Adding Sound to the Bachmann Class 70  Heavy Freight Diesel Locomotive



This page shows how a Legomanbiffo LokSound V4 decoder and speaker will be fitted in place of the original TCS decoder to add a sound system to Class 70 number 70006.


   To see how the Class 70 lights were updated, click here


Class 70, 70006, viewed through the dirty window of a Northern Rail Class 156 at Carnforth in September 2010


Scope of the project:

Speaker Location: My preferred speaker solution would be an ESU 50334 20x40mm or a good base reflex unit, mounted facing the track between the bogies, but I'm not sure if it can be done.  Second choice is to use the space provided below the roof fans, with the speaker facing upwards. However, in this location, I would hope to seal the speaker around its rim to the underside of the roof around the fan vents.  No local enclosure would be used, instead the locomotive body will provide isolation between the front and back of the speaker.

Big problem with option 2 above! I've just checked the available width in the upper body shell directly below the fans. It's around 18mm and would not accommodate a 20mm wide speaker!  So I will have to very seriously see what might be possible below the motor between the bogies. I have bitter experience of using smaller speakers than a 20x40mm in past projects, with less than satisfactory audio performance..... until they were replaced by a 20x40!  .......See how it was done below!

LokSound function output interfacing: The original TCS (non-sound) decoder had 6 full performance function outputs for lighting control. The LokSound V4 has 4 equivalent function outputs plus two low current "logic level" outputs that can be converted to full performance outputs by using ESU's adapter PCB, which contains two simple amplifiers for this purpose. It is proposed to use a pair of small surface mount N-channel MOSFET devices to perform a similar function to ESU's adapter board amplifiers, but in a much smaller space.

Mounting the speaker :

A 22mm long aperture can be cut in the now redundant switch panel on the loco underside between the bogies. To increase the available depth below the motor, the chokes and capacitors (not required for DCC operation) can also be removed. This makes enough room for a full size ESU 50334 speaker enclosure to be comfortably accommodated between the lower locomotive frames.


The original underside structure, with the bogies removed.


Top of the original switch mounting PCB, before removal.


The motor underside after the original suppressor chokes and capacitors have been removed.

(The 100k resistor simply maintains the motor contacts separation, in place of the original capacitor.)


Underside view of the new aperture.


Topside view of the new aperture


Topside view of the speaker enclosure in place


Underside view showing the ESU 50344 20x40mm speaker glued in place within the enclosure.

(The inside surface of the frame girder is cut away to clear the speaker cone.)


When the speaker glue had set, the ends of the cylinders that were originally mounted below the switch panel, were re-attached to the end of the speaker enclosure to improve the appearance between the frames, when seen from a 3/4 side view.

The speaker is not at all obvious when the loco is on the track.


Circuit Diagram proposed for the sound equipped locomotive: 

Small N-Channel MOSFETs (2N7002) are used to convert the low current logic level Aux3 and Aux4 function outputs to have a normal switching capability.


The SOT23 device was glued upside-down to the PCB and thin wires soldered to the inverted gate & source legs.

This is the Aux3 interfacing MOSFET connected to the base resistor of the original PNP transistor enabling the rear lights.


Showing the Aux4 device adjacent to the 21 pin decoder socket.


Excellent, the MOSFETs work fine without the need for any additional hold-off gate resistors!


Proposed function button assignments & CV changes required: 


An edited version of Bif's Function list

(Changes in red)

CV changes required    (See ESU LokSound V4 manual)

Prog line Change outline CV changes  (x/yyy = CV32 to x and CVyyy as indicated)
F0 Lights (Day Running…..forward via ESU headlight FO) 5  No change  
F0 Lights (Day Running…..reverse via ESU rearlight FO) 6  No change  
F1 Sound On/Off. (See also Engine Priming Start, in V4 Helpsheet  NOTES)      
F2 Playable High Note Horn.      
F3 Playable Low Note Horn.      
F4 Buffer Clash/Buffering up when moving slowly.      
F5 Air Release (Air Dump).      
F6 Drivers door slamming open/closed.        
F7 Compressor Startup (High Engine Rev's).        
F8 Dynamic Braking Fan.       
F9 Automatic random Flange/Wheel Squeal.      
F10 Dispatcher's whistle.      
F11 Lights (Night Running, programming line: forward only via ESU Aux1 FO) 17  CVA 0 to 4, CVK 0 to 4 3/257 0 to 4;   3/266 0 to 4
F12 Not used….programming used for F11 reverse only via ESU Aux2 FO. 18  CVA 0 to 8, CVD 16 to 4, CVK 0 to 8 3/273 0 to 8;   3/276 16 to 4;   3/282 0 to 8
F13 Rear Lights off…..(output only on if button is not pressed via ESU Aux3 FO) 19  CVD 64 to 128, CVK 0 to 16 3/292 64 to 128;     3/298 0 to 16
F14 Strobe lights (double strobe output via ESU Aux4 FO)   20  CVK 0 to 32 3/314 0 to 32
F15 Not Used.      
F16 Not Used.      
F17 General Electric Skipfire Idle.      
F18 Triple Detonators.      
F19 Aux1 set to "not used" 25  CVK 4 to 0 3/394 4 to 0
F20 Aux2 set to "not used" 26  CVK 8 to 0 3/410 8 to 0


Action CV32 CV changes
Set Aux4 to double strobe 0 CV299 = 6  (double strobe)    CV302=31 (max brightness)   CV303=128 (LED)
Adjust double strobe timing NA CV112 reset to 20 for the desired effect.
Set Volume (BEFORE running sound!) NA CV63 = 50   (This is set to a speaker breaking max at 192 upon arrival.)
Drop engine volume (sound slot 1) 1 CV259 dropped from 128 to 100 (to increase the relative volume of horns and wheel flange squeal effects etc)



Active Button Sound Slot Assignments (& Lighting Controls): 

Most of the sound slots (at least all of those I've checked) are set at maximum volume, so to make any of them sound louder compared to the diesel engine sounds, I've had to reduce the volume of the engine sounds in sound slot 1.

Sound Control Function list

Sound Slot
F0 Day running lights -
F1 Sound On/Off and engine start/stop. 1
F2 Playable High Note Horn. 23
F3 Playable Low Note Horn. 24
F4 Buffer Clash/Buffering up when moving slowly. 4
F5 Air Release (Air Dump). 5
F6 Drivers door slamming.   6
F7 Compressor Startup.   7
F8 Dynamic Braking Fan Enable.  8
F9 Flange/Wheel Squeal Enable. 9
F10 Dispatcher's whistle. 10
F11 Night running lights -
F13 Switch off rear lights -
F14 Double strobe "shunting warning lights" -
F17 General Electric Skipfire Idle Enable. Logical Function sets to shift mode
F18 Triple Detonators. 18
F19  Set zero momentum (for test purposes only) -


Motor Control Settings

CV CV Value Description
2 1 Start voltage
3 180 Acceleration delay
4 95 Deceleration delay
5 200 Max. speed setting
6 75 Med. speed setting  (<100 expands slow speed part of range)
49 19 Ext Config.1 includes 40kHz motor pulse frequency
51 3 Slow driving I component
52 52 Slow driving K component
53 105 Max speed adjustment
54 45 Normal driving K component
55 75 Normal driving I component (proportional to motor momentum)
56 255 100% range speed control
124 5  Ext Config.2 includes Enable adaptive regulation frequency

Smooth running is achieved across the entire speed range, but I'm sure many other combinations will achieve the same result!


Some Unique Class 70 Sound Effects:

Two of the sound effects on Biff's decoder are new to me......

1) Skipfire idle engine sounds..... The class 70 employs a novel fuel saving technique. When the locomotive is stationary and the engine is idling, a computerised system arranges for fuel to be injected in rotation to only a few cylinders at a time. This greatly reduces fuel consumption during idle, but keeps the engine turning and warm, ready to revert to normal operation quickly, when power needs to be applied. The sound is of a distinctive intermittent nature, with a lower frequency content than when the engine idles normally. When enabled, via button 17 in Biff's decoder, the engine switches to this mode immediately if in stationary idle state or automatically a few seconds after entering idle mode when the locomotive comes to a halt. When the locomotive is next moved, first the engine reverts to normal mode and after the revs build up, the loco gradually moves away.

2) High air mass cooling fans supporting the dynamic braking.......  Another unusual Class 70 feature for a U.K. mainline freight diesel is the dynamic or rheostatic braking system. When the driver applies the brakes at medium or high speed, the axle mounted electric motors are disconnected from the normal driving electrical supply and re-connected to a large bank of fan cooled resistors. In this mode, the motors act as electrical generators, turned by the driving wheels, while feeding electrical current into the resistor bank. This action has a significant  braking action on the driving wheels as while the wheels are turning, the motor current induces torque in the motor rotor assemblies that works against the direction of motion. However, to prevent the resistive load from overheating, powerful fans blast cooling air through the resistor bank as long as the motors continue to rotate during the braking period. This creates a distinctive "screaming air sound".  Button 8 enables this feature in Biff's decoder. The fan sounds are only produced when the speed control is shut down to slow or stop the locomotive from medium to high speeds. Slow-speed braking results in conventional braking sounds only.

Both effects work very well in the Bachmann model and provide a lot of added interest to the driving experience. I've tried to illustrate the effects in the test track video below!



The finished Class 70 on the test track.


  To see a Youtube video of the Class 70 in action on the test track, please click here



Supplier website links:


Rails of Sheffield    The Freightliner Class 70 unit 70006 was purchased from this excellent web supplier.
Rapid Electronics    Excellent source of sensibly priced electronic components.
DC Kits - D&E Videos    The source for Legomanbiffo sound equipped LokSound V4 decoders
Maplin    Convenient local source for resistors and transistors, although the prices reflect that convenience.


Photos of the real class 70 were taken at Carnforth in September 2010.   The photos of the model were taken hand held on the kitchen worktop.


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