Resistor Circuit for Speakers
Kirk Hobart
>i like to record bass guitar thru the amps line out w/ no speakers. i've
>heard of a resistive circuit to take the load normally delivered to the
>speakers. is this simply a resistor = the speaker resistance?
>mike senkpiel - sy...@nr.infi.net
You are correct. If your speaker is 8-ohms, then buy an 8-ohm power
resistor. Be sure the resistor's power rating is at least as high as
your amplifier. When you are jammin', the resistor may get too hot to
touch, but that is normal.
--
Kirk Hobart Santa Barbara, California hob...@rain.org
Andreas Beermann
>Andreas Beermann (an...@moire2.u-strasbg.fr) wrote:
>: hob...@rain.org (Kirk Hobart) wrote:
>: >mike senkpiel <sy...@nr.infi.net> wrote:
>: >
>: >>i like to record bass guitar thru the amps line out w/ no speakers. i've
>: >>heard of a resistive circuit to take the load normally delivered to the
>
>Andreas,
>The original poster said he wanted to be able to use the line output from
>his amp with the speaker disconnected. Knowing that, your response will
>not satisfy his requirements.
You are right - but perhaps he could put the Loudspeaker in another room
in order not to hear anything from it... OK, just a joke!
A loudspeaker has got a very complex input-impedance due to the mechanical
effects. These mecanical effects can (an do!) feedback to the electrical
side of the loudspeaker (Remember: you can use a loudspeaker as a microphone).
This produces a lot of distortion, and other "interesting" things that are hated
by high-end-fans but that are wanted by musicians.
An Amp only with an resistive load wild sound very "clean". You get a MUCH better
sound when leaving the Loudspeaker in the circuit (which will be - by the way - no
danger for the amp because it is designed to drive the speaker)
>Consider this: A source pumping 100 Watts RMS into an 8 Ohm load will
>only be placing 28.3 Volts RMS across the load.
What I meant was not the shock hazard but the danger of shortcuts which would kill
the amp.
>Finally, I believe the circuit you show as being useful for a line output
>is not. This circuit's output voltage will vary as you crank the amp and
>its output impedance and voltage will change as you adjust its pot.
>I believe the usual line output circuitry presents a constant impedance
>to a load at a voltage level referenced to a particular output level from
>the amp.
You're right! Same problem as you would record the bass with a microphone beneath the
loudspeaker what is the most common way to do it (you get the best sound this way).
As far as I know the cheap "loudspeaker simulators" just have a 8 or 4 Ohm resistor
inside. The more expensive ones have some additional coils and capacitors to improove
sound. One should open such a thing and see whats inside ;-)
Andreas
Michael R. Przytarski
>mike senkpiel <sy...@nr.infi.net> wrote:
>>i like to record bass guitar thru the amps line out w/ no speakers. i've
>>heard of a resistive circuit to take the load normally delivered to the
>>speakers. is this simply a resistor = the speaker resistance?
>>mike senkpiel - sy...@nr.infi.net
>You are correct. If your speaker is 8-ohms, then buy an 8-ohm power
>resistor. Be sure the resistor's power rating is at least as high as
>your amplifier. When you are jammin', the resistor may get too hot to
>touch, but that is normal.
Problem is the 8 ohms is inductive resistance while a resistor is not.
Any 1st year ele. student can tell you that a inductive resistance of
8 ohms is really around 5.7 ohms. Not to mention the effect it would
have on reflective loading.
Not to say a 8 ohm resistive load is bad, just don't turn it up past
say 50% or so. if you do you may have to post a message to us asking
how to fix your amp.
M. Przytarski
Lewis Sound
Michael R. Przytarski
>This is not the whole truth! A Loudspeaker is NOT a simple 8Ohm Resistor. Its schematic
>contains a lot of coils and capacitors.
Thats called the crossover, and most drivers bypass the crossover.
The crossover is only used for setting the frequency ranges of the
speakers, example is the tweeter or midrange. A driver would use one
if it was being used as a sub.
M. Przytarski
Lewis Sound
Mark Garvin
>mike senkpiel <sy...@nr.infi.net> wrote:
>>i like to record bass guitar thru the amps line out w/ no speakers. i've
>>heard of a resistive circuit to take the load normally delivered to the
Many manufacturers make speaker emulators now. Try Groove Tubes,
Marshall, Harry Kolbe (NYC) and THD for a start. The problem you
might have is that speakers for musical instruments should be
considered from two viewpoints:
Load: You could get by with an 8 ohm resistor but the response
will be different. Amp designers and techs use these on the bench
all the time. Usually less prone to cause oscillation in an amp
than is a reactive speaker load. Careful that you always have the
speaker load attached. No speaker load sometimes == burnt transformer.
Flyback spikes cause arc'ing in tubes and transformer.
Freq Response: Most MI speakers have peaks around 2khz and then
dive at around 4k to 8khz. Refer to response charts for your
favorite speakers. The speaker emu's above will do this
for you as well. Otherwise, expect a relatively flat 'hifi'
sound. Some bass players like it. Few guitarists do.
Also see Craig Anderton's 'Stack in a box' in Guitar Player
...hmmm...Feb 93 or thereabouts. Also available in kit form
from PAIA. It has a 4-stage active low-pass filter for speaker
emulation. Probably just a Buttorworth, for the technical here.
All addresses, phone numbers for above could be found in Guitar
Player mag or their 95/96 equipment review magazine.
Regards,
Mark Garvin
Composer/Design Engineer
New York, NY