A custom super-high-current GFA-555 Dual-mono upgrade

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.

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Pretty.

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. :^}

 

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What’s this? Two power switches? Yes, one for each mono amp section.
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2x Triad VPT100-5000 500VA 50V+50V Power supply capacitors upgraded from 15,000uF to 22,000uF. Dual bridge topology for reduced rectifier noise. Solid copper buss-bar.
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Input board with “The Works”. Matched input transistors, matched gain resistors (Dale RN55D), Upgraded zobel with Polypropylene cap, Nichicon FG cap for feedback to ground, also bypassed with .1uF MKP, Heatsinks on drivers, Bourns bias pots, etc… The four large caps in the corners are the local power supply bypasses. These reduce noise, and supply high-frequency currents to the input board. The main filter capacitors are poor at providing current at high frequency, due to their distance from the circuit; connected by long wires. These caps seem to make a big improvement in the sound, so I am including this as a standard feature of my audiophile rebuilds from now on. The output modules get local bypasses too. (555 II already has them)
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Safety earth from DC star ground to chassis. The resistor provides a buffered path to chassis ground to shield the circuitry, while the capacitor shunts RF. In the event of a fault, the bridge rectifier will conduct any voltage in excess of +/- 1v to the chassis. Not shown; is an earth-ground wire that goes to this same point. Note, this is NOT a safety-earth breaker, which should never be done. Earth ground must always connect straight to chassis. For more, please read… http://sound.whsites.net/earthing.htm
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Local power supply bypass capacitors added to output modules, paralleled with MKP bypasses mounted underneath. This is such a nice improvement to the sound that I now include this as part of my standard audiophile rebuild package. Capacitors are Panasonic ED; an ultra high-endurance 105C 12,000 hour capacitor.
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Upgraded binding posts, RCA jacks and a locking Neutrik PowerCon inlet. http://www.neutrik.com/en/audio/powercon/powercon-20-a/ I think this is much better connector than the ubiquitous IEC type.

One thought on “A custom super-high-current GFA-555 Dual-mono upgrade

  1. Wow… really … wow. This is beyond craftsmanship. I was going to try to understand this more to do it myself but I just cannot replace the depth of your experience. Given your attentions to detail I can see you are passionate about what you do.
    I have an old GFA-555 that was purchased in the 80s then shelved after about three years of use.
    I know you have a lot of activity, but I was hoping you may consider looking at it.
    Thank you.
    Kelly

    Like

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