About RMS
Audio Design

Private Projects from before RMS Acoustics & Mechatronics

The following two projects cover the extremes in the range of audio related design activities that I worked on during my regular career at Philips, ASML and the TU Delft. They acted as learning vehicle to better understand both the technology behinds audio systems and to develop listening skills "ears!" to be able to judge the musical properties op hi-end audio equipment.

The MFB loudspeaker has served for many years. The design shown is the original design with analog electronics and is published for those who do not like class-D amplifiers and DSP's.
In 2019 the electronics were replaced by a Hypex Fusion amp FA 253 with a dedicated MFB add-on PCB with a DSP controller.

The project as described is the status of the first design. The sound was less pleasant as expected. With measurements a strange intermodulation was visible with 50Hz separated spikes around the sighnal frequency, which was the cause of the unpleasant (harsh) sound. In 2021 this was solved by including two e-chokes from Tentlabs.

MFB loudspeaker

large-boxlarge-box measurement

The first project that is described is a large MFB loudspeaker with active 4th order Linkwitz-Riley filters and many active protections, like a maximum excursion limiter of the woofer. The project is not directly suitable for "DIY", as several components are from many years ago which means that they have to be replaced by more recent units. A possible address for a useable accelerometer is for instance available with Piratelogic. Anyway I hope that it will inspire you.



This second project is based on the wish to design a tubeamplifier using the recent knowledge on control of transistor amplifiers and active mechatronic systems. Because I have built many tube amplifiers in my highschool time, sometimes over a weekend, I thought that I could manage it now also in a few weeks. Due to the regular job and the wish to make it perfect it took me however 3 years before it was at the present state. Key features are a fully balanced design. Ring-core output transformers from Amplimo, designed by Menno vd Veen, Instrumentation input circuits with MOSFET input Op-Amps and two active control circuits, one to set the bias current and one to balance the DC value of the anode voltages over the driver tubes. The input triodes ECC83 and the professional driver triodes E88CC are original vintage Philips tubes and the power tubes KT88 are from Svetlana.



© 2021 Rob Munnig Schmidt