Recently, I wanted to build some compact and battery-operated headphone amp (Just for fun!!!)
Then I browse thru this website: TCAAS - The Class-A Amplifier Site and saw JLH1969 class A schematic and realized that I hadn't build any OTL amp yet.
I've been building some decent design amps and those topologies all utilize dual-rail power and dual battery operate amp is problematic (don't like the virtual ground design)
thus I saw many scale-down versions of JLH1969 class A amp (This is the one particularly good for low voltage operating JLH 1969 battery amp) and simulated it.
It's turned out very well!
Thus I went to the local electronics store bought some goodies and start build on the perf board
Also found a small case and rechargeable Li-ion 9V battery for the amp
It sounds pretty good with my DT990 pro 250 Ohm headphones!
Although I have finished this project I've some questions want to ask.
1. I know the output capacitor in OTL amp is to block circuit DC from the headphones but I still measured some 10-15 mV only at the left channel. This makes tiny noise at the left channel that sounds like scratching ear. I've swapped many output cap and didn't fix them. Never occurs at right ch tho.
2. I use MJE340 as my output stage device and quiescent current is set at nearly 70 mA. MJE340 works at about 300 mW.
I saw the datasheet said the Thermal Resistance(Junction−to−Case) is about 6.25 C/W. Should I use the heatsink?
Then I browse thru this website: TCAAS - The Class-A Amplifier Site and saw JLH1969 class A schematic and realized that I hadn't build any OTL amp yet.
I've been building some decent design amps and those topologies all utilize dual-rail power and dual battery operate amp is problematic (don't like the virtual ground design)
thus I saw many scale-down versions of JLH1969 class A amp (This is the one particularly good for low voltage operating JLH 1969 battery amp) and simulated it.
It's turned out very well!
Thus I went to the local electronics store bought some goodies and start build on the perf board
Also found a small case and rechargeable Li-ion 9V battery for the amp
It sounds pretty good with my DT990 pro 250 Ohm headphones!
Although I have finished this project I've some questions want to ask.
1. I know the output capacitor in OTL amp is to block circuit DC from the headphones but I still measured some 10-15 mV only at the left channel. This makes tiny noise at the left channel that sounds like scratching ear. I've swapped many output cap and didn't fix them. Never occurs at right ch tho.
2. I use MJE340 as my output stage device and quiescent current is set at nearly 70 mA. MJE340 works at about 300 mW.
I saw the datasheet said the Thermal Resistance(Junction−to−Case) is about 6.25 C/W. Should I use the heatsink?
Fix left channel offset by add extra ground wire at the rear part, seem output see the different ground resistance cause offset.
However, left channel randomly occur ear scratching (wind?) noise still there, may try replace new output npn transistor and add heatsink😭
However, left channel randomly occur ear scratching (wind?) noise still there, may try replace new output npn transistor and add heatsink😭