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Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Hi all,
I am putting together an audio amplifier using an LA4705 IC that I have lying around.
But I am needing some guidance over the value of the input AC coupling capacitor when using a potentiometer for volume control. I know that a high pass filter is formed between the capacitor and the input resistance of the IC which is stated as 30kohm in the datasheet.
But how does this change when using a volume control potentiometer like this?
Also should the capacitor be placed before or after the potentiometer?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
That capacitor is there because the IC data sheet says it needs to be. Which means the input mudt be allowed to put its own DC voltage on the input without being upset by any earthed components on the input. So leave the cap there. The input resistance is stoill 30k, the input time constants are still what was calculated.
Now, the addition of the pot is confusing you about the source impedance. The worst that the pot can add to it is 2.5k - that's two halves of a 10k pot in parallel. Usually it will be less. That's small compared to 30k, you can safely ignore it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yup. Also, the capacitor goes after the volume control. If you put it before the volume control, the volume control would short out the DC bias mentioned by Dr. Slack.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Dr. Slack wrote ...
That capacitor is there because the IC data sheet says it needs to be. Which means the input mudt be allowed to put its own DC voltage on the input without being upset by any earthed components on the input. So leave the cap there.
Steve Conner wrote ...
Yup. Also, the capacitor goes after the volume control. If you put it before the volume control, the volume control would short out the DC bias mentioned by Dr. Slack.
Thanks guys, that sounds about right becuase just after I posted this thread I was experimenting with it and putting the capacitor before the potentiometer caused the amp to go into some sort of current limiting mode drawing about 2.5amps continuously.
Shame that the datasheet only includes block diagrams and not full schematics.
Here is the amp on stripboard mostly made out of junk parts. I was sure to pay close attention to the layout, grounding and current paths to prevent oscillation problems that I have had on previous amplifiers. But I've yet to move and rotate the input jack to the very left side of the board just to prevent any chance of the external input cable picking up anything from the speaker wires.
I just have one more question regarding the datasheet, they refer to something called "Rg" several times. For example "Operating Characteristicsat Ta = 25°C, VCC= 13.2 V, RL=4Ω,f=1kHz, Rg = 600Ω", and other times the value of Rg has been anything from 0 to 10kohm
Does anyone know what "Rg" could stand for? All I can think of is resistor gain but this IC doesn't have any user adjustable gain pins.
Update: Looking at the datasheet charts it appears to mean input resistance, would that be referring to the impedance of the audio source or the the impedance of the input pins?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Why not post the data sheet, saves us guessing?
<edit> OK you've given us the number
why not post the data sheet, saves us digging it out all of alldatasheet (how is that site supposed to work, do they get a pay per ineffective click?)<\edit>
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Amplifier volume pot and coupling capacitor size confusion
