![[Under Construction]](images/undercon.gif){kind=small}
Mark Levinson No.38 Preamplifier
INTRODUCTION
This preamplifier was one of the most popular of the time, costing about USD5000 at the time of release. I picked up the board locally when the agent closed down, for comparatively peanuts money. But alas, some work needed to be done...
DESCRIPTION
My immediate job was to reverse engineer the fairly complex board to at least get sound out of it. The amplifier stage is made up of all op-amps and instrumentation amps. A 12 bit DAC is used to control gain at the output of the input stage. This is achieved thru an optical encoder coupled to the on board microcontroller. Speaking of which is based on the Motorola 6811 family. The microcontroller handles all functions on board, namely, the display, buttons, switching, remote control decoding, and volume control. Power is supplied from a single 60VA toroid from Piltron. Mostly linear regulation, but oddly the microcontroller is supplied from a switching regulator.
At first I worked without any schematic(!) Through observation and sheer determination I managed to get at least the volume control working. This without any form of schematic! By the way, the volume knob is a 2 channel optical encoder, not a potentiometer as some thought. Later on I managed to get a copy of the schematics from kind fellow DIYAudio forumers. Upon examination of the schematics I noticed the many glue logic required to form the display and control panel. Numerous chips providing functions such as latching, bit shifting, multiplexing and the like. This proved a daunting task for routing the PCB for them. To make matters worse many of the chips are obsolete. Well, at least I managed to get the remote transmitter IC...
I'm currently using a microcontroller to replace all the glue logic. I don't intend to copy the original design. I will interface a LCD panel instead of the obsolete Omron LED display, and a different button configuration. The most difficult part of this project is to interpret the data format required to interface the microcontroller to the main board. There has been a problem synchronizing the data frames, thus the display interface cannot be properly read yet. Some advanced functions of the preamp cannot be implemented if the data does not synchronize. On the bright side, I have successfully emulated all other front panel controls and LEDs.
Work is still in progress, the last picture shows the front panel interface. I used a Siemens DLG1414 VFD to display the info before emulating on the LCD. It still shows gibberish probably because of non-sync and different data format from the original Omron PD4435 used in the preamp. More work will be done in this area.
Mods will come later after the preamp is fully functional.
SOUND QUALITY
The sound of this preamp belongs to the typical Mark Levinson stable. It has a smooth high end coupled to a tight bottom end. Overall hypnotic quality, typical op-amp sound... Actually I would attribute most of the character to the OPA604 op-amps used, as I had previously experienced in a self modded Sony CD player. Fairly satisfied with the performance for the price paid.
PICTURES
(Still under construction!)