We combine the historic 300B with another classic, the large octal based 6SL7GT, for the input stage- all triodes running in pure Class A operation. To complement this 'retro' feel, we implemented vacuum tube rectification and regulation in the power supply along with plenty of capacity. We also developed an additional (and innovative) ripple rejection circuit to filter out power supply noise making this an incredibly low-noise preamplifier- especially critical for headphone users.
Complementing this lovely and rich sounding line-driver is the ALPS audiophile volume control, gold plated RCA connectors, high purity silver wire, mil-spec printed circuit boards with extra thick copper and plating, and custom designed output transformers which we wind right here in our own magnetics department. It all adds up...
For 2002, we returned to our classic and elegant "Manley-Blue" pewter grey color and the chassis was reworked to accommodate a few updates we had in mind.
Whether you use headphones or loudspeakers this preamp will deliver the goods and spoil you for any other preamplifier.
Notes for current version not covered in the manual:
The transformer switch puts the output transformer in or out of circuit. You HAVE TO have it switched to "IN" to use the headphone jacks. It is recommended, in the spirit of "less is more", to switch to "OUT" when using the main line outputs.
The SUB out is a post-volume control output which can be used to drive a powered subwoofer unit so that the level of the subwoofer follows the volume level of the preamplifier.
Inputs: 5 stereo pair unbalanced RCA jacks
Outputs: 2 stereo pairs MAIN out same as SUB out unbalanced RCA jacks
RECORD output: 1 stereo pair pre-volume control unbalanced RCA jacks
HEADPHONE Outs: 2 front panel 1/4" headphone jacks
Optimized headphone impedance switching: 30-400 Ohms to 300-4000 Ohms
MUTE: Selecting HEADPHONE operation mutes LlNE outputs
Output Vacuum Tubes: 2 x 300B Electro-Harmonix
Input Tubes: 2 x 6SL7 Sovtek
Rectifier Tubes: 2 x 5AR4 Ruby
Regulator Tubes: 2 x OD3
Gain: factory set at 12 dB at max Volume
Internal Variable Feedback: 8.5 dB of global NFB is used @ 12 dB of gain, adjustable trim -1.7dB to +1dB
Noise Floor: typically -70 dB (1 Hz - 100 KHz)
S/N Ratio: typically 105 dB (A WGT, 20 Hz - 20 KHz)
Frequency Response: 5 Hz to 50 KHz 1dB
Channel-to-Channel Separation (L to R): @1KHz 48 dB; @10KHz 35 dB; @20KHz 34 dB
Channel-to-Channel Separation (R to L): @1KHz 50 dB; @10KHz 40 dB; @20KHz 37 dB
THD: 0.08 % @ 1KHz at 1V rms out
Distortion THD+N: -80 dB (.01 %) zero "crossover" distortion, mostly 2nd harmonic
Input Sensitivity: 250mV (-9.8dBu) yields 1V out
Input Impedance: 100 kOhm Noble volume control
Maximum Input Level: 5.5dBu or 1.459 Vrms @ 1KHz, volume control at maximum
Output Impedance (LINE STAGE): 100 Ohms
Maximum Output Level: 17.2dBu or 5.636 Vrms @ 1KHz, volume control at maximum
Output Power (100 Ohm Headphones): 1W (10 V RMS) (28 V P-P)
Output Power (600 Ohm Headphones): 1W( 25 V RMS) (70 V P-P)
Power Consumption: 170 Watts (1.4 A @ 120VAC)
Operating Mains Voltage: Factory set for 100V, 120V or 220-240VAC operation for original destination country's mains voltage.
Operating Mains Voltage: changeable with power transformer re-wiring and fuse value change.
Mains Voltage Frequency: 50~ 60Hz
Mains Fuse Value: 3A SLO-BLO (100, 120VAC operation), 1.5A SLO-BLO (220-240VAC operation)
Dimensions: W=19" x L=13"
Top Deck Height: 4"
Transformer Height: 3 1/2"
Suggested shelf height minimum: 10" (depending on height of 300B tube)
Shipping Weight: 28 Ibs.
FREQUENTLY ASKED QUESTIONS
FAQ #1: I want to roll tubes! What can I do with the rectifier tubes?
Answer: In the original design 300B preamp in the 1990s, we used a Chinese 5AR4 which rarely gave problems; how reliable these tubes are now and how they would work in the latest power supply configuration is something we haven't revisited. We switched from the original Russian 5AR4 at some point as these proved unreliable, and sometimes if they shorted would cause an internal part to fail. The 5U4 has a little higher ratings and is generally more rugged construction. The difference in plate voltage on the tubes with a 5U4 vs 5AR4 is considerably less than 10%; whether you can hear this difference or not is something we won’t comment on, but in actual measurements of the unit, there isn’t any discernible difference. GZ34 rectifiers work fine also. 274B is very similar. 5AR4/GZ34/U77 are internally constructed differently than the other group, but they will work also.
FAQ #1a: What about the heater current? 5AR4 rectifying tubes have a heater current of 1.9 amps. Can I convert the rectifier tubes to 5U4GB which have a 3 amps heater current? Will the power transformer be too overloaded and overheated?
Yes, we looked into this before, according to the transformer datasheet is should be OK.
FAQ#2: What about those solid-state rectifier plugins?
Answer: this unit was designed around the vacuum tube diodes. Two potential problems with plugging in solid-state replacements: 1) higher B+ voltage, 2) instant warmup. The output tubes and the rest of the circuitry will get hit with full high voltage before they are warmed up, which may not be a problem with a preamp but is of concern for power amps. Some output tubes (like 300B’s) are expensive. Also, since the other tubes aren’t drawing any current, the high voltage supply may go over-voltage for some of the capacitors until the tubes warm up. To be honest we haven't tried the solid-state plugins as we designed this unit for tube rectification. Probably most of those issues could be mitigated with a few resistors and a few hours of engineering work.