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Sansui 4900Z impedance dilemma

T

tulgen

New Member
Hi All,
One of my friend bought a 4900Z in a very bad condition. It was a long repair, including output transistor, preamp transistor, so called "protection circuit" (even the resistors were blown), some resistors and capacitors.
Anyway, after All adjustments, I tested the system with 8 ohms speakers, as the service manual says 8 ohms only. But on the back plate it states 4 to 16 ohms. My friend used the system with headphones for a while. Though I recommend to use 8 ohms speakers, he managed to find 4 ohms ones. And immediately after he powered the system smoke came out, he says. When I checked the right channel tarnsistors C2579 and A1104 are gone and also A1015 on the right protection circuit.
So though the service manual states 8 ohms, can 4900Z handle 4 Ohms as started in the back plate? The voltage in my country is 220V 50Hz and the serial number of the 4900Z is 831110445.
Thanks in advance...
 
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That's sad, after all your work.
Then your question. Although no expert, a properly working amplifier in healthy state will be able to play any loudspeaker regardless off nominal impedance. If the amplifier in question can not deliver the current needed at a certain volume at certain impedance the rail voltage will drop down more in the effort and it will clip. This should not automatically lead to shorted transistors. Maybe when you leave it playing like that, at the maximum of the capabilities of the transistors it will, maybe even after a few minutes.
It can be that this the Sansui 4900Z receiver is more sensitive to this scenario but in my opinion, on turn on of the amplifier with a sensible volume setting, this means something was still not good with the amplifier. Or something totally different happened?
 
It would have been fine with 4 ohms at low volumes. I had a 7900Z. Some stated 4-16 ohms on the back and others 8-16. I can tall you I cranked the 7900Z many times with two sets of 8 ohm speakers (= 4 ohm load) and never had an issue. YMMV.

Rob
 
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