(Speakers not included!)

1960’s Altec / Coil Audio 250T3 LCR Modified Vintage Console System with Dual Sidecars

This system centers around a heavily modified Altec 250T3 control console, expanded with two custom sidecars that transform the original design into a flexible modern recording and mixing environment. Modifications and service by Coil Audio.

The package combines:

• A 16-input LCR mixing console
• A custom attenuator / routing sidecar
• A stand-alone 8-channel preamp sidecar

Together they form a system capable of tracking, monitoring, and mixing simultaneously, while maintaining the character and architecture of the original Altec design.

The console uses the Altec 9475 amplifier modules and transformer-based signal paths found in classic Altec consoles still in use today at Electric Lady and Dan Auerbach’s Easy Eye Sound. With Coil Audio’s updated layout, it allows signals to move between microphone preamps, recording systems, and the mix bus in multiple ways.

System Overview

The system consists of three main components:

1. Main Console – 16-input LCR mixer
2. Attenuator Sidecar – input pad network and routing
3. 8-Channel Sidecar – additional preamp consolette

Together they provide:

• 16 microphone preamps
• 16 direct outputs
• 16 monitor returns
• 8 mono + 4 stereo console channels
• Left / Center / Right mix bus

The entire system contains:

• 23 Altec 9475 amplifier modules
• • 16 used as mic preamps
  • 6 used in the mix buses
  • 1 spare module included

It also includes 19 Peerless and Coil line transformers for line inputs and monitor outputs throughout the system.

Main Console16-Input LCR Mixing Architecture

At the center of the system is a 16-input console feeding a Left / Center / Right master bus.

The input section consists of:

• 8 mono channels
• 4 stereo channels

Each mono channel feeds a stepped rotary attenuator and can be assigned to:

• Left
• Center
• Right

using an LCR toggle switch above the attenuator.

The stereo channels accept line level inputs and can operate in two modes:

• Stereo Left / Right
• Dual Mono, routing both channels to the center bus.

LCR Mix Bus Design

Each of the three mix buses uses two 9475 amplifier stages:

1. Booster amplifier for passive mix make-up gain
2. Program amplifier to drive the output

Between these stages is a stepped master gain attenuator allowing a wide range of gain staging options, from clean operation to intentionally driven saturation.

Each bus also has:

• Switchable insert points
• Master attenuators
• Individual outputs
• Fine trim (+/- 0.1db) for each of the 3x buss outputs

The console can also operate in Stereo Mode by turning the center master attenuator fully counter-clockwise. In this mode the center channel is split to the left and right outputs.

Bus Inserts and External Amplifiers

Each mix bus includes insert send and return points, switchable from the front panel.

These allow external processors to be inserted directly into the LCR buses.

Another modification allows the console’s internal booster amplifiers to be bypassed and replaced with external preamps, allowing different amplifier stages to drive the mix bus.

The Two Sidecars

The console’s flexibility comes primarily from the two sidecars, which control how signals move between microphones, the console, and a recording system.

Each sidecar serves a different purpose.

Sidecar 1 – Attenuator & Routing Sidecar

This sidecar sits ahead of the console’s mic preamps and provides input control, routing, and direct outputs for the eight mono channels.

Each channel includes:

• 250Ω input for dynamic microphones
• 1.2kΩ input for condenser microphones
• Line-level input
• Three-position input pad (-6 dB, -12 dB, -19 dB)
• Phase reverse
• Coil pad attenuation

Direct outputs are controlled by a 12-position stepped attenuator (1.5 dB per step).

Preamp Routing

The sidecar also includes a Preamp to Mix / Preamp to Direct Out switch, which determines how the console preamps are used.

Preamp to Mix
The microphone signal passes through the console’s 9475 mic preamp, feeds the channel attenuator, and enters the LCR mix bus for traditional console mixing.

Preamp to Direct Out
The preamp output is routed to a direct output for recording (such as Pro Tools or tape), while a line-level return from the recorder feeds the console channel for monitoring.

This allows the console to record through its preamps while simultaneously monitoring the recorder through the mix bus.

Sidecar 2 – 8 Channel Preamp Consolette

The second sidecar is a stand-alone 8-channel preamp unit that expands the input capability of the console.

The main console itself contains eight microphone preamps feeding the mono channels, while the four stereo channels are line-level inputs only. In its base configuration, the console therefore provides:

• 8 mono mic/line channels
• 4 stereo line channels

The 8-channel sidecar adds eight additional Altec 9475 microphone amplifier modules, allowing microphone inputs to be used on channels that would otherwise be line-only. With the sidecar attached, the system effectively expands to 16 microphone preamps available for tracking.

The sidecar includes:

• 8 Altec 9475 amplifier modules
• The same custom pad network design
• Stepped output attenuators
• Its own dedicated power supply

Because it is a separate unit, the sidecar can be used in multiple ways:

• Integrated with the console to expand the system to 16 microphone inputs
• Operated independently in another room as a remote preamp front end

This design keeps the signal architecture consistent across the system while allowing the console to scale from an 8-preamp console with stereo line inputs to a 16-preamp recording system when the sidecar is connected.

Example Workflow

The system was designed to allow tracking and monitoring at the same time.

Tracking

1. Microphones connect to the attenuator sidecar
2. Signals pass through the console’s 9475 mic preamps
3. The routing switch sends the signal to direct outputs
4. Direct outputs feed the recording system

Monitoring

1. The recorder sends signals back to the console as line inputs
2. Those signals enter the console through transformer-coupled line inputs
3. The console mixes them through the LCR mix bus

This allows the engineer to monitor the recording while tracking.

Mixing

Once recording is complete:

• Signals return from the recorder to the console
• Channels are assigned to Left / Center / Right
• External processors can be inserted into the mix bus
• The console outputs the final mix through its master outputs.

Transformers and Signal Path

Throughout the system are Peerless and Coil transformers used for:

• line inputs
• monitor outputs
• routing between sidecars and console.

The signal architecture is illustrated in the console flow chart diagram in the documentation, showing how signals move between:

• microphone inputs
• line inputs
• mix buses
• outputs.

Restoration and Upgrades

All amplifier modules in the system have been restored.

Each 9475 module has been:

• completely recapped
• re-resistored with matched resistor pairs
• updated with stabilization modifications created by the original designer, John Hall

Any out-of-spec transistors were replaced with NOS mil-spec parts, and a supply of spare transistors is included for long-term maintenance.

Power Supplies

The system includes two custom Accopian linear power supplies:

• one for the main console
• one for the 8-channel sidecar.

These supplies are designed to provide stable power to the amplifier modules.

Physical Layout

The main console sits in its own desk, while the two sidecars are mounted in separate rolling racks with matching tops.

Because the sidecars are independent racks, the system can be arranged in multiple layouts such as:

• straight line
• split left and right
• U-shaped mixing position.

Summary

This system combines a vintage Altec 250T3 console with extensive modern modifications and custom sidecars, creating a flexible recording and mixing environment built around classic transformer and amplifier circuits.

Key capabilities include:

• 16 microphone preamps
• 16 direct outputs
• 16 monitoring returns
• 8 mono and 4 stereo console channels
• Left / Center / Right mix bus
• switchable bus inserts
• external bus amplifier option
• modular sidecar architecture

The result is a console system designed for simultaneous tracking, monitoring, and mixing, while maintaining the architecture and amplifier topology of the original Altec design.