Subbing a resistor for a speaker

[Randall]

Customer wants to disconnect the speaker in his Hammond M3 organ for use in his studio so there is no mic bleed. What is the best way to do this? It's a pair of 6V6s, so figure maybe 20 watts max? Can I just sub in a 30 - 50 watt 8 ohm resistor, or does changing from an impedence load to a resistive load play a part in this?

[Leo_Gnardo]

Customer wants to disconnect the speaker in his Hammond M3 organ for use in his studio so there is no mic bleed. What is the best way to do this? It's a pair of 6V6s, so figure maybe 20 watts max? Can I just sub in a 30 - 50 watt 8 ohm resistor, or does changing from an impedence load to a resistive load play a part in this?

I'd start simple - just a big resistor. Remember it's a good idea to provide some heat sink. Those Dale & similar gold-anodized hi tech looking resistors are just fine, and you can get 'em at reasonable price from Parts Express for instance, but don't believe just because it says 25W that it will handle that happily all by itself. Bolt one inside a 1590B foot-pedal box, even better one with some heatsink fins on it, that will keep the resistor and its accompanying wires from suffering in case your organist goes flat out E Power Biggs whilst laying down tracks in the studio while on "mute" mode. While we're on wires - I'd suggest using teflon insulated wires, because I've learned the hard way.

Of course there are fancy load 'resistors' that may approximate the way real speakers act - feel free to indulge if you want to spend lots of money and drive yourself crazy. Several threads exist about this I'm sure - search "ideal" speaker attenuator, something like that.

[nevetslab]

Customer wants to disconnect the speaker in his Hammond M3 organ for use in his studio so there is no mic bleed. What is the best way to do this? It's a pair of 6V6s, so figure maybe 20 watts max? Can I just sub in a 30 - 50 watt 8 ohm resistor, or does changing from an impedence load to a resistive load play a part in this?

I gather there is a Direct Out from the organ, so you're just looking to mute the speaker? Depending on where that direct out is from, using the dummy load will at least remove the sound from the speaker in the studio. If the direct is across the speaker, via some attenuation, it probably WILL change the sound of the organ, since it's now a constant resistance, and not an impedance, which has a curve to it.

[Randall]

The direct out is a rigged kluge involving a Radio Shack car stereo box of some kind. I don't remember exactly, it's been I while since I saw it. I will have another look when I install the amp later. Perhaps there is a better way to acheive a direct out, but I was thinking it was off the speaker connection.

Edit: I found this simple circuit for a line out. I get that it is a current divider, and that the 8 ohm load resistor is dissapating 99.9% of the current than the 21K parallel resistance, but what is the purpose of the 1K resistor? And what is the cap doing?

http://www.dairiki.org/HammondWiki/LineOutFromASpinet

[Helmholtz]

but what is the purpose of the 1K resistor? And what is the cap doing?

I think the 1k is used to protect the output (and cap) against accidental shorts when the 20k pot is fully up.
Not sure about the cap. It doesn't have a frequency shaping effect, so it could only block DC. In normal operation it should not be necessary.

[Chuck H]

Just to add to the what others are saying it's been common practice in guitar amp attenuators (and I see no reason for this to be a different application) to use a resistance a bit higher than the rated speaker load. I think the idea here must be that a rated speaker load is only observed in passing at very low frequencies and then for a time in the midrange frequencies with the impedance at other frequencies being higher (sometimes MUCH higher). So the nominal impedance of an actual speaker load is being considered as higher than it's rated impedance. It's also been observed (by some reports) that such attenuators that use a higher value resistive load than the rated speaker load requirement sound better than those which use a resistive load matching the rated speaker load. YMMV.

The Ho/Ultimate attenuator now uses an inductive load according to their literature. But I know that in 2009 when I asked about the units load at their NAMM show booth I was told they were using a 32 ohm resistive load for any amplifier plugged into the unit. The Trainwreck Airbrake load changes with attenuation settings, but also uses a similarly higher resistive load at most settings. Both attenuators have a good reputation for sound quality. Possibly others (Bad Cat leash?)

[Helmholtz]

I am not in favour of using a dummy resistor having more than double the nominal output impedance, because that would result in excessive screen dissipation at high output power.
The impedance of a guitar speaker stays below twice its nominal value over most of the guitar's fundamental frequency range (up to around 2kHz), meaning that higher values at the frequency extremes don't contribute that much on average.
Regarding reliabilty it seems safest to stick with the nominal load impedance.
(The Ultimate Attenuator looks like a sloppy design anyway.)

Also the application concerned is not really a power attenuator that feeds a guitar speaker, so not necessarily the same sound criteria apply as with a guitar amp > power attenuator > guitar speaker arrangement.

[Randall]

So, any opinion on what would be the best value resistor for this application?

[Leo_Gnardo]

So, any opinion on what would be the best value resistor for this application?

What's the speaker impedance? I'd go with that.

[nevetslab]

So, any opinion on what would be the best value resistor for this application?

To show graphically what Helmholtz just stated, where in normal use, the sounds from the organ will rarely dwell at the 'nominal' impedance value, and more like twice that value as a average would be just fine. For an 8 ohm tap, 16 ohms or something in that general range that's handy, with the wattage & heat dissipation accounted for in the install, would suffice.

Here's a plot on the Ampeg BXE-115HL4 bass cabinet I have in the shop for general use with amps in for service. I calibrated the scale with resistor load connections on my 4kW load bank, so you can see the variation of a typical speaker (in this case, a ported cabinet).



In this instance, the speaker is a nominal 4 ohm value. Twice that is 8 ohms, so you can see from the graph a value twice your speaker will more than suffice for the task.

[Helmholtz]

So, any opinion on what would be the best value resistor for this application?

What's wrong with sticking to the Hammond circuit? If you prefer, double the value. I gave you a good reason not to go higher.

[Randall]

I went with a 8 ohm 50 watt ceramic tube resistor with standoff mounting flanges. $13.