Rather than create another "beginner question" thread, I'm adding another question here. Thanks in advance for any help.
It appears that one of the trimpots (KT2) on my Chinese JLH69 board isn't working correctly. With 24V going into the board, I am unable to get the current below about 1.6 amps. I tried another 202 trimpot and got the same issue - both trimpots bottom out at about 1.6 amps. Would simply increasing the resistance value of KT2 get the current down to 1.2 amps?
If increasing the resistance value of KT2 is the way to go, would a fixed-value resistor work in place of the KT2 trimpot? Is the substitution a simple as inserting the fixed resistor into the outer holes of the trimpot's PCB location? I'm thinking of simply swapping fixed resistors until I get the current amount that I'm looking for.
Some pictures to help with my issue. The first is the amp (which I'm sure is quite familiar). The second is a schematic drawn by a YouTube source based on my amp's circuit - I've added basic notes to help my understanding of the situation.
It appears that one of the trimpots (KT2) on my Chinese JLH69 board isn't working correctly. With 24V going into the board, I am unable to get the current below about 1.6 amps. I tried another 202 trimpot and got the same issue - both trimpots bottom out at about 1.6 amps. Would simply increasing the resistance value of KT2 get the current down to 1.2 amps?
If increasing the resistance value of KT2 is the way to go, would a fixed-value resistor work in place of the KT2 trimpot? Is the substitution a simple as inserting the fixed resistor into the outer holes of the trimpot's PCB location? I'm thinking of simply swapping fixed resistors until I get the current amount that I'm looking for.
Some pictures to help with my issue. The first is the amp (which I'm sure is quite familiar). The second is a schematic drawn by a YouTube source based on my amp's circuit - I've added basic notes to help my understanding of the situation.
The easiest way would be to increase what seems to be R8 in your drawing. If 220 ohm is fitted then try 270 or 330 ohm.
This one:
This one:
Thank you, Mooly!
For giggles, I tried the amp @ 24V with 1.3 amps. I was surprised at how cool the amp was after driving a pair of 4 ohm speakers for an hour.
My new 24V power supply is a Mean Well LRS-100-24 with an adjustable voltage output. If you're willing to make a recommendation, what voltage/amperage would you suggest for driving 4 ohm speakers?
Here's the spec sheet for the power supply:
Mean Well LRS power supply info
Here's the speaker if you're curious:
Parts Express "Samba"
Samba impedance graph (sorry that it's a bit dark) -- and thanks again for any help.
For giggles, I tried the amp @ 24V with 1.3 amps. I was surprised at how cool the amp was after driving a pair of 4 ohm speakers for an hour.
My new 24V power supply is a Mean Well LRS-100-24 with an adjustable voltage output. If you're willing to make a recommendation, what voltage/amperage would you suggest for driving 4 ohm speakers?
Here's the spec sheet for the power supply:
Mean Well LRS power supply info
Here's the speaker if you're curious:
Parts Express "Samba"
Samba impedance graph (sorry that it's a bit dark) -- and thanks again for any help.
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Class A behaves a bit differently to the more usual Class B and AB. Heat dissipation is worst at no signal and decreases as you drive it harder into low impedance loads as an ever greater part of the bias current is 'diverted' to the load.
Heat dissipation is the big problem in deciding what voltage and current to use. I wouldn't really go much over 24 volts with the transistors so close together on that bracket, even if the main heatsink is large. We also listen at mostly low levels:
https://www.diyaudio.com/community/...h-voltage-power-do-your-speakers-need.204857/
So I would suggest you see what actual sort of output you really need and work to that.
You can also experiment empirically and see how low a bias current you can use for the levels you listen at. You might be surprised.
only 02 days after power on original JLH . I ready modified it become like this and feel more better.
Thanks for your reply, Mooly. I am also a "low level listener" who values quality over quantity. Unfortunately for me, my understanding of circuitry is minimal at best.
Is it not necessary to increase the power supply from 12V for the JLH '69 design to perform to its full potential when driving a 4 ohm load? Are the figures in table 1 simply to achieve the rated power output for a given load or do the different voltages/currents influence the amp's "sound"?
Let me put it another way:
At the same volume level, driving a 4 ohm load, would there be any audible difference between 12V @ ~500mA vs 24V @ ?mA? I use a question mark for the 24V current since I'm awful at math - sorry.
Thank you for any insights on this topic. I have an ACA Mini that sounds nice with my 4 ohm speakers, but I'm interested in hearing how the JLH design compares at the same volume levels.
Is it not necessary to increase the power supply from 12V for the JLH '69 design to perform to its full potential when driving a 4 ohm load? Are the figures in table 1 simply to achieve the rated power output for a given load or do the different voltages/currents influence the amp's "sound"?
Let me put it another way:
At the same volume level, driving a 4 ohm load, would there be any audible difference between 12V @ ~500mA vs 24V @ ?mA? I use a question mark for the 24V current since I'm awful at math - sorry.
Thank you for any insights on this topic. I have an ACA Mini that sounds nice with my 4 ohm speakers, but I'm interested in hearing how the JLH design compares at the same volume levels.
12 volts as a supply is getting a bit low and the output would be limited to about 3.5 volts rms which would be around 1.5 watts into 8 ohms and 3 watts into 4 ohms.
Remember the supply voltage sets the available output voltage. The amp is always biased so the midpoint voltage is half supply. That means on 12 volt supply the output can only go from 6 volts (the midpoint) to 12 volts and from 6 volts down to zero volts. In practice because of losses it falls short of that by a volt or so in each direction. So the output is more like -/+5 volts (so a 10 volt swing) which is our 1.5 and 3 watts into 8 and 4 ohm.
That sets the voltage limits.
To deliver that wattage into say 4 ohms means the current in the load will be as a maximum 5 volts (the highest voltage swing at the output divided by 4 ohms. That gives a load current of 5/4 = 1.25 amp as a peak maximum.
Because of the way the JLH works that means the bias current needs to be half that current value or 1.25/2 = 0.625 amp. If you set the bias higher than that then all you achieve is higher heat dissipation. If you set it lower then the output will clip and not reach its maximum voltage swing into 4 ohms.
So put simply the bias current need to be at least half the peak load current. As long as that condition is met the amp will sound the same. If you set the supply voltage and bias current to achieve say 3 watts into 4 ohm and then you go and double the supply voltage and leave the current the same then the amp will sound the same. Increase the bias current and the supply voltage and it will still sound the same. You will only lose quality when the supply voltage and bias current are inadequate to meet your required output voltage and current as that will cause distortion as the amp clips.