HT GPS + Audio: Revised Schematic

This is a tweak to the previous design, based on some road testing.

An attenuator on the output of the MAX4467 voice amp allows gains below unity. Right now, the MAX4467 has Av=5 and the attenuator cuts it back by about 1/5, so the overall gain is about unity. I have a bunch of surplus electret mic capsules and some have come through really hot; this allows backing the gain way down with the mic amp set to Av=1.

That requires stiffening the Vcc/2 supply by swapping in a 33 µF cap for the original 1 µF unit. If you don’t do that, the amp turns into a oscillator: the attenuator jerks the Vcc/2 supply around, which feeds back to the non-inverting input of the MAX4467. In principle, the gain should be less than unity, but I wouldn’t bet on it.

The MOSFET relay sometimes didn’t quite turn on from the piddly 4 mA available through the ICOM IC-Z1A’s mic power supply; it was vaguely temperature dependent. I returned to an ordinary optocoupler with a CTR of about 100% driving a 2N2907 PNP transistor, as in the first-pass design that you never saw.

The two 2N2907 devices allow either a through-hole TO-92 or SMD SOT-3 package, depending on what you have and the power dissipation you need. In my situation, the SMD version suffices, with less than 100 mV of VCE saturation.

Let me know if you need the Eagle PCB files or PCB layouts.

Clicky for a bigger image…

GPS + Voice HT Interface schematic - revised 15 July 2010

GPS + Voice HT Interface schematic - revised 15 July 2010

[Update: I'm not convinced the Vcc/2 supply is stiff enough. I ripped out the attenuator and cut the amp gain to 1.0. If I get some really hot capsules, I'll think it over a bit more.]

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  1. #1 by John Rehwinkel on 12-August-2010 - 12:26

    One trick to get a stiffer low-power Vcc/2 supply is to run the resistive divider voltage through an op-amp strapped as a voltage follower. I’m fond of this trick when I have a “spare” op-amp available (e.g. I’m only using three on a quad op-amp chip).

    • #2 by John Rehwinkel on 12-August-2010 - 12:31

      Looks like you don’t have op-amps to burn, so you could also just try the simpler approach of buffering your voltage divider with an emitter follower.

      • #3 by Ed on 12-August-2010 - 13:43

        The catch is that Vcc/2 really needs stiffness for both source & sink current; a transistor gets you halfway there. An op-amp would be ideal, but, sheesh, that board is packed already. I must cut some stuff out or start putting parts on the solder side. Ick.

        In the last couple of days, though, I threw the divider overboard and cut the gain back. The audio is still somewhat hot, but eliminating the grand feedback loop through Vcc/2 seems like a win.

        The catch with too much audio gain is wind noise: immense sub-sonic buffeting that slams the op amp (in the MAX4467) against both rails. So a lower mic gain is much better.

        I’m still unhappy with the design, but I now have three similar boards working. More fiddling is in order, which means a few more weeks of riding… so it’s not all bad!

        • #4 by John Rehwinkel on 12-August-2010 - 14:33

          As for the subsonic overload, this is a common problem in audio and communication circuits. The usual approach is to filter out those frequencies. One of the tiny surface-mount op-amps like MCP606 or LT1880 is probably overkill.

        • #5 by Ed on 12-August-2010 - 14:53

          Part of the problem is duplicating the wind on the bench; it’s pretty fierce out there on the road!

          I’m thinking of tossing the 4467 and using an ordinary op amp, at which point cutting everything below about 200 Hz would make a lot of sense. The DC blocking caps help, but an actual passive filter would be a step in the right direction.

          All I need is a Round Tuit, which are in short supply right now. That rotted washer drum is still soaking up thread lube like it’s a sponge, for example. Sigh.

  2. #6 by John Rehwinkel on 12-August-2010 - 15:14

    I looked at Maxim’s page on that IC, and it does say “for use in noisy environments”, but I think they mean electrically noisy, not acoustically noisy. Looking at the datasheet, at the gain-vs-frequency plots, it has lots of subsonic gain. I think the easiest thing to try is just replace C4 (the mic coupling capacitor) with something (much) smaller. That’s a passive high-pass filter you already have, it’s just the cutoff point is too low.

    • #7 by Ed on 12-August-2010 - 16:16

      Surprisingly, setting Av=1 (for a particular set of mic capsules, sigh) eliminates the wind noise at normal riding speeds with just a simple foam windscreen.

      I had used Av=10 (for another set of capsules) and that didn’t work at all: audio OK, noise intolerable. Attenuating the output by 1/10 cooled off the audio, but the wind noise was still bad; I think the 4467 was slamming against the supply. Different capsules, different gain: better results.

      So there’s a delicate relation between the specific capsule (hard to control with surplus parts), the amp gain, and probably the foam windscreen size. Cutting C4 back can’t possibly hurt (ha!), so I ought to do that, too.

      I should gimmick up an earbud to serve as a sound source to test the mic capsules. Then I could actually measure the audio-to-electrical response and get some idea of what’s going on… another dang project!

  3. #8 by Bob Paddock on 12-August-2010 - 21:43

    • #9 by Ed on 13-August-2010 - 08:11

      I actually have a couple Railsplitters in the stash, but … dang, that board is already too complicated!

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