L Series Linear Amplifiers

Each of our L series amplifiers contains regulated power supplies, a Pre-Amplifier PWB, Output PWBs, and an output transformer system. The L series Output PWB is a current source. Up to 50 PWBs may be combined and still maintain proper sharing of current. This is the configuration of an L50, our largest L series amplifier. Voltage feedback is used to create a voltage source or resistive source.

These amplifiers, together with the M series, are dual rated. That is, they are capable of both continuous as well as burst output. During continuous output into a reactive load, the heat sinks warm up. The heat sink temperature is sensed by the Pre-Amplifier and used to determine a safe current limit. While driving short tone bursts, or resistive loads, the heat sinks are cooler, therefore the current limit is increased by as much as 2.5 times. This function is automatic and on-going, cycle by cycle. This enables the amplifier to safely maximize output power under all conditions.

The enhanced power output during a 1 second tone burst is obtained by a slow average current limit in the power supply. The system is well protected, and there is no combination of input signal and passive loads which will damage these amplifiers. Active loads must be less powerful than the amplifier rating.

Models L2, L6, and L10 are standard rack mount units. They require only a single phase main power source. The larger models (L20 - L50) are all powered from a three phase line.

In order to optimize the high frequency performance and to fit in a small chassis, the L2 output transformer is only large enough for full power operation down to 500 Hz. The high end is limited mostly by the slewing rate of the output transistors. This gives usable continuous rated performance with the L2 up to 200 kHz (about 100 VA).

The L6 has an output transformer capable of -3 dB continuous rated performance from 40 Hz to 150 kHz.

In models L20 and larger, each bank of 10 Output PWBs is wired independently to its own output transformer. This has enabled us to obtain the same superior high frequency performance from an L50 as we can achieve from an L10.

The output transformers have multiple secondaries with taps to match various loads. These secondaries are put in series and parallel combinations. This is done with relays which are always "cold switched" since the relays are commanded to move only after the amplifier has been automatically inhibited. The various load impedance taps may be selected by a front panel switch or controlled remotely. At least 70% of the secondary copper is used at all times.

Selectable Source Impedance

Our switching amplifiers are voltage sources. By taking current feedback, the output can be made to have a known source impedance. This impedance is also known as damping factor and may be made as great as twice the matching load impedance.

Our L series amplifiers have a switch to select the characteristic of the source impedance. As a voltage source, the output voltage tends to remain constant as the load impedance varies. As a current source, the current does the same. As a resistive source, the amplifier appears to have a resistor in series with the output of a voltage source. Full power is still available for the load. The value of this synthesized resistor is approximately the same as the matching burst-rated load impedance. The resistive source has the advantage of damping out ringing and other transients. Transducer calibration in small tanks is an ideal application of the resistive source.

Linear, Low Distortion

Our L series amplifiers are very linear, even without feedback. In the current source mode (open loop), midband distortion is typically 1.0%. Voltage feedback reduces this to less than 0.1%. The dynamic range of more than 100 dB makes them useful for a wide variety of applications.

Low Frequency Option

On Models L10-L50 the low frequency performance may be specified to -3 dB at 40 Hz, rather than the standard 200 Hz. This option requires that we build a larger output transformer system. This may extend delivery to 4 months, depending on delivery of core materials and it will lower the high frequency limit to about 60 kHz when operating at the CW power level and about 30 kHz when at the Burst level. The amplifier size and price is unchanged, but its weight increases slightly. This option is not available on the L2 due to size constraints. Our standard model L6 has an output transformer with a -3 dB operating bandwidth at the CW power level of 40 Hz to 150 kHz.

Specifications For Standard L Series Amplifiers