402-VLZ3 Internals

Well, got my Christmas present today and already have taken it apart... 8-)

The power supply voltage regulators are indeed LM337 and LM317. Inspecting the mic preamplifier sections I see they use NJM2068 and TL072C opamps. The minimums supply voltages for those are respectively 4V and 7V.

How low can you go?

My 402 arrived yesterday but I won't open it before christmas! Meanwhile I'm still searching the web for hints, howtos etc... see my delicious links.

During the holidays I will lift the input pin  of the voltage regulator found inside the mixer (I mean the output of the diodes bridge) and feed the regulator with various voltages to see how low it can drop without impacting the Mackie analog circuits (don't know yet what will be the criteria). I guess Vin is 25.4 Vpeak (18 Vrms) minus two diode voltage drops. In order to minimise the risk of messing with the Mackie internal analog and injecting noise, I'll stay en amont of the regulators even if that means a lesser power efficiency (shorter battery life). What do you think?

Voltage regulation

Well, it seems the way to go when running from batteries is with low dropout voltage regulators (LDO): LM317s need at least 3V dropout!

Initial design and gathering information

What I have in mind: two supplemental power sockets drilled on the case (will there any room for those? It seems so compact... )

• Dual power for the analog circuits
• "Phantom in" for the phantom power: for now I plan to try stacked 9V batteries (no DC-DC circuit)

From the specs we read dual power supply 18.5 VAC @ 350 mA and "8 W power consumption" Hmm that doesn't add up... is this a reactive load ? C'est une blague: for those interested in knowing when V*I doesn't give Watts see Power factor on Wikipedia.

I found (thanks Google) the Mackie 1604 VLZ service manual complete with detailed schematics of the 1604 power board: this will greatly help me in the analysis of the 402 power section (I guess Mackie engineers don't reinvent the wheel with each new line product...) The 1604 power board gives +/- 16 V, it's a start.

Anyone found the 402 service manual?  8-)

The first thing I'll do is power measurements at the VAC input. Does anyone with experience can advice me for the cell type? Will lead gell cell be OK? Each cell gives 2.1 V so 8 cells would give 16.8 V, enough to drive the voltage regulators? (LM317 et LM337 in the Mackie 1604) or is it too tight and won't have enough headroom to allow cells discharge voltage drops (what is the discharge curve of a lead cell?)

So the next steps are

• Complete drawing of the analog et phantom power circuits with components identification
• Finding viable injection points of external DC power (for analog and phantom)
• Finding working DC values
• Determining cell chemistry and topology.

C'est tout!

Introduction

So I've just ordered a Mackie 402-VLZ3 (mostly for my educational videos, I'm a physics teacher). The obvious question on every thread about this mixer is "Could it be battery operated?" Well this blog will explore the feasibility of such a modification.

Of course one could use a 12 VDC inverter and plug the provided external brick transformer, but I want to hack and void my warranty...

Anyway at 100$ a pop, I won't refrain myself! This is gone be fun. Gentleman, start your soldering iron!

I'm an amateur so don't hesitate to post your advices in the comments section. There's a lot of knowledgeable experts on the net, don't hesitate to chime in: I'm here to learn.

This will be a spare time project and I don't have any idea at what pace I'll progress: I've got a full time job AND three preschool kids AND a wife 8-)