I'm back working on the amp. I wanted to add some kind of speaker protection, and there are lots of great choices out there, but in keeping with my objective of keeping costs low I wanted to use parts I already had on hand. So I came up with a circuit that's not small, it's not vast, but it's kind of half-vast.
First, there's a fuse in series with the speaker. I'm driving restored AR2AXs, and backs in the 60s AR recommended an obscure industrial fuse but I substituted 1 A AG3 fuses because that's what I had on hand. While 1 A is lower than most commercial amps would ever use, I was still worried about what would happen if one of the output FETs shorted. So I dug through the junk box and found some 48 VDC coil relays that would fit on the chassis, in fact they're the same footprint as the AC relay. I also had some 51 V 3 W Zeners, but unfortunately the maximum Zener current is something like 29 mA and the relay coil draws about 25 mA which was too close for me so I paralleled two of them.
The assumption of the circuit is that if one of the output FETs shorted and the output fuse didn't blow for some reason the supply fuse would - although I'm currently using 1 A output output fuses and 2 A supply fuses so I can't imagine why this would happen but whatever. At that point the K1 relay voltage would collapse and the speaker would be disconnected. Additionally the relay doesn't pull in until the total supply voltage is something like 85 V, so along with the good enough soft start that gives a speaker connect delay.
First, there's a fuse in series with the speaker. I'm driving restored AR2AXs, and backs in the 60s AR recommended an obscure industrial fuse but I substituted 1 A AG3 fuses because that's what I had on hand. While 1 A is lower than most commercial amps would ever use, I was still worried about what would happen if one of the output FETs shorted. So I dug through the junk box and found some 48 VDC coil relays that would fit on the chassis, in fact they're the same footprint as the AC relay. I also had some 51 V 3 W Zeners, but unfortunately the maximum Zener current is something like 29 mA and the relay coil draws about 25 mA which was too close for me so I paralleled two of them.
The assumption of the circuit is that if one of the output FETs shorted and the output fuse didn't blow for some reason the supply fuse would - although I'm currently using 1 A output output fuses and 2 A supply fuses so I can't imagine why this would happen but whatever. At that point the K1 relay voltage would collapse and the speaker would be disconnected. Additionally the relay doesn't pull in until the total supply voltage is something like 85 V, so along with the good enough soft start that gives a speaker connect delay.
Previously I had written about temperature rise on the plate/heat sink/chassis and the fact that I thought my Amazonian insulators weren't doing a great job. I got some mica insulators and things are definitely better. Here you can see that at idle the temperature rise is lower and the heat is spreading better.
This image was taken with 30 mV bias which is 150 mA, times about 48 v supply equals about 7 W idle power per FET. The room is about 70 deg F giving a temperature rise of 64 deg F or 35 deg C. 35 deg C divided by 7 W puts the thermal resistance from the top of the package to ambient at about 5 deg C per watt which lines up pretty well with the nomograph I previously posted.
(By the way, question to those who built this as a griddle amp - what thermal resistance results are you getting?)
At 10 W out the top of the FET is about 45 deg F or 25 deg C hotter which is reasonable.
Laying the chassis on its side you can see the plate is spreading the heat pretty well.
So I'm reasonably satisfied with the results from a $12 plate of aluminum.
The next step was intended to be distortion measurements but I made the mistake of installing a new version of REW and I can't get the dang output to work no matter which device I choose. I finally gave up and decided to install the amp and listen instead! So far I've been very impressed, it's noticeably cleaner than the old faithful NAD 7240PE it replaced.
So long story short, what a fun project, thanks to everyone involved!
This image was taken with 30 mV bias which is 150 mA, times about 48 v supply equals about 7 W idle power per FET. The room is about 70 deg F giving a temperature rise of 64 deg F or 35 deg C. 35 deg C divided by 7 W puts the thermal resistance from the top of the package to ambient at about 5 deg C per watt which lines up pretty well with the nomograph I previously posted.
(By the way, question to those who built this as a griddle amp - what thermal resistance results are you getting?)
At 10 W out the top of the FET is about 45 deg F or 25 deg C hotter which is reasonable.
Laying the chassis on its side you can see the plate is spreading the heat pretty well.
So I'm reasonably satisfied with the results from a $12 plate of aluminum.
The next step was intended to be distortion measurements but I made the mistake of installing a new version of REW and I can't get the dang output to work no matter which device I choose. I finally gave up and decided to install the amp and listen instead! So far I've been very impressed, it's noticeably cleaner than the old faithful NAD 7240PE it replaced.
So long story short, what a fun project, thanks to everyone involved!
I gave the Griddle Amp away so don’t have it to measure anymore. I recall it was barely warm - maybe 28C. It cost $10 at Walmart. It’s cast iron so not the best conductor but there is a lot of thermal mass and surface area.