MMSR77

Manual

Thank you for your purchase of a Boscoe Music MMSR77 preamp!

The MMSR77 is a reproduction of the 2-band preamp used in late-1970s Music Man StingRay basses. Each preamp is individually tested before shipping and is designed to help recreate the response and character of the original StingRay electronics.

The standalone MMSR77 is intended for installations where you are providing your own potentiometers, output jack, battery connection, pickup wiring, and other associated hardware. It can be used to replace an existing preamp or as the basis for a new custom installation.

Pickup and Control Selection

The MMSR77 will work with many different pickups and wiring configurations. For the closest approximation of the original late-1970s StingRay sound, however, the pickup and control values are important.

The original circuit was designed around an Alnico V Music Man-style humbucker wired in parallel, with approximately 1750–1950Ω DC resistance and approximately 700mH inductance. Early StingRay pickups varied somewhat in winding count and specifications.

For a vintage-style installation, Boscoe Music recommends:

Pickup: Music Man-style Alnico V humbucker, coils wired in parallel
Volume: 25kΩ, A25K or B25K
Treble: 1MΩ reverse-audio taper, C1M
Bass: 100kΩ audio taper, A100K
Power: 9V nominal

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

A pickup wired in series can still be used with the MMSR77, but its higher output, resistance, and inductance will produce a different response from the original parallel-wired StingRay pickup.

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

Why the Potentiometer Values Matter

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

Using substantially different potentiometer values or tapers may alter the usable range of the controls or make them behave differently from a vintage StingRay.

For the closest reproduction of the original control response, use the recommended values above.

Power

The MMSR77 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 or an appropriate regulated 9V supply intended for onboard instrument electronics.

If operating the MMSR77 from two 9V batteries at 18V, they must be connected in series. Do not exceed 18V.

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

When a standard mono instrument plug is inserted into the jack, 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 clear photographs of the original wiring. Pay particular attention to:

pickup connections
bridge ground
output jack wiring
battery wiring
potentiometer connections
cavity shielding and grounding

These photographs can be extremely useful if you need to identify an original connection later.

Disconnect the battery before beginning installation, and please complete all wiring before reconnecting the battery.

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

The MMSR77 may be a close fit in some four-string StingRay-style basses with narrow crescent-shaped control cavities. Leave enough wire length to reposition the preamp and controls as necessary.

Arrange the installation so that:

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

Protecting the Preamp From Shorts

The solder joints and component leads on the underside of the MMSR77 must not contact conductive surfaces.

This includes:

conductive shielding paint
copper foil
metal control plates
potentiometer bodies
output jack hardware
other exposed metal

Use a suitable nonconductive insulating material between the underside of the preamp and any conductive surface.

A self-adhesive neoprene or other insulating pad works well because it provides both electrical insulation and some mechanical cushioning.

Do not allow adhesive material to interfere with the screw terminals or components, and do not force the PCB tightly against other hardware.

MMSR77 Connections

Current-production MMSR77 preamps provide two GROUND screw terminals, one on each side of the PCB.

Both GROUND terminals are electrically common and are provided to make wiring and component placement easier. Either or both may be used as convenient.

Typical connections to GROUND include:

pickup ground and shield
bridge ground
volume potentiometer ground
output jack Sleeve
potentiometer-body grounding
cavity shielding ground

The MMSR77 connections are:

Control Connections

VOL CW
Volume potentiometer CW lug

VOL WIPER
Volume potentiometer center/wiper lug

TREBLE CCW
Treble potentiometer CCW lug

TREBLE WIPER
Treble potentiometer center/wiper lug

TREBLE CW
Treble potentiometer CW lug

BASS WIPER
Bass potentiometer center/wiper lug

BASS CCW
Bass potentiometer CCW lug

Signal and Power Connections

PICKUP INPUT
Signal from pickup

OUTPUT JACK
Signal to output-jack Tip

9V+
Battery positive

GROUND
Circuit ground

The two GROUND terminals may be used interchangeably.

Using the Screw Terminals

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

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

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

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

It is possible for a wire to be inserted underneath the internal box clamp rather than inside it. In that situation the screw may feel tight even though the wire itself is not securely clamped.

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

Do not overtighten the terminal screw. It only needs to be tight enough to securely retain the conductor.

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 or B25K potentiometer may be used depending on the preferred control taper.

Connect:

CW (lug 3) → VOL CW

Wiper (lug 2) → VOL WIPER

CCW (lug 1) → GROUND

Either MMSR77 GROUND terminal may be used.

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 a useful control response similar to the original StingRay circuit.

Bass Potentiometer

Use an A100K audio-taper potentiometer.

Connect:

CW (lug 3) → Not connected

Wiper (lug 2) → BASS WIPER

CCW (lug 1) → BASS CCW

Left Handed Basses

If you have a left handed bass you will want to use potentiometers with the opposite taper and swap the CW and CCW wires.

Volume: A25K → C25K (B25K works for both right and left)

Treble: C1M → A1M

Bass: A100K → C100K

Note that you will also have to connect the outer wires in reverse as well. Treble CW lug connects to TREBLE CCW on the preamp. Bass CCW lug is unwired and the CW lug is wired to the BASS CCW terminal, etc. Wiper connections remain unchanged. Please contact us if you have any questions about how to wire your lefty bass.

Pickup Wiring

Connect the pickup output lead or leads to PICKUP INPUT.

Connect the pickup ground, shield, and any bare shield wire to either GROUND terminal.

The original StingRay used its humbucker coils in parallel.

For the Sterling by Music Man H-1 pickup configuration tested by Boscoe Music, parallel wiring is:

Red + White → PICKUP INPUT

Black + Green + Bare → GROUND

Sterling by Music Man basses have been produced with different pickups and wiring configurations, so do not rely on wire color alone.

If you are unsure, identify the pickup coils with a multimeter before connecting them.

Some Sterling models are wired with their pickup coils in series from the factory. A series connection can be used with the MMSR77, but it will not present the preamp with the same pickup characteristics as the parallel-wired original StingRay configuration.

Some vintage-style pickups may require additional attention to grounding and shielding. For example, vintage-style construction may leave the pole pieces electrically isolated from ground.

For additional pickup models and tested wiring arrangements, see the Pickup Wiring Reference.

Output Jack and Battery Wiring

For a conventional battery-switched installation, use a TRS (stereo) output jack.

The three jack contacts are:

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

Output Jack

Connect:

Tip → OUTPUT JACK

Ring → Battery negative

Sleeve → GROUND

Battery

Connect:

Battery positive → 9V+

Battery negative → Output jack Ring

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

How the Battery Switching Works

Connecting battery negative to the Ring terminal is intentional.

A normal instrument cable uses a TS mono plug. When that plug is inserted into the TRS output jack, its Sleeve contacts both the jack's Ring and Sleeve contacts.

This connects battery negative to circuit ground and switches the MMSR77 on.

Removing the cable breaks the battery-negative connection and turns the circuit off.

For this reason:

Always unplug the instrument when it is not being used.

Grounding

All circuit grounds ultimately need to be electrically common.

The MMSR77's two GROUND terminals provide convenient locations for these connections.

At minimum, verify the following:

pickup ground/shield → GROUND
bridge ground → GROUND
volume CCW → GROUND
output jack Sleeve → GROUND
potentiometer bodies → GROUND

If the bass uses conductive cavity shielding, the shielding should also be connected to circuit ground.

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 normally electrically bonds the potentiometer bodies together.

Verify continuity between the potentiometer bodies and MMSR77 GROUND with a multimeter.

Do not simply assume that mechanical contact provides a reliable electrical ground, particularly if painted, coated, oxidized, or nonconductive hardware is present.

Through-Body / Rear-Routed Installation

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

Connect the potentiometer bodies to circuit ground using suitable ground wires. A convenient method is to daisy-chain the three potentiometer bodies and connect the chain to either MMSR77 GROUND terminal.

A ring-terminal grounding harness, similar to the one supplied with Boscoe Music 18mm-shaft MMSR77 Kits, may also be used.

Bridge Ground

The bridge should be electrically connected to circuit ground.

This is normally accomplished by a wire running from beneath the bridge into the control cavity.

Connect the bridge ground to either MMSR77 GROUND terminal.

If the bass produces substantial hum that changes when you touch the strings or bridge, verify the bridge-ground connection.

Shielding

Conductive shielding paint or copper foil can reduce noise in some installations, although additional shielding is not always necessary.

If shielding is used:

make sure it is electrically connected to circuit ground,
verify continuity between separate pieces of shielding,
and make sure the underside of the MMSR77 cannot touch it.

A well-shielded cavity that accidentally contacts the underside of the PCB can short the circuit, so insulation around the preamp is particularly important.

Output Protection

The MMSR77 includes the 1kΩ output protection resistor that Music Man incorporated into later versions of the StingRay preamp.

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

The protection resistor is part of the normal MMSR77 circuit and should remain in place.

Replacing an Existing StingRay-Style Preamp

The MMSR77's connections and control functions are based on the late-1970s StingRay preamp and are intended to make replacement in an original-style wiring harness straightforward.

Do not assume that an existing instrument has remained completely original, however. Basses that are several decades old may have been rewired, repaired, or modified.

Before transferring wires from an existing preamp:

Photograph the original wiring.
Identify each wire by its function, not merely its color.
Confirm pickup, output-jack, power, and ground connections.
Transfer the controls according to the labels on the MMSR77.
Verify the complete circuit before connecting the battery.

Before Applying Power

Before connecting the battery, inspect the complete installation.

Verify:

Pickup signal → PICKUP INPUT
Pickup ground/shield → 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+
All potentiometer bodies are grounded.
No loose wire strands can contact adjacent terminals.
Every wire is actually captured inside its screw-terminal clamp.
The underside of the MMSR77 is insulated from conductive surfaces.

Only after this inspection should the battery be connected.

Complete Wiring Diagram

The diagram below shows a typical MMSR77 installation with a single humbucker wired in parallel, three controls, a switched TRS output jack, and a 9V battery.

The pickup colors shown in the diagram should represent only the specifically identified pickup configuration.

For the Sterling by Music Man H-1 humbucker configuration tested by Boscoe Music:

Red + White → PICKUP INPUT

Black + Green + Bare → GROUND

Pickup wire colors can vary by manufacturer, model, and production run. Always verify your pickup before making connections.

For additional pickup models, see the Pickup Wiring Reference.

Testing the Installation

Before closing the control cavity:

Connect the battery.
Plug a known-good instrument cable into the bass.
Connect the instrument to an amplifier at a moderate volume.
Verify that the Volume control operates through its full range.
Verify that the Bass control boosts and cuts normally.
Verify that the Treble control boosts and cuts normally.
Listen for excessive hum, intermittent output, or crackling.
Gently move the wiring and confirm there are no intermittent screw-terminal connections.
Unplug the instrument cable and complete the physical installation.

If possible, perform this test while the electronics are still accessible. Troubleshooting is considerably easier before the cavity is closed.

Troubleshooting

No Output

First verify that the instrument cable is fully inserted.

Because the output jack completes the battery circuit, the preamp will not normally power up until a plug is inserted.

Check:

Battery positive → 9V+

Battery negative → output jack Ring

Output jack Sleeve → GROUND

Output jack Tip → OUTPUT JACK

Then verify PICKUP INPUT and the pickup ground.

Check every screw terminal with a gentle pull on the wire. A conductor inserted underneath the internal clamp may look correctly installed while making little or no electrical contact.

Intermittent Output

Inspect the screw-terminal connections first.

Make sure each wire is actually captured by the internal box clamp and cannot be pulled from the terminal.

Check for:

loose strands,
partially inserted conductors,
wires under mechanical tension,
output-jack contacts touching other hardware,
or the underside of the PCB contacting conductive material.

Excessive Hum or Noise

Verify:

bridge ground
pickup ground/shield
output jack Sleeve ground
potentiometer-body grounding
cavity shielding ground, if present

On a through-body installation, make especially sure the individual potentiometer bodies have been grounded.

If the hum changes substantially when you touch the strings or bridge, inspect the bridge-ground connection.

Also make sure the underside of the preamp is not touching conductive shielding.

Pickup Sounds Much Louder or Darker Than Expected

Check whether a four-conductor humbucker is wired in series rather than parallel.

The MMSR77 will work with a series-wired pickup, but the resulting response will differ from the original StingRay configuration.

Refer to the Pickup Wiring Reference for tested pickup wiring.

Bass or Treble Control Does Not Behave Correctly

Verify both the potentiometer value/taper and its wiring.

For a vintage-style installation:

Treble → C1M

Bass → A100K

Make sure the CW, Wiper, and CCW terminals have not been reversed.

Also verify that Bass CW is intentionally left not connected.

Volume Control Does Not Behave Normally

Verify that the volume potentiometer is approximately 25kΩ and that:

CW → VOL CW

Wiper → VOL WIPER

CCW → GROUND

Substantially different pot values may change the interaction between the pickup and preamp.

Battery Drains Quickly

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

If the battery continues to drain rapidly, verify that battery negative is connected to the output jack Ring and has not been permanently tied to Sleeve or GROUND.

Also inspect the output jack to make sure its contacts are not being unintentionally shorted together.

Circuit Works Outside the Bass but Not After Installation

Inspect the underside of the MMSR77.

If the preamp works before being placed in the cavity but stops after installation, a solder joint or component lead may be contacting:

conductive shielding paint,
copper foil,
a potentiometer body,
the output jack,
or another piece of grounded metal.

Insulate and reposition the PCB before testing again.

Need More Help?

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

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

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