I have assembled the stereo version of the amplifier in the attachment. My problem is when I give power to the amp, after connecting the speaker, the series bulb is in "ON" state indicating DC voltage in output. If I give power first and then connect speaker there is no DC voltage at output and if I connect an audio input the output can be heard from amp without no audible distortion. Both channels have same problem. Please help me to correct the problem.
Could "almost" call it "normal" 😉
Some (many?) amplifiers "wake up stupid" (hey, just like me 🙁 ) and for a second or two have NO control of output DC.
After they settled up (which takes a couple seconds until al ll internal capacitors, specially electrolytics) charge up to working voltage you will have normal few mV at speaker out, but until then you may have up to full rail voltage across speakers, which pulls rail voltage down thanks to series bulb.
Quite a few, including yours, can´t recover from that low rail voltage and stay there, blocked and pulling high current.
Now if you first turn them ON without speaker and only later connect it, no problem.
Straight connecting without bulb is the same, now it still has enough voltage to "wake up".
Leave as-is, it would mean full redesign which is not the point here.
In normal use (no bulb) it will work fine.
Note: that amp is very simple/crude.
It "works", as in it supplies rated power to speaker, BUT:
* it NEEDS a Zobel network across speaker out (0.1uF cap in series with 10 ohm)
* be certain to epoxy thermal compensation diodes to heatsink so they detect/track its temperature.
* it has NO short protection at all.
No big deal in a fixed home or, say, Club or Church installation where wires are connected once to terminals at both ends (amp and speaker) and left there but dangerous in any PA or mobile DJ application where they are mover around, plugs connected/disconnected all the time, somebody can trip or step on cables on the floor, plugs may be poorly soldered/assembled, etc.
Just triple check wiring before turning amp ON.
Some (many?) amplifiers "wake up stupid" (hey, just like me 🙁 ) and for a second or two have NO control of output DC.
After they settled up (which takes a couple seconds until al ll internal capacitors, specially electrolytics) charge up to working voltage you will have normal few mV at speaker out, but until then you may have up to full rail voltage across speakers, which pulls rail voltage down thanks to series bulb.
Quite a few, including yours, can´t recover from that low rail voltage and stay there, blocked and pulling high current.
Now if you first turn them ON without speaker and only later connect it, no problem.
Straight connecting without bulb is the same, now it still has enough voltage to "wake up".
Leave as-is, it would mean full redesign which is not the point here.
In normal use (no bulb) it will work fine.
Note: that amp is very simple/crude.
It "works", as in it supplies rated power to speaker, BUT:
* it NEEDS a Zobel network across speaker out (0.1uF cap in series with 10 ohm)
* be certain to epoxy thermal compensation diodes to heatsink so they detect/track its temperature.
* it has NO short protection at all.
No big deal in a fixed home or, say, Club or Church installation where wires are connected once to terminals at both ends (amp and speaker) and left there but dangerous in any PA or mobile DJ application where they are mover around, plugs connected/disconnected all the time, somebody can trip or step on cables on the floor, plugs may be poorly soldered/assembled, etc.
Just triple check wiring before turning amp ON.
Last edited:
In a quick glance, the amplifier doesn't seem well designed and would have a lot of DC offset.
there seems to be random unneeded resistor values shoved in places trying to
correct the DC offset.
when it could have been eliminated with proper design.
there seems to be random unneeded resistor values shoved in places trying to
correct the DC offset.
when it could have been eliminated with proper design.
Hmmm , this is the original sch , dual bootstrap by RCA , a
Hung-Chang Lin
concept : https://www.hilberink.nl/amps/amp6.jpgAttachments
Taking out 3 of the transistors and 4 diodes, remove the transformer center tap, and connect the speaker to the negative rail instead of the center tap will solve the problem. Certain resistors & capacitors will need revision or deletion. https://www.diyaudio.com/forums/solid-state/236256-retro-amp-50w-single-supply-42.html
Apex AX6 will work fine at 52 v instead of 50. Protection provided by a $3 3300 uf cap series the speaker. Miller 4606 coil replaced by 10 turns 22 ga wire wound around the speaker cap.
I run mine at 70 v rail to increase wattage/channel to 70 from the 50 W original. Also uses a 53 v transformer I had already.
Apex AX6 will work fine at 52 v instead of 50. Protection provided by a $3 3300 uf cap series the speaker. Miller 4606 coil replaced by 10 turns 22 ga wire wound around the speaker cap.
I run mine at 70 v rail to increase wattage/channel to 70 from the 50 W original. Also uses a 53 v transformer I had already.
Last edited: