Addendum added to the text January 13, 2025 - scroll to see -
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Addendum January 13, 2025
LOL -- a reader emailed and wants more signal power -- and asked whether they could just swap in a pair of medium power BJTs like the BD139-140 to get more output power. Sadly no. I have a higher power version of this amp in a receiver called Regen 5 plus one other on this Blog. However, there is another way to get more power without greatly increasing the parts count:
Above — My new Popcorn PA version that goes to 700 mW clean signal power without adding a ton more parts. The finals = a TIP 122 &127 pair. 2 more diodes get added to the bias stack to forward bias the 2 pairs of emitter followers. To avoid a big voltage drop, I moved the 22 Ω resistor outside the TIP power followers on the DC positive rail since these BJTs draw much more current than the basic version of this audio PA. Further, a 470 µF AF bypass capacitor gets added to filter the output pair on the positive DC rail.
The TIP 122-127 are low voltage complementary power Darlington transistors in a standard TO-220 package giving a current gain of ~ 1000. They only cost ~ 60 cents per transistor even in Canada with our nearly worthless Canadian dollar. A BD139-140 transistor usually cost 25-30 cents more.
Above — Schematic and pin out of the TIP 122-127.
Above —DSO screen capture at about the maximum clean signal power ~ 700 mW.
Above — FFT screen capture with the AF amp driven to 700mW. The harmonic distortion is not as low as the original audio PA shown in the video. However, this is the compromise of using the TIP Darlington transistors + easy-peasy diode biasing scheme with 4 diodes. The better way to bias the power followers is via a transistor like I did in the audio PA Regen 5 back in 2015. Regardless, the TIP pair are a compromise compared to 2 separate BJTs pairs with optimum circuit design.
Still, to, the harmonic + crossover distortion will still be on par; or better than most of the audio amps you see in many lower power consumer electronics -- or a typical home brew Ham /SWL radio receiver.
Thanks for the email. If you have a question,or request, please email.
My readers are the best! Popcorn Todd
nice video quide Vasily...Dinos 73
ReplyDeleteThanks Dinos. I appreciate that you take the time to comment. It seems that few people do anymore. Best to All ! Popcorn Todd
ReplyDeleteTIP122 and TIP127? thanks, the other night I was looking through some of your files for an audio amplifier, just to see, and saw your prior project with an NE5532 driving a 2N3904/6 pair. I'm not sure if I like how it turned out yet... but that's the point of trying it out. Alas I did it on perf board and it's a bit harder to rework--next version might have to go onto copperclad. I need to first build a clean 1kHz tone generator (almost anything followed by a good filter) and then see if my Picoscope can give good FFT's. I don't trust my ears farther than I can throw them.
ReplyDelete[On mine I elected to do a bridge drive so as to avoid the output coupling capacitor. I have not done an A/B with LM386's to make sure it's "musically" correct for my ears. I think I'll order some of these TIP transistors first, then go from there.]
[FWIW using LM386's in a bridge wasn't too hard but it's more work than it's worth, trying to avoid a 470uF coupling capacitor, as it uses far too many parts! but it was fun.]
Big thanks for your comments! Great to see builders experimenting with audio PA amps! Bridging the LM386 has become popular in the past 2-3 years - - and I have only done that once to an LM380 pair many years ago. Perhaps one day, I'll look at it with the 386.
ReplyDeleteIn my view, the best way to avoid an output cap -- and to get more power -- is to use split DC supply such as +/- 15 volts. I did this in all my guitar amps. Bridging amps is so car audio ( where they are limited by the single supply car battery) is very cool. Of course , modern design switch MOSFETs finals in class D and such ---- plus usually involve some DSP .
Current powered audio speakers are nothing short of amazing -- but cost $$$. It's nice to make classic, simple designs with home brew. Thank you and Best!!!
I like the idea of split supply, and should resort to that. I've been mostly interested in ham radio, hence using +13.8V.
DeleteOn my bridge LM386 I resorted to a matched pair of 2N3906 PNP's, matching first beta and then checking base-emitter diode voltage--albeit with a cheap multi-meter. First attempt had 1.8mV DC across the speaker, second was 10.8mV, second time I think I did less matching... but I did change the design too, that may have had an impact. Anyhow. I don't think I have what I wanted, as looking over the soldering, I just found a mistake... hmm. Still. It's a DC coupled diff pair to measure DC across the speaker, with heavy filtering, using that output to feed back to the LM386's input pins that are supposed to be AC grounded in this bridge drive.
I had measured about +/-0.1V DC error from 6 different LM386's, so I figured, +/-0.3V worst case matching error; with a gain of 20V/V not a lot of DC feedback signal is required. If the input to the amplifier was biased up by say 15mV and then the diff pair could generate around +/-30mV to feed back, that should find a point where DC offset is zero.
Once I got the filtering to a good and low frequency (not so trivial!), it isn't too bad--but shows the real drawback to the LM386: hiss and/or noise. For ham radio on the HF bands, you'd never notice, band noise would dominate. And it's not that bad into a speaker. But it's there and easily heard. Your 2N3904/6 doesn't have that, doesn't require matched transistors, and so far, has less distortion by a few dB. Not as much power; am waiting for those TIP's to come in to see what can be had.
I'm not sure how much further I want to go with bridge drive on LM386's; now that I spotted my error, maybe one more try before wrapping up. I think I am getting to about 1.8W out of LM386-1's on 13.8V (pushing way too hard) before distortion, that's a heck of a bump. The bridged 2N3904/6 was about 0.46W (Vpk^2/16 as I was using an 8Ω speaker).
Power something like that... at one point I got curious and my 8Ω speaker measures 8Ω at DC. At 100Hz I think I measured a 1Ω bump (and barely any audio) while at 10kHz I measured something like an 8Ω additional resistance with a bit of inductance starting to show up. I'm going to go back to that rabbit hole later, so I question my power findings, but one experiment at a time.
Fun stuff!