That does make sense that it might still hum because as drawn originally you have the L and R grounds linked by the relay board. You then connect the PSU to the board which introduces another ground route via the boards supply back into both channels grounds. It all forms a loop.
The final test you can do is to remove the speaker positive from the boards and link the speakers direct to the JLH amps. All you have in place then is the PSU feed to the board. That has got to be silent. I think you have to try that to give us a starting point.
Also just to cover all bases when the board is not connected to anything it should all be floating or isolated. If you measure on ohms from the boards ground to the amp grounds there should be 'infinity' meaning no connection exists. That rules out routes caused by mounting screws into the chassis etc etc.
AL100 are germanium which has a max junction temp of 100C vs 200C for silicon, SOA says at 60C junction temp it can dissipate 25W at 80C 12W.
I can't get head or tails from this setup.
Routing speakers wires directly from the amplifier to the terminals, bypassing the protection boards, still hums.
Removing the input wires, then the humming stops. Ground from the source is shared. Its an 8-channel output board from a DSP module.
And:
You're not the only one 🙂
You have to take it back to the state when it was all good before all of these issues with the relay board. Then begin again from that point, first if necessary just proviong that putting the board in the case doesn't cause a problem.
Then one connection at a time beginning with the boards - supply lead and nothing else.
Nobody said it was easy 🙂 But it works now. I'm scared to turn it on every time but it's a nice experiment.
@Mooly
I think I find something.
Using the relay board, grounds are connected of both JLH amp boards. Then there is a connection to the ground lift circuit (10 ohm/ 100nF) for both boards.
Without the relay board there is no connection between both grounds apart from the route of the input signal.
I assume this route is not optimal and has some raised resistance.
When I directly link the second board ground to the ground lift circuit, which is connected to the first board, the humming stops. (orange line to the ground lift circuit)
This is without the relay board. (I assume this link is also created by the relay board and the shared ground)
Using the relay board will make a link between its power input and speaker ground which cause the humming.
Next, I connected only the speakers positive side through the relay board and bypassing it on the negative side.
The second amp board is still connected to the ground lift circuit.
powering the relay board from the + - input of the first amplifier board bring it immediately into DC protection.
powering it from the LM1875 protection circuit gives a buzzing sound.
....
I think I find something.
Using the relay board, grounds are connected of both JLH amp boards. Then there is a connection to the ground lift circuit (10 ohm/ 100nF) for both boards.
Without the relay board there is no connection between both grounds apart from the route of the input signal.
I assume this route is not optimal and has some raised resistance.
When I directly link the second board ground to the ground lift circuit, which is connected to the first board, the humming stops. (orange line to the ground lift circuit)
This is without the relay board. (I assume this link is also created by the relay board and the shared ground)
Using the relay board will make a link between its power input and speaker ground which cause the humming.
Next, I connected only the speakers positive side through the relay board and bypassing it on the negative side.
The second amp board is still connected to the ground lift circuit.
powering the relay board from the + - input of the first amplifier board bring it immediately into DC protection.
powering it from the LM1875 protection circuit gives a buzzing sound.
....
Yes, the connection of the boards via the grounds is most likely the issue but if you follow what I keep outlining that should not happen 🙂 because you are using the board as a delay circuit only and it does not even need to be ground referenced for that.
Think it through... from the position of it all working hum free and then adding the board.
If you hold the relay board in your hand and connect one single wire from the boards negative supply input, which will be ground if you feed it from one of the JLH supplies the amp can not then suddenly start to hum. Get it to that state.
Now connect the positive supply to the board with it still free and not in the amp. It still should all be hum free.
Fix the board in the amp. Still hum free... it should be.
Now just wire the positive speaker feeds only through the relays. That still should be hum free.
Board is hum free if speakers are bypassing the relay board and ground between both boards are connected to the ground lift.
Noise free.
noise free
I try to do this tonight.
I discovered that one of my crocodile wires is broken...so it probably gave me some false conclusions.
Since you mention it, be careful with crocodile wires purchased on Amazon and other Aliexpress, they look good, the clamps are correct but on all the samples that I had in my hands, the wires only contain a few conductive strands and the wires are not welded, but just pinched clumsily on the crocodile clip.
Remove all wiring. Connect speakers directly to amp. Connect only power to protection board. In a few seconds hear relay click. The protection board works.
Establish if you hear a hum on speakers, If you do hear hum, then you have a grounding problem in the amp. If no hum then remove speaker connections from amp and connect speakers to protection board. Turn on power hear relay click. Listen for hum if you hear hum there is something odd on the protection board, if not then turn off. Connect amplifier output connection to protection board. Turn power on, relay clicks, there should be no hum.
Establish if you hear a hum on speakers, If you do hear hum, then you have a grounding problem in the amp. If no hum then remove speaker connections from amp and connect speakers to protection board. Turn on power hear relay click. Listen for hum if you hear hum there is something odd on the protection board, if not then turn off. Connect amplifier output connection to protection board. Turn power on, relay clicks, there should be no hum.
Good 🙂
I was going to say this has to work... but... and easy to fix...
Looking back on the thread did you go from using a 100 ohm resistor on each channel to charge the output cap to using a N/C relay that shorts the output to ground? I think you did.
If you did that then the relays need to have their common connection point separated both from each other and from any ground points in the relay board circuit they connect to. In other words the relay contact pins need to be all isolated from everything and then wired individually wired to each JLH. A sharp craft knife is good for cleanly removing print if needed.
@Mooly
I think I have a decent working amp now. The hum is a bit louder on the JLH board then on the LM1875, but in both cases I need to put my ear next to the speaker to hear it.
I saw that I put a 100 ohm resistor on the ground lift iso an 10 ohm. This cases some hum which took about 1 minute to disappear Most of the test where just power on and off, so I only notice the hum not the fact that it faded out.
Anyway I put a 10 ohm resistor on the ground lift.
3 crocodile wires had a bad connections. This caused many wrong test interpretations. Time to replace them.
I started to replace all connections one by one and see if i got any improvement. I ended with all wires going through the relay board. powered from the input of 1 jlh board (its 25Vdc, the board has an 25v rated input cap after a diode, so its just just.)
The board with 10 ohm ground lift, 10ohm resistor on the NC contact of the relay and both speakers + and - through the protection board seems to work with a slight hum only audible when you put your ear next to the speaker.
The only issue now is that the protection board takes a while to switch off after the power has been cut. The discharge of the power supply capacitors take around 3-5 seconds. Not sure if its going to be any problem.
Thanks for your time and suggestions it helped me to debug and learned a few things on the way.
I think I have a decent working amp now. The hum is a bit louder on the JLH board then on the LM1875, but in both cases I need to put my ear next to the speaker to hear it.
I saw that I put a 100 ohm resistor on the ground lift iso an 10 ohm. This cases some hum which took about 1 minute to disappear Most of the test where just power on and off, so I only notice the hum not the fact that it faded out.
Anyway I put a 10 ohm resistor on the ground lift.
3 crocodile wires had a bad connections. This caused many wrong test interpretations. Time to replace them.
I started to replace all connections one by one and see if i got any improvement. I ended with all wires going through the relay board. powered from the input of 1 jlh board (its 25Vdc, the board has an 25v rated input cap after a diode, so its just just.)
The board with 10 ohm ground lift, 10ohm resistor on the NC contact of the relay and both speakers + and - through the protection board seems to work with a slight hum only audible when you put your ear next to the speaker.
The only issue now is that the protection board takes a while to switch off after the power has been cut. The discharge of the power supply capacitors take around 3-5 seconds. Not sure if its going to be any problem.
Thanks for your time and suggestions it helped me to debug and learned a few things on the way.
Pleased to hear you are getting there 🙂
The slow switch off is only a problem if it allows unwanted noise to get through as the rails discharge. A lot of threads and posts have passed my way since you started on this 🙂 so its difficult to pick up the line of thought of how we configured it all.
The only way to get an instant drop out is to power the relay board from AC (of suitable voltage, maybe the JLH supply) where the AC detect on the board is then functional and this triggers the relay the instant AC is removed.
It might be possible to retain your DC powering but use the AC detect line to pick up the AC from the transformer.
Don't do anything in a hurry and if there isn't really an issue in use then its fine as is.
I'm thinking you could cut this line as shown and connect it to the transformer secondary AC. Untried of course but the theory is sound enough at this point.
The slow switch off is only a problem if it allows unwanted noise to get through as the rails discharge. A lot of threads and posts have passed my way since you started on this 🙂 so its difficult to pick up the line of thought of how we configured it all.
The only way to get an instant drop out is to power the relay board from AC (of suitable voltage, maybe the JLH supply) where the AC detect on the board is then functional and this triggers the relay the instant AC is removed.
It might be possible to retain your DC powering but use the AC detect line to pick up the AC from the transformer.
Don't do anything in a hurry and if there isn't really an issue in use then its fine as is.
I'm thinking you could cut this line as shown and connect it to the transformer secondary AC. Untried of course but the theory is sound enough at this point.
Good day all. I am up to page 206 of 487.
I am primarily interested in the original 69 circuit and have two E(vil)bay kits on order, one 2N3055 TO-3 and one 2SD718.
Of special interest is the PNP output version as I have 10 1980s production MJ490s. I also have 10ea 80s production 2N3055s and a few MJ3055s (TO220 package). So, this should be an interesting build.
I will be driving Heresy speakers, so 10W is plenty. I am curious to see how this compares to my DH-200s, which were designed by JLH as well.
I am primarily interested in the original 69 circuit and have two E(vil)bay kits on order, one 2N3055 TO-3 and one 2SD718.
Of special interest is the PNP output version as I have 10 1980s production MJ490s. I also have 10ea 80s production 2N3055s and a few MJ3055s (TO220 package). So, this should be an interesting build.
I will be driving Heresy speakers, so 10W is plenty. I am curious to see how this compares to my DH-200s, which were designed by JLH as well.
Last edited:
Good morning friends... It was during the Covid lockdown days that I noticed how the John Linsley-Hood ('JLH') Class-A amplifier had come past its Golden Jubilee, and that needed a 'celebration'. Naturally I wanted once again to build an amp that I had built for most of of my friends back in the 1970s. (Ah, then we were all young, and went chasing after high power amp designs...that's another story!) Now I wanted to go 'deep into' JLH's classic design and decide to build the version I thought best reflected my preferences. But sadly scores of useful links were dead, and the DIYAUDIO thread had grown humongous (just a couple of hundred posts to reach 10,000!). So I set about reading that and culling info from the collected wisdom of the group. Also, I dug up my old PC backups.
This took time, and what originally was planned as a Golden Jubilee Tribute to the Master is ready for posting only by now. Better late than never, I guess...
Through my blog posts I wish to share my collection of published material by JLH, with the stress on the "Simple Class A amplifier" first published in the Wireless World, April 1969, issue. My feeling is that for the ardent JLH fan and DIYer, my blog will serve as a "single window" to archival information, other relevant material, and gleanings from the various forums. Hope most of you will find this compendium helpful in your DIY pursuits.
https://jlhclassicdesigns.blogspot.com/2024/11/the-jlh-gateway-golden-jubilee-tribute.html
This took time, and what originally was planned as a Golden Jubilee Tribute to the Master is ready for posting only by now. Better late than never, I guess...
Through my blog posts I wish to share my collection of published material by JLH, with the stress on the "Simple Class A amplifier" first published in the Wireless World, April 1969, issue. My feeling is that for the ardent JLH fan and DIYer, my blog will serve as a "single window" to archival information, other relevant material, and gleanings from the various forums. Hope most of you will find this compendium helpful in your DIY pursuits.
https://jlhclassicdesigns.blogspot.com/2024/11/the-jlh-gateway-golden-jubilee-tribute.html
I am aware of that sir, but common parlance has accepted JLH, and I thought I would stick to that ... simple, short and sweet! (By the way, it is Laurence ... )
I am sure you too are likely to have old material in your collection. Do share any that I might have missed. I am counting on senior members like you to further help 'refine' my posts.
https://en.wikipedia.org/wiki/John_...insley-Hood (9,War Two, at Reading University.
I am sure you too are likely to have old material in your collection. Do share any that I might have missed. I am counting on senior members like you to further help 'refine' my posts.
https://en.wikipedia.org/wiki/John_...insley-Hood (9,War Two, at Reading University.
The JLH Gateway: Part-2
https://jlhclassicdesigns.blogspot.com/2024/11/the-jlh-gateway-golden-jubilee-tribute-2.html
https://jlhclassicdesigns.blogspot.com/2024/11/the-jlh-gateway-golden-jubilee-tribute-2.html