MMSA78

Manual

Thank you for your purchase of a Boscoe Music MMSA78 preamp!

The MMSA78 is a reproduction of the original late-1970s Music Man Sabre 2-band bass preamp. It recreates the original circuit using the now-discontinued LM4250CN opamp, period-correct circuit values, and modern construction with screw terminals for easier installation and service.

Each MMSA78 preamp is individually tested before shipping.

In addition to the familiar Volume, Bass, and Treble controls, the early Sabre circuit includes a Bright function that can be controlled by a simple toggle switch or push-pull potentiometer.

The MMSA78 is intended primarily for 1978–1981 Music Man Sabre basses using the early toggle-switch electronics, or for people looking to recreate them. Later Sabre basses use a different electronics arrangement based on the StingRay preamp. Our MMSR77 preamp is a suitable replacement for the 2 band versions of those later Sabres.

The MMSA78 can also be used in custom instruments with one or more pickups.

Pickup and Control Selection

The MMSA78 will work with many different pickups and wiring configurations. For the closest approximation of the original late-1970s Sabre response, however, pickup type, pickup wiring, and control values are important.

For a vintage-style installation, Boscoe Music recommends:

Pickup(s): Music Man-style Alnico humbuckers with their coils wired in parallel
Volume: 25kΩ, A25K or B25K
Treble: 1MΩ reverse-audio taper, C1M
Bass: 100kΩ audio taper, A100K
Bright switch: SPST, SPDT, or suitable push-pull switch
Power: 9V nominal

For a vintage Music Man-style pickup, approximately 1750–1950Ω DC resistance when wired in parallel and approximately 700mH inductance is a useful target.

Examples of vintage-style replacement pickups include:

Aguilar AG 4M and AG 5M
Nordstrand MM4.2 and MM5.2
Seymour Duncan SMB-4A and SMB-5A

Vintage Sabre-specific replacement pickups are relatively uncommon, but a vintage-style StingRay pickup with similar electrical characteristics can work very well.

The MMSA78 can also be used with higher-resistance pickups, series-wired humbuckers, Jazz Bass pickups, Precision Bass pickups, and many other configurations. The circuit will still operate normally, but the pickup's resistance and inductance affect its interaction with the preamp and therefore the final response.

For tested pickup models, coil identification, magnetic polarity, and series/parallel wiring information, see the Pickup Wiring Reference.

The Original Sabre Signal Path

The original Sabre uses more switching than a single-pickup StingRay.

In a typical early Sabre installation, the pickup selector and phase switching occur before the preamp. The resulting pickup signal then feeds the MMSA78 through its PICKUP INPUT connection.

The MMSA78 therefore does not replace or duplicate the pickup selector or phase switch.

If you are replacing the preamp in an original Sabre, preserve the existing pickup-selection and phase-switch wiring unless it also requires repair.

If one pickup or one pickup-switch position does not work, but the instrument works normally in other positions, inspect the pickup switching before assuming the preamp is at fault.

If your Sabre retains its original phase-reversal function, remember that an out-of-phase setting can intentionally sound thinner and lower in output when both pickups are selected. This is caused by cancellation between the pickups and is not a preamp fault.

Why the Potentiometer Values Matter

The control values are part of the original Sabre circuit and affect how the preamp responds.

For the closest reproduction of the original control behavior, use:

Volume — A25K or B25K

Treble — C1M

Bass — A100K

Substantially different potentiometer values or tapers can change the usable range of the controls and the way the pickup interacts with the circuit.

The Treble control in particular should use a 1MΩ reverse-audio taper potentiometer for the intended control response.

Power

The MMSA78 is designed for normal operation at 9V and is safe to operate at up to 18V.

Higher voltage may provide additional headroom, but does not necessarily improve the sound.

For normal operation, use a standard 9V battery.

A conventional active-bass installation uses a TRS output jack to switch the battery automatically. Battery negative connects to the Ring terminal of the output jack rather than directly to ground.

When a standard mono instrument plug is inserted, the plug connects Ring to Sleeve and completes the battery circuit.

Unplug the instrument cable when the bass is not being used. Leaving a cable inserted keeps the preamp powered and will discharge the battery.

Before You Begin

Before disconnecting any existing electronics, take several clear photographs of the original wiring.

Photograph:

both sides of the original preamp,
pickup connections,
pickup selector,
phase switch,
Bright switch,
bridge ground,
potentiometer wiring,
output jack,
battery wiring,
and cavity shielding.

Older instruments may have been repaired or modified during their lifetime. Do not assume that wire colors or existing connections match another instrument.

Whenever possible, identify wires according to their function and destination, not just their color.

Disconnect the battery before beginning installation.

Complete all wiring before reconnecting the battery.

Before cutting wires or mounting the MMSA78 permanently, test-fit the preamp and other components in the cavity.

The MMSA78 measures approximately 50mm × 18mm × 11mm.

Arrange the installation so that:

wires are not under tension,
the screw terminals remain accessible,
the preamp is not pressed against switches, potentiometers, or the output jack,
and the underside of the PCB cannot contact conductive shielding or metal.

Do not shorten original wiring unnecessarily.

Protecting the Preamp From Shorts

The solder joints and component leads on the underside of the MMSA78 must not contact conductive material.

Possible sources of accidental shorts include:

conductive shielding paint,
copper foil,
metal control plates,
potentiometer bodies,
switch bodies,
output jack hardware,
and other exposed metal.

Use a suitable nonconductive foam, neoprene pad, or other insulating material beneath the preamp when necessary.

If replacing an original Sabre preamp that was wrapped or supported by insulating material, the original insulation may be reused if it remains in good condition.

Do not force the PCB into a cavity where components are pressed against surrounding hardware.

MMSA78 Connections

The MMSA78 uses clearly labeled screw terminals for the pickup, controls, output, power, ground, and Bright-switch connections.

The primary connections include:

PICKUP INPUT
Audio signal from the pickup-selection circuitry.

9V+
Positive battery supply.

GROUND
Circuit ground.

OUTPUT JACK
Audio output to the Tip connection of the output jack.

BRIGHT
Bright-switch control connection. Grounding this terminal activates the Bright function.

The remaining terminals connect the Volume, Treble, and Bass potentiometers as described below.

Using the Screw Terminals

Before inserting a wire, loosen the screw enough to open the internal clamp.

Strip only enough insulation to allow the conductor to be fully inserted.

Make sure no stray strands of wire remain exposed where they could contact an adjacent terminal.

Insert the wire fully and tighten the screw until the conductor is securely clamped.

After tightening each terminal, gently pull on the wire to make sure it is actually secured.

The screw terminals use an internal box clamp. It is possible to accidentally insert a wire underneath the box clamp instead of inside it. In this situation the screw may feel tight even though the conductor itself is not being held securely.

If a wire pulls free, loosen the terminal, reposition the wire inside the clamp, and tighten it again.

Do not overtighten the terminal screw.

When working with the PCB, hold it carefully by the edges rather than using the capacitors or socketed LM4250CN opamp as handles.

Potentiometer Terminology

When working with the controls, the potentiometer terminals are identified as follows:

CCW (lug 1) — counterclockwise lug; on the left when viewed from the bottom
Wiper (lug 2) — center lug
CW (lug 3) — clockwise lug; on the right when viewed from the bottom

Volume Potentiometer

Use a 25kΩ potentiometer.

An A25K audio-taper or B25K linear-taper potentiometer may be used.

Connect:

CW (lug 3) → VOL CW

Wiper (lug 2) → VOL WIPER

CCW (lug 1) → GROUND

Treble Potentiometer

Use a C1M reverse-audio taper potentiometer.

Connect:

CW (lug 3) → TREBLE CW

Wiper (lug 2) → TREBLE WIPER

CCW (lug 1) → TREBLE CCW

The reverse-audio taper is important to obtaining the intended control response.

Bass Potentiometer

Use an A100K audio-taper potentiometer.

Connect:

CW (lug 3) → Not connected

Wiper (lug 2) → BASS WIPER

CCW (lug 1) → BASS CCW

Pickup Wiring

Connect the output from the pickup selector or pickup wiring network to PICKUP INPUT.

Connect pickup shields and other appropriate ground connections to GROUND.

The original StingRay and Sabre pickup systems used Music Man-style humbuckers with their coils wired in parallel.

For the closest vintage response, parallel wiring is recommended.

As an example, for the Sterling by Music Man H-1 pickup configuration as tested by Boscoe Music, parallel wiring is:

Red + White → PICKUP INPUT

Black + Green + Bare → GROUND

Do not rely on pickup wire color alone. Wire colors can vary between manufacturers, models, and production runs.

If you are unsure of the pickup wiring, identify the individual coils with a multimeter before making connections.

For a dual-pickup Sabre-style installation, the individual pickups normally connect to the pickup-selection circuitry first. The selected signal then connects to the MMSA78 PICKUP INPUT.

For tested pickup wiring, see the Pickup Wiring Reference.

The Bright Switch

The MMSA78 reproduces the original Sabre Bright circuit.

Unlike the Bass and Treble potentiometers, the Bright function is controlled by grounding the BRIGHT terminal.

BRIGHT open / not grounded → Bright OFF

BRIGHT connected to GROUND → Bright ON

The Bright terminal does not require battery voltage.

Do not connect BRIGHT to 9V+.

A simple SPST toggle switch, SPDT toggle switch, or push-pull potentiometer switch can be used.

Wiring an SPST Bright Switch

An SPST switch has two electrical terminals.

Connect:

BRIGHT → one switch terminal

GROUND → the other switch terminal

The order of the two terminals does not matter.

When the switch closes, BRIGHT is connected to GROUND and the Bright circuit is engaged.

When the switch opens, the Bright circuit is disengaged.

Wiring an SPDT Bright Switch

A standard SPDT ON-ON switch has three terminals:

one Common
two switched terminals

Connect:

BRIGHT → Common

GROUND → one outer switched terminal

Other outer terminal → Not connected

When the Common is connected to the grounded outer terminal, Bright is ON.

When the Common is connected to the unused outer terminal, Bright is OFF.

If the switch operates in the opposite physical direction from what you prefer, move the GROUND wire from one outer terminal to the other.

No special ON-ON-ON switch is required for the standard MMSA78 Bright function.

Wiring a Push-Pull Bright Switch

A push-pull potentiometer can be used when you want the Bright function without adding another visible toggle switch.

Most push-pull controls contain a DPDT switch with six terminals. Only one side of the switch is required.

Use one set of three terminals as an SPDT switch:

BRIGHT → Common

GROUND → one switched terminal

Other switched terminal → Not connected

Whether Bright is engaged with the knob pushed in or pulled out depends on which switched terminal is grounded.

Push-pull switch layouts vary between manufacturers. Use a multimeter in continuity mode to identify the Common and switched terminals before wiring.

Where to Connect the Bright-Switch Ground

The Bright switch can be connected to any reliable point on the instrument's circuit ground.

Suitable locations include:

the MMSA78 GROUND terminal,
a grounded potentiometer body,
a common ground bus,
or another verified circuit-ground connection.

If using a potentiometer body as the ground point, verify with a multimeter that the potentiometer body is actually connected to circuit ground.

A switch mounted to a metal control plate should not automatically be assumed to be grounded merely because its mounting hardware touches the plate.

Verify continuity when in doubt.

Understanding the Original Bright Function

The original Sabre Bright circuit is intentionally subtle.

The vintage circuit uses a very small 1.8nF capacitor together with the original switching network. Its frequency response is concentrated in the very high treble range.

As a result, the difference between Bright OFF and Bright ON may be much less dramatic than the Bass or Treble controls.

This is normal.

The effect is generally easiest to hear when:

using fresh strings,
playing slap or pop techniques,
listening through a full-range system,
using headphones,
using a cabinet with a tweeter,
or recording directly into an interface.

A traditional bass cabinet with limited high-frequency response may reproduce very little of the frequency range affected by the Bright circuit.

Do not assume that the Bright circuit is defective simply because its effect is subtle.

For the clearest comparison, use a clean signal path and repeatedly switch Bright on and off while playing material with strong high-frequency content.

Testing the Bright Switch With a Multimeter

If the Bright function does not appear to work, first verify the switch electrically.

Disconnect the battery.

Set a multimeter to continuity or resistance mode and measure between BRIGHT and GROUND.

With Bright OFF, the connection should be open.

With Bright ON, BRIGHT should have continuity to GROUND.

If the switch changes correctly between open and grounded states, the external Bright-switch wiring is functioning.

If BRIGHT remains grounded in both switch positions, the Bright function will remain engaged continuously.

If BRIGHT never connects to ground, the Bright function will never engage.

Output Jack and Battery Wiring

For conventional battery switching, use a TRS stereo output jack.

The three contacts are:

Tip — audio output
Ring — battery negative
Sleeve — circuit ground

Connect:

Output jack Tip → OUTPUT JACK

Output jack Ring → Battery negative

Output jack Sleeve → GROUND

Connect:

Battery positive → 9V+

Battery negative → Output jack Ring

Do not permanently connect battery negative directly to GROUND in a conventional switched-jack installation.

How the Battery Switching Works

A standard bass cable uses a TS mono plug.

When the plug is inserted into a TRS jack, the plug's Sleeve contacts both the jack's Sleeve and Ring connections.

This connects battery negative to circuit ground and powers the preamp.

Removing the cable breaks that connection and switches the preamp off.

For this reason:

Always unplug the instrument cable when the bass is not being used.

Grounding

All circuit grounds ultimately need to be electrically common.

The following should normally be connected to circuit ground:

MMSA78 GROUND
pickup shields
bridge/string ground
output jack Sleeve
Volume CCW
potentiometer bodies
conductive cavity shielding, when present
Bright-switch ground

These connections do not all have to terminate directly at the same screw terminal. They may be joined elsewhere in the instrument as part of a common ground network.

Potentiometer Body Grounding

How the potentiometer bodies are grounded depends on the instrument.

Metal Control Plate

When the controls are mounted to a conductive metal control plate, the plate may electrically bond the potentiometer bodies together.

Verify continuity between the potentiometer bodies and circuit ground with a multimeter.

Do not simply assume that mechanical contact guarantees a reliable electrical connection.

Paint, oxidation, washers, coatings, or other hardware may interfere with continuity.

Through-Body or Rear-Routed Installation

When potentiometers are mounted directly through wood, the bodies are not automatically electrically connected together.

Use suitable ground wires to connect the potentiometer bodies to circuit ground.

A daisy-chain arrangement connecting Volume, Treble, and Bass to the MMSA78 GROUND connection works well.

Bridge Ground

The bridge should normally be electrically connected to circuit ground.

Connect the bridge-ground wire to the instrument's common ground network.

A missing bridge ground is a common cause of hum that changes when you touch the strings or bridge.

Shielding

Conductive shielding paint or copper foil can reduce noise in some installations.

If shielding is used:

make sure it is connected to circuit ground,
verify continuity between separate shielding sections,
and make sure the underside of the MMSA78 cannot touch it.

A properly grounded conductive cavity can still short the preamp if exposed PCB connections touch the shielding.

Always provide adequate insulation beneath the board.

Some vintage-style pickups may require additional attention to shielding and grounding.

For example, some vintage-construction pickups do not electrically ground their pole pieces.

Output Protection

The MMSA78 includes a 1kΩ output protection resistor similar to the protection Music Man incorporated into later versions of its active electronics.

This resistor helps protect the increasingly scarce LM4250CN opamp if the instrument output is accidentally shorted.

It is part of the normal MMSA78 circuit and requires no additional wiring or modification.

Replacing an Original Sabre Preamp

The MMSA78 is designed as a replacement for the early Music Man Sabre preamp used in the late 1970s and early 1980s.

Before removing the original preamp:

Photograph all wiring.
Identify each connection by function.
Label wires where useful.
Preserve the original pickup selector, phase switch, potentiometer, output-jack, and Bright-switch wiring whenever possible.
Leave as much original wire length as practical.

If possible, transfer wires from the original preamp to the MMSA78 one connection at a time.

Do not assume that an older instrument still has completely factory-original wiring.

If the bass uses the later Sabre electronics with blade-style switching and a StingRay-style preamp, the MMSR77 is generally the more appropriate direct replacement.

Before Applying Power

Before reconnecting the battery, inspect the entire installation.

Verify:

Pickup or pickup-selector output → PICKUP INPUT
Pickup shields → GROUND
Bridge ground → GROUND
Volume CW → VOL CW
Volume Wiper → VOL WIPER
Volume CCW → GROUND
Treble CW → TREBLE CW
Treble Wiper → TREBLE WIPER
Treble CCW → TREBLE CCW
Bass Wiper → BASS WIPER
Bass CCW → BASS CCW
Bass CW → Not connected
Output jack Tip → OUTPUT JACK
Output jack Ring → Battery negative
Output jack Sleeve → GROUND
Battery positive → 9V+
Bright switch connects BRIGHT to GROUND only when engaged
All potentiometer bodies are grounded.
Existing pickup selector and phase-switch wiring has been preserved.
No stray wire strands can contact adjacent terminals.
Every conductor is actually captured inside its screw-terminal box clamp.
The underside of the MMSA78 is insulated from conductive material.

Only after completing these checks should the battery be connected.

Complete Wiring Diagram

The diagram below should show a complete MMSA78 installation including:

pickup input,
Volume,
Treble,
Bass,
Bright switch,
TRS output jack,
battery,
bridge ground,
and circuit ground.

For an original dual-pickup Sabre, the existing pickup selector and phase-switch circuitry normally remain ahead of the MMSA78 PICKUP INPUT.

Original Vintage Music Man Sabre Wiring

Testing the Installation

Before closing the control cavity:

Connect the battery.
Plug in a known-good instrument cable.
Connect the bass to a clean amplifier or audio interface at moderate volume.
Verify Volume operation.
Verify Bass boost and cut.
Verify Treble boost and cut.
Verify all pickup-selector positions.
Verify the phase switch if present.
Compare Bright OFF and Bright ON.
Listen for hum, distortion, crackling, or intermittent output.
Gently move the wiring and verify that all screw-terminal connections remain stable.
Unplug the instrument cable and complete final assembly.

Test the instrument while the electronics are still accessible whenever possible. Troubleshooting is much easier before the cavity or control plate is closed.

Troubleshooting

No Output

Check the basic power and signal connections first.

Verify:

Battery positive → 9V+

Battery negative → output jack Ring

Output jack Sleeve → GROUND

Output jack Tip → OUTPUT JACK

Pickup-selector output → PICKUP INPUT

Check every screw-terminal connection with a gentle pull.

A conductor inserted underneath the internal box clamp may appear correctly installed while making little or no electrical contact.

No Output From One Pickup or Selector Position

If other pickup positions work normally, the MMSA78 itself is probably passing signal.

Inspect:

the affected pickup,
pickup-selector wiring,
phase-switch wiring,
and connections between the pickup switches and PICKUP INPUT.

The pickup-selection circuit operates before the preamp.

Both Pickups Together Sound Thin or Quiet

If the bass has a phase switch, verify its position.

When two pickups operate out of phase with each other, frequency cancellation can produce a thinner sound with reduced output.

This may be the intended function of the phase switching rather than a fault.

Bright Switch Does Nothing

First remember that the original Sabre Bright effect is very subtle.

Use a clean amplifier, full-range system, headphones, or a cabinet with good high-frequency response.

Play bright material such as slapped, popped, or aggressively plucked notes while switching repeatedly between the two settings.

If there is still no noticeable change, disconnect the battery and use a multimeter to verify:

Bright OFF → BRIGHT is not connected to GROUND

Bright ON → BRIGHT has continuity to GROUND

If the switching does not change electrically, inspect the switch wiring.

Bright Is Always On

Disconnect the battery and measure continuity between BRIGHT and GROUND.

If continuity exists in both switch positions, the BRIGHT terminal is being permanently grounded.

Inspect for:

an incorrectly wired switch,
a solder bridge,
a grounded unused switch lug,
or a BRIGHT wire touching grounded hardware.

Bright Works in the Opposite Toggle Direction

This is not an electrical problem.

On an SPDT switch, move the GROUND connection from one outer switched terminal to the other.

This reverses which physical switch position engages Bright.

Bright Switch Introduces Hum or Noise

Verify that the Bright switch uses a reliable circuit-ground connection.

Check continuity between the switch ground and MMSA78 GROUND.

If the switch is grounded through a potentiometer body or metal control plate, verify that those parts are actually connected to circuit ground.

Also inspect the Bright wire for loose strands or contact with unrelated terminals.

Bass or Treble Control Does Not Operate Correctly

Verify the potentiometer values and wiring.

For a vintage-style installation:

Treble → C1M reverse-audio

Bass → A100K audio

Check the CW, Wiper, and CCW connections.

Remember:

Bass CW → Not connected

Volume Control Does Not Operate Normally

Verify that the Volume potentiometer is approximately 25kΩ.

Connect:

CW → VOL CW

Wiper → VOL WIPER

CCW → GROUND

A substantially different potentiometer value can change both the control behavior and the pickup/preamp interaction.

Pickup Sounds Much Louder or Darker Than Expected

Check whether the pickup coils are wired in series rather than parallel.

Series wiring increases resistance and inductance and generally produces a hotter, thicker, darker response.

The MMSA78 will operate with a series-wired pickup, but the classic late-1970s Music Man configuration used parallel coils.

Refer to the Pickup Wiring Reference for tested wiring arrangements.

Excessive Hum or Noise

Verify:

pickup ground and shielding,
bridge/string ground,
output jack Sleeve,
potentiometer-body grounding,
Bright-switch ground,
and cavity shielding.

Also make sure the underside of the MMSA78 is not contacting conductive paint, foil, a control plate, potentiometer, switch, or output-jack hardware.

Battery Drains Quickly

Unplug the instrument cable whenever the bass is not being used.

Verify that battery negative connects to the output jack Ring and has not been permanently connected to GROUND or Sleeve.

Check the output jack for bent contacts or accidental shorts.

Circuit Works Outside the Bass but Stops After Installation

This strongly suggests an accidental short or pinched wire.

Inspect for contact between the underside of the MMSA78 and:

conductive shielding,
copper foil,
a metal control plate,
potentiometers,
switch bodies,
or output-jack hardware.

Also check for wires pinched beneath a control plate or cavity cover.

Reposition and insulate the preamp before testing again.

Final Installation

After the electronics have been tested successfully:

Confirm every screw-terminal wire is secure.
Position the MMSA78 so that it cannot contact conductive surfaces.
Insulate the underside of the board.
Arrange wiring so that it is not pinched or under tension.
Make sure the cavity cover or control plate does not press directly against the PCB or components.
Reinstall the control plate or cavity cover carefully.

Test the instrument once more after final assembly.

Need More Help?

For pickup-specific wiring, coil identification, magnetic polarity, and series/parallel information, see the Pickup Wiring Reference.

If you have any questions about installation, compatibility, or troubleshooting, please contact us!

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