Please, please forgive the newbie amateur post. I purchased some of the red LM4780 boards designed by Peter Daniel and Brian GT many years ago. When I turn on the power, you can hear a soft turn-on thump, but it will not pass signal. I assume the chips are in muted mode. After looking at the datasheet, I am still unclear as to the method of disabling the mute, so the amp stays on. Pins 14 and 20 are the mute pins; do I apply voltage or ground to turn off the mute?
See data sheet pages 9 and 16, and also figures 1 and 21.
https://media.digikey.com/pdf/Data Sheets/Texas Instruments PDFs/LM4780(TABD).pdf
"Mute resistance set up to allow 0.5mA to be drawn from each MUTE pin to turn the muting function off.
RM is calculated using: RM ≤ (|VEE| − 2.6V)/l where l ≥ 0.5mA. Refer to the Figure 21 curves in the Typical Performance Characteristics section. Mute capacitance set up to create a large time constant for turn-on and turn-off muting. "
So if around 1mA or more is drawn from each mute pin to -Vee, the amplifier will be enabled (turned on).
You can use a single resistor and connect the two mute pins together, if you draw twice that amount of current.
Then RM < ( |VEE| − 2.6V ) / 2mA
So if Vee is -20VDC, then the maximum resistance mute resistor (it should really be called the UNMUTE resistor)
connected between both mute pins and -Vee is:
( 20 - 2.6 ) / 2mA = 8.7k..........so use something less, like 8.2k.
https://media.digikey.com/pdf/Data Sheets/Texas Instruments PDFs/LM4780(TABD).pdf
"Mute resistance set up to allow 0.5mA to be drawn from each MUTE pin to turn the muting function off.
RM is calculated using: RM ≤ (|VEE| − 2.6V)/l where l ≥ 0.5mA. Refer to the Figure 21 curves in the Typical Performance Characteristics section. Mute capacitance set up to create a large time constant for turn-on and turn-off muting. "
So if around 1mA or more is drawn from each mute pin to -Vee, the amplifier will be enabled (turned on).
You can use a single resistor and connect the two mute pins together, if you draw twice that amount of current.
Then RM < ( |VEE| − 2.6V ) / 2mA
So if Vee is -20VDC, then the maximum resistance mute resistor (it should really be called the UNMUTE resistor)
connected between both mute pins and -Vee is:
( 20 - 2.6 ) / 2mA = 8.7k..........so use something less, like 8.2k.
Last edited:
When grounded is in mute state. Tied to minus rail through a resitance it plays music as the datasheet seems to say.
My amp is designed with 2 chips, one for each channel. The negative voltage into the board is -35 vdc. So I should connect both pins together and feed from -35vdc through a 16.2k to each chip?
The LM4780 is basically two LM3886 dice in one package, so you can follow all the recommendations for the LM3886. If you use 100 uF from the MUTE pin to ground and 33 kΩ to V- (assuming ±28-±36 V rails) the LM4780 will start up cleanly.
If you're interested in more detail on how the MUTE circuit works, have a look in the Grounding section of my Taming the LM3886 guide.
Tom
If you're interested in more detail on how the MUTE circuit works, have a look in the Grounding section of my Taming the LM3886 guide.
Tom