3, Interfaces, fuse problem

Read more on the Acoustat 3 in our home audio section

Avi Bukobza2004-02-23 22:21

One of my best customers owns a pair of Acoustat 3 speakers. Frequently the fuses blow. After replacing the fuse it works fine. When I looked for the source of the problem I noticed that the resistance mesured with a simple ohm meter shows 1.5 ohm, in both speakers.

The spec sheett shows impedance of 4 ohm. Looks like some thing is shorted, but still works fine between the fuse replacement. I am in Israel and they must be repaired here, I am most thankful in advance for your assistance in solving this problem. My questions are:

1. What is the normal resistance I am supposed to see when I mesure the input terminals?
2. What is the resistance I am supposed to mesure on the HF and LF Transformers?
3. If something is wrong with the input transformers, Can I buy from you new ones?

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Andy Szabo2004-02-23 22:21

When you measure the resistance of a loudspeaker with a simple ohmmeter, you are measuring DC resistance only. Your measurement of 1.5 ohms is okay for the input of the MK-121 interface. The primaries of the individual HF and LF transformers will measure less than one ohm. The specification of ”4-ohms impedance” is an AC value. The actual value varies considerably with frequency, but 4-ohms is a good average. This is the load seen by the amplifier when playing music.

The good news is that I don\'t think there is anything wrong with the speaker. Several things could cause the fuse-blowing problem. The most likely is that you are using a 3-amp slow-blow fuse, which is the value that Acoustat recommended for many years. However, in later years, this value was discovered to cause ”nuisance blowing” with large amplifiers, and the recommended value was increased to 5-amps slow-blow.

Advise your customer that a 5-amp fuse will still provide some protection in case of a catastrophic failure in the speaker or amplifier. However, sustained high levels of music can cause thermal damage to the interface without blowing the fuse, especially with 200+ watt amplifiers.

Another possibility, though much less likely, is that a small amplifier is being used (less than 100 watts), and it is being driven into severe clipping. This can also cause fuse blowing. The obvious solution to this problem is a new amplifier!

If this does not solve your problem, please write again!

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