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Tech’s Files: TESTA FET


Illustration: Eddie Ciletti

By the title of this month’s column, you’d think I’d discovered a new Star Wars character, but really it’s something I’ve been doing lately, and if it isn’t obvious, it will be. The technical revolution has made dinosaurs of not only recording equipment, but also some critical components. There are many reasons why components become difficult to find, or worse, obsolete. All of this means that from low-noise, high-gain preamp parts to cheap-and-dirty effects ICs, keeping some of our quirky toys alive means scrounging for parts in unusual places.

Ironically, it’s easier to buy vacuum tubes — and related paraphernalia — than it is to get certain ’70s-era integrated circuits (ICs), Field Effect Transistors (FETs) and even plain-old transistors. This has many audio design engineers scrambling for suitable parts.

A New Niche

Just when you thought there was no hope for your ’70s-vintage stomp boxes, Cool Audio ( steps in to fill the void. The company makes a variety of application-specific devices (like converters and discrete transistors), yet two products that caught my eye were bucket brigade devices — analog delay lines used to make the classic tapeless echo/delay sounds.

Linear Systems ( offers the LSK170, its version of a low noise, low-capacitance JFET amplifier (discontinued by Toshiba) that is the darling of the condenser microphone crowd.

The MCI Project

This leads me back to that cryptic column title: I just spent the past month sourcing FETs for three different products: an MCI Auto-Locator III, an ADR F760 compressor/limiter (built into the Raindirk console featured in the July 2008 “Tech’s Files”) and a mic preamp kit used in my electronics class. While a curve tracer is the ideal device for grading and matching semiconductors — transistors, FETs and even zener diodes — let’s examine a method of sorting/grading FETs with basic test equipment.

Simple circuits require components with tighter tolerances, especially if you want identical performance from two or more channels. Cases in point are compressor/limiters like the Teletronix LA-2A and the UREI 1176, both of which rely on a “simple” voltage divider for gain reduction. When these are used on mono sources, any idiosyncrasies can be cherished and embraced, but as soon as two channels of “anything” are strapped together, consistency becomes a necessity. The problem with replacing the key components in such signal processors is that their respective factory-replacement photo-resistors and FETs are prematched to an in-house reference. Sometimes, components like these are given proprietary part numbers that can’t be cross-referenced.

Similarly, MCI tape machines and Auto-Locators use FET switches: non-mechanical, solid-state “relays” that turn circuits on/off in response to “remote” commands. Mechanical switch and relay contacts do “black and white” switching well, but over time they are prone to corrosion and eventually become unreliable and noisy. With electronic switching, the transition from on to off (and vice versa) can be made “gray,” a soft transition that, in the worst case, makes a thump instead of a loud pop or snap.

The Sorting Hat

Figure 1 shows a simple schematic that was built on a solderless prototyping board to test a wide range of FETs. A sine wave is applied to each FET’s input (gate) while monitoring the output (drain) via oscilloscope for the clipping threshold. Figure 2 shows four “windows” that represent the range of FET behavior. Fifty FETs of each type were tested, yielding a standard deviation bell curve, with most of the FETs falling within the middle range, with just a few on either side.

For purely audio purposes, a simple circuit with no negative feedback (very similar to the eval circuit) yields a gentle overload characteristic. Visual pre-testing is the easiest way to ensure that each preamp behaves in a similar manner. FETs that don’t make the grade can be made to work by tweaking the resistor values, especially the bias (source) and load (drain) resistors.

When a control voltage is added to the FET switching and stereo gain-reduction equation, more precise tolerances are required to ensure the identical response from each device. To refine the sorting process, the oscillator was turned off and a voltmeter attached at the junction of the drain and the load resistor, yielding the numeric results in the table shown in Fig. 2.

Finding Zero

As most auto-locators guide a tape transport toward the locate point, the wind speed is typically ramped down. The MCI A/L III uses FET switching to select a range of suitable speeds. Four of the five switching FETs are on a 16-pin Intersil 5011 IC that is no longer available. The fifth FET, a P1086, is similar to the 5011’s characteristics; it’s still available but is not easy to find. The Fairchild data sheet provided a clue — “sourced from process 88” — that led me to a readily available alternative, the J175. The obsolete 5011 was replaced by four matched FETs and four diodes built on a 16-pin DIP header. The fifth matched FET replaced one on the circuit board.

After projects like this, my workbenches are always a mess. Now a new sorting process begins: Transferring several paper charts stuffed with FETs into drawers without them being “de-graded.” And that’s the easy part!

Eddie thanks Steve Sadler for the MCI tip and Dave Meyers for suggesting NelsonPass’
site with accompanying power MOSFET tutorial.


Active = Amplification

Active devices can amplify — such as vacuum tubes, transistors, etc. — while passive devices — resistors, capacitors and inductors — can only manipulate the signal (divide, affect frequency response, block DC, etc.). The amount and quality of amplification is pre-determined by design and confirmed in post-production by testing.

When “plumbing” is the analogy, N-channel FETs (the solid-state equivalent of vacuum tubes) have three parts The gate (grid) is the “valve” that controls the amount of water/current that flows through the source (cathode) and the drain (plate). You can think of the drain’s load resistor as a bungee cord hooked into the ceiling (the power source). The amount of “strength” a FET needs to pull the bungee down toward ground is being measured. In an audio circuit, this “biasing” process optimizes the FET to pull the “bungee” halfway down so that the signal can symmetrically swing as high up as down.
Eddie Ciletti