the amp is a radioshack mpa250. I hooked it up, and took the top cover off to see how big the transformer and caps were. theres two 80v, 10000uf caps in there, with a 40.5v transformer... i checked the output of the transformer before the bridge rectifier, its reads 40v after the bridge rectifier it goes up to 110v... im not sure if im reading that, or even testing it right... The main problem im having is that it seems the transformer and capacitors arnt big enough for the amp, im seeing voltage drops around 20 - 25v, and the lights dim, i guess you have to cut back somewhere (the amp was 180$) i was thinking about putting a bigger transformer inside http://www.antekinc.com/an-10440-1000va-40v-transformer/ (outputs around 10a rms) its 100$, i really think its worth it to do these simple upgrades, the amp has a good design and sounds nice. The transformer hum's loudly (yes its grounded the right way) i think its not strong enough... the caps are rated for 80v, and their pushing 110v dc into them... thats a big problem... i think this amp is starving for power, once these upgrades happen i think its gonna really shine. that is some backround info, but my question is how big of a cap can i put inside there, i mean the peak current draw from that transformer is probably 20a...and the new bridge rectifier i have is good for 25a or so... i just want to know if i could fit a farad cap in there, or something alittle smaller... please get backk to meh, i wanna see this amp work at its ability!! thx in advance.

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    \$\begingroup\$ Do you have a comprehensive circuit diagram of this module? Precisely what are you powering it from? What load do you have connected? Do you know that starting a sentence with a capital letter is nice? What measurement tools are you using? Why are you trying to fix this device? \$\endgroup\$ – Andy aka Mar 19 '15 at 19:20

I'm not sure if there's a question in there anywhere, but here are a few points of explanation:

  • The voltage rating on the transformer is an RMS value. 40 VRMS corresponds to a rectified peak value of about 55 VDC.

  • The two filter caps are in series, with 55 V across each one, or a total of 110 V across the pair. This is fine.

  • The amplifier is rated at 125 W (per channel) into 8Ω and 175 W into 4Ω. This implies that the output voltage is at most 31.6 VRMS and the maximum current is 6.61 ARMS.

  • That means that the power supply must be able to deliver at least ±50 V (peak) to the output stage, at an average current of 13.2 A, 18.7 A peak.

If you're seeing the DC bus voltage drop from 110 V to 85-90 V, then yes, the power supply may be a bit undersized. But if you beef it up, the output devices may not actually be able to handle the extra power, and they'll overheat and fail. Keep an eye on the heatsink temperature!


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