Fooling an amp speaker output - speaker ohm values (mod)

[Tone2TheBone]

One time I made an 8 ohm resistor setup and paralleled it to one side of a speaker so that I could fool an amp into thinking it was seeing an 8 ohm speaker (it was a 16 ohm speaker). I don't remember how I did it but I do remember the resistor being big, long and dark gray. An electronic tech here at work hooked me up with the set up. Do any of you "techs" see anything wrong with this idea? Do you have any improvements to this?

[Spudman]

I've been reading something about that recently. The Classic 30 likes a 16 ohm load and somehow the other speaker output jack is an 8 ohm and the the amp is set up to default to certain values depending upon which jack is being used and if both jacks are being used. I'm not sure I understand exactly how it's done but there is a way to set up a dummy load on one jack then use the output from the other jack (8 ohms) to match your 8 ohm cabinet. Your amp might work the same way?

I'll bet someone here knows how to do it right.

[Tone2TheBone]

Yeah Spud that sounds similar to what I'm talking about. Specifically I'm trying to fool my 8 ohm output amp into seeing 8 ohms from a 16 ohm speaker and resistor hooked up across the terminals. In essence it should be near 8 ohms if i have the resistor values right across the terminals. Like I said this resistor I have is huge in appearance but I don't know enough about electronic parts to be exactly sure what it is. I'm sure it would handle the power ok...or at least I think it would that's why I created this post.

[tunghaichuan]

Yeah Spud that sounds similar to what I'm talking about. Specifically I'm trying to fool my 8 ohm output amp into seeing 8 ohms from a 16 ohm speaker and resistor hooked up across the terminals. In essence it should be near 8 ohms if i have the resistor values right across the terminals. Like I said this resistor I have is huge in appearance but I don't know enough about electronic parts to be exactly sure what it is. I'm sure it would handle the power ok...or at least I think it would that's why I created this post.

Tone,

I dont see a problem with what your doing. To use your 8 ohm output using a 16 ohm speaker, you would want to put a 16 ohm power resistor in parallel with your 16 ohm speaker to give you a nominal 8 ohm impedence. Generally you want 2X-3X the wattage rating for the power resistor. So if your amp is 30 watts, you'd want a 60 watt rated resistor. This doesn't take into account using a speaker in parallel with the resistor though. That big resistor you have is probably 100w or so.

Edit: forgot to add that both the speaker and the resistor will be dissipating power, so you will get less SPL out of the speaker. Of course, this maybe what you're going for.

tung

[Tone2TheBone]

Tone,

I dont see a problem with what your doing. To use your 8 ohm output using a 16 ohm speaker, you would want to put a 16 ohm power resistor in parallel with your 16 ohm speaker to give you a nominal 8 ohm impedence. Generally you want 2X-3X the wattage rating for the power resistor. So if your amp is 30 watts, you'd want a 60 watt rated resistor. This doesn't take into account using a speaker in parallel with the resistor though. That big resistor you have is probably 100w or so.

Edit: forgot to add that both the speaker and the resistor will be dissipating power, so you will get less SPL out of the speaker. Of course, this maybe what you're going for.

tung

What's SPL? Thanks a bunch tung! Yeah I'm pretty dang sure this resistor is high in wattage now that I've read what you're saying. I remember someone telling me long time ago that it had to be that way. Thanks again for commenting!

[duhvoodooman]

Yep, as Tung says, no reason why this simple approach wouldn't work, so long as you get the right wattage resistor. But keep in mind you'll be dissipating half of your amp's power output as heat from the resistor! But that's only about 3dB volume loss, which is no big deal....

[tunghaichuan]

What's SPL? Thanks a bunch tung! Yeah I'm pretty dang sure this resistor is high in wattage now that I've read what you're saying. I remember someone telling me long time ago that it had to be that way. Thanks again for commenting!

SPL = sound pressure level

You're more than welcome.

Take a look at this device:

http://www.blueguitar.org/new/schem/misc_mod/volreduc.gif

I don't know if that will help you, but I thought I'd post it in case you were interested.

tung

[Tone2TheBone]

Yep, as Tung says, no reason why this simple approach wouldn't work, so long as you get the right wattage resistor. But keep in mind you'll be dissipating half of your amp's power output as heat from the resistor! But that's only about 3dB volume loss, which is no big deal....

Thank you Mr. Tinkerer of All things Electronic. :beer:

[Tone2TheBone]

SPL = sound pressure level

You're more than welcome.

Take a look at this device:

http://www.blueguitar.org/new/schem/misc_mod/volreduc.gif

I don't know if that will help you, but I thought I'd post it in case you were interested.

tung

Hey that's neat what does that device do exactly?

[tunghaichuan]

Hey that's neat what does that device do exactly?

It's supposed to knock down your volume a bit so you can crank the amp up and get more natural distortion.

tung

[M29]

Does a speaker always keep the designated ohm rating? I thought they changed during use. A resistor would be consistent, which would be ok I suppose. I may be way off here but I thought I heard somewhere that the ohm rating fluctuated during use.

M

[Simon]

M, it should not change significantly over time, if that's what you mean... But the impedance ("ohm rating") is not a fixed number either, see the explanation in the first few paragraphs and image here:
http://www.churchsoundcheck.com/imp1.html

[M29]

Hello Simon,

My mistake I mixed it up with impedance.

Thank you for the link:AOK:

M