A mad man's approach to attenuation

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Guitarski

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Joined 2013
#1
Hi,

Many things have been said about dummy loads, but I couldn't find anything on the issue of using car audio speakers as a dummy load. I'm posting it here, because it has to work for a bass guitar tube amp.

Mine is a 300W amp that will do 4 and 2 Ohms. Resistors that will handle 300W didn't seem cheap (to me) and someone suggested using cheapo car audio speakers as dummies. I'm looking at a 30 Euro 300W (max) set, rated at 4 Ohm (the pair, or per speaker..). I suppose it's cost effective and removes any issues with differences between resistance and impedance.

The purpose of the amp is recording, using a direct box's (Avalon u5) speaker input, so I'd still be left with 300W of noise I don't need. Could I perhaps do some fatal damage to the cones of the speakers, to shut them up a bit, or is that going to affect the electrical part as well (I suppose not, but I'm a musician, not an engineer)? It's a destructive approach and I don't like it, but a dummy load box that can handle 300W gets very expensive, the ones I've seen.

So the question is, is this a mad man's approach to attenuation (don't answer that) and is it going to work?

thanks so much/regards
Guitarski
 
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#5
There are no car speakers for 30 euro, that can stand 300 watts for more than a couple of seconds. What happens then is that the voicecoil burns, and will short the amp. And they will make lots of awful noise when dying, both the speakers and the amp, with a lot of smoke 🙂
 
#11
jan.didden said:
Do you really have to go through a big power amp to get your guitar signal into a recorder??

Jan
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Actually 300W isn't much for bass. I would have settled for 1W, but my amp was the least expensive amp with a truly driven tube sound.





jan.didden said:
So what's wrong with a tube preamp? I'm not trying to derail your thread, just trying to understand this - I'm not a guitarist/tube guy.

Jna
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Nothing wrong with a preamp. My amp's DI (pre-power) sounds great. The 300W simply came with the package. Whether or not it's got anything to do with the power stage, all tube amps (expect a certain 15W Model ) sound better (to my ears) than any solid state or hybrid bass amp, even at the DI level.

So for car audio, would I need 300 RSM, or what's it called? I'd hate to muck around with resistors, not knowing how to match resistance with impedance.

Thanks/regards
Guitarski
 
#13
seppoa said:
Btw, better to use a small Fender Champ, or maybe a Deluxe then, if you want to record with tube sound, instead of a killer 300 watt tubeamp, that cannot be overdriven at low volumes...
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Sorry, no Fender amps for me 🙂. Matter of taste, I suppose. Thanks for the suggestion. ;-)
300W Is not killer at the bass level. 500 is just about avarage. Bassists need 10x the power that guitarists do.
Nowadays you have 500W Solid state amps that fit into a shoe box. There's an Orange amp, of lunch box size that does 1000W.
Because I'm a recordist and not touring, I don't mind the bulk of an all tubes amp, for relatively little power.

Thanks/regards
Guitarski
 
#15
I've build a 300W resistive dummy load for testing and what not, and added an attenuated buffered output to connect the thing to e.g. a sound card for spectrum analysis etc. Cost me about 60 euros in mouser parts. Some 150W resistors, big *** heatsink and a nice Hammond box. Buffer opamp and surrounding components are junk bin parts.

A resistive load is different from the impedance from a speaker in the sense that the speaker impedance varies with frequency and a resistor doesn't. But how the impedance varies depends on the speaker. So yes, a resistor will 'sound' different, but so will different speaker chassis. And as mentioned, the 30 euro car speaker will burst into flames with 300W. That's usually some momentary peak rating, not a real RMS value.

Do you live in the Groningen area? You could try my load and see (hear) how it functions.
 
#18
merlinx76 said:
I just bought a 300W 4 ohm dummy load resistor for $28. I didn't think that was all that much money. Here is a link to the one I got:

C300K4R0E Ohmite | Mouser
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That's great. Greetings to Canada. Did you connect it to your amp just like that? No heat sink, or anything?
How do you mount such a thing? Does it dangle from your speaker cable? 😱

Thanks a lot, Funk, but I'm near Amsterdam.

A resistive load is different from the impedance from a speaker in the sense that the speaker impedance varies with frequency and a resistor doesn't. But how the impedance varies depends on the speaker. So yes, a resistor will 'sound' different
Click to expand...

So with a dummy impedance is not an issue because no sound is produced? And so for dummy purposes resistance = impedance?
 
#19
I only ran it up to 250 watts or so for a minute, so I just had it laying on the table. But it gets hot fast even at less than 100 watts. I was running a steady sine wave through it though so it was seeing that wattage continuously, not a varying signal like it would if it was playing music. I don't think there is any way to hook up a heat sink. You should definitely stand it in the open air away from any surfaces if you are pushing any significant power through it.
 
#20
It's tricky. This resistor (made for short intervals, like motor starting, breaking) can withstand 400 C. Its temperature coefficient is 400 ppm/C, whatever that means, so the ohmage may be off at high temps. All in all not something I can try and see if my amp doesn't explode.

Thanks/regards
Guitarski
 
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