As promised, here's my AT-to-12V conversion.
This started as a Hipro PSB-HP200 from the mid 90's. It came from a horribly outdated system and looked too nice to trash.
All the old caps went, FB ckt was completely changed. Too many times, "12V" conversions are nothing but a load resistor on 5 to keep things stable or the duty cycle high enough for the '494 to stay powered. This is an AT supply; the regulator is powered off the 12V output, before the output inductor. So if the load falls too low, not only is there discontinuous current, the '494 also picks up and drops off repeatedly as the supply 'pulses'.
The new feedback loop is directly from the now 13.8-14.2V output, completely dedicated. _Zero_ attempt was made to "make do" with the old group reg ckt. You can see where I cut traces and changed/removed components.
Here's a list of the 'mods', not necessarily in order:
Calculated and wound new transformer
Rewound output inductor to match new operating conditions and current
Replaced all caps/upsize the ones in the output and doubler-rectifier
15A line rectifier instead of 4A unit
MJE13009L's instead of 2SC4242's
Reduced the value of the base drive resistors from 4.7 to 2.7 ohms, to drive transistors harder
Two SBL3045CT secondary rectifiers instead of one UF10100
Larger line and load wires
Reversed fan and bent heatsink fins for better cooling, could be better
Sealed vents no longer used, to direct airflow
"Tinned" traces and added jumpers where high currents flow
Cleaned all that junk glue off the components
Resoldered everything
Removed and reinstalled components that were too far off the board, trimmed leads, etc.
Cut out punched grille for fan and replaced with wire fan guard
The load test consisted of the 12-to-120 inverter, a few large light bulbs, a big audio dummy load, and some 'ignition coil' ballast resistors. 60A, as detailed below:
Inverter- 15A
Dummy load- 14A
Ballast resistors- 23.3A
Light bulbs- 8.33A
Total- 60.66A/~850W
Anything from that load all the way down to just the fan is stable. Short ckt protection is provided by a novel hookup of the driver transformer. By way of an extra diode and other parts from the center tap, a derived voltage is obtained. This is related by turns ratio to the primary current. It will even save the supply from a short at turn-on. The new transformer was a success- the primary snubber resistor (R11) runs cooler than with the original, smaller TX.
Voltage setpoint was 14.2, under load it was 14.0. The mess with all the test leads would make Unique jealous... or confused
I don't have pix of that, it was sort of "improvised."
With the fan shown, 30A is much more reasonable. If I stack two fans and/or change heatsinks/airflow, it'll be able to run flat out continuously. Transformer windings were sized for 30A out, so I'd have to 'cheat' and specify a duty cycle limit on anything over 30, even with a hurricane through it...
Now the amp. It was once a Memphis 16-ST500D, one of those PWM subwoofer things. I got it in '03, from someone after it blew up. It appears that the board shifted and the tab of one of the output MOSFETS grounded out. The cascading fault killed all the power semis in the power amp. Two of the MOSFETS in the power supply burned their leads off and split themselves. The four others were also shorted.
While trying to fix it, a cap must have been charged somewhere and I backfed and blew out the 'special' PWM driver. Special meaning no markings on the two ICs, other than "B52" and "F18". The little driver board itself was all SMT, so playing with that was over.
I put a TDA7240 in there for 'temporary purposes' to use it as a utility amp. Sort of awkward, it needed two 18V/2A laptop adapters to make full output. In late December of 2010, I bit the bullet and started the design of the new amp and the redesign of the PS.
The amp is capable of 2x110W into 4 ohms and 1x220W into 8 ohms bridged. I have not tested its output at 2x8 ohms yet, but expect around 2x75W. Tone control ratios are straight out of my Sansui 881 and it sounds fantastic. With TL072/71 opamps, it's also dead quiet.
More amp details:
SG3525 driving '50N06s, fixed at 35kHz, 49%- mostly due to how many 'filars' I used in the TX and not wanting to tempt the parasitics in my 'point to point' wiring
Dual positive and negative rectifiers feeding caps to provide +/-36V
Resistor-fed zeners for +/-15V opamp supplies
The amp itself is nothing real special, typical diff amp feeding a class A CC voltage amp. The finals are NJW21193/94s, driven by MJF15030/31s. The preamp is on its own little board, since it is mostly DIP. Not the kind of thing I wanted on the main perfboard.
Sorry for writing a book here, I just didn't want two separate threads...
-Paul
This started as a Hipro PSB-HP200 from the mid 90's. It came from a horribly outdated system and looked too nice to trash.
All the old caps went, FB ckt was completely changed. Too many times, "12V" conversions are nothing but a load resistor on 5 to keep things stable or the duty cycle high enough for the '494 to stay powered. This is an AT supply; the regulator is powered off the 12V output, before the output inductor. So if the load falls too low, not only is there discontinuous current, the '494 also picks up and drops off repeatedly as the supply 'pulses'.
The new feedback loop is directly from the now 13.8-14.2V output, completely dedicated. _Zero_ attempt was made to "make do" with the old group reg ckt. You can see where I cut traces and changed/removed components.
Here's a list of the 'mods', not necessarily in order:
Calculated and wound new transformer
Rewound output inductor to match new operating conditions and current
Replaced all caps/upsize the ones in the output and doubler-rectifier
15A line rectifier instead of 4A unit
MJE13009L's instead of 2SC4242's
Reduced the value of the base drive resistors from 4.7 to 2.7 ohms, to drive transistors harder
Two SBL3045CT secondary rectifiers instead of one UF10100
Larger line and load wires
Reversed fan and bent heatsink fins for better cooling, could be better
Sealed vents no longer used, to direct airflow
"Tinned" traces and added jumpers where high currents flow
Cleaned all that junk glue off the components
Resoldered everything
Removed and reinstalled components that were too far off the board, trimmed leads, etc.
Cut out punched grille for fan and replaced with wire fan guard
The load test consisted of the 12-to-120 inverter, a few large light bulbs, a big audio dummy load, and some 'ignition coil' ballast resistors. 60A, as detailed below:
Inverter- 15A
Dummy load- 14A
Ballast resistors- 23.3A
Light bulbs- 8.33A
Total- 60.66A/~850W
Anything from that load all the way down to just the fan is stable. Short ckt protection is provided by a novel hookup of the driver transformer. By way of an extra diode and other parts from the center tap, a derived voltage is obtained. This is related by turns ratio to the primary current. It will even save the supply from a short at turn-on. The new transformer was a success- the primary snubber resistor (R11) runs cooler than with the original, smaller TX.
Voltage setpoint was 14.2, under load it was 14.0. The mess with all the test leads would make Unique jealous... or confused


With the fan shown, 30A is much more reasonable. If I stack two fans and/or change heatsinks/airflow, it'll be able to run flat out continuously. Transformer windings were sized for 30A out, so I'd have to 'cheat' and specify a duty cycle limit on anything over 30, even with a hurricane through it...
Now the amp. It was once a Memphis 16-ST500D, one of those PWM subwoofer things. I got it in '03, from someone after it blew up. It appears that the board shifted and the tab of one of the output MOSFETS grounded out. The cascading fault killed all the power semis in the power amp. Two of the MOSFETS in the power supply burned their leads off and split themselves. The four others were also shorted.
While trying to fix it, a cap must have been charged somewhere and I backfed and blew out the 'special' PWM driver. Special meaning no markings on the two ICs, other than "B52" and "F18". The little driver board itself was all SMT, so playing with that was over.
I put a TDA7240 in there for 'temporary purposes' to use it as a utility amp. Sort of awkward, it needed two 18V/2A laptop adapters to make full output. In late December of 2010, I bit the bullet and started the design of the new amp and the redesign of the PS.
The amp is capable of 2x110W into 4 ohms and 1x220W into 8 ohms bridged. I have not tested its output at 2x8 ohms yet, but expect around 2x75W. Tone control ratios are straight out of my Sansui 881 and it sounds fantastic. With TL072/71 opamps, it's also dead quiet.
More amp details:
SG3525 driving '50N06s, fixed at 35kHz, 49%- mostly due to how many 'filars' I used in the TX and not wanting to tempt the parasitics in my 'point to point' wiring
Dual positive and negative rectifiers feeding caps to provide +/-36V
Resistor-fed zeners for +/-15V opamp supplies
The amp itself is nothing real special, typical diff amp feeding a class A CC voltage amp. The finals are NJW21193/94s, driven by MJF15030/31s. The preamp is on its own little board, since it is mostly DIP. Not the kind of thing I wanted on the main perfboard.
Sorry for writing a book here, I just didn't want two separate threads...
-Paul
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