AGC

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Automatic Gain Control automatically evens out variations in volume. This is useful on a TV, where ads are often much louder than programs.

Electronic AGC can give a consistent volume output level, if relatively sophisticated. But these are far outside of typical DIY skill sets.

What's presented here is a basic passive AGC that almost anyone can make. This can't produce an even output level, rather it just reduces the volume changes to some extent. That's as much as can be achieved with a simple approach.


The circuit

Bulb agc 2889-3.jpg

The circuit is very simple: a filament light bulb is wired in series with each speaker. As the TV's output volume varies, the bulb's resistance changes over a ratio of about 8:1, partially stabilising the speaker volume level.

o-------+
to      |
        |
       (X)  lightbulb
        |
        |
       [_]<  speaker
        |
tv      |
o-------+

The lightbulbs can either be fitted inside the set to work with the already built in speakers, or external speakers can be plugged in and light bulbs added.

Bulb life

Filament bulbs last decades in this application, as long as they're not knocked or run white hot.


Bulb ratings

Ideal bulb ratings depend on the output power of the set.

Small portable TV (0.2-0.5w): 1.2v - 1.5v torch bulb (for use on a single 1.5v cell), preferably about 0.15A.

Most TVs (3-5w): 4v 0.5A, or as near as possible

Home cinema systems (30w per channel RMS): 12v bulb, 12-18 watts.

For 1w 15 ohm speakers (a minority of old sets): 3v 0.2A torch bulb


Side effects

A series light bulb has other normally minor effects on sound output.

Maximum volume output is reduced. This isn't often a problem, but if you do experience inadequate max volume you can increase it with a switch to short the bulbs.

The sound response characteristics of the speakers are altered slightly due to variations in impedance with frequency of moving coil speakers. This isn't normally noticeable with TV transmissions, but if you run tv sound through a good hi-fi you may want to add a switch to bypass the lamps for non-tv use.


FAQ

Are there any components that can be added in parallel with the speaker to improve the agc further?

  • Passive components exist, but they all cause sound distortion, so they aren't really useful.
  • Active circuits can be used though, such as a rectifier & reservoir fed mosfet, if you have the skills.

Do the light bulbs fail after a bit?

  • No, operated the way they are in this setup they last decades. Small filament bulbs aren't very robust though, and should not be knocked.
  • If the bulb lights up bright white in use, your light bulb specs are not ideal. Replace with a slightly higher voltage/current/power bulb to avoid eventual failure.

If this doesn't even out the sound, what's the point?

  • Reducing the amount of volume variation significantly reduces the incidence of annoyance / inconvenience.


Safety

AGCs that plug into the TV and run external speakers aren't hazardous. If fitting bulbs inside a TV, TVs contain several hazards, even when unplugged. Only work inside a TV if you know how to do it safely.


See Also