Amp through cab Vs reactive load and cab sim

Hi peeps,
I spend a lot of my time practicing at night with a Victory Kraken (50w) on low power mode and considering that its a bastard loud tube amp, it actually sounds pretty good. I wondered whether it would be worth investing in a reactive load of some sort and use an IR instead as I could do with cranking the amp a little more and I could play through headphones when the wife and kids are around.

So, my (noob) questions are;
1- is it worth it (tonally) to go down this route?
2- any suggestions on gear? (Small budget :wink:)
3- I don’t have a PC/mac/laptop to use, is it possible/worth it to do it without one?
4- should I just get a reactive attenuator like the fryette powerstation 2?

Many thanks in advance!

1 Like

Do you want to use a tube amp with zero sound? If so, you can use a load box. Especially if you’re a high gain player where the differences can be mostly dialed out via EQ if you need to.

I’ve done extensive A/B tests with this a few years back, and older non-reactive loads like the Palmer Speaker Simulator really eff up your sound. Using something like Match EQ in Logic will show you exactly what’s being removed, and also allow you to dial back in the removed portion of the frequency spectrum for a tone which is nearly indistinguishable from the original, miked / amplified cabinet. Again, this works best for high gain playing where dynamics aren’t an issue because everything is squashed anyway. So in short, you can even use a cheap non-reactive box if all you’re looking for is a good distortion tone coming back from your speakers or headphones with zero cab volume.

I haven’t tested the newer generation of load boxes like the Two Notes Torpedo and the Universal Audio OX. But if we’re expecting them to sound identical to a particular amp / cab pairing, that’s going to be tough. It’s not just the fact that the amp is plugged into a speaker. It’s the specific model of speaker that’s plugged in which creates differences in the amp’s response. If you load down your amp with a generic reactive load box, even a good one, it’s not the same same as loading it down with a 4x12 filled with greenbacks, versus a 2x12 filled with Celestion V30s, and so on. Each of those cabinets will produce a different response from the amp.

That being said… unless you’re trying to exactly duplicate the sound of a particular amp through a particular cabinet, it probably doesn’t matter that much. A good reactive load box will probably still produce a sound that’s realistic and better than what the Palmer generation of devices produced. Andy Wood uses the OX and loves it, for example.

For a deeper dive on impulse responses and cabinets, here’s a post from a few years back:

1 Like

Cheers for the reply, gives me something to chew on!

No prob. I should also mention that we DI the amp with an IR on it for all our recordings — everything you hear from us. However, we do the DI with the speaker still plugged in. So it’s still loud in the room, and the amp is still respoding as it normally does.

When you A/B the miked track versus the IR track this way, they are identical. Tiny differences in mic position from day to day produce a much more drastic difference in tone coming back than the difference between the IR and the mic track done from the same mic position.

I know this isn’t relevant for your scenario because you’re looking for quiet, but just as an illustration of why the speaker being plugged in affects tone even when you’re not actually miking the speaker. There are some audio samples of this in the article above.

It makes sense that the having the speaker connected is an important part as the act of adding an attenuator interrupts the interaction between amp and load… its electrical voodoo and I don’t even pretend to understand it… thanks again for your post Troy!

The speaker load thing is nothing but frequency-dependent gain, which is another term for EQ. It’s easily replaceable with a tuned EQ circuit.

Even with the speaker connected, you’re missing the cabinet response, which is most of what the IR is putting back. The famed ‘speaker interaction effect’ is pretty small in comparison.

This may be what you’re already saying, but when I tested this the frequency rolloff from the non reactive load was huge. A giant dip with a wide Q at around 100hz. And a similarly large drop in the treble that efffectively low-passed the entire top end. I can dig up the Logic files with the gain-normalized tests of a popular attenuator model if anyone is interested.

You can definitely add it back with EQ but the trick is knowing what to add back. The compensation knobs on the Palmer don’t come anywhere close to adding back the correct curves, for example. If you’re not A/B-ing with the miked cab I don’t know how you would know what compensation curve to dial in for the speakers in question.

I’m guessing it looks something like this, but upside down?

The general idea of the speaker impedance effect is that higher impedance in the speaker itself generates greater amplification, and the speaker has a frequency-dependent impedance curve.

This is the free resonance of the speaker mounted in the cabinet. If you shake the cabinet, this is the frequency the speaker wants to vibrate at. This motion generates current in the voice coil that opposes the motion of the magnet, which looks just like resistance to the transformer, and so generates a high-Q EQ bump at the resonant frequency, and gives a bit of extra thump.

This is due to the inductance of the voice coil. Inductors increase impedance with frequency.

Neither of these effects was intentionally designed into the amplifier, they are largely just unavoidable consequences of speaker design, but it turns out most of us like the way they sound. Engineers generally aim at fidelity rather than interesting coloration, and linear amplification was a mythical beast at the time guitar amplifiers were invented, so this was all entirely accidental.

The impedance effect looks like a big deal in the plot, but that’s often misleading because of this plot:

jc12-50f_speakerg

This a typical frequency response of a guitar speaker cabinet, on the same plot as a typical impedance curve. Take a look at the frequency axis and compare the interesting parts of the two curves. Notice that guitar speakers are not full-range transducers. They drop off pretty sharply below a few hundred Hz and above 5k. This means that much of the boost from the impedance effect is not reproduced anyway. Very often, the bulk of the resonance peak is off to the left of the corner frequency of the cabinet response, which blunts its impact considerably. Any negative feedback incorporated by the amplifier also flattens the impedance EQ effect. That’s why this is ‘tube tone’. Solid state amplifiers generally have lots of negative feedback, and tube amps generally don’t.

In practice, the overall effect is a bit more thump and a bit more sparkle, aka ‘tube tone’. It’s definitely noticeable, definitely an improvement IMO, but if it was gone, the tone would be just a little boxier, and that’s about it. It’s hard to say whether it’s turned on or not without side-by-side comparison.

The EQ effect of the speaker cabinet on the other hand, dominates the impulse response. If you’ve heard the phrase, ‘the guitar lives in the mids’, this is why. Guitar speakers are very effective high and low pass filters. What guitarists think of as ‘treble’ and ‘bass’, everyone else thinks of as ‘mids’. The range of frequencies that come out of your pickups is very broad, and if you’ve never heard or recorded the direct guitar signal without a cabinet or cabsim, you’ll have to take my word that there is way more treble than any human wants to be exposed to. If you ever forget to turn your cabsim on, believe me, you will notice.

These curves are all very similar. The peak can move up or down in frequency a little, get a little wider or narrower, taller or shorter. The high-frequency ramp can have a different slope. Sometimes the frequency range between the two drops down to the baseline, sometimes not. The sonic effect is subtle and you don’t have to get it ‘right’, you just have to like it. Pick a speaker you like and copy the curve. Speakers are often chosen to complement the amp circuit. If it’s a bassy circuit, use a less aggressive resonance peak, that sort of thing. But this is custom EQ circuit territory: it requires no controls, and needs higher-order filters than you’ll get from a TMB tone stack or whatever. You’d just use a circuit (or software) designed specifically for this purpose. Or nowadays, an IR.

I built myself a resistive dummy load and an impedance simulator circuit because I like to crank up my little Class A into power tube saturation at 3 am. So I just take a lineout from the load box into the impedance circuit, then into a tiny solid state power amp, then back into the original guitar speaker. Does it sound like the amp cranked, just quieter? I don’t know, I’ve never turned the amp up past 1 without a dummy load. But it raises the hair on the back of my neck and I like it very much. It makes me orders of magnitude happier than the Amplug or V-amp I use when traveling. Plus, if I get signed, I can replace the tiny power amp with a giant one and a wall of cabinets and go play Edinburgh without changing my setup. (I guess I’d need to start a band or something, too.)

This was probably completely OT and a total thread hijack. Feel free to delete it. Sometimes I don’t know when to shut up. Plus this is all massively oversimplified, so to anyone out there who knows what they’re talking about and is enraged right now, so sorry.

Edit: I just reread the OP, and I guess this is actually on point. My message is this, @PickingApprentice. This is definitely doable in analog. If you want to use headphones instead of a cabinet, you’ll also need a cabinet simulator. If you’re ok with playing cranked yet quietly through a guitar cab, you can skip the cabsim. If you are a DIY kinda guy with some electronics experience, you can build this yourself for less than $100 (load box, impedance sim, cab sim, solid-state amplifier, I’m happy to share my designs). If you can find an off-the-shelf solution for this (somebody must have done it by now), I’d expect it to be in the $1k-$2k range.

2 Likes

I think the Suhr reactive load was designed to match the impedance curve of a 4x12 with greenbacks, I don’t know about other manufacturer’s options.

Yep! Or right-side up depending on which way you throw the slider in Logic’s “Match EQ”. Not as peaky and closer to 100hz, which is probably the interaction of the mic / cab, but yes.

Great post. The blog article I’ve linked to above includes pretty much the exact frequency curve you’ve linked to here with the speaker impedance overlayed on the frequency response. I think we’ve all spent a lot of time looking at these. This one was the difference between white noise and the same noise fed through the cabinet / sm57 recording chain:

The only thing I’d add to your excellent overview is that I personally find what bad attentuators do to the sound to be way more noticeable than the loss of a little sparkle. Yes the speaker rolls off in the vicinity of 5k but the amp is putting out so much in that region that even if it’s at -20db or something, like it is in the Match EQ plot, it’s still super audible to me and an important part of the treble response of the amp and the weird phase cancellations inside the cabinet. When the attenuator shows up and that part of the curve is now gone, the only way to really get it back accurately is to use something like Match EQ. Again, something like the “deep / flat” and “bright / mellow” switches on the Palmer don’t really do it and were either just educated guesses or averaged approximations. I don’t know how the unit was designed.

Thanks for saying so.

From what I understand, Palmer only models the cabinet response and not the impedance response. Apparently deep vs. flat is supposed to represent open back combo vs. closed back 4x12. Some units have adjustments for British vs. American and the suchlike. I’ve never tried one.

I imagine the impact of the impedance response is both equipment-dependent (because the impedance effects are strongest at the edges of the cabinet responses, and those vary from model to model) and volume-dependent (because the impedance curves broadly mimic the Fletcher-Munson curves). My goal was good tone at low volume, so that’s how I did my AB testing, but there are lots of applications that I haven’t tested, so my observations may be skewed.

In any case, the reasonable options for non-digital use are either a reactive dummy load or resistive load plus impedance simulator EQ. The latter is easier and cheaper to DIY (inductors are a bitch), so I do that. I adapted the circuit from a Marshall patent I found on the internet. I found a few others out there as well. I’ve read that most solid-state amps include something similar, even the fairly cheap ones.

This is my slave amp. The impedance simulator (aka ‘reactance emulator’ for the google-minded) is the little daughter board at the top right. It’s about the size of a postage stamp. The other board is the amplifier itself.

Here’s the frequency response of the impedance simulator.

Awesome!!!

So this is just a tiny amp that accepts the loaded-down output of your regular amp? What’s doing the loading, another box with a giant resistor in it, feeding this one? What do you plug this into, your usual cabinet?

So you’re dialing in the EQ compensation curve with the little “simulator” circuit, and that’s giving you your bottom and treble back?

Yup. That’s all correct.

Primary amp -> resistive load -> impedance EQ -> clean power amp -> speaker cab

If the primary amp has a speaker cabinet, you can use it. Or you can use any other cabinet you want, as long as it works with your slave amp. I use the cabinet from a retired modeling amp. If you want to use a PA or computer speakers or something, you’d put a cabsim after the impedance EQ and take the output from that.

Here’s the load box.

The switch is for dummy load vs bypass to an external cabinet. The line out works in both configurations. It has a volume and a tone control. I built it a few years before I built the impedance simulator, so I wanted to add some brightness back. Now I’d leave the tone control out. I always have it set flat.

The resistor should have a power rating at least twice the rated output power of the primary amp. The primary amp is a 4 watt Vox, and I like to run it around 4 on the dial, so a 10W ceramic resistor is plenty. It’s pressed against the bottom lid for heat-sinking purposes, but it doesn’t get too hot. For a 50W primary amp, you’d want at least a 100W resistor, or four 25W ones, like the ones in the foreground. This wouldn’t necessarily require a much larger enclosure, but it would have to be heat-sinked better.

You can use the primary amplifier’s rated output impedance as a guide for the resistor value, but it’s not super crucial. Bigger will be brighter, but stay within reason. DC loads don’t resonate, so this is actually easier on the primary amp than a reactive load or cabinet, then again running your amp at 10 is harder on it than running it at 1, so moderation in all things. And watch that the resistor doesn’t overheat if you crank it up.

You probably want the slave amp to be as clean and neutral as possible, but experimenting is best. You can use a 100W Class D car stereo amplifier to push a stack of cabinets with a Fender Champ, or tame your HiWatt so you can play it in the nursery. The general idea is the same, just the size of the slave amp and the size of the power resistor vary.

My primary amp doesn’t have reverb, but you can think of the line out of the dummy load as an effects send, and I put EQ, reverb, tremolo, etc. after all the distortion. I’m essentially using my amp as a pedal.

This is all just a variation on the Garnet Herzog, in case you’d like to do some short reading. It involves ‘American Woman’, so that’s kind of fun.

1 Like