JBL 5101 Microphone Preamp — Pair, Fully Serviced — Circa 1970–72 — Genuine JBL Transformers

Offered here is a pair of JBL 5101 microphone preamplifiers, original gray-faceplate versions, serial numbers 6630152 and 6630186. These are early production units dating to approximately 1970–72, with serials 34 apart — almost certainly from the same production run. Circuit The 5101 is a discrete, transformer-balanced FET preamp with a single gain control. Input impedance is 150 ohms; output impedance is 600 ohms balanced. It is a straightforward, no-frills design — no phantom power, no polarity switch, no pad — but the combination of FET gain stage and transformer I/O gives it a character that more complex signal chains sometimes lack. FET-based preamps from this era occupy an interesting middle ground sonically — quieter and faster than tube designs, but with a natural, slightly rounded character that solid-state op-amp designs of the same period often lacked. Paired with transformer I/O on both input and output, the result is a preamp with real low-end body and a gentle, transformer-induced softening of transients that tends to sit well in a mix. The 150-ohm input impedance will interact with microphone output transformers in ways that affect tone — a characteristic that ribbon mic users in particular may find interesting. Genuine JBL 5901 mic input transformers and 5195 output transformers are included and installed. Service Work Both units have been comprehensively serviced: Full power supply rebuild: all electrolytic capacitors replaced, rectifier diodes replaced, zener diode replaced All other electrolytic capacitors replaced as needed throughout Pilot lights replaced Pots cleaned and lubricated Both units tested with a live microphone source Connectivity XLR input jacks have been installed on both units. XLR output tails have been added. All wiring is pin-2 hot, standard balanced configuration, using Switchcraft XLR connectors — ready to connect to any modern interface or patchbay snakes without adapters. Frequency Response Both units were swept using REW and a Fireface 800 interface. Transformers were selectively swapped across the available inventory to optimize unit-to-unit consistency. Midband response (100 Hz–5 kHz) tracks within 0.1 dB between the two units. Minor transformer-related variation exists below 50 Hz, as is typical of vintage units of this type. High-frequency response rolls off gradually above the midband — characteristic of transformer-coupled designs and part of the sonic signature. Cosmetics Faceplate paint shows chipping and some light staining consistent with age and prior studio use. Chassis and internals are clean. Photos show the actual condition accurately. Sold as a pair. Separation considered if the pair does not sell.