I've been interested in this amp for some time for basic high powered systems but was concerned about driving 4 ohm loads. Posted some comments here about the design and output devices used:
http://www.audiokarma.org/forums/showpost.php?p=8748108&postcount=71
http://www.audiokarma.org/forums/showpost.php?p=8748587&postcount=72
And:
http://www.audiokarma.org/forums/showpost.php?p=8751045&postcount=73
Decided to measure the unit, all voltages measured with:
Fluke 87 true RMS multimeter
HP function generator for sine source
Cheap B&K scope for viewing clipping
Loaded both channels with 8 ohm 1% 100W resistors made up of a string of four 2 ohm resistors, 25W each.
Did not set up to measure distortion, it is generally recognized that a good looking sine by eye on a scope should be less than 1% distortion, and just visible clipping is often also just below 1% distortion.
Set it up for testing and first measured the power line voltage:
idle = 120.9 Vac
both channels driven 100W/ch = 118.8
Line voltage is steady enough that a Variac is not needed.
Measured the raw DC at the 6800 uF 65V power caps at:
B+ = 57.8
B- = 57.7
Measured RMS output voltage and calculated power are given in the attached image.
It is as expected, just meets spec in the middle of the band, and is perhaps a bit low at 20Hz.
Checked Sherwood's spec and it is for 105 W 40 to 20 KHz less than .9 percent THD, and I'd say
that it meets the spec since the onset of clipping measurements are all over 105W and the rating
is for .9% distortion.
The reduced output at 20 Hz is due to the small (for the rated power) PSU caps at 6800 uF each.
The caps are not able to maintain the supply voltage between peaks of the transformer rectified output,
this is completely normal and larger caps would help. 10,000 uF each would be a decent upgrade
20,000 uF if 4 ohm loads are going to be used - not recommended.
I did not want to overly stress and break this unit since it is obviously a budget model not intended for extended full power use. I brought up the power just long enough to take the measurement, then backed it down. Even so the heat sink got VERY HOT, I could hold my hand on it but I really did not want to leave it there. I should get a temperature probe. No matter what these kinds of temps are not good, it clearly needs either a bigger heat sink or fans.
Interesting to note that this unit has an extra thermal sensor on the heat sink that I assume is to shut it down to prevent output device failures. Does anyone know if the older ones had the sensor? It is toward the middle of the heat sink and has a black and white wire leading from it. If this is a new feature that prevents failures when driving 4 ohm loads or when an object is accidentally left on top of the unit blocking cooling then one does not have to be as concerned about reports of failed outputs.
http://www.audiokarma.org/forums/showpost.php?p=8748108&postcount=71
http://www.audiokarma.org/forums/showpost.php?p=8748587&postcount=72
And:
http://www.audiokarma.org/forums/showpost.php?p=8751045&postcount=73
Decided to measure the unit, all voltages measured with:
Fluke 87 true RMS multimeter
HP function generator for sine source
Cheap B&K scope for viewing clipping
Loaded both channels with 8 ohm 1% 100W resistors made up of a string of four 2 ohm resistors, 25W each.
Did not set up to measure distortion, it is generally recognized that a good looking sine by eye on a scope should be less than 1% distortion, and just visible clipping is often also just below 1% distortion.
Set it up for testing and first measured the power line voltage:
idle = 120.9 Vac
both channels driven 100W/ch = 118.8
Line voltage is steady enough that a Variac is not needed.
Measured the raw DC at the 6800 uF 65V power caps at:
B+ = 57.8
B- = 57.7
Measured RMS output voltage and calculated power are given in the attached image.
It is as expected, just meets spec in the middle of the band, and is perhaps a bit low at 20Hz.
Checked Sherwood's spec and it is for 105 W 40 to 20 KHz less than .9 percent THD, and I'd say
that it meets the spec since the onset of clipping measurements are all over 105W and the rating
is for .9% distortion.
The reduced output at 20 Hz is due to the small (for the rated power) PSU caps at 6800 uF each.
The caps are not able to maintain the supply voltage between peaks of the transformer rectified output,
this is completely normal and larger caps would help. 10,000 uF each would be a decent upgrade
20,000 uF if 4 ohm loads are going to be used - not recommended.
I did not want to overly stress and break this unit since it is obviously a budget model not intended for extended full power use. I brought up the power just long enough to take the measurement, then backed it down. Even so the heat sink got VERY HOT, I could hold my hand on it but I really did not want to leave it there. I should get a temperature probe. No matter what these kinds of temps are not good, it clearly needs either a bigger heat sink or fans.
Interesting to note that this unit has an extra thermal sensor on the heat sink that I assume is to shut it down to prevent output device failures. Does anyone know if the older ones had the sensor? It is toward the middle of the heat sink and has a black and white wire leading from it. If this is a new feature that prevents failures when driving 4 ohm loads or when an object is accidentally left on top of the unit blocking cooling then one does not have to be as concerned about reports of failed outputs.
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