Aiken's Reactive Dummy Load.

[Mike Lind]

Yes, you can use Bipolar caps (non-polarized electrolytic) and they will usually work as long as you know how to read the specifications and see beyond them because necessary information is not always available.

An example:

A common 100VDC bipolar cap rates at 35VAC and they tend to have much higher ESR than film caps. They have a shorter lifespan and are more susceptible to heat. Using several of them in parallel is a good strategy.

You can also create your own non polar cap by connecting two polar caps back to back. Ripple current figures for polar caps are available but observe that these figures are usually specified at 100k and drops significantly with frequency.

For Hi-Fi purposes electrolytic should be avoided in the audio path but a guitar amp is not Hi-Fi so they will work when used appropriately.

 

[savt22]

Hi,
I hope this thread is still alive! I'd really like to build one of these!

I'm in the UK and I'm finding it hard to find all of the components listed here.
Could someone look at the components I've chosen to see if they would be suitable?

For the high power resisters, I'm getting from eBay - will the quality be good enough?
50ohm, 75w - https://www.ebay.co.uk/itm/362590935070
8ohm, 200w - https://www.ebay.co.uk/itm/332996533995
50ohm, 50w - https://www.ebay.co.uk/itm/401514561019

This is the capacitor I've chosen, will that work well?
200uF bipolar - https://www.ebay.co.uk/itm/183388254384

And for the inductors, I'm sourcing from falcon acoustics. Will they handle the current and be sufficient?
11mA, 5A - https://www.falconacoustics.co.uk/a...r-air-core-audio-9mh-10mh-audio-inductor.html
0.5mH, 5A - https://www.falconacoustics.co.uk/a...-power-air-core-041-050mh-audio-inductor.html

Any thoughts?
Thanks in advance!

Just in case anyone in the UK is thinking of building one and looks at this page, I built one with the Falon AC100 (11mA) and an "INDUCTOR AIR-CORE 0.47MH 0.2R Inductors/Chokes/Coils - GM85457" from eBay, with the B&W cap and the listed resistors, and it works great. It fits quite neatly inside a 187x118x57mm project box too which are also easy to grab on eBay.

 

[Nodnarb]

Hope everyone is well as it’s been a while since I last posted, and the world is kind of different now.


I did end up taking my proposed design of my load box and made one. My results ended up not providing the same impedance output that my Marshall 1960 cabinet does but it is pretty close. I do view if I increase the values of the resistors and possibly unwind the inductor I could match a little closer, but the end result that I have has pretty decent feel and the sound is pretty good too (I end up distorting it through a high gain amp with some EQ on it as all IR files require some type of notching even when mic'd up for real).


Some bullet points in regards to my project:


) I used Room EQ Wizard (REW) https://www.roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html to do some measurements for comparison. I used some terminal/screw in/twist connections for the tester harness (in photos). I used both a ceramic traditional resistor and a "low inductance" power resistor in the tests since REW indicated zero inductance. Both yield basically the same graphs however I did get into a situation where it appeared that the base resistance of the results would increase significantly like instead of the values being near 16ohm they would be in the high 20s or even higher. I was not able to replicate it after tearing the harness apart and putting it back together with the ceramic resistor. That research did lead to me being surprised by the open/closed back results. My sensor resistor ceramic measured 99.8 and power low inductor resistor measured 100.1 on my ohm meter.


-- I get it now, but notice the drastic difference in the curves when the back is off the 4x12 cabinet. The resonance peak moves from about 110hz down to about 80hz and spikes big time. Not shown but I also had way different curves when I ran the cabinet 4x12 as a 16 ohm cabinet over a 4 ohm cabinet wiring.


-- Notice when the back is closed the resonance frequency on the way down kind of acts like a roller coaster with some dips. I replicate this no matter what resistor or setup I use in the REW test. Soon as you pop the back off it goes away. Put the back on, roller coaster comes back.


-- I understand the REW test is using a small (1 watt) from the sound card and doing the same test with a full load clean solid state amp at 50w or higher might yield different results and using more pro equipment to do the curve tests


-- I didn't realize how bad I was at soldering. I know that the circuit could use some TLC regarding the lead lengths and n00b soldering end result


-- I used the resistor values I did for the resonance portion of the circuit to tame the resonance peak and it really tamed it so I may end up replacing the 65 ohm resistor in my circuit for a higher value.


-- I was so concerned about cooling before I started. So much that I spent the extra $ on the case (and part because not many aluminum cases I could find could fit everything). I even went total overboard and placed all of the power resistors on a 6mm aluminum plate in addition to the thermal paste. Either this was overkill or proper choice because when pushing a decent load (my 6505+ on post gain 6) the power resistors result in near zero heat and are actually "cool" to touch.


-- Concerned with heat, most of the components are raised. The two resonance caps are suspended in air resting on a pvc modified gutter clip.


-- Nearly all screws/bolts/nuts are either aluminum, some stainless washers, or plastic including some zip ties.


-- I know the series power resistors isn't ideal, but I was going for 15 ohms since the other components would add series resistance and get me closer to 16.


-- I got all parts from mouser, erse audio, parts express, lowes, and amazon. Erse customer support was good because on the website the shopping cart system was charging very large $ for shipping and they were able to remedy that. Cost was under $200.


-- I used a ton of techniques and design items from many contributors here and I thank all that contributed


-- A comment in a thread here implied a debate about resonance frequency doesn't matter since you don't play those notes a lot. 110hz is an open A string and us metal guys that is actually one of the higher frequency we will play so the resonance portion is pretty important.



Here is a ten second snip using 6505+ with my Load Box processed with an Ownhammer Mesa V30 IR and light EQ

https://www.dropbox.com/s/k13fgjxvg17uvlu/OverdoseWithLoadBox.mp3?dl=0


Here is a ten second snip using 6505+ with SM57 on my 1960 processed a tad bit more

https://www.dropbox.com/s/nic1v2xs27hgx7a/WhatDoesntBreakSM57.mp3?dl=0


For quick demos and to make it so my own and family ears don't bleed, using the Load Box looks to fit the bill and the "reactive" portion of the circuit makes it way more realistic.

 

[Husky]

One thing to remember
Doesn't matter if it's aluminum, aluminum will still interfere with the inductors, get them as far from any metal you can. Maybe you have noticed but test the same circuit without a box and your curve will change.

Hope everyone is well as it’s been a while since I last posted, and the world is kind of different now.


I did end up taking my proposed design of my load box and made one. My results ended up not providing the same impedance output that my Marshall 1960 cabinet does but it is pretty close. I do view if I increase the values of the resistors and possibly unwind the inductor I could match a little closer, but the end result that I have has pretty decent feel and the sound is pretty good too (I end up distorting it through a high gain amp with some EQ on it as all IR files require some type of notching even when mic'd up for real).


Some bullet points in regards to my project:


) I used Room EQ Wizard (REW) https://www.roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html to do some measurements for comparison. I used some terminal/screw in/twist connections for the tester harness (in photos). I used both a ceramic traditional resistor and a "low inductance" power resistor in the tests since REW indicated zero inductance. Both yield basically the same graphs however I did get into a situation where it appeared that the base resistance of the results would increase significantly like instead of the values being near 16ohm they would be in the high 20s or even higher. I was not able to replicate it after tearing the harness apart and putting it back together with the ceramic resistor. That research did lead to me being surprised by the open/closed back results. My sensor resistor ceramic measured 99.8 and power low inductor resistor measured 100.1 on my ohm meter.


-- I get it now, but notice the drastic difference in the curves when the back is off the 4x12 cabinet. The resonance peak moves from about 110hz down to about 80hz and spikes big time. Not shown but I also had way different curves when I ran the cabinet 4x12 as a 16 ohm cabinet over a 4 ohm cabinet wiring.


-- Notice when the back is closed the resonance frequency on the way down kind of acts like a roller coaster with some dips. I replicate this no matter what resistor or setup I use in the REW test. Soon as you pop the back off it goes away. Put the back on, roller coaster comes back.


-- I understand the REW test is using a small (1 watt) from the sound card and doing the same test with a full load clean solid state amp at 50w or higher might yield different results and using more pro equipment to do the curve tests


-- I didn't realize how bad I was at soldering. I know that the circuit could use some TLC regarding the lead lengths and n00b soldering end result


-- I used the resistor values I did for the resonance portion of the circuit to tame the resonance peak and it really tamed it so I may end up replacing the 65 ohm resistor in my circuit for a higher value.


-- I was so concerned about cooling before I started. So much that I spent the extra $ on the case (and part because not many aluminum cases I could find could fit everything). I even went total overboard and placed all of the power resistors on a 6mm aluminum plate in addition to the thermal paste. Either this was overkill or proper choice because when pushing a decent load (my 6505+ on post gain 6) the power resistors result in near zero heat and are actually "cool" to touch.


-- Concerned with heat, most of the components are raised. The two resonance caps are suspended in air resting on a pvc modified gutter clip.


-- Nearly all screws/bolts/nuts are either aluminum, some stainless washers, or plastic including some zip ties.


-- I know the series power resistors isn't ideal, but I was going for 15 ohms since the other components would add series resistance and get me closer to 16.


-- I got all parts from mouser, erse audio, parts express, lowes, and amazon. Erse customer support was good because on the website the shopping cart system was charging very large $ for shipping and they were able to remedy that. Cost was under $200.


-- I used a ton of techniques and design items from many contributors here and I thank all that contributed


-- A comment in a thread here implied a debate about resonance frequency doesn't matter since you don't play those notes a lot. 110hz is an open A string and us metal guys that is actually one of the higher frequency we will play so the resonance portion is pretty important.



Here is a ten second snip using 6505+ with my Load Box processed with an Ownhammer Mesa V30 IR and light EQ

https://www.dropbox.com/s/k13fgjxvg17uvlu/OverdoseWithLoadBox.mp3?dl=0


Here is a ten second snip using 6505+ with SM57 on my 1960 processed a tad bit more

https://www.dropbox.com/s/nic1v2xs27hgx7a/WhatDoesntBreakSM57.mp3?dl=0


For quick demos and to make it so my own and family ears don't bleed, using the Load Box looks to fit the bill and the "reactive" portion of the circuit makes it way more realistic.

 

[Mike Lind]

REW is a good software and the point of failure is usually a combination of audio card, sense resistor and not using a professional amplifier for the test signal. However, using consumer type equipment correctly will get you reasonably close.

As long as the coils are not saturating measurements should be similar from a 1w or a 50w source. Iron based coils will change their behavior depending on current applied but as stated previously placement is usually a bigger factor.

More free air for the speaker elements will get you closer to specs so your measurement is to be expected.

Yes, resonance matters. I learned it the hard way when I had Fredrik Thordendal from Meshuggah in my Mastering studio and I wanted to cut some of the resonance peak. He called it the “obb” effect and if you listen to their albums, you understand why.

 

[Bruce Clement]

Question: I want to build one of these for a 10w max amp. Naturally, I don't need such big resistors and inductors. How do I calculate the ratings for the inductors? If a 5A rated inductor can handle a 100w amp, how many A (or mA) should I get for a 10w amp?

Thanks!

 

[reaiken]

Question: I want to build one of these for a 10w max amp. Naturally, I don't need such big resistors and inductors. How do I calculate the ratings for the inductors? If a 5A rated inductor can handle a 100w amp, how many A (or mA) should I get for a 10w amp?

Thanks!

The problem you're going to run into is that large value inductors with small current ratings typically have relatively high resistances, which completely kills the Q, so your reactive load won't come anywhere close to matching the theoretical impedance curve.

 

[Bruce Clement]

Thank you.

 

[torquil]

Just for interest, here is the complex input impedance for my dummy load (same circuit as most of the others in this thread), measured using the Bode Plot II function available when connecting a Siglent SDS1104X-E oscilloscope and a Siglent SDG1062X signal generator. After a few years of using this dummy load with a 200 uF capacitor, these new measurements showed that the nominal input impedance was actually quite high in the flat region between 300 Hz and 1 kHz, around 11 or 12 Ohms. So I added a 100 uF for a total of 300 uF which gave the more reasonable result below, around 9 Ohm. It also shifted the resonant peak down to around 90 Hz from around 110 Hz.

The Bode Plot function in the scope is really meant for comparisons of input and output signals from a device, but can also be used for impedance measurements such as shown below, as long as you provide voltage and current measurements to the scope. In my case I'm using a differential probe for the voltage measurements.

Next up is the complex impedance of the 12" speaker in my Peavey Triumph 60 open back combo amplifier:

I think the Peavey speaker had a sticker on the back which has fallen off, so I don't know exactly which speaker it is, but I think it is a "Scorpion" speaker. NB: Please note that the plot scales are different! I have not tried to match the speaker load or anything, just including the two measurements for curiosity.

 

[ItsTonyB]

Awesome thread with loads of great info.

I've built one of these and it's working great however if I touch the enclosure while holding the guitar I get a whistle through my monitors.

Just a quick question should the input be grounded to the chassis?

 

[Johndh]

my guess would be that the load box case should not be grounded to the input. That's how I build my attenuators. But is be very interested to know given what yours is doing, is it grounded or not and if it is, how is it?

 

[ItsTonyB]

my guess would be that the load box case should not be grounded to the input. That's how I build my attenuators. But is be very interested to know given what yours is doing, is it grounded or not and if it is, how is it?

The slave out now. I forgot to solder one end of the wire!

It would only whistle if I was monitoring through my daw.

I opened it up and checked it over and noticed I hadn't soldered the cable. Foolish mistake!

 

[Badside]

Hot damn! Just needed to chime in. I finally built mine, using a 10mF resonance inductor with 200uF cap, and 0.47mH presence inductor, ran a line out into my HX Stomp with some Celestion Plus IRs

First with my recent 6V6 JTM45 style build and damn if that wasn't the most dynamic and enjoyable bluesy tone I've ever had. Cranking the treble and lowering bass got me into instand Angus Young territory.
Then with my 2204 and that was the most fun I've had playing it at home ever. So bold and mean and punchy.

All of this... with my headphones.

 

[Mike Sch]

First post here...very interesting thread. I'm accomplished at tube gear design and LTSpice literate so decided to give this a go and tweak it to my tastes. But I did have one outstanding question that I wanted answered before proceeding: Does the impedance curve change with wattage applied?

So I built a test rig for REW that could do the measurements while putting 3W rms across the load and also when putting 50W rms across the load. I measured my 2xG12T75 enclosed cab from 30Hz to 10KHz.

The results are that the curves overlay from 30Hz thru the resonant peak and to about 175Hz. From there the 50W curve gradually begins to separate from the 3W curve while holding the same "shape". At 1KHz the 50W curve is about 1ohm greater, at 5KHz its about 4 ohms greater and at 10KHz about 8 ohms greater. Which is very similar to comparing open back vs enclosed in that freq range.

So, I'll probably model my build in LTSpice to average the curves for this freq range.

Side note: I also modeled the parasitic inductance of the power resistors most people here are picturing and it is insignificant in this design (a good thing!). So no need for expensive non-inducting power resistors. Of course the parasitic resistance of the inductors is quite significant, so it must be modeled.

I also measured my open back 1x12 amps and found the resistance peaks match the resonant specs for the Vintage 30 and G12H30 Heritage. The G12T75 pair displays a higher resonant peak than spec since they're in an enclosed cab.

But given how much lower the resonant peaks are for the open backs, I'm looking at doing a switchable design to accommodate since I have some open back IRs I'm really interested in and want the most authentic feel for those. LOL, we'll see if the switchable design was worth it in real life once I finish the build.... The good news is that the low peak and the rest of the curve can be modeled virtually independent from each other, so the additional parts aren't going to break the bank (additional lf inductor in series, additional lf capacitor in parallel, switch).

 

[James Freeman]

Still use mine.
It has become an indispensable tool for recording, measurement, and comparison with Modelers.
I'm really happy that a DIY option is available, since the commercial offering is quite expensive.

 

[Bruce Clement]

First post here...very interesting thread. I'm accomplished at tube gear design and LTSpice literate so decided to give this a go and tweak it to my tastes. But I did have one outstanding question that I wanted answered before proceeding: Does the impedance curve change with wattage applied?

So I built a test rig for REW that could do the measurements while putting 3W rms across the load and also when putting 50W rms across the load. I measured my 2xG12T75 enclosed cab from 30Hz to 10KHz.

The results are that the curves overlay from 30Hz thru the resonant peak and to about 175Hz. From there the 50W curve gradually begins to separate from the 3W curve while holding the same "shape". At 1KHz the 50W curve is about 1ohm greater, at 5KHz its about 4 ohms greater and at 10KHz about 8 ohms greater. Which is very similar to comparing open back vs enclosed in that freq range.

So, I'll probably model my build in LTSpice to average the curves for this freq range.

Side note: I also modeled the parasitic inductance of the power resistors most people here are picturing and it is insignificant in this design (a good thing!). So no need for expensive non-inducting power resistors. Of course the parasitic resistance of the inductors is quite significant, so it must be modeled.

I also measured my open back 1x12 amps and found the resistance peaks match the resonant specs for the Vintage 30 and G12H30 Heritage. The G12T75 pair displays a higher resonant peak than spec since they're in an enclosed cab.

But given how much lower the resonant peaks are for the open backs, I'm looking at doing a switchable design to accommodate since I have some open back IRs I'm really interested in and want the most authentic feel for those. LOL, we'll see if the switchable design was worth it in real life once I finish the build.... The good news is that the low peak and the rest of the curve can be modeled virtually independent from each other, so the additional parts aren't going to break the bank (additional lf inductor in series, additional lf capacitor in parallel, switch).

Mike, what values did you settle on? Thanks.

 

[Mike Sch]

Mike, what values did you settle on? Thanks.

Just finalized it tonight after modeling with LTSpice, doing a "breadboard" build (actually just the parts laid out on a small piece of plywood and soldered together), then doing iterative REW tests using a power amplifier with both the circuit and the actual cab I want to emulate (so I didn't get any inconsistencies in measurements due to re-setting up and re-calibrating), and adjusting a few values for reality vs LTSpice.

That said, I built the Aiken "more accurate" version as seen on his page, but used my own values. So, referring to his schematic here are my final values for an 8 ohm load and 150WRMS design target (meaning for use with a 100W amp):

R1 6 ohm 200W (Arcol)
R2 33 ohm 50W (Arcol)
L1 .7mH / .6 ohm / 200W (Dayton 20awg)
R3 3.3 ohm 50W (Arcol)
L2 1.2mH / .82 ohm / 200W (Dayton 20awg)
L3 12mH implemented as 5.6mH / .499 ohm / 250W (EARSE IQ series 18awg) + 6.5mH / .535 ohm / 250W (EARSE IQ series 18awg) in series
C1 182uF implemented as 2x 91uF (EARSE Pulse X) in parallel
R4 150 ohm 15W (Arcol)

These values result in considerably over-specking for a 200WRMS design target on a modest budget. IMO, the caps are not the place to cut corners on this design but the inductors don't need to be top of the line, hence the choices I made.

A very important note about L3 where I used 2 inductors in series: It is critical to orient them at 90 degrees to each other if they are anywhere close to each other, otherwise mutual inductance will totally ruin the design. In fact if you lay them side by side, they create an air coupled transformer and you will blow fuses/tubes in the amp. (note they can be placed end to end as long as they are wired in phase and the air gap between them is adjusted to get the desired inductance value)

I saved a few bucks doing it this way since EARSE only sells 12mH in the expensive Super Q, which is total overkill for this project IMO (though can't fault spending more just to avoid the issues described). That said, based on the measured inductance of the 5.6 + 6.5 when physically butted together in series and in phase, I got a total of about 22.4mH due to additive mutual inductance (coupling factor of about .85). So in theory you could use a pair of very inexpensive ERSE 3.3mH 19awg butted together to get the desired 12mH (actually calcs to 12.2mH) and avoid need for spacing with 90 degree orientation.

Note the same principle applies for L1 and L2, if they are anywhere close to each other they need to be oriented at 90 degrees to each other to minimize mutual inductance.

As for doing a pair of 91uF instead of a single 180uF, it was purely price and availability, though it also doubles the current handling capacity.

When all is said and done, all that the "more accurate" accomplished for me after modelling probably 6 hrs in LTSpice was to move the tracking error (actual cab vs load box) into the 4KHz-10KHz range instead of the 300Hz-4K range. The idea being the 300Hz-4KHz range is the most important for electric guitar. That said, I saw a post buried in here where someone said they have both the simple version and the "more accurate" and report very little difference in use. Meaning just build the simple one IMO.

If you do want to tweak, the equivalent of L3, C1, R4 in the simple version can be modeled in Spice to place the LF peak where you want it (my cabs measured about 75-80Hz for open back 1x12, about 104-108Hz for 2x12 closed, about 115-118Hz for 4x12 closed). Note that the rest of the trace is very similar for the cabs. Fortunately you can just change L3 to place the LF peak and then change R4 to set how high the peak is, and it doesn't mess up the rest of the design.

The idea behind multiple inductors is to be able to switch them in and out order to place the LF peak for the various cabs. LOL, I'm building it that way, but it will be interesting to see if there is enough difference to have made it worthwhile. I'll report back when I get all of this stuffed into the chassis and try it out...

 

[Mike Sch]

EARSE only sells 12mH in the expensive Super Q

Looks like I was wrong, just rechecked EARSE site and they have 12mH in the 19awg IQ series p/n ELC54-19-12000 available in qty 1 for $6.62. It specs at .948Ω - 250w making it a great choice for this application IMO. I wish I would have seen it before I purchased...they didn't have it in 18awg and I assumed they also didn't have it in 19awg - wrong.

BTW, modelling in LTSpice shows the identical impedance curve when using a Super Q and damper resistor (R4 in Aikens "more accurate") or using the cheaper version and appropriate damper resistor value to compensate for higher internal resistance of the cheaper IQ series part.

Also, its possible that a less expensive capacitor might be suitable as long as its low ESR and high ripple current and at least 100 volt.

At some point I'll probably build a min cost version for 100WRMS design target (meaning for use with a 60W amp) just to see if it performs any different than with higher spec parts - a quick guestimate says it should be doable for about $50 plus chasis. i suspect performance will be very similar, but we'll see...

 

[7thString]

Wow epic thread. Thanks to everyone for all the info. Contemplating a build now... I can build pedals fine but this whole higher wattage / heat dissipation thing is new and interesting to me. If anyone wants to chat IRs I'd be happy to help, and have a 1960TV cab inbound to make some new ones with. Always found the ownhammer / celestion IR offerings a bit too dark, personally.

 

[Mike Sch]

Contemplating a build now... I can build pedals fine but this whole higher wattage / heat dissipation thing is new and interesting to me.

I'm just wrapping mine up, and doing the "more accurate" version is a total PIA build wise. The reason is that you want to have each of the inductive elements mounted at 90 degrees to the others in order to minimize mutual inductance, which for this project is a big deal. Obviously the inductors themselves, but also the power resistors if not using the more expensive non-inductive parts.

All to say, I'd recommend building Freeman's recipe for the simple version and call it a day. Someone in the thread says they have both versions and the differences are slight when playing them.

So back to mounting inductive elements at 90 degrees to each other, with the simple version you have three inductive elements, the two inductors, and as a group the wound power resistors. You also have the X, Y, Z axis to mount the three elements and spacing of them is not so critical (though more spacing the better). Add the extra elements of the "more accurate" version and you still only have X, Y, Z axis to work with but you have 4 inductive elements, therefore spacing at least two of them becomes a big deal.

Try to implement switchable main inductors in order to move the LF peak, and the physical layout becomes pretty hairy. I had to resort to iterative tuning of the physical spacing of the LF inductors placed on axis in order to leverage mutual inductance. Plus had to do the layout to get enough physical space between the two HF inductors placed on the same axis in order to keep the entire layout in compliance with the three axis principle. Total PIA for very little improvements. So build the Freeman recipe IMO.

That said, I would not skimp on the cap(s) for this design though I have found the lower priced 20awg inductors to be quite satisfactory even for 150wrms (sine) designs. If you go with lower priced inductors, be sure to model the design in SPICE in order to adjust the damping resistor values to compensate for the differences in internal resistance between the premium parts and the lower priced parts. If you need/want help with that, just post your inductor specs and I can model it for you and give you the revised damper resister values....

If anyone wants to chat IRs I'd be happy to help, and have a 1960TV cab inbound to make some new ones with. Always found the ownhammer / celestion IR offerings a bit too dark, personally.

Personally I'm liking the latest from Own Hammer. I'll be shooting IRs of my homebrew 2x12 (a 1936 but 1" deeper) loaded with G12T75 in a couple of weeks. But I'm not holding my breath after hearing all of the DIY IRs floating around the net. There is an art to get something usable and LOL, my guess is I will be way off the mark even though I have great gear to capture the impulse... That said, would be happy to trade IRs with you and we can see how we did!