MXR invented the Dyna Comp the OTA Comp that re-invented the wheel of compression. Do not get me wrong it was not an improvement of the best but it was a circuit that could be made affordable and worked really well with a guitar. If it had been used for vocals it would have been laughable. Just this week Keeley brought our a new comp that sounds killer and is still based off of it. These days if one makes a clone one starts with the Ross Compressor a very early clone of the MXR as it, without digressing too much is a noticeably smoother sound.