Yes, I can set each channel and it remains set after shutdown and startup.
It mostly holds steady but seems to increase with temperature. I've bolted it down in the heatsink so I can safely run up the temperature and will report back when it gets hot.
It mostly holds steady but seems to increase with temperature. I've bolted it down in the heatsink so I can safely run up the temperature and will report back when it gets hot.
Try heating the bias transistors slightly. If that decreases the bias current, mate them to the MEHSA strip with heatsink compound.
Also try driving it hard for about 10 seconds into a load and instantly reducing the signal to 0 to see if the idle current lags at high current. You can expect a slight lag but it should start falling very nearly the point where you cut the signal.
Also try driving it hard for about 10 seconds into a load and instantly reducing the signal to 0 to see if the idle current lags at high current. You can expect a slight lag but it should start falling very nearly the point where you cut the signal.
Here are some numbers, I'm not sure what value they have if any.
Power supply voltage: 12.2
I set the biases at room temperature (~72° F) to:
Ch 1: 0.9mv
Ch 2: 1.1mv
Ch 3: 1.1mv
Ch 4: 1.1mv
I ran the amp up to 110°F measured near the middle of the heatsink and measured the biases:
Ch 1: 1.6mv
Ch 2: 2.0mv
Ch 3: 2.2mv
Ch 4: 2.5mv
I'll have to wait untill tomorrow to drive it hard, dummy loads are still on my shopping list and it's too late to be making a racket. Will using a reactive load cause any issues?
For heating the bias transistors, would you suggest hot air or touching the iron to their case for a couple seconds at a temperature around 100°C or so? That's still 50° C below their max operating temp.
Power supply voltage: 12.2
I set the biases at room temperature (~72° F) to:
Ch 1: 0.9mv
Ch 2: 1.1mv
Ch 3: 1.1mv
Ch 4: 1.1mv
I ran the amp up to 110°F measured near the middle of the heatsink and measured the biases:
Ch 1: 1.6mv
Ch 2: 2.0mv
Ch 3: 2.2mv
Ch 4: 2.5mv
I'll have to wait untill tomorrow to drive it hard, dummy loads are still on my shopping list and it's too late to be making a racket. Will using a reactive load cause any issues?
For heating the bias transistors, would you suggest hot air or touching the iron to their case for a couple seconds at a temperature around 100°C or so? That's still 50° C below their max operating temp.
You can heat them with a few seconds of hot air. Most of the plastic cases can easily survive but I've seen some that would melt.
If heating reduces the bias current, get them down onto the MEHSA insulators (don't let the legs short to the insulators). Use heatsink compound to ensure that they are well coupled (thermally) to the strip.
If heating reduces the bias current, get them down onto the MEHSA insulators (don't let the legs short to the insulators). Use heatsink compound to ensure that they are well coupled (thermally) to the strip.
Each one rapidly dropped between 0.2mv and 0.4mv then quickly rose to above the voltage it started at. Roughly 5 seconds of hot air at 90°C.
Each bias transistor has been thermally mated the the mehsa stripes.
I ran each channel at full un-clipped output into a 4 ohm sub at 35 hertz for 9-10 seconds then paused the track. I monitored the voltage across the source resistors and power consumption on my power supply. Each of these measurements dropped back to idle almost instantly.
I ran each channel at full un-clipped output into a 4 ohm sub at 35 hertz for 9-10 seconds then paused the track. I monitored the voltage across the source resistors and power consumption on my power supply. Each of these measurements dropped back to idle almost instantly.
OK. I'll drive it hard when I get some dummy loads before it goes into service.
On the topic of dummy loads, I'm thinking four 2ohm 100 watt resistors should cover most of my needs with the ability to wire them in series/parallel for different loads and higher power dissipation. Does that seem appropriate or would you suggest something else?
Thanks for guiding me through this 67 post journey, I truly appreciate it.
On the topic of dummy loads, I'm thinking four 2ohm 100 watt resistors should cover most of my needs with the ability to wire them in series/parallel for different loads and higher power dissipation. Does that seem appropriate or would you suggest something else?
Thanks for guiding me through this 67 post journey, I truly appreciate it.
I've used 2 ohm 100w in series to make 4 ohms. I had 2 pairs of those on my bench.
The hollow cylindrical type can take a lot of power (music) when using a fan to force air through their core.
Use high temperature wire or keep the insulation off of the resistors. If any contaminant gets on the resistors, the resistors tend to burn open at that point on the resistor. I used SIS wire but only because I had a roll laying around.
The hollow cylindrical type can take a lot of power (music) when using a fan to force air through their core.
Use high temperature wire or keep the insulation off of the resistors. If any contaminant gets on the resistors, the resistors tend to burn open at that point on the resistor. I used SIS wire but only because I had a roll laying around.
Awesome, I'll find some hollow ones and rig up a cooling solution. I appreciate the tip on keeping them contamination-free, it'd be a big bummer to learn that lesson the hard way.
They make mounting clips for them. If you don't have any, you can easily make some from coat hangers.
I was just looking at the various mounting options. I think I'll stick with a DIY solution like you suggested that doesn't obstruct the hollow core as much as the off-the-shelf solutions appear to.
This is what I'm looking at currently. Does the enamel coating offer protection from contamination above that of the variable ones with their exposed resistive element?
L100J2R0E Ohmite | Mouser
This is what I'm looking at currently. Does the enamel coating offer protection from contamination above that of the variable ones with their exposed resistive element?
L100J2R0E Ohmite | Mouser
Those are the ones recommended in the tutorial.
They are also the ones (actually just one) that burned open where the wire's insulation melted on it.
Mouser is out of stock but DK has them.
They are also the ones (actually just one) that burned open where the wire's insulation melted on it.
Mouser is out of stock but DK has them.
