LM3875 Newbie Question re: sensitivity

[Gringrath]

HI Everyone

I know how to set the gain of an IGC amp. But how do you measure or calculate the sensitivity ie. the minimum RMS input signal required to give full output without clipping? I can measure this easily on my tube amps using a sine wave generator & scope, but feeding a 300 - 800 mV sine wave into these chips makes them red hot very quickly even with a large heatsink?

Does any know the sensitivity of the IGC circuit with 10K input and 220K feedback?

I wan't to try and match the sensitivity of some of my tube amps, so I can use the IGC's to power additional speakers for a 5.1 surround system.

[GregGC]

Originally posted by Gringrath
HI Everyone

I know how to set the gain of an IGC amp. But how do you measure or calculate the sensitivity ie. the minimum RMS input signal required to give full output without clipping? I can measure this easily on my tube amps using a sine wave generator & scope, but feeding a 300 - 800 mV sine wave into these chips makes them red hot very quickly even with a large heatsink?

Does any know the sensitivity of the IGC circuit with 10K input and 220K feedback?

I wan't to try and match the sensitivity of some of my tube amps, so I can use the IGC's to power additional speakers for a 5.1 surround system.

Pretty close guess would be + V-5V. If you have a 30 v rails it'll start cliping at 25 IF the other power considerations are met.
If you want to measure it remove the load and see when it'll start clipping by increasing the V source. So, for the prev. example if you have a gain of 22 and the V ps is 30V I expect to start clipping at around 1.2V @ the input.

[Bozzz]

Hi Grin,

Read the part "generalized audio power amplifier design" (p 13/14) in the LM3875 datasheet. Equations also apply for Inverted configuration. (right?)

Hope this answer your question.

[macboy]

...feeding a 300 - 800 mV sine wave into these chips makes them red hot very quickly even with a large heatsink?

Then there is something very wrong with your amplifier. It is probably not stable.

Figuring the sensitivity is simple. If you simply assume that the clipping voltage is equal to the power supply voltage (it will be a few volts less actually), then the peak input voltage required is equal to the rail voltage divided by the voltage gain of the amp (which you determine by design). To find RMS, divide that number by the square root of two (1.414).

So if you have an amp with a gain of 20 and voltage rails of +-30 V, then the peak input voltage to clipping is 30/20 = +-1.5 V peak (or 3 Vp-p if you wish). The RMS voltage is then 1.5/1.414 = 1.061 Vrms.