Blues Jr. -- Billm Mods, Preparation -1

Posted by Jim Black on 1/16/2015 to Mods
Frequency response is what it is all about. Before I rip into the Fender Blues Jr., I want to document the frequency response from the factory.
I have decent ears and I can hear subtle differences; however, I can't do an A-B comparison on this mod. Only 250 of the Legacy Blues Jr were made. Even if I could find one for sale, I'm not sure I want to own two.
Thankfully, there are reasonably priced tools just for this purpose. Let's have a look on how to use them.
It takes software and hardware to take these measurements. You'll need a generator/measurement software package, a calibrated mic, a decent audio interface and your windows PC.
I found a good software package online called TrueRTA. It has more than what I need for this test. Parts express has a nice selection of calibrated mics for decent prices. I picked the Dayton EMM-6. You can see it at the bottom of the photo to the right. The mic comes with a downloadable calibration file that the TrueRTA uses.
The audio interface I used was a Roland Audio interface with two balanced analog inputs. I think any USB interface with a windows driver would work.
Finally, I used a run of the mill laptop with a USB 2.0 interface.
I spent around $250 excluding the Windows PC.
The way I setup the amp and mic was to point the mic in-line straight at the speaker 1 meter away. Most Sound pressure measurements are made this way. The main difference between my setup and a measurement lab is they use an anechoic chamber. The chamber removes all surface reflections. Only the source audio gets to the mic. This is ideal.
I just need to get a basic idea of what this amp outputs so we can skip the hard science for this experiment. Besides, I've never played a gig in an anechoic chamber.
I set all the controls of the amp at 12 o'clock. The reverb was off and the Phat switch was off. The TrueRTA was set for a full sweep between 20hz and 20khz. This is the ideal range of human hearing and pretty standard audio frequency test range.
The result of this sweep is shown below. It shows exactly what I was hearing. The graph shows low frequencies on the left and high frequencies on the right. The height is amplitude in decibels.
The guitar frequency range is from 82hz (open low E) to 1.2khz (22nd fret high E).
The frequencies at 100hz are down around -40dB and the frequencies around 1khz are -10dB.
This is quite a range and shows the bass and mids are not as prominent as I'd like them. This measurement takes into account the amplifier, the speaker and the room.
There is another tool I found that is a tone stack simulator from Duncan Amplification. The graph below shows a curve based on the tone stack in the Blues Jr. This is a classic Fender tone stack. This proves the existing configuration can produce the lows and mids I want.
So what is missing from the Blues Jr? Let's dig in and find out! Next time we have to deal with rusty screws...

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