Adcom GCA-510, Minimalist integrated amp made even more minimalist

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.)

Listening, Recording and balance controls have been replaced with hole plugs. An ugly volume knob from the parts bin completes the look.
Listening, Recording and balance controls have been replaced with hole plugs. An ugly volume knob from the parts bin completes the look.

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.

4 thoughts on “Adcom GCA-510, Minimalist integrated amp made even more minimalist

  1. Hey. Had one of these back on 94/95. Picked it up in Okinawa, Japan. Had my Vandersteen 1c’s and they were a perfect match! Sold them before I left to a coworker and he still had them today!

    I have picked up 2 since! One was shot. Had water damage, possibly cat pee! Recently got the second and its nearly perfect! Have it pushing some Vienna Haydn’s and sounding great! Switch out some Theil CS .5’s and Totem Rainmakers. All sound amazing!

  2. Hi were you ever able to find a manual for this amp? I picked up one on eBay but don’t know what the bias voltage needs to be set at. Also I’m getting 65mV dc on the right speaker channel. Any idea?

    1. No I haven’t found a manual, but also haven’t looked real hard either. The schematic is essentially the same as GFA-535 but with a different bias spreader with improved thermally tracking.

      It think you’re confusing DC offset with bias. 65mV is normal DC offset and is not related to the bias. (If you insert shorting plugs into the RCA’s you will probably see that DC offset goes down further.)

      Bias current is measured by measuring the voltage drop across the emitter resistors. Like the GFA-535, the GCA-510 should be biased so you see about 5V across any one of the emitter resistors. See my FAQ here for a detailed procedure.

      1. Thanks for the fast reply. I was using the Left and Right test points of the GCA-510 to adjust the bias voltage. The amp was sounding like it should and figured the bias voltage was off. As for the DC offset at the speaker I was seeing 65mV on the right channel and 2mV on the left.

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