Erse in Koss


Staff member
Jan 14, 2011
Gillette, Wyo.
Halfbiass...Electron Herder and Backass Woof
Mark, if you remember I melted the solder connections on Al's universals....maybe you were absent in that time period..


Veteran and General Yakker
Jun 24, 2011
Jacksonville, FL
Well, that 60/40 Kester melts at 370 degrees. That is a warm day in anybody's book.
What gets those components hot is the heat energy. Energy (Joules) = Power (Watts) X Time (Seconds). If your pumping 2,500 Watts of power to your crossover for a 3 minute song, that would be 450,000 Joules of energy. Dividing that many Joules by 1,055 converts the energy to 427 BTUs.
There is a formula, Q=m*s*t (heat input in Kcal=mass of lead in Kg*specific heat of tin/lead (0.60)*temp to be raised in C) to figure out how many BTUs are required to melt a certain mass of lead. If you used a gram of lead to solder, and a gram of tin/lead has 0.0048262548262548 Moles, raising the temperature 300 degrees F, which is 149 degrees C (above room temperature) is 67.59 Kcal. You only needed .08 Kcal.

So ya, that solder melted...

To prevent the solder from melting:
1. Listen to shorter songs
2. Turn down the volume
3. Use more solder next time
4. Lower the crossover points so more power goes to the woofer.
5. Increase the load resistor to raise the impedance of the system

I recommend #5
If Current (in amps) = the square root of Power/Resistance. Then the square root of 2,500W/8 Ohms = 17 Amps. If you increased the resistance in the crossover to 16 Ohms, the Amps would drop to 12.5, below the threshold of melting the solder in three minutes.


New Around These Parts
Mar 20, 2023
Ottawa, Canada
Author / Humourist
Sorry for being so very late to this thread. I've only just come across it.

I'm the online resource for information regarding the Koss CM Series of speakers (CM/1030, CM/1020, CM/1010, CM/530).

I've provided help to quite literally hundreds of people who have contacted me over the last two decades.

I can be reached at

If I might comment on this thread...

By now, all original Koss CM crossovers will need to be refurbished. The resistors should be replaced (they corrode from the inside out and eventually go open circuit - making it seem like a driver has failed) as well as the caps (they drift out of tolerance - resulting in output imbalances between drivers).

Ratings and values can be found (although, like the OP did, some may need to be replaced by a pair to get the right value) but tolerances on the original resistors might be impossible to find. Go with what's available. I attest that doing so will still yield like-new sound.)

I can provide schematics and other information to those who need it.

As to the discussion about the caps' 250V rating... the schematics call for 50V. No harm in going bigger, but it is overkill. Just make sure the speakers are fused as required. In addition, all of my CM/1030s (I have six pairs) have a thermistor to protect them - factory original but not shown on the schematics. The worst I've ever done is blown a fuse, or two.

And by the way, the Koss CM line are very very good speakers. And while the asking prices, like a lot of other "vintage" gear, have been going up and up, they are still a great bargain. But they will need some work. Contact me. I can help.