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LT9914 Opto Coupler Location Next to Tremolo Speed Control

Vox reintroduced the Cambridge name to a line of solid state

amplifiers released in 1999. There were three Cambridge models: the fifteen watt, 1x8" V9169 Cambridge 15, the thirty watt, 1x10" V9310 Cambridge 30 Reverb and the thirty watt 2x10" 9320 Cambridge Reverb Twin. All three included a 12AX7 tube in the preamp section, making these the first in a long series of "hybrid" amps produced by Vox.

Channel Switcher
The Cambridge 30 had a single input "channel switcher" design. A push button on the control panel toggled between the first and second channel.

Channel one included only volume, bass and treble controls. The circuitry of this channel was all solid state, making it the "clean" channel.

Channel two featured the 12AX7 tube. In addition to gain, volume, treble and bass controls, channel two also included a gain boost switch and an "MRB" (mid range boost) switch.

The circuit design for channel two was a blend of transistorized and tube circuitry. The 12AX7 was arranged in the audio signal path between two solid state integrated circuits. This first triode of the 12AX7 was used as a basic gain stage, the second triode provided gain recovery for the MRB mid boost circuit and the tone controls.

Reverb and tremolo were available on either channel.

Reverb
The Cambridge 30 included a three spring analog delay line that was mounted inside the chassis. The reverb drive and receive signals were provided by a single NJM4558DD integrated circuit.

Optical Tremolo
The term "Achilles heel" is often used to describe the presence of an unintended weakness, in spite of overall strength, that leads to total failure. The LT9914 opto coupler used in the tremolo circuit became the Achilles heel in the Cambridge 30. The failure of this part leads to a severe loss of output that has been dubbed by many in internet chat rooms as "Cambridge-itis."

Opto couplers are included in the tremolo circuits of many amps. An opto coupler consists of a small lamp and a photo resistor.

An amplifier's tremolo "drive" circuitry sends pulses of voltage to the opto coupler lamp, causing it to flash on and off. The tremolo speed and tremolo depth controls regulate the speed and brightness of these flashes. These light pulses are directed into the sensor of the photo resistor, causing changes in the resistance created in the device. When the lamp is off, the photo resistor applies no resistance to the preamp circuit and the audio level is unaffected. When the lamp is on, the photo resistor introduces an increased electrical resistance to the preamp signal, causing the volume of the audio signal to drop. The opto coupler uses a lamp and a photo resistor to convert flashes of light into rhythmic pulses in volume, or tremolo.

The LT9914 opto coupler in the Cambridge 30 was an "inverse " design. Unlike the example above, the photo resistor in the LT9914 had high electrical resistance when no light was applied to the sensor and low electrical resistance when fully illuminated. To maintain normal audio output, the Vox design required the opto lamp to remain at full brightness, even when tremolo was not in use. This led to premature lamp failures in the opto coupler.

When the opto coupler bulb fails in the traditional tremolo circuit, the amp continues to operate at normal level, only the tremolo stops working. When the bulb fails in the Cambridge 30 opto coupler, the amp loses 90% of its output power and is stuck in the "quiet" side of the tremolo cycle.

There is a simple test to determine if the Cambridge 30 9914 opto coupler is defective. Perform this test at your own risk. The opto coupler is on the main PC board near the tremolo speed control (see photo above and at left). There are four leads coming out of the opto. Place a jumper wire between the two leads that face toward the center of the amp. If the amp returns to normal level while these leads are jumped, you probably need to replace the opto coupler.

Replacing the opto coupler requires that the main PC board be removed from the chassis. Suitable opto replacements include another LT9914, a Vactral p/n VTL53/2, Vactral p/n VTL5C3 or a Fender Opto p/n 16282.

30 Watt Power Amplifier Chip Rather than designing a power amp stage using discrete electronic components, Vox chose to utilize a TDA2050A integrated circuit power amplifier chip for the Cambridge 30 amp. The TDA2050A chip functioned as a complete solid state 30 watt Class AB power amp. The chip's internal circuitry included over 40 electronic components. All that was required for operation was a 50 VDC supply, an audio input signal, an inverse feedback loop connection and a ground. The TDA2050 was bolted to a large finned heat sink to protect the chip from thermal overrun and is designed for no lower than eight ohm loads.

Increasing the RMS Output Power to 56 or 68 Watts
Some have suggested that replacing the TDA2050A power amp chip in the Cambridge 30 with either a Texas Instruments LM3875 or LM3886 would offer a significant increase in the RMS power rating. One blog suggested that the LM3875 would increase output power from 30 watts RMS to 56 watts RMS and the LM3886 to 68 watts RMS. A brief view of the Cambridge 30 power supply and the voltage demands of either chip suggests that this is not the case.

The Cambridge 30 power supply provides 50 VDC (-25VDC and +25 VDC) to the TDA2050A chip. This voltage is consistent with the engineering data provided by the SGS Thomson, the manufacturer of the TDA2050, that states that the V_S, or maximum supply voltage required to develop the rated 30 watt RMS output is +25VDC (see chart below).

The engineering data for both the LM3875 and LM3886 chips indicates the the V_S, or maximum supply voltage is 92 VDC, nearly twice that of the TDA2050A (see table below). Unless the Cambridge 30 power supply is upgraded to provide a 92 VDC supply voltage, the output power of the LM3875 or LM3886 chips will be limited to what they can produce with a 50 VDC supply. This is still about 30 watts.

North Coast Music offers the following cosmetic spare parts
and optional foot switches for the Cambridge 30 amplifier.

Genuine Vox Factory Spare Corners Genuine Vox Factory Spare Handles Gold String White Grill Piping Fascia Strip

Genuine Vox Black Grill Vox logo for the Cambridge 30 (NCM-026) Vox VFS2 Foot Switch Pedal