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Resistor watt ratings and substitutions
Resistor watt ratings and substitutions
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Old 30th August 2012, 10:19 PM   #1
HiFiHarv is offline HiFiHarv  United States
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Default 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.
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Old 31st August 2012, 04:12 AM   #2
Damon Hill is offline Damon Hill  United States
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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.
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Old 31st August 2012, 06:35 AM   #3
benb is offline benb  United States
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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.
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Old 31st August 2012, 03:24 PM   #4
HiFiHarv is offline HiFiHarv  United States
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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?
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