Another old homebuilt amp, 100K Volume pot - substution OK with lower value? - Page 1
Forum section: electronics / beginners

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: November 07, 2011, 10:54:31 am
content: Yet another old amp but this one has a scratchy volume control. Opened it up and discovered a 16mm 100K (log) dual gang mounted on the PCB with the control through the front panel, so any replacement size mismatch is a problem. My dilemma is, the nearest (Penrith, Sydney) Jaycar stocks no 100k dual gang pots in 16mm only a 24mm one which would be difficult to workaround. Save from going online and ordering the exact right part from somewhere else, I'd like to know what is wrong with putting say a 50k or even 10k (log) pot in place of the 100k?Looking or should I say rummaging through stuff trying to find the schematic for the amp kit PCB, I seem to think there was only mounting instructions. It has a whole bass, treble and balance stage, and not complicated to draw the circuit but it's only the volume pot that needs a talking too...

author: amspire
author_rank: Super Contributor
timestamp: November 07, 2011, 11:46:50 am
content: It should be OK if it is a volume control.You just want to look at what is the signal going to the Pot coming from, to make sure it can drive a lower impedance like 10K. If it is from an Opamp output, it should be OK. It is is a valve amp, it may not be able to drive 10K. Same is possible if it is from a transistor collector, but if it from a transistor emitter, it will be fine.The simplest test is if you put in the new Pot and the volume hasn't decreased, there is a very good chance that it is fine.Also there may be a series capacitor going into the Pot that was chosen for the 100K, so a lower capacitor may be required. Anything on the Pot output will be fine as it is.Now if you need to replace any tone control Pots, then the 100K value is important.Richard.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 07, 2011, 11:47:40 am
content: It should work fine, although there may be some interaction with the balance control, making it more or less sensitive. You also may prefer to get a linear pot and configure it for pseudo-log operation with fixed resistors. Google can give you some example circuits for psuedo-log volume control circuits.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: November 07, 2011, 10:10:57 pm
content: Thanks for the replies. Nar, it's not that old that there are valves involved but that is good information to know.Will try a 50k pot and see what happens.Thanks again.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 07, 2011, 11:17:35 pm
content: Hi edAs ejeffery suggested, using linear pots modified with fixed resistors to change the curve of the pot is a good way to go since linear pots are more widely available. See the web pages at ;sound.au.com/index.html, (Elliot sound pages). There is plenty of excellent info here on this topic (A better volume control) as well as much much more.cheersjohn

author: ciccio
author_rank: Frequent Contributor
timestamp: November 08, 2011, 08:54:19 am
content: Quote from: amspire on November 07, 2011, 11:46:50 amNo, a bigger capacitor (it must be of lower impedance).Regards

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: November 08, 2011, 08:13:08 pm
content: Thanks. I'll try that out and see what happens.Quote from: lowimpedance on November 07, 2011, 11:17:35 pmThere sure is a lot of information on that site.