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Block Box Analog Design

A new concept in compression and limiting, that just happens to be decades old
One of the best-kept secrets in the recording industry is BACK and M1 is ready to share it with the world.  Spectra 1964 was an integral part of many classic recordings.  Their op-amps and EQs were utilized in the consoles of some of the most legendary studios in the world.  Over the past 45+ years, their Model 610 compressor has found a devout following, including the likes of Tchad Blake, Dave Cobb, Vance Powell and many more.

Spectra 1964 has offered the world's finest hand-built audio equipment since 1964 and we still adhere to the same rigorous standards. Spectra is considered "vintage" now, but really it's the forefront of modern recording technology!

The Spectra Legacy was acquired by a partnership of two former employees in 2007. The same manufacturing techniques which were implemented by Spectra under its original ownership are still used today. All Spectra equipment is hand-tested and hand-soldered in our Ogden, Utah facility, by a small team of specialists. All Spectra equipment undergoes rigorous hands on testing before leaving the factory. Much of the testing equipment used was hand built in the original factory over 40 years ago. Our factory specifications have not changed. You can rest assured that your brand new Spectra equipment will sound just as good as your favorite vintage unit.



STX 600

500 Series Complimiter Microphone Preamplifier

STX 600

The STX 600 is Spectra 1964’s latest effort that adds to the fifty year Complimiter history, in an affordable 500 series format. At the heart of the STX 600 resides the original, and famous, 601 Complimiter circuit that was introduced in 1969. As the fastest analog compressor limiter available, the 601 allows for dramatic improvement in headroom, as well as reduction of system noise and distortion. The 601 is the same circuit that is used in the original 610, as well as the recently introduced C610, 611, and venerable V610.

The STX 600 is a true Complimiter design based on the same 601 compressor limiter circuit as found in both the C610 and V610. The unit features the same ultra‐fast limiter/compressor performance in a small affordable 500 series format. Overall gain the STX 600 is 62dB, which allows for use as a microphone or line preamplifier. Switchable 48 volt phantom and phase control further adds to the versatility of the unit.

  • INPUT IMPEDANCE: 1200 Ohms
  • OUTPUT LOADING: 600 Ohms to Infinity
  • INPUT LEVEL: TYPICALLY -50dBu to +10dBu
  • MAXIMUM OUTPUT:+18dBm @ 600 Ohms
  • SIGNAL-TO-NOISE: -80dBu, Below +4dBu Output,-40dBu Input 20Hz-20kHz, Unweighted
  • FREQUENCY RESPONSE: +/- 2dB, 20Hz-20kHz
  • LIMITER ATTACK TIME: Less than 90 Nanoseconds
  • COMPRESSOR ATTACK TIME: 100 Nanoseconds to 1.2 Milliseconds
  • TOTAL HARMONIC DISTORTION: .03%, 30Hz-20kHz, @ +8dBu
  • POWER SUPPLY REQUIREMENT: Less than 80 Milliamps @ +18dBu


Compressor and peak limiter with high resolution detented controls, lower distortion, noise, and a +24dBu output


In 1969, Spectra unveiled the Model 610 Complimiter, a revolutionary audio compressor and limiter.  At the heart of the Model 610 was the 601 Compressor/Limiter module.  This module allowed users to employ The Model 610’s compressor and limiter circuitry independently or simultaneously. Its low noise floor, ultra-fast peak limiting and intelligent compression earned the 610 its status as a classic studio compressor that is used by some of the best-known engineers in the industry.

The Spectra 1964 Model V610 Complimiter can be considered an evolution of the original Model 610, utilizing the very same 601 module with updates to fit in 21st Century workflows that include tracking, mixing and mastering.  With amplifier stages based around the low-noise 110A amplifier circuit, the V610 is capable of 65dB of gain and +24dBu, making it a deft mic preamp in addition to its Complimiter functions. 

In mastering applications, the V610 has detented/switched controls for precise recall of all parameters.  The ultra-fast, fixed-timing peak limiter circuit operates fully within a 180 nanosecond range, transparently eliminating voltage transients without applying compression and effectively increasing the headroom of any device placed after the V610 in the signal chain. 

The compressor circuit has a fixed attack around 100 nanoseconds, and variable release timing from 50 milliseconds to 10 seconds with full bandwidth at ratios from 1.1:1 up to 100:1, making it capable of both extremely transparent compression and hard compression.  For tracking and mixing, the V610 can be used to do anything from subtle, automatic gain control, to invisible peak limiting for safeguarding analog-to-digital converters, or to dial in aggressive, fast-release, compression to add color to your sonic palette. 

Though the heart of the Model V610 was designed nearly 50 years ago, its brilliant design still has specifications that go unrivaled in today’s industry, proving Spectra Sonics founder William G. Dilley was decades ahead of his time.  And now, a new generation of engineers is discovering the Spectra Sonics sound for themselves. 

“I'm hesitant to disclose to other mastering engineers what a wonderful game changer the V610 is.” - Garrett Haines

“They sound HUGE! I used the hell out of them on the new Jason Isbell record!!” - Dave Cobb

"The 610 has been my longtime 'go to' device for dirtying up drums, parallel compression, and other general 'grit enhancement' tasks. Its ability to pump and overdrive make it perfect for things like that. The 610's not-so-dirty little secret is that, with proper settings, it can also be one of the cleanest, most transparent compressors available. Unique and invaluable. A classic." - Pete Weiss

"They've been on every record I've engineered since, (1993), and most that I've mixed." - Tchad Blake

"Behind me in the rack were mounted two 610 compressors that were a perfect fit across the stereo buss. How I missed that sound. Now, it’s back!" - Jack Douglas


Original Version Of The Classic Compressor And Peak Limiter

Spectra 1964 Model C610

The Spectra 1964 Model C610 Complimiter is a modern spin on the V610 Complimiter. While the V610 is a near-identical recreation of the classic Model 610 Complimenter by Spectra Sonics, the C610 offers updated features like higher bandwidth Cinemag transformers, external power supply, selectable input padding and more.


As its name implies, the C610 Complimiter can be used as a compressor or a peak limiter—either separately or simultaneously. Peak limiting mode lets you stealthily remove transient peaks. Mode 2 offers peak limiting with limited compression, which applies mild compression in addition to removing peaks. In compression-only mode, gain reduction is constantly applied for smooth, subtle gain reduction.

After it was originally released in 1969, the Model 610 Complimiter quickly earned a reputation as the fastest, quietest, and most versatile analog dynamics processor on the market. Now, over 50 years later, the Spectra 1964 Model C610 Complimiter delivers the same iconic tone and versatility with a number of major improvements.


The C610 utilizes the 601 compressor/limiter module, with a 101 Preamplifier for make-up gain. Ideal for tracking, bus processing, and mastering, the Spectra 1964 C610 Complimiter puts the limitless tone-shaping capabilities of the Model 610 Complimiter at your fingertips.

Featuring an updated design, the C610 offers lower distortion, less noise, and improved output levels. High resolution detented controls make dialing in the perfect tone quick and easy. The slope control can be used to dial in the compression ratio, while the release time allows you to dial in lighting-fast peak-limiting, soft and slow compression, or anything in between. The new C610 Complimiter also features a dedicated input pad for proper gain staging.



Desktop Complimiter

Spectra Sonics 611

Every once in a while, an idea comes along that just works. An idea that makes so much sense that it's hard to understand why nobody else did it before. Enter the Spectra 1964 Model 611 Complimiter.

Spectra Sonics is known for their proprietary Complimiter circuit. The Spectra 1964 Complimiter circuit features the fastest analog peak-limiter in the world. For decades engineers have used this circuit to "invisibly" remove transient peaks from their tracks. Removing even inaudible transient peaks improves the headroom and performance of equipment downstream of the Complimiter. Now imagine applying this principal at the start of your recording chain BEFORE peaks can cause any performance issues.

The Spectra 1964 Model 611 Complimiter is a hand-built, discrete design utilizing custom transformer I/O that accepts a balanced line input, microphone input, or high-impedance ¼" input and is designed to be placed in FRONT of the mic preamp in your signal chain, thereby eliminating transient peaks as close to the source as possible. Removing peaks before the preamp allows the preamp to more accurately reproduce the source before the recording device.

Additionally, the compressor portion of the Model 611 Complimiter can be used to control the overall dynamic range – everything from evening out a bass track, to making a kick drum boom, or crushing a drum room mic. Plus, its small, portable extruded enclosure makes it easy to dial in sounds in the studio or control room alike. Perfect for producers and session musicians on the go.



Passive Direct Box

Spectra 1964 BB-DI

Plug into something better.
The Spectra 1964 BB-DI is a passive direct box designed around Spectra 1964’s proprietary, custom-wound, Hi-Z, Cinemag transformer. This transformer was developed as means of achieving voltage transfer at the front end of the Spectra 1964 Model 611 circuit. It was soon discovered that it had its own merits outside of the circuit.

As a DI, the BB-DI has a very even frequency response that accurately reproduces low end without oversaturating like some DI’s. The result is 100Hz to 500hz with great harmonic clarity without being too flat or clinical. 

A custom, extruded aluminum enclosure, and molded rubber end caps make the BB-DI the perfect choice for both studio and stage. Wherever you need a durable and tonally robust DI, the BB-DI will have you covered.

Design: Passive, HI Z Input, Balanced Output, High Quality Isolation Transformer Type
Signal Loss: 26dB, Minimum
Maximum Input Level: +15dBu, -10dBm Output, (Nominal)
Frequency Response: 20Hz-20kHz, +/- 1dB*
THD: Less than .01%, 20Hz-20kHz*
Dimension: 5”D x 3”W x 1.75”H
Weight: Less than 8 ounces

*Output Reference Level: -10dBm


STX100 MK2

500 Series Microphone Preamp

Level Or

The Spectra 1964 STX100 Mk2 500 Series Preamp is a compact, affordable 500 series version of the classic Spectra 101 amplifier circuit—the same discrete preamp design used in the original consoles from the 60s. The STX 100 features an expanded dynamic range, reduced self noise and distortion, and enhanced harmonics for unparalleled performance.


With an impressive 64 dB of gain and an extended frequency response from 10 Hz all the way up to 50 kHz, STX 100 delivers deep, rich lows and silky smooth highs with unparalleled dynamic range. And with convenient features like a 10 dB pad and continuously variable input and output controls, the STX 100 is versatile enough to use in any scenario—for both mic and line level signals.


Traditional analog preamps measure input signals using program peaks, which can reduce your dynamic range by up to 20 dB, limiting your headroom and potentially introducing unwanted distortion. The STX series measures input signals using a VU system, which is not limited by the transient response of a signal. Drums and other percussion instruments can create transient peaks up to 24 dB above the program material average, which would easily create distortion on a traditional preamp. But the STX 100 uses two stages of independent LED overload indicators to effectively eliminate distortion from transient peaks.


Bring your tracks to life with the new and improved Spectra 1964 STX 100. Capture drum recordings that sound like you’re standing in the room right next to the kit. Record guitars with full, rich harmonics that cut through the mix. Take your vocal recordings to the next level with class-leading dynamic range—capture everything from the quietest whisper to the loudest scream. For even more tonal versatility, the STX 100 preamp is custom designed to integrate with the STX 500 two-band passive EQ module.

STX 100 Specifications:
  • Gain: 64dB
  • THD: Less than .01% @ 20 Hz - 20 kHz
  • Signal/Noise: Better than 120 dB, (unweighted)
  • Maximum Output: +18 dBm, 20 Hz – 20 kHz
  • Power Requirement: Less than 50 mA, +18 dBm
  • Input/Output Control: Continuously Variable/10 dB Pad
  • Input Impedance: 5K Ohms, Nominal
  • Output Impedance: Less than 50 Ohms
  • Frequency Response: 10Hz-50KHz
  • Phantom: 48 Volts
  • Polarity: 180 Degrees/Reverse/Mic or Line Input

STX 500

2-Band Passive Equalizer

Stretch 500 Series Compressor

The STX 500 is the rebirth of the Model 500 2-band passive equalizer. The STX 500 has four frequency selections per band and up to 12dB of boost/cut in 2dB increments. This passive EQ becomes the active feedback element of the STX 100 amplifier. Combined with the STX 100 there is zero insertion loss and introduces virtually zero distortion when boosting or cutting.

Each band also features an individual bypass. High frequency centers are 2.5KHz, 5KHz, 10KHz, and 15Khz. Low frequency centers are 50Hz, 100Hz, 200Hz, and 15KHz. Perfect curves evoke the feeling of classic recordings with every knob turn.

PLEASE NOTE: The STX 500 is designed to connect directly to the STX 100 preamp. The STX 500 has no VPR edge connector of its own to plug into an API-style rack. Therefore it cannot be used in a standalone configuration.