Krell KBL DUAL MONO Preamp Stack with Dual Power Supplies

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General:

We've managed to assemble the holy grail of Krell Preamps from the 1990's across two  different purchases after lots of looking!

We started with just a single stereo KBL. A few years later we finally came across another KBL which almost completes our stack.

You see, there are switches inside the KBL allowing you to reconfigure it to a fully balanced mono preamp! Ganging two together gives you the best reference configuration possible. Yes, you do end up with two volume controls but thats the best you can get. 

Be sure to watch our video below detailing the setup and design.

Best of all, we have two power supplies so each KBL will have its own power source. So essentially you end up with a FOUR chassis system!

Complete this setup with a pair of Krell MDA-300 monoblocks that we just listed for an outstanding Krell system. We've included pictures in this listing with the amps, but they are obviously not included in this listing.

Back in the 1990's Stereophile gave the KBL a Class A rating in stereo configuration. Had they reviewed the Fully Balanced version we have here, it would have been off the charts. 

Recommended Cables:

Kimber Kable - RCA Interconnects

Kimber Kable - XLR Interconnects 

Kimber Kable - 14AWG (Gauge) Power Cords

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's Class A. As did Krell's KSA-250 power amp, I feel the KBL to redefine the term "neutrality." Dynamic and transparent, this preamp is faithful to the musical material, revealing a recording's best and worst aspects."

Specifications - KBL Preamplifier:
Solid-state, class-A, DC-coupled, line-level, stereo preamplifier
S/N ratio: 100 dB (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 9 dB)
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
Year: 1990

Click here for a PDF copy of the Krell KBL & KPA Owners' Manual 

Our Testing Process for Preamplifiers at SkyFi:

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 1KHz 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 1KHz 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.