Transformer power?


2016-11-06 11:28 am
So I went to local flea market and bought this for $4, ripped it apart and found nice speaker, heatsink and transformer. Transformer was main reason for buying this type of audio systems, name of system was Durabrand 5.1 sp-690w. Heatsink is 9x14cm speaker says 50w 8ohm and transformer is 8x4.5x7cm how can I know how much power can this transformer handle. I will use it only for headphone amps so I know i wont stress him at all but Im curious how to calculate. In this system transformer was powering 7 lm1875t's. Transformer fuse is 250V/1.5A.



2016-11-06 11:28 am
Really good bargain. And, 7 "genuine" LM1875.
I cannot find any good photo on the Internet of this system. Normally on the rear of the cabinet, near the power input, you have a label stating the maximum power consumption. Do you have such a label?

I throw everything out except radiator speaker and transformer. I didnt want to bother to desolder lm1875t's, speaker was working before ripping apart.

Weight is 1.8kg
Last edited:


Paid Member
2003-06-12 7:04 pm
Maine USA
Power transformers are historically 5VA/pound.

The properties of iron have changed little (in ways that matter for our types of iron) and copper has changed not at all. Insulation is better and commercial pressure is much higher, so ratings/pound have gone up from 5VA/lb. But comparable types will all fall in a narrow band. E-I versus torroid makes some difference. Cheap-junk versus lasts-forever can be a big difference. Industrial transformers often fall under industry guidelines and may seem under-spec for the weight (but will stand hours of overload).
Transformer with no load gives me 14.5-0-14.5 and after rectifier filter board -20-0-20 or 40v. I assume this is 12-0-12 transformer because voltage would drop to 12-0-12 unrectified under load. Am I correct?

Sounds right. Nominally some 13Vac. The 20Vdc rectified is unusually high for nominally 12Vac but you may have peak rectification of some spikes as well. With a slight loading of the 20Vdc it should drop to some 18Vdc.