I'm a digital guy, not an analog audio guy...

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And this one's got me puzzled, maybe someone out there could shed some light on it?
Here's what I'm building: I need a microprocessor controlled, differential audio signal attenuator. Basically, the user can set a timer with some BCD switches (read by my ATTiny2313). When a song starts playing, an external interrupt signal goes high, which starts a timer countdown based on the values on the switches. When the timer hits zero, the ATTiny uses a SPI interface (or something similar) to control a digital pot to attenuate the audio signal. Sounds simple, but I'm having a horrible time finding an audio attenuator. I was going to use the MAX5410, but the audio input has to be within DC range (0-5VDC) and non differential. My audio signal is a differential audio signal (R+,R-,L+,L-, and GND) that is 1Vp-p, so it would be out of range. Here are a few options I've found, can anyone shed some light on which one would work best, or give me another idea?
1 - Use the MAX5410. Since the MAX5410 has two channels per chip and the SPI can be daisy chained, just hook two 5410's together, so one 5410 attenuates R+ and R-, and the other attenuates L+ and L-. Hook up an op amp to the audio inputs to bias the signal up to DC ranges.
Problems: The output will then be biased, so I'd have to de-bias the output using ? (capacitor high-pass filter? another op amp?). The differential signal will be attenuated separately, which could introduce noise, or basically defeat the whole purpose of a differential signal.
2 - Use a differential chip. I've found one so far, the MAXIM DS4420. That fixes my differential problem, but the input still has to be within -0.5V to 5VDC. Is that cutting things too close?
Problems: Might be outside spec to put in a 1Vp-p signal, do I still have the need-to-bias-and-de-bias problem as I did with the 5410? This chip only attenuates 35db, not down to mute, which is what I want, but I could put two chips in series to get the same effect?

Any help would really be appreciated, and I can provide more details upon request. (Also, if you know of a better location for me to post this question, I'll be happy to move it)

MAX5410 datasheet: http://www.maxim-ic.com/quick_vi...
DS4420 datasheet: http://www.maxim-ic.com/quick_vi...

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A problem simply handled by correct design of power supplies in the first instance.
Namely superimpose a +5V on top of conventional =/- 15V rails for the analogue circuit.
Work out what the burden is on the 5V supply and arrange =/- 2.5 V opamp voltage followers with perhaps some additional transistor power stage. That will provide the necesary 5Volt supply to run the digital controls ie digital potentiometer and microcontroller while at the same time maintaining balanced supplies for analog circuit and ensuring the nalog signal is within the operating range of the digital pots without introducing unnecesary modifications into the signal path.

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I'm building an add-on board to an existing audio system, so I don't really have much control over the power coming in. I'm assuming I can find a +/- 1V power supply somewhere since the audio signal is 1Vp-p, and I know I have a 5VDC signal because the existing audio system powers a standard SCSI hard drive (amongst other things) with a 5V/12V molex power connector.

Which IC are you recommending I use with your solution then, the 5410 or the DS4420? I guess I'm not following your message on how the addition of the power stages nad voltage followers will put the analog signal within range for the pots, are you meaning to put the analog signal as input to the voltage followers, and if so, are you still talking about using a differential audio signal, or combining, or discarding one half?

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fowdawgg wrote:
I'm assuming I can find a +/- 1V power supply somewhere since the audio signal is 1Vp-p, and I know I have a 5VDC signal because the existing audio system powers a standard SCSI hard drive (amongst other things) with a 5V/12V molex power connector.

You won't find +/- 1V power supply. If any audio output, differential or not, is 1Vpp, it does not mean it must be centered at 0V GND level. It may internally have half of the supply voltage like 2.5V as the internal zero level.

You can convert differential audio to single-ended for volume control and back to differential if you like. In fact it may be your only way, as the audio really is differential, it may have nothing to do with ground potential, only the +ve audio signal is relative to -ve audio signal.

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Well that's good news if it proves correct. I'll check the actual voltages on the line. What is the standard voltage level for a commercial line-out (unamplified) audio signal transmitted out an XLR port (to hook into an amp or mixer)? If it's not centered around zero, but rather something above zero (and below 5V) then I've got it made.

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Standard output levels are 775mV into 600 ohms: one milliwatt, for 0dB. Pro audio gear will expect to supply up to +18 or +24dB; that's several volts. But pro audio will also expect to be balanced. Some equipment will be unbalanced and some will be happy to have the antiphase leg grounded if plugged into an unbalanced input. It all depends...

One way to convert balanced to unbalanced signals is with a transformer; the input across both and the output with one leg grounded. Another way is to use the differential inputs of an operational amplifier, with suitable gain control - you may need in both cases to manage the output levels and DC offset voltages.

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Tell me more. So far I think its some sort of automatic gain control that fades out music under microprocessor control (so the announcer can page someone at the airport?). If you're getting a contract to make 100 of these (or 1000), then engineering happens and we worry about cost of parts, etc. I gather the audio is 1v p to p and differential.... thats not a line level signal like from a board to an amp (that more like 20v p to p), and its too hot for a mic level signal which is 40 to 60 db below line level (1/100th or 1/1000th). If its just one you need, I'd plug the differential signal into a mic transformer and the output will be hotter and single ended, suitable for running thru a pot (even a motorized one like they sell at marlon p jones and skycraft) or cots compressor/limiter. Commercial models have 'ducking' for voice announcements. Computer controlled volume chips are avail.. 'logdacs' that have 10 or 12 bits of control in .1 or 1 db steps. Like I said, tell us more about your project.

Imagecraft compiler user

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How much do you need attennuation and how many steps? If only on/off control, use a relay. Heck, there are even commercial audio amplifiers that have digital volume control built with relays!