Building the John Linsley Hood JLH 1969 Class A audio amplifier

mosfets

mosfets

Active Member
There is a channel on YouTube called Steve's Trivia by an interesting old fella named Steve Wagner. He did a video series on the JLH 1969 amplifier. I thought to email him and ask for a trade and see if he was interested in making a few videos on the Mike Bitner amplifier called Symasym. Wagner could mail me a few of his JLH amps and I would mail him a few of my Symasym amps. So I sent him the proposal via email and he responded a few days later asking for my mail address. He told me not to send him anything, he was no longer interested in small amplifier boards. In fact, he said he would box up everything he had on the JLH1969 and post it to me. A month later I get a box in the mail with almost 20 board kits of the amp, some assembled and tested and others in kit form, untouched. Wow! I thanked the man and offered other things in return but he said keep the stuff and don't worry about it. I got to business quickly and dialed in on a set of unused circuit boards made by Zero Zone - Aliexpress. There were four in the box and one of them was assembled and tested by Mr. Wagner. In his YouTube series, I believe it to be amplifier #2, that is what I called it: JLH1969 Amp #2. Then I set out to build it.

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I studied the little PCB Steve assembled and the associated companion schematic drawing he created and provided. I decided the tiny Zero-Zone PCB was my best option to start with. I had three blank PCBs and one assembled & tested PCB as a sort of guide. I entered the schematic into LT SPICE and began simulations. I took inventory of the parts I had on hand. I studied the papers by John Linsley Hood and read all manner of forum posts on the internet regarding the JLH1969. I modified the SPICE parameters of the components to match the devices I had on hand. I augmented and adjusted some of the components to take advantage of the parts I had and to exploit the JLH circuit. I ordered the power transformer and began drilling and machining the case and heat sinks I had on hand. The case has lots of white grease in the grain of the aluminum from previous experiments on the heat sinks especially. Thermal paste residue. It may look odd to some or perhaps a bit messy. Sorry. I left it that way but if I ever take the lid off again, I promise to clean it up nice. I started soldering the PCB together. I made a layout wiring plan for the insides of the enclosure. I bought a screw terminal barrier strip after I received the toroidal power transformer. After a few weeks of slow build-up, it became an assembly job that took a day or two or three to finalize.

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I had the power supply caps on hand for another build. I had the enclosure for a few years! I had the bridge rectifier. I had all the caps and resistors on hand with the exception of the output coupling cap. I kind of went hardcore on the output cap deciding an automotive grade would be most robust and up to the task. The transistors have been sitting here in parts drawers for years. The output binding posts and RCA jacks were local. The rubber feet on the back were from Amazon. The power supply switch is from Digikey but I will not list the part number as it is not a good device IMO. The copper bar on the PSU caps is from Amazon of all places. It is drilled tapped and threaded 3mm in some spots. The output devices are mounted with 3mm as well.

Bridge KBPC3510
XFMR 1182S15
Output cap EGPD500ELL302MM35H
PSU caps ALS70A113DB063
PRP Resistors
Nichicon caps
Wima coupling cap
Enclosure eBay
KSA992 Hfe - 430
MPSA06 Hfe - 215
NJW0281 Hfe 100(matched)
VR1 - 1 Watt!

EDIT: Two of the caps are Elna. You know by looking at the board on the right.
EDIT 2: There are two ground lift switches on the back of the device. I think they are C&K Unimax. The enclosure came with the IEC socket/fuse combo machined and mounted. The power cord is from Cables Canada or whatever that company is called. You can get one anywhere.
EDIT 3: There are two thermal cut-off switches that cut the mains AC when the heat sinks reach 75° or 80° ...I can't remember the exact temperature or part number. Sorry.

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am I just missing something or is there no thermal compensation for the bias? I see pots have been added, presumably to set the center voltage and the bias.
 
I love this amplifier. I can't believe it took me over thirty years to build one. I have wanted to build a JLH amplifier ever since I learned of it back in college in the early 1990s. I found it in a magazine in the university library and I photocopied the schematic, mulling it over for almost a year before the subject was let go. Then it was back in my face around 2000 on the internet. Still, I pondered it and moved on to other projects.
I can say all kinds of things about the sound of this amplifier but the subject has no interest to me. All I can tell you is, it is one of the best-sounding amplifiers I have ever built or listened to. I absolutely adore it.
I conclude the JLH 1969 is one of the best DIY amplifiers an audio hobbyist could ever construct. IMO the sheer volume of detail on the internet regarding this amplifier is a testament to its paramount ranks in the DIY audio community around the world. The simplicity and flexibility of the JLH 1969 circuit lend it to easy deployment in the DIY realm. The version I built is an absolute sonic treasure.

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So, was one the already completed, and one your build above? Interested to see the progress as he has the same initials as me, so it would be cool to have an amp like that…
 
R7 & R1 should be a single 1-watt 220 ohm resistor but I used what I had on hand. * R11 is the 1-watt Bourns potentiometer *. P2 is the 500mW Bourns potentiometer. C7 and R13 are added to prevent oscillation because new transistors are faster than the originals back in 1969. C1 is Wima. Q1 should be low noise and high gain (Hfe). The output transistors should be gain(Hfe) matched, if not, read Hood's article to set them up. The NJW0281 is a very musical transistor, almost legendary IMO, perfect for this type of application. C5 is of paramount Importance. * R12 is in the simulation to measure current, it is not in the design! * Q2 may run hot, use something that can dissipate a handsome amount of power. I deployed a heatsink on my Q2s as seen in the pics. It is a Class A design, it runs best when idling hot or very warm. Basically what we have here from 1969, is Hood's attempt to make a transistor amplifier that sounded like his tube amp. Read all the Hood articles and enjoy the lore that goes back to a time just before I was born. The stuff is plastered all over the internet so I find no purpose to add such links here. Instead, I just want to share with you my success with what I used.

EDIT: Q2 may come in a can with a metal hat. Depends on what you decide to use. It may or may not require a heat sink. If you use a TO92 transistor for Q2 just Google TO92 Heat Sink and you will find some options. I recently picked up some new ones from China. Attached.
EDIT: attached more pics

I'll say it in a different way: This is the first amplifier I should have ever built way back in 1992. Yet I am lucky to come around after all these years and discover it's true beauty.

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Very nice project! I built one on perf board about 20 years ago, and I loved how it sounded, but it melted down a couple times, and eventually I gave up on it. These boards look much nicer than what I put together. I still remember how hot it got, and how great it sounded.
 
I love this old guy. He gave me a great gift(s). I have never contacted anybody on YouTube but something compelled me to reach out. It paid off in spades.

 
a little surprised they don't pull the feedback from after the cap to compensate for it. Most of the other cap coupled stuff I've fooled with does. Makes a huge difference in preventing LF roll-off and dealing with what the cap itself adds into the mix.

another thing I've found with cap coupled amps, the "set to 1/2 voltage" thing may not be quite optimal. Its close, but if you really want to dial it in, wind it up until it just starts to clip and adjust it for symmetrical clipping. It usually ends up slightly below 1/2 voltage at idle, but under load with power supply sag it ends up pretty darn close.
 
I tried the set to 1/2 voltage trick with one of the Marantz 1060 cap coupled amps I had on the bench. It did come in darn close.
I couldn't tell the difference when listening. Setting it using a scope or just setting it at half voltage sounded the same to me at high volume.
 
I doubt its going to make enough extra power to notice, but on a low power amp I figure it makes sense to squeeze what there is to be squoze.
 
TONS
mosfets said:
There is a channel on YouTube called Steve's Trivia by an interesting old fella named Steve Wagner. He did a video series on the JLH 1969 amplifier. I thought to email him and ask for a trade and see if he was interested in making a few videos on the Mike Bitner amplifier called Symasym. Wagner could mail me a few of his JLH amps and I would mail him a few of my Symasym amps. So I sent him the proposal via email and he responded a few days later asking for my mail address. He told me not to send him anything, he was no longer interested in small amplifier boards. In fact, he said he would box up everything he had on the JLH1969 and post it to me. A month later I get a box in the mail with almost 20 board kits of the amp, some assembled and tested and others in kit form, untouched. Wow! I thanked the man and offered other things in return but he said keep the stuff and don't worry about it. I got to business quickly and dialed in on a set of unused circuit boards made by Zero Zone - Aliexpress. There were four in the box and one of them was assembled and tested by Mr. Wagner. In his YouTube series, I believe it to be amplifier #2, that is what I called it: JLH1969 Amp #2. Then I set out t
Click to expand...
Tons of these for decades from and thru ChiFi. Finished amps. Some listing for thousands$$. Problem and situation is, a MAJORITY have counterfeit outputs. You posted this based on the famous Hood circuit. But its all moot if you can't get and find authentic output transistors that havent been made for 30~50 years.

Fake *******ING "MOTOROLA" Transistors

Hi... I bought two transistors MJ15024 and two MJ15025. The MJ15025 is from OnSemi, and it appears to be OK... The other is from "motorola", and their top just jumped out when I attached it in a plain heatsink :mad: :mad: Look... By the inside, can you tell me if it appears to be realy fake?
www.diyaudio.com www.diyaudio.com

Counterfeit Transistors

ESP - The Audio Pages. Counterfeit transistors are in epedimic numbers world wide. Find out how to identify the real from the frauds. Page 2 of article
www.sound-au.com www.sound-au.com
 
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However, if modern replacements, and any modifications due to modern parts have been made (which appears the OP may have made), I don't see this as a major issue.
 
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