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Choosing Trimpot for volume control

Nigel Goodwin

Super Moderator
Most Helpful Member
But it occurred to me that a potentiometer at the output stage would have challenges because of impedance mismatch problems and also the high current.
Impedance mismatch isn't a problem, as matching is rarely required, and must not be matched in this kind of case.

The high current IS a problem though, hence the need for a big pot.
 

audioguru

Well-Known Member
Most Helpful Member
Do you want the sound to be good to play music or is it for a bad-sounding intercom at a gas station?
A resistor in series with a speaker ruins the "damping factor" of the amplifier. Then low frequencies sound like a bongo drum. You can ask the gas station attendant, "What? What" until you understand what the bonging sounds are saying.

If you connect a linear 100 ohms variable resistance in series with an 8 ohms speaker then at 100 ohms the speaker sound is about half. If you turn the resistance to half at 50 ohms then the sound is almost the same as at 100 ohms.
If you turn the resistance almost to a short at 8 ohms then the sound is noticeably less than when it is a short. So you have poor control of the sound level since a logarithmic volume control is needs.

If you connect a 100 ohms variable resistance as a voltage divider then it is heated a lot by the audio but feeds the speaker almost like a logarithmic volume control. At halfway the sound will be half and have a boomy sound like a bongo drum.
 

audioguru

Well-Known Member
Most Helpful Member
As usual you're vastly over exaggerating AG, while it will ruin the damping factor it's unlikely that anyone will even notice.
That is because the average person is tone deaf (never heard a live musical or good hifi system) and the average sound system is cheap (but costly) garbage. They think that it is normal for a sound system to sound like the cheap intercom at a gas station.
 

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