Pot wiring. Pot questions. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

RF Burn · 20th Jul 2017, 5:44 pm

Connecting a log pot 'backwards' does not give anti-log, that was a mistake I made before I realised the truth. In any case if that were the case then there would be no need for special anti-log pots!

Also, logarithmic pots, with moulded (carbon, cermet, whatever) tracks, are made up from just two linear sections rather than a smooth log taper which is very difficult to produce. It is possible to hear the effect on the volume and hence most serious audio designers will use the 'modified pot' arrangement with linear parts to obtain a much more realistic logarithmic change of resistance/volume. Placing a resistor across the wiper and 'top' end of the pot synthesises a log taper and a resistor across wiper to 'bottom end' of the pot synthesises an anti-log taper, the only disadvantage of this arrangement is that the impedance presented by the pot to the driver stage will vary as the pot is turned, this will give an even more interesting characteristic to the volume control if it follows a high impedance (i.e. valve anode) signal source.

http://sound.whsites.net/project01.htm

That page also contains a very novel stereo balance control which solves all the problems of having to use special pots or tolerate poor balance adjustment.

Craig Sawyers · 20th Jul 2017, 6:43 pm

Quote:

Originally Posted by RF Burn

Also, logarithmic pots, with moulded (carbon, cermet, whatever) tracks, are made up from just two linear sections

Well, actually that might have been the case back in the day, but now the available tapers are much more sophisticated:

http://www.alps.com/prod/info/E/PDF/...eter/TAPER.PDF

Craig

Diabolical Artificer · 22nd Jul 2017, 6:59 am

Ah, see the thread has burst back to life. I guess whether a pot is truely logarythmic is academic as most amplifiers are rarely turned up past the transition.

RF Burn brought up a point I've also been mulling over. We have a valve voltage gain stage with a pot to vary the signal seen on the control grid. My practice is to put a grid resistor on as well as the pot, I usually use a 1M.

Obviously with the pot in parallel the input Z is going to change. So what is best practice and while were at it and what value pot? I've read a 1M pot favoured by vintage amp/radio builders is too high and that 10 - 100k is better....

A.

RF Burn · 23rd Jul 2017, 1:25 pm

Quote:

Originally Posted by Craig Sawyers

Well, actually that might have been the case back in the day, but now the available tapers are much more sophisticated

Look carefully at the taper graphs that Alps show, the "curves" are mainly composed of conjoined straight sections...

20th Jul 2017, 5:44 pm	#21
RF Burn Hexode Join Date: Apr 2008 Location: Ramsgate, Kent, UK. Posts: 252	Re: Pot wiring. Pot questions. Connecting a log pot 'backwards' does not give anti-log, that was a mistake I made before I realised the truth. In any case if that were the case then there would be no need for special anti-log pots! Also, logarithmic pots, with moulded (carbon, cermet, whatever) tracks, are made up from just two linear sections rather than a smooth log taper which is very difficult to produce. It is possible to hear the effect on the volume and hence most serious audio designers will use the 'modified pot' arrangement with linear parts to obtain a much more realistic logarithmic change of resistance/volume. Placing a resistor across the wiper and 'top' end of the pot synthesises a log taper and a resistor across wiper to 'bottom end' of the pot synthesises an anti-log taper, the only disadvantage of this arrangement is that the impedance presented by the pot to the driver stage will vary as the pot is turned, this will give an even more interesting characteristic to the volume control if it follows a high impedance (i.e. valve anode) signal source. http://sound.whsites.net/project01.htm That page also contains a very novel stereo balance control which solves all the problems of having to use special pots or tolerate poor balance adjustment.

22nd Jul 2017, 6:59 am	#23
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,658	Re: Pot wiring. Pot questions. Ah, see the thread has burst back to life. I guess whether a pot is truely logarythmic is academic as most amplifiers are rarely turned up past the transition. RF Burn brought up a point I've also been mulling over. We have a valve voltage gain stage with a pot to vary the signal seen on the control grid. My practice is to put a grid resistor on as well as the pot, I usually use a 1M. Obviously with the pot in parallel the input Z is going to change. So what is best practice and while were at it and what value pot? I've read a 1M pot favoured by vintage amp/radio builders is too high and that 10 - 100k is better.... A. __________________ Curiosity hasn't killed this cat...so far.