How many inputs do you really need? For my second system, at my workbench, I really only need one, and that’s to listen to whatever is playing on the main system.

Few people have heard of this product, but for a short time, Adcom made a great little 50WPC integrated amp called the GCA-510. (Good Control Amp ,where’s the F?) It was essentially a GFA-535 with a smaller, lower-voltage power supply, and a passive preamp built in. Apparently, Adcom was going after the UK market, land of small but high-quality integrated amps. (More people sharing walls with neighbors, and we Yanks are so rude with our big amp stacks.)

Some people say the GCA-510 is essentially a passive line controller connected to a GFA-535 in one box, but that’s not exactly true. Passive line controllers used with separate amps often suffer impedance mis-match issues, which are dependent on the source output impedance, and the amp’s input impedance. A relationship that shifts with the turning of the volume dial.

Bottom line, amps are not generally designed with passive line controllers in mind.

The GCA-510 is different. The amplifier is set up with more gain than a separate amplifier, and the input impedance to the amp itself, after the volume control is a relatively high 39Kohms. This allows one to put a simple volume pot in front of the amp, while maintaining a good impedance relationship. This setup does not suffer the usual problems with passive preamps, like rolled-off frequency response. (BTW, many receivers and integrated amps have always been like this.)

The amp circuit is basically identical to two GFA-535’s on one board, except for the biasing scheme, which uses a transistor junction for thermal compensation, instead of a thermistor, and it is an excellent system indeed! The bias hardly drifts at all from stone cold to hot.

In fact, this might actually be the best-sounding version of the GFA-535! Notice the power supply caps, which are mounted right on the board, near the power outputs, instead of being remotely attached by wires.

Despite it’s quality, they didn’t sell many, and were discontinued. The UK market is saturated with excellent integrated amps, so that’s no surprise! Few knew the Adcom name.

So, I saw one come up cheap on Ebay, in fair condition, with a bad volume pot. Should be fun to tinker!

Of course, I couldn’t leave well enough alone, so I installed a pair of GFA-535 power transformers to bring the power up to approx 80-90WPC. (GFA-535 is underrated.) This raises the rail voltage, and so the idle heat goes up too. Fortunately, the bias tracking is so good it doesn’t run away. But it does run pretty hot and could use more heatsink. I’ll be using it with near-field monitors, so I won’t be taxing it. Don’t do what I’ve done here if you intend to crank it loud for long periods!

It also received my usual mods: Hand-matched input transistors, dual bridges with snubbers, bigger filter caps, better caps in key areas, matched gain resistors, etc.

Let’s get minimal; I don’t need all those inputs, and their switches, or even a balance control! After studying the original circuit, I concluded that a 20K volume pot should work nicely, and it’s all I need.

Out came the circuit board containing all the signal switching, making room for the bigger power supply. I installed a single pair of RCA input jacks, removed the balance control and installed a single 20K Alps RK27 stereo pot. I used really well-shielded coax to wire up the pot.

It sounds amazing! Really dynamic and impactful.

This project gives me a cool idea…. It should be possible to modify an ordinary GFA-535, adding a volume control and increasing its gain, thereby turning it into a minimalist integrated amp! What a sweet little setup that would be. No preamp, no op-amps, one input.