Greetings to everybody and thank you for letting me post my rather stupid questions. Hopefully I can contribute to the forum as I learn more.
I'm currently trying to build a stereo amp using a pair of Chinese JLH 1969 kits. At this point, I've stuffed a PCB from one of the kits, but I want to be sure of my power source before learning how to adjust/test the circuit. I don't want to fry anything. Despite plowing through loads of posts from various sources and watching Youtube, I still haven't found a simple step-by-step guide to building this amp.
Does anybody have a link to a set of basic instructions on how to build a kit like mine? Are there any instructions out there on how to build a basic power supply for this amp?
Is it okay to use a "wall wart" transformer that provides 12V dc from standard US household AC? If the "wall wart" is okay (even if just for adjusting/testing), what is a typical 12V current rate - 1A? 1.5A? 2A? I'm not trying to modify the amp circuit, so whatever voltage/current gets the "standard" performance is fine.
Thanks for any help. Again, I apologize for such basic questions.
I'm currently trying to build a stereo amp using a pair of Chinese JLH 1969 kits. At this point, I've stuffed a PCB from one of the kits, but I want to be sure of my power source before learning how to adjust/test the circuit. I don't want to fry anything. Despite plowing through loads of posts from various sources and watching Youtube, I still haven't found a simple step-by-step guide to building this amp.
Does anybody have a link to a set of basic instructions on how to build a kit like mine? Are there any instructions out there on how to build a basic power supply for this amp?
Is it okay to use a "wall wart" transformer that provides 12V dc from standard US household AC? If the "wall wart" is okay (even if just for adjusting/testing), what is a typical 12V current rate - 1A? 1.5A? 2A? I'm not trying to modify the amp circuit, so whatever voltage/current gets the "standard" performance is fine.
Thanks for any help. Again, I apologize for such basic questions.
I think you have to use the schematic to figure out where each part goes.
With 12VDC, 2A would be ok. Some use laptop supplies for the power.
With 12VDC, 2A would be ok. Some use laptop supplies for the power.
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Welcome to diyAudio 🙂
The standard JLH69 requires a 27 volt DC power supply and the current requirement was 1.2A for the original design (per channel). So for reliable use you need at least a 3amp capable supply which is way above a typical wall wart capability.
A switching type supply like a laptop brick providing 24 volts might be a good option. The Meanwell one used in the popular ACA amp comes to mind.
Do not under estimate just how hot these amps run, they are Class A dissipate near to 35 watts per channel under no signal conditions. That means you need suitable heatsinks.
Here is the original article that should answer many of your questions.
The standard JLH69 requires a 27 volt DC power supply and the current requirement was 1.2A for the original design (per channel). So for reliable use you need at least a 3amp capable supply which is way above a typical wall wart capability.
A switching type supply like a laptop brick providing 24 volts might be a good option. The Meanwell one used in the popular ACA amp comes to mind.
Do not under estimate just how hot these amps run, they are Class A dissipate near to 35 watts per channel under no signal conditions. That means you need suitable heatsinks.
Here is the original article that should answer many of your questions.
here is some more reading material for you... https://sound-au.com/tcaas/index-1.htm
Tho this type of amps looking fairly easy to assemble, you must bear in mind they operate in 'class A' and HUGE heatsinks are mandatory... proper power supply too... as others stated laptop bricks might do the work, simple wallwart transformer just won't do....
What do you plan to achieve with it? School project? Headphone amp? Main livingroom system? Another problem with 'chifi' kits might be fake parts in kit...
Anyway, if you are just starting with this hobby I would recommend some 'chipamp' kit if you want to blindly follow assembly by PCB, and then slowly graduate to more complex projects...
Tho this type of amps looking fairly easy to assemble, you must bear in mind they operate in 'class A' and HUGE heatsinks are mandatory... proper power supply too... as others stated laptop bricks might do the work, simple wallwart transformer just won't do....
What do you plan to achieve with it? School project? Headphone amp? Main livingroom system? Another problem with 'chifi' kits might be fake parts in kit...
Anyway, if you are just starting with this hobby I would recommend some 'chipamp' kit if you want to blindly follow assembly by PCB, and then slowly graduate to more complex projects...
Thank you for your kind replies, guys. I'm eager to hear how this amp sounds. I'm assuming the amp acts like a pair of BBC LS 3/5a monitors - it might not have great "stats" but it's musical at reasonable volumes in a smaller space.
Fully agree.
I must add that it is one of the amps that is the easiest to operate but like any simple amp, each component of the amp becomes extremely important.
The simpler the recipe, the more difficult it is to get the quintessence out of it, but whatever it is, I have never heard this amp sound bad.
A couple of pics of the situation up to this point. Could someone confirm that the aluminum heat sink shown looks big enough for normal use? I have another for the other (as yet unstuffed) board:
This is to share what I assume is a rather poor job of soldering. Be kind - this is the first PCB soldering job. I understand that resistors have no polarity, so they may not all be "aligned" the same way. I've checked the 3055's for potential short circuits, otherwise I assume I'll find out if something doesn't work when I attach a power supply. If anybody has basic "pre-power up" checks to suggest, feel free to share.
This is to share what I assume is a rather poor job of soldering. Be kind - this is the first PCB soldering job. I understand that resistors have no polarity, so they may not all be "aligned" the same way. I've checked the 3055's for potential short circuits, otherwise I assume I'll find out if something doesn't work when I attach a power supply. If anybody has basic "pre-power up" checks to suggest, feel free to share.
I'm not sure on the heatsink tbh although it is enough to see where it is all at. I see two presets on the board so I assume you can alter the bias current. Lower current means cooler running. A current of just 300ma will still get over 1 watt into 8 ohms which is actually a lot louder than you might think.
If you keep the back of a finger on the transistor for 6 or 7 seconds before it starts to be to burn you then its OK.
Your soldering looks fine 🙂
If you keep the back of a finger on the transistor for 6 or 7 seconds before it starts to be to burn you then its OK.
Your soldering looks fine 🙂
The latest dumb question:
I've hooked the amp up to a 12V 1.25a "wall wart" converter and the amp sounds good and barely gets warm. The speaker being driven is a small (5.25" two way) 6 ohm load. Plenty of volume for me at this voltage.
I managed to figure out how to set the quiescent voltage for KT1 @ 6V. I assume this is the correct value for this load.
I'm now trying to set the current for KT2 (circled in the picture). With the power transformer I mentioned, at what value should this resistor be set? Should the multimeter be connected to Ground and the positive lead of the big capacitor (C6) like it was with the KT1 adjustment?
Again, thanks for your help.
I've hooked the amp up to a 12V 1.25a "wall wart" converter and the amp sounds good and barely gets warm. The speaker being driven is a small (5.25" two way) 6 ohm load. Plenty of volume for me at this voltage.
I managed to figure out how to set the quiescent voltage for KT1 @ 6V. I assume this is the correct value for this load.
I'm now trying to set the current for KT2 (circled in the picture). With the power transformer I mentioned, at what value should this resistor be set? Should the multimeter be connected to Ground and the positive lead of the big capacitor (C6) like it was with the KT1 adjustment?
Again, thanks for your help.
Apologies for my question (that has probably been asked far too many times). I found an answer in an old thread here.
Just being a bit timid - I don't want to fry anything...
Just being a bit timid - I don't want to fry anything...
That'll probably happen sooner or later anyway to everyone, so don't worry about it.
Yet another basic question:
I've wired up a six-pin stereo pot as a volume control for the two amps. Both amps are set at the same input voltage and each KT1 is at 1/2 input voltage, but I still can't verify the current setting for either amp (KT2 level).
At this point, volume output of one amp be much higher than the other. I assume each amp's volume output is tied to its current setting (whatever that is)?
I thought it was just a matter of connecting the multimeter in-series to incoming power and then adjusting the KT2 resistor to set current. I can turn the KT2 resistor screw several times and get no change in the current level. What am I doing wrong?
I found this post from Mooly on how to connect/set up the multimeter:
Thanks for any help and for your patience with my questions.
I've wired up a six-pin stereo pot as a volume control for the two amps. Both amps are set at the same input voltage and each KT1 is at 1/2 input voltage, but I still can't verify the current setting for either amp (KT2 level).
At this point, volume output of one amp be much higher than the other. I assume each amp's volume output is tied to its current setting (whatever that is)?
I thought it was just a matter of connecting the multimeter in-series to incoming power and then adjusting the KT2 resistor to set current. I can turn the KT2 resistor screw several times and get no change in the current level. What am I doing wrong?
I found this post from Mooly on how to connect/set up the multimeter:
Thanks for any help and for your patience with my questions.
The gain (or volume) is not affected at all by the bias setting.
The high bias current just eliminates crossover distortion.
What kind of trim pot do you have (brand and model), and is it one turn or 20 turn?
If it is a 20 trim pot, just keep turning. If it clicks, that is the maximum (or minimum) setting.
If it's a single turn trim pot, it will stop at the maximum setting.
But make sure which direction you are supposed to be turning it.
Some may go counterclockwise instead, to increase the bias current.
The high bias current just eliminates crossover distortion.
What kind of trim pot do you have (brand and model), and is it one turn or 20 turn?
If it is a 20 trim pot, just keep turning. If it clicks, that is the maximum (or minimum) setting.
If it's a single turn trim pot, it will stop at the maximum setting.
But make sure which direction you are supposed to be turning it.
Some may go counterclockwise instead, to increase the bias current.
An update...
I tried a new multimeter and was able to measure/adjust the current level for the KT2 trim pot. I have the KT2 trim set at 1A for now. My amp sounds pretty good and doesn't seem to get too warm. I have no idea of how hot my little JLH's "should" get, though.
Here's my question now:
I have two of the Chinese JLH circuit shown above wired to a common power supply socket. For reasonable-volume listening (quality over quantity), what KT2 current should be set for a laptop power supply with a rated output of 12V 3A? I think the power supply in question is a switching-type. I assume KT1 (quiescent current) will be 1/2 of whatever KT2 is set at.
The more posts I read elsewhere on the forum (like the long JLH thread) the more confused I become. Is it true that if a class A circuit like the JLH is not fed sufficient current, it no longer functions as a "class A" circuit? I hope my 12V 3A supply can provide enough juice to perform well (I'm interested in sound quality, not quantity).
With my power supply, I assume my rms wattage will be lower than 10W, but I can't get the right values.
I've struggled with the calcs based on the original Wireless World article. As I have no engineering background, even "basic knowledge" to an EE is exotic to me.
Thanks for any help you're willing to provide...
I tried a new multimeter and was able to measure/adjust the current level for the KT2 trim pot. I have the KT2 trim set at 1A for now. My amp sounds pretty good and doesn't seem to get too warm. I have no idea of how hot my little JLH's "should" get, though.
Here's my question now:
I have two of the Chinese JLH circuit shown above wired to a common power supply socket. For reasonable-volume listening (quality over quantity), what KT2 current should be set for a laptop power supply with a rated output of 12V 3A? I think the power supply in question is a switching-type. I assume KT1 (quiescent current) will be 1/2 of whatever KT2 is set at.
The more posts I read elsewhere on the forum (like the long JLH thread) the more confused I become. Is it true that if a class A circuit like the JLH is not fed sufficient current, it no longer functions as a "class A" circuit? I hope my 12V 3A supply can provide enough juice to perform well (I'm interested in sound quality, not quantity).
With my power supply, I assume my rms wattage will be lower than 10W, but I can't get the right values.
I've struggled with the calcs based on the original Wireless World article. As I have no engineering background, even "basic knowledge" to an EE is exotic to me.
Thanks for any help you're willing to provide...
If you are running the JLH on 12 volts DC you need only set the bias current to approx 450 milliamps. There is no point setting it higher because the 12 volt supply puts a limit on the voltage that can be put across an 8 ohm speaker.
On 12 volts the absolute maximum across the speaker can not be higher than -/+6 volts (6 volts positive swing and 6 volts negative swing). The AC coupling of the speaker to the amp 'removes' the DC voltage and we are able to ground reference our readings... so we see -/+6 volts maximum across the speaker.
In practice because of losses it will be more like -/+5 volts and the current in the speaker a maximum of I=V/R which 5/8 equals a peak current of 0.625 amps.
-/+ 5 volts output (the maximum you can expect on 12 volts supply) translates to a power output of 5/1.414 (1.414 is square root of 2, a magic number we use all the time) which is 3.5 volts. The maximum power into 8 ohms is (3.5*3.5)/8 which is 1.5 watts. That is the most 'power' you can deliver into 8 ohms on your 12 volt supply.
On most class AB amps we can over bias them into Class A if we wanted. If we under bias them or even reduce the bias current to zero the amp essentially still performs almost as well with just a tiny increase in distortion.
The JLH is very different because of the nature of a Class A push pull output stage. If you under bias it you put a limit on the maximum current it can deliver to a load. For example if we bias the JLH to 100 milliamps it could theoretically deliver seemingly twice that value -/+200 milliamps to the load. That would be at around -/+1.5 volts peak to peak voltage swing at the output and a power of just 130 milliwatts into the 8 ohm load. Beyond that point the JLH would distort terribly and simply 'crush' the output voltage peaks to that -/+1.5 volt level.
On 12 volts the absolute maximum across the speaker can not be higher than -/+6 volts (6 volts positive swing and 6 volts negative swing). The AC coupling of the speaker to the amp 'removes' the DC voltage and we are able to ground reference our readings... so we see -/+6 volts maximum across the speaker.
In practice because of losses it will be more like -/+5 volts and the current in the speaker a maximum of I=V/R which 5/8 equals a peak current of 0.625 amps.
-/+ 5 volts output (the maximum you can expect on 12 volts supply) translates to a power output of 5/1.414 (1.414 is square root of 2, a magic number we use all the time) which is 3.5 volts. The maximum power into 8 ohms is (3.5*3.5)/8 which is 1.5 watts. That is the most 'power' you can deliver into 8 ohms on your 12 volt supply.
You need to read up on the different types of amplifier classes (A, AB and B) to get a handle on what is going on.
On most class AB amps we can over bias them into Class A if we wanted. If we under bias them or even reduce the bias current to zero the amp essentially still performs almost as well with just a tiny increase in distortion.
The JLH is very different because of the nature of a Class A push pull output stage. If you under bias it you put a limit on the maximum current it can deliver to a load. For example if we bias the JLH to 100 milliamps it could theoretically deliver seemingly twice that value -/+200 milliamps to the load. That would be at around -/+1.5 volts peak to peak voltage swing at the output and a power of just 130 milliwatts into the 8 ohm load. Beyond that point the JLH would distort terribly and simply 'crush' the output voltage peaks to that -/+1.5 volt level.
Totally disagree on that. A properly constructed JLH can deliver truly excellent sonics.
Thanks very much for your replies, guys.
I neglected to mention that my speakers are rated a 6 ohms. Given my awful understanding of mathematics (I'm an old history major) it'll be interesting to see the results of my "calculations".
In all honesty, I'm going springboard off Mooly's generous posting of his calculations. 🙂
EDIT: So six ohms gets me a figure of around 835mA, right? I wonder how I can cross the street without getting killed...
I neglected to mention that my speakers are rated a 6 ohms. Given my awful understanding of mathematics (I'm an old history major) it'll be interesting to see the results of my "calculations".
In all honesty, I'm going springboard off Mooly's generous posting of his calculations. 🙂
EDIT: So six ohms gets me a figure of around 835mA, right? I wonder how I can cross the street without getting killed...
Lets see...
5 volt peak across 6 ohms is a peak speaker current of 0.833 amp (so your spot on there 👍) and a power output of 2 watts rms. The amp would need biasing to around 500ma to play it safe although theoretically half of the 833ma (so approx 415ma) should suffice. I would go a bit higher though.
Just for info... impedance such as for a speaker is not a constant, it varies with frequency. A resistor on the other hand is fixed in value and gives the same loading at any frequency.
Here is the impedance plot of a small 2 way speaker, the B&W CM1. You can see it varies between around 50 ohms all the way down to 5 ohm and yet this is classed (by B&W) as an 8 ohm nominal impedance.
The efficiency of the speaker is really what makes the difference between how loud one speaker is vs another.
5 volt peak across 6 ohms is a peak speaker current of 0.833 amp (so your spot on there 👍) and a power output of 2 watts rms. The amp would need biasing to around 500ma to play it safe although theoretically half of the 833ma (so approx 415ma) should suffice. I would go a bit higher though.
Just for info... impedance such as for a speaker is not a constant, it varies with frequency. A resistor on the other hand is fixed in value and gives the same loading at any frequency.
Here is the impedance plot of a small 2 way speaker, the B&W CM1. You can see it varies between around 50 ohms all the way down to 5 ohm and yet this is classed (by B&W) as an 8 ohm nominal impedance.
The efficiency of the speaker is really what makes the difference between how loud one speaker is vs another.
Thank you very much for your help, Mooly. I've been listening to the amp set at around 835mA and it's already sounding better. It will be interesting to hear things after I change the bias to around 415mA or so as you suggest (I think it's currently set at about 600mA).
I can see why this amplifier circuit is a classic.
I can see why this amplifier circuit is a classic.