Saturday, 17 June 2017

Roland 121 filter & 521

A viewer requested a comparison between the Roland eurorack 521 and the vintage 121.

My understanding is that the  521 filter is not meant to be a clone of the vintage 121m
The docs state it was inspired by both the Roland system 100m and the 700m.
I like the sound of both 521 & 121.
Well done Malekko & Roland.
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The 121 has a lovely bubbly character.
Possibly due to the BA662A OTA (Operational Transconductance Amplifier).
It's actually a VCA chip that Roland also used in the TB303.

The BA662A is long obsolete so I wonder what replacement was used in the 521

 This is from the TB303 service manual.
The OTA is a flexible chip... used in the NLC Dual OTA-VCO
and the Cluster matrix mixer
What the OTA  shares with your common Op-Amp is:
1. a single output.
2. They both have inverting & non-inverting inputs.
3. They both have power lines (V+ & V-)

What makes the OTA different is that it has 2 extra inputs (Ibias & Iabc)
and it's output is a current not a voltage.
So, to put it another way, a OTA is a voltage to current amplifier.

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