Legacy of 4066 ICs

The 4066-series CMOS analog switch ICs were widely used in audio equipment designs of Golden Age of Hi-Fi, particularly in preamplifier stages and high-fidelity cassette decks. However, these devices exhibit relatively high on-state resistance, which varies nonlinearly with signal voltage and supply conditions. This nonlinearity introduces undesirable artifacts in the audio path, significantly degrading sound quality.

Specifically, the use of 4066-series switches can result in the generation of high-order harmonic distortion (THD) that varies with frequency, as well as intermodulation distortion (IMD) dependent on signal amplitude. Both effects contribute to significant degradation and a reduction in overall fidelity.

A more suitable approach for high-quality audio circuits is the use of analog micro relays, which minimize nonlinearity and preserve signal integrity.

Drop-in replacement for 4066 IC

We are proud to announce “ASH Modul” - drop-in replacement of 4066 Chip. The name ASH comes from an abbreviation of german Analog Schalter that means analog switch. It’s a classic and very useful transistor-driven relay circuit.

How works transistor-driven relay circuit

Let’s thinking of the transistor as a digital switch managed by an control signal. When the control signal goes high, current flows into the base. This current turns the transistor on. It now acts like a closed switch, creating a low-resistance path from its collector to its emitter. The current charges the relay coil. The magnetic field pulls the internal switch, causing it to change state. When the control signal goes low, no current flows to the base. Without this current, the transistor is in its cutoff state. It acts like an open switch and there is no path for current to flow through the relay coil. The relay is not charged and remains in its default state.

Layout and components

This compact 2-layer FR4 PCB was newly layouted for the ASH module. With dimensions of 24.80 mm × 18.50 mm and a thickness of 1.6 mm, the board optimizes space utilization while maintaining mechanical stability for through-hole component assembly. It is manufactured from FR-4 TG150 material. Assembly and soldering under controlled factory conditions ensure consistent quality and a clean, professional finish.

Selecting the right element base

As a relay was chosen a japanese non-latching, double-pole double-throw (DPDT) signal relay from Panasonic. With features like gold-plated contacts with very low resistance at 75 mΩ. These mechanical relays are mainly used in signal control applications such as communications and measurement equipment. Audio gear could use this relay for managing low-power signal routing, benefitting from its low coil power requirement and very good switching performance.

As a switch was chosen 2SC1815L Bipolar NPN Transistor. Exactly this type and not PNP opposite one, is often preffered at such circuits, especially in audio. A few key advantages depend on the nature of the structure NPN-transistors: they can switch on and off faster and they differ during saturation in more lower noise figures compared to their PNP competitors.

As a current-limiting resistors were chosen Vishay Dale CMF Model - Industrial Metal Film Resistors with exceptionally low noise; typically 0.10 μV/V.

The PCB was manually routed to minimize the overall module height without reducing its electrical performance.

Features

• Drop-in replacement for 4066-series switch
• High linearity (low on-resistance)
• Low noise figures
• Industrial PCB assembly and soldering

Upgrade your tapedeck / preamp

Desolder old CMOS switch and solder ASH Modul instead, without any modifications. Check the following list for compatibility.

How to order

A selective list of decks suitable for upgrade