Class A stereo amplifier - Page 1
Forum section: electronics / projects

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 01, 2018, 01:52:41 am
content: HelloI wanted to share my project of class A stereo amplifier.The design was an experiment at first but later I transformed it into a fully working stereo amp.KD502 is a vintage Czechoslovakian power transistor.60V,20A, 150W NPN transistor.Similar to a western 2N3772 but with better gain and ~2MHz Ft.In reality it seems to be able to handle much higher power, the case is fully copper (which gives low 0.866K/W Rjc).Power is mainly limited by 155°C Tjmax (lower than most modern transistors).The die is huge.With 200°C Tjmax it would be a 200W transistor.BD139 is just a medium power NPN transistor used for gain boost.Nothing fancy. Doesn't require cooling(bit warm).BDX18 is a MJ2955/2N2955 under a different name.This one was made by Hungarian TUNGSRAM back in the Eastern Bloc days.Design is as following:The symmetrical +-15V is mainly for opamp.Rails are fully regulated, +30V is created by uA723+BD139+BDX18 combo(BDX18 + BD139 create NPN Sziklai pair).+15V is barely loaded so I used a simple 7815.6800uF 50V capacitor, 30V transformer, KBPC5010 rectifier.Why PNP BDX18 for regulation ?Collector is case, this way I don't need any isolation.Isolation increases thermal resistance which is a big problem for class A amp.Case is +30V potential.10A fuse is used for short circuit protection.Current limiting had to be removed in order to allow isolation-less connection.Why the capacitor on output ?I had trouble keeping the DC offset low enough.Opamp offset gets amplified which creates too large DC offset.I tested DC coupled output but the distortion was horrible.Fan cutout is not pretty. Simply drilled a lot of holes in a circle.Will have to correct this when I purchase some dremel-like device.Fan reduces the temperature of output stage.Without the fan it was 95/100°C first/second transistor (case temperatures).Fan lowered this to 75°C/80°C which is much more comfortable.BDX18 stays at 85°C, high but there's still bit of headroom.Improvised high power resistor may be noticed (floating radiator).Two 1W resistors in parallel, glued(thermally conductive glue) to a heatsink.This powers the fan(standard 12V PC fan) from 30V rail.Testing stage on bench power supply (DIY, 2x2N3055 + uA723,0-30V 0-6.5A).The backround noise is from camera."TO DO" list:-better fan cutout-new resistor for the fan-better power cord-VU meters (for looks)-possible bigger radiator for BDX18

author: John Heath
author_rank: Frequent Contributor
timestamp: February 01, 2018, 04:12:40 am
content: I had a similar idea but more brute force simple with a go big or go home design. 300 watts class A and cheap as dirt. Straight off the lines with no isolation transformer. Maybe you can use some of it if it works for you.

author: Gyro
author_rank: Super Contributor
timestamp: February 01, 2018, 11:38:28 am
content: I like the 'balls-out' engineering!You might find something of interest on the Class A amplifier site (John Linsley Hood design based...http://sound-au.com/tcaas/index.htmor Nelson Pass's site...http://www.firstwatt.com/articles.html

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 01, 2018, 06:09:45 pm
content: I've done something similar, but with power MOSFETS and replacing the 8 ohm resistor with a constant current sink. The sink can be really simple and have a pot so you can adjust the idle current. These simple circuits can do a remarkable job if you don't mind the inefficiency.

author: Zero999
author_rank: Super Contributor
timestamp: February 01, 2018, 06:49:36 pm
content: Is it really necessary to have a regulated PSU? It's quite likely the power supply rejection of the op-amp is sufficient.There's DC in the potentiometers, which can cause noise, when they're adjusted. It's better to AC couple signal through the potentiometers.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 01, 2018, 09:50:10 pm
content: I have played with something similar 20-25 years ago as well.AD audio grade opamp, MOSFET, current sink (passive or MOSFET in current source mode - with a few thermally coupled diodes in gate bias to stop it from running away, also a lot of paralleled J-FETs.)It's great fun and very rewarding when it actually works.You can almost convince yourself that it has pure sound but years later reality catches up and it belongs on a skip.Leo

author: John Heath
author_rank: Frequent Contributor
timestamp: February 01, 2018, 10:25:39 pm
content: Quote from: Conrad Hoffman on February 01, 2018, 06:09:45 pmYes and I am liking it. Constant current source not 8 ohm resistor. It is tempting to tweak the constant current source a bit for higher current at low voltage then lower at high voltage difference but hey it is a class A amp so leave it alone. Just make it a constant current source as you said. It will need a fan to keep it at a reasonable heat but that is the price if one wants to say true class A amp.

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 02, 2018, 03:32:03 am
content: Quote from: Leo Bodnar on February 01, 2018, 09:50:10 pmI did test it on my usual home theater system.Sound quality was very good.I also did a frequency sweep(full audio frequency range) to test for any noticeable distortion(8R resistor for load).Oscillograms were clear.I may mod the amplifier into standard Class A topology later.I just need a good way of radiator/amplifier case isolation.It's easier to isolate entire radiator from the case than to isolate the transistors.In early stage I used 2N3055 for voltage regulation which required the output stage radiator to be isolated.It was messy and there was trouble with mounting (radiator is heavy) so I purchased BDX18 just for this project.In general I prefer bipolar over mosfet. Less components needed for basic operation (two resistors for minimalistic class A amp, no work point stabilisation) and can operate on low voltages.TESLA KD502 output characteristics look like this:This transistor was used in audio amplifiers but in power supplies as well (it's high power after all).The opamp I use is LM833. It's similar to NE5532 in performance and uses the same pinout.Regulated PSU has noticable advantage.Some of the heat is moved away from output stage.With the PSU you have +30V supply, without you would have ~36V average.Few more volts times current and you get plenty of extra heat on output stage.Main filter cap(6800uF) shows ~4Vpp ripple, quite high if you ask me.Voltage drops is what keeps the BDX18 cool enough.Normally I would use 10000uF cap for such load but the electronics shop didn't have one.I had to use a trick in PSU in order to make it operate properly, uA723 has separate capacitor connected through diode.Voltage on this capacitor is not dropping as much which gives uA723 some extra voltage for Ube drops.I really like vintage technology. A part of my collection:TG70 were the only power transistors in Poland for quite some time.TEWA TG70 is germanium PNP, 10W,30V,1.5A, Ft=100KHz.It was even used in new devices in early 70's because early silicon power transistors had poor reliablity("fixed" by reducing their rating).Currently working on AB class amp using USSR germanium transistors.I have MP10(NPN),MP37(NPN),MP42(PNP).They are not complementary so I have to pick a good MP10/37 and pair it with a weak MP42.MP42 has higher gain,higher current and slightly higher power (150mW vs 200mW IIRC).USSR had a long story of germanium.My 1992(USSR collapsed in 1991) C1-107 oscilloscope still has one or two germanium transistors in deflection circuitry.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 02, 2018, 04:13:29 am
content: The first transistor radio made TR1 in 1955 employed a class A output stage running on battery power. Almost a carbon copy of a 5 tube radio of the same time. I imagine the battery did not last that long but then again how long will a cell phone go on one charge.

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 02, 2018, 04:42:35 am
content: Those are NPN ?In Poland there were no NPN transistors until the introduction of silicon transistors in 1967.Mesa transistors.Push-Pull amplifiers at the time used transformers or imported NPN transistors."Tramp" portable radio, Japanese transistors were used for radio frequency stage.TG5 is a small germanium transistor, 75mW,30V,10mA Ft=800KHz.Fast germanium transistors came with new technology.TG5 is Alloy-junction transistor.TG39 is Alloy-junction/diffused and has "big" 40MHz Ft, it replaced imported transistors in radio receivers.TG70 is an Alloy-junction transistor yet it was still in production in early 70's.USSR produced alloy-juction transistors pretty much until collapse.Polish engineers actually managed to design a PNP-only Push-Pull amplifier which had no transformer.It was a concept which was never actually built.Another example built and tested:Works but has unsymmetric output.http://www.forum-trioda.pl/viewtopic.php?f=8&t=15635&hilit=german+tewa+pnp

author: mikerj
author_rank: Super Contributor
timestamp: February 02, 2018, 11:29:31 am
content: I'd like to see actual testing to show the inefficiency does actually lead to improved performance over a good class AB design.

author: Gyro
author_rank: Super Contributor
timestamp: February 02, 2018, 11:43:18 am
content: I think you're talking a different performance, rather than improved... but one that sounds nice. The figures certainly won't say 'improved'.Personally I'm a fan of sounds nice.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 02, 2018, 12:08:16 pm
content: From my fleeting experience with this design it sounds really good at lower levels with music that does not have a lot of dynamic range but distorts easily and unpleasantly (unlike tubes) at  normal sound levels where cheapo AB keeps chugging along.If you don't crank it up it easily beats your average Yamakai. It is DIY built in one evening so it always sounds nicer than it measures.LeoP.S. I am probably a bit random but the facts are - I built mine, I thought it sounded awesome, 20 years later I think it's nothing special and it's waiting to be chucked out.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 02, 2018, 04:14:46 pm
content: Quote from: Leo Bodnar on February 02, 2018, 12:08:16 pmThe class AB , more B than A , with two transformers was so good for battery transistor radios that it lasted 25 years unchanged. Class AB is the best of both worlds. What happened to class D ? The switching strategy for audio is great theoretically but it never seemed to get off the ground.

author: Audioguru
author_rank: Super Contributor
timestamp: February 02, 2018, 04:26:45 pm
content: One earlier post said there was no visible distortion on a 'scope. Oh yeah? I can't see 2% even harmonics distortion on a 'scope but I can hear it. Less than 0.05% distortion sounds good.This amplifier like many vacuum tube class-A amplifiers produces plenty of even harmonics distortion at high levels that some people say makes it sound "warm" and "musical".I think that any kind of distortion is bad. Also I can smell the massive heat produced by this amplifier.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 02, 2018, 04:41:56 pm
content: There's a trick that happens when you use a power MOSFET and the circuit is set up right. Since the turn-on of the MOSFET will be several volts, the opamp zero output point isn't at zero. It doesn't go through its output crossover point at low signal levels, and by the time it does, it doesn't matter. I don't know about the design belonging in the skip. Mine had excellent measured performance and made a dandy headphone amp or office amp.

author: cvanc
author_rank: Frequent Contributor
timestamp: February 02, 2018, 05:04:00 pm
content: Great to see all this homemade stuff. It's both great fun and a great opportunity to learn at the same time. Well done, Pawelr98!

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 03, 2018, 01:24:18 am
content: Quote from: Audioguru on February 02, 2018, 04:26:45 pmI purchased analog scope instead of digital one(high-end analog VS chinese digital) so measuring THD won't be that easy.I agree that distortion is a bad thing.At the same time I often prefer "modified" sound instead of original.I prefer headphones over speakers due to prices and portability.FiiO E07K Amp+DAC combo.I have set slight treble bump because many headphones boost the bass bit too much.At some point I experimented with my UNITRA FS032 hardware equalizer.Music can be more enjoyable if you are able to customize it.Lowering bass levels is certainly one of the things I like, especially on tracks with good vocals as it makes them stand out.My only comparsion at the moment is Panasonic SA-HE70, class "H+" (that is AB with adjusted supply voltage).At some point I had Unitra WS432(exported to west as Dynamic Speaker SA 940) which is a classic AB class ampifier.<0.2% THD@1KHz at rated output (2x35W).

author: John Heath
author_rank: Frequent Contributor
timestamp: February 03, 2018, 02:24:52 am
content: Before starting I like the scope in the picture posted. 150 MHz and it is real not a digital simulation therefore at 150 MHz what you see is what it is not software connecting dots.The amplifier can only do so much. What about the speaker? A negative resistor could be used to make most speaker close to perfect within a power range , theoretically. To say why is a long story , apologies in advance.Cassette tape motors used to have mechanical weights with contacts to govern the speed , centripetal force , like the old steam engines. One bright spark came up with the idea of making a theoretically perfect electric cassette motor that will in theory will turn at a RPM that equals the volt to the motor regardless of load. A perfect motor is too expensive so he put a op-amp simulated negative resistor in series with the motor. This causes the motor to act like a perfect motor without the cost of making a perfect motor. The copper wire resistance of a motor is 10 ohms so you set the negative resister to 10 ohms in series and it becomes a perfect motor where voltage equals RPM regardless of motor load. This idea worked so well that most cassette cap stand motors were negative resistor type for the last ten years of cassette players.With this in mind could the same be done for speakers? No one has tried it to my understanding but it seems plausible. An 8 ohm speaker having a DC resistance of 2 ohm would require a op-amp simulated negative resistor of 2 ohm in series to make it a perfect speaker. A tiny 3 inch and 12 inch speaker both Z = 8 should act the same. If the little 3 inch speaker does not care for bass notes then the 2 ohm negative resistor will rip the voice coil right out of it trying to make those bass notes, ha.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 03, 2018, 02:57:48 am
content: Quote from: John Heath on February 02, 2018, 04:14:46 pmIt didn't become common until digital audio became mainstream. Much easier to justify the cost of a DSP to convert PCM into a bitstream when you need the DSP for other purposes anyways.Chip scale digital amplifiers even started appearing in USB Type C headphones - apparently logic can scale down to the point where it ends up using less power than precision analog circuits that do not scale well. They more or less made a Delta Sigma DAC that connects directly to the headphones.

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 03, 2018, 04:00:17 am
content: Quote from: John Heath on February 03, 2018, 02:24:52 amSchlumberger 5228It's a 250MHz scope made by French Sefram.Which is why I purchased it over a chinese digital oscilloscope.Some RF guy was measuring signals close to 500MHz on it.All three channels have full bandwidth, third channel has "crippled" functionality.0.1V/div and 1V/div, no 50R termination and no custom V/div.It's not my only scope.C1-107, 5MHz single channel.XY + external triggerIt was my first scope before I gathered money for a proper high-end scope.There's a built-in multimeter. Needs some repairs and adjustments but some functions work correctly.ADC,display is all discrete TTL + analog.Do you guys purchase NOS devices instead of new ?I usually do because the quality is better and you don't get counterfeits.There's Czechoslovakian TESLA, Hungarian TUNGSRAM or MEV and Polish Unitra-CEMI.Plenty of NOS hardware at low pricing.A pack of 4xKD502 + shipping costs 17PLN.With current 3.35PLN/USD it gives ~5USD.Occasionally I get some western Motorola or even RCA.Additionally I take apart electronic junk and strip it from all components.They go into big storage wall for components.Too bad that I mainly get ATX pc power supplies. I mainly get low voltage caps and high voltage transistors from them.My first amplifier was 2xTDA2003 amp for a TV set around 2 years ago.Only the chips and the PCB was new.All capacitors and resistors were used components.Works to this day.Sound quality is good for just watching TV.Main problem is that the switching supply noise comes through sound card of my HTPC (DVB-T tuner installed).Semi-fixed by adding 27R resistors on amplifier input (low but does the job).Switching power supplies are a big problem.I constructed a DIY AM radio.AA144 germanium diode for detection, BC516 + LM386 for audio amplification.Poland has 1MW Long-wave (225KHz, Polish Radio Channel 1) transmitter in Solec Kujawski(it has enough power to be received in Japan or USA as most "competing" transmitters got turned off in recent years).This city is right next to Bydgoszcz which I live in.Over here the signal is excellent.Once time I decided to take my radio to Gdansk(~140KM north) where I study at Polytechnic(Technical University).Station could be picked up but I had lots of noise which covered the audio signal(broadcast could be heard in backround).At one point in history we had 2MW long-wave transmitter (which had effective power output of 3MW) in Konstantynow.Highest construction on earth (646meters) until collapse in 1991.https://en.wikipedia.org/wiki/Warsaw_radio_mastNow I want to build a more advanced radio.TDA2822M for audio amplification. It can run down to 1.8V which means higher volume with the same 18650 cell(LM386 is barely running at such voltages).BC516 for RF amplification this time.AM detection using the same AA144.I learned radio technology from russian DIY youtube channels along with USSR radio-magazines (mainly schematics).I know cyrillic (self learned) so I can understand plenty of stuff just from Polish-Russian language similarities.Circuit #8 is what inspires meTwo-stage RF amp replaced by a single BC516 (PNP darlington) and audio amplifier replaced by TDA2822M.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 03, 2018, 05:44:02 am
content: I am liking diagram # 6. One resister , wow. Is that a complete radio or just the last IF and audio out?

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 03, 2018, 09:18:53 am
content: Quote from: John Heath on February 03, 2018, 05:44:02 amThis chap seems to be an awesome educator.https://translate.google.com/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=https%3A%2F%2Fwww.litmir.me%2Fbr%2F%3Fb%3D284795%26p%3D66&edit-text=Translation is a bit off but you get the idea.Leo

author: John Heath
author_rank: Frequent Contributor
timestamp: February 03, 2018, 03:00:25 pm
content: Quote from: Leo Bodnar on February 03, 2018, 09:18:53 amNice link. This is old school thinking at it's best. The foundations that got us to where we are today. I especially liked the positive feed back to increase Q factor. A tricky balancing act if it is a little too much positive feed back but it can not lock up as it is a AC coupling not DC. The hand drawn art work is a nice touch as well.

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 04, 2018, 01:19:28 am
content: I did construct a modified #8 circuit.Two stage RF amp-> single BC516Audio amp-> TDA2822MLower diode in the AM detection got replaced by low value resistor.I had trouble with audio distortion. Connected oscilloscope in 50R(integrated 50R terminator) mode over the lower diode.Distortion was gone and the waveform was looking very good compared to usual 1MR mode.Replaced the diode with 56R resistor and it works very well now.The radio seems to be much more noise-proof.Lowering the impedance does wonders sometimes.I guess it's because of the BC516 gain. Small-signal darlingtons have insane gain.BC516 has a minimum of 30K.The AM modulated waveform on analog scope looks beautiful.Old books/magazines are a good source of knowledge.In eastern bloc the semiconductors were scarce and expensive.Which is why many designs are minimalistic.Very often the available devices were "second-rate" or to be honest "factory trash".Example would be transistors with single digit gain.Purchasing an entire device was often impossible.You had people building entire TV's from parts."Factory Trash" CRT was often used as a base for a working TV.I learned my radio stuff from here:

author: John Heath
author_rank: Frequent Contributor
timestamp: February 04, 2018, 03:17:47 am
content: The CRT was the base. You start from a mounted CRT and work your way backwards. It made me laugh. Not an easy way to make a TV but then again the CRT is the hardest part to make. Transistors with a gain in the single digits !The video posted was well thought out. Great graphics. I could see how this would be a valuable leaning tool. Found another one .Even without understanding the language I am getting half of what he is saying. This is mostly do to the great graphics.Also I found a respectable picture of your scope. That is a nice scope. 250 MHz not 150 MHz as I said.

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 04, 2018, 04:42:08 am
content: Those russians have entire series. The title of the series would be "Prostaya Elektronika" which means "simple electronics".I decided to self-learn cyrillic because of my russian oscilloscope.Comes useful when searching on russian websites.An example of a "second-rate" diode.Normally marked as BYP680-100.5A 100V general purpose diode.This one is marked as BYP100, you won't find it in any datasheet from the time.What kind of load can this "second-rate" diode handle ?Dunno really. May have lower current rating, higher leakage current or decreased max voltage.Sometimes "second-rate" devices were repurposed or used for "bottom of the line".Broken transistors used as diodes (case was the same, collector leg was cut).TG1 (small germanium PNP) is the second example.Beta:9-20Ft=300KHzFor comparsion TG5(from the same family, most popular):Beta:25-80Ft=800KHzTG1 doesn't even have Vcemax rating. Only a Vcbmax of -10V is given.There's only a graph which shows Vcemax in relation to Rbe.Current and power rating is the same as for entire family.In reality it was a bad device:Sometimes you could get a transistor which had single digit beta but otherwise performed OK (eg. could take same power as proper device).Such devices were often sold in hobbyist shops etc. A "not fully functional" mark was often given but sometimes not(which may cause problem today as the stock of such shops spreaded over private owners).I was born after communism era (1989, I was born in 1998) but I often deal with equipment/parts from that time.At my university there's plenty of vintage equipment from that era.When browsing old books/magazines you can find very interesting projects.I recall a self powered AM radio with amplifier from a polish magazine.A voltage multiplier was used to turn radio waves into power for germanium transistors.I have a friend who attends technical highschool which specializes in radio communications.He managed to build a working FM radio on a single transistor.Superregenerative receiver.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 04, 2018, 07:28:26 pm
content: For self powered a radio could tune it the strongest station out there and use it for power. With this power it can tune in weak stations and amplify them. This means the strong transmitter station is providing power to listen to weak AM stations. You should send a letter of thanks to the strong transmitter AM station for providing power to listen to other AM radio stations , ha.I have head of 1 transistor AM radios that will drive a speaker ! But I have never seen a 1 transistor FM radio. Do you have a diagram of it?

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 05, 2018, 01:42:56 am
content: Superregenerative receiver is one option.LM386 is used for audio signal amplification.But as always the russians built something interesting:http://gef79.narod.ru/Shem/Radiopriem/64.htmQuality is good enough to be connected with stereo decoder.This is not Superregenerative but something else. I have a description of the device in Polish.It seems to be a generator which tunes to 1/2 of station frequency.Signal from antenna synchronises the generator.I managed to get such information.GT311E is a very high frequency germanium transistor.Such transistors are commonly used in soviet TV's or oscilloscopes (as in my case).This one is tuned for OIRT band (65.5-74MHz) so values would have to be adjusted for CCIR band (87.5-108MHz).Eastern Bloc was using OIRT band which created problems after transition to new CCIR band.I still work on one OIRT radio which refuses to work with CCIR correctly.100+MHz works OK but lower band is unstable.Will have to inverstigate using the popular SDR receiver.It can be used as spectrum analyser which can check superheterodyne frequency.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 05, 2018, 03:40:16 am
content: So it would be a 100 MHz AM receiver tuned just off the station frequency so that it would act like a frequency slope detector , FM decoder. Yes?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 07, 2018, 12:20:23 am
content: Here's a circuit I did "way back". It worked great, but I know at least one person had some stability issues with it, so watch grounds and such.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 07, 2018, 03:00:52 am
content: I could not help playing with it. Apologies in advance. Assume regulated rails so we do not need a zener for bias pot. One zener and one resistor gone. Set gain to 1 therefore feed back resistors not required. Two more resistors gone. Assume ground isolation condensers are on the power supply not the op-amp. I realize this is cheating but I am not above cheating when it comes to simplicity. Two more condensers gone. This leaves only two resistors plus 1 pot and 1 condenser. It is perfect.

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 07, 2018, 03:37:57 am
content: Quote from: John Heath on February 05, 2018, 03:40:16 amSuperregenerative receivers do use slope detection.It can be used for receiving high-frequency AM radio broadcasts.The russian design however is different.It seems to be Analog Phase Locked Loop(APLL) on a single transistor.It synchronises with the second-harmonic of the input signal.The design works with Silicon transitors as well.Quote from: Conrad Hoffman on February 07, 2018, 12:20:23 amDoes this really have <0.02% THD ?IRF530 is a cheap transistor.Drain on case=no isolation needed for the main device.The load transistor would need isolation though.For a headphone amp this won't produce much heat so it shouldn't be a problem.I will try to go and purchase CEMI UL1111.I found one electronics store in Gdansk which sells vintage parts.This chip contains differential pair(along with three standalone transistors) which is useful for various designs.It cannot withstand high voltages (15V Vce) but it's a cheap matched pair.Pretty much a clone of western CA3046.High Ft is an additional bonus. May use those for RF applications.For AB class I usually go with one-chip amplifiers.TDA2003,LM386 and recently TDA2822M.My first discrete AB class is most likely going to be the Ge USSR amp.Germanium amplifiers can be pretty much rail-to-rail amplifiers if done correctly.Very useful if supply voltage is limited.TDA2822M is good as it has 1.8V minimum (popular LM386 is 4V) but I heard that Ge can go even lower.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 07, 2018, 04:42:41 am
content: The measurement was probably for 1 kHz but I don't remember as it was so long ago. The number is 0.002%, not 0.02%. The design does produce significant heat if biased for anything useful. I built it on a fairly substantial heat sink, probably 70 mm x 70 mm x 130 mm. Though it looks simple, a fair amount of work went into the design and substituting or changing anything will probably cause some type of problem. Do that after getting it working as drawn.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 07, 2018, 08:48:42 am
content: Easy solution for the bottom MOSFET in  current source mode is to use two-three diodes in series with a gate trim pot. This is how I did it in 1980s. Glue or thermally couple the diodes to the MOSFET. This will stop the thermal runaway.Good news is you might not need thermal control. Bad news is MOSFET would have melted by then.There are two thermal effects working against each other (Vgs and Ids) but equilibrium is usually in few dozen amp range.Ferrite beads on gates will stop oscillations.LeoP.S. I did not use load resistance in current source because this would have made it active current source. It was just MOSFET in constant current mode.

author: John Heath
author_rank: Frequent Contributor
timestamp: February 07, 2018, 01:00:57 pm
content: Quote from: Conrad Hoffman on February 07, 2018, 04:42:41 amIf I needed an amplifier and it was important I would go with the original Hoffman design. I can well imagine my boiled down version would have problems. But it is fun to try and eliminate components , somewhat like a cross word puzzle.

author: Pawelr98
author_rank: Regular Contributor
timestamp: March 04, 2018, 09:51:09 pm
content: Time for Germanium.My first discrete AB class amplifier.MP37 NPNMP42 PNPTG5S PNPSchematic which I received from university professor I know(he's vice-dean of other department on my university).He also provided me with a better schematic but for now I experiment with this one.TG50 is a different transistor. I decided to use MP42 because TG50 was made in Poland while MP37 is USSR-made.MP37 is much weaker than MP42.The best MP37 paired with the worst MP42(gain wise that is).MP42 is higher current,higher power and higher gain.

author: mikerj
author_rank: Super Contributor
timestamp: March 05, 2018, 10:41:38 am
content: On the subject of simple amplifiers, can anyone remember a crude class D design using (I think) BC108s on the output? It was published in one of the UK electronics magazines, probably back in the 70's, and had pretty terrible distortion but I built it and it did work. I've not been able to find the design since however.

author: Pawelr98
author_rank: Regular Contributor
timestamp: May 03, 2018, 12:11:22 am
content: Started working with tubes.Soviet 6S19P triode. Main use was series voltage regulation in military aircrafts.Isn't the glow nice ?Regulated grid and anode voltage.Anode voltage is 180V zener + KSC5027 NPN transistor (800Vce).Grid regulation is 4x15V zener.I have a audio transformer which was originally used in Stern 7e86, an Eastern German Vacuum tube radio from 50's.The radio used 6V6 (in Eastern Germany) or EL84 (version for Polish market) as output tube, 40mA@240V so it should be fine with 6S19P.170V anode(~320R primary, 185V before transformer),-60V grid which results in 44mA anode current.Close to original value used in the radio.