Audio Research SP-6B Vintage Tube Preamp w/ Original Box - Serviced

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General:
The SP-6 is a classic tube preamp from Audio Research. It would be a perfect match with the Audio Research SP6B tube preamp we have listed.

These are wonderful full featured preamps worthy of any high quality system and are an amazing bargain. Like all SP models, they contain a phono section for connecting your turntable with an all tube gain stage.  If you dont run a turntable consider an Audio Research piece staring with LS.  

On the front you will find just what is needed and nothing more. There are no tone controls or silly bits to alter the sound quality. Just a volume, balance, input selector, and stereo selector mode.  

On the back you’ll find all the necessary single ended inputs and outputs and some convenience outlets, some of which are switched from the front and great for connecting a modern or classic amplifier. 

This particular SP-6 was lovingly serviced here at SkyFi for ultimate performance and reliability for years to come. Included with the preamp will be its original box and manual.

SkyFi Cosmetic Notes:
The original feet on this unit wear worn and deformed. We have replaced them with reproduction feet of similar height.

SkyFi Technical Notes:
This SP-6B was very well cared for. It was a single owner unit and was sent in for factory upgrade at least twice. This specific unit started as an SP-6. It was upgraded to an SP-6A in 1979 according to the service documentation that we received. We do not have documentation of the exact date that it was upgraded to an SP-6B but the included Schematic/Parts list is marked 1980. The rear panel model number sticker reads SP-6B and it looks like it was placed over the top of the original model number sticker.

This preamp was serviced here at SkyFi. We replaced all of the aging electrolytic capacitors, cleaned all of the controls, verified all of the regulated power supply voltages, and aligned the bias according to the service manual. The preamp is fitted with a set of unmarked 12AX7s, all from the same manufacturer which have been selected for their exceptional triode balance.

The phono stage is configured for operation with most MM cartridges. We have set the rear panel gain control to the low setting for increased volume control range on efficient systems.

Brand Background:
Audio Research continues its dedication to unparalleled musical expression and crafting components of the highest quality and lasting value for its customers. Our long-term dedication to service is peerless in this industry (Audio Research, with few exceptions, can repair anything ever produced during its entire history). We continue to seek out new and exciting ways to make music more real and more enjoyable. More than just a name or a marketing tool, Audio Research has come to represent exceptional performance and long-term value. Experience Audio Research – experience High Definition®.

Ownership:
Single Owner

Connections:
RCA, Power outlets, Ground Terminal

General Sound:
Big beautiful musical sound with little fatigue.

Working Condition:
Working perfectly and tested in our lab and listening room.

Included:
Unit, Box, Manual

Packing:
Original Manufacturers Packing

Specs:
Frequency response: 5Hz to 30kHz
Total harmonic distortion: 0.01%
Gain: 60dB (MM), 26dB (line)
Signal to noise ratio: 66dB (MM), 90dB (line)
Output: 2V (Pre out), 60V (Pre out Max)
Valve complement: 6 x ECC83

Dimensions:
18.90 x 5.28 x 10.24 in

Weight:
22 lbs.

Approximate Age:
1979

Link to Manual:
Click Here

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Phono Interconnects

Testing Process:
We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect the PCBs for discoloration from resistors or transistors that may have been running hot. On vintage units we often spot check select capacitors for value and ESR.

When we first power on a preamplifier we connect its RCA output to a Sencore PA81 Power Analyzer which simulates real world loading conditions and gives us an oscilloscope interface.

The first order of business is checking that the volume control works smoothly throughout its entire range with acceptable channel balance. This is accomplished by feeding a 1 KHz sine wave into one of the preamp’s line level inputs while monitoring the preamp’s output on an oscilloscope.

We then switch to a 1 KHz square wave to test the tone controls, loudness function, and filters where applicable. During this step we are watching for equal alteration of the test signal by both channels. This also helps us identify dirty controls that will need treatment.

Once the basic line stage functions are verified, we test each input individually. This is especially important for devices that use relays to select their sources. If the preamp is equipped with a phono stage we test that as well. We use an inverse RIAA filter which allows us to feed a reference test signal into the phono input with the proper RIAA equalization and level. A square wave or sine sweep is used to verify that the device’s phono stage is faithfully reproducing the RIAA curve. If the preamp under test has balanced inputs and/or outputs these are tested as well.

We finish up our bench testing with a listening test with our bench amplifier and reference speakers. During this test we check for hum or hiss that may not have shown up in earlier testing. We also check that all of the tone controls and filters perform as expected.

If the preamplifier has remote control functions these are also tested. Preamps with tube circuits or complicated power supply topologies are connected at our long term test rig for extended stress testing under real world conditions.