dummy speaker load.

[Alan0354]

I know to make a dummy load to simulate speaker is hard. But what is a simple way to make a dummy load that is "similar" the the impedance of a speaker? I am not an expert, the most simple thing I can think of is a resistor of something like 4 or 5 ohm in series with a parallel combination of an inductor and capacitor. The inductor has value similar to the speaker and the capacitor the get the resonate frequency of a typical speaker.

This is the simulate a resonance circuit with peak like a typical speaker, then the resistance of the resistor at off peak frequency. May be a resistor Rp in parallel with the LC tank to lower the Q.

What do you guys think? If you have schematic of existing dummy speaker load, please post it for me.

Thanks

[d95err]

This thread will tell you all you need to know...

Aiken's Reactive Dummy Load. - The Gear Page

[Alan0354]

Thanks, I looked at the thread, the OP want to load the tube amp so the line out sounds like an amp with speaker in the direct recording. That is not what I am after.

What I am trying to do is using it as a dummy load. I have a Pro Reverb that has two 8 ohm speaker in parallel. All I want is to replace one of the speaker with the dummy load and using one speaker and reduce the output loudness by half. Basically I want to have a switch to switch between one speaker or dummy load so I can get the equivalent of either a 40W or a 20W amp. I just need to have a load that does not change the sound of the one speaker. So I would expect the requirement will be a lot more relaxed and the design should be simpler.

I tried using a straight resistor, it is really not too bad. I seemed to loss a little of the rawness and a little high. Maybe that is due to the lack of the resonance peak to divert more power to the speaker at resonance frequency. I think all I want is the impedance graph of a speak and design a load to curve fit the impedance.

[Chuck H]

If you don't think the HF impedance curve is important your diagram is fine. Using values readily available at Parts Express Crossover Components in the Speaker Components Department at Parts Express | 21 you can build an 8R "active" load either way like this.

I've noticed that using air core inductors for the HF inductor can cause EMF feedback with single coil pickups and possibly amp circuitry. I solved for this by using two 250mH inductors in series, out of phase stacked together with a copper plate in between to minimize cross induction. Then the EMF from the 500mH circuit is humbucking.

[J M Fahey]

Your requirements are much higher than his, who worst case may "cheat", load the amp with *anything* (a cheap simple resistor) and tweak low level signal afterwards, while you want "same response" at full power, and listen to it through the remaining speaker, a much harder task .
And lowering power by 3 dB is very little.
I'm more and more convinced that Power Scaling is the way to go, only consistent way to *really* reduce power and nothing else.

[Chuck H]

Juan, I think it's important to note that the amp would indeed only be 3db less loud at half power, but we're also taking half the speaker surface out of the equation too! I think the difference should be worth the effort. Lot's to consider if you really wanted to nail the frequency response and the amplifiers reactance though. Roughing it will have to do. Plotting the actual speaker being bypassed would mean using multiple components in series or parallel to nail the actual impedance curve and then, of course, the other speaker doing the rest of the work now has the whole cabinet to itself, which will change things too. Getting closer than a plain old resistor is worth the effort IMHE but a generalized guitar speaker-ish plot should be fine.

[Enzo]

Remember, the OP didn't say half power, he said half loudness. A resistor will show how well that will work, all adding the L and C components will do is massage the tone.

[Chuck H]

Absolutely. Good point on the plain ol' resistor giving the general clue on actual loudness.

I can't speak as to exactly what "half as loud" even means. Half the dB's would probably seem waayy less than half as loud. And of course half power is waayy more than half as loud. There may be an actual formula for it but I think it mostly comes down to a perception by the user.

I do know that back in the day, when the Fender Twin Reverb was the almighty stage king for well healed players, a common trick was to pull two tubes and then disconnect one speaker to correct the load. It did indeed sound like half the amp was shut down.

[Enzo]

When we speak of loudness in any sort of practical sense we are concerning ourselves with SPLs. Perceptions are not measurable.

[Alan0354]

I want to clarify, I should have said "half power loud". Yes, using a dummy load for one of the two speaker just make my Pro Reverb loudness down to a Deluxe Reverb......from 40W to apparent 20W. Not much lower in loudness.

But as everyone experience, no one method can lower the loudness all the way down and still sounds good. I did a few power scaling, if you go down beyond 1/3 power, it will sound different. So are attenuators.

I even tried unplugged one speaker and using my 8 ohm THD Hotplate to replace the speaker when put it in high attenuation setting. It still does change the sound a little. But remember, attenuator are made to optimize the sound when you plug the speaker through it, not necessary a dummy load simulator.

So far, I tried a simple resistor, THD Hotplate as dummy load. Seems they are no better if even as good as just using the THD Hotplate and use it at -4dB ( more than reducing half power already).

[Alan0354]

Anyone have experience with the Weber attenuator. Their Mass supposedly have a speaker type of load. Anyone know how's the inside looks like?

[Tom Phillips]

...Anyone know how's the inside looks like?

It's a speaker magnet structure, voice coil and spider mounted in an abbreviated speaker frame. Basically a "speaker" without a cone and the large part of the basket that supports the cone. I assume that a mass (weight) is added to take the place of the cone.

[Alan0354]

It's a speaker magnet structure, voice coil and spider mounted in an abbreviated speaker frame. Basically a "speaker" without a cone and the large part of the basket that supports the cone. I assume that a mass (weight) is added to take the place of the cone.

But that still change the power handling capability and the reactance. The resistance of the real cone change everything. Or else it will be too simple by just cutting the cone off a cheaper speaker and use it as a dummy load. That was my first thought way at the beginning and I dismissed that out right!!! I thought make the shoot absorber to simulate the speaker cone will be too hard.

It is like comparing an loaded motor and an unloaded motor, the impedance and the impedance profile is totally different.

[Tom Phillips]

Yep. The Weber mass is not a perfect simulation of a real speaker. There is a photo of the "Speaker Motor" at Attenuators . It appears that they have simplified since the original design. A while back they also stopped selling the stand alone speaker motor and I believe it is now only available as part of their attenuator products.

[olddawg]

I use a Weber Mini Mass 50 watt in clubs all the time. It has a 3 position treble compensation switch on it. I had mine made with a foot switch to turn it on and off. Sometimes I use it like a boost for leads. It also has a line out. It does color your sound a bit, especially if it is turned way down. It is best used just to take the "edge" off of an amp not squash it. I hear you can use them for a dummy load with no speaker attached although I have never done it. I would research that. Generally I use mine in practice and in small clubs for the first set while things are filling up and settling in. The 50 watt version for me, with the foot switch option was $125. I could not build anything near that quality for that price. It is very rugged in a metal case with switch craft jacks and such. Very usable and a great bang for the buck. Many times I have it on turned all the way up. Then switch it off for leads. Keeps the stage volume down and the amp cranked. Works great on my 18 watt Marshall and my VJ. That's another idea. You might want to pick up a VJ for $100 and play through it and one 12" speaker. You might be surprised. Nobody believes I'm playing through a $99 5 watt amp in the clubs but I do it all of the time. I have played the HOB with that VJ head through a 2x12 cab. The sound guy loved it. Same thing for 1200 people at The Coach House. Just got miked up.