Hi there people.
A year ago i rediscovered electronics after 15 years of just selling them. in 95 i studied electronics to become a radio/tv repairman but being a rebelling student not too focused on school and then not giving electronics any attention for 15 years haven't given me the most knowledge or experience with fixing and understanding it.
I found a JLH 2x10W amplifier pcb kit with components on ebay and started working from there. I buildt the JLH in 92-93 at school, but we kinda never got it working as it should. We then used 2n3055, and this one from ebay also came with those.
I replaced the outputs with MJE15003's. The two other transistors is a A970 and a c2240. I dont understand the workings about transistor schematics / datasheets and to know what to look for to understand if these are gonna work good with this circuit, but i trusted the guy putting the kit together. I've measured all the resistors and can confirm that their values are correct compared to the schematic on the original circuit at http://www.tcaas.btinternet.co.uk/jlh1969.pdf
Now if these values still are correct with the "new" transistors is something else. That's where my knowledge comes short.
The problem is...
My amplifier doesn't get warm at all. I can feel only a slight difference between it being off or on, but i remember the one we buildt at school became HOT. I DO have slightly better heatsinks, but i first buildt it with comparatively small heatsinks and still almost nothing.
R1 and R2 are 100 and 560 Ohm, since i'm using 8 Ohm speakers. Also i replaced all the electrolytics with Elna Silmic II. The sound is very nice, but being a class A amplifier, it SHOULD generate heat and since it doesn't, i assume the quiescent current is too low.
I've given it a rather large power supply, regulated it at 27V exactly and thrown in 1F capacitors for each channel.
How to solve this? Do i mess with the R1/R2 or is will that screw up the voltages over the rest of the transistors as well?
This problem haunts me and i would really, really appreciate help.
Again, i know i should know this, but i don't.
EDIT: I forgot to mention, in a desperate move, i replaced the c2240 with a 2sc3421 which i use in the 2x40w JLH (which DOES get hot and sounds beautiful) but it didn't change anything.
Ben Cato Malkenes.
A year ago i rediscovered electronics after 15 years of just selling them. in 95 i studied electronics to become a radio/tv repairman but being a rebelling student not too focused on school and then not giving electronics any attention for 15 years haven't given me the most knowledge or experience with fixing and understanding it.
I found a JLH 2x10W amplifier pcb kit with components on ebay and started working from there. I buildt the JLH in 92-93 at school, but we kinda never got it working as it should. We then used 2n3055, and this one from ebay also came with those.
I replaced the outputs with MJE15003's. The two other transistors is a A970 and a c2240. I dont understand the workings about transistor schematics / datasheets and to know what to look for to understand if these are gonna work good with this circuit, but i trusted the guy putting the kit together. I've measured all the resistors and can confirm that their values are correct compared to the schematic on the original circuit at http://www.tcaas.btinternet.co.uk/jlh1969.pdf
Now if these values still are correct with the "new" transistors is something else. That's where my knowledge comes short.
The problem is...
My amplifier doesn't get warm at all. I can feel only a slight difference between it being off or on, but i remember the one we buildt at school became HOT. I DO have slightly better heatsinks, but i first buildt it with comparatively small heatsinks and still almost nothing.
R1 and R2 are 100 and 560 Ohm, since i'm using 8 Ohm speakers. Also i replaced all the electrolytics with Elna Silmic II. The sound is very nice, but being a class A amplifier, it SHOULD generate heat and since it doesn't, i assume the quiescent current is too low.
I've given it a rather large power supply, regulated it at 27V exactly and thrown in 1F capacitors for each channel.
How to solve this? Do i mess with the R1/R2 or is will that screw up the voltages over the rest of the transistors as well?
This problem haunts me and i would really, really appreciate help.
Again, i know i should know this, but i don't.
EDIT: I forgot to mention, in a desperate move, i replaced the c2240 with a 2sc3421 which i use in the 2x40w JLH (which DOES get hot and sounds beautiful) but it didn't change anything.
Ben Cato Malkenes.
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I just used my multimeter and measured the currents drawn from the 27V directly, it's 0,59A. This should be 1.2A for a 8 Ohm load, right?
I have seen some drawings with this amp where the 560 Ohm resistor is replaced with a 1.2K pot, but all i got are those multi-turn pots, isn't it too high a current for those?
Thanks for helping
Ben.-
Linear class A into 8 ohms would require 850mA
quiescent to swing rail-to-rail from 0 to 27V....
Considering JLH currents are non-linear in a way
that runs far hotter than Linear A when its doing
nothing, you would probably need at least 1A.
JLH takes more base current for one transistor
to reach the rail than it does for both to stay
perfectly idle in the center. A slightly excessive
bias becomes the necessary evil of that curve.
Got workaround for that, but its a whole other
topic. Let's fix your more serious problem first.
quiescent to swing rail-to-rail from 0 to 27V....
Considering JLH currents are non-linear in a way
that runs far hotter than Linear A when its doing
nothing, you would probably need at least 1A.
JLH takes more base current for one transistor
to reach the rail than it does for both to stay
perfectly idle in the center. A slightly excessive
bias becomes the necessary evil of that curve.
Got workaround for that, but its a whole other
topic. Let's fix your more serious problem first.
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I am in disagreement with JLH over R1 R2 not paired of the same value.
Mismatch increases parasitic loss within the bootstrap for same bias...
I don't have a clue how much base current an MJE15003 would need
to conduct 1.7A peaks? But that requirement would dictate how much
bootstrap current you must have, at a bare minimum.
Mismatch increases parasitic loss within the bootstrap for same bias...
I don't have a clue how much base current an MJE15003 would need
to conduct 1.7A peaks? But that requirement would dictate how much
bootstrap current you must have, at a bare minimum.
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For 100mA base current max, boot resistors might be about 68R apiece?
I'm figuring 135R for dropping 13.5V @ 100mA in the quiescent state.
And bootstrap wants to tap right in the middle for best performance.
So this splits to a pair of 68R or maybe 75R. Gotta figure Beta will be
riding a hotter curve than 25C... Dude, its JLH! Get me a potholder...
I'm figuring 135R for dropping 13.5V @ 100mA in the quiescent state.
And bootstrap wants to tap right in the middle for best performance.
So this splits to a pair of 68R or maybe 75R. Gotta figure Beta will be
riding a hotter curve than 25C... Dude, its JLH! Get me a potholder...
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Hi again and again thanks for helping.
R1 and R2 together make up the bootstrap, right? I ask since as i said in the initial post, i've forgotten so much, consider me a beginner who knows Ohm's law and how to solder. :-D
I lost my old school books and today's books is not nearly as thorough as they were in 91-94. I've found some pdf's of old books and i got JLH's book of amplifiers and some other analogue / amplifier books, but it's hard to re-understand it all by yourself. (i think).
So if R1 and R2 is what's called the bootstrap, are you suggesting i lower the values of both? Won't that also raise the collector voltage on TR3? or is that needed to also in the process raise the collector current of TR1?
When it comes to R/C impedances in the circuit, i'm lost. That's way above me i think. :-/
Also: Jonathan, i feel i owe you a reply. I did post in the other JLH thread some half year or so ago. I cannot remember if it was about this specific issue, but i didn't get it solved even though Geoff and some other people with high knowledge replied (again, can't remember if it was about this specific issue) but i thought if i made a topic with this specific issue, it would be easier for others with the same problem to also find it in the future. I might be wrong but that's why i chose to make another topic.
Ben.-
R1 and R2 together make up the bootstrap, right? I ask since as i said in the initial post, i've forgotten so much, consider me a beginner who knows Ohm's law and how to solder. :-D
I lost my old school books and today's books is not nearly as thorough as they were in 91-94. I've found some pdf's of old books and i got JLH's book of amplifiers and some other analogue / amplifier books, but it's hard to re-understand it all by yourself. (i think).
So if R1 and R2 is what's called the bootstrap, are you suggesting i lower the values of both? Won't that also raise the collector voltage on TR3? or is that needed to also in the process raise the collector current of TR1?
When it comes to R/C impedances in the circuit, i'm lost. That's way above me i think. :-/
Also: Jonathan, i feel i owe you a reply. I did post in the other JLH thread some half year or so ago. I cannot remember if it was about this specific issue, but i didn't get it solved even though Geoff and some other people with high knowledge replied (again, can't remember if it was about this specific issue) but i thought if i made a topic with this specific issue, it would be easier for others with the same problem to also find it in the future. I might be wrong but that's why i chose to make another topic.
Ben.-
Feedback from TR4 raises current through TR3 to restore voltage balance.
The collector you asked, voltage behavior won't be changed, only current...
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The circuit if built correctly will run at 1.1 amp quescient current with a 27VDC supply. There is nothing wrong with the biasing arrangement. See if you do not have a mistake in your circuit somewhere.
Ic/Ib of different power devices will have a slight difference in bias current but not what you are experiencing. 1.1 amp through the power devices would generate 30 watt of heat and you will be aware of it getting hot.
Recheck everything carefully.
Ic/Ib of different power devices will have a slight difference in bias current but not what you are experiencing. 1.1 amp through the power devices would generate 30 watt of heat and you will be aware of it getting hot.
Recheck everything carefully.
I am pretty sure i have measured all the components, but too be positive before i promise anything, i will measure them tomorrow. It's too dark here now and i can't find my flashlight.
Thanks a lot for the continued help.
Ben.-
Hi again, and sorry for it taking so long. I have measured and also double checked the color codes on the resistors and they are all correct compared to the schematic.
Considering i've replaced the output transistors with MJE15003's and the other transistors with A970 / c2240 (also tried to switch the last one with a 2sc3421 with no change), will the voltages and currents still remain the same or do some of them require a higher voltage?
Ben.-
The 2SC2240 and 2SA970 uses a different pin configuration than the typical BC types.
I've found the datasheets on both the A970 and it's E-C-B. Same with the c2240 and the c3421. One thing i haven't tried is to check the output transistors for failure, but could the amplifier deliver wonderful music if they were broken? The amplifier sounds really nice even though it's not getting warm.
Also if earthing was a problem, wouldn't it be audible?
Also if earthing was a problem, wouldn't it be audible?
Hi Shamron, have you come to any conclusions because if you say everything is right, then I am a little lost here. All indications are that if this amp is running in class A it has to be hot, not a little but a lot. Unless you have hugely efficient heat sinks.
Earthing problems normally manifest itself as hum or output off-set, maybe others would disagree.
Earthing problems normally manifest itself as hum or output off-set, maybe others would disagree.
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