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Old 24th May 2015, 04:20 PM   #1
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Default Digital potentiometer. Distortion and line level

Hi. I just added a MCP4231 8-bit digital pot to my headphone amplifier design. With some voltage dividers and blocking caps add a 2.5V offset, everything works just excellent and I enjoy using a rotary encoder instead of a regular potentiometer I have a couple of questions though, I've payed no attention to distortion when choosing a digital pot, I just used one I had laying around. Is it possible for the MCP4231 to add audible distortion or should I have nothing to worry about? I'm also a little dissatisfied with the lowest audible level I can choose. I have a gain of 2.5 and an input signal of about 1V peak and the lowest pot setting is still quite loud actually. 1V/256*2.5*0.707=1.1mVrms which is really quite audible into sensitive headphones. Is there a way to get way more attenuation some how? There aren't that many higher bit digital pots to be found, but perhaps cascading two pots could work? An attenuation of 256^2 would be plenty I think :P Any ideas?
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Old 24th May 2015, 04:27 PM   #2
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You can use the volume control from TI/BB:
Volume Control | Products for Volume Control
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Old 24th May 2015, 05:10 PM   #3
jcx is offline jcx  United States
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finer step volume controls aren't really the answer to bad gain structure

if you are using sensitive iem you could have as little as 200 mVrms drive some to 120 dB SPL

since many sources today are as much as 2 Vrms you really need low noise attenuation - not Voltage gain

someone should make audio output step down transformers optimized for these iem

cheaper is the "hiss buster" output resistor divider "power" attenuator
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Old 24th May 2015, 05:26 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Just add a (switcheable) passive attenuator at the input.

Or as jcx alludes to at the output, reducing the noise.

rgds, sreten.
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Old 24th May 2015, 06:57 PM   #5
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I see now that there are several dedicated volume control chips that does exactly what I want. Thank you
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Old 24th May 2015, 07:06 PM   #6
jcx is offline jcx  United States
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you really should look into, try to understand "gain structure" for a better audio system

headphones bring this to the front because of their large range of sensitivities - you really have to design the amp for a restricted range of headphones or build in some method to switch system gains or attenuation in as much as 20 dB steps to accommodate even just dynamic headphones
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Old 24th May 2015, 07:09 PM   #7
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My sources can only output 1Vp maximum, my cellphone is only outputting like 3-400mVp so I need a gain of minimum 2.5 for my current headset. My amplifier has an adjustable gain switch of 2.5, 6 and 40 in addition to the digital pot.
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