Resistor Sound Quality? - Page 13 - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th July 2006, 04:38 PM   #121
diyAudio Member
 
serengetiplains's Avatar
 
Join Date: Feb 2004
Location: Canada
Christer, Vishay makes bulk metal foil trimpots.
  Reply With Quote
Old 9th July 2006, 05:02 PM   #122
poobah is offline poobah  United States
diyAudio Member
 
poobah's Avatar
 
Join Date: Nov 2005
Yes... trimpots are evil.

The thermo-electric voltage from wiper to element is also a pain.

Some dirty pot tricks:

1) don't use one... calibrate in software.

2) use a matching style sacrificial pot right next to the real one in your circuit. arrange it in such a way that the wiper currents flow in opposite directions... this will help to cancel thermal EMF's.

3) don't use one.

4) In voltage dividers... never use a pot as a potentiometer... always configure it as a rheostat.

5) don't use one.

6) the tempco's are horrible... use proportionate values in each branch of a voltage divider to help cancel the effects.

7) don't use one.

8) the tempo's are still horrible... the wattage rating only implies that amount of power which will keep the smoke inside the device. Stay under 5% of the wattage rating where you can.

9) don't use one.

10) forget about multi-turn pot's. They are really just single-turn pots with a gearbox... so you think you are doing a great job. The gearbox is loaded with mismatched coefficients of thermal expansion... an inaccurate temp sensor at best.

11) don't use one.

12) use wire-wounds when you can... this avoids much of the horribleness.

13) don't use one.

14) always sweep the wiper 5 or 10 times from min to max... this knocks down the lumps and let's you think you did a better job.

15) don't use one.

16) always tap the pot when you're done adjusting it to relieve stiction. This will kill your setting though.

16) Did I remember to say, "don't use one"?

17) Vishay metal foils are excellent... try finding one... and then try to buy it.
  Reply With Quote
Old 9th July 2006, 05:35 PM   #123
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
I have had no problems with my tantalum film pots in hundreds of units sold. Expensive trim pots are worth the cost.
  Reply With Quote
Old 9th July 2006, 05:42 PM   #124
poobah is offline poobah  United States
diyAudio Member
 
poobah's Avatar
 
Join Date: Nov 2005
Where are you getting them? And, do you know the tempo's off the top of your head?
  Reply With Quote
Old 9th July 2006, 06:36 PM   #125
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
NEC, Japan.
  Reply With Quote
Old 9th July 2006, 07:19 PM   #126
poobah is offline poobah  United States
diyAudio Member
 
poobah's Avatar
 
Join Date: Nov 2005
Thanks...
  Reply With Quote
Old 9th July 2006, 08:06 PM   #127
diyAudio Member
 
Join Date: Sep 2002
Location: Sweden
Thanks for the answers. I think I have asked once before with no results at all. I guess one has to ask when the right guys are online.

So metal film or tatntalum sounds to be the choice if available. For most of us that would label under the exotic though, since they seem not so common. I will keep in mind that they migt be worth looking for in critical situations, though. Any opinions on carbon vs. cermet? Those are usually the only types easily available.

SY has an excellent point. In contrast to the material in the film it is not so easy to find out from tech. specs. though. I guess the best one can hope for is that more expensive pots are on the average better designs in this respect.

Poobah, you had many very interesting points. Especially the odd numbered ones, although I think I knew most of those already.


Of course, as long as we are doing DIY and not designs for mass production, there is often also the option to first use a trimpot to find out the right value and then replace it with fixed resistors, assuming it is a parameter that need not be readjusted later. I guess that would be too expensive in production though, even for most of Johns designs.
  Reply With Quote
Old 9th July 2006, 08:22 PM   #128
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
I'm surprised that poobah didn't mention the other thing with trimpots. Never pass all of your current through them. Always use the trimpot in series with a fixed resistor, all in parallel with a fixed resistor. That way, you can ensure that the trimmer only gives the exact range of adjustment you genuinely need and that all its awful problems are swamped by the majority of the current going through the fixed resistors. It's not usually difficult to get a 10:1 ratio of current, thus effectively dividing the trimpot's problems by ten, and making them useable.

I wrote a little spreadsheet that plots the variation in total resistance against wiper rotation for the above scheme so that I can plug various values in to see if I like the look of the law and the range of variation. It allows me to force a standard value of trimpot to give exactly the weird range of variation I need.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 9th July 2006, 08:25 PM   #129
poobah is offline poobah  United States
diyAudio Member
 
poobah's Avatar
 
Join Date: Nov 2005
Christer,

What I've done often where extreme accuracy is needed is to place 2 resistors in parallel.

Load the board in production with only one of the resistors about 1% (or whatever the math says) over the target value. Then for calibration, derive a chart for resistor values versus calibration value, and hand load the second resistor. This actually is not too bad of a method... doesn't take that long.

OH!

18) Don't put trimots on a board because some nitwit will have to screw with it.

I actually had moron call me once wanting to return a circuit board that he had burned up. His complaint was that there was a stripped screw "that wouldn't draw up tight"... the small screw on a muti-turn pot.

19) Always reverse the screw slightly after adjusting a muti-turn pot... this relieves residual tension in the mechanism and prevents the setting from changing after the first jolt or temperature excursion.

20) Plan for an open wiper (back to the rheostat)... configure your circuit such that the ejection seat won't fire in the event of an open.
  Reply With Quote
Old 9th July 2006, 08:37 PM   #130
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Good point poobah!

Why is it that every budding wannabe technician has to mess with the trim pots first???

When asked, it never happened.

-Chris
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Quality Control differences = variations in sound quality? KT Class D 3 4th June 2014 12:02 AM
Signal path Resistor Quality rtate Solid State 121 28th November 2008 07:48 PM
Resistor Sound - How is this possible??? fmak Parts 81 24th August 2003 12:38 PM


New To Site? Need Help?

All times are GMT. The time now is 10:13 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2