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Old 23rd February 2005, 01:05 AM   #1
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Default DS1802 with LM3875

I'm trying to use a DS1802 volume control with my LM3875 stereo amp. It works, but not good enough to use. If I push the up button, the sound gets louder gradually, but then all the sudden, on the last press that brings it to max, the volume jumps to about twice as loud as the last step. It should go up 1 db at a time.

What causes this, and can it be fixed? I'm using the schematic from Maxim, but without the little opamp based preamp on it.
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Old 23rd February 2005, 08:43 AM   #2
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Without having the schematic, I'd guess it's a matter of impedances. If the DS1802 feeds an impedance that's too low, it's likely this would happen.

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Old 24th February 2005, 12:05 AM   #3
vmac011 is offline vmac011  Serbia
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Here is some good reading

http://users.verat.net/~pedjarogic/audio/spot/spot.htm
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Old 24th February 2005, 12:13 AM   #4
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Here is the schematic:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/846

figure 5.

I built the exact thing but without the opamps.

Do you think the opamps are needed to generate the correct attenuation?
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Old 24th February 2005, 03:35 AM   #5
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Default Look at Pedja's circut

Not sure why you might be having the problem with the large volume jump, but as suggested in the post above, try Pedja's circut, it is what I've used it with both a CMOY headphone amp and an LM3875 gainclone. In both cases I left everyting as is and just added the volume control before the input. Also on Pedja's site the supply indicates +/15 V...this will quickly kill your chips. I used a pair of regulators (LM317/337) to regulate the supply voltage to +2.8V and fed -2.5V to GND...worked like a charm.

I also fed a loopback signal from my PC soundcard through the DS1802 and back in. The results in SpectraLab were quite good, vey clean signal compared to the original. Unfortunately it's been dissasembled awaiting new life in my preamp project.
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Old 24th February 2005, 11:13 AM   #6
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The sample circuit of Maxim is for single power. If you use +/- rails, or a virtual ground, you don't need the whiston bridge at all.

If the voltage (input and working) is below 5V, everything will be fine. If working voltage is higher than 5V, then you need to use two regulators, or two resistors as it drains very little current, to devide the voltage. Then, input voltage still cannot go over 5V.

The 5V limit seems a huge problem of this great IC, because line level can go over 5V. In practise, however, it'll work at 7.5V, high enough to avoid input clipping with line level.

Seems your input level is too high - higher than 1802's working voltage.

I've been using it in my portable amps for years. The only problem is Digikey etc. do not carry it and Maxim's sales department sucks. Requesting some samples from Maxim is pretty easy and fast though.
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Old 24th February 2005, 09:02 PM   #7
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Wonder if it's because I am using a portable CD player as the input. It has a pretty good power headphone amp built into it. Maybe when I try using a standard DVD player with line level out I will have better luck with it.

Maybe I'll try the other circuit too.

One final question about these chips. Is there a way for it to retain the volume setting in memory or something when I turn it off, or am I stuck leaving it on all the time if I want to keep the volume where I left it?

almost forgot, my power supply is a 5V switching wall wart

another edit: tried it with a line level, still nothing. I think I'll try that other circuit, or just go back to a good old potentiometer.
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Old 25th February 2005, 01:22 AM   #8
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Default Line Level May Be Too High

The line level output of your DVD player. and likely the output of your CD player may both be too high vor the DS1802 to handle. If I recall it doesn't like anything higher than 3V above GND. This is where lowering GND in Pedja's circut comes into play.
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Old 25th February 2005, 01:27 AM   #9
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According to that website, his circuit will clip with over 2V input, which I sure don't want.


Do I need to use that odd supply, or can I just somehow supply +/-2.5VDC, or, even better, use my 5V power supply I have already.

edit: just rereading, if that supply destroys the chips, why is it used in that schematic?

and also, could it have anything to do with input impedence of the amp? my amp uses 10k input impedence, and has a 500 ohm resistor from signal to ground so in case a wire accidently is unplugged, the amp doesn't send a large DC voltage to the speaker to destroy it. Is this all correct values or could this be causing my problem also?
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Old 25th February 2005, 07:30 AM   #10
Fixup is offline Fixup  United States
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Of course you can power it with your 5V, but if the following amp circuit does not have a virtual ground, then you do need the whiston bridge (bad stuff).

The easiest way to keep the volume position is to keep its power supply. As it drains very little current, this is not a problem even with batteries. This is the way I use in my portable amps. Otherwise you'll need a microcontroller, then all the beauty of this button controllable IC is gone.
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