 years ago

The amp is good, but if I may, I'd make a few suggestions. Keep all currents through the differential amplifier low. These devices heat up and when it's a discrete version there isn't any thermal coupling and that'll cause your output to drift too. Also add a low pass filter at the input, and also one in the negative feedback to further restrict the upper bandwidth limit. You did good by keeping the overall gain low (11). I'd push it slightly down to let's say 7-8. This reduces the output thermal drift and improves the crossover distortion. Ah and speaking of crossover distortion replace one of the two diodes with a 330ohm resistor to establish some idle current through the power transistors. Even in class B it's always good to push them slightly in the class A region. Even a few mA of idle current should do the job ;)

thebugger

8 years ago

That diode should be mounted on the power transistor's heatsink, as it'll act as a temperature compensation. When the output wants to pull the DC offset in one direction due to thermal drift, the diode will pull in the other, thus stabilising the regimes. In reality far complex circuitry is used with transistors or thermistors, but that should do the job as well

hurz

8 years ago

The diodes are not needed. There is no ush pull stage directly following which need crossover elimination. Replace it with an ntc to get a proper op at outstage. Good luck for a stable OP over temperatur. BTW, the current feedback is a standard voltage feedback.

AmplifierFAN

8 years ago

For the deff amp I'll use BD139/140 so it won't be a problem. This amp follows only the topology so it's not that great :/. Thank you allot for the feedback appreciate it!

hurz

8 years ago

What looks very ugly are the four emitter follower. Most ugly are the two at imput which get DC bias from unknow input source! The scecond pair before power stage seems totaly waste of material, they are at least inside the regulating loop, which the first pair is not. Everything outside the loop will dominate the THD. All in all I can not recommand to build this amp! Or can you give a good reason whats better to normal amps?

AmplifierFAN

8 years ago

http://www.andiha.no/articles/audio/dcfig/dcff15.gif

hurz

8 years ago

And you see the differences and hear what I said? Emitter follower proper biased at input by Ra. Center emitter follower are voltage to CURRENT controled source. The diodes are completly useless. Right?

AmplifierFAN

8 years ago

I'm not a expert I'm a student i experiment and it will take me a while to understand fully what you mean. Btw yes i know they are usles better for me when i build the real thing tomorrow

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