Here’s a custom GFA-555 I built for Mark in Michigan. This is a fully dual-mono conversion with all the tweaks. Each channel has its own, dedicated 500VA transformer, for 1000VA in total, replacing the single, 850VA stock unit.
With any transformer upgrade, care must be taken not to exceed the safe operating area of the output transistors and upstream circuitry. Power is voltage multiplied by current, so if we are increasing the current available from the power supply, we must also reduce voltage so not too much power is generated. The stock GFA-555 runs at about 83V on the rails, with a single, shared 850VA transformer. At full power, the rail voltage sags some, and the amp produces about 260w into 8 ohms, and 325w into 4 ohms. With the new transformers, rail voltage is lowered to about 75V. At full power, there is less sag, and a little less 8 ohm power, about 225W. However, at 4 ohms, there is more power, 400W! That’s pretty close to the ideal doubling of power at half impedance. The supply is stiffer, and not sagging as much under load, and modulating with the music. This is great for the sound!
A small caveat: Amps with power supplies upgraded in this manner should not be used with a nominal 2 ohm load, as it may develop more power than the outputs can handle. More margin of safety can be had by using a lower voltage transformer, like a 45v+45v instead. But with any reasonable set of speakers, I don’t think the risk is any greater than stock.
How about the sound? Well, here’s a quote from the owner…
So far, I’ve had the system on about 5 hours or so today. Even with such low hours I can tell the difference in background noise – it is black noise. Absolutely nothing on either speaker. Just nice clean sound, the bass is more defined than before. The separation of instruments is very noticeable. I put on a CD by Leonard Cohen – his latest and last one before he passed away, called “You Want it Darker”. I’m very familiar with this CD as I listen to it in the car and on my iPad. His voice seemed very smooth on the car radio and iPad – much better sounding than an 82 year old voice should recorded at his home in poor health. When I hear the CD through the Adcom the music is fantastic, but his voice sounds so different – much more realistic. His enunciation of certain words comes through load and clear via the Adcom – something you do not hear on lesser sources like radio and iPad. -Mark
That’s what I like to hear. :^}