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Old 5th September 2009, 01:20 AM   #1
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Default MAX5486- digi pot help!

okay so asking here has been my last resort with this chip....

I have a few MAX5486's that I have now built two sets of and they both have the same problem (used new components for each set too), so I don't think it's the design.

Basically there is just very annoying crackling/distorted sound when running sound through the chip. I've triple checked everything and caps, wiring, etc. are fine. I've tried different power sources too, more bypassing, etc. It's almost as if the chip is not happy sharing with the audio ground (for the low level inputs). I need to share grounds though cause I'm running this single supply for a single supply amp.

any help much appreciated, been hurting my head over this for a while now. PDF is over here: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5288

volume goes up and down with the respective switches. It just sounds terrible. I noticed putting a .1uf or smaller cap between the LL/RL and ground helps a good bit, but it still cracks a little after that.

Last edited by audiomodder; 5th September 2009 at 01:25 AM.
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Old 5th September 2009, 04:22 PM   #2
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Good morning Modder,

That's a neat looking little IC! I just took a quick moment to go through the datasheet, though I must admit it was brief.

Do you have a copy of your system schematic handy? What is your audio source? My first thought is that the output buffers may be clipping, either due to excessive input, or Vcc being too low. It looks like they expect a high-impedance load, also, at their outputs.

Jim
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Old 5th September 2009, 06:31 PM   #3
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Hey Jim,
it is a great little chip isn't it? I am really excited to use it and would absolutely love to get it working. When my project is done I will post it here, it will be reeeeally cool

The datasheet is too brief with a very mediocre schematic. My system schematic needs updating badly and would take a very long time to do that since I have added a lot to it over the past year. But I can explain it..

I've tried different audio sources, and I would like to use this chip as the standard volume control in my audio system, so I would need it to work with many sources. The digi pots I've worked with before managed this very well. I am testing with a computer audio source and a portable MP3 player (iriver T30 player). I am using a basic (though great sounding) single supply amp, the LM4752. The amp is powered by 24v and I'm using a 7805 to pull off +4.9v (using diodes to protect and drop the +5v a bit) to power the MAX5486. However, I have also tried powering it through an external 5v wall supply (same result) and a CR2302 3v lithium battery (which it didn't seem to like probably due to start up power drain).

Clipping does sound like a possibility but the input is completely standard and VCC is a stable well-bypassed +4.9v.

Do the inputs need to be bypassed at all? I'm wondering if maybe this chip isn't supposed to work with standard line-level inputs(?) Also, the outputs on the schematic show the usual cap before going to the amp but there are resistors in there as well....though when I looked up the amp example in the schematic these are apparently necessary for this chip and not the MAX5486. Either way, I'm out of ideas :/

It's a fairly simple chip so I really don't know what else to check. I have the 100pf cap bypassing the bias, the 1uf cap for the BIASCAP bypass, and VDD tied to Vlogic, and Vss tied to ground (for single supply mode). Also tried tying SHTDWN, MUTE, and MODE to VDD even though they are already defaulted to this on startup. No change of course. Haven't wired up the LEDs yet either, so it can't be with that.

Thanks for looking into this! and if you want to order some I recommend getting them soon, they took a while for me cause they were on backorder

Last edited by audiomodder; 5th September 2009 at 06:33 PM.
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Old 20th September 2009, 07:22 AM   #4
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ok so 1uf ac coupling caps on the input and output helps get a solid audio output at FULL volume, but once I start to use the switches as a volume control the quality goes to crap again. Any ideas?
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Old 20th September 2009, 03:36 PM   #5
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Hi Modder,

Sorry, I fell behind a bit with checking the forum. Hmm... that's a tough one. Yeah, I agree - the datasheet application schematic is a little vague.

So, it sounds good with the digital pot turned all the way up? But in any setting lower than maximum it falls apart again? Hmm ok here's something you can try, but I must warn you, there's a chance this could damage things.

I couldn't find it in the datasheet as I am limited for time this morning, but where do they list the recommended cap value on the bias pin? You said you have 100 pF here and it's going to ground? Try a different value, I'm not sure which direction would be good. Maybe larger.

Would you have access to an oscilloscope? I'd be interested in seeing the output. About your amplifier: have you tried driving it with a signal of similar level? Maybe it's an amplifier issue? Also, does your digital pot and amplifier have a common ground?

Jim
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Old 20th September 2009, 04:11 PM   #6
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Looks like you are using it with a single suppy, while the audio signal of course is bipolar. The coupling caps help in a static situation, but when you start changing the level, the caps charge up to a certain DC and that then becomes the offset and (asymetrically) limits output level, giving distortion.

The solution is to bias the input to the pot AFTER the input coupling cap with 2 100k resistors to 1/2Vcc for max dynamic range.

Do you have a schematic? Even one hand-drawn and scanned? It's a pain to 2nd-guess someone else's circuit ...

jd
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Old 20th September 2009, 08:03 PM   #7
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ah, SUCCESS! Here are the solutions:

1) Input and output (WR and LR) must have 1uf caps

2) Bias needs to be tied to LL and RL (which is ground in the case of my single supply amp) BUT this means you cannot share the same power grounds if using this chip and a single supply amp (messes up the bias of course). Not sure if the 100pf cap is necessary (probably not) but I left it in there anyway since it says to have it in the datasheet (in the pin description area).


So this is great, I have it running off a CR2032 lithium battery since my whole setup is single supply and that's the best way to keep the grounds separate (since my setup is battery powered).

There seems to be some distortion at full output right now but I'm not sure if that's due to the chip yet. I'll have to do more work to check this, but janneman, it sounds like what you're saying is entirely correct.

I'll whip up a schematic to post here a bit later.
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Old 21st September 2009, 10:28 PM   #8
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ah rats,
so the CR2302 battery didn't last long to power the chip since it fell below the 2.7v threshold I've attached a schematic below of my setup, anyone have any ideas on how to make this work with my single supply 12v battery?? adding another large battery to my setup is going to kill it, since it won't be very portable (not to mention practical having to charge them both.. :/ )
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File Type: png lm4752.PNG (47.2 KB, 519 views)
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Old 22nd September 2009, 02:37 AM   #9
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agh, I can't figure out how to edit my last post. Someone pointed out an error in the schematic I made by accident...I mistakenly swapped the W and H MAX5486 pins. Here's the attached corrected schematic.

I am also attaching another schematic with new corrections I am going to try that were suggested to me. These might solve the single supply problem..
I will report back!

Also note, I found that 1M ohm was the wrong value for all the LEDs. this drops the voltage to ~1.5v which most LEDs won't work with. Instead, I used a 470ohm resistor and most LEDs worked fine minus one Blue LED I had which was probably higher consumption.
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File Type: png lm4752 without corrections.PNG (46.8 KB, 497 views)
File Type: png lm4752 corrections.PNG (49.5 KB, 501 views)
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Old 22nd September 2009, 06:32 AM   #10
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Quote:
Originally Posted by audiomodder View Post
agh, I can't figure out how to edit my last post. Someone pointed out an error in the schematic I made by accident...I mistakenly swapped the W and H MAX5486 pins. Here's the attached corrected schematic.

I am also attaching another schematic with new corrections I am going to try that were suggested to me. These might solve the single supply problem..
I will report back!

Also note, I found that 1M ohm was the wrong value for all the LEDs. this drops the voltage to ~1.5v which most LEDs won't work with. Instead, I used a 470ohm resistor and most LEDs worked fine minus one Blue LED I had which was probably higher consumption.
That bias point should be on half supply. As the name implieas, and is shown even in the schematic. THEN decouple it to ground.

jd
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