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koolscooby 25th December 2003 05:11 AM

Homebrew poweramp...
hey all,
i recently recieved a home brew amp that a buddy gave me to fix, one of his friends built it in 1991, he says that it blows fuses (on the output), but all i can figure is that he either
1) Put the wrong fuses in it, if so, how do i determine the fuse amperage?
2) The output caps (7300 microfarad i forget the voltage, IIRC, i'll have to check ...) aren't big enough to withstand the output needs
3) Wrong impedance speakers...
any thoughts on this? the fuses only blow after a while of the amp being on... kind of flukey... its very nicely built and very organized, i'll have to take some pictjures, and perhaps draw a schematic, also, how can i determine what the amp was bjuilt for (Ohmage of speakers, 4, 8, etc...), it uses 2 IRF640's as output MOSFETS (In parallel), its a stereo amp..., just looking for some input :))

Merry Christmas to those of you who celebrate!!! :) (12:11 AM here, 12/25/03)
thanks guys :)

[btw i love these forums still... do more reading than posting tho ;) ]

SkinnyBoy 25th December 2003 06:43 AM

me thinks the amp was designed for 8 ohm speakers and the guy if running 4 ohm ones off it.. :)

koolscooby 25th December 2003 05:25 PM

hehe this could be verry opssible, and i'm contacting the owner at the moment :)... just wanted that input, thanks, btw, how can i increase the impedance? bigger caps is my first guess, but i am not sure, (a bit new to diy stuff :))

SkinnyBoy 26th December 2003 12:53 AM


Originally posted by koolscooby
how can i increase the impedance? bigger caps is my first guess, but i am not sure, (a bit new to diy stuff :))

if you wanted it to happily run 4 ohm speakers you would really need more output devices, a bigger transformer, and more heatsinking... but, if you leave the existing transformer, you will still only have the same amount of power available so you should be alright just adding more output devices on the existing heatsinks, and adding more storage capacitors would probably help.... but you might wanna check all this with someone else... :rolleyes:

sreten 26th December 2003 07:45 PM

The obvious question : what size are the fuses ?

also what are the rail voltages of the amplifier ?

:) sreten.

koolscooby 26th December 2003 10:08 PM

well, he may have put the wrong ones in, he told me they were 6 amps.... and i'm not sure weather he put slow blow ones in like he should have (Right?, for the output), and fastblow ones in on the internal of the amp (its hard for me to explain, they are in paralell)... so if soemthing in the amp breaks, the ones in the amp blow, if the speakers pull too much etc, the ones on the fuse boxes blow, i think it'd be more effective to debug if i drew a schematic, but the amount of time it'd take me is alot :-/ well we'll see, maybe i'll get adventrous :)

i'm not sure on the voltage rails, i will take some measurements later... but only if i knew what the voltage rails were (i repeat, i'm new to electronics kind of), i know most of the fundamentals, just not some of the terms such as voltage railes, which is why i'm on this wonderful place :)

i'm also having problems getting ahold of him for the speaker ratings because hes away for the holidays etc...

thanks alot
happy holidays

sreten 27th December 2003 12:23 AM

Slowblow go in the power supply, fastblow for the speakers.

6A speakers fuses are fairly serious ratings, I'd suspect some
fault condition. Badly wired speaker crossover or a short /
intermittent short in the speakers cables.

6A fuses should not blow unless you are a volume fiend.

(6A into 8 ohms = nearly 300w, 150w into 4 ohms)

Rail voltages need to be over 50V per rail to achieve the above.

Rail voltages are the power supply voltages.

:) sreten.

koolscooby 27th December 2003 01:24 AM

oh , i thoguth slowblow for the speakers due to bass notes and stuff, that if it needed alot of energy, it cuold peak without blowing....

could definately be bad speakers etc... or intermittent short int he cables, i have not seen the speakers he last tested this on, but it was a while ago

he told met he amp was designed to be around 160 watt/channel output, so at your calculations, it seems it was designed for 4 ohms... and he hasn't tried it for 5 years, so i guess i should get out soem speakers and do some tests :)...

i'll check the rail voltages a little later tonight, wokring on some things here....

another question : ... there are 2 caps in paralell 7300microfarad for the output, is it true that these caps only actually are applyed to the positive output pole?

thanks alot :) (for dealing with my n00bness), i'll get back to you about the rail Voltage readings

koolscooby 27th December 2003 04:39 AM

ok i took some pics of this amp... itl ooks very nice, organized etc... caps are 7300uF at 75VDC... rails ont he transformer...
Primary: 120v
secondary: 40v @ 6amp...
output mosfets are irf640's i believe, 2 in parallell for each channel... he must have thought of everything, there is evena thermistor on the heatsink for the fan to turn on! :) In the picture of the output boards, there is a fuse on both the + and - for each channel (l+r), they are rated 6 amps at 250volts
but the ones on the outside of the case, shown in other pictures, are unknown, guessing 6 amps... its weird that the secondary of the transformer is 40volts, even though you said it should be 50 or so...

here are some pics...;%20Amplifier/

tahnks alot! :)

Easyamp 27th December 2003 04:42 AM

Fast blow fuses are used on equipment, such as speakers, where the peaks (as well as failures) are more instantaneous than loads on the supply of the amplifier.

The amplifier before the load should be adequately fused with a fuse rating less than the maximum continuous rms output of the amplifier so that it should always blow first (as long as the speakers rating is higher than this).

Slow blow fuses are used in supplies because the peak current loads demanded by audio amplifiers are conditioned by the output caps in the supply into a more continuous and lower current then the amplifiers output peaks.

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