Research: Audio taper trim pots about ohis HOT 2 CLOSED

Source:

http://www.geofex.com/article_folders/potsecrets/potscret.htm is a good page talking about this.

Qucs simulations

I also did some Qucs simulations:

Source impedance is 10R. Pot is 1k (split into R2 and R3, with equations doing the heavy lifting.) The taper resistor is 1/10 of the pot (100R) for a large change in slope. The load impedance is 100k.

The source and load impedances are 100x outside of the pot (source = 1k/100 = 10R, load = 1k*100 = 100k), to minimize the impact of the variable impedance the tapered pot will represent.

Having said that, I also tested with a 10k and 1k load resistance, and the graph barely changed. This makes sense since the load is in parallel with R_taper; as long as R_load is 10x R_taper (or, 1k in this case), it will be dominated by R_taper and not make much effect on the whole graph. Meaning, our load impedance can be a wide range, so long as it's ~10x R_taper or more.

Qucs Result: Output voltage as a function of sweep

Blue line is the resultant voltage, graphed linearly. The red line is the same, graphed logarithmicly. A perfect log pot would result in a straight red line, so curvature in the red line shows how not-logarithmic the graph is.

So, this isn't a great logarithm, but it's WAY better than a linear pot. Notably, it gives a lot of precision at the low end, which is what I really want.

Qucs Result: Input impedance presented to the source, function of sweep

This shows the impedance of this pot as presented to the source. Note that it varies by >90%, from 1k down to below 100R. This is why this architecture isn't normally used.

But, as long as your source can handle driving the minimum presented impedance, then it should be ok.

Analysis

I like the simplicity of a single pot.

Microphone trimmer

The source on the Mic trimmer is our own opamp, so we know the source impedance is very low, plenty low to drive the pot.

The load is the microphone input to the radio, which is likely to be at LEAST 600R, more likely 1k or 10k. I graphed a 600R load and it's still pretty darn good.

So, I'm calling this good.

Headphone/Speaker trimmer

I don't get to specify the source on this one. The source is likely to be:

• Line: 600R
• Headphone: 16R or less
• Speaker: 8R or less

So the worst cast to consider is Line output, 600R. Unfortunately, even with the volume turned all the way up, this results in a voltage divider between source impedance (600R) and R_taper (100R), which results in a 1/6 decrease in voltage, even at full blast.

However, I specify the load here, our headphone amp, which is 10k. So I can go to a 10k pot/1k R_taper safely. That makes the voltage divider 600/1000, or a little less than half voltage.

Going to 100k/10k makes R_taper and R_load equal, effectively dropping R_taper by half. This results in a much flatter graph (not a bad thing), and a full volume much closer to 100%:

Conclusion

• Mic Trim: 1k pot, 100R taper resistor.
• Headphone Trim: 100k pot, 10k taper resistor.

from ohis.

Lessons learned:

• The load impedance doesn't matter TOO much. It's primary impact will be to change the value of R_taper, since it's in parallel with R_taper, and the impact that has on the graph.
• The source impedance DOES matter, A LOT. It impacts the maximum output volume, by defining a series voltage divider with R_taper. So as long as R_taper >> R_source, it'll be ok. 10x seems a good definition of >> here.
  • The impedance presented to the source varies between R_pot (all the way down) and R_pot in parallel with R_taper (all the way up). Above, I talk about the worst case of this being R_taper. But if there are any frequency dependent parts of the output of the source (eg: an RC DC block), this needs to be considered.
• The ratio between R_pot (larger) and R_taper (smaller) will affect the curve. Larger ratio, the "more log like" the resultant taper will be. But it also results in a steeper curve at the low end. The web page listed at the top of this ticket graphs values up to 6x. I've gone with 10x. Somewhere in that range seems reasonable.

But, as long as the above is considered, I think this works well to make "an audio taper trim pot."

from ohis.