If you are just a casual music listener, who doesn´t really care if the sound is top quality or just good enough, maybe this is not for you.
But if you really care about your music, and your demands are high, read on.

CM-50 Mk II
Hifi 50W poweramp module.
The module is ready to use and easy to connect.
The CM-50 Mk II is exactly what we use (but two) in our high-end stereo power amplifier, CM-130 Mk II.

Send orders or questions to email@valutronic.se

CM-50 Mk II is the 2011 improved version of the highly successful CM-50 we have been producing since 2005.

If you are handy with a soldering tool, and has some experience from building electronics, you can build a really top-quality amplifier using the CM-50 Mk II´s as building blocks.

The input sensitivity is high:
0.91 V input = 20 V output = 50 W output power.
So, a CD/DVD player or computer soundcard is enough for full power without any need for a pre-amplifier.
Drawings and more info about the connection, power supplies, volume controls et.c., click here.

Unlike the mainstream amplifier modules you can buy at electronics dealers, this one is made to meet very high demands.

The sound quality is really among the best you can find, no matter what cost.
It has the ability to reproduce even tiny details in the signal you feed to it.
For example, I had a recording that I had listened to for many years. When I first heard it through the CM-130, a stereo amplifier built with two CM-50 modules, I suddenly could hear the tounge of the female singer as it lifted from her palate. A tiny, tiny clicking sound that got lost in the two amplifiers I had before, one NAD (3020e) and one Denon (don´t remember which model).
Also, ambience sounds and other small details come out perfectly, the amplifier is ”totally open”. If a sound exists in the input signal, it will reach your loudspeakers without being muffled or softened.
So, if you have the ability to do some soldering, build your own DC supply and fix a suitable cabinet, you can save a huge sum of money compared to what this high quality sound would cost ready-built.

Just as its predecessor its main feature is top-of-the-line sound quality. But we have also added a few extras, see ”Tecnhicalities” below.
We build each unit by hand, using only carefully selected discrete components to ensure highest possible sound quality and long-time stability.

So, if you are looking for a power module with really high sound quality, and is handy enough to put together an amplifier, this module is for you.

Technicalities...

First, how the first CM-50 came to be:

I developed the CM-50 because I needed an amplifier ”as perfect as possible” for loudspeaker testing purposes.
I had listened to quite a few amplifiers, but was not really satisfied. Either the sound was not perfect, or the price was far too high for my budget. (= about the same as for a small car.)
And as I had engineered electronics for many years I knew that there is really no correlation between price and sound quality. A complicated and expensive design can deliver just mediocre sound, while a more simple design can have really high-quality sound.
So, what I set out to produce was a really high-quality product at a moderate price. I had designed audio products for many years, so I thought I would be ready in a week or two.
But I soon realized that the combination of extreme high sound quality, low noise and high stability was a tougher challange than I had expected. The really difficult thing was to find out what components were best suited for each task – balanced input, low- and high current sources, voltage amplification, current amplification... and maintaining high speed and stability at the same time.

After a few weeks and several different designs I gave up. It delivered very good, but not really top-of-the-line, and so it was dismissed. Then, a few months later, I started again. And gave up. And started again, and... I made about 30 prototypes in two years... until the CM-50 was finally ready. Now the sound quality was really as good as can be. You could listen to the amplifier for hours without getting tired, even tiny details were reproduced perfectly, noise was almost non-existent and odd harmonics distorsion far below the levels where it can be heard. I was thrilled by the wonderful sound, and made a webpage like this one.

Luckily our customers were also thrilled by the detailed and totally open sound. We even had two customers who bought our ready-made model (CM-130) and ”retired” their many times more expensive Lynn amplifiers, which are often considered to be among the best in the world.

I had reached my primary goal -- perfect sound quality -- but had to make some sacrifices to get there:
Normal short-circuit protection is either ineffective (no real SOA protection) or, when also compensating for transistor voltage, can introduce distortion. So, I decided not to use it, and instead give distinct warnings against short-circuiting the output.

When using the standard temperature stabilization with the MOSFET´s I ran into a problem: The temperature/gate voltage curve of a MOSFET is different than from a normal transistor, so the temperature sensing transistor circuit had to be re-designed. I finally made it, so that the quiescent current was kept within reasonable limits (high enough to avoid cross-over distortion, and low enough to avoid temperature run-away, between 15 and 70 °C. (The high temperature is needed because the heat-sink gets much warmer than air temperature.)

However, there is a certain drift over time, the MOSFET and sensing transistors don´s age identically. Even if it is almost never done, re-adjustment should be done every five years or so for safety. Increased heat due to this drift is one reason why many amplifiers break down, even if this takes 10 - 30 years.

Almost all modern hifi amplifiers use a +, – and ground voltage supply. If plus or minus is lost (a blown fuse on only one of the supply lines) it usually results in a DC voltage output, and if a loudspeaker is connected, it coud be harmed by the cone being pressed out- or inwards too much.
This is a common problem with most amplifiers, and CM-50 was no exception. However, as far as we know, it has not caused any problem. But nevertheless, if it could be avoided it would add to the safety of the amplifier.

Conclution: The CM-50 produced top quality sound, but a certain caution was recommended not to harm it or the loudspeaker connected to it. Also, if a customer used a non-24 V transformer, the CM-50 needed re-adjustment as the quiescent current was supply voltage dependent. Otherwise a lower voltage could produc distortion and higher voltage over-heating.
So, the CM-50 was a wonderful amplifier, but you had to be very careful and connect everything exactly right to avoid damage to the amplifier or loudspeaker.

So, what is kept from the original CM-50?

We use the same fast low noise / low capacitance transistors that made the CM-50 reproduce even the smallest details in the music played, and the same linear output MOSFET:s. All parts of the signal path are able to handle 20 kHz (and some...) at full output power without feedback. So there is no latch-up or TIM distortion.
Every hifi amplifier can handle 20 kHz, but NOT without feedback (= ”open loop”). And this is important.

An amplifier with a frequency range up to 50 kHz with a 100 x feedback can have an open-loop frequency response to only 500 Hz. If the drive stage only reaches 500 Hz (-3 dB) with its normal 0.1 mA input it can be pushed to 20 kHz by applying 4 mA from the previous stage to its input, and this is what happens in many integrated amplifiers. However, when this is done, the collector-base capacitance can be charged so that the output current stays on for a short moment when it should be cut-off. This causes ”latch up” distortion, and if the driver stage is complementary it can also increase average current level in the driver and output transistors, shortening the life of the amplifier.
Some small details in the signal may also get lost in the process.

Also, when the transistor ”speed limit” is reached, TIM (Transient InterModulation distortion) appears.
It is a distortion that is not perceived as ”bad sound”. Instead you soon get ”listening fatigue” -- you feel the need to lower the volume or turn the sound off completely. ”Your ears need a rest” even if the volume is not excessively high.

As neither Latch-up or TIM is easy to spot when you listen to an amplifier for a few minutes in a shop, many amplifier designers don´t care about it. Power amplifiers using IC:s with low open-loop speeds are common, as they are cheaper to build. And it is difficult for a customer to return an amplifier just because the customer ”gets tired” of his own music...
But we care. For the CM-50 MK II we therefore selected only high speed discrete components. It is more expensive, but you get a amplifier you can really enjoy. The sound is ”open” and clear, no details are lost. And you can listen for hours without problems.

And what has been changed?

The MK II has short-circuit protection, no-drift current stabilisation and protection against single side voltage loss.

Short-circuit protection:
I did some intense thinking and came up with a short circuit protection circuit with four transistors. It doesn´t interfere with sound quality, and is only active during the half-period when it is needed. This stops it from introducing any distortion.

No-drift current stabilisation:
A special amplifier I designed for a railroad company needed stable quiescent current over a very wide temperature range as the amplifier was used outdoors. In Sweden this means -30 to +40 °C. The normal way to compensate was not enough. Instead, I came up with a very different solution. A design similar to a constant current circuit but with a high current diode. A resistor takes care of the cuiescent current, the diode takes care of the higher signal current. It worked perfectly.
And it also brought along three extra features: No current adjustment potentiometer needed, total long time stability and voltage independendence. The cuiescent current is the same in the whole +/- 15 to 40 V range. And the cuiescent current will not change over time. No need for re-adjustment ever.

Finally I came up with an idea of how to disconnect the input current for both output MOSFET´s as soon as one of the supply voltages fails. The output just goes silent. No risk for loudspeaker damage.

All these three features are now incorporated in the amplifier.
So, the sound is as perfect as before, but the MK II is easier to install, safer if something fails in the power supply, and the long-time stability is perfect.

Except for the CM-50 Mk II modules themselves, you need a power supply, a cabinet with enough cooling, cables, input and output sockets, maybe volume controls et.c.

Drawings and more info about the connection, power supplies, volume controls et.c., click here.

If you fell that you don´t understand everything, you should ask someone who does to help you.
Without sufficient knowledge there is a risk that one can´t complete the project.

The size of the module is about 85 x 85 x 40 mm. It consists of a PCB and aluminum L-profile. Mounting holes are 40 mm (about 1.6'') apart.
Connection is done via a screw terminal.

Cost: Two CM-50 MK II including shipping to EU countries = 2225 SEK.
When this text was written (august 2011) this corresponds to 240 Euro / 210 GBP / 347 USD.

If you also want to build really high quality loudspeakers, have a look at our SQ-50´s. Sound quality out of the ordinary.
Not only deep, but also clean bass, open and uncoloured mid and crisp and clean treble.
The goal of the design was the same as for the CM-50 amplifier: As precise reproduction of the input signal as possible.
It is not surprising that they are used and enjoyed in thousands of homes and many recording studios.