gainclone lm4780's supply voltage, please advice.

Sorry to those that if I posted this question in the 'wrong' thread previously.
This may be related to sound performance of chip amp and also about power supply, so I don't know where should I ask.


I have a 2x25Vac transformer, will it be too high for lm4780?
25Vac x (240/230) x 1.414 rec = around 36Vdc on load.
National Semiconductor datasheet shown 25vdc 4ohm, 30vdc 6ohm and 35vdc 8ohm for their THD test.

I have 8ohm, 6ohm and may be 4ohm speakers in the future, so I have to keep switching different voltage supply of transformer ranging from 18Vac to 25Vac, every time I pair the amp with different impedance of speaker for better sound? There are some other people recommended 18Vac so it can play 4ohm better, but how if I pairing this 18Vac gainclone with 6ohm and 8ohm speakers?

Will internal regulator of lm4780 lower down the higher voltage when I plug 4ohm speaker into it?
 
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25Vac will be OK with a transformer of 10% regulation or better, since the max supply is 42V. To get the very best out of the chip into different impedances then you'd need to operate with different supplies, but I'd just go for the 25V supply and see what happens. Chances are, it will be fine, depending on the kind of music you play - if you listen to highly compressed grunge then it might thermally limit. There's no voltage regulator built in, just thermal regulation to prevent over temperature on the die.
 
25+25Vac is the usual maximum for driving 8ohm speakers.
One can go a tiny bit higher with a low regulation transformer. But that will make the chip run a bit hotter.

25Vac is too high for 4ohm speakers on the dual chip which is strangled by the thermal resistance from junction to sink.

6ohms might work OK for domestic listening if you use a big heatsink, but even 8ohm speakers will require a big sink.
 
25+25Vac is the usual maximum for driving 8ohm speakers.
One can go a tiny bit higher with a low regulation transformer. But that will make the chip run a bit hotter.

25Vac is too high for 4ohm speakers on the dual chip which is strangled by the thermal resistance from junction to sink.

6ohms might work OK for domestic listening if you use a big heatsink, but even 8ohm speakers will require a big sink.

well, these are all just paper exercises, the reality is, we really do not know how heavy his traffo will be and the internal resistance, and how the power utility situation is in his locality....

finally we do not know his listening habits, if he plays soft or plays loud...

all these have an impacts.....

i remember the Pioneer Spec 2 power amp which i covet till this day has a 4/8 ohm speaker selector switch which selects the traffo secondary voltage based on speaker impedance.....
 

tatus

Member
2008-06-23 2:05 pm
For the sonic quality of the amplifier, the more supply voltage the better. And it will drive 2,4,8 ohms up to a certain power level regardless of its supply voltage. Plus, extra voltage when driving 4 ohms also gives extra headroom until clipping.

It is really only a thermal consideration. The point is that driving a 4 ohm speaker does not require a supply voltage as high as needed for 8 ohms. On the other hand, driving 8 ohms requires less output current as driving 4 ohms. If you want to build an amp that can handle both 4 and 8, you will need to provide the supply voltage for 8 ohms AND the current for 4 ohms (or, as mentioned, a switch that selects the one or the other).

The consequence will be that, driving 8 ohms, the amplifier is oversized in terms of output current capability (no problem, just extra silicon), and driving 4 ohms, it is oversized in terms of supply voltage. In the latter case, the amplifier will run hotter as necessary as it. (Every linear amplifier transforms supply voltage, that is not transferred to the speaker, into heat. And, as explained before, an 4 ohm speaker needs less voltage.) For 36VDC and 55W @ 4 ohms, this should be in the range of 100 watts. Now look at the thermal resistance and the maximal junction temperature of the LM4780, and you can choose a heat sink that can handle this situation and you're done. (Again: no problem, just extra aluminium ;-) Roughly: at 45°C maximum room temperature you need a heat sink with Rth < 0.25K/W, to keep the chip below 150°C.