LM4950 Resistor Substitution

supersonik · 30th August 2009, 01:01 AM

I'm building my second chipamp, using an LM4950. In the datasheet for the mono BTL configuration, the circuit specifies 20k ohm resistors. I don't have any of these lying around, can I use 10k, 21k, or 22k resistors? How will this affect the sound?

DigitalJunkie · 02:18 AM

I built a couple small stereo amps using that chip.I experimented a bit,and decided to use higher value resistors so that the input impedance would be higher.
IIRC,I used 47K resistors,and one 470K resistor between pins 1-3 to get the gain to 10x. the other half of the amp is just unity gain,following the output of the first half.
It seemed to me that increasing the value of the resistors improved the sound,perhaps because my source wasn't straining to drive the lower impedance.

One note,don't make the bypass cap on pin 8 too large,IIRC 22-33uf was about as large as I could go without having it start to act 'funny'. (I think the datasheet suggests 10uf?)

Another note,IIRC i had better results when using lower values for the standby divider,running it "hotter" (more current through the divider chain) seemed to turn on/off the amp more smoothly,without any thumps and pops. Give it a shot with a couple of 1K resistors.

-Just remembered,I built a parallel amp with some of these chips. 0.47ohms on each output,paralleled to a 4ohm driver (IIRC the chips are rated for 8ohms). It worked well,not much power though.

supersonik · 30th August 2009, 12:43 PM

Thanks for your advice, but it's a little hard to follow which resistors you mentioned go where..... Would you mind listing which resistors you used where? On the datasheet they are:

RinA: Resistor between Audio Input and pin 1
RFA: Resistor between Pins 1 and 3
RinB: Resistor between Pins 3 and 9
RFB: Resistor between Pins 7 and 9

I would experiment myself to see which work, but I don't have access to any measuring equipment other than a muiltimeter...

Minion · 30th August 2009, 03:55 PM

Two 10K resistors in series will make 20K , I do this all the time when I don"t have exact Values so i series or paralell values to get the desired Value ...

30th August 2009, 01:01 AM	#1
supersonik diyAudio Member Join Date: Aug 2009	LM4950 Resistor Substitution I'm building my second chipamp, using an LM4950. In the datasheet for the mono BTL configuration, the circuit specifies 20k ohm resistors. I don't have any of these lying around, can I use 10k, 21k, or 22k resistors? How will this affect the sound?

30th August 2009, 02:13 AM	#2
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	I built a couple small stereo amps using that chip.I experimented a bit,and decided to use higher value resistors so that the input impedance would be higher. IIRC,I used 47K resistors,and one 470K resistor between pins 1-3 to get the gain to 10x. the other half of the amp is just unity gain,following the output of the first half. It seemed to me that increasing the value of the resistors improved the sound,perhaps because my source wasn't straining to drive the lower impedance. One note,don't make the bypass cap on pin 8 too large,IIRC 22-33uf was about as large as I could go without having it start to act 'funny'. (I think the datasheet suggests 10uf?) Another note,IIRC i had better results when using lower values for the standby divider,running it "hotter" (more current through the divider chain) seemed to turn on/off the amp more smoothly,without any thumps and pops. Give it a shot with a couple of 1K resistors. -Just remembered,I built a parallel amp with some of these chips. 0.47ohms on each output,paralleled to a 4ohm driver (IIRC the chips are rated for 8ohms). It worked well,not much power though. Last edited by DigitalJunkie; 30th August 2009 at 02:18 AM. Reason: fuzzy details coming back to me.

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30th August 2009, 12:43 PM	#3
supersonik diyAudio Member Join Date: Aug 2009	Thanks for your advice, but it's a little hard to follow which resistors you mentioned go where..... Would you mind listing which resistors you used where? On the datasheet they are: RinA: Resistor between Audio Input and pin 1 RFA: Resistor between Pins 1 and 3 RinB: Resistor between Pins 3 and 9 RFB: Resistor between Pins 7 and 9 I would experiment myself to see which work, but I don't have access to any measuring equipment other than a muiltimeter...

30th August 2009, 03:55 PM	#4
Minion diyAudio Member Join Date: May 2006	Two 10K resistors in series will make 20K , I do this all the time when I don"t have exact Values so i series or paralell values to get the desired Value ...

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