A simple preamp, designed primarily as a line booster.
Not long after completing my subwoofer in 2006, I discovered a problem: my then-modern PC audio output, using a soundcard of early-2000s vintage, did not have enough gain to drive my audio system over a 20-foot unbalanced cable run. The cable attenuation was too large to overcome. I took stock of my supplies and realized I could remedy this using the Elliott Sound Products Project 88 preamp as a line booster.
One small hitch: I didn't have a negative-rail voltage regulator IC on hand at the time. Opamps usually have good supply rejection, but rather than use an unregulated supply, I figured a variation of Project 102 would do the trick. It is a simple transistor regulator that clamps the base at a desired step-down voltage using a Zener, pretty much the same application as a pass-transistor in a high-current LMxxx voltage regulator application.
For two paltry two opamps I could have skipped the transistor entirely and just let the Zener regulate directly, but I wanted the experience of working with yet another new and previously unfamiliar circuit. One application, two new projects to try out.
The preamp circuit came together without too much fuss:
Figure 1. The preamplifier circuit.
The two opamps happen to be cheap TL082s, because that was what I had available. TL072s would otherwise be preferred due to improved noise performance, and are equally cheap. Local supply decouplers and a ceramic bypass capacitor are located between the two opamp ICs. While probably not required for TL082s, this is a standard topology for opamp power pins and would be required for higher-speed units such as NE5532s or OPA2134s, both of which are popular in audio circles.
For input decoupling I created two 5µF bipolar units by wiring 10µF pairs with the negative terminals connected back-to-back. Dedicated bipolar units are nice if you have them in stock, but I did not. Electrically, this is the same thing. Meanwhile the large resistor/capacitor pair in the upper-left corner are a ground lift to isolate the preamp board ground from the supply ground, which is earthed.
Figure 2. The power supply regulators.
Everything about this supply amounts to killing a fly with a shotgun. The board could easily feed a small stereo power amplifier, let alone two dual opamps sourcing perhaps 50mA total. But, I wanted the experience, and it works. The rectified DC enters at the two large capacitors, and then hits the regulators. A simpled resistor-zener network clamps each base to 12V away from 0V, holding the outputs at +/-12V. The outputs then pass through a pair of 470µF capacitors and a 1Ω resistor per rail, wired to create a CRC filter.
If the TO-220 transistors' labels were legible, the part number would show jaw-dropping specs, but that was because they were pulls from a different project that went sideways and was abandoned. Here, they are mounted to a heatsink from a switching power supply using beryllium oxide insulators. (At least, I think that's what they are. In retrospect, aluminum oxide - also known as sapphire - is a possibility.) BeOx ceramic is a strong, sparkling white material with excellent dielectric and heat conducting properties. The powdered form also turned out to be a spectacular lung carcinogen, so production is subject to hazardous material regulations that make BeOx rare and highly specialized in modern days. I stumbled across bin of these TO-220 insulators at a surplus shop, and bought as many as I could afford.
The interior of the preamp-booster is as follows:
Figure 3. Overhead view of the premamplifier layout.
The preamp took shape in a layout that turned out to be a repeat performance in future projects. AC enters at the back left portion of the chassis, and the hot lead passes through a fuse before bouncing through the power switch on the front panel. The bridge rectifier is crowded into a vertical position in the front corner to keep the AC wiring as far away from the preamp board as possible.
After passing through the power supply regulators, a feed is routed over to the preamp board, which has all I/O jacks on the back, and volume and balance controls on the front.
With the preamp completed and assembled, there isn't a whole lot to see. No form, all function, really:
Figure 4. Front view of the completed preamplifier.
Left to right are the power switch, a power indicator LED, volume control, and balance control. There is no source selector switch because I didn't need that feature for this application.
From the rear, the unit presents itself less gracefully:
Figure 5. Back view of the completed preamplifier.
The steel chassis and proved difficult to modify with my limited machining skills at the time, hence the ugly cutouts. The first RCA-type jack pair accepts the source input and the other two pairs are wired in parallel to the output. The cutouts may be ugly, but the RCA jack grounds are fully isolated from the chassis, so the function is fine.
The IEC power inlet is located at the far right-hand side of the image.
While funcitonal at best and crude at worst, the unit does work. For a while, it amplfiied my PC soundcard's output and send the result to both my stereo system (for those blissful times when everyone else was out of the house) and my V2.0 DoZ headphone amplifier (most other times).
The unit is not presently in use, but I keep it around for testing power amplifiers. The opamp-based design makes it relatively immune to output short-circuits and ensures that a serious wiring error in the amplifier will only blow cheap JFET opamps in the preamp, rather than feeding back to the source device. §
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