Another random kit send-in to the channel, we have a class A amplifier to try. Let's see if this thing is any good. PO box 362 Vandalia OH 45377 for sending goodies to the JAT channel.
Hi John, if you can't fill the sine wave signal on the dso screen with the 1-2-5 vertical divisions you can always press the vertical nob to go to fine step and fit the sine wave nicely. Just thought I share that with you. Keep up the good work!
My first amplifier was single ended AD149 when I was a kid in 1970.. The sound was high ended as well! The designer was Finnish Hannu Miettinen 3>. the heat sink was a aluminium plate folded into a box!
At high output and high frequencies its becoming slew-rate limited, hence the pointed waveform. Probably won't affect real signal where the larger signals are low in frequency. The driver needs more standing current to fix this.
good review John as always. the scope showed the 2nd order harmonics at higher frequencies - interestingly how did it sound when playing music reaching those frequencies?
Thank for the efforts you put into your channel content! This design was respected back then, can you trace the schematic so we can compare to the original, and maybe simulate in LTspice?
Hi John, Thank you for what you do on your channel. I bought 2 jlh boards slightly different to yours and they don't have any trimming pots in the design. Have not tried them yet but if I can't set the current does this mean they are no good. Keep up the good work.
Some thoughts John!! Might be worth using a higher voltage, 24 maybe, not exactly over the top, adjusting the QI as you go. Another interesting thing would be to pop in the original transistors as supplied. The original circuit by Mr Hood was very highly acclaimed in it's day. Makes you wonder what has been done to mess it up. Might be interesting to compare the circuits and bring it up to the original speck. Something to do if you have a few hours free!!
Hi John, thats a neat kit. I was wondering if there is some type of oscillation going on at or near 12k? How is the feedback in this amp, a little or a lot? Might want to play around with that and last are your transistors beta's (gain) close? Just a few thoughts. Great video and its fun to debug as well. Take care.
Shame about the high Hz thd. Thought that class amp might really be great, although not sure if I could hear the diff. Love your vids for long time. Please keep it up, as I learn something new every vid. Thanks!
What would be a good cheap class A or A/B amp kit, with a clean 10-20 watts @ 8 ohm, stereo. I'd like to get into this , I built a headphone amp, and kinda got the itch!
The 2N3055 you showed at the beginning appears to be a "pull." Notice the indents around the mounting holes that were made by star lockwashers. I tinkered with John Linsley Hood's designs decades ago and got fine results, but had to match the outputs carefully with a curve tracer. If I remember correctly, subbing different driver transistors made a difference in the amount of high frequency distortion. Nice job with all your videos. They take me back to younger days and have sparked a new interest in experimenting again. How times have changed.
@@kalusovsky What's so tricky about it? Just make sure your thermal bias compensation has the correct curve across the operating temp range at frequencies up to neutralization across the range of possible load characteristics. Simple.
From what I see concerning the polarization of the output stage, it seems that this amplifier, although it is said to be class A, however, it works as class B. What's your opinion?
Bahhhhhh ahhhh ha ha ha ha haaaaa! I got caught with that same insulator issue when I assembled this amp. My voltage measurements were all but a carbon copy of yours. I scratched my head trying to figure out if I put a wrong resistor in the wrong place. I did what you did but only drew up a partial BS schematic only realize that the damn collectors were floating in space. It's funny because I seen it before you even turned it on and I was lamo! Why would they add those to the kit in the first place? FYI... All you had to do to raise the heat sink away from the pcb was to use the thin cardboard back off of a notepad. That deals with any possible shorting issues with the heat sink to the pcb. I also used heat shrink tubing on the emitter and the base legs to play it safe. I caught the feedback cap fix on your second video and I think I will go back and check the slew rate just to be sure I don't have this issue. I was actually going to use this to rebuild a small bass guitar practice amp. Nice job on your videos! Thanks....
Glad I’m not the only one who hates the blue resistors and use a meter to confirm values. It’s about time for us end users to standup to the manufacturers or standards and get this problem fixed. I much prefer the cream colour or even light blue is somewhat acceptable.
What pulls up the base on the high side transistor? Is it boot strapped? I don't understand the 4 small electrolytic capacitors on the board, between the signal transistors, and the output transistors. You mentioned, that the output capacitor is one of those 220 µF capacitors, but, are other capacitors in parallel, to give more capacitance for output coupling? I would imagine, that the blue one on the far right would be the boot strap, with the other 3 in parallel, to give a total of 660µF of output coupling capacitance. It would make more sense to me, to use that huge axial 2,200µF 35 V capacitor as the output coupler, just to allow lower frequency output band width. I'm sure that the 1µF capacitor on the input is the input coupling capacitor, and I would put that up to at least 10 X that. Increasing the input and output capacitance by 10X, may necessitate increasing the value of the bootstrap capacitor as well, if so equipped. It might be worth it for me to get one of these, just to tinker with it. I'm curious now.🤔
Sweet, last song on the King Crimson album. Just saw Sean Lennon with Les Claypool at a concert play the In the Court song, great rendition, great performance overall, would have preferred epitaph though.
If you ever get a chance try feeding signal into the feedback capacitor ( remove it from ground ). As is gives 100% DC feedback at this point it should be OK. Modern op-amps should drive this. Remember to invert speaker terminals if being strict. This should by ear alone. It could go either way. This feeds signal direct to the input transistor emitter in common base which is valued by many. If liked the input might be arranged for lower hiss ( values of resistors ). I have to say the JLH is very close to being a bad idea that works. I used BC327-40 337-40 in mine with 2N3055. Looking back that might have been the limit of the 377. I used 8 ohms. I had 0.03% THD even at 20 kHz. I had to use 33 pF base to collector to stabilise it ( 377 ). It almost didn't need it. Could I recomend you find some Dynaco A25 speakers. A better speaker to suit your experiments. They make small differences larger. Hopefully my memory of the circuit is right? The 2K2 feedforward resistor is to get better hf distortion. Being class A it isn't helping the base switch off. My 2N3055 were low cost Indian types. They measured like a more modern device. I paid less than $1 a piece. Mine was built dead bug. The JLH is far more sophisticated than it looks. That means no one understands it. I got 7 watts 8R from mine.
I think the high frequency distortion is due to insufficient slew rate. The voltage cannot increase fast enough to keep up with the waveform. If you were to decrease the input signal amplitude you would see the distortion decrease or disappear all together. The amplifier can only increase the voltage so fast. You need an amplifier with a higher slew rate.
Hey John, awesome video! I am looking for an IC to use on the power stage of the guitar amplifier I am building. Which one do you recommend to get a high output power and volume? I am planning to use a notebook power supply (19V DC) or a simple 12V adapter. Thanks :D
You cannot get particularly high power from 12V or even a 19V power brick. Even at 24V, you'd be limited to about 30 to 40W of clean power. In some amps you can bridge them to double the output providing you are mindful of speaker impedance. Yeah, I know that distortion won't matter much, but with low voltage transistor amps it's clipping of the sound rather than distortion components like added by valves. The key to high volumes from small amps is the speaker efficiency... never underestimate how many amplifier watts can be "saved" by a speaker with over 90dB/W/1m. Most modern speakers are far lower in efficiency, so you need double, triple or even more power to get the same loudness.
The Bifet op-amps such as the TL071 would provide a higher performance over the 741 standard. Has a high slew rate however that could lead to stabilization problems in some conditions. Use of 2 feedback resistors with one shunted with a bypass capacitor may need to be fitted in this case. This is to be used with additional current boost transistors as the op-amp is not a power amplifier.
It has too few components just for the transistors driving and probably has no negative feedback circuit which could add extra stability and wider bandwidth but it lowers down the amplification.And since it is class A amplifier this means mainly power losses and if has hifi quality the output power would be from 2-5 Watts.
Could the problem be the inductivity of your 2200 mf output capacitor? Try to shunt it with a say, ceramic one and see how that performs at high frequencies. Just a thought ...
The HF problem could be the o/p transistors. Apart from the ft of these not being specified on datasheets (it's probably so bad) some forums suggest that this is an obsolete device that loses gain at higher frequencies. As if that lot isn't enough it's these kinds of transistor that are regularly faked. It's very difficult to buy a genuine to3 transistor these days.
Where you probe to get 9v ( half of supply) is it at output? And the current measure around 1.2 amp if the power supply doesn’t have current meter can I put meter in series with the supply and adjust the pot to get that 1.2 amp current right John?
16:28 They should have removed the base and emitter pads from the top side of the board then you could mount the heat sink flush. A few clicks in a CAD package and that wouldn't have been an issue just shows whoever designed that PCB had no attention to detail.
Here's a question, because I don't know the answer :) Is the harmonic distortion due to the shape of the waveform being changed to a more rapid change at it's peak voltage?
From Foyer analysis theory: ANY change in the waveform from a pure sine wave represents a harmonic. Since harmonics are easier to measure, we use that for the "purity" of the amplified signal.
hmm...thoughts on a pair of these; as a high impedance (250ohm and up) stereo headphone amp? I know the heat would be crazy, BUT I have more than a few old heatsinks sittings around...could even actively cool them. What changes would you make? Probably would spend more on the power supplies(gotta keep it true dual mono right?:)) Also thought of running HO 14v car audio batteries....could get expensive quick though. I wouldnt use the the amp all the time, just for those "critical" listening times....know what I mean?
Thanks for the quick reply. I have been looking over the "kits" being offered with the Hood design. Pretty sure that I will be getting at least one "pair". There does seem to be a "mirror" image board offered which would be nice to keep the inputs and outputs on the same side. What caveats should I be looking for running the amp at low current? I did find a kit that is actually being sold sold as a dedicated headphone amp even the same output transistors. www.ebay.com/itm/2pcs-Two-Channel-JLH-1969-A-CLASS-A-Stereo-Amplifier-Board-ST2N3055-MKP-DIY-Kits/171380740698?_trksid=p2485497.m4902.l9144 It does look pretty "sparse" though. It does mention "1.25A" as "output"... Finally on a somewhat different note. I am also considering building a pair of stereo kits like the one that you have in this video for a three-way active speaker design that I am building. Iwill use a smaller A/B amp on the lows(50Hz or so and below, depending on where the speaker actually rolls off after it is built). Do you know of any kits or designs that you would recommend? Something like this would be ideal, but again very open to suggestions. smile.amazon.com/3-Way-Active-Crossover-Kit-Linkwitz-Riley/dp/B01N6FVDL6/ref=sr_1_29?ie=UTF8&qid=1522880689&sr=8-29&keywords=active+crossover+3-way Thanks again for your expertise, advice, and videos....
Sorry for the slow reply. If current gets too low, it will come out of class A operation which could mean more distortion. A Linkwitz-Riley filter is great for active crossovers. I didn't inspect that product. Unfortunately time prevents me from giving detailed answers and inspecting all products.
Hello,... Love your videos, maybe that funny high frequency waveform is due to the output capacitor (poor quality)... Could you try somewhere in the future add 470n to a 1uf in parallel with the output cap? This is something I read about quite a few years back. Keep up the good work!
Ray Lewis: I agree. Several theories in comments. Come on John, troubleshoot & repair. Also, I think Matt intended for you to COMPARE the OE transistors and the ones he added to kit.
The minute i saw the al-u-min-i-um bracket i thought it needs to be electrically insulated from the board because it would surely make contact the PCB pad points for the transistors and so short things together!
This is , I guess, old JHL 1969 class a amplifer, it could sound great but the parts selection is critical. That cheap 2200uF output coupling cap is degrading the sound for shure.
It is JLH for John Linsley Hood, a welknown amplifier designer from that time. There is a lot of material on the net, and many have build this design. There is even at least one how-to-build video here on RU-vid. The original design used 2N3055 transistors.
Nice video dude really like your stuff but I see your piles getting big if you want to you can send me the pile high could put it to use maybe I can do some experiments with myself getting contact me if you would like to do that
Yep, if the ink scrapes off its likely fake. 3055 are ancient and have been cloned for decades, so they may be 'reliable fakes'. Still never worth using.
I think the shoulder washers were just included with the insulators, not needed for the kit. Curious about the cause of the hf waveform issue, C1?. Also how the original outputs would have compared. ae01.alicdn.com/kf/HTB1DTKpSXXXXXcNaXXXq6xXFXXXH/Simple-Class-A-JLH-1969-Power-Amplifier-Kit-Two-channel-ST2N3055-Amplifier-Board-DIY-Kit.jpg
It looks like an extremely poor example of the JLH design. The original design put out 10 to 30 watts RMS depending on the power supply voltage. The original design can be found online and if the components are sourced through a reputable company like RS components you will end up with a class A amplifier that has distortion of about 0.05% I think the trimmers were original designed to bias the output transistors. The Paul Kemble page gives more information as this is a legendary design. I met John Linsley Hood when he was alive. A brilliant man www.angelfire.com/sd/paulkemble/sound3b.html
Hi John, these old Quasi-Complementary-Design Amplifiers are quite often a pain in the ass, especially for higher frequencies. Remember: the positive half of the Output stage runs as current-amplifier with a gain >1. The only way to get a fairly good symmetry in such a design is having a huge amount of negative feedback, but for this you need to have a input stage with a high gain. I think this is not possible with only two Transistors for Input- and Driverstages... About 4 Decades ago i had designed and build such a kind of Power Amplifiers, running on 60Vdc with 4 pcs. of 2N3772 from RCA for each channel in the Output, for driving 2 ohm loads. They deliver about 160 Watts @ 2 ohms and the Distortion was fairly low, even at 20kHz. But the fundamental Design i used was very different from this amp you have shown. I had used a 2-transistor differential-stage in the input, followed by a single transistor voltage amplifier, driving an complementary current amp stage to drive the output transistors. So i had a huge amount of gain and could use the negative input of the differential amp for a effective negative feedback loop. All in all there where 11 transistors in each channel....
@@JohnAudioTech what limited the power to 24 volts Approximately? Isn't the transistors rated for higher voltage? I know little about this, but usually more in equals more out! If not anything but heat!! Maybe I should stick with D class, I'm running off battery, so more effiecent the better, I'd likely never hear the difference anyway! What I'd really like is 5 channels, and a clean 25w rms x 4 with 100w sub channel, its not going to be that loud, about like a good factory car system, so being its for basically a camper, I should just find a car amp! I have 12v and 24v avalible, thanks!!
Question - would anyone care about harmonics starting at 12KHz? The second harmonic would be 24KHz - that would be pushing the limits of most people's hearing. I suspect the source of the problem is the gain is falling at high frequencies so the negative feedback isn't able to "clean up" the amp.
you got the idea wrong. even though the distortion is well beyond hearing capability, the original waveform is not the same anymore. it's about the output compared to the input. ideally, output = input. but since nothing is ideal then output should be as similar as the input
There's two ways of looking at a signal - time domain and frequency domain. You're looking at the time domain and the distortion of the waveform. I'm looking at the frequency domain where those deviations would require hearing beyond what most can hear. If you look at how the ear and in particular the cochlea works, it seems like the ear works in the frequency domain. Since the frequencies needed to reproduce/recognize that distorted waveform are outside of normal hearing, I question if it has any significant impact on the sound quality. Or, it put it another way, if you were to pass that signal though a 20KHz low pass filter, would it still be distorted? I don't think so.
why dont you get proper test gear , i mean, testing an amp on scope isnt exactly the best way to do it , i think i would get better analyzer with just my sound card , its pro one , still think its better then that scope I think the sound of this particular one comes from the transistors type , should sound a bit crunchier , if that is germanium there , they do sound a bit harsher on a higher note . Still , if you get falling amps from shelves , i have 10+ years of Music production XP and officially educated in Acoustics and electronics , so you can send it to me for further analyzing , i would return the favor with my skill if the price of shipping doesnt exceed the actual component value . I watched a lot of your videos and they all end up on a same not( while being interesting dont get me wrong , that is why i watch it ) , but quite different amps from digital to full analog , with same conclusions , FFT , maybe SNR , some harmonics . IF you like , just saying , good job as always .
Bullshit, the harmonics are no problem. 12khz - so 24khz is the next frequency. Tell me one, who can hear that, also, the peak was not large enough. The amp is absolutely fine. That is, why your first test did also sound good in your opinion. Because it was good. Have fun.
