Grace Design m303 Active DI

1 Welcome

Thanks for purchasing the m303 Active DI. While it is designed to be simple and easy to use, we do recommend that you familiarize yourself with the setup and operational details contained in this manual. As always, with this or any other Grace Design product, please don't hesitate to reach out if you have any questions. We are always available by telephone Monday - Friday, 9AM to 5PM MST, or by email at info@gracedesign.com. Also, other information including technical documents and firmware can always be found on our website – www.gracedesign.com. Thanks for reading and enjoy your m303!

2 Important Safety Information

2.1 General

• Ordinary Protection: This equipment should not be exposed to dripping or splashing.
• Avoid placing objects filled with liquids, such as vases or glasses, on this equipment.
• Pollution Degree 2
• Maximum Relative Humidity: <80%
• Operation temperature range: -10 °C to 60°C
• Storage and transportation temperature range –40 °C to 70 °C
• Maximum altitude: 3000m (9843 ft)
• Equipment suitable for continuous operation

2.2 Safety Marking Symbols

! This symbol, located in this manual, refers to important instructions. Read this manual thoroughly before operating this equipment.

2.3 Service Information

The Grace Design m303 contains no user serviceable components. Contact Grace Design for repair and upgrade information. In the event that your Grace Design m303 needs to be returned to the factory, contact us for a return authorization number.

2.4 California Proposition 65 Warning

This product may contain metallic nickel, a chemical known to the State of California to cause cancer and birth defects or other reproductive harm. Wash hands after handling.

3 Overview And Features

3.1 Features

• Phantom powered active DI with full galvanic isolation
• Custom wound Lundahl transformer with magnetic shield
• Ultra low noise output -118dBu (A weighting)
• Wide bandwidth 20-80kHz (-3dB)
• Excellent low frequency headroom THD+N -10dBu out @50Hz, <-80dB (0.007%)
• 1M ohm input impedance works with any pickup or line signal source
• Peak indicator LED 3dB before clipping
• Power indicator LED
• Efficient design only draws 4ma of phantom power current
• Through signal connector
• Fully ESD and RFI protected
• Input pad for high level signals
• High quality gold contact toggle switches
• Rugged 1/4" wall extruded aluminum chassis

4 m303 Overview

The m303 is a fully-isolated active DI, which converts a high impedance unbalanced signal to a low impedance balanced signal. It is perfectly suited for connecting instrument pickups (passive or active) and line level signals such as from a keyboard, synthesizer or laptop computer.

Passive pickups require a very high input impedance to ensure that there is no alteration of the tone quality of the instrument, which the high impedance FET input amplifier of the m303 provides. The unique power supply design of the m303 provides power for the active FET input amplifier while also supplying complete galvanic isolation from the XLR output connection. This provides the best possible provisions for minimizing ground loop noises.

In the event that the m303 is connected to high level signals, the input pad can be used to protect the m303 from overload. The PAD allows the m303 to receive a maximum level of +27dBu (27V peak to peak), which is sufficient for any line level source. However, even with the pad enabled the m303 is not designed to be connected to power amplifier outputs.

The audio signal is isolated through the use of a custom audio transformer from Lundahl in Sweden. Lundahl transformers are legendary for their sonic fidelity and transparency. The m303 transformer features a high nickel core which ensures high headroom and low distortion. The mu-metal magnetic shield protects the m303 from hum and buzz interference from nearby equipment.

The m303 power supply converts the incoming 48V phantom power to a bi-polar DC supply for the input amplifier and HPF. This power supply is isolated, so that when you switch the GND lift switch, the instrument input ground is completely isolated from the output XLR ground. This ensures the highest possible rejection of ground induced noises.

The m303 features a built-in 2nd order (12db/octave) high pass filter (HPF), which removes any subsonic noise from the signal before being sent to the audio transformer. Subsonic information in the signal can easily cause high flux density in the output transformer which can significantly increase the distortion in the audio band. The HPF protects the downstream signal path from the adverse affects of unwanted low frequency information and provides maximum signal level handling of the DI. This is especially useful with piezo type pickups which can be very sensitive to instrument handling noise.

4.1 m303 Front Panel Connections, Controls, and Indicators

A graphical representation of the front panel shows the following controls and indicators:

• PAD Switch: UP position will add 13dB of attenuation. Normally the pad will not be needed. In the case where a very high level input signal is being used, the PAD can be switched on to avoid overloading the m303.
• INPUT jack: The input of the m303 is unbalanced, for use with a cable that is wired with a TS (Tip-Sleeve) plug. The tip is hot and the sleeve is ground. If using a TRS (Tip-Ring-Sleeve) connector, the Ring will be grounded. The input impedance of the m303 is 1MΩ with the PAD off and 115kΩ with the PAD on. For passive pickups it is best to leave the PAD off as the lower impedance can have significant effects on the tone quality. If using a very high output source like an active pickup system or keyboard, the PAD may be necessary.
• PEAK indicator LED: The red peak indicator LED will illuminate when the input level exceeds +11dBu (PAD Off) and +21dBu (PAD On).
• THRU jack: The THRU jack is wired in parallel with the INPUT jack. It is intended to be used as a parallel output for feeding a stage amplifier.
• Power indicator LED: The green Power Indicator LED will illuminate when +48V phantom power is applied.

4.2 m303 Rear Panel Controls and Connections

A graphical representation of the rear panel shows the following controls and connections:

• Ground Lift Switch: The GND-ISO switch disconnects the Input ground from the Output ground for true (galvanic) isolation. When in ISO, the m303 can withstand large ground potential differences without causing hum or buzz.
• OUT XLR connector: The OUT XLR connector receives the +48V phantom power and sends the low impedance balanced line outputs to the PA system.

5 Installation

5.1 Open and inspect your box

Please take care when removing the m303 from its OEM packaging. Our packaging is designed with ‘Korrvu©' suspension inserts, which provide the best protection from the dubious underworld of freight handling. The protective packing is 100% recyclable so please, if you don't save the packing materials, recycle them.

5.2 Connecting your m303

Typical DI connections are illustrated with diagrams showing a bass guitar connected to a stage amp and the m303, and a stereo keyboard connected to two m303 units, all feeding into a mixer or interface.

Diagram 1: A bass guitar is connected to the INPUT of the m303. The THRU output of the m303 is connected to a stage amp. The balanced OUT XLR connector of the m303 is connected to a mixer or interface.

Diagram 2: A stereo keyboard is connected to two m303 units, one for the left channel and one for the right. Each m303's balanced OUT XLR connector is connected to a mixer or interface.

6 Operation

1. First make sure you have properly connected your instrument to the m303, your m303 through to your amp (if using), and the DI XLR output to your mixer or interface.
2. Next activate 48V phantom power on your mixer or interface. Note: it is always best to activate 48V power after all the connections are made, and deactivate 48V power before disconnecting.
3. The green LED power light will be on, indicating the m303 is ready to go.
4. If you are using a passive pickup, you probably won't need the PAD, if you are using a higher level active source, you may need to engage the PAD circuit.
5. If there is hum or buzz on your signal, try setting the GND-ISO switch to the 'ISO' setting, which will isolate the input ground from the output ground.
6. Enjoy!

7 Electrical Specifications

m303 Specifications (preliminary)

Grace Design reserves the right to update or change the electrical specifications at any time.

8 Block Diagrams

A block diagram illustrates the signal flow:

Instrument Input -> Input Amplifier -> 20Hz HPF -> Peak Indicator -> Lundahl Output Transformer -> Balanced Out.

Also shown are the Input Pad (-14dB), SIG/CLIP INDICATOR, POWER INDICATOR, ISOLATED POWER SUPPLY, and GROUND LIFT.

9 Dimensions

The dimensions of the m303 are provided with diagrams:

• Rear Panel: 1.77" (45mm) height, 3.19" (81mm) width.
• Front Panel: Shows the same width and height, with controls labeled PAD, INPUT, PEAK, THRU, ON/OFF.
• Overall dimensions: 5.30" (134.5mm) length.

A Grace Design logo is also displayed.

10 Warranty Information

• Grace Design warrants this product to be free of defective parts and workmanship for a period of five years from the original date of purchase. This warranty is transferable.
• Exclusions include: normal wear and tear, misuse, customer negligence, accidental damage, unauthorized repair or modification, cosmetic damage, and damage incurred during shipment.
• During the warranty period, Grace Design will repair or replace defective parts or workmanship at no charge, provided the customer has proof of purchase and the original factory serial number.
• Product registration online at www.gracedesign.com within 10 days of purchase is recommended for efficient warranty service. Alternatively, registration can be done by telephone at 303-823-8100 (Monday-Friday 9:00am to 5:00pm MST).
• This warranty is in lieu of all other warranties, expressed or implied, including warranties of merchantability or fitness for a particular purpose.
• Grace Design shall not be liable for lost profits or any incidental, consequential, or exemplary damages, even if aware of the possibility of such damages. Liability will not exceed the purchase price of the product.
• This warranty grants specific legal rights. Other rights may vary by state. Some states do not allow limitations on implied warranties or consequential damages.

11 Manual Revision History