In Europe, little is known about Walter Woods and his peculier personality. Today, Walter has almost stopped his activity. Most Americans think that France is far away and WW amps are not officially exported. So to get one, you have to scan the classifieds ads.
Walter had only one aim for his amps: build a tiny head amp, powerful and light. With efficient equalisation, this amp met success with double bass jazz players who like the sound, and also vibraphonists and some guitarists.
To build such a small amp, the designer used unusual techniques in audio domain and optimised the volume of the case. The consequence of such a design is that a Walter Woods amp is expensive. Here is the evolution of this amp, nicknamed "the small red box".
The 1970's years
Min Amp 1 channel
The mono channel preamp is identical to the Mi100, his successor. This pream colors the sound, so my advice is to use it at low level. Master volume at maximum, adjust the volume of the instrument with the input gain. The power amp delivers around 50W under 8 Ohms. A limited power for use with a double bass.
One of the Min amp's originalities is the use of the lateral side to support some plugs and the mains light. On the rear panel, a magnificient red anodised heatsink remind us of the Quad amps design. On the other hand, the back and the top of the the box are constituted with plywood gained.
The Min Amps are rare. The first owner of the model shown was Herb Mickman. For this series, Walter Woods had engraved the customer's name on the heatsink. Because of this fact, each Min Amp is unique.
Today The Min Amp looks like a collector's item and exists only in 110V version.
The technical side
The Power Amplifier is mechanically very complicated. The vertical mounting has been choosen to keep the size compact. This choice makes any maintenance difficult, and changing a burned transistor on this machine is a real punishment.
The Min Amp is equipped with a switching power supply. Its probably the first case of an instrument amplifier equipped with a such technology. This light and compact power supply replaces the heavy transformer present in each amp.
On the schematic, there is a mysterious rectangle named "switching power supply". In fact, this is an auto-oscillator working around 30kHz. The auto oscillator is derived from the relays vibrators, used to supply tubes radio cars in the 50's. Purists will note two points: the frequency of the oscillator varies according to the load and the transformer heats up because it works in the magnetic saturation.
At this time, no mains filter was present in the amp, leading to the presence of parasites on the mains, beyond present day norms.
The 1980's years
Mi 100 - 2 channels
The preamp has 2 channels. The user manual explains that the equalisation frequencies (bass, medium and treble) are slightly different for both channels. These corrections are efficient (+/-15dB).The front panel offers the following controls: Gain, bass boost +/++, Bass, Medium, Treble, treble boost +/++, phase inverter, effect loop, Master volume.
The preamp, derived from the Min Amp had bad distortion (between 3 and 10%). This addition of harmonics is the secret of the Walter Woods tone.
The power amp, inspired from the RCA schematic, published in 1959 is a big standard. It delivers 75W/8ohms and 105W/4ohms. A 110/220V switch exists now on the rear face.
This model, with typical couloured sound has met with much succes and lot of bass players still use it.
The technical side
The pre-amp circuit board is vertically mounted. This solution is possible by the use of specific jacks and potentiometers. This choice optimises space but makes it less accessible for difficult repairs.
For this pre-amp, Walter decided to use a couple of transistors "Mos + bipolars" to substitute audio integrated circuits. This conception, allowing cost reduction is responsible of the typical sound of these amps.
The power supply, key of the concept, delivers now more than 100W and has a "soft start" circuit.
Mi 225 - 2 channels
At the begining of the 80's, the Mi 225 expands the Walter Woods range. With an identical look and preamp, it has a more powerful supply and a digital class D power amp. This is an event because this is probably the first digital amp dedicated to musical instruments. With more than 200W, the race for power has begun.
The Mi225 D-Class is a complex machine, equipped with first generation integrated circuits. This dedicated circuits needed lots of external components and possessed bad audio performances.
The complexity of electronic circuitery will forced Walter to increase the depth of the box and its weight (4kg). External sign of recognition, the orange mains light helps to differentiate the Mi225 from his little brother.
Walter Woods range - 1983
The 1990's years
Mi 100 - 1 channel
This model, from the begining of the 90's, has one channel and 2 mixing inputs. The dimensions of the box are identical but the lateral sides are lengthened to protect the knobs. The faceplate , now covered in red skinplate, is a solid finish and pleasant to touch.
The pre-amp is modified. It now uses integrated circuits and posseses decent characteristics in distortion and noise. The medium equalizer is equipped with a parametric to answer the customers' demand. The preamp is equipped with a symetric XLR output.
The technical side
The analog power amplifier is entirely new and uses MosFet power transistors. At the end of the 80's, these transistors were in vogue and using them in amps equated with high tech.
The power supply is improved. It uses also MosFet transistors. So the schematic is simplified. This amp delivers the same power as the previous model.
Mi 100 - 1 channel
This model, identical to the previous is equipped with 2 mixable channels. It is from from the 90's and is the best sounding Walter Woods to my mind.
A word about the technical:
As usual, in the purest tradition of "copyright", references of power components are erased by Walter, a frustrating gesture for maintenance labs.
The printed circuit board may be mounted as a stereo version (2x50W/8Ohms). This configuration is rare, the mono 100W version being the most common.
Mechanically, the case is still very designed and is made entirely of aluminium. Thanks to this material, light weight is garanteed and also a good thermal conduction. Parts are assembled with stainless steel screws and riveted nuts, an expensive but noble solution.
The Electracoustic séries
The middle of the 90's see the arrival of a new generation: the Electracoustic series, easily recognizable by his new frontplate. The pream stays identical to the previous series but the power amp is definately in digital technology. Some models are proposed, the anounced output power are enormous: 350, 600 and 1kW!
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