Dave's Inductive Dummy Load for Practice and Recordng

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Dave's Inductive Dummy Load for Practice and Recordng

Postby dlvoots on Thu Dec 29, 2011 9:53 pm

This is another "How To" story for those who love to tinker with amplifiers. It started with a topic posted by greg about what he wanted for Christmas:

greg wrote:A small box that I can plug my AC30 head into, and into which I can plug Headphones, or a line out, or both at the same time, without losing any of the sound of the amp.....


Upon reading his topic, I realized that I had one! And except for the headphone jack, it was exactly what he wanted for Christmas! I designed and built it back in the 90's to solve two issues -- I wanted a way to record my AC30 without miking the cabinet and I wanted to keep my wife from complaining about the high decibel guitar playing in the house.

This is one of those 80% / 20% stories. I had read several articles on building dummy loads, and eventually realized that using non-inductive power resistors would get me 80% of the way there for about 20% of the time and effort. The trouble is that after building a couple prototypes, I realized that it was really that last 20% that contained the tone I was looking for.

Simple resistors don't do the trick for two main reasons; inductance and something called "damping". The amplifier is quite happy when it has a resistor as a load, because the resistor simply converts the energy from the amp into heat. That's the 80%. The tone magic is in the missing 20%.

Speakers don't just convert energy from the amp into sound -- the the inductance in the speaker voice coil "fights" the signal from the amp tooth and nail. The voice coil has to use the energy from the amp to generate a magnetic field. As the magnetic field builds up, it interacts with the magnet in the speaker and causes the speaker cone to move. Generating the field takes a finite amount of time, so you could actually say that the speaker cone movement lags the signal from the amp by a bit. Not much, but enough to make a difference. Additionally, when energy from the amp stops or changes, the magnetic field doesn't stop or change at the same time, and the speaker fights the change.

It takes a finite amount of time for the field to collapse. As it collapses, it turns the speaker into an electric generator of sorts. The generated electricity from the speaker heads right back to the amplifier, goes backwards through the output transformer and interacts with the output tubes. If the amp sends more signal at the same time, the backwards energy coming from the speaker alternately cancels and reinforces parts of the amp signal. It is distortion in a pure electrical engineering sense, but players like me call it tone.

Damping is a bit of a beastie that I never understood until quite recently, and it seems to be related to how the amplifier fights back at the speakers, who are busy fighting any and all changes from the amplifier. To the best of my understanding, telling the story is a play with two acts.

Act One is about the cone and its movement. Speaker cones are primarily made out of stiff paper -- when the voice coil begins to move, the portion of the cone closest to the voice coil begins moving with it. So far so good. The problem is that the rest of the cone doesn't start to move at the same exact time. It catches up quickly, but by that time the voice coil has inevitably moved the cone in a different direction. The cone fights the physical movement of the voice coil in the same manner that the inductance in the voice coil fights the amplifier (think of it almost like a "physical inductance"). As the cone resists movement of the voice coil, it contributes distortion to the magnetic field, which feeds backwards into the amplifier. Again, more distortion in a pure physical and electrical sense. I would call it tone.

Act Two has to do with how the amp attempts to rein in all of this speaker fighting the amp nonsense. To minimize this backwash of interaction from the speaker, most amps are designed to resist it or somehow compensate for it (one of the many side effects of a negative feedback loop in the power amp). The measurement of how well the amp does it is called damping. Highly damped amp designs keep a pretty tight rein on it (most solid state amps are in this category). Not so highly damped amps allow more speaker interaction back into the amp (many tube amps are in this category). Highly Damped versus Not So Highly Damped is yet another of the myriad of characteristics that make up an amplifier's distortion voice.

Over time, I realized that a lightly damped (or no damped) amp / speaker system got me into the rarefied air of that last 20%. It moved me closer to the tone I wanted.

I also found that using a really long cable from the amp to the dummy load tends to inhibit the amplifier's ability to dampen speaker interaction and increase this type of distortion / tone. It is not a lot, but I can just hear the difference. My ears perceive it as an increase in the looseness or jangly-ness of the sound. Remember, it is not about efficient power transfer, so the longer and thinner the better. I used 30 feet of 18 gauge (or thinner) speaker wire for years, all rolled up into a bundle in the back of the amp. Years ago, I read how amp guru Ken Fisher used an 18 foot piece of old vacuum cleaner power cord as his speaker cable. Said it gave him the most jangle of any cable he tried.

So how does this play into making a good inductive dummy load? Simple -- Use a really icky cable from the amp and don't use resistors.


Dave's Inductive Dummy Load
=============================

I found that I could make a really excellent dummy load by simply using high power/small diameter speakers in place of resistors. I purchased three 5" 8 ohm 50 watt "Hot Spot" speakers. I wired them up in series / parallel and put them into a sealed cabinet with a 100 watt L-Pad or Fader as the "fourth speaker". I surrounded the speakers with scraps of old foam carpet underlayment to dampen the sound. The sealed cabinet has no openings except for the front panel, which has the control panel. The control panel has an input jack for the amp, a variable output for the speaker, and a variable line out for recording.

There is no magic to selecting of speakers for use in this application. I had four criteria when I bought them -- small size, high power, large magnets, and cheap. The three hot spot speakers did the trick at around six bucks each. The 100 Watt L-Pad is available in the US from MCM Electronics. The link is here:

http://mcmelectronics.com/product/DISTRIBUTED-BY-MCM-50-070-/50-070

I wired the line out and the speaker independently of each other. That way I can play at a low volume and record at a high level, or play at a high volume and record at a low level. It looks like this:

Inductive_Load_External.jpg

The front is held in place with four screws. Remove the screws and the front easily pulls out. The control panel is made from a scrap piece of 1/8" acrylic. I painted it grey and applied dry transfers as labels, followed by a generous overspray of clear acrylic. I was able to mount all of the wiring on the back of the control panel.

The three wires that run into the cabinet are individual feeds that go to the three speakers. As you can see from the picture, the foam underlay completely fills the inside of the box.

Inductive_Load_Internal.jpg

Oops! Two more pictures to go and a three picture limit per post. I'll be right back to continue the story.

Dave
Last edited by dlvoots on Sat Dec 31, 2011 2:54 pm, edited 10 times in total.
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Re: Dave's Inductive Dummy Load for Practice and Recordng

Postby dlvoots on Thu Dec 29, 2011 9:59 pm

To continue.....

The foam underlay is simply stuffed into the cabinet and removes quite easily. I cut various diameter holes in the underlay to gently hold the speaker from top to bottm. The underlay does a pretty good job of absorbing sound and vibration from the speakers. I removed one of the three speakers to give an idea of how they are mounted.

Inductive_Load_Speakers.jpg

The story wouldn't be complete without a schematic. As you can see from the diagram, the first two 8 ohm speakers are wired across the amp jack in series. The third 8 ohm speaker is wired in series with the fader. The two branches are in parallel with each other, making the box look like an 8 ohm load to the amplifier.

As with speaker selection, there is no magic associated with the values I used for the line out control and limiting resistor. I had a 500 ohm pot sitting in my junk box and a 470 ohm resistor seemed to work nicely. The pot can be anything up to 5K, and the limiting resistor can be anything up to 22K or so. Higher values on both tend to attenuate the higher frequencies, so you can build to suit your taste.

I didn't see a need for a headphone jack in my original version, but since greg requested one in his Christmas Wish List, I have included it in the diagram as an optional component. If you have a set of mono headphones, a normal input jack will suffice. If you decide to use stereo headphones, simply wire the left and right channels together and connect it as a single "hot" lead to the same point in the circuit.

Inductive_Load_Schematic.jpg

Disadvantages? Yeah, big and heavy and probably over-designed. It carries about the same weight as your average 50w tube head, but since it was built to practice with and record, it doesn't do much traveling. One end of the basement to the other is the usual journey.

One of the things I learned early on was that if you want to design and build something AND expect it to work reliably, you'll never be able to factor in all the variables in its use that will conspire to make it fail. How do you get around the dilemma? You derate everything by at least half. Given that I used three 50 watt speakers and a 100 watt L-Pad, it could potentially sink 200 watts of maximum power. However, I wouldn't feel comfortable using it with anything larger than 100 watt head. If you use smaller or cheaper components to build something less powerful -- and it is entirely within reason to do so -- simply derate everything accordingly.

If you have any questions about this one or about building your own, let me know. Enjoy, and hope you had a good holiday!

Dave
Last edited by dlvoots on Fri Dec 30, 2011 9:37 pm, edited 4 times in total.
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Re: Dave's Inductive Dummy Load for Practice and Recordng

Postby mAx___ on Fri Dec 30, 2011 3:49 pm

Dave, that's an excellent way of doing it!
Thanks a lot for taking the time to explain it in so much detail, great post!
Happy New Year!

M.
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Re: Dave's Inductive Dummy Load for Practice and Recordng

Postby murky69nz on Fri Dec 30, 2011 10:45 pm

Awesome Dave, thank you!

Couple of questions if it's not too much trouble (as my high school physics isn't too sharp any more!):

1) Does it matter what resistance output speaker (the one you plug in) you use? Looks like you're using the 8 ohm tap from the amp into the box, and perhaps an 8 ohm output speaker as well? Seems that the overall resistance the Amp sees will drop a little when the output speaker is blended into the circuit?
2) At it's most attenuated, is the output speaker silent?
3) At it's least attenuated, it looks like the output speaker will be taking about 25% of the amp's power - is this right?
4) If one is using a 16 ohm amp tap (for arguments sake, lets say that's the only tap available), then best replace the dummy speaker and L-pad with 16 ohm equivalents correct?
5) If one isn't needing a line or headphone out I assume there's no issues with omitting that part of the circuit?

Appreciate you time, and thanks in advance!
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Re: Dave's Inductive Dummy Load for Practice and Recordng

Postby dlvoots on Sat Dec 31, 2011 9:02 pm

M,

Welcome to the forum!

murky69nz wrote:1) Does it matter what resistance output speaker (the one you plug in) you use? Looks like you're using the 8 ohm tap from the amp into the box, and perhaps an 8 ohm output speaker as well? Seems that the overall resistance the Amp sees will drop a little when the output speaker is blended into the circuit?


Your are correct; I designed it to present an 8 ohm load to the amp. The 8 ohm L-Pad / fader is also designed to be used with an 8 ohm speaker or speaker cabinet. L-Pads are a special type of variable control, in that they are designed to present an 8 ohm load to the amplifier regardless of where the control is set. If you hang an ohmmeter on the input to the 8 ohm L-Pad and an 8 ohm speaker on the output and turn the knob, you'll see that the ohmmeter will read within a small percentage of 8 ohms regardless. Likewise with the total impedance of the box -- the amp will see something in the neighborhood of 8 ohms regardless of the L-Pad setting,

MCM also sells a 16 ohm fader, but at half the power handling capability (50 watts).

If you build it as an 8 ohm box, you can still plug in a 16 ohm cabinet or a 4 ohm cabinet into it and the amp won't really see the difference. The total impedance of the box might be a an ohm or two higher than 8 ohms for the 16 ohm cabinet and maybe an ohm or two lower than 8 ohms for a 4 ohm cabinet, but still within the range that will keep the amp happy.

murky69nz wrote:2) At it's most attenuated, is the output speaker silent?

Yup.

murky69nz wrote:3) At it's least attenuated, it looks like the output speaker will be taking about 25% of the amp's power - is this right?

Correct. The purpose of the box is to attenuate the power without "messing" with that speaker-to-amp wrestling match and degrading tone. Think of it as a way to play the amp at "Gig Levels" in your bedroom or during recording without actually rattling the windows. The three actual speakers in the box fight the amp together and suck away 75% of the amp's power. The 8 ohm L-Pad gives you control over that other 25%, going from nothing to the full 25% that is left.

murky69nz wrote:4) If one is using a 16 ohm amp tap (for arguments sake, lets say that's the only tap available), then best replace the dummy speaker and L-pad with 16 ohm equivalents correct?

Actually there are two options. It just depends on what power handling capacity you need to end up with and whether a single speaker in the box (versus three) gets you to tone happiness. If you're using the 16 ohm output on the amp, you could substitute 16 ohm equivalents and have the same configuration. The MCM 16 ohm fader is only a 50 watt maximum control, so you'd just derate the total power handling capacity accordingly.

Alternately, you could remove the first two speakers in my version and turn it into a 16 ohm dummy load that would take an 8 ohm cabinet (i.e. one "real" 8 ohm speaker buried in the foam, an 8 ohm L-Pad and an 8 ohm cabinet). Configured that way, the "real" speaker in the box would only suck half the power as it is busy fighting the amp before it gets to the 8 ohm L-Pad and on to the external speaker(s). One "real" speaker in the box may fight the amp enough to get you into that 20% -- only you can say whether it gets you there. Try it with one 8 ohm internal speaker in series with the 8 ohm L-Pad and see (remember to derate the capacity; one 50 watt speaker and the 100 Watt L-Pad would have a potential 100 Watt maximum -- for reliability, don't use anything bigger than a 40-50 watt amp).

If it doesn't turn you on, then get the 16 ohm equivalents and go with the first option. And remember to derate it all by a factor of two because of the 50 watt capacity of the 16 ohm L-Pad -- maybe a 50 watt amp maximum for the box also.

murky69nz wrote:5) If one isn't needing a line or headphone out I assume there's no issues with omitting that part of the circuit?

Correct - you don't need to include the line out or the headphone jack. Simply omit either one or both. I never had the headphone jack, but it was in greg's Christmas wish list so I added it to the diagram.

Any other questions, just let me know. Good luck!

Dave
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Re: Dave's Inductive Dummy Load for Practice and Recordng

Postby murky69nz on Mon Jan 09, 2012 10:48 pm

Thanks Dave, much appreciated!

Been using hotplates for a while, but have been looking to get one of the "new generation" attenuators (Alex/Aracom etc). Will give this a go first me thinks..... :D
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