Preliminary Review of MRC Sound Decoder.
by Larry Maier

• BUY HERE
• SOUND SAMPLES

MRC has entered the DCC sound realm with the introduction of their AD370 [HO] Diesel Sound Decoder. This unit is designed to provide an entry level sound decoder at a price point that most modelers can meet. The decoder is shown below attached to a 1" speaker with enclosure from Tony’s Train Exchange. The AD370 is supplied with its own ¾" speaker, but it was reviewed with the larger speaker to better allow us to judge the sound quality.

The AD370 measures 44.0mm X 17.5 mm X 7.0 mm (1.75 X 0.69 X 0.28 inches) and will just fit side to side in a scale diesel shell. It is rated for 1.5 amp motor operation and has three 0.1 amp accessory outputs controlled by F1 – F3.

The motor drive portion of the AD370 is a basic DCC decoder. While it complies with NMRA standards, it only has limited functions. These include short and long address modes along with advanced consisting. You can program the start voltage and the maximum running voltage, but there is no provision for mid-speed adjust or custom speed curves. Programmable acceleration and deceleration are included, as is one CV for control of the lighting effects. Included lighting effects are "Rule 17", ditch lights, and a strobe light effect. These effects are turned on or off by setting CV64 to 0 (special effects on) or 1 (special effects off). The "Rule 17" effect applies to the headlight and reverse light, while the ditch lights are intended for use in conjunction with the horn and the strobe effect is for a cab mounted strobe light. With the limited capability, this unit is very easy to program. We were able to program it both on the programming track with proper data display and on the main. The AD370 supports 14, 28, and 128 step speed control. We tried it out in an Atlas GP-40 (#8913) and found that the AD370 provides competent speed control. Low-end speeds allowed realistic yard speed control of the engine. You can set the maximum top speed voltage to limit your engine to realistic top-end speeds. As a DCC controller, the AD370 is basic but certainly adequate.

The significant additional feature that the AD370 brings to your layout is the addition of diesel sound without the cash outlay that most other sound units require. The sound repertoire consists of 6 sounds: bell, long horn, short horn, uncoupling, diesel idle and diesel run. When placed on the track and powered up, the AD370 remains silent. Sending any command from the cab to the address of the AD370 starts the diesel idle sound. Once started, there does not appear to be any way to turn it off or mute the sound other than removing track power. Increasing the throttle causes a sudden change from idle to run sound. The transition is masked by turbo whine so there is an illusion of the prime mover spooling up. Once the transition to the running diesel sound has been made, there is no further change in the engine sounds as the throttle is varied from slow to top speed. Conversely, when the throttle command reaches near 0, the engine sounds suddenly transition from running to idle. The change is again masked by turbine (?) noise, but in this case we had the feeling that the engine was suddenly idling, and we weren’t sure how it got there. There is no sense of the prime mover coasting down to idle. The change from idle to running occurred at speed step 8 in our sample. The quality of the engine sounds is reasonably good. There is the appropriate deep rumbling sound along with the various noises of mechanical parts dashing about within the engine. The diesel running sounds are generic, and may not be exactly correct for your particular engine. In particular, this is not an Alco diesel sound.

The bell has a clear sound with the appropriate sharp transient from the clapper hitting the bell, but the initial bell sound has a slight rising pitch that sounds as if there may be a slight error in the digital playback. The horn sound is for a multi-chime diesel horn and sounds reasonably good. You can clearly hear the individual chimes and the hiss of the air. However, you can only give a short blast or a long blast. The length of the horn is pre-determined, and cannot be changed by how long you hold down the function button. F2 will give a long blast on the horn when operated. F3 gives the complementary short sound. To blow for a crossing, you must move back and forth between these two controls rather than tapping out the familiar pattern on a single horn control button.

The sounds are triggered whenever a function transitions. This means that if you have your cab set to push on / push off (i.e. latching) then the associated sound will play when you turn the function on and when you turn it off. We found that a quick press-press sequence would prevent double playing under these conditions. Of course, this can be useful if you want to play a sound twice in a row, such as two long horn blasts.

The biggest complaint that we found with the AD370 is the sound volume. It is LOUD, and there is no provision to adjust it. We found working with the AD370 on our bench painfully loud at times. The high volume also results in some sound distortion because the speaker is over-driven. Running the sound at full tilt boogie all the time results in significant heating of the sound amplifier. We measured a surface temperature of 65.5 C (150 F) with the AD370 in the engine running mode. While this is within the acceptable range for electronic circuitry, it is quite hot for normal operation, particularly since we were using 14.5 volts for our DCC supply. Systems with higher voltages will result in hotter temperatures.

Since it seemed that quieter would be better, we experimented with putting a resistor in series with one speaker lead. A 15 ohm 1/8 Watt resistor will drop the sound level to a much more enjoyable (in our judgment) level. As a further benefit, use of the resistor will result in a decrease in the temperature of the audio amplifier. After the resistor installation, we measured a temperature of 46.4 C at engine idle compared to 57.3 C prior to the resistor installation. The lower volume also yields better audio fidelity since the speaker is not over-driven.

Conclusion

If you are used to the sound capabilities and the lighting and performance features of advanced sound units, the AD370 is not comparable. However, if your layout currently has no sound and you have been waiting for a less expensive entrance into DCC sound, then the MRC AD370 may be worth a try. You might also consider the AD370 for those locomotives that you want to run occasionally, but don’t want to invest the full price of a top-level sound system. It gives reasonable control, lighting, and sound performance for a price that is only slightly above many non-sound decoders. When you consider that the price includes the speaker, it compares even more favorably with non-sound units.

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