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Old 18th December 2009, 09:55 PM   #31
tomchr is offline tomchr  United States
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Don't ever leave digital input pins floating. They tend to accumulate charge and you'll end up with some voltage there. Sometimes you get a '0' other times a '1', and most of the time somewhere in between that will cause metastability (if you're lucky) or latch-up and dead chips (if you're unlucky).

I'm not sure why you bother to ramp the volume control manually. Toggling the MUTE pin will do the same thing. The chip actually does ramp the volume on mute/un-mute.

It's interesting... I just completed my preamp that I have designed over the past couple of years and finally decided executing on. Input select via relays controlled by an ULN28xx driver, LME49720 buffer, 1 Hz high-pass (DC block), PGA2320 volume control, LME49720 output buffer. I'm using 7-segment LED displays (yes, I'm old school) to indicate the volume. The display is bright when input or volume is changed but dims to a discrete dark red after 2 seconds (at which point the uC goes to sleep). Everything is controlled by a ATmega165. I use encoders for volume control and input select along with an IR receiver. The LED dimming is done without resorting to PWM as I didn't want any noise radiators in the amp.

Beware that with the PGA chips, you need to turn all powers off when you power off your amp. In my amp, the digital +5V stays live when the amp is in stand-by. I spoke with TI's applications engineers about this. They were a bit concerned about latch-up if just the 5V was left on. So I'm switching it with a MOSFET.

Sound quality: I never realized how much difference a preamp could make. Seriously! I like it. It's open and accurate but without the aggressive hissy treble that I have heard from other amps. I have compared it with a friend's Parasound P3 (based on OPA2134, PGA2310). The Parasound is a solid preamp! But mine is way more laid back and less tense to listen to. My friend was actually tempted to swap the OPA2134's in the Parasound with LME49720. The less aggressive treble made metallic instruments like cymbals, hi-hat, etc. sound less hissy and more metalic.

One thing to keep in mind is that the PGA23xx parts are not super low noise. They're about 10 dB noisier than the LME49720 (that granted are state-of-the-art). But it is noisier than a 100 kOhm potentiometer. I am only able to measure the difference. In actual use, I can't tell a difference. I've attached the noise plots for academic interest. The plots show the noise floor before and after the volume control with the volume control set to 0.00 dB.

~Tom
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Last edited by tomchr; 18th December 2009 at 09:59 PM. Reason: Clarification
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