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Amp/Pre Frequency Response graphs and data


pfsmith0
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I took my HD500 back into the lab to measure the frequency response of the various amps and pre's and have attached the procedure and results below in the message. If you want all 450k of data you can find it here.

 

My goal is to measure the Cab DEP parameters but first I need to find a flat, neutral amp/pre to use. From the data I've collected it looks like the Class A-15 full may be a nice first try.

 

A couple of interesting things to note:

  1. The "No Amp" measurement shows just how wideband and flat the raw HD500 is. Very nice.
  2. You can clearly see the amps that have most of their gain in the preamp stages vs the output stages. For example, looking at the 3 Solo-100 models you'll see that the full model has about the same gain thruout while the pre model gain increases as you go from clean -> crunch -> overdrive. But something like the Brit-45 has lots of gain in the power amp stages.
  3. All the tone control knobs were set to 50% which, after looking at these responses, is probably not the ideal setting for what I was looking for. The "lumpiness" of some of the responses is most likely due to too much bass and treble (not saying it sounds good or bad, but just that it's not very flat).
  4. In many of the amps, the MID control is a midrange cut, not boost.
  5. Pre's can have a substantially different response than the full model. For example, the Divide 9/15 Pre could make a pretty good bass amp. Actually, it would make a pretty good acoustic amp as well. Don;t discount the pre's. They provide a whole 'nother set of sounds.
  6. Quite a few of them have their own "thump" responance in the 80-100 Hz region (to let those low open E-G notes ring out).

 

I also indirectly measured the distortion of the models by looking at how small the signal needed to be, or how high the Drive control could be set before hitting 1% THD. I arbitrarily used that as a threshold to indicate a "linear system" so that frequency response could be easily measured. Using that metric I grouped the amps/pre's into categories of low, medium, and high gain. The results are probably no surprised to most of you. Note, however, the word "gain" is used to signify the amount of distortion, not gain per se. An amp can have more gain and stay linear (and therefore louder) than an amp that heavily distorts the signal. It's all a matter of headroom. But I'll use the word "gain" here to mean more gain = more heavily distorted as is common amongst guitar folks.

 

I will start another thread showing the frequency response of the various cabs with an amp. Then another showing the various mics with a cab and amp. Then another showing what the cab DEP parameters are doing. it'll take a little while so be patient. In the meantime, enjoy.

 

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

Sorry, the PDF file slightly too big to attach. You'll find it here.

 

25May 2017: Edited to update Dropbox links

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I measured the EQs last year. You'll find the info here.

 

The first page of the pdf file describes a little of how I measured it. The APx525 (by Audio Precision) is a piece of high performance audio test equipment. It'll measure all sorts of things - THD, SNR, gain, levels, frequency/phase response, etc. It has balanced analog, unbalanced analog, serial data, AES/SPDIF inputs and outputs. For this I plugged the unbalanced output into the Guitar Input of the HD500 (set to normal) and then fed the SPDIF output of the HD500 back into the test equipment. That way I only go thru one set of converters to get a better idea of the raw performance of the HD500. Inputs are set to Guitar/Variax (to reduce noise) and the mixer is set to everything centered at 0 dB. I can then setup whatever signal chain I want, drive it to whatever frequency/level I want, and measure what's coming out. Fairly simple, but tedious.

 

Measuring amps are a little tricky because distortion, which is desirable for guitar amps, creates problems getting accurate frequency response. So I had to reduce the input to quite a low level to keep the system non-distorting. Then, what level do I set all the knobs? I picked these:

  • Channel Volume = 100%
  • E.R. = 0% (early reflections - reflections can change the response based on how long it takes to sweep the frequency range, which doesn't reflect amp performance at all - better to just turn it off)
  • Tone controls - set all to 50%, mid-level seemed like a good idea at the time
  • Drive - raise it until I got 1% THD
  • Amp DEP - hum = 0% the rest leave at default settings

That'd about it.

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Great work! It seems like more an academic exercise than anything else. I mean how do these numbers and graphs help someone get a good tone from their rig? I played a $4000 Ibanez Evo today. It sounded Horrible! I don't know how Vai does it with that thing! I definitely didn't need a graph and a bunch of numbers to determine that the EVO is not for me. So far, my ears and my emotions tell me everything I ever need to know about my gear and my tone.

 

Can you help me understand why these numbers and graphs are important and how they are helpful? How do you use his info to help you with your tone?

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Well, you have to be armed with some knowledge first. 2nd order distortion terms are warm sounding while 3rd order terms are harsher and more in your face. The list will guide you to pick which ones will give you what you want. In general, power amp gain sounds harsher than preamp gain, so the list can help you pick which amp saturates in which way (by comparing the Full and Pre gain and THD). If you want a bright sound then pick an amp that's naturally bright like the Super O or Class A-30 TB. There are quite amps suitable for bass besides the Flip Top. Try the  Divide 9/15 Pre or PhD Motorway Pre. Who would have thought the Hiway 100 Pre is one of the cleanest amps in there? It was enlightening to me to discover the Mid control on the Blackface is a midrange cut. I otherwise wouldn't know how to use it appropriately.

 

You can certainly audition all the amps and pre's until you find one suitable but this list can guide you to focus on the few that lend themselves to what you're looking for. It saves time. It helps reduce the learning curve.

 

That's about all I can think of. Power amp gain vs preamp gain. Knowing which are naturally bright or have an extended frequency response. Which are clean. Which are dirty. Not only that, but they can be ranked as to how dirty they are. It's much more than academic to me. It will help me pick the amp suitable for the occasion without spending an inordinate amount of time sifting thru them all.

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 That's about all I can think of. Power amp gain vs preamp gain. Knowing which are naturally bright or have an extended frequency response. Which are clean. Which are dirty. Not only that, but they can be ranked as to how dirty they are. It's much more than academic to me. It will help me pick the amp suitable for the occasion without spending an inordinate amount of time sifting thru them all.

oh....I am not attempting to be obtuse or contrary but I have to ask, you need a graph and numbers to determine what amp is clean and which one is dirty? A graph and numbers to pick an amp suitable for the occasion? It seems like these graphs and numbers would be helpful to a deaf person who is dialing in a great tone, but if you have working ears....I don't get it. But hey, if it works for you, get on down!

 

I know that music can be boiled down to math and frequencies and frequency responses and Impulse responses (I just learned about that one!) but to me, music is about touch, about emotion, it's intangible nostalgia. You close your eyes crank it up and when it's good you start to feel like you are floating. You FEEL good tone, you don't graph it.

 

Rather I should say I feel good tone, and I won't be graphing it at this point. But I am always willing to get my learn on about ways others ar able to pimp their tone.

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It was enlightening to me to discover the Mid control on the Blackface is a midrange cut. I otherwise wouldn't know how to use it appropriately.

 

That is one of those things missing from the current HD manuals and documented on the old ones

 

http://line6.com/support/manuals/podxt/PODxt Manual - Version 2 - English ( Rev H ).pdf

 

 

One of the interesting things about the Bassman is just how interactive the Middle and Treble controls are.

The Middle control isn't a bandpass, as in most tone control setups. Instead, it's almost like a second

treble control. The two are additive, so if you're running PODxt's Middle knob higher

than halfway up with this model, you'll find that the Treble control might give you more

bright than you really want. On the other hand, when you turn the Middle knob down,

you'll probably want to boost the Treble. The Bassman, like many of the amps modeled

 

and talking about the Blackface Lux

 

The Deluxe itself has only Bass and Treble controls, leaving us, once again, with the prospect of a couple

knobs with nothing to say for themselves. But fear not; in this case, we've set up PODxt's Middle knob

so you can add some post-Amp Model Midrange contouring for a little more flexibility,

while Presence adds, well, Presence. Once again, set the Middle knob to its "neutral" 12

o'clock position and the knob to 0 for the classic Deluxe sound. Tweaked up

 

and of course even on the paper manual that I got :) I wonder why those things are still not there...

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Very nice and extensive work. It's appreciated, and, as you said, there are some interresting observations in these base lines already.

You did not mention that Cab models where off, but I assume you did to keep the Cab out of the amp measurements.

Also, If i understood your description right, you dialed in Drive to get to 1% THD.

Did you do this independently for pre and full amp measurements or not, and did you by chance record the Drive value you needed?

I think it would be interesting to get those numbers of each amp model as well.

Thanks again for all the work you're putting into this.

Martin

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  • 3 weeks later...

These are interesting. The /13 is questionable at flat knobs. It is definitely the quirkiest to set up, but I find it similar to,the deluxe once it's dialed how I like it.

 

I think the mic tests may be more useful. Curious to see how they compare with manufacturer specs.

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oh....I am not attempting to be obtuse or contrary but I have to ask, you need a graph and numbers to determine what amp is clean and which one is dirty? A graph and numbers to pick an amp suitable for the occasion? It seems like these graphs and numbers would be helpful to a deaf person who is dialing in a great tone, but if you have working ears....I don't get it. But hey, if it works for you, get on down!

I know that music can be boiled down to math and frequencies and frequency responses and Impulse responses (I just learned about that one!) but to me, music is about touch, about emotion, it's intangible nostalgia. You close your eyes crank it up and when it's good you start to feel like you are floating. You FEEL good tone, you don't graph it.

Rather I should say I feel good tone, and I won't be graphing it at this point. But I am always willing to get my learn on about ways others ar able to pimp their tone.

You need a graph and numbers to figure out the EQ, that's for sure! LOL

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Back to the question the graphs and give you lots of various bits of information. You can pinpoint trouble areas in the frequency spectrum and know exactly what freq to cut or boost. You can find out if you have an EQ that is up to the task, meaning is -12dB of cut enough or do you need more.

 

I know one manufacturer that was asked why there seemed to be a high freq buzz in their amp models. A graph was done and showed a huge spike, which I believe had something to do with the littering in the power supply.

 

For most part it should really be about what sounds good to your ears. Plug in to a few pedals, into your amp, make a few adjustments, and done. However, there are some issues with the HD genre of models that warrant a deep dive to find out what is really going on. Resonance default settings at 100%, that accentuate trouble frequencies, and the Master default, which adds power amp crossover distortion are some of the issues that are finally being understood.

 

Bottom line, sit tight. If things go right all these issues will be addressed with the next big release. Of upgrade and add non-HD amps and the problems mostly go away.

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  • 2 months later...

pfsmithi0,  Great work! Thank You!

My final evaluation is always about how it sounds to me, but it is great to have some real numbers to help me understand why some patches sound different than others. Again; Thank You!

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  • 2 years later...

Interesting master-thesis about ampsimulations and it's components.

Unfortunately (for you) it is in German language, but maybe it can be translated by Google?

Otherwise the comparing graphics might be nearly self explaining on pages 38, 53, 54 which illustrates the 3 basic tests A (all knobs like Marshall, 212 cabsim), B (tweaked to match), C (cabsim off, Bogner 212)

 

https://www.hdm-stuttgart.de/~curdt/Schempp.pdf

Abstract

The goal of this study is to analyze whether digital ampsimulations are able to reproduce the sound of real analogue tube amplifiers and possibly replace them completely. They are tested with different settings in different scenarios and heard solo and within a mix of a pop-song. Furthermore the components of the whole signal chain which surround the ampsimulations are analysed and possible deficits revealed.

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