Tone Matching just the Amp, not the Speaker?

Bill Piersall

Bill Piersall

Experienced
Going this week to a buddy's house to tone match a bunch of cool amps. I know this has been asked somewhere before, but my search didn't bring up an answer for me. How can I tone match an amp without matching the speaker as well? If I mic the cab, the TM would include the speaker, and I'm guessing you would then use it without a Cab block. If I want just the Amp, and the ability to add different cabs to the chain, what is the best way to do this? I used to own a THD Hotplate, thinking I should have kept it to use as a dummy load and used it's Line Out to do the TM? I know most of his amps do not have EFX loops, which would be pre Power Amp anyway, and none have Slave outs. How do Cliff and Co. match amps when they do it? Want to get it right because he has a killer Somatone, a Tophat, a few different Engl's, Victoria and a Redplate on the way, some stuff I would love to add to my rig. He also has a killer array of pedals, wondering if I'm using just drive pedals and loving the tone, can that be matched as well?
 
An attenuator or dummy load with a direct out is how you match just the amp, put a Cab block after the TMA block for monitoring the reference amp and Tone Match through the same IR. Being able to change cabs on a Tone Matched amp is really cool.
 
MaxTwang said:
An attenuator or dummy load with a direct out is how you match just the amp, put a Cab block after the TMA block for monitoring the reference amp and Tone Match through the same IR. Being able to change cabs on a Tone Matched amp is really cool.
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Looks like I need to get my Hotplate back, lol.

AlbertA said:
http://fractalaudio.com/downloads/manuals/axe-fx-2/Axe-Fx-II-Tone-Match-Manual.pdf
http://fractalaudio.com/downloads/manuals/axe-fx-2/Axe-Fx-II-ir-capture.pdf
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Thanks, I have read these and understand how to tone match program material and shoot IR's of just a cab. I guess where I'm confused is in trying to isolate the amp in a TM. If you mic a combo amp or head/cab, the TM would include the speakers contribution to the sound. You don't really have the option to use different cabs with that TM then.

Thanks for all the input guys.
 
If you are handy with electronics you can just build a voltage divider in parallel to the speaker load to get a line level signal.
 
Your not really going to get an "amp" match doing all that stuff. If tone matching worked for the actual amp models no one would own an axe fx. Matching the "amp" is something fractal does and it seems proprietary.
 
An IR is simply an eq curve. Shooting an amp only match with eq alone wont sound like an amp, it'll sound like a gtr DI with an eq on it.
 
toolfanem said:
Your not really going to get an "amp" match doing all that stuff. If tone matching worked for the actual amp models no one would own an axe fx. Matching the "amp" is something fractal does and it seems proprietary.
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Of course. It's important to point out that Tone match will only match the linear response using an existing modeled amp that ideally has similar topology to the amp that he's trying to match.

The user will have to match the drive/gain characteristics on their own which is all explained here:
http://www.fractalaudio.com/downloads/manuals/axe-fx-2/Amp Matching Tutorial.pdf
 
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toolfanem said:
An IR is simply an eq curve. Shooting an amp only match with eq alone wont sound like an amp, it'll sound like a gtr DI with an eq on it.
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I assume the OP knows this. The intention is to match the linear response of the physical amp and the modeled amp.
 
Im interested in doing this to capture the tone of my other guitarists amp and then make an ir of his cab so we can have a sound as close as possible for tracking purposes with the Axe-fx. using this method would in theory work? Or am I completely mistaken?
 
AlbertA said:
I assume the OP knows this. The intention is to match the linear response of the physical amp and the modeled amp.
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I know on it's own, the Amp match won't sound great, same with the most amp models we have from the factory. When you add the Cab IR, it should sound like the Amp matched, going through the Cab IR, at least in theory. Have any of you TM'd an amp using a mic in front of it's speaker? If so, wouldn't running a Cab block be the same as running the entire signal through the 2nd Cab? Wouldn't you lose the ability to try different Cab IR's?
 
IAMLORDVADER said:
Behringer: ULTRA-G GI100

Would this get the job done safely? getting the sound of the head directly into the Axe without blowing both?
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No
You can tell by the look of it really: it does not contain a proper dummy load - boxes that do are bigger like the THD hotplate, etc
Also you can see there's no impedance spec or switch. You use these units inline with a real cab typically, or with a separate dummy load if you make one (e.g. A big resistor with cooling)
I checked the Behringer manual to make sure I am not wrong. See http://www.behringer.com/assets/GI100_P0137_M_EN.pdf
 
AlbertA said:
If you are handy with electronics you can just build a voltage divider in parallel to the speaker load to get a line level signal.
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I saw a link from the Wiki page where Cliff had described the same type of Voltage Divider you suggest. I had posted there, but it's such an old post I don't think it's being monitored. Cliff's post was talking about ways to separate the Amp and Cab and the use of Load Boxes vs. Resistive Dividers

I cut and pasted the questions I had for this thread.



FractalAudio said:
Exactly!

The amps in V6 were matched this way. The speaker was not part of the equation. The benefit to this technique is that the cabinet is completely separated. "Profiling" lumps the speaker IR with the amplifier output IR since linear functions are not separable. That's why those products don't work well when you try to run into a power amp and conventional guitar cab.

You don't really want to use a load box though, if you can help it. A real speaker is a much more realistic load. All you need is a DI box that can handle speaker level signals. You don't even really need a DI box. All you need is a resistive divider to knock the voltage down to line level. I have a simple little box that just has some resistors and a pot to reduce the level which I then send to Input 2.
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You seem to indicate that the Resistive Divider Box you use might work better than say the THD Hotplate, or other attenuator/load boxes. Could you shed some light on why this would be?

Also, does the Resistive Divider Box have to be able to handle the full speaker out load from the amp, or because it's a tap off the speaker and the speaker is left intact, is it less of a load on the box?

You mention a DI Box that can handle a speaker level signals, how does this differ from something like the THD Hotplate? Who makes a high quality DI Box that would function the way your Resistive Divider Box does?
The reason I ask is I'd like to build a box like yours, but have no idea how to go about it. I want to make sure I use resistors with a high enough rating that I won't burn them up or possibly damage the amp. Is there any way you could post a wiring diagram and picture of yours, or possibly a web link if it's available online?

I have the opportunity to match a friends great collection of amps, but I want to be sure I do it in a way that captures the tone the best way possible, and don't want to damage any of his amps in the process! Got all my money tied up in the Axe Fx 2, MFC, and RCF speaker etc, lol. Nothing left to cover any damages. :D

What method would I use to achieve results closest to what is used for the Factory Amp Models?

I was digging on the net and I came across this design:
Vintage Amps Bulletin Board • View topic - Dave's Inductive Dummy Load for Practice and Recordng

Any thoughts? Would this be worth trying, or be any better or worse than the THD Hotplate or the Resistive Divider Box you suggest? With real speakers instead of resistors for the load, would the amp react better than using fixed resistor values?

Sorry for all the newbie questions, but I'd love to end up with some really high quality Cab Ir's and Amp Match blocks that would be worth sharing with the forum, so any input would really be appreciated!


Thanks







Thanks
 
Bill Piersall said:
You seem to indicate that the Resistive Divider Box you use might work better than say the THD Hotplate, or other attenuator/load boxes. Could you shed some light on why this would be?
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A guitar amplifier's frequency response can change (sometimes dramatically depending on its topology) based on the load it has to drive. A speaker presents a complex impendance to the amp, which varies with frequency. You can simulate a speaker load by using passive components (LRC) which is what the THD hotplate and similar devices do; think of it as a generic impedance - it won't exactly match the impedance curve of your specific speaker cabinet. A dummy load (or resistive only load) will present a constant impedance to the amp at all frequencies which is not representative of a speaker at all. So if the goal is accuracy, there is nothing more accurate than using the actual speaker as the load.

Bill Piersall said:
Also, does the Resistive Divider Box have to be able to handle the full speaker out load from the amp, or because it's a tap off the speaker and the speaker is left intact, is it less of a load on the box?
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No, the voltage divider is just a measurement probe, so it will consume an insignificant amount of power compared to the speaker.

Bill Piersall said:
You mention a DI Box that can handle a speaker level signals, how does this differ from something like the THD Hotplate?
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Explained above. THD hotplates and dummy load are designed to take a brunt of the amp output to lower the power seen by the speaker and therefore lower its acoustic output. The DI Box in question is just a measurement probe.

Bill Piersall said:
Who makes a high quality DI Box that would function the way your Resistive Divider Box does?
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I don't know of any. You can easily put one together with a couple of resistors and some alligator clips :) Or you can get fancy and make a box with jacks and a knob :)

Bill Piersall said:
The reason I ask is I'd like to build a box like yours, but have no idea how to go about it. I want to make sure I use resistors with a high enough rating that I won't burn them up or possibly damage the amp. Is there any way you could post a wiring diagram and picture of yours, or possibly a web link if it's available online?
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You won't need high power resistors as it will just be a probe.
Say the output voltage of the amp is say 50V max, then a 41k/1k voltage divider should be enough (50/41 = 1.2V which is around +4dBU).
The power dissipated by your probe will be: P = V^2/R = (50V)^2/42K Ohm = 60mW - so standard 1/8W resistors are all you need.

The wiring diagram is the simplest circuit you could imagine (R1 = 41K, R2 = 1K)
Resistive_divider.png


If you want to get fancy you can put a potentiometer in addition to R1 to get control over the output voltage.

Bill Piersall said:
What method would I use to achieve results closest to what is used for the Factory Amp Models?
Click to expand...

Look at all the PDFs from Fractal (posted in this tread and sticky's in the forum as well as the video from Cliff).
 
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