Want to replace blown amp board with generic on Fisher P294B Stereo

Thread Starter

izon

Joined Mar 17, 2013
217
Thanks.. hadn't known of the whining noise problem. Could you tell me again what current is
drawn by the old amplifier. Or to put it in more direct form, if I used an AC adapter type power supply, what
ratings would I need to look for on the product? Of course 120 vac in and then 12vdc out but at what
wattage or amperage would be proper? Do AC adapters also us SMPS in them and do they also cause a
whining noise.... guess I've never noticed any from them.
 

Audioguru again

Joined Oct 21, 2019
6,673
A cheap lightweight power supply (adapter) or voltage booster uses a high frequency oscillator and a small high frequency transformer instead of the huge and heave ordinary 50Hz or 60Hz transformer. An audio circuit might produce that switching frequency.

Modern audio amplifiers use class-D so that their power consumption is low but they also oscillate at a high frequency.

Then the high frequencies from the power supply and the class-D beat together and make more audio frequencies.

A new amplifier (TPA3116) powered from 22VDC or 24VDC produces about the same output power (27W per channel into 8 ohms) as the original amplifier but if the power supply is only 12VDC then the output power will be much less at about 7W per channel into 8 ohms.
 

Thread Starter

izon

Joined Mar 17, 2013
217
Thanks much At the risk of being too stupid, if I go with using 24 volts to supply the amp,
what current does the AC Adapter need to be rated at with 24vdc output.?
 

Audioguru again

Joined Oct 21, 2019
6,673
With a 24VDC supply, the TPA3116 stereo amplifier produces 27W per channel into 8 ohm speakers and the amplifier is about 90% efficient so the total power is 27W + 27W + 6W= 60W. Then the power supply maximum current will be 60W/24V= 2.5A.
If your speakers are 4 ohms then the output power and power supply current are doubled.

Use a large and heavy old style power supply not a small and lightweight switchmode power supply.
 
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