Here’s an awesome amp I just finished for a customer in NYC. This was really fun to build! He wanted a high-current version of the GFA-545, so we went all-out with dual Avel 500VA transformers, gigantic 27,000uF filter caps, and a truly dual-mono layout throughout. This amp has dual power cords, fuses and power switches.
The sound is just incredible! Perhaps the best one yet. Super clean and powerful, and never seems runs out of gas, right up to clipping. I’d say it has an effortless sound to it. I measured 162W at 8 ohm and 272 at 4 ohm just before clipping! That’s only 50W short of a perfect doubling of power at half impedance. That is one stiff power supply!
One must always be very careful when upgrading the power supply of an amp, so as not to overload it and possibly blow it up! The Safe Operating Area of the transistors must be considered. The original 2sa1492 and 2sc3856 have excellent SOA—as good as newer models—but I opted to replaced them with better-matched On Semi NJW3281 and NJW1302, so that current-sharing is maximized.
SOA of transistors in preceding stages must also be considered, as they could be under more stress driving upstream transistors that are now pushing more current. The triple-darlington output section of the MKII Adcoms makes for a good sort of buffer between it and VAS stage preceeding it. (I don’t recommend this mod for MK1 Adcoms)
A stock GFA-545 II usually gives about 130W into 8 ohms. This mod bumps that to about 160W. So, we are not pushing the output section too hard, though we have given away some SOA headroom. But it’s got three pairs of output transistors, which is pretty typical for an amp at this power level.
This upgrade is not without risk. If you want to attempt the same, and want a little more safety, Avel makes a 500VA 35+35V transformer as well, and that would lower the rail voltage and give more SOA headroom. It would have a little less 8 ohm power, but would also have even less sag.
The customer’s requirement was to drive a pair of studio monitors that are kind of famous for being difficult speakers to drive. The Amphion One 15 is rated at 8 ohms, but of course all speakers have a curve. This one has a somewhat unusual impedance dip to nearly 2 ohms at 20KHz. (Zobel network maybe?) So at high frequencies, the amp is driving way less than 8 ohms, which should ideally not interfere with the amp’s reproduction of other frequencies. I reckon what these speakers like is an amp that can deliver big current at high frequencies.
The point of this upgrade is not so much the increased power output, but to improve the power supply regulation. The fact that it delivers nearly double power into halved impedance means the supply is barely sagging. This is an important feature for speakers with complex impedance curves, such as electrostatics, or speakers with complicated crossovers.
Dual bridges feed the caps. (Less hum than typical center-tapped, single rectifier layout)
Wima MKP 0.1uf 250V bypasses
Dale RS drain resistors
Dual bridges feed the caps. (Less hum than typical center-tapped, single rectifier layout)
I had to hone flat the transistor mounting surfaces. This is a common issue with the 545, but not the 535 for some reason. The tapped screw holes are warped out and proud of the surface. This makes for poor thermal contact.
Transistors are NJW3281 and NJW1302. Such excellent transistors, and really consistent manufacturing. These match within 2% at idle, half and full power.
