Audio Test Fun!

 Exciting Audio Tests!

So as promised here are the results of the audio tests I ran on the Aviator Cub and the Micropro Mach2. Each of these tests was intended to confirm or contradict what I heard during the side-by-side comparison. All of the tests are "black box" tests, in that, I did not open up or modify either amplifier. Also, just to be clear, I am not an employee of or competitor of Quilter. Just an audio guy with some equipment and a lot of curiosity.

None of what is here is meant to imply that one thing is better than another. The final judgement has to be your ears, and we all respond differently to audio, which is what make this all fun (and sometimes frustrating). As engineers, we would love to have a test, or suite of tests that could accurately predict sonic perfection, but sadly, no such test exists. The best we can do is try to explain what we hear by measuring it. Audio is very subjective.

Frequency response

The first step in my mind was to get plots of the frequency response of each amplifier voicing, using the FX send as the output to avoid any cab sim shaping circuitry at the direct outs. Each amp was set neutral, with all EQ knobs centered, effects off, Limiter off and levels set well below clipping. 

In all cases, the input signal was set to -20dBv (100mVrms) and swept from 20Hz to 20kHz

The Cub

All three voices have a gentle roll off below 100Hz, with some interesting shaping in the mid-band. The Tweed voice (blue) is the most neutral and has the highest level in the low-frequency range but a couple of dB. The Black voice (red plot) shows a mid notch, with the Blonde voice (green) significant;y notched around 300Hz. All three voices tend to boost frequencies above 2kHz, with eh Blonde and Black having more boost than Tweed. Note that all three voices are at about the same level (other than in the mids) so I would expect that each is about equally as loud.

These measurements tend to confirm what I was hearing, that the Blonde and Blck voices are the brightest, and Tweed, while still bright, was the warmest of the three. Recall that in my listening tests, I preferred the Tweed voice overall

Micropro Mach2

What a very different set of voicing curves! First, from having played this amp for several years I know to expect that each of the six voices was at a different power level, with Tweed set for about half of the power of the other voices.

The Tweed voice (blue) is at the lowest level, confirming the power reduction, but look at that LF roll off! Starting about 200Hz it has about a 10dB per octave reduction. The mid-band is flat, with slight roll off above 8kHz. The legend at the bottom of the graph calls out the individual voices by color. In general, this also confirms what I heard in the listening tests, that the MP2 was a "warmer" or "less bright" amp than the Cub, probably due to the generally increased bass and the rolled off treble. (in all cases, Hi Cut was Off).

The Direct Outs

So, that's what the FX Send has, but what about the Direct Outs? The test was repeated, but this time taking the signal from the Line out of the Cub, and the XLR of the MP2. 

In the Aviator Cub, the Tweed voice (blue again) is more similar to the MP2, with that fast roll-off below 100Hz. Once again the HF end is boosted relative to the bass, which would retain the brightness of this amp. The HF roll-off is explained in the Micropro manual as being necessary to maintain PA stability. It makes sense to limit HF content going into the PA.

And once again, looking at the MP2 below we see very different curves. The difference in level is there (side note, these tests were performed with an 8ohm load attached, which in the MP2 lowers the available output from the XLR. The Cub is not affected by load). 

Plot colors are the same as before - and again - more low end, less HF (the MP2 manual says the additional HF roll-off is for PA stability). I've used the XLR out at gigs, and it sounds fine BTW. As I mentioned, the XLR out on the MP2 is affected by the load and here's how much:

With an 8ohm load connected and set to Tweed, we see about a 12 dB drop in level. It's also interesting to note that on the MP2, the XLR level is controlled by the Master volume knob, On the Cub, the Line Out has a separate level control, and is not affected by speaker level.

Did I like the Cub?

I found the Cub to be brighter than the MP2, but I did like the Tweed voice quite a bit, once I got the EQ where I liked it. Here's where I netted out:

I added some bass, rolled off the mids and treble and added just a touch of reverb (I'm not crazy about the Cub reverb - I miss the Dwell and Tone settings of the MP2). With the Gain set to about 3 o'clock, Limiter at 2 o'clock I liked it a lot, and could hear the "squishiness" of the Limiter.

If I was after a purely Fender-like sound, the Cub could be the way to go. It is certainly still grab-and-go friendly, and loud enough (I was not able to test this live since the owner needed it back...) , In my opinion, it really would benefit from having a Hi Cut knob - or even just a pushbutton - to tame the highs. A footswitch for the Limiter would be nice too. 

Block Diagram

Here's my interpretation of the signal flow in the Aviator Cub. Based on the way the limiter is working and the effect of the active EQ circuits, I'm pretty confident that the EQ comes before the Limiter, circuit-wise. I may explain why in a subsequent post. In the MP2, the Master is BEFORE the Loop, so changes in output volume affect the signal in the loop as well. The Cub doesn't do that (Yayy for the Cub!) Also note that the Line out and Headphone out are before the Master volume, so again, this is unlike the MP2 as noted previously.

In another post I will show some quick-and-dirty acoustical test results, and I will explore the functionality of the Limiter in more detail. 

Thanks for reading!