Class A audio amplifier kit test & review

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.

[MarkTillotson]

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.

[hannunorppa]

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!

[SmithKerona]

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!

[olipito]

So you played a King Crimson song from the 1969 album, because the board is called 1969? Or maybe just because King Crimson is awesome.

[flashUSA]

ahhhh ha ha ha ha 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....

[markcummings]

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.

[carlfranz]

For those of us whose eyes are not what they used to be... those $6 component tester kits are a godsend.

[jrs]

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.

[EliteRock]

Might be nice driving the tweeters in bi-wireable speakers, with maybe a class-d (TPA3116?) handling the bass-mid unit.

[interbudelblag]

a lot of harmonics are also on lamp amplifiers so it can be beneficial to hearing a "good quality" amp.

[trixie]

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.

[Bushougoma]

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.

[astorina]

Very nice video and the following one (I viewed it before that one) is also very instructive
Thank you John
Have a nice Easter
Antonio from Brussels

[noelj]

This circuit reminds me of a DIY magazine amplifier that I've built around 15 years ago using the infamous 2N3055. It didn't sound correct anyways.

[MichaelBeeny]

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!!

[iblesbosuok]

Yap, 2N3773 works much better than 2N3055. Good job on review and magnificent video.
Cheers from Indonesia

[bigbread]

John, I think you should design a class ab amp for education and sell it as a kit to your viewers.

[rochtourigny]

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?

[manueladao]

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?

[nigelpearson]

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.