Discovered a way to "erase" the crossover distortion in class B amps. - Page 1
Forum section: electronics / projects

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 05, 2025, 09:58:08 am
content: The class B amps, as are the majority of push pull solid state amps, get the output impedance increase at the crossover region. To cancel the nonlinear transition I parallel with another output stage of small power linking with a capacitor. The capacitor will advance the phase of the current in the auxiliary output so that when the primary is at the crossover, the auxiliary has already passed that region and its output impedance is lower to overtake the loading.Bellow is a simulation in class C to observe better the behavior although non biased outputs do not erase in perfection as in class B.In real world, I used 2X TDA7293, where the second with unity gain output paralleled with a 10uF switched.The scope pictures shows stand alone crossover distortion and the second with paralled, sorry it's noisy.See the pictures herehttps://www.audiosciencereview.com/forum/index.php?threads/tda7293-exempt-of-crossover-distortion.66453/

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 05, 2025, 10:28:17 am
content: I am unable to attach pictures, I don't know why?

author: jwet
author_rank: Frequent Contributor
timestamp: October 07, 2025, 03:53:12 pm
content: You have "reinvented" a technique that has been used for a long time but in different ways. Your capacitively coupled techniques has some downsides in phase distortion but certainly helps. There are various names for this kind of stuff in the literature. If you read Cordell or Self, you'll find this stuff. There is another class of amps that was called current dumping that actually used a class A and class C in a hybrid configuration. It faded into hisotry.Many high quality Amps (Luxmann is one I recall) use class A type biasing for the first few watts and then a class A/AB bias at higher power. Its possible to do it all in one set of output transistors by having the predrivers feed the big power devices and also go to the speakers directly. The idea is that the first 5 watts of an amp is what really matters and then all the big headroom and punch don't have crossover needs.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 08, 2025, 12:11:37 am
content: Thank you for giving attention to the subject.Quad current dumping or Technics class AA are low power class A amps assisted by class B to over power.I don't have any class A in my circuit.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2025, 12:42:07 am
content: Quote from: Kokoriantz on October 05, 2025, 09:58:08 amWhen properly biased into class-AB, the output impedance *decreases* because both transistors are contributing and transconductance doubles. Between selecting the proper emitter ballast resistors, and bias voltage or idle current, the variation in transconductance may be minimized.Local feedback around the output transistor pairs should improve the situation, however it is very difficult to get any improvement on properly biased emitter follower outputs.

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 08, 2025, 04:08:00 am
content: When the transconductance doubles in crossover region, this means the amp is in classAB. The distortion is much higher than classB where it decreases less than 50% in crossover region.

author: MariuszD
author_rank: Regular Contributor
timestamp: October 08, 2025, 06:21:32 am
content: QuoteIn the simulation, smoothing the zero-crossing point seems to make a big difference, but if you look at the distortion waveform (output with first harmonic cancelled), you can see that the improvement in linearity is not significant.Properly biased Class B amplifier is much better than this improved class C.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2025, 06:36:59 am
content: Quote from: Kokoriantz on October 08, 2025, 04:08:00 amThis discussion will be difficult if we do not agree on the definitions of class-AB and class-B. Douglas Self defines class-B as:My definition of Class-B is that unique amount of biasvoltage which causes the conduction of the two outputdevices to overlap with the greatest smoothness and sogenerate the minimum possible amount of crossoverdistortion.And then he defines class-AB as class-B with a higher conduction angle, but in the same paragraph describes it as the same thing with conduction between 50% and 100%. His "blameless" design has conduction of 100% because the fixed bias voltage will always make the "off" transistor slightly on, unless Vbe saturation or emitter ballasting of the on transistor reduces it?

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 08, 2025, 06:39:36 am
content: The classC is only to show visible improvement. Look at the TDA7293 without and with scope pictures.

author: mawyatt
author_rank: Super Contributor
timestamp: October 08, 2025, 01:24:33 pm
content: We always thought of Class B for a complementary output pair as a conduction angle of 180 degrees, thus both devices are never conducting together. Class A/B is when the conduction angle is greater than 180 degrees and over a small range (Class A part) both devices are conducting. Class C is when the conduction angle is less than 180 degrees and often employed in older RF transmitters where the output is "tuned" to the desired signal frequency to reduce harmonics.Do recall a few amplifiers which used a separate Class A in the middle of the waveform and Class B/C for the rest of the waveform cycle, these were the "ultra-linear" power amp types.Best

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 08, 2025, 01:59:41 pm
content: Looking at the schematic in the first post, it's not clear what's being accomplished.... basically at low frequencies T3 and T4 do nothing for the output. At high frequencies T3 and T4 will act in parallel with T1 and T2, giving a lower output impedance and maybe lower distortion. But why would one do this, as opposed to just connecting T3/T4 directly in parallel with T1/T2 so the benefit is gained at low frequency as well?Quote from: David Hess on October 08, 2025, 06:36:59 amWho is Douglas...?Quote from: mawyatt on October 08, 2025, 01:24:33 pmAgreed, class B is where conduction angle is very close to 180, if there are two output transistors in a push-pull configuration then their conduction should not overlap significantly.

author: moffy
author_rank: Super Contributor
timestamp: October 08, 2025, 03:03:37 pm
content: Quote from: mtwieg on October 08, 2025, 01:59:41 pmFirst ran across his articles in Wireless World, a pioneer in low distortion power amp design, one just has to Google his name well worth it.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 08, 2025, 05:30:42 pm
content: Quote from: Kokoriantz on October 08, 2025, 04:08:00 amThis is a bit misleading at best.First of all, voltage gain error from finite transconduction is a bigger change to decrease 50% than to double. The gain error is g_M / (1 + g_M). The change from a 50% decrease in transconductance is equivalent to infinite g_M.More importantly, it's the behavior of the distortion. Most distortion is like compression which is large signal distortion. The fractional distortion decreases with smaller amplitude. Crossover distortion in a class B amplifier is small signal distortion and actually gets worse at low amplitude. The advantage of class AB is that you can mostly eliminate the small signal distortion. The distortion onsets at a letter amplitude which and significantly decrease the worst case _relative_ distortion.Finally while I understand Doug Selfs approach, generally class B is considered 180 degree condition angle for each output device. And I get the idea that there is a sweet spot near there with minimum distortion. But generally biasing has moderate variation with temperature and process. If you under bias the distortion gets much worse very quickly.Class AB give you much more stable biasing as well as the lower small signal distortion. With emitter ballasting and feedback you can reduce crossover distortion further. That's why most desigs for low distortion are class AB with significant quiescent current.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: October 08, 2025, 05:57:47 pm
content: Doug Self has several books related to audio-electronics.And this is not audiophoolery. His books are technically very solid, with detailed technical advice.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2025, 06:19:49 pm
content: Many years ago I designed an audio power amplifier like nothing published even now. I used current mirrors to drive a common emitter output stage with current feedback, so something like a complementary current feedback amplifier, but with a current feedback differential input stage. It worked, but at the time I had no way to evaluate distortion.It could be considered class-AB but the idle current flowed through the transistors continuously, probably to no advantage. It achieved a 600 kHz bandwidth while using 2 MHz output transistors. I might revisit it some day, but would change it to use current feedback only in the output stage as an inner feedback loop, something like the output stages with gain that Douglas Self described.Quote from: mtwieg on October 08, 2025, 01:59:41 pmHe wrote a couple of the better books about audio circuit design. He might be famous for his "blameless" audio power amplifier design.https://www.amazon.com/Small-Signal-Audio-Design-Douglas/dp/1032366257https://www.amazon.com/Audio-Power-Amplifier-Design-Sixth/dp/0240526139https://www.amazon.com/Design-Active-Crossovers-Douglas-Self/dp/1138733032And with Peter Baxandall, maybe you know of him?https://www.amazon.com/Baxandall-Self-Audio-Power-Classic/dp/9490929034Bob Cordell is another audio design author, but I find his book on power amplifier design to be less thorough.https://www.amazon.com/Designing-Audio-Power-Amplifiers-Cordell/dp/1138555444

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 09, 2025, 02:53:05 am
content: To get crossover distortion free class B output stage is to drive with current source a single EF complimentary.Such configuration did exist in late 60's in German car radios with AD161/162, in hifi, the last model I know was Dual cv12 2x4W music with AC180/181, same price tag as its precedent 2x20w rms.I first tried it with C5359/A1987, fake but matched pair hfe 160,161 as it requires high gain transistors. See the picture bellow.To be reproductive, I made it by 3 pairs of D44h11/D45h11 and driven with a mosfet VAS IRF510 +50ma CCS. These very low cost TO220 transistors have hfe of 300 in STmicro DS but I got 5 pairs from Ali original ONsemi hfe of 155 to 165 for $1.5 free ship. I have the PCBs done $4 for 5pcs.I'll post a video on YouTube explaining how to built one.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 09, 2025, 07:42:10 am
content: Me too. Its shame that WW disappered from the shelves.I spent my meagre pocket money on it.I learned a hell of a lot. Scroggie any one?Doug Self always replied to my, often naive, letters (co ww). Nice chap.

author: moffy
author_rank: Super Contributor
timestamp: October 09, 2025, 08:12:54 am
content: Quote from: Terry Bites on October 09, 2025, 07:42:10 amIt was a sad day when Wireless World passed, it was innovative in design, authorotative, with some of the best contributors around, a unique publication.

author: nfmax
author_rank: Super Contributor
timestamp: October 09, 2025, 08:23:23 am
content: I knew Peter Baxandall slightly, back in the early 80's, through his contacts with P.J. Walker at QUAD: we met at AES meetings in London from time to time. I have a copy of a letter he wrote describing his 'flux-nulling' design of a transformer-coupled balanced line receiver he developed for KEF audio. Rummaging around, I found a copy of his obituary notice in, I think, the IEE magazine (as was)Scanned copies of Wireless World are available on the invaluable worldradiohistory.com website. It had its ups an downs, but WW was one of the key factors that me into this business, 'when I were a lad'.

author: peter-h
author_rank: Super Contributor
timestamp: October 09, 2025, 11:25:25 am
content: Quad used this "little amp feeding the output of the big amp" technique in their, IIRC, 405 (current dumping) amp, in the late 1970s.I used to build 500W and 800W audio amps back then, using about a dozen 2N5239 transistors, 6 on top and 6 on the bottom. In front of this lot was a LM318H, with overall feedback, which was so fast that the THD was below 0.01% at full power, all the way to 20kHz1978... The design was based on a circuit diagram of am amp made by BGW, published in some US mag with no component values.

author: moffy
author_rank: Super Contributor
timestamp: October 09, 2025, 11:41:24 am
content: Quote from: peter-h on October 09, 2025, 11:25:25 amThat's a pretty impressive spec of 0.01%@20kHz@full power. Between the heatsinks and the transformer it must have weighed a ton.

author: peter-h
author_rank: Super Contributor
timestamp: October 09, 2025, 11:53:54 am
content: You can see the supply rail voltageThe above one was a mono amp, 500W, One big transformer, two huge capacitors. Yes probably 20kg. The case is 3mm aluminium panels, on a 10x10mm aluminium frame.Later I built an 800W mono one for a friend who ran a mobile disco. He gave me £500 for it.Then I built several 2x400W stereo amps.There was a circuit which detected the DC level on the output and opened a relay if it looked like an output transistor(s) blew S/C. The speakers, Electrovoice Eliminator, cost as much as the amp!Fun days

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 10, 2025, 02:17:58 am
content: This is a simulation of the circuit I described above. It has 0v bias and running at 50hz in open loop. You can notice it is exempt of crossover region. The bias voltage, not current is necessarily here only for charging the capacitors of the transistors in less time for higher frequencies. 1v is sufficient for lowest distortion at 20khz with 32ma current bias.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: October 10, 2025, 01:49:42 pm
content: @KokoriantzAre you planning in actually producing and selling your designs?

author: Kokoriantz
author_rank: Frequent Contributor
timestamp: October 10, 2025, 05:53:43 pm
content: I am retired EE, 68, son of TV repair, and my ears are no more valid to design audio equipment. To end in grandiose after 60 years of passion, I gathered what best I created, 6amps (2 classD, 2 chip, 1 classA, 1 classB) and a universal 16 bit DAC to plug in multiple type of chips, will publish on YouTube and make all PCBs available at cost price from JLCPCB.