General: |
The Dynaco St-120 is one of the few amplifiers that were made that has a regulated power supply. It will deliver about 60 watts / channel into 8 ohms. However, when driving 4 ohms the amplifier actually delivers less power which is very uncommon for solid-state amplifiers. The reason for less power is due to a current limiting circuits within the amplifier and the power supply. Under normal operation, the amplifier just gets warm to touch. |
Input: |
The input to the amplifier is a class-a voltage amplifier, configured as a darlington
configuration. This is where the Dynaco St-120 differs from almost any other audio
amplifier. The amplifier does not utilize a differential input. The output of the input section is coupled to the driver section by a 35uF capacitor so that the driver section may have its own centering voltage. Since the driver section has a impedance of about 5000 ohms, the -3db point for this stage is about 1hz. There is bootstrap capacitor between the input and driver stages to boost the input sections voltage to prevent clipping of the positive half of the audio signal. |
Driver: |
The driver section is realized by utilized using a TIP41/TIP42 power transistors. The bias for the driver section is one of the stranger methods employed. Essentially the bias voltage, created by two diodes, is between the TIP's emitters. Most power amplifiers place the bias voltage generator on the base of the driver transistors. |
Output: |
The output section utilizes two 2n3055 transistors in a quasi-complimentary design, with only one emitter resistor that has a value of .47 ohms. Why only one emitter resistor, on the positive 2n3055, is a mystery to me. The driver section is direct coupled to the output section which helps to maintain a good low frequency response for the amplifier. The output of the output transistors are then connected to a series connected capacitor. This capacitor has a value of 3300uF and has two main uses. First is to strip the DC portion of the output voltage. Second is to allow for protection of the speakers, in the event that the amplifier should fail and the output peg to the positive rail. May speakers have been destroyed by amplifiers that do not account for the failure of the amplifier to the rail voltage. The output capacitor does have its drawbacks, one of which is the low frequency response, which is about 6hz. |
Protection: |
The output SOA protection of this amplifier is quite strange. Instead of using the common current limiting transistors. This amplifier will starve the output transistors of current as the current requirements of the output transistors increases. I cannot think of another amplifier that uses this design, and based on my listening tests, it works reasonably well. One of the problems of this design is that its difficult to increase the current allowed the output transistors without generating excessive heat from the starvation resistors. If you have the schematic these are resistors (r17/r16). I decreased these values to 150 ohms to give a bit more current allocation to the outputs. This is fine since the output transistors (2n3055) are pretty tough transistors and have an excellent SOA. |
Power Supply: |
The DC power supply is rectified by a bridge rectifier made from (4) 3amp diodes. From there the DC power goes through a reasonably sophisticated voltage regulator. The regulator has current limiting in the event that the amplifiers that follow are overloaded. The voltage supplied to the amplifiers is about +72 volts. There is not a negative supply since the amplifiers divide this voltage by two to center the supplied voltage. |
Final comments: |
Each amplifier has 6 transistors and the power supply has 3 transistors. So for transistor conservation Dynaco did a great job. The frequency response of this amplifier is not too bad either considering that there are three sections with capacitor coupling. All in all, a well recommended amplifier for moderate power requirements. The one thing missing that I add for people, is a power light to the front of the amplifier. In the stock amplifier it only has a neon indicator on the power switch which is located on the side of the amplifier. |