Catalinbread Manx Loaghtan on the bench

We got a lot of great feedback and kind words from our Internet friends about our writeup on the Ibanez TS-9 Tube Screamer, so my brother and I figured we should get back at it and put another guitar pedal on the bench and see what we could learn. We had a request to do a fuzz pedal and we thought that would be fun, so this time we're looking at the Catalinbread Manx Loaghtan. My understanding is that this pedal is meant to be similar to the Ram's Head Big Muff, though with a Baxandall type tone control.

In the comments we received about our previous write-up, some mention was made of looking at even- versus odd-order harmonics and the relative contribution of each to the total distortion produced, with the idea that this relationship might help quantify some of the differences in timbre that we hear produced by different effects. Luckily, the DSA we're using has the Instrument BASIC option and I was able to write a small program, EVEN-ODD.BAS, for the instrument to compute these relative contributions to total harmonic distortion from the measured power spectra. I think this was a great write-up to have this new tool available for and I appreciate the timely advice!

As before, I started with swept-sine frequency response measurements to characterize the operation of the bass and treble controls. After that, I moved on to take distortion measurements with a fixed-sine input to characterize the sustain control. I noticed some unexpected interaction between the tone controls and the distortion product blend and that this interaction depended as well on the supply voltage. I thought this interaction was interesting, so I made some additional measurements to try to characterize it.

A Catalinbread Manx Loaghtan guitar effect pedal sits on an electronics
work bench connected to a dynamic signal analyzer. Also visible are a manual
for the DSA, a glass of water, keyboard, soldering iron, and some hand-written
notes.

Methodology

I was able to use essentially the same instrument setup for these measurements as I did for the Tube Screamer measurements, with the exception that for certain measurements the power supply voltage was adjusted as noted below. The DSA configurations for frequency response and distortion measurements are available as FREQRESP.STA and DISTORTN.STA.

Findings

I started out with a quick measurement of current draw, something I now wish I had measured on the TS-9 as well. I found that, at least when powered from the external power jack, the pedal draws 2.18mA at 9 Volts even when switched off. When switched on, the current increases to 2.91mA. I imagine most or all of the increase is accounted for by the LED lighting up. A typical alkaline 9V battery has a capacity of about 600mAh at low currents like this and so should be good for something like 20 hours of use, though the supplied voltage may be quite low (perhaps 5 Volts) at the end.

I suspected a true-bypass switching arrangement on a boutique pedal like this, but made a quick frequency response measurement with the pedal switched off to check. The measurement confirmed my suspicion: dead flat response across the measurement range. I saved this trace as FRBYPASS.DAT.

I switched the pedal on and took a baseline frequency response measurement with both the treble and bass controls at their middle positions. This showed a more or less flat response between 78 and 3.8kHz, though the bass response was a little higher and technically rolled off 3dB from its peak by 1.3KHz. I think with some knob tweaking, you could dial this in as flat as you please.

A frequency response trace labeled Baseline Frequency Response shows 
fairly flat response and approximately no net gain or loss in the pass band.

Turning the bass knob all the way down rolled the bass off smoothly below 580Hz. Sorry I didn't match the scales in the print below; the response with this setting peaks at about the same level in the treble region as the baseline, it just looks higher because the top of the scale is down 20dB, as you can see in the scale labels.

A frequency response trace labeled Bass Cut Frequency Response shows
a distinct reduction of bass response and essentially unchanged treble 
response superimposed over the previous baseline trace.

Turning the bass knob all the way up gave a less-pronounced but still respectable boost to the bass frequencies down to 66Hz.

A frequency response trace labeled Bass Boost Frequency Response shows
a bit of boost in the bass frequencies tapering into a close match through
the mids and into the treble region superimposed over the previous
baseline trace.

I tried the treble boost next. Turning that knob all the way up gives a more pronounced boost than the bass knob did, out to about 7.2kHz.

A frequency response trace labeled Treble Boost Frequency Response shows
a pronounced increase in treble frequencies and close match in bass and mid
frequencies superimposed over the previous baseline trace. The bump up in
treble frequencies along with the small peak in bass frequencies that this
trace has in common with the baseline begins to highlight a small dip in
the mid-range frequencies.

Turning the treble knob all the way down resulted in high frequencies rolling off from about 910Hz.

A frequency response trace labeled Treble Cut Frequency Response shows
a pronounced decrease in treble frequencies, a tiny boost in mids, and a close 
match in bass frequencies superimposed over the previous baseline trace. The
small dip apparent in the treble boost trace is not apparent in this trace.

I took complete measurements at each combination of treble and bass at their minimum, middle, and maximum settings. I used the DSA's marker feature to measure the peak response and find the -3dB corner frequencies of the resulting pass-band. For some measurements, a little dip in the mid-range created two peaks in the pass-band. In these cases, I measured 3dB down from the highest peak and also 3dB down from the lower peak. The corner frequency of the lower peak is in parentheses. With both knobs in their maximum positions, the mid-scoop was pronounced and I broke out separate measurements for the bass peak, mid scoop, and treble peak on this setting.

Treble setting Bass setting Peak response Corner frequencies Data file
Minimum Minimum -5.4dB 360 to 1,800Hz FRMINMIN.DAT
Minimum Middle 2.3 81 to 910 FRMINMID.DAT
Minimum Maximum 7.8 68 to 425 FRMINMAX.DAT
Middle Minimum -2.4 580 to 4,400 FRMINMID.DAT
Middle Middle 1.7 78 to 1,300 (3,800) FRMIDMID.DAT
Middle Maximum 7.5 66 to 370 (2,900) FRMIDMAX.DAT
Maximum Minimum 6.6 1,400 to 6,700 FRMAXMIN.DAT
Maximum Middle 5.8 (74) 1,600 to 7,200 FRMAXMID.DAT
Maximum Maximum three segments, detailed below FRMAXMAX.DAT
Bass peak 7.2 65 to 360
Mid scoop -2.9 600 to 1,400
Treble Peak 5.7 1,800 to 7,300

Next, I turned my attention to distortion measurements. I wanted to investigate the effect adjusting the sustain knob would have on the overall output level and distortion products at various input levels. In each set of measurements, the output level stayed the same until the input level fell below a certain threshold, then it fell along with the input level. True to it's label, the sustain knob adjusted where this point fell: high output would be maintained at lower and lower input levels the higher the sustain setting.

I noticed that even-order distortion was most pronounced at the highest and lowest input levels, with the least amount of even-order distortion products near the input level above which the output level stopped changing. This showed what I thought was an interesting effect: though the output level stayed the same as the input signal fell off, the character of the distortion changed smoothly throughout the process.

Minimum sustain setting

Source Fundamental Distortion File
Even-order Odd-order Total
-70 dBVRMS -45 dbVRMS 10% 1% 10% DMIN70.DAT
-50 -18 26 6 27 DMIN50.DAT
-30 -7 9 28 29 DMIN30.DAT
-10 -4 10 38 39 DMIN10.DAT
-5 -4 12 39 41 DMIN5.DAT
0 -3 24 37 44 DMIN0.DAT

Middle sustain setting

Source Fundamental Distortion File
Even-order Odd-order Total
-70 dBVRMS -26 dBVRMS 29% 1% 29% DMID50.DAT