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Product Review - Mosfet 32B Power Amplifier - August, 1995

By John E. Johnson, Jr.


Mosfet 32

Mosfet:32B Power Amplifier. Two channel power amplifier, 250 watts rms per channel into 8 Ohms, 500 watts rms per channel into 4 Ohms, frequency response 5 - 75 kHz + 0.1 dB, - 3.0 dB, harmonic distortion less than 0.1%, slew rate 75 v/microsec, size 4 1/4"H x 17 1/8"W x 12 1/4"D, weight 32 pounds. Black metal chassis, grounded AC plug. $649. Sound Valves, 185 N. Yale Avenue, Columbus, Ohio 43222-1146, Phone (614) 279-2383.

Once in a while, an audio product comes along that not only performs well, but is a bargain. The Mosfet:32B, by Sound Valves of Columbus, Ohio, is one of those products. It is a 250 watt rms per channel stereo power amplifier, with all sorts of inputs and output configurations, for $649. Let's start out by explaining why the price is so reasonable. A notable hi-fi amplifier company decided to move its operations out of the United States, and Sound Valves bought the entire inventory of circuit boards for a very low sum. They built the Mosfet:32B with these parts and are offering the amp with the savings passed on to the customers (that's you and I), factory direct. If it only offered a plain vanilla 250 watts per channel, it would still be a good price, but this amplifier comes with balanced as well as unbalanced inputs, switching for bridged mono output, dual mono (one input drives both amplifiers for bi-amping), ground lift (when ground loop hum is a problem), and input level controls. (See the figure which shows the back of the amplifier). The power switch is on the front of the amp as are three LEDs (one which is green for power on, and the other two indicate green for safe operating, red for overload).

The Mosfet:32B uses a 1 kv-ampere toroidal power transformer (greater efficiency for its size, less stray magnetic field), and 32 mosfet (transistor) output devices (16 per channel). We tested the amp with a variety of loads including planar magnetic speakers, as well as standard, but not very efficient cone type speakers. The balanced XLR inputs provided a hum free output, and use of the ground lift lowered the amount of hum when the RCA inputs were used (input type is switched by a pushbutton on the back). We drove the amplifier very hard, and it became somewhat hot, even though there is a small and very quiet fan that ventilates the heat sinks (it comes on only when the amp warms up, and varies its speed depending on the temperature). Therefore, it is important to keep this amp well ventilated when installing it into a permanent spot. The sound of the fan is not noticeable when using the amp. The only way we could tell it was operating was to feel the draft in the fan's port.

The sound quality of the Mosfet:32B is very good - above average. Clean highs, and deep lows. Mosfets have often been compared to tubes in their sonic character - not as being identical, but being similar in a tangential sort of way. Mosfets are somewhat more musical than bipolars (another type of transistor), or at least it is easier to get satisfactory sound out of them. Bipolars can be made to sound good (superb, in fact), but it requires very careful engineering, raising the cost. The advantage of bipolar output devices is their enormous potential to deliver high current into low impedance loads. This is one other reason that the Mosfet:32B is only $649. It does not require the extra engineering that would otherwise be necessary for high quality sound if it were to use bipolar output devices. However, it won't drive 2 Ohm loads very well. Also, if the bridged mono output configuration is used, then only an 8 Ohm load is appropriate (one of the limitations of bridging a stereo amplifier). Ah, well, nothing is perfect.

The LEDs on the front of the amplifier normally are green, when any significant amount of signal (more than 200 milliwatts) is passing through the amp circuitry. If the amp is overdriven (when the harmonic distortion reaches or exceeds 0.5%), the LEDs turn red. There is no problem if they only flash red now and then (such as a transient loud sound might cause). However, if they remain red, this means the volume should be turned down, or there is a more serious problem such as overheating (a circuit takes over and cools the amp down before it can be used further), blown fuses, or shorted speaker load.

The Mosfet:32B at $649 is one heck of a deal. Thirty day money back guarantee. Three year parts and labor warranty. Since they have only a finite number of the circuit boards, the units will not be on the market for very long. So, if you are really interested in having one, do it now. Good sound, well engineered, and a bargain hunter's delight (but they don't take credit cards - one more way of keeping the price low). Highly recommended.

Summary (Five stars is highest rating):

Sound Quality:
Video Quality: NA
Pros: Astonishing number of features for the price.
Cons: Gets pretty hot, even with the fan.

John E. Johnson, Jr.


Manufacturer Response:

Dear John,

Thank you for your review of our Sound Values Mosfet 32B amplifier. We have a bit more background-design info to share with you, however, regarding your "Cons" comment that the 32B "gets pretty hot".

True, the sides of our 32B do get "pretty hot". But it is important to compare apples with apples on this point. First, unlike a "typical" power amp design, the sides of the 32B ARE its heatsinks. Secondly, the hottest part of any "fin" type heatsink will be its thick section (i.e., nearest where the output devices are mounted). Conversely, the coolest part of the heatsink structure will be its fin section (i.e., at the end of the thermal path).

In our 32B, the fins are turned INWARD. And while this internal fins-forced air cooling design actually combines to produce more efficient cooling than a conventional heatsink, one must remember that the massive (thickest) section of the 32B heatsink is directly in the thermal path and is now turned outward . . so touching the 32B's sides is exactly like touching the inner heatsink area of a "conventional" amplifier. In this comparison, both amplifiers would doubtlessly seem "pretty hot" - because you are putting your hands very near the hottest part of the amplifier (i.e., where its output devices are mounted). Using this basis of comparison, we feel the 32B's heat generation is on a par with its comparably powered competitors.

Additionally, another reason the 32B might seem to run warm is its relatively high bias idle current. Biasing a solid-state amplifier harder into Class A operation helps reduce odd-order crossover distortions - a type of distortion which can be particularly fatiguing to listeners (even in minute amounts). While setting the bias lower allows an amplifier to "not work so hard", it may also have the effect of making a solid-state design almost unlistenable at low volume levels. We opted to run our 32B a bit harder for "sound" reasons.

One final note . . .Careful individual in-house matching-grading of all 32 mosfet output devices further helps the 32B to run at maximum thermal and sonic efficiency. Each amplifier is carefully hand-assembled, bench-checked, heat run for 48 hours, then rechecked before shipment. The result is a finished product with only 2 field failures to date - a record we are very proud of.

Again, thank you for reviewing this "bargain hunter's delight".

John Peterson
Sound Values

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