A dual-mono design  including dual power circuits and volume controls, and  an output transformer for low output impedance.

The original 604S appears to have been designed with a single MC Phono input (utilizing an internal 600 Ohm USA made TRIAD step-up transformer) and two line-level inputs. However, these were sold as kits so the back plates in many images show configurations where builders have installed additional inputs, especially MM phono. These alterations may also have been made at the Shindo location as they provided a building service for kit purchasers and were open to user requested modifications.

Description, Shindo product catalog, 1981:

The MODEL 604S employs the assembly circuit of the Western Electric T604 single-ended amplifier, resulting in a single-ended configuration from input to output. This amplifier is especially recommended for those who love single ended amplifiers, such as those using the 300B tube. It uses a TRIAD HI-7 MC input transformer and an HO-6 output transformer, and has a 600 ohm output, making it reliable enough for professional use. What's even better is that it also has TUNER and AUX inputs, expanding the range of sources that can be used. With the increasing number of FM stations and the rise of stereo AM, the presence of a TUNER is undeniable these days. Furthermore, the AUX is invaluable for playing tapes in the best possible condition. For multi-source oriented audiophiles, the 604S is a valuable new item that enables functionality at a high level.

For use with MC cartridge equalizer amplifiers only

Technical Specifications, Shindo product catalog, 1981:

• EQ: NFCR type equalizer
• Vacuum tubes: GE 6072A/12AY7 x 2AX7/5751 RCA 5Y3 x 1
• Controller: ALLEN-BRADLEY
• Power Transformer: Our original high-regulation SN transformer rectifier
• Tube rectifier, USA OIL capacitors, MALLORY type filter
• Input/Output transformers: TRIAD
• Capacitors: SPRAGUE, CORNELL DUBLIER, DALE
• Power Supply circuit: ALLEN-BRADLEY
• Power cable wire: BELDEN
• Sockets: CINCH
• Terminal boards: USECO
• Chassis: Steel (with wooden case)
• Inputs/Outpust : SWITCHCRAFT
• Tantalum: WET+ Tantalum CORNELL-DUBILER
• Input/Output: RCA Pin Jack
• Measurements (with case): 160 (H) x 510 (W) x 315(D)
• Measurements (no case): 125 (H) x 460 (W) x 315(D)
• Weight: 16 kg.

Additional Technical Details:

• One set pre-outs RCA
• One set 600 Ohm outs (RCA)

The next version is probably from 1985 based on the casework, the lettering, and the fact that names rather than numerical designations started around this year. The model number being included after the name will be common for for the remainder of the '80s

The REC-OUT function has been removed, the other functions remain.

Description:

None available

Technical Specifications:

None available

Additional Technical Details:

This version of the Allegro was sold on the used market with the following specifications:

• Vacuum tubes: 12AX7/5751, 6072A/12AY7, and a 6X4 rectifier
• Impedance: PHONO (MC) 2~600Ω
• Impedance: AUX 100kΩ
• Maximum PHONO Input Power: 1200mV
• Output Level/Impedance: 10V/600Ω
• Total Harmonic Distortion: 0.01% (at 1V output)
• PHONO RIAA Deviation: ±0.01% (20~20000Hz)
• S/N Ratio: 119db
• W50×H16×D31
• 11.0kg

By 1990 the front panel has changed to a glass panel and acquired the Swan logo.  The front no longer says 'Model 604 S' but it's still stenciled on the back of the case.

Additional line-level inputs have been added as well as a MM phono input.

Description, Shindo Marketing Material, 1990:

The preamplifier is said to determine 70% of the sound quality. The Allegro 604S is a preamplifier specifically for MC cartridges with a built-in step-up transformer, created to reproduce 100% of the emotion of live performance. The circuit is based on the EMT 927 vacuum tube equalizer amplifier "139", and the amplifier is an adaptation of Western Electric's T604 circuit. Its excellent separation and high signal-to-noise ratio of 119dB maximize the rich expression of analog sound, making it a high-quality amplifier for those appreciate the difference.

Technical Specifications, Shindo Marketing Material, 1999:

• Input Sensitivity / Impedance
• Phono Input MC: 0.2 mV /2Ω–600Ω
• Output Impedance: 10V/600Ω
• Maximum Input: 1,200mV (1kHz)
• Total harmonic distortion: 0.01%
• PHONO RIAA deviation: ±1dB
• Frequency response: 20Hz-20kHz
• SN ratio: 119dB
• Power supply: AC I00V/1A
• Power consumption: AC 100V, 50/60Hz, 50W
• Dimensions: 500(W) x 160(H) x 310(D) mm
• Weight: 12 kg

Additional Technical Details:

At some time in the early 1990s the Allegro was changed from stereo to dual-mono. This was common in their designs at that time, an attempt to gain even further channel separation.

In April of 2005 a Shindo importer briefly posted online that the Allegro had been "[r]edesigned into two separate chassis for better channel separation" and indicates that it remains in the Shindo pre-amplifier lineup.  This was probably a mis-communication with Shindo Laboratory as the Allegro had been removed from the Shindo website in late 2004 and they largely were moving back to single-chassis designs by 2005.

Description, Shindo website, 1999:

This preamplifier features a beautifully designed chassis and a monaural configuration that delivers superior channel separation, alongside circuitry further evolved to reproduce the full emotional impact of a performance. It embodies the hallmarks of our preamplifiers—exceptional channel separation and a high signal-to-noise ratio, both essential for music reproduction—faithfully conveying the rich musical expression found in analog records and CDs. It is a preamplifier designed for the discerning listener who truly appreciates the difference.

Technical Specifications, Shindo website, 1999:

• Input Sensitivity / Impedance:
• Phono Input MC: 0.2 mV / 2Ω–600Ω
• MM: 3 mV / 100kΩ (RCA jacks)
• switchable between 2 inputs
• Other Inputs: AUX, FM, TV, CD (RCA jacks)
• S/N Ratio: 119 dB
• Frequency Response: 20 Hz–20 kHz
• Output Level / Impedance: 8 V / 600Ω (XLR jacks), Rec Out
• Power Consumption: 100 V – 40 W
• Power Supply: AC 100 V, 50/60 Hz
• Vacuum Tubes Used: ECC83 ×1, ECC82 ×1, E80CC ×1, 6BM8 ×1, 6X4 ×1
• External Dimensions: W 440 × H 70 × D 265 mm (per unit)
• Weight: 10 kg (per pair)
• Remarks: Built-in step-up transformer for MC cartridges and output transformer.

Additional Technical Details:

A Limited Edition run in 2008 supposedly used up the stock of single-unit  Allegro chassis remaining from its discontinuation in 1992.  The batch used a new circuit design however so specifications are unknown.

The number of units imported into the United States is unclear.  Dealers have variously stated the numbers to be 10 or 12.

Description:

None available

Technical Specifications:

None available

Additional Technical Details: