Universal Relay Line Buffer

I decided to design and market a universal amplifier relay interface buffer I’ve had several amplifiers in with hum or noise induced from softkey circuits. This is sometimes mistaken for a bad filter capacitor or oscillation.  These relay buffer systems are sometimes called an “ARB” (amplifier relay buffer) or “SoftKey” system.

This universal relay buffer is an improved universal relay interface. It has an optical isolator that drives a complementary pair of transistors. The advantage of the complementary pair is extremely high current gain along with very low stable saturated switch voltage.

The input of the relay buffer or softkey board is at standard low-power TTL levels. The input switching threshold is about 2-3 volts dc. The high threshold and modest impedance makes this board far less critical for hum, noise, and unwanted interference.

This system is truly universal. The output is 100% ground isolated. It will work in high voltage positive relay systems like the SB220 Heathkit, or high voltage negative systems, or any relay system from a few hundred volts down to a few volts. Adding a simple inexpensive bridge rectifier allows use with up to 250Vac coil relays. The relay coil path is fully isolated from ground inside this unit for line voltage safety.

While some systems work with high voltage ac relays, they are not fully ground independent. If they are ground independent, they have a few volts drop at in dc applications. Amplifiers like the SB200 Heath and 30L1 Collins are particularly sensitive to relay buffer threshold voltage and stability. This system handles every problem in one small board. 

Example of a Problem

This bias voltage pattern is in a Collins 30L-1 using a popular soft-key. This pattern is caused by a small amount of bias system 60Hz hum modulating an FET out of conduction on peaks.

Grid Bias Softkey 30L-1

60Hz pulses from FET falling in and out of conduction.

Bias systems in Collins 30L-1, Heathkit SB-200, and Yaesu FL2100 amplifiers, along with some other amplifiers, are directly dependent in the relay keying circuit resistance and voltage to ground. The relay coil path must drop less than ~1 volt. The control system must also be immune to line ripple and grid current variations.

This board works on negative or positive relay lines up to 400Vdc using a saturated transistor in a unique complementary array.

The relay control terminals are fully ground independent. This isolation not only eliminates hum-trigger issues, it also is allows full power line voltages to be applied between the output port and input system. A common external bridge rectifier will adapt this relay buffer to ac line operated relay systems up to 250Vac, such as old power line operated Dow Key or open frame relays.

Amplifier relay or amplifier control line current is conservatively rated at one ampere, making it the best on the market for high voltage or higher current relay systems. This unit is also ideal for variable-current voltage-drop-critical relay lines; e.g., Collins 30L-1, Yaesu FL2100, and Heathkit SB200 or SB201. These amplifiers pass relay current, bias divider current, and control grid currents out through the relay jack. Since this current is variable and sometimes with ripple, the external control line must be very stable, immune to hum and noise, and have very low voltage drop.

This board will fit inside any amplifier including but not limited to:

Drake L4 and L7 relay buffer

Swan relay buffer

Ameritron Relay buffer

Collins 30L-1 and 30S-1 Relay buffer

Yaesu FL2100 relay buffer

Amp Supply LK450, LK500, LK550 relay buffer

This buffer’s input circuit is an improvement over other popular designs. The input control threshold is at TTL logic levels, in the 2–3-volt range. This high threshold prevents false or erratic lows, making the system more stable in systems with control loop resistance, hum, and/or noise.

The input current is a few mA. This helps keep relay and switch contacts clean while also being safe with sensitive solid state radio outputs. Any radio pulling below two volts will reliably and safely activate the URB (universal relay buffer) system.

Additionally, this relay buffer has virtually zero latency when compared to relay transfer times. This makes it superior to buffers using mechanical relays for hot switching.

This board is designed to fit inside all popular amplifiers, although it can be boxed and used externally.

URB-1 relay interface buffer $34 plus postage Order now

Softkey SB220, SB200, 30L-1, Henry, Ameritron

W8JI Relay buffer for all popular amplifiers


Amplifier relay buffer size


Relay interface buffer

URB-1 Connections


Example of Unit Boxed With Rectifier for Old School 4-1000A Amplifier 

Unit boxed by customer for use with his homebrew 4-1000A.

Relay Buffer Soft-key AC relays

AC with addition of bridge rectifier for old amplifier systems

AC amplifier relay

Outside of AC relay buffer interface