Analysis of the Classic Stack amplifier model in the original Roland Micro Cube

Published: Nov. 27, 2018

The Roland Micro Cube comes with a very nice 'British stack' amp model (British type hi-gain rock). Despite the poor quality of the built-in speaker (high resonance frequency, distortion at moderate sound levels) the Micro Cube 'rocks'.

This article contains measurements of the original Micro Cube's Classic Stack amplifier model. The amp's linear frequency response and post-distortion filter ('post-gain') were measured. From the measured data the pre-distortion filter ('pre-gain') response were calculated.

The measurements were carried out on the headphone output of the Micro Cube. They are valid between 70 Hz and 10 kHz (enough for guitar).

Block diagram of the Classic Stack amp model
(filter block may contain several filters in series)

The pre-distortion filter very closely follows the preamp filtering of 80s Marshalls: JCM800 2203/2204 and the Silver Jubilee. I think it's closer to a Silver Jubilee, than a 2203, because 2203 has much more bass and a bit more top-end in the preamp.

The post-distortion EQ contains the cabinet response with some low-end resonance - as one would expect from a modeling amp. The high-frequency roll off is a bit early, the peak should be at 3 kHz and not at 2 kHz.

The Classic Stack amp model has a strange 'feature': the volume knob acts as a tone control too! At low levels (~2) we have enormous bass, but as we turn up the volume, the tone becomes very thin. It is not an overall loudness control or compensation, because only the Classic Stack model has this odd feature.

Frequency response of the whole Classic Stack model, Roland Micro Cube
Linear frequency response of the Classic Stack amp model (Gain@5, Vol@2, Tone@5)
(This graph is meaningful for clean tones only)

Pre filter of Classic Stack model in Roland Micro Cube
Pre-distortion filter, gain at 5 and 8 (the preamp simulation includes the effect of bright capacitor)

Post filter of Classic Stack model in Roland Micro Cube
Post-distortion filter, Volume @ 2 and Tone knob set to 5 (half) and 10 (full)

Post filter responses for Classic Stack
Post-distortion filter response for Vol@2 (blue) and Vol@5 (red), Tone @5. Notice the bass loss at higher level!

The measurements were made with Audacity back in 2016. The tool for the graph plotting was Room EQ Wizard. Good work needs time.🙂

Csaba Horvath

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