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Kenwood KR-4070 receiver: output problem

mrk229 said:
Glad it is working. I am not surprised that you are getting more power than specified. Kenwood did not say what the AC line voltage was when measured, I bet it was lower than what comes out of the wall in most homes today. A higher input voltage gives you a higher voltage on the output transistors which can translate to a higher power output.

I don't think you would see 18 more watts though. How did you determine the output power?
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Thanks. I hooked speaker output up to an 100w 8 Ohm dummy load and sent a 1000Hz tone to line-in (Aux). Voltage (Vrms) was measured on the scope, and volume was dialled up until clipping occurred. Voltage and resistance was then used to calculate output power. Is that how you would do this?
 
Martinvb said:
Thanks. I hooked speaker output up to an 100w 8 Ohm dummy load and sent a 1000Hz tone to line-in (Aux). Voltage (Vrms) was measured on the scope, and volume was dialled up until clipping occurred. Voltage and resistance was then used to calculate output power. Is that how you would do this?
Click to expand...
VRMS at highest output without clipping. Check resistance across dummy load with multimeter. 58 watts is about 21.54 VRMS into an 8 ohm load

Watts = VRMS squared / 8 (load resistance).
 
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The higher the amplitude (vrms) of the input signal, the quicker the volume control will take the amplifier to full output.
 
mcdoug3721 said:
VRMS at highest output without clipping. Check resistance across dummy load with multimeter. 58 watts is about 21.54 VRMS into an 8 ohm load

Watts = VRMS squared / 8 (load resistance).
Click to expand...
Yes, I did it according the procedure that you describe here. As I don’t have a dedicated tone generator, I used a function generator app on my iPhone that was connected to the Kenwood via an Airport Mini. The volume on the iPhone was pre/post repair set to 100% (minus one notch, just to be sure that no distortion would be present in the Airplay signal). To complete the picture I could perhaps still measure the input signal and assure that it does not exceed the max line input voltage.
 
The input needs to be enough to drive the amplifier to full output. You have done that. Verify the load resistance and the output voltage at clipping and your calculation should be accurate. No need to be concerned about the fact that you are feeding a higher signal into the AUX port so long as 'line level' voltage is used.

Input sensitivity of the receiver represents the minimum input signal required to drive the amplifier section to full output. So in theory it takes around 150mv minimum input to achieve 40 watts per channel output. Input voltages above that will drive the amplifier to full output sooner (lower volume setting - higher resistance). Line level voltages vary greatly. The volume pot is a variable resistor (potentiometer) that allows variable amounts of the input signal to reach the amplifier input stage by dropping the signal. At 'full' volume the AUX input is fed directly to the amplifier with little or no drop in voltage. At '1' the input voltage is throttled way down and will vary on an algorithmic scale as you turn the knob.
 
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