Power rating of an IGC

[Dr.H]

Hi All,

I recall the LM3875 being rated at 50W.

However, using a sinewave input of 2V-7V, I am able to get no more than 14.5V output into an 8.5 ohm load before distortion (as seen in the sinewave output on a scope) starts to set in.

This would correspond to an output power of V^2/R, which is 24.7W, much lower than the rated 50W.

Any explanations?

My setup:
IGC-LM3875, 10k input R, 220k feedback R
300W toroidals, 22V AC, no regulation, about 31V DC
32mV DC offset, 12cmx7cm aluminium heatsink,
1000uF Panasonic FC caps, pin7 to ground, No input cap

Thanks.

Ryan

[carlosfm]

Try adding more capacitace to the PSU.

[UrSv]

First of all you gain is higher than 20 which means that you are trying, with a supply of 31 V, to get at least 40 V out with 2 V in and if possible some 140 V out with 7 V. This means that already at 2 V in you amp is clipping like mad. This seems like a good reason why you don't see what you expect. Secondly the 14.5 V corresponds to perfect sine wave RMS power of (14.5*14.5)/(2*8.5) which is 12.35 W and not 24.7. Third the power rating into 8.5 Ohms with a full 31 V per rail is not 50 W but some 42 W.

Adding more caps to the supply will perhaps make it burn to pieces more easily.

[GregGC]

Originally posted by Dr.H
Hi All,

I recall the LM3875 being rated at 50W.

However, using a sinewave input of 2V-7V, I am able to get no more than 14.5V output into an 8.5 ohm load before distortion (as seen in the sinewave output on a scope) starts to set in.

This would correspond to an output power of V^2/R, which is 24.7W, much lower than the rated 50W.

Any explanations?

My setup:
IGC-LM3875, 10k input R, 220k feedback R
300W toroidals, 22V AC, no regulation, about 31V DC
32mV DC offset, 12cmx7cm aluminium heatsink,
1000uF Panasonic FC caps, pin7 to ground, No input cap

Thanks.

Ryan

At what freq. did you get your measurments? What is the rail voltage when the amp starts clipping? It could be that your transformers ar not big enough for that power you need. Last, put a bigger heatsink.

Greg

[Dr.H]

Interesting answers thanks,

CARLOSFM: Adding more caps not good for the sound.

URSV: I guess you're assuming that the maximum output voltage should correspond to the rail voltage? I'll have to go bruch up on my understanding of amps.

GREGGC: Sinewave of 1kHz, did not measure rail voltage when distortion starts. I would have thought that 300 VA transformers are more than adequate and my heat sink also seems large enough, since the chip did not start to fry.

Thanks again
Ryan

[carlosfm]

Originally posted by Dr.H
Interesting answers thanks,

CARLOSFM: Adding more caps not good for the sound.

Ryan

Did YOU try it?

[Dr.H]

CARLOSFM: Mmm, another interesting answer. No I have not, I've relied on my fellow DIYers.

I would admit that since I have soldered the caps directly to the pins of the LM3875, it is a bit crowded and therefore any new caps will have to be added at a greater distance from the chip than the original caps.

[janneman]

Originally posted by Dr.H
CARLOSFM: Mmm, another interesting answer. No I have not, I've relied on my fellow DIYers.

I would admit that since I have soldered the caps directly to the pins of the LM3875, it is a bit crowded and therefore any new caps will have to be added at a greater distance from the chip than the original caps.

Your problem is not the caps. It should be clear from Usvr post what the problem is. You cannot get more than 14.5v peak from an amp with 14.5 V peak supply. Infact, because the amp needs some voltage for itself, the max output will most probably be 12v or so with 14.5 v supply.
What you need (if you want more power in 8.5 Ohms) is more supply voltage. No amount of cap upgrade of heavier xformers will change this basic fact.

CarlosFM, I'm sure you know all this. Don't put him on the wrong foot.

Jan Didden

[Kuei Yang Wang]

Konnichiwa,

Originally posted by Dr.H

I recall the LM3875 being rated at 50W.

Yes, and that with a powersupply of +/-35V (with zero dropout at higher load - eg. regulated) and a chip temperature of 25 Centigrad, implying a humungous heatsink.

Originally posted by Dr.H

However, using a sinewave input of 2V-7V, I am able to get no more than 14.5V output into an 8.5 ohm load before distortion (as seen in the sinewave output on a scope) starts to set in.

As one would expect from an Amplifier with a "soft" Powersupply and a possibly too small heatsink which would trigger the protection circuitry of the chip on sinewave testing. Note that the original gaincard is also rated as 25W RMS into 8 Ohm.

If you use the transformer you mention using one channel driven the PSU Voltage will drop to a point where the lowest points in the Supply line is around 20V. The LM3875 has around 3V dropout (clipping) voltage at that point, giving theoretically undistorted peaks of sinewaves of around 16V and thus 11.3V RMS max and 16W RMS (theoretcially).

In reality the figures may very well be a little worse or better, depending upon implementation, rectifier, cpacitor quality etc, so 25W ballpark when sinewavetesting is just fine for what you got. More interesting however, for as long as 10mS the Amplifier could (again theoretically) sustain an output of nearly 30W which is twice the power provided during sinewavetesting and for 5mS up to 40W can be supplied.

So, in reality with music (which tends to contain short peaks well above the average or RMS level) will be able to play as loud as would possible with an amplifie rhaving a "stiff" supply and around twice the outputpower when sinewave testing.

So for once the term "Music Power" actually has meaning....

Sayonara

[carlosfm]

Originally posted by Dr.H
CARLOSFM: Mmm, another interesting answer. No I have not, I've relied on my fellow DIYers.

I would admit that since I have soldered the caps directly to the pins of the LM3875, it is a bit crowded and therefore any new caps will have to be added at a greater distance from the chip than the original caps.

That's what I meant.
On the PSU.