OK, I think very similar questions have been asked but I couldn't find one stated in this way. I'd like to keep it very simple/straightforward, so here goes.
I have a tube amplifier expecting to see an 8 ohm load.
I have an 8 ohm speaker.
If I put a 4 ohm power resistor in series with the speaker and then an 8 ohm power resistor in parallel with the speaker what will happen?
It seems like it should still be an 8 ohm load but the wattage through the speaker will go down to about 1/4 but will this have adverse effects on the amplifier? Will the sound quality be absolutely miserable? No, I don't want to just turn the amplifier down. I know a resistor isn't the same as a speaker, but I don't see how this is drastically different than replacing the power resistors with the same impedance speakers.
Thanks for your help.
I have a tube amplifier expecting to see an 8 ohm load.
I have an 8 ohm speaker.
If I put a 4 ohm power resistor in series with the speaker and then an 8 ohm power resistor in parallel with the speaker what will happen?
It seems like it should still be an 8 ohm load but the wattage through the speaker will go down to about 1/4 but will this have adverse effects on the amplifier? Will the sound quality be absolutely miserable? No, I don't want to just turn the amplifier down. I know a resistor isn't the same as a speaker, but I don't see how this is drastically different than replacing the power resistors with the same impedance speakers.
Thanks for your help.
No harm will happen to the amp or speakers. Mostly this will reduce the amplifier damping factor to about 3.
Most speakers will not sound as they should with this level of source impedance (2.7 Ohms), especially in the bass.
Why do you want to do this?
Cool, thanks. So I'm trying to keep close to the exact "tone" out of the amplifier but reduce the SPL. Is there a better (standard) practice of doing this?
Sure, use an attenuator at the input instead. Two resistors, one series, and then one shunt to ground.
What is the source output impedance, and the amp input impedance?
If you don't know, then a 10k series and 10k shunt would reduce by at least 6dB.
If you want a lower output than this, make the shunt resistor smaller, maybe 5k to 2k.
If you are overdriving the amp and want that sound, then you have to go with your original way, though.
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So I know that makes perfect sense to get the SPL down but I don't want to adjust any signal level(s) going into or through the amplifier because that will ultimately adjust the signal coming out of the amplifier. Any other options?
Speakers are complex loads that vary with frequency. Connecting them through an L-pad, the amp will see a less varying load. This is often done with tweeters.
Adjusting the input level is no different than what a volume control does. If you have an output stage that can do more power than you need and sounds good then you have a good thing.
Some other options... you can remove one of your gain elements, use a simpler driver stage, use more feedback, remove a stage and use less feedback, or just use an attenuator.
It is possible to do it the way you suggested without changing the response but these other options won't change the response and they use less power.
Some other options... you can remove one of your gain elements, use a simpler driver stage, use more feedback, remove a stage and use less feedback, or just use an attenuator.
It is possible to do it the way you suggested without changing the response but these other options won't change the response and they use less power.
What kind of amplifier and speaker are these (make and models, please)? What are they used for? If it's for electric guitar or bass (as your mention of tone suggests), that's fine, though the thread'll get moved to Instruments and Amps.
But yes, doing what you described is perfectly fine. Just be sure your connections are good, as running a tube amp without a load connected can cause too-high voltages in the output tubes and transformers which may cause damage.
But yes, doing what you described is perfectly fine. Just be sure your connections are good, as running a tube amp without a load connected can cause too-high voltages in the output tubes and transformers which may cause damage.
As the OP said this is a tube amplifier, it likely doesn't have a high damping factor to begin with, so this could result in little or no change to the damping factor.
The L-pad will reduce damping factors higher than 2, and will increase those that are less than 2.
The only reason I can think of to do something like this would be for, say a tweeter in an active setup that has noise issues.
You didn't say how powerful the amplifier in question is, but these resistors can get pretty big in a hurry if you wanted to do this to a full range speaker.
Lastly (and I might be completely wrong here, someone chime in) with limited lifetime / expensive valves, wouldn't you want all your power delivered into the speaker rather than being burned off by resistors?
You didn't say how powerful the amplifier in question is, but these resistors can get pretty big in a hurry if you wanted to do this to a full range speaker.
Lastly (and I might be completely wrong here, someone chime in) with limited lifetime / expensive valves, wouldn't you want all your power delivered into the speaker rather than being burned off by resistors?
OK, so like benb had alluded to the actual application is for guitar tube amplifier and speaker...I thought since this was a "theoretical" question this section would be appropriate.
So yeah, I guess I was trying to make my own power attenuator. I actually didn't know what an L-pad was but apparently that's exactly what I was thinking of in my head. So yeah, DrDyna, in case you don't play guitar the sound that you can get by pushing your guitar amplifier to "10" is ideal...sometimes the volume that produces is not. So basically I wanted to make a chassis with some switches and resistors for my amplifier that would let me play on 10 (11...) but keep the volume much lower when I wasn't playing in a band setting.
benb, this is for amps I've made, so something similar in size/output to a Fender Bassman with a 15" speaker. I was planning on using all 50 watt power resistors with a heat sink. Just wanted to get some opinions going before placing mouser order so thanks everyone for help! If anyone else has other opinions happy to hear.
So yeah, I guess I was trying to make my own power attenuator. I actually didn't know what an L-pad was but apparently that's exactly what I was thinking of in my head. So yeah, DrDyna, in case you don't play guitar the sound that you can get by pushing your guitar amplifier to "10" is ideal...sometimes the volume that produces is not. So basically I wanted to make a chassis with some switches and resistors for my amplifier that would let me play on 10 (11...) but keep the volume much lower when I wasn't playing in a band setting.
benb, this is for amps I've made, so something similar in size/output to a Fender Bassman with a 15" speaker. I was planning on using all 50 watt power resistors with a heat sink. Just wanted to get some opinions going before placing mouser order so thanks everyone for help! If anyone else has other opinions happy to hear.
What is producing the distortion you like at 10? You might be able to reproduce it at another level by re-biasing a stage.
Aha, ok. Is overdriving the amplifier the only way to do that, or could you do it with a crunch pedal or something?
I think even in a modest size guitar amplifier, you're going to be into some serious resistor sizes, because not only does this depend on the volume being pegged, but your average power will be pretty high when you're in clipping, so you'd probably need something like 50 watt resistors to be safe with something like a 30 watt amplifier. The other thing is.. the resulting distortion might actually sound different with resistors drawing the current instead of a driver, because a driver's impedance varies, so there could end up being notes where a real driver will pronounce the crunch harder or softer than resistors, if that makes sense.
Maybe that's wrong, and I'm sure someone will say something if it is, but that's my partially educated musing, lol.
I wonder if there's a really simple solution using the amplifier's schematic, to just get it to start being non-linear sooner. Out of my depth at that point, though.
The distortion comes in multiple preamp stages as well as the power section...there is just something about the sound of pushing a guitar amp to (and probably past) its normal operating limits that sounds fantastic but yet hard to replicate without actually doing it...it's an age old question/pursuit, really. Not to mention, I would need to be able to instantly change back to full volume settings at practice/shows so rebiasing or altering gain stages would be a little tricky.
Some companies use master volumes or power scaling but in my opinion any signal reduction before the power tubes just isn't the same.
Some companies use master volumes or power scaling but in my opinion any signal reduction before the power tubes just isn't the same.
Oh, and yeah DrDyna...that kinda was my thought/worry to begin with. I imagine there's gotta be some tradeoff's for functionalityand it won't sound EXACTLY the same I just didn't know if the idea itself was completely ridiculous to begin with...and maybe it still is. With tube amplifiers you definitely have to keep the impedances as close to matching as possible or it will have very adverse effects.
So yeah, that said...that's why I was thinking if I need an 8 ohm load, a 4 ohm power resistor in series and then an 8 ohm power resistor across the speaker (shunting) that keeps the load at 8 but yet the voltage across the speaker will be dropped significantly...but since speakers have different resistances at different frequencies I didn't know if the resistors would drastically alter the amplifier's output or worse, possibly be harmful to it. But since it sounds like worst case it just won't sound exactly the same and/or make A LOT of heat I might as will give it a shot...
So yeah, that said...that's why I was thinking if I need an 8 ohm load, a 4 ohm power resistor in series and then an 8 ohm power resistor across the speaker (shunting) that keeps the load at 8 but yet the voltage across the speaker will be dropped significantly...but since speakers have different resistances at different frequencies I didn't know if the resistors would drastically alter the amplifier's output or worse, possibly be harmful to it. But since it sounds like worst case it just won't sound exactly the same and/or make A LOT of heat I might as will give it a shot...
Yeah, might be worth a shot to try it. It's reasonably safe at least, just watch the heat if your resistor values are close to maximum amplifier power.
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