A fun project: Adcom GFA-535 SUPER-HIGH-CURRENT MONOBLOC

Hi folks, this is a fun little amplifier project I slapped together yesterday… (Well, carefully slapped together.)

I have lots of spare parts for Adcom amplifiers around, including a fully restored output module for a GFA-535 II, and a few chassis with power transformers.

So, I thought it would be fun to piece together a mono-block amplifier, capable of driving a 2 Ohm load using TWO power transformers for just one channel! This should give close to double the current to just one amp channel. This is the same basic idea as my Super High-Current GFA-535 II that I created from the remains of a GFA-2535. (The 2535 is basically four channels of GFA-535, with a double-size power supply, and I modified it to drive only two channels.)

wpid-20150812_145831.jpg

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The original power supply uses one transformer for each stereo channel. A true dual mono design! (This is one of the things that makes the 535 such a great bargain.) Each transformer is center-tapped and creates a typical split-supply using a bridge rectifier, like so…

diode-bridge-split-supply

But I want to use two transformers on just one channel for double current! The new arrangement uses one whole transformer for the positive supply, and another for the negative. Thus, the load is shared between two transformers. They are connected like this now…

Super-High-current GFA-535 power supply

I didn’t have any suitable high-current diodes lying around, so I just used one half of a bridge rectifier for each half of the supply, leaving one pin disconnected.

wpid-20150812_140140.jpg

With all this current flying around, the output modules had better be able to take it! More output transistors are needed. Fortunately, the GFA-535 and 545 share the same board, except the more powerful GFA-545 uses 6 output transistors instead of 4. This is convenient, because all I had to do is install more output transistors in the empty spots.

wpid-20150812_140121.jpg

Results:

I connected it to my test load, and took some measurements at 8, 4 and 2 ohms. Mind you, this is power just before clipping. I lack a means to actually test distortion, so output at say 0.1% may be a little lower.

8 Ohms – 104 W

4 Ohms – 160 W

2 Ohms – 207 W !!!

Wowie! Great result! A stock GFA-535 typically gets about 80 watts into 8 ohms using the same measurement technique.

The big surprise is 207 Watts at 2 Ohms! Now, I don’t intend to use it for extended periods at 2 ohms. 6 output transistors is just barely sufficient for this level of power, and it’s not really enough heatsink.

As a test, I hooked it up to my monstrous 15″ TC Sounds subwoofer, that has two 4 ohm coils, replacing the Crown K2 that normally provides 1600W to the sub.
Really terrific output! I was surprised how loud it could drive the sub without clipping. (Made me question; do I really need 1600W on this thing?) That was at 8 ohms, and about 100W, so I hooked the coils in parallel for 2 ohms and 200W, and this little amp just POUNDS! Heat buildup was not too bad, and I drove it like this pretty loud for about an hour.

It did seem to sound a bit flabbier at 2 ohms, but this is no surprise. I’ve always thought this was due to damping factor, but now I am not so sure. As an experiment a while back, I stuck an 8 ohm test load in series with my sub and the Crown K2 to purposely degrade the damping factor to basically 1. I didn’t hear much of a difference then. But the sound of the Adcom at 2 Ohms reminded me of similar observations made back in my car-stereo days. Subs hooked up at 2 ohms always seemed to sound flabbier, especially in the very low bass. So I suspect damping factor actually has little to do with it, but I’m not sure what’s going on. Perhaps it something to do with beta droop with more current, and the need for more feedback when that happens, or power supply sag. I’m not sure, and welcome any commentary on the subject at my Facebook Page.

UPDATE: 09-16-2015
Sold to Gary in Arlington, VA! He’ll be using it as a center-channel amp for a 5-channel surround system based on Klipsch LaScalas and Belles. Sounds like a cool system.

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