Rogue Audio M-150 Tube Monoblock Amplifier Pair


Like New in Original Boxes with all accessories. Plenty of positive reviews on the internet so I’m not going to go into much detail.

Their slightly more powerful brother, the M-180, was a Stereophile Class A Recommended Component.

This unit was bench tested carefully and it passed with flying colors. All tubes were tested and all capacitors were inspected and tested for leakage. It was driven to its full power rating while monitoring distortion without hiccups. The tubes were tested on a calibrated BK747 tester and measured strong.

Listing includes the amplifiers, power cords, and original boxes.

Here is the feature description from enjoythemusic dot com

Technical Details

Mark O'Brien started designing his own tube gear about 20 years ago. Having worked at Bell Labs in engineering physics he had access to superb lab resources as well as lots of help from electronics engineers who had received their degrees during the golden age of tube electronics. It's obvious to me that Mark has learned his craft well. It starts with the monoblock layout. It's nearly impossible for me to accept an amplifier as Reference class unless it is of monoblock construction. Two totally independent and isolated power supplies and separate signal paths result in ideal channel separation and stable imaging under dynamic drive conditions.

Both the single-ended (RCA) and balanced (XLR) inputs are connected to a wide-bandwidth Jensen input transformer. Although it's strictly not necessary in the case of the RCA input, Mark believes that it does a neat job of eliminating potential ground loops and smoothing out the top end. One leg of the secondary is grounded, so that only the positive going portion of the balanced signal is used. The signal is then fed to a long-tailed pair phase inverter consisting of a 12AX7 dual triode. The driver stage is more sophisticated than what one typically sees in a 1950s vintage amp. It's a series-regulated push-pull circuit, also known as a Mu follower or totem pole, which uses a pair of 12AU7 dual triodes. The output stage is comprised of two pairs of KT88s operating in parallel push-pull configuration. The output transformers are quite beefy and feature an ultra linear connection. They use E-I laminate cores of grain-oriented silicon steel and are custom wound to a frequency response spec of 10 Hz to 50 kHz (+/- 1dB). The windings are bifilar and are interleaved to reduced leakage inductance. The output stage mode may changed on the fly from UL to triode by the mere flick of a switch on the back panel. Of course, maximum output power is reduced by about one half in triode mode.

Instead of the more common cathode bias, a fixed bias scheme is used. There are no cathode resistors, a dedicated –100 VDC bias supply is provided. This is a far more efficient biasing scheme than self bias since the entire B+ voltage is useable as plate voltage. A very nice touch is the built-in current meter. The quiescent current of each output tube may be adjusted individually using a set of pots located under a plate near the meter. A special tool is provided for that purpose. The entire process only takes a couple of minutes to complete if there are a few pots that need tweaking. Note that the bias is set on the low side at the factory as a precaution because it is a function of the AC main voltage. The recommended bias current is 40 mA and it should be set accurately only after each amp has been running for at least one hour. One knock against self bias is that it is not self-regulating as is cathode bias with tube aging. I found it necessary to tweak the bias a couple of time during the first week of use. After that, there was little drift; I would recommend a once-week bias check under heavy usage conditions and a bimonthly frequency for the average user.

The power supply is solid-state rectified and features voltage regulation for the front end preamp tubes. The other important feature, mandatory really, for a fixed-bias amp is the soft start circuit which ensures that high voltage is applied slowly while the bias supply kicks in. Each output tube is individually fused as an added safety feature to prevent runaway currents should the bias supply be interrupted.

Both 4 and 8-Ohm impedance taps are provided. These ratings should not be confused with the amp's internal source impedance which determines its damping factor. The source impedance is said to be about 0.4 Ohm — quite low for a tube amplifier with moderate levels of global feedback (about 18 dB) — and resulting in a damping factor of 20 into an 8-Ohm load. I'm so fed up with insulated EU style binding posts that look fancy but are difficult to tighten. Hence, it was really nice to see good old US style un-insulated posts of hexagonal profile that I can really tighten down on with a nut driver.

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