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FAQ - Upgrades and Tweaks Circuit and Component tweaks

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Old 31st August 2013, 11:07 AM
Richard Richard is offline
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Default Adjusting Overall gain from Pre3 using the Shunt Pot Mod

Adjusting Overall gain from Pre3 using the Shunt Pot Mod

Pre3 has 15.5dB (6x) overall gain with a standard volume control (pot) at maximum position. This is to allow its use with a variety of power amps such as Quad II needing 1V or more. It is better to have a little more gain than not enough as the extra can easily be lost but not easily found!

It may be too much gain for driving a sensitive power amp though, and result in an over-sensitive volume control, so here is how to adjust it and improve the volume control at the same time.

The Shunt Pot Mod uses 2 fixed series resistors before a reversed pot. This gives a hard wired signal path which doesn't use the pot wiper for the signal connection (just for the ground connection) and many think the sound is improved.

The SPM attenuates the signal at the max volume position and controls the remaining signal in a log-like way. 6dB is lost as a practical minimum but this is very useful if we wish to lower gain and have an improved volume control at the same time, one of those rare win-win situations!

Below is a table to allow the easy choice of a pot and series resistors to give various levels of gain (or indeed remove all gain) from Pre3. The Alps Blue pot is recommended and works very well with small (0.6W) metal film resistors.

The greyed out selections will work and may be useful to test a level of attenuation but are not optimal for reason of low or high input resistance. By all means try them and leave them in if satisfactory or change the pot if not.

Below is a comparison of a 50K Alps Blue pot used in the Standard way and wired in the Shunt Pot Mod way. 47K, 100K and 240K series resistors are used in this example to show how the max vol is lowered and the remaining signal spread evenly in a log-like way over the pot movement shown at 1/4 1/2 3/4 and Max turn.

See also Shunt Pot Volume Control for better sound in FAQ Tweaks section
Attached Images
File Type: jpg SPMpot.jpg (22.0 KB, 149 views)
File Type: jpg SPM.jpg (54.0 KB, 146 views)

Last edited by Richard; 13th October 2016 at 09:31 PM. Reason: attach pics

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