Custom Designs

M2 Circuits does not only repair analog electronics, we also build them!

Below you will see some custom gear we have designed and built. Some work has been documented over on our YouTube channel here.

M² Type 2103 RIAA Preamplifier

The Type 2103 is an RIAA preamplifier designed for moving magnet cartridges. It has an accurate response curve and low noise floor (-70dBV or better) with an available 50dB of gain. The preamp is of a discrete design based on discrete opamps that were found in early Midas and Calrec recording consoles. The gain is split into two stages. The first 30dB fixed gain amp takes care of the RIAA response using active feedback equalization. This achieves a near perfect curve with excellent channel matching (better than 1dB) that conforms to the RIAA standard. The second stage is an adjustable 20dB linear amplifier that features an output drive capable of driving a 600ohm load. The two volume controls can be used to independently adjust the channel’s volumes to better match other levels in your hifi. They are placed between the two gain stages so that the output impedance of the preamp remains constant.

The unit was constructed using the M2 Modular Card series, a collection of plug-in PCBs designed in-house. Each PCB has a job and together they work like building blocks to create a flexible prototype platform. The modularity allows for easy changes to be made down the road if new revisions are made.

Shown below are the unweighted, weighted, and noise floor graphs taken using the QA401 Analyzer System. The noise measurement is done with shorting jacks on the inputs, and with the volume controls fully CW. The THD of the preamp is 0.01% or better at 1kHz.



Power Amplifier Load

As mentioned on the Repairs page, some of the test equipment we use is custom designed and built. One of those is a test panel designed to test power amplifiers.

For those more technically inclined, or just curious, we have provided the documentation for it here – AmplifierPowerLoadDoc_M2.

By taking a differential measurement across the load resistors, we are able to keep the loads completely floating. This is especially important with power amplifiers that do not reference the speaker output to ground. The calibrated meters allow for us to read the power output in Watts. The power amplifier test panel also interfaces with the audio test panel, so that the output may be monitored audibly.

The PCBs (Printed Circuit Boards) were custom designed for the test panel. In the future more of our custom builds will become modular and card based. Using cards allows for lots of flexibility, and many of the cards can be used in a variety of designs.


The custom PCBs used in the load panel.

Response characteristics, the upper frequency is limited by the QA401 used to measure the response.

This page is in the progress of being updated with more custom work, stay tuned for more!