Krell KBL Dual Chassis Preamplifier - Serviced

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General:
The Krell KBL is a dual-chassis flagship line-level preamplifier that divides itself into a primary audio chassis and outboard power supply. In 1990's Stereophile gave the KBL a Class A rating. MSRP was $4500 in 1991.

By having a two chassis design, it minimizes power supply interference and and it can have an incredibly low noise floor.

The circuit topology utilizes fully complementary, direct-coupled Class-A amplification stages engineered without output coupling capacitors to maintain direct signal path continuity.

Inputs include both XLR and single ended RCA connections. This allows it to take full advantage of a high quality phono stage with XLR outputs.

Also worth noting, If you are lucky to own two KBLs, there are switches inside allowing you to reconfigure it to a fully balanced mono preamp! Ganging two together gives you the best reference configuration possible. You end up with two volume controls but thats the best you can get. 

Included will be its cable to link the power supply and preamp together, plus its original box.

From a review written by Lewis Lipnick for Stereophile:

"The KBL is undoubtedly the finest preamp ever to come from Krell. It's priced in the same league as the No.26 ($4500 for the Krell _vs_ $4850/$5450 for the Levinson unbalanced/balanced), is a "line-level only" component which can be combined with a matching phono stage (Krell KPA _vs_ Levinson No.25), and offers balanced operation (as does the No.26 with its optional input card). It can also be mated with another KBL for dual-mono operation.

...I feel the KBL belongs at the top of _Stereophile_

SkyFi Cosmetic Notes:
Light wear is present on the faceplate and rear corners of the preamp. The power supply has wear and marks. Overall condition is good.

SkyFi Technical Notes:
This unit was fundamentally functioning on arrival but we noticed that the power supply voltages where not quite right. We replaced all of the heat sink mounted pass transistors in the power supply which restored the proper +/- rail voltages. The Power supplies LED was replaced and is red.

Ownership:
Single Owner

Connections:
XLR, RCA, IEC Power,

General Sound:
Smooth, uncolored, undistorted natural and clean

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

Included:
Unit and box

Packing:
Original Manufacturers Packing

Original MSRP:
$4,500.00

Specs:
Solid-state, class-A, DC-coupled, line-level, stereo preamplifier
S/N ratio: 100dB (A-weighted)
Input impedance: 10k ohms
Output impedance: 0.5 ohms
Frequency range: 0.1Hz-250kHz
Voltage swing: 65V peak-peak (23V RMS)
Voltage gain: 3x (approximately 9dB)

Dimensions:
2.25" H by 19" W by 12" D (preamplifier) and 2.5" H by 9" W by 12" D (power supply)

Weight:
27 lbs.

Approximate Age:
1991

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - BALANCED XLR Connectors - Better

Kimber Kable - BALANCED XLR Connectors - Best

Kimber Kable - Power Cords - Better

Kimber Kable - Power Cords - Best

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.