Audio Amplifier using LMX324ASD

[ling1995]

This article give the circuit diagram about audio distribution amplifier using LMX324ASD which produced by MAXIM and some information.

Look at the following circuit diagram,
The datasheet of LMX324ASD
Here three channel output distribution amplifier uses a single LMX324ASD. Each output stage independently drives an amplifier through the 50 µF output capacitor to the 5.1 kohm load resistor.The inputs are at 1/2 VCC rail or 2.5 V.A voltage gain of 10 (1 Mohm/100 kohm) is obtained in the first stage, and the other three stages are connected as unity-gain voltage followers.

[pandy]

And your question is?

btw resistor values suggest battery power source - trust me - 1Mohm resistor is no good for HQ audio...

[ling1995]

And your question is?

btw resistor values suggest battery power source - trust me - 1Mohm resistor is no good for HQ audio...

I would like to made such one audio amplifier.But I always feel there maybe are some wrong here. What's sad, I don't know how to describe where is wrong . Maybe my thought is wrong.So I would like to ask someone if there any suggestions before I make it.

Appreciate !

[pippas]

I'm not sure what you think this device is?......it is a' low voltage rail to rail quad op amp.'

An op-amp may be used as an audio preamp, but it is not a power amp capable of driving loudspeakers. Some op amps can drive some headphones directly (but not this device - it only has an output load rating of 2000 Ohms)

I think pandy is right to suggest it is intended for battery power - the bias components selected will draw very little power for example.
But in this case the 1Mohm resistor is used to provide negative feedback, so will not affect the audio quality.
When compared to the effective impedance of the virtual earth input used here, 1Mohm is pretty low impedance, so won't affect signal quality.

This is not a device to drive loudspeakers directly.... it will need an additional power amplifier stage

[pandy]

But in this case the 1Mohm resistor is used to provide negative feedback, so will not affect the audio quality.
When compared to the effective impedance of the virtual earth input used here, 1Mohm is pretty low impedance, so won't affect signal quality.

One of basic rules for low noise design is to avoid such high resistance in signal path.

[pippas]

Looks as if the configuration is set for a non inverting gain of 11 , and an input impedance of 100K.
In that case they have chosen to bias both inputs with the same impedance - standard practice of course - and in that case the 1Mohm feedback resistor is the required value for that particular circuit.

In effect, the impedance around that resistor will be dominated layout wise by the low output impedance of the op amp, so there should be no problems with 1Mohm there.


It's unusual to find input impedances that high with modern audio circuits - with the exception of passive guitar inputs and the like - but the schematic doesn't say what this circuit is intended for....

[pandy]

Looks as if the configuration is set for a non inverting gain of 11 , and an input impedance of 100K.
In that case they have chosen to bias both inputs with the same impedance - standard practice of course - and in that case the 1Mohm feedback resistor is the required value for that particular circuit.

In effect, the impedance around that resistor will be dominated layout wise by the low output impedance of the op amp, so there should be no problems with 1Mohm there.


It's unusual to find input impedances that high with modern audio circuits - with the exception of passive guitar inputs and the like - but the schematic doesn't say what this circuit is intended for....

Once again - divide by 10 and you will land closer to audio, virtual ground is another problem but this is purely theoretical discussion as OP didn't provided feedback if he is interested in audio amplifier or audio power amplifier.

[pippas]

I'm not sure how you would 'divide by 10' if you want to use a single stage with in input impedance of 100K and a gain of 11?

Personally, I would use 2 stages - one as a high input impedance buffer, followed by a gain stage at lower impedance. That way you can avoid the use of 1Mohm resistors

...But if you only want to use a single stage, I can't see how you can avoid high value resistors - if you want the gain and the high input impedance ?...

[pandy]

I'm not sure how you would 'divide by 10' if you want to use a single stage with in input impedance of 100K and a gain of 11?

Personally, I would use 2 stages - one as a high input impedance buffer, followed by a gain stage at lower impedance. That way you can avoid the use of 1Mohm resistors

...But if you only want to use a single stage, I can't see how you can avoid high value resistors - if you want the gain and the high input impedance ?...

Question is do you really need high impedance - IMHO high impedance may be suboptimal in many practical aspects of audio (IMHO high impedance is desired only for narrow group of sources, mostly piezoelectric one but not only).
And yes, i also prefer to buffer signal before remain part of signal path.