Resistor wattage for stepped attenuator - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, 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 23rd February 2011, 10:04 AM   #1
Dagwood is offline Dagwood  New Zealand
diyAudio Member
 
Dagwood's Avatar
 
Join Date: Feb 2010
Location: North of Auckland
Default Resistor wattage for stepped attenuator

Hi all

I'm working on a simple passive pre-amp with just a 25K stepped attenuator and a 4 pole, 5 way rotary switch.

Questions???

2 mains sources will be Arcam CD player and an Ipod, is 25K ok feeding into Pete Millets DCPP amp?

Also,I am having trouble matching the values required using Dale vishay RD60N 1/2 watt resistors, will there smaller brother at 1/8 watt be ok for this setup

Part attached (RN55D5101FRE6 Vishay/Dale Metal Film Resistors - Through Hole)

Cheers
__________________
If you keep digging in the garden of failure you will eventually find success
  Reply With Quote
Old 23rd February 2011, 10:46 AM   #2
Did it Himself
diyAudio Member
 
richie00boy's Avatar
 
Join Date: Nov 2003
Location: Gloucestershire, England, UK
1/8 watt is many times more powerful than needed, so ok.
__________________
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more.
  Reply With Quote
Old 23rd February 2011, 12:21 PM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
I hope you were asking in jest.

P = V*I = V^2 / R = I^2 * R
For an input attenuator you already know the maximum input voltage and the resistance of the attenuator.
Choose the P = V^2 / R version and insert your numbers.
example.
For a CD player with a maximum output of 2.2Vac feeding a 50k attenuator the maximum power delivered to the attenuator is 2.2^2 / 50000 = 0.000097W ~ 100uW
__________________
regards Andrew T.

Last edited by AndrewT; 23rd February 2011 at 12:24 PM.
  Reply With Quote
Old 23rd February 2011, 06:14 PM   #4
diyAudio Member
 
Join Date: May 2010
Quote:
Originally Posted by AndrewT View Post
I hope you were asking in jest.

P = V*I = V^2 / R = I^2 * R
For an input attenuator you already know the maximum input voltage and the resistance of the attenuator.
Choose the P = V^2 / R version and insert your numbers.
example.
For a CD player with a maximum output of 2.2Vac feeding a 50k attenuator the maximum power delivered to the attenuator is 2.2^2 / 50000 = 0.000097W ~ 100uW
Would that not be the *minimum* (typical) power in W, ie at 50 Kohm resistance? What about at near 0K resistance (full clockwise, with the lowest value resistor, or nearest "10")?

In any case, 1/8 w should be fine over most, if not all, values; there will be an input impedance at your amplifier to load down the source. Use the value for your amp's input impedance in the above formula to determine if your lowest value resistor is robust enough.
__________________
" ... Go back to the beginning of a technology before the priesthood was established; that was the time when people were communicating information, not proving why there needs to be Priests. This is why the old texts tend to be so good. ..."

Last edited by Johnny2Bad; 23rd February 2011 at 06:26 PM.
  Reply With Quote
Old 23rd February 2011, 06:54 PM   #5
Dagwood is offline Dagwood  New Zealand
diyAudio Member
 
Dagwood's Avatar
 
Join Date: Feb 2010
Location: North of Auckland
Thanks guys, I'm not an EE so appreciate any help with formulas, will know for future reference now.
So, lowest point on the step will be a 7ohm resistor, 2.2^2 / 7 = 0.6914285W of dissipation?

Cheers

__________________
If you keep digging in the garden of failure you will eventually find success
  Reply With Quote
Old 23rd February 2011, 08:19 PM   #6
Dagwood is offline Dagwood  New Zealand
diyAudio Member
 
Dagwood's Avatar
 
Join Date: Feb 2010
Location: North of Auckland
Thanks Johnny2Bad

I wanted to complete the passive pre-amp before I completed the power amp for testing. But I guess it's chicken and egg really, I could always just rig the power amp up on a standard dual pot to check the levels?
I'm not really sure how to measure the amp's input impedance sorry?? can it be worked out from the schematic?? DCPP Amp

Cheers for your help
__________________
If you keep digging in the garden of failure you will eventually find success
  Reply With Quote
Old 23rd February 2011, 08:53 PM   #7
Did it Himself
diyAudio Member
 
richie00boy's Avatar
 
Join Date: Nov 2003
Location: Gloucestershire, England, UK
No the lowest point still has all the preceeding resistors before it, so the calaculation Andrew did stands and is the dissipation of the entire bunch of resistors, so as you can see 1/8 watt really is many times more than is needed as 5 ways will give over half a watt of capacity.
__________________
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more.
  Reply With Quote
Old 23rd February 2011, 09:51 PM   #8
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
Fuel to the fire, and meat to the dogs...

I'll disagree.

The 1/2 watt resistors will simply sound better in most instances.
Why?
All sorts of reasons, and also the construction of the resistors counts as well.
There was a discussion of resistors in the John Curl Blowtorch part II thread - might be worthy of a review...

Since people generally report a difference in sound between vintage Holco metal films and other metal films, and again a difference with things like bulk film (Vishay for example), I'd not consider "resistors to be resistors".

Feel free to chomp and thrash...




_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 24th February 2011, 04:43 AM   #9
Dagwood is offline Dagwood  New Zealand
diyAudio Member
 
Dagwood's Avatar
 
Join Date: Feb 2010
Location: North of Auckland
Thanks Bear

I had a scan through JohnCurlBlowtorchpartIIthread, man my head is sore!

Ok I get Simons results (sort of) and have read through a few of the data sheets

I still think I will use the Vishey Dale RN55D's as I have to consider product cost,
availability at required values, cost of shipping to NZ, size and overall performance (datasheet attached)

http://www.vishay.com/docs/31027/cmfmil.pdf

Cheers
Ben
__________________
If you keep digging in the garden of failure you will eventually find success
  Reply With Quote
Old 24th February 2011, 05:12 AM   #10
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
Ya ok, everyone makes choices in this thing... as I said I prefer the 0.5W part... I think it was also said that paralleled resistors had benefits as well...

But it may be that for most folks this is minutia that matters not.

I've had good results with lowly asian made (good quality) 0.5w MFs... I tend to attribute that to the fact that they are 0.5w devices, but I also use a super high quality switch and tend to build "discrete L" attenuators, not the usual "pot" type units. The discrete L puts only two resistors at a time in the signal path, not a ladder. That may make a difference too.

Btw some people say it was the copper end caps and the way they were made that made the original vintage Holco resistors sound the way they did... dunno.

_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  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
Stepped attenuator resistor calculation software pmillett Tubes / Valves 5 13th October 2013 04:27 PM
Burr-Brown PGA2311, stepped resistor attenuator Miller-8 Solid State 5 30th October 2007 07:44 PM
What wattage for resistor in attenuator DC Dave Parts 1 5th February 2004 05:43 AM
Resistor values of stepped attenuator for BoSoZ stappvargen Pass Labs 1 21st February 2003 01:29 PM
DIY stepped attenuator (how to compute resistor values?) jarthel Solid State 8 16th June 2002 10:54 AM


New To Site? Need Help?

All times are GMT. The time now is 01:47 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