Preamp and Power Supply


For my purposes, there’s no reason to go haywire on an over-designed preamp. Modern audio op-amps offer exceptional performance, so that was a slam-dunk decision. My favorite op-amps are LME49710 and the dual LM4562. I find them easier to apply than the AD797, which is exceptionally prone to parasitic oscillation above 10 MHz.

Here are the main features of my preamp:

  1. Two-stage RIAA phono section with feedback equalization.

  2. Teledyne 122C military relays for source switching.

  3. Low overall line section gain (10 dB).

  4. Step attenuator for volume control.

  5. The “Better Balance Control” arrangement from ESP Audio.

  6. Cardas RCA phono jacks connected via micro Teflon coax.

The preamp runs off an independent power supply, with a small toroidal transformer, rectifier, and filter.

A Simple, Modern Preamp

Main Power Supply

The main power supply uses an Antek 400 VA toroidal transformer in a steel shield, 35A bridge rectifiers (one for each channel), and 30,000 uF of capacitance for each supply rail. A careful grounding plan was enforced, with a main single-point ground in the middle of the chassis. I also used a “ground break” circuit to separate the signal ground from the safety ground, but without adding a safety hazard.

To limit inrush current, I used a positive tempco inrush limiter with a time-delay relay that shorts it out after a second or so.

(Click to down- load power supply and protection schematic)

Output Protection

For system reliability, I wanted speaker output protection. The best way to do that is to monitor the power amp outputs and use a suitable relay to disconnect the load when DC or a thermal overload is detected. (The power amp protects itself with rail fuses that blow when the output is shorted.) I used a simple, old transistor array for this circuit, the TA7317. It provides a power-on delay, a quick disconnect when power is turned off, and disconnect when signals below about 0.4 Hz are present. The relay was a lucky find on ebay, a very expensive military relay: Deutsch ET4103602BG-D.  It’s a high-current 4PDT relay in a hermitic enclosure. Pairs of contacts are used in parallel.

(Click to down- load preamp schematic)

Headphone Amplifier

I’ve always liked to have a convenient headphone jack right on the front panel. This amplifier is a buffered opamp: An LME49710 with an LT1010T buffer. Distortion is below my instrumentation threshold (<0.002%) and it drives 32-ohm headphones easily. It’s also stable into large capacitive loads. It lives on the same board as the preamp power supply.

(Click to down- load headphone amp schematic)