Quatre dg-250 amplifier

I have owned 4 gain cells over the years.

The GC 300 was the best sounding model and the best capable of driving speakers with difficult impedance properties. The next best was the DG250C. Quatre/QMI licensed the 250/300 designs and produced variations and made changes the designer did not sanction. In particular, the 500 I heard sounded terrible and these were prone to blow up.

Mechanical hum may be caused by the transformer mounts - grommets broke or bolts loosened up.

Some 300s were retrofitted into 250 chassis. These are more desirable than the flimsy chassis QMI used on the factory manufactured models.

Wow that name is truly a blast from the past. I had one in for service a long time ago. It was pretty well built. A lot of room under the cover. Unfortunately, I don't have a circuit diagram for it. I remember visiting Quatre and talking at length to the head of the company but for the life of me I can't remember his name. Big heavy set guy that's all I remember. The amp wasn't bad. That was around the time I was working for a company that made speakers for John Iverson in the San Fernando Valley while going to college. That was a trip!

my dg100 died after only a few years after buying it out the door of the factory in the San Fernando valley. The factory replaced the circuit boards with 250 models, and it has worked
since without any problems. Great bass no hum or vibration as mentioned earlier.

My question is what is the output impedance? I've run it into
B&W DM5's, both as a parallel pairs and singularly- 8 v's 4 ohms. As it is now "long in the tooth", I'm concerned about which configuration is healthier. BTW I also use an APT preamp.

I would be rather careful what you choose to connect to it as some of those amps were not particularly stable with all loads. They liked to blow up a lot. I would probably keep it between 4-8 ohms. No low or complex impedance i.e. no electrostats or ribbons. Resistive as opposed to capacitive.
Wow an Apt Holman Preamp. Make sure the Apt isn't outputting any DC. If I'm not mistaken, that was a AC (capacitor coupled) preamp.

In SD?
Try Stereo Design off of Clairmont Mesa, near where it crosses the 805.

http://stereodesign.com/

Been there 20+ years and have a huge operation.