My Latest

[ir]

I scrapped an old Akai 20W amplifier and used many of it's parts - most importatnly the case and transformer to build my new LM4766 based amplifier

All thats left to do is the digital vu meters and the input select indicator LEDs but those two can wait as i was so eager to use the thing it's all set up in the lounge and i have no intention of moving it

I painted the case a nice flat black, then coated it in laquer for a nice finish whilst retaining the 80's style silver buttons for a little contrast (p.s. i REALLY hate modern silver coloured electronics - tvs, stereos etc ALL AWFUL. i'd rather spraypaint a $1000 tv then leave it silver). also, i have since added a knob to the old tuning dial on the rioght of the amp so it's look a little less ghetto now

anyways....

here tis next to its smaller cousin the 20+20W LM1876 amp

[ir]

inside...

[ir]

p.s i have a question

after being left on for numerous hours the transformer get's hot. i mean REAL hot - even when the amp hasn't been outputting anything for some time (i.e idling) or running at very low volumes.

i know the tranny was only designed for a 20+20W amp but it's rated at 120VA so should be sufficient for my 40+40W version, especially as i don't push it very hard at all

the current draw whilst idle should be negligible. anyhoo's i'll give it a measure some time...

[Morse]

Hi ir;

Nice work on the amp! Any observations on the sound quality of the LM4766? I've got a few over here I've been thinking of trying out....

On the issue of trafos running hot, remember that there are two sides to a trafo. Excessive current draw on the secondary can cause overheating, as can excessive voltage or a DC offset applied to the primary. Now, assuming that the current draw is not excessive, you might take a look at your AC mains voltage and whether or not there is a significant DC offset present. Also, take a look at the rating of your trafo's primary; is it above the actual mains voltage? In PS trafos for valve amps, the Hammond 2xx line is notorious for hot operation (and humming) when run at 120VAC (they're only rated for 115VAC) so I have added an autotransformer that drops 6.3VAC to my PS filter stage for my valve amps using the Hammond 2xx PS iron.

One final thought; have you measured the temperature of the PS trafo?

Good luck and all the best,
Morse

[ir]

well i'm in New Zealand - we run 240V. i assume the transformer worked fine in the old amp which goes to say it's something in my amp.

i haven't mearsured the temperature nor do i have any means to do so but i will say that last night when i turned it off after having idled for about 4 hours, it was too hot to touch for more then a few seconds.

i'll test it later by chucking the multimeter in place of a fuse.

as for the sound? Brilliant! no hum, tonnes of volume, sweet highs and deep lows - i also wired up the Akai's 'loudness' switch too which adds a bit of oomph. i have'nt even got a preamp in there (yet) so it's running flat and it sounds damn good.

i must say tho, it has more bass then the old LM1876 amp. which was built in almost exactly the same way. the old one had 4700uF on the power amp board plus 3300uF at the supply. this one has 4700uF on the board and 220uF at the supply (totally unecessary i know, but i had them and i was bored). they both run the same voltages by the way +/-32V

[ir]

well no luck with the multimeter - the batterys gone flat. stupid non auto-off digital thing

anyways...

i have a theory. i may have miscalculated the value of the mute resistor. it supposed to draw 2mA thru the mute pins (combined) to take the IC out of mute - well its definitely drawing at least 2mA as it's definitely not muted. however if i chose a wrong resistor, it could be pulling a lot more thru thus creating the heat?

i don't know. i don't know a thing about electronics. i just makes things and they work

[Tube_Dude]

Hi Ir!

If your transformer is too hot even when the amp is not playing music:

1- the amp must be in oscilation...check with a scope.

2- One (or more) diode bridge rectifier in the power supply is shorted...inspect!

[uvodee]

that Tube Dude is correct all the way.

I built 3 sets so far and they are fed by 200 VA. When the cd player stops playing the amp s cool down very quickly and they never get HOT, just warm = hand touch is ok on the relatively small heatsinks or in one set the enclosure serves as heatsink.

Jean-Pierre

[Morse]

Hi ir;

If the PS trafo is from the original amp, and it never ran too hot there, then you're right, the trafo is fine.

Yep, Tube-Dude's got a very good point; if the amp is oscillating (if it's high enough freq oscillation you won't be able to hear it) then it's drawing a lot more current than you think. If you don't have an oscilloscope, check to see if the chip itself is running hot. If it is (signal or no signal) then it's oscillating.

Did you use .1uF decoupling caps at the chip's pins?

Good luck with the problems!
All the best,
Morse

[ir]

well the heatsink is warm - the chip itself is not. and as you can see from the internal pic, the heatisnk could easily warm up after being near a hot tranformer for several hours. or of course it could warm up by doing it's job correctly by keeping the chip cool

the 0.1uf caps are on the bottom of the board dirctly on the pins - altho i'll pull it all to bits and see that it's all fine. i may have overlooked something. ADD does that to a person....

thanks for your help guys