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The Secret Weapon: Transformer Match

FractalAudio

Administrator
Fractal Audio Systems
Moderator
One of the most important "advanced tweaks" in the amp block is the Transformer Match parameter (XFRMR MATCH). This control sets the relative turns ratio of the virtual output transformer.

Each amp model has a default turns ratio embedded in the model data. The Transformer Match parameter adjusts that ratio relative to this default value. Turning it down reduces the ratio and makes the transformer "undermatched". Turning it up increases the ratio and makes the transformer "overmatched".

Why is this important? In a "classic" designed power amp the transformer turns ratio is selected to provide the full power the output tubes are capable of to the load. IOW, when the tubes are delivering their maximum current the voltage at the plates is near zero (the voltage swing is maximum) for the NOMINAL speaker impedance.

If the transformer is undermatched the tubes won't fully saturate. If the transformer is overmatched the tubes will saturate early. Most guitar amps are slightly undermatched.

This wouldn't really matter than much if the load were a simple resistance but a speaker is a reactive load. As mentioned in my other posts the impedance has a low-frequency resonance and a high-frequency boost. If the transformer were perfectly matched then the plate voltages would reach maximum excursion when the current was at a maximum only at those frequencies where the speaker impedance equals the nominal impedance. When the speaker impedance is greater than the nominal impedance the tubes saturate early. This has the effect of distorting the highs and lows before the mids.

If you decrease the matching the highs and lows don't distort as quickly and the amp will sound more "open". However the distortion can be harsher since power tube current limiting is harder than voltage limiting. If you increase the transformer matching the highs and lows will distort sooner and the amp will sound more "compressed. The resulting distortion will be smoother due to the softer nature of the voltage limiting.

As explained above a classic design selects the turns ratio to give the maximum power for the nominal speaker impedance. However the "nominal" speaker impedance is just that and the actual speaker impedance varies considerably from model to model. The impedance of a typical 8-ohm speaker can vary 20% or so. If the actual impedance is lower then this effectively undermatches the transformer and vice-versa.

Furthermore the transconductance and maximum current capability of the power tubes varies. For example, the amp models in the Axe-Fx that use EL34s were modeled with Mullard tubes (as these are considered the best sounding). A set of JJ EL34s will actually produce slightly more current and saturate earlier. This effectively overmatches the output transformer.

Some amps deliberately overmatch the output transformer, i.e. Trainwrecks. This results in a smoother power amp distortion with more compression of the highs and lows. The amount of overmatching is considerable, typically about 50%

Now, the effect of the transformer matching is only evident when the power amp is distorting. Non-MV amps get most of the distortion from the power amp so the effects of altering this parameter should be readily apparent. Master Volume amps get most of their distortion from the preamp so the effects of altering this control may not be as noticeable if the MV is turned down.

Small adjustments can make a big difference. I typically never adjust more than 20% (0.8 to 1.2) and usually less than 10%.

If you find your tones are slightly too open and harsh turn up the Transformer Match slightly. Conversely if you find your tones too compressed turn it down a bit. Be warned that turning it down may seem to sound "better" because the volume will increase (our old friends Fletcher and Munson again) but then when you play loud it will be boomy and harsh. You want some of those high and lows to be distorted early so that things aren't too scooped.
 

boogieman75

Experienced
One of the most important "advanced tweaks" in the amp block is the Transformer Match parameter (XFRMR MATCH). This control sets the relative turns ratio of the virtual output transformer.

Each amp model has a default turns ratio embedded in the model data. The Transformer Match parameter adjusts that ratio relative to this default value. Turning it down reduces the ratio and makes the transformer "undermatched". Turning it up increases the ratio and makes the transformer "overmatched".

Why is this important? In a "classic" designed power amp the transformer turns ratio is selected to provide the full power the output tubes are capable of to the load. IOW, when the tubes are delivering their maximum current the voltage at the plates is near zero (the voltage swing is maximum) for the NOMINAL speaker impedance.

If the transformer is undermatched the tubes won't fully saturate. If the transformer is overmatched the tubes will saturate early. Most guitar amps are slightly undermatched.

This wouldn't really matter than much if the load were a simple resistance but a speaker is a reactive load. As mentioned in my other posts the impedance has a low-frequency resonance and a high-frequency boost. If the transformer were perfectly matched then the plate voltages would reach maximum excursion when the current was at a maximum only at those frequencies where the speaker impedance equals the nominal impedance. When the speaker impedance is greater than the nominal impedance the tubes saturate early. This has the effect of distorting the highs and lows before the mids.

If you decrease the matching the highs and lows don't distort as quickly and the amp will sound more "open". However the distortion can be harsher since power tube current limiting is harder than voltage limiting. If you increase the transformer matching the highs and lows will distort sooner and the amp will sound more "compressed. The resulting distortion will be smoother due to the softer nature of the voltage limiting.

As explained above a classic design selects the turns ratio to give the maximum power for the nominal speaker impedance. However the "nominal" speaker impedance is just that and the actual speaker impedance varies considerably from model to model. The impedance of a typical 8-ohm speaker can vary 20% or so. If the actual impedance is lower then this effectively undermatches the transformer and vice-versa.

Furthermore the transconductance and maximum current capability of the power tubes varies. For example, the amp models in the Axe-Fx that use EL34s were modeled with Mullard tubes (as these are considered the best sounding). A set of JJ EL34s will actually produce slightly more current and saturate earlier. This effectively overmatches the output transformer.

Some amps deliberately overmatch the output transformer, i.e. Trainwrecks. This results in a smoother power amp distortion with more compression of the highs and lows. The amount of overmatching is considerable, typically about 50%

Now, the effect of the transformer matching is only evident when the power amp is distorting. Non-MV amps get most of the distortion from the power amp so the effects of altering this parameter should be readily apparent. Master Volume amps get most of their distortion from the preamp so the effects of altering this control may not be as noticeable if the MV is turned down.

Small adjustments can make a big difference. I typically never adjust more than 20% (0.8 to 1.2) and usually less than 10%.

If you find your tones are slightly too open and harsh turn up the Transformer Match slightly. Conversely if you find your tones too compressed turn it down a bit. Be warned that turning it down may seem to sound "better" because the volume will increase (our old friends Fletcher and Munson again) but then when you play loud it will be boomy and harsh. You want some of those high and lows to be distorted early so that things aren't too scooped.

I noticed this with the tweed fender amps. Particularly with the deluxe tweed. Which sounds pissed off enough as it is! :) I love it!
 

Patzag

Fractal Fanatic
Wow. Great post and great explanation of this powerful parameter. Never used it. I'll definitely experiment!
 

lauke-lux

Power User
Thanks for these explanations. I generally undermatch fender DLR & Twin & Bassman amps to 95%, and rarely any other amps .
Should I understand though that for example the trainwreck is already 50% overmatched in it's standard implementation in Axe FX (i.e. at 1.0 for transformer match in the amp block) or is the transformer match in the amp block standard at 1.5 ? Sorry for the question but can't look on the Axe Fx as I'm 7000km from my dearest guitar engine.
 

Slammin Mofo

Inspired
Thanks for these tech notes, Cliff! I just love that you take some time out of your busy schedule to impart useful theoretical knowledge to us. It's much appreciated!

A question that popped up in mind as I was reading the very last part of your post was, if there is a possibility of modelling an "ideal" transformer match function that makes up for the volume lost/gained in the process of altering the relative turn ratio?

I know that the saturation switch uses two modes: authentic, which allows for a drop in volume, just like in the real world, and the ideal mode, which compensates the volume to the level it was before the engagement of the saturation switch. But in this sat switch example, it's all about a fixed network of zener diodes (arredondo mod), if I remember correctly, so that particular implementation is maybe much easier than the (potentially impossible) transformer match volume compensation.

Again, it's just a thought, maybe even an impossible one at that, and I'm fully aware that there are a thousand more pressing things to do than to care about volume compensations that can be done manually, anyway.

I'm happy as it is! So many amazing and effective tone shaping switches and knobs are at our disposal, and I'm often surprised at how powerful they really are at times. it's truly a great time to be a guitar player!!
 

Stratoblaster

Fractal Fanatic
This parameter is one of the ones I almost always tweak for a finishing touch to get the amp character and feel exactly where I want it; a little adjustment either way is all it takes for sure. This control has been a go-to control for me since it first was introduced. There is a reason why a lot of amp makers spec there own transformers in their designs; it's a big part of the amps' ecosystem and it's immensely useful to be able to customize this component in the AFX. Appreciate the write up...
 
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Krisz

Member
Great to hear some of the details behind why and how this powerful control does its magic. I instinctively set it to around 0.9 for a more open sound - now I know why...
 

cyberk9

Inspired
I personally use the dog crap out of the Transformer Match parameter ..the more to the left I go the more Metallic the amp starts to sound to me...I like it to sound Chug Chug with some metallic thrown in...Some of my amps are as far down as .411..........without the KABOOM or smoke of blowin something up...cool...
 

Pwrmac7600

Power User
So I was looking around in axe edit trying to find this parameter and I don't know if I am missing it, but I can not locate the transformer match parameter to be able to adjust it. Is this accessible through axe edit?
 

Rex

Legend!
So I was looking around in axe edit trying to find this parameter and I don't know if I am missing it, but I can not locate the transformer match parameter to be able to adjust it. Is this accessible through axe edit?
Look for XFMR Match.
 
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