Resistor watt ratings and substitutions

HiFiHarv · 30th August 2012, 09:19 PM

I know it is OK to replace a resistor with one of a higher watt rating, but can there ever be a sonic penalty for doing this in a HiFi amp? I am talking here about a resistor used to replace a removed pot on the input of a tube amp. I swear that it sounds worse now. Any chance this is due to the 1/2 watt resistor instead of a 1/4 watt pot? I know it shouldn't matter, just wondering if anyone else has experienced a phenomena like this when going higher than spec'd with the watt rating of a resistor. I know, I know, listening is so subjective. It's probably my imagination, or the sound of the resistor itself. Even after break in time though, it just doesn't sound right.

Damon Hill · 31st August 2012, 03:12 AM

Absolutely none.

It all depends on exactly how you "replaced" the pot, which is a three terminal device, with a two terminal device. You've changed the circuit, perhaps not for the better.

benb · 31st August 2012, 05:35 AM

What are the resistance values of the resistor and the pot it replaces? I wonder about the measured values as well as the nominal or "stated" values. Fixed resistors are usually worst-case 5 percent tolerance nowadays, but pots can be a lot more off. What signal source is driving the amp? A different value resistor could change the frequency response of the source, and if there's a true difference, I'm thinking it's because there's been a substantial change in the actual resistance value. The power rating (by itself) has no effect.

HiFiHarv · 31st August 2012, 02:24 PM

Thanks guys, for your comments so far. I've been thinking about this since and am wondering if ,yes, it may be a case of changing the value of the input impedance. OK, here are the details of the situation. Many years ago I removed the input pots and capacitors on a pair of Fisher 30A mono amps. Have been using them that way for more than a decade. I recently was informed that the circuit needs the pot or a comparable resistor to give the input section a reference to ground. You all probably know this, but I am just a tinkerer trying to be a better DIY'er. I guess I have just been lucky since the amps were getting their reference to ground from whatever preamp I was using at the time. I always made sure that preamps had DC blocking caps at their output, but never knew to think about impedance. So, I recently soldered the resistors between the pos and neg of the input jacks. The pot removed was 500K and the resistor is 470K. As I understand it, too low of an input impedance of the amp can diminish low frequency response. So, maybe I just prefer the lighter, tighter, and subjectively brighter sound of the amps while they think the input impedance is the lower value of the preamp's output? Or doesn't it work that way? I never noticed any low frequency roll off. Actually, I felt the bass was great for low powered tube amps. Are we talking really low hertz here? like below the output of musical instruments? If so, maybe this roll off is a good thing, eliminating the amps need to reproduce those power robbing lows. I have checked the input impedance of other tube amps and They go all over the map with 470K or 500K pretty common. Could I check the output impedance of whatever source sounds best with these amps and set the amp's inputs to that resistive value instaed of 470K? A crap shoot, I know, but I'm thinking these things need a lower input impedance. Who am I to argue with the Fisher engineers though? What do you think?

30th August 2012, 09:19 PM	#1
HiFiHarv diyAudio Member Join Date: Aug 2009 Location: Sarasota, Florida	Resistor watt ratings and substitutions I know it is OK to replace a resistor with one of a higher watt rating, but can there ever be a sonic penalty for doing this in a HiFi amp? I am talking here about a resistor used to replace a removed pot on the input of a tube amp. I swear that it sounds worse now. Any chance this is due to the 1/2 watt resistor instead of a 1/4 watt pot? I know it shouldn't matter, just wondering if anyone else has experienced a phenomena like this when going higher than spec'd with the watt rating of a resistor. I know, I know, listening is so subjective. It's probably my imagination, or the sound of the resistor itself. Even after break in time though, it just doesn't sound right.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Tantalum Resistor Watt?	batsong	Tubes / Valves	1	3rd May 2010 06:12 PM
What resistor power ratings should be used in guitar effects pedals?	Test Electrix	Instruments and Amps	2	4th February 2010 02:22 AM
Normal temp of a 2 watt resistor?	turbo_slug	Parts	18	30th May 2008 11:25 PM
Newbie question regarding fuse's and resistor ratings	neil_kaye	Parts	7	2nd July 2006 07:29 PM
Resistor wattage ratings	whitey_woltrap	Multi-Way	0	22nd March 2002 07:20 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	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

31st August 2012, 03:12 AM	#2
Damon Hill diyAudio Member Join Date: Mar 2001 Location: Auburn, WA (somewhere between Seattle and Tacoma) Blog Entries: 1	Absolutely none. It all depends on exactly how you "replaced" the pot, which is a three terminal device, with a two terminal device. You've changed the circuit, perhaps not for the better.

31st August 2012, 05:35 AM	#3
benb diyAudio Member Join Date: Apr 2010	What are the resistance values of the resistor and the pot it replaces? I wonder about the measured values as well as the nominal or "stated" values. Fixed resistors are usually worst-case 5 percent tolerance nowadays, but pots can be a lot more off. What signal source is driving the amp? A different value resistor could change the frequency response of the source, and if there's a true difference, I'm thinking it's because there's been a substantial change in the actual resistance value. The power rating (by itself) has no effect.

31st August 2012, 02:24 PM	#4
HiFiHarv diyAudio Member Join Date: Aug 2009 Location: Sarasota, Florida	Thanks guys, for your comments so far. I've been thinking about this since and am wondering if ,yes, it may be a case of changing the value of the input impedance. OK, here are the details of the situation. Many years ago I removed the input pots and capacitors on a pair of Fisher 30A mono amps. Have been using them that way for more than a decade. I recently was informed that the circuit needs the pot or a comparable resistor to give the input section a reference to ground. You all probably know this, but I am just a tinkerer trying to be a better DIY'er. I guess I have just been lucky since the amps were getting their reference to ground from whatever preamp I was using at the time. I always made sure that preamps had DC blocking caps at their output, but never knew to think about impedance. So, I recently soldered the resistors between the pos and neg of the input jacks. The pot removed was 500K and the resistor is 470K. As I understand it, too low of an input impedance of the amp can diminish low frequency response. So, maybe I just prefer the lighter, tighter, and subjectively brighter sound of the amps while they think the input impedance is the lower value of the preamp's output? Or doesn't it work that way? I never noticed any low frequency roll off. Actually, I felt the bass was great for low powered tube amps. Are we talking really low hertz here? like below the output of musical instruments? If so, maybe this roll off is a good thing, eliminating the amps need to reproduce those power robbing lows. I have checked the input impedance of other tube amps and They go all over the map with 470K or 500K pretty common. Could I check the output impedance of whatever source sounds best with these amps and set the amp's inputs to that resistive value instaed of 470K? A crap shoot, I know, but I'm thinking these things need a lower input impedance. Who am I to argue with the Fisher engineers though? What do you think?