The nice thing about the LM3900 is that it operates on a single supply voltage. Electronic music circuits tend to be bipolar though, with signals having excursions above and below ground, or 0V. A desireable interface circuit for this situation is one that can accept the bipolar input, then offset it to the middle of a unipolar voltage range, for subsequent signal processing. Using a single +15V supply, the following LM3900 circuit accepts +/-5V input waveforms, and offsets them to +15V/2, or 7.5V. The output is the same 10 Vpp input, centered around 7.5V, with a phase inversion of 180°.
This variant of the circuit allows precise trimming of the actual VZR:
This type of circuit was part of a realization that LM3900 circuits can use any number of voltages as a Vertical Zero Reference, or VZR. This enables bipolar processing in a unipolar supply, and allows custom high-quality reference voltages to be employed (the reference diode circuit, above) that are not directly connected to an actual ground -- potentially avoiding noise currents seen on a busy ground return bus.
This variant of the circuit allows precise trimming of the actual VZR:
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