Frequency Response of Amplifier Tone Controls

[pfsmith0]

There was a recent (now deleted) thread discussing what the Presence Control does and how it may enhance the sound. In an effort to add something productive to the discussion and to document how the various tone controls operate, I took my HD500 into the lab and measured the Frequency Response of various amplifiers. You can find the full report here (3M big, too big to attach here). 

 

A couple of points to note are mentioned in the front page of the summary as follows:

 

Test setup: APx525 -> Guitar Input -> HD500 -> SPDIF Output -> APx525

Input level: -52dBV. This is very small but was required to keep the high gain amps in their linear region. SNR was still better than 60dB.

HD500 settings:

• Inputs: Guitar/Variax
• Guitar: normal
• Channel Volume = 100%
• E.R. = 0%
• Drive level adjusted to give the best THD+N (usually below 0.2%) for the cleanest looking graphs
• No cabs (and therefore no mics, no Cab DEP parameters)

Amp DEP settings:

• Master Volume = 100%
• Hum = 0%
• All other controls (Sag, Bias, Bias X): default values (50%)

Tone controls were set to give the flattest reasonable response. As one control was varied, the others were kept at this “flat†setting. This doesn't represent a setting that necessarily "sounds good" but it provides a good starting point for comparisons.

 

Also, the low level input allows the underlying frequency response of the amps to be seen. For larger signals, like guitar level signals, distortion terms will become more noticeable and add more high frequency content, perhaps making the amp sound different from what you may expect by looking at these graphs. So, use these just as a guide. Or use them to figure out what the controls do. Some are cuts, some are boosts, some are both. Some operate over a much wider frequency range than you'd expect. And now you can see what the FAC control does!

 

Finally, I did not measure the effects of how these controls may effect the dynamic response of the amp. Adjusting them might effect the "feel" of the amp. I do not know. These measurements will not tell us. Enjoy, I hope you find them useful.

-----------------------------------------------

Edited Jan18, 2018 to update Dropbox link.

[cruisinon2]

There was a recent (now deleted) thread discussing what the Presence Control does...

They deleted it? Lol...nannies

[CipherHost]

Thank you pfsmith0.

[mcbeddall]

Nice one smithy

 

Why did it get deleted ?

Couldn't they have just removed the offensive parts?

 

It was one of the most helpful threads I've ever read, I was thinking they should have condensed it and pinned it

[duncann]

Thanks pfsmith. Very interesting.
 
p.s. "They" didn't delete anything:

 

Hi friend aerosol_d7,
 
even if personally I'm still enjoying the results of the "presence" trick, I deleted my thread.. I was still not 100% sure about the validity of my theories, so it seemed right to not divulge them for too long before having a more consolidated evidence..
 
I admit that sometimes for a limited time I use the forum as a testing laboratory, or as a mind extension (although a few times it turns out to be more like a mind reduction.. ah ah..) if you prefer, ie I propose publicly some of my ideas and if I see that they get a really good level of approval then I realize that I'm probably on the right track, and I go ahead..
 
however I am very interested in the upcoming results of pfsmith0 tests and measurements, I believe that he will open a new thread dedicated to the topic, and I'm sure it will be really interesting..
 
so our personal "presence" and feedback "resonance" will be further useful over there

[MartinDorr]

Very interresting to see these graphs and the surprising variation in how some of the Amp models behave to tone stack changes.

Great effort and thanks a bunch to get this kind of insight!

If I interpret the measuring conditions right this would apply to clean tones given that THD was minimized using Drive control.

The question for me would be what, if any, differences to expect when Drive is adjusted to something that just starts to break up or when in full distortion territory?

Does anyone out there has experience in that regard?

[aerosol_d7]

Nice work, pfsmith

we were waiting for It, now, DO NOT delete it!!!!

 

:p

[aerosol_d7]

The presence of Brit J45s and plexi leads don't seem to behave as expected....

 

Do you have measurements for the others? For ex: treadplate (the one I use most)

[Brazzy]

Nice Thread pfsmith0. Your threads are always interesting.

[pfsmith0]

MartinDorr, you are correct that these graphs/measurements are mostly valid for clean tones. If you increase the drive control then, if I'm reading the companion thread correctly, the bass, mid, treble controls shape the signal before the drive circuitry while presence shapes the signal afterwards. They will have different effects and will sound different.

[duncann]

I'm being off topic here.. sorry!

 

I miss an info that I would really like to know:

how did you post the above link which shows just a clear and short title instead of the full and long url address?

 

One way is to highlight the phrase you want containing the link, and then use the link button.

[MartinDorr]

MartinDorr, you are correct that these graphs/measurements are mostly valid for clean tones. If you increase the drive control then, if I'm reading the companion thread correctly, the bass, mid, treble controls shape the signal before the drive circuitry while presence shapes the signal afterwards. They will have different effects and will sound different.

 

Yes, and considering how Presence 'normally' works in a tube amp, one would think that it is a power amp model function.

That does not seem to be the case though in the HD because Presence also impacts tone when you select a preamp only model.

My memory may be off on the later (have not used preamps for  while now).

[duncann]

I suppose that by saying "highlight the phrase you want containing the link " you mean using the windows "copy" function after having highlighted the full url,..

 

however if I press the link button it opens a window where I can only write or paste the url, and I can't specify any title that will be show instead of the full url

 

No. Copy the url like you normally would, then highlight the words in your post you want the link to refer to, then hit the link button and paste the url in there.

[pfsmith0]

I thought about including preamp measurements but there was just too much data to be taken. And I didn't even get to ALL the amps.

[MartinDorr]

I thought about including preamp measurements but there was just too much data to be taken. And I didn't even get to ALL the amps.

 

I did not intend to request any preamp measurements. Just wondering and thinking loudly what L6 may have thought making Presence also available for preamps considering that Presence seems to be a negative feedback loop in the power amp circuitry (in tube amps that actually have it).

Besides guessing what Presence normally does, i am perfectly happy with having another dial to get a better tone independent of 'right' or 'wrong' ;-)

Thanks again for the trouble getting some of the hard evidence on how these models and their dials behave.

Martin

[BBD_123]

@pfsmith

 

Sorry - only just seen this thread - thanks for the analysis! I will look at the full pdf properly this weekend, but at first pass, the results appear to contradict Hurghanico's hypothesis that the presence control is *not* effectively a treble control.

 

@Hurghanico - please do not delete entire threads. Everybody's contributions are lost. Just edit your own comments if need be :-)

[BBD_123]

my hypothesis was not that the presence control is *not* effectively a treble control..

 

my hypothesis instead was that the "presence" doesn't affect (by adding or removing) frequencies beyond those already controlled by the treble control, and its influence spans more or less all the frequency range of the bass mid and treble controls of a tube amp..

 

 

Ah, okay, I see what you mean (reading your new post has reminded me what you said originally). However - I think it's pushing it to say that the analysis shows that the presence control 'spans more or less all the frequency range of the bass [control]'. Mid and treble, yes. Bass, not so much.

[BBD_123]

not so much pushing it in my original words, in fact in one of my previous posts regarding the "presence" influence on the other controls response, I said "with more emphasis going towards the high mids and treble"

 

Okay, but I can only respond to what you write, and that contradicts what you wrote above.

[BBD_123]

I don't want to argue about this because it's not worth the effort :-)

 

But... I do think that you are slightly over-stating the importance of the presence control as a fundamental of tone shaping. I *do* agree that it is very important and perhaps both misunderstood and under-used. But I don't go all the way with you. I also agree with other commenters here that resonance DEP is an important part of some tones and simply zeroing it is to limit what you can do.

 

I'm 65% with you :-)

[BBD_123]

My recollection from the thread that you have unfortunately now deleted is that you said something like: for great tones, max presence and zero cab resonance DEP. Perhaps I misunderstood you, or perhaps you are reconsidering what you actually wrote. Regrettably, it is now impossible to tell.

[DunedinDragon]

I can't say I can recall every nuance of the conversation, and honestly it's not all that important.  What I do recall is Hugh's initial discovery of the relationship between resonance and the presence control motivated many of us to examine it for ourselves and provide feedback with what we discovered.  And I have to say it was a significant finding as it lead to a number of improvements in how I now develop my patches.

 

I think his summary which he then posted later is a very accurate accounting of both his discovery along with several other inputs by others here of how those controls interact.  Ultimately a very useful exercise.

[Giorgos02]

I thought about including preamp measurements but there was just too much data to be taken. And I didn't even get to ALL the amps.

 

Thank you very much for all your wonderful work pfsmith0. 

 

I 've thought many times in the past to do something similar like measuring what each tone knob of my 2 favorite amps (FBall & Treadplate) do, and we can finally have a clue about what's going on when we tweak these EQ knobs!

 

So do you plan in measuring more amps, or you just measured some in order to give us a clue on how things work? (I'm just curious  :) )

[pfsmith0]

At some point I may add more amps, but I just don't have the time at the moment. Got a new puppy and I'm a little overwhelmed with how much time she takes.

[duncann]

...Got a new puppy and I'm a little overwhelmed with how much time she takes.

 

Ha! You don't know the half of it yet. Especially if she's hyperactive. Good luck, and most of all have fun!

[JTSC777]

I have just finished spending 2 hours with 4 of my favorite home brewed patches turning the presence control all over the place whilst playing my PRS guitar and I can tell you it has an effect on treble mid and especially the low end of the cabinet modeling if you are playing through any kind of FRFR speakers. Some amps more than others of course but it is a real difference maker for sound creation.

Thanks to all on this and the other thread who have contributed so much in this area.

[EIM93]

Sorry to bump an old thread, but the dropbox link in the first post is dead, could someone reupload the whole material?

 

Thanks in advance =)

[pfsmith0]

Links have been updated.