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2845 Power Amplifier Monoblock Pair

50W Class-A
Parallel Single-Ended Triode Tube Power Amplifier
Recommended Minimum Speaker Efficiency: 86dB 1W/1m

The 2845 is quite possibly the best performing open loop single-ended triode amplifier available.

It's no secret that at Bandwidth Audio, we love open loop zero-feedback single-ended Triode (SET) amplifiers. Characterized by their openness, immediacy, and accuracy to the source, these typically modest output power amplifiers are usually reserved for those with highly efficient speakers.

 

The 2845 Amplifier breaks down the low power SET barrier. With 50 watts of output power from a pair of parallel 845 directly-heated triodes, Its outstanding low frequency control, exceedingly open musical presentation, and exceptional noise performance means it can drive both high and low efficiency speakers alike, to audio nirvana.

The 2845 amplifier is custom built to order

$26,999.99 / Pair

110-120VAC or 220-240VAC Mains

Your specific serial number is assigned at the time of order, once 50% deposit is received.

Option - Vacuum Tube Cage

Coming Soon

Option - XLR True Balanced Input

$749.99 / Pair

Option - Signal Capacitor Upgrade

$599.99 / Pair

*The amplifier pictured shows both the XLR Balanced Input and Mundorf Signal Capacitor upgrade options

This amplifier represents a four and a half year long journey, pulling together our knowledge and experience designing, engineering, and manufacturing world-class tube audio amplifiers.

 

The initial design goal of the 2845 was to build an amplifier that sounded like our 22A3 5-watt amplifier, but with more output power, bringing the coveted open loop single-ended triode sound to customers with less efficient speakers. Like our 22A3 Amplifier, design emphasis was placed on low frequency accuracy and speaker control which improves the bass presence and weight, an often-lacking area with SET amplifiers. During the design it quickly became clear that the 2845 would far exceed this goal, and listening tests during our final engineering stages revealed just how impressive this amplifier is.

 

The 2845 is a very special amplifier characterized by a massive soundstage with thunderous dynamics that feels immediate, bringing performances into your room like no other. The parallel 845 output stage and in-house developed output transformer have even better low frequency speaker control than our 22A3 amplifier, giving the 2845 even more low end authority, accuracy, and speed at any listening level. Yet, it has the delicacy characterized by the very best low power SET amplifiers with extremely low noise, especially for an amplifier of this type, providing a greater sense of scale and depth than nearly any other SET amplifier. The 2845 is truly the best of both worlds.

 

The ground up circuit design provides extremely low distortion in the preamp and driver stages, allowing the sound signature of the 845 output tubes to come through. In fact, thanks to the amazing preamp and driver stage as well as our Bandwidth Audio designed output transformer, the distortion profile of the amp is dominated by the 845 tubes itself.  Nothing else gets in the way.

845 SET Amplifier Pair Tubes Glowing
Design

Design

 

The design is centered around a pair of 845 directly-heated triode tubes working in a Class-A parallel single-ended output stage. This early 1930s jumbo triode not only delivers higher output power capability, but improved audio linearity with less distortion for the same output power when compared to smaller triodes of the era. With the 845s working in parallel, a design balance is reached, achieving both higher output power and lower output impedance. This low output impedance improves the quality of the power at low frequencies, where loudspeaker movement needs to be well controlled.

To take full advantage of the 845 tube, a massive power supply has been designed which is capable of delivering over 1000V of ripple free power to the output stage. Our bespoke US-made audio grade power transformer weighs just under 24lbs, and provides the necessary internal AC voltages with very low AC waveform distortion due to its extra large core and low operating flux density.

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Three 5AR4 / GZ34 rectifier tubes provide AC to DC conversion for both the 845 power supply as well the independent preamp and driver stage power supply. These tube rectifiers offer a slow and controlled startup, which reduces stress on the expensive capacitors since they are charged more slowly up to the high operating voltage.

After the rectifiers, both the output stage and preamp stage voltage rails are first filtered using extremely high performance polypropylene film filter capacitors as default. Rather than more cost-effective electrolytic capacitors, these film capacitors not only do a better job of creating a quieter DC voltage, their low ESR allows them to better tolerate the pulsed ripple current generated from the rectifier action. Therefore, the power supply filtering performance is maintained for many years longer than with electrolytic capacitors.

2845 Amplifier Hand Wired

The 2845 output stage power supply uses a cascaded filter topology with two low loss 10H 200mA filter choke inductors to reduce high voltage supply ripple and noise to a minimum. The supply impedance is extremely low, creating a very stiff supply. The preamp uses its own dedicated filter stages and an additional filter choke, so there is no cross coupling between the preamp and output stages through the power supply. This high performance power supply is partly what allows this 2845 amplifier to achieve its nearly 100dB Signal to Noise Ratio (SNR) at the speaker, all without the use of negative feedback.

2845 Hand Wired Tube Sockets

The requirements of 2845 push every parameter into areas well outside typical tube audio design.

 

To drive the 845 output stage to full power, an internal high gain preamp stage must be capable of developing over 400V peak-to-peak signal swing. The gain required from a line-level input source is nearly on par with a phono preamp, yet sitting adjacent to this sensitive preamp is a high voltage power supply generating over 1000V.

 

This presents quite a design challenge and is yet another reason why many SET amplifiers, especially high power or parallel designs, often sound dull and have poor SNR.

 

Our preamp and driver stage design takes a modern approach, using active current source loading of the 6SN7, 6AH4, and triode connected EL34 / 6CA7 tubes. This achieves the high gain and desired signal swing with exceptional linearity, providing low distortion and low noise. In the case of the triode connected EL34 / 6CA7 driver, it alone can provide up to 2.5 watts of single-ended audio power to drive the 845 tubes to their full potential. A weak driver stage is the Achilles heel of most 845 amplifiers.

 

Active current source loading was selected beacuse we wanted to avoid the typical interstage transformer or plate choke preamp approach. More magnetics in the signal path often leads to a reduction in bandwidth and highly non-linear frequency dependent distortion profile in the preamp drive.

A great circuit design is only as good as its output transformer.

 

While there were many challenges to designing the 2845 amplifier, one of the biggest hurdles was the output transformer.

 

For single ended tube amplifiers, the output transformer design is especially difficult because the transformer must carry the idle bias current for the output stage. This current sets up a large static magnetic field in the core which makes the necessary core size larger than an equivalent power push-pull amplifier. The larger core means more copper with a longer winding length per turn and therefore a physically larger diameter copper coil. Physics dictates that these large coils will have both higher winding capacitance and leakage inductance, reducing the high frequency bandwidth. Furthermore, the longer winding per turn increases the copper resistance, which increases both the losses inside the transformer and the amplifier's output impedance. 

 

This leads to the typical output transformer design compromise; reduce core size and sacrifice low frequency bandwidth in order to achieve acceptable high frequency bandwidth. 

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For 2845, there would be no compromise, especially since we wanted a design capable of exceptional open-loop performance, which would not have any self correction to compensate for poor transformer bandwidth. After a multi-year long journey in itself, including considerable investment in our own high power laboratory equipment which can measure output transformer performance under real operating conditions, we gained a deeper practical hands-on understanding of output transformer design backed up by our own mathematical models. This resulted in our own bespoke output transformer specifically designed for the 2845, which is capable of full bandwidth below 20Hz.

 

These exceptional and massive 22 pound output transformers use a proprietary in-house developed winding scheme that we call Bifilar Wound Perfect Distributed Secondary™ to achieve low distortion, extremely wide full power bandwidth, and a secondary copper winding resistance of just 0.23Ω for the 4Ω speaker tap. The open loop performance of this transformer is more closely aligned with closed loop SET amplifiers using very high levels of feedback.

Furthermore, this transformer offers virtually identical bandwidth between its 4Ω, 8Ω, & 16Ω speaker taps, accessible directly from the binding posts on the rear of the amplifier. Many amplifiers of this type require the user to install jumpers or bridging straps to reconfigure the transformer for their speaker impedance, but not here. With 2845, ease of use was paramount.

Bandwidth Audio 2834 Pair Side by Side
Features

Features​

The 2845 Power Amplifiers were designed to be easy to live with.

 

A 24-position stepped attenuator volume control allows precise matching of the individual left and right audio channels. Unlike a normal variable potentiometer, the stepped attenuator clicks into position setting the volume precisely and accurately through a fixed resistor network, which removes any background noise associated with a conventional volume control. Perfect stereo balance is made by setting each amplifier into the same "click" of the volume control. 

 

A custom US-made 2.5" large format VU or Volume Unit meter on the front illuminates when the amplifier is powered on and indicates the relative power output to the speaker. It offers compression and controlled needle movement to represent the output level. Each meter is calibrated to display 0dB at 50 watts of output power, allowing the amplifier headroom to be observed. 

 

Analog ammeters monitor the bias current through each of the 845 tubes independently. 

Setting the bias of the output tubes takes seconds since no external metering is required. The bias meters also serve as an in-circuit tube tester. If there is a discrepancy in the current of each meter, the output tubes are no longer matched.

2845 VU Meter Detail
2845 Bias Ammeters and Input Selection

Open Loop or Closed Loop Modes:

While the 2845 Power Amplifier was designed to be primarily operated in open loop, feedback-free mode, some may desire to run the amplifier in closed loop mode.  The built in feedback switch provides users with the ability to add feedback, which will reduce distortion and output impedance while linearizing the amplifier at higher speaker excursions.

Optional Vacuum Tube Cage:

The optional tube cage protects people and pets from coming into contact with the hot vacuum tubes at the front of the unit and protects the tubes from accidental breakage. The cage is installed from above with posts that simply slide into the receptacle locations on the chassis with a tight friction fit.

Optional XLR True Balanced Input:

As well as the standard Single Ended RCA input connection, these amplifiers can be outfitted with a balanced input, which adds an XLR connection to the back.

This option uses a high-performance tuned line input transformer for a true balanced input. The transformer provides high Common Mode Rejection Ratio (CMRR) to remove noise picked up on long input cable runs to the amplifier.

If the optional balanced input is not equipped, the XLR Jack, input selection switch, and line input transformer mounting locations will be blocked off with black decorative block-off plates. This allows the option to be installed in the future, if so desired.

Optional Mundorf Signal Capacitor Upgrade:

During the design cycle, we talked with the folks at Mundorf who allowed us to sample their catalog so we could put together a premium signal capacitor package for the 2845. We wanted to find capacitors that would offer a perfect pairing.

 

Mundorf film capacitors are arguably the most transparent on the market, and after our own testing, we selected their Supreme Classic and Supreme Classic SilverGold Oil series to further elevate the signal path of the 2845. These film capacitors feature unique foil winding patterns for excellent high frequency behavior with nearly unmeasurable loss. Each capacitor is individually potted in metal cans to reduce microphonics and EMI noise over a standard film capacitor.

2845 Feedback Open Loop or Closed Loop Switch

Construction

The 2845 is our finest constructed power amplifier, utilizing the highest quality components for an amplifier that will provide exceptional long term reliability and ease of serviceability so it can remain at peak sound quality for decades.

Like all of our power amplifiers, a 0.125-inch thick aluminum alloy chassis rigidly supports all of the components and heavy transformers while absorbing heat from tubes and components so they run cooler. An aluminum transformer cover and bottom cover panel, as well as stainless steel hardware, ensures the entire chassis assembly is non-ferromagnetic for excellent shielding as well as minimal pickup from stray AC noise. The chassis is finished in Bandwidth Audio's signature grey powder coat for a hard wearing durable finish.

The side of the chassis are supported with select grade solid purpleheart hardwood, which is finely worked and inlayed with figured curly maple accents. These end caps are not only decorative, but the very high density purpleheart makes the chassis ultra-ridged and acts as a dampener, preventing the chassis from absorbing vibrations.

 

Hand-wired point-to-point and turret board construction improves ruggedness, making component replacement simple.

The circuit is all hand-wired using point-to-point method for the  amplifying stages at the tube sockets. This keeps signal wires extremely short, reducing noise and increasing stability. Tube sockets are mounted directly to the chassis rather than a circuit board, so there is no risk of cracking a PCB or solder joints when installing tubes.

 

The power supply is kept to the back of the chassis and built up on extra thick high temperature grade FR4 boards with components soldered between high quality solid brass turrets. These turrets provide a secure and permanent mechanical solder joint for the high voltage components for both safety and robustness. If components need to be replaced down the road, the turrets provide an infinitely re-solderable connection without the risk of damaging tiny PCB board traces.

 

Special attention is made to the grounding and power supply return paths to reduce crosstalk between stages and maintain signal-to-noise integrity. 

2845 Wood sides
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Documents
Specifications


  • SINGLE ENDED RCA INPUT

  • OPTIONAL: BALANCED XLR INPUT WITH TUNED LINE INPUT TRANSFORMER

  • OPTIONAL: HIGH PERFORMANCE SIGNAL CAPACITOR UPGRADE PACKAGE USING MUNDORF SUPREME CLASSIC AND SUPREME CLASSIC SILVERGOLD OIL

  • PSEUDO INTEGRATED DESIGN WITH VOLUME CONTROL ACCEPTS  LEVEL SOURCES

    • 24 POSITION STEPPED ATTENUATOR VOLUME CONTROL WITH DALE RESISTORS

  • CUSTOM US-MADE LARGE FORMAT 2.5" ILLUMINATED VU METER

    • LOGARITHMIC COMPRESSION METER DRIVER CALIBRATED TO 0DB FOR OUTPUT MONITORING

  • CUSTOM ANALOG AMMETERS FOR OUTPUT TUBE MATCHING AND BIAS MONITORING

  • BANDWITH AUDIO'S BESPOKE 22 LBS ULTRA HIGH PERFORMANCE BIFILAR WOUND PERFECT DISTRIBUTED SECONDARY  SINGLE ENDED OUTPUT TRANSFORMER​

  • INSULATED 5-WAY GOLD PLATED BINDING POSTS

    • 4Ω, 8Ω, & 16Ω SPEAKER IMPEDANCE TAPS

  • BANDWITH AUDIO'S BESPOKE 24 LBS AUDIO GRADE LOW FLUX DENSITY POWER TRANSFORMER

  • SOLEN FAST POLYPROPYLENE FILM CAPACITORS USED IN FIRST STAGE POWER SUPPLY FILTERING FOR LOW ESR AND LONG CAPACITOR LIFE

  • THREE, 10 HENRY 200MA US-MADE FILTER CHOKES FOR ULTRA-QUIET POWER SUPPLY

  • ETFE HIGH TEMP MIL-SPEC AEROSPACE GRADE WIRE​​​

  • MOGAMI W2330 COAXIAL SHIELDED INPUT WIRE

  • PRP PR9372 US-MADE METAL FILM NON-MAGNETIC AUDIO GRADE RESISTORS IN THE SIGNAL PATH

  • 1/8" ALUMINUM CHASSIS AND NONMAGNETIC STAINLESS HARDWARE

  • SELECT PURPLEHEART & CURLY MAPLE HARDWOOD CHASSIS SIDE CAPS

    • HAND OILED AND FINISHED WITH A UV PROTECTIVE SATIN CLEAR​

  • SOLID MACHINED ALUMINUM FEET WITH VIBRATION ABSORBTION PADS

  • HAND WIRED

    • POINT-TO-POINT SIGNAL LEVEL WIRING AT THE TUBE SOCKETS

    • HIGH TEMPERATURE EXTRA THICK FR4 CIRCUT BOARDS WITH SOLID BRASS TURRETS FOR POWER SUPPLY COMPONENT MOUNTING

  • IN-HOUSE CURVE TRACED MATCHING OF 6SN7 and 6AH4 TUBES

    • ​IN-HOUSE MATCHING OF TUBES ACROSS GM, MU, RP, & IA​​

  • EL34/6CA7 DRIVER TUBES HEAT SOAKED BEFORE MATCHING FOR IA & GM

2845 Feature List

2845 Typical Specifications

Rated Output Power

50-Watts RMS 

 

THD+N at 50-Watts 1KHz

2.75% Open Loop

2.5% Closed Loop

Full Power Bandwidth (50-Watts)

15Hz - 26kHz at -3dB Open Loop

<10Hz - 27kHz at -3dB Closed Loop

 

THD+N at 1-Watt 1KHz

0.3% Open Loop

0.2% Closed Loop

 

1-Watt Bandwidth

11Hz - 36kHz at -3dB Open Loop

<10Hz - 45kHz at -3dB Closed Loop

 

Input Voltage for Rated Output - Single Ended RCA

1.25V RMS Open Loop

2.35V RMS Closed Loop

 

SNR 1-Watt into 8ohms

82dB A-Weighted Open Loop

88db A-Weighted Closed Loop

 

SNR 50-Watts into 8ohms

99dB A-Weighted Open Loop

105dB A-Weighted Closed Loop

 

Gain Single Ended Input - RCA

24.0dB Open Loop

18.5dB Closed Loop

Gain Single Balanced Input - XLR (if equipped)

21.8dB Open Loop

16.3dB Closed Loop

 

Negative Feedback (In Closed Loop mode only)

5.5dB

 

Input Impedance

100K ohms

 

Speaker Taps

4, 8 & 16 ohms

 

Power Tubes

2x 845

 

Preamp Tubes

1x 6SN7, 1x 6AH4GT,  1x EL34/6CA7 (triode connected)

 

Rectifier Tube

3x GZ34/5AR4

Power Consumption (with 845 bias set to 60mA both tubes)

375 Watts - Idle

390 Watts - Full Power

 

Dimensions

Chassis Footprint (L,W,H): 23” x 16.5” x 11.25”

Exterior Envelope, includes knobs and switches (L,W,H): 25” x 16.5” x 11.25”

 

Due to the open loop design of this amplifier, actual performance will vary depending on vacuum tube quality & performance.

Specifications subject to change without notice.

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