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I want to build a spectrum analyzer for my car stereo. 5 bands, left & right channels represented individually, 20 LEDS per band using cascading LM3915s per band and channel. (Yes, 20 LM3915s, 200 LEDs in total, I'll be busy for a MONTH soldering!)

From what I've read, changing the volume of the radio "head unit" dramatically effects the display. So how about automatic gain control? It would automatically ramp up the gain for soft volumes (like when I'm at a red light and there's a cop in the next lane) or attenuate the signal when I want to crank it up & bang my head.

I found thid online:

Automatic Gain Control Circuit

Now, if I want to build a 2 channel (separate left/right) 5 band led display, would I need to build just TWO AGC circuits, or would I need one for each band for a total of ten AGCs?

Second, is this circuit acceptable for my purpose of processing an audio signal to maintain the signal strength for the LED display regardless of volume settings?

Next question, could I use a single TL074 quad op amp instead of 2 ICs (072 dual op amp & single 071) and just ground out the + input of the unused op amp and short the - input to the output? It would save cost, real estate on the circuit board and complexity.

Finally, where would I put this? Before the unity gain buffer/filter stage, between the buffer and bandpass filters, after the filters but before the peak detectors/rectifiers? My intuition say before everything, so the buffers/filters actually have a signal to process. That would be cheaper also, I'd only need to build 2. But since I'm building this to provide a signal to 5 "processors" wouldn't that diminish the signal for each? Kind of like wiring 5 bulbs to a single circuit, or 5 speakers to a single line, each additional one diminishes the output of the others - or would the unity gain buffers of each one compensate for all that? Honestly, I've never done anything like this before, so I now to y'all's experience and expertise.

All of this would be driven by the radio's unused speaker outputs, as the actual speakers are driven by amplifiers thru the radio's line-out jacks.

I don't know if this matters but the head unit is advertised at 75 watts max x4, 18w RMS.

JRE
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Super-Dave
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  • Other than a lot of flashing LED's , what would you hope to accomplish with this? Normally one wants to equalize the levels to one's hearing sensitivity which changes in amplitude according the Fletcher Munsen curves and just use a fixed Line output so that volume does not demand such a large dynamic range or require AGC. https://en.wikipedia.org/wiki/Equal-loudness_contour – Tony Stewart EE75 Oct 09 '21 at 04:30
  • Tap off before the volume control and you won't need AGC. Don't forget to add a dimmer for night driving. – Transistor Oct 09 '21 at 06:38
  • If you have access to the line-level outputs, then use them. Then you don't need AGC. – Mattman944 Oct 09 '21 at 06:38
  • If you put AGC on each frequency band, you no longer have a spectrum analyzer. You just have a "color organ". – Dave Tweed Oct 09 '21 at 13:11
  • Note that another method of AGC when using an LM3915 is to feed the LM3915 reference with a heavily filtered peak detector. – Mattman944 Oct 09 '21 at 14:58
  • Mattman, the head unit is a car stereo. Line outs are still governed by the volume knob. I want AGC so when I adjust the volume to low levels, the LEDs don't diminish to just one or 2 blinking, nor do I want all the LEDs to go full throttle at high volumes. I'm not looking for accuracy, just a constant state of input regardless of volume. Some of the songs in my USB playlist play louder or softer than others. Unlike radio broadcast, not all of my music is compressed at the same rate. – Super-Dave Oct 14 '21 at 15:03
  • Transistor, the head unit uses micro surface mount components. The tip of my solder iron is wide enough to touch 2 to 3 solder dots simultaneously. I'm building this device using thru-pin PCBs, 2.54mm spacing. Even if I could understand what component or circuit does, I couldn't splice a wire into it without destroying the radio. – Super-Dave Oct 14 '21 at 15:07

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