Guitar Amp Reactive Load Box

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john12ax7

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Has anyone built one? The design is straightforward,  I plan on modeling my actual 4x12. But it is a bit of a logistical issue getting appropriately rated parts.

Here is a good reference on the topic for those interested
https://www.aikenamps.com/index.php/designing-a-reactive-speaker-load-emulator
 
john12ax7 said:
Has anyone built one? The design is straightforward,  I plan on modeling my actual 4x12. But it is a bit of a logistical issue getting appropriately rated parts.
I don't understand, what's the difficulty? You may not easily find 100W resistors, but it is quite common to aseemble series/parallels arrangements.
For inductors, you can adjust them by taking a few turns off.
 
Resistors not the issue.  But the currents in the reactive components can be very high. To make something universal you are in the range of 5-10A for inductor Isat.  Also high ripple ratings for caps. Plus a suitable enclosure,  adequate heat sinking, perhaps  a fan,  etc.

A 50W design at 16 ohm is no big deal.  Part availability goes way down for 100 and 150W at 4 and 8 ohm.
 
Do you plan to use a reactive load and then a digital cabinet simulator or make the cabinet simulator analog too? In either case reactive load might not sound any better compared to resistive load. I'm assuming the plan is to also attenuate the signal and use it for direct recording or listening with headphones.
 
The plan was reactive load plus IRs, though an analog speaker sim could be interesting too. Reactive loads sound and respond better imo.
 
Heikki, I disagree. the Speaker impedance is compensated in the power amp by the feedback but tube amp does not have tons of feedback. You can clearly see the speaker impedance if you measure the frequency response of the power amp. I have measured more than 30 power amps for a project i´m working on.
I also confirm that load boxes sound different I have access to fractal, suhr and ox.

I built 8 load boxes for Matias Kupiainen from Stratovarius, he is using them on his studio.

for the low freq peak I found that Erse Super Q work well. It is very difficult to have a high and sharp enough peak, low ESR cap can help a little but I don´t think is really necessary. you can unwind some turns to get the value needed.
the material of the case is important if you use aluminum the inductance will be lowered and if you use steel it will increase. better to measure it assembled.


for High freq I use J.W. Miller that you can find at mouser. same for the power resistors.

I would avoid load boxes as OX and two notes they don´t have the low freq peak(it is just shelf and lower in freq). In a tone talk Dave Friedman and John Surh were saying that the lack of that impedance could create high currents.
 
Did you ever consider using electrical heating elements as resistors , depending on volts/wattage they are in and around the kinds of ohmage you might need , even with a 100w amp they wont heat up much . Stove top cooker elements might do or even an old fashioned electric 'bar' fire would handle 2kw peaks easy.
 
https://youtu.be/XJTMKUCB4hE?t=888 I think Alan Holdsworth did something like that.

power resistors are not that expensive and you can use thermal paste to help with the dissipation.

we burnt a couple of fractals. they use 3 2.2ohm resistor in To220 format attached to an aluminum heatsink.
other thing I don´t like is they drive the fan from the speaker signal. I think the amplifier will see a change in the load when the diodes start conducting. That fast change will produce some current spikes in the output transformer. I have no idea how big they will be but I blew the fuse of an old 1959 marshall and a friend of mine burnt the output transformer of his mesa 2C+ I don´t know the reason but both were the fractal load.

it is easy to produce feedback if you don´t use an isolation transformer in the front of the amp in case you are reamping. be sure you don´t have super sonic oscillations.
 
12afael said:
for the low freq peak I found that Erse Super Q work well.

Thanks,  these seem like a good option.  Did you ever find actual saturation levels for these?  The website is kind of vague,  saying "saturation levels up to 20A", but no data on individual parts.
 
Few years ago when I was asked to design a reactive load box and I did some testing with reactive loads. I even had loaner Two Notes Torpedo Live so I could get an idea what the speaker impulse responses sound like. My unpopular opinion at the time was that you can get just as good sounds with resistive load and some EQ. For marketing you of course want to say that high output impedance of a tube amp responds differently to a resistive load compared to reactive load, which is true but doesn't necessarily make it sound better. My opinion still is that reactive loads aren't the honey for your earballs that marketing promises.
 
john12ax7 said:
Resistors not the issue.  But the currents in the reactive components can be very high. To make something universal you are in the range of 5-10A for inductor Isat.  Also high ripple ratings for caps. Plus a suitable enclosure,  adequate heat sinking, perhaps  a fan,  etc.

A 50W design at 16 ohm is no big deal.  Part availability goes way down for 100 and 150W at 4 and 8 ohm.
I routinely design cross-overs for 1kW+ systems. Sources for inductors are many. i use exclusively film caps, cause NP lytics are no good and prone to failure.
 
On the Marshall forum there is a DIY design that people a lot.

https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/

Let us know what you think!
 
Few years ago when I was asked to design a reactive load box and I did some testing with reactive loads. I even had loaner Two Notes Torpedo Live so I could get an idea what the speaker impulse responses sound like. My unpopular opinion at the time was that you can get just as good sounds with resistive load and some EQ. For marketing you of course want to say that high output impedance of a tube amp responds differently to a resistive load compared to reactive load, which is true but doesn't necessarily make it sound better. My opinion still is that reactive loads aren't the honey for your earballs that marketing promises.

the two notes lacks of the impedance peak arround 100hz. I think it is very important. The fractal have a switch to emulate an open cab lowering the frequency of that peak. I noticed that palm muting is where you notice more the effect of that peak.
I totally agree that you can get great tones without a reactive load. It have a lot to do on how the impulse responses were recorded. if you use a tube amp probably most of the effect will be already in the impulse. if you use a solid state amp with enough feedback then the reactive load will add the missing part.

Thanks,  these seem like a good option.  Did you ever find actual saturation levels for these?  The website is kind of vague,  saying "saturation levels up to 20A", but no data on individual parts.

I don´t remember seeing it. but it is an open core, I suppose the saturation would be high.

 
abbey road d enfer said:
I routinely design cross-overs for 1kW+ systems. Sources for inductors are many. i use exclusively film caps, cause NP lytics are no good and prone to failure.

Can you recommend some good sources for inductors and caps then? I have not found any 10mH that explicitly guarantee Isat of 7A+. And film caps for 200uF are $60-80+, so interested in more economical options as well.

peterc said:
On the Marshall forum there is a DIY design that people a lot.

https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/

Let us know what you think!

Good info, thanks. It's more geared towards attenuation,  but something to consider incorporating. Having a future proof box to cover various needs would be appealing.
 
john12ax7 said:
I have not found any 10mH that explicitly guarantee Isat of 7A+.
Air core inductors simply do not saturate.

And film caps for 200uF are $60-80+, so interested in more economical options as well.
https://www.lamaisonduhautparleur.com/fr/condensateurs/1440-condensateur-mkp-pb-100-f.html
https://www.lamaisonduhautparleur.com/fr/condensateurs/771-condensateur-mkp-pb-220-f.html
I have this company (SCR) making custom 190uF.
 
An update,  found a good deal on a Suhr box so picked that up as it is essentially a clone of the basic design. Still plan to my build my own with some extra features but this gets me up and running immediately.  Spent a good bit of time experimenting with mic pres and IRs finally getting something close to a miced cab.  A step up for recording vs a resistive load.

Here is another thread I found for others who are interested
https://www.thegearpage.net/board/index.php?threads/aikens-reactive-dummy-load.1072793/
 
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