Metrum Acoustics Forte!


Cees Ruijtenberg explains why Metrum Acoustics Forte amplifier sounds so relaxed and natural... “Most amplifier gain stages are build with a so called diferential circuit. The typical properties are a very high gain (120dB) but limited bandwidth (typical 1-5 Hz) Necessary bandwidth or gain can be only accomplished by feedback. The drawback of such circuits is lack of control in case of complex loads but also a very clinical and unnatural sound.”

Circuits without these negative properties can be realized by using tube circuits as they will have the right gain and bandwidth without any kind of feedback. A drawback however is the aging of tubes.

For the Forte amp we have created a circuit which has the good properties of both worlds like  a full bandwidth which exceeds 30kHz , low distortion and lower gain . The name of this circuit is "TSA" which stands for "Tube Simulated Amplifier" and is build around high voltage fets. A possible disadvantage is the need for the same high voltage (200-300 Volts) as needed for tubes but the biggest problems like aging is eliminated.

In the attached pictures you can see the nice properties of the TSA circuit. First of all the open loop gain of the TSA circuit. As already mentioned without any kind of feedback we already have a typical bandwidth of 33kHz but even more exiting is the amount of distortion which never exceeds 0.03% . This distortion is dominated by the second harmonic and not by the worse third harmonic.

For the Forte  we are using two gain stages so both of them are tuned for 13dB gain and together they will give a gain of 20x. The effect of this setting is shown in the last picture. As a result each stage has a bandwidth of 800kHz but is limited in the final design by using a mild first order filter at the input of the amplifier. As a result we have an rock solid stability, a tube like holographic sound but at the same time a transistor like  damping factor.