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FAQ - Upgrades and Tweaks Circuit and Component tweaks

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Old 2nd October 2010, 10:27 PM
Richard Richard is offline
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Default Shunt Pot Volume Control for Better Sound and/or Gain Reduction

This is a volume control which uses a fixed resistor as the series part of the potential divider and a log pot as an variable resistor for the shunt part. Note that the pot is "inverted" from its normal wiring; the signal-in and earth connections are the other way around from standard.

It gives the signal a hard-wired path. The earth connection is made via the pot wiper. Any noise at the wiper junction with the track will be isolated/reduced by the track resistance between the wiper and signal.

(Subjective comment - the sound is noticeably clearer. A metal film resistor used as series gives a brighter more detailed sound whilst a carbon film type gives a warmer sound. Some of the pot character remains and pot/resistor combinations can be tried to taste. An Alps Blue with a metal film series resistor makes a very enjoyable control with sweet and open high frequency detail and warm bass.)

To keep a similar "feel" to the volume control law choose a series resistor approx 90% the value of the pot track, so 91k with a 100k pot, 47k with a 50k pot, or 18k with a 20k pot. (Only small wattage resistors are needed, eg 1/4W or 1/2W will be fine.)

Three considerations.

1) A reduction in maximum volume of a little under 1/2 voltage, -5.5dB. This may be an issue with insensitive power amps or low output sources.

2) An increase in source impedance due to the series resistor when the control is turned past approx the 3/4 -8dB position. (This would only be a consideration if the control needed to be used at this level or higher.)

3) Variable input impedance of 91k to 191k, 47k to 97k, or 18k to 38k. This is not usually a problem but the driving source should be borne in mind. A high impedance valve output source may not be happy into the lower load but would be fine into the higher. A solid state source should be fine into either and may actually prefer the lower.

Use as a passive preamp.

If a 100k pot with 91k resistor is used as a passive pre to drive a power amp the source impedance at the worst point will rise after the 3/4 -8dB position from 25k for the pot alone to 47k for this arrangement. (Typical log 12 o'clock 1/2 way position would be -17dB and give approx 12k source impedance with either pot alone or this arrangement.)

A better arrangement for use with low output impedance sources would be a 20k pot with 18k resistor which will have a worse case of 9k and be quiet into loads of 100k to 1M. (Typical log 12 o'clock 1/2 way position would be -17dB and give approx 2.5k source impedance with either pot alone or this arrangement.)

Using Veroboard or Plain matrix board (from Maplin) for the pot and resistor connections

Below is shown a way of using Verostrip or plain 0.1" matrix board to hold the resistors and allow the easier connection of large cables. If using board with copper strip, note the position of the break in the strips, separating the pot tags. Note also that the 2 ends of the resistors are on separate tracks and so are not connected together.

The one shown is in a Pre3 (which has 6x gain = +15.5dB as standard) and uses values to lower the overall gain as well as provide the sonic advantages of the Shunt Pot Mod. In this case, 47K series resistors are used with 20K log pot to give a reduction of -10.5dB at max vol position. Alps pots are available from WD or RS etc and 0.6W metal film resistors from Maplin or RS etc work well with it.

Use the potential divider formula to work out other reductions; (Shunt) divided by (Shunt plus Series) = Ratio. (Log Ratio) x20 = dB or to fine tune start with one suggested below and adjust the resistor value greater for less gain or lesser for more gain.

* To do the shunt mod for sound benefit only, use 39K or 47K series resistors with the 50K pot. This will reduce max gain by -5dB or -6dB respectively at full volume and give a similar "feel" to the standard pot whilst keeping loss of gain to a minimum.

* To reduce gain by -10.5db use 47K series resistor with a 20K pot. This works well using Pre3 with a typical WD/WAD/other line level power amp.

* To reduce gain by -15dB use 47K series resistor with a 10K pot. May be useful with very sensitive power amp and/or speakers.

* To reduce gain by -20dB use 91K series resistor with a 10K pot. May be useful with very sensitive power amp and/or speakers.

It would be possible to use higher value series resistors with higher value pots when reducing gain but noise and the sonic result may be different.

Attached Images
File Type: gif Shunt-Pot-Mod-Alps.gif (18.2 KB, 449 views)
File Type: jpg potshuntboard2.jpg (50.0 KB, 197 views)

Last edited by Richard; 31st March 2017 at 10:40 AM. Reason: add pic

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