General: |
The Dynaco St-410 is a brut of an amplifier. It will deliver about 225 watts / channel into 8 ohms. Into 4 ohms it delivers about 350 watts. The amplifier has a built in two speed fan. This fan helps to keep the heatsink from getting to hot under high power output conditions. You can hardly hear the fan when its in low speed mode. The switch from low->high speed happens when the heatsink temperature goes above 130 degrees. In addition the power supply will shutdown if the heatsink temperature rises above 185 degrees. |
Input: |
The input to the amplifier is a typical differential pair. The current source
supplying the differential amplifier is set to about 2.8mili-amps using a transistor and
14v zener diode. From there the signal pass into a second differential amplifier that is
used for the amplifiers class-A gain section. The class-A gain section was nicely implemented by utilizing a active current source, which supplies about 6mili-amps to the class-A amplifier under low signal conditions. An interesting part of this amplifier is that as the voltage to be supplied by either the class-A transistor or the current source increases, the current the transistor can supply increases. That's what transistor Q4 does, should you have the Dynaco schematic available for reference. Between the Class-A gain transistor, and the current source transistor, is a normal adjustable bias generator composed of two transistors. Most amplifier designs can get this accomplished using only one transistor or a series of diodes. I do not understand why Dynaco decided to design a bias that used two transistors where one will do? |
Driver: |
The driver section is realized by using (2) TIP41's and (2) TIP42's. No quasi-complimentary stage here. This is where the ST-410 gets interesting. The drivers are placed in series. This done for two reasons. First is to split the rail voltage in half to keep the transistors SOA in a good range. Second is to split the rail voltage in half to supply the output transistors correctly. I like amplifiers that place the transistors SOA within the range of the transistor capabilities where full power is possible. The driver stage is directly coupled to the output stage. |
Output: |
The output section utilizes (2) 2n5630 and (2) 2n6030 power transistors. These are reasonably powerful transistors though I would have instead liked to have seen the mj15023/mj15024 instead. Again, these outputs are placed in series with each other instead of in parallel. By doing this, a better SOA is possible when compared with placing the transistors in parallel. On the Dynaco ST-416 they doubled the number of output transistors to allow for even more current output before current limiting takes effect. |
Protection: |
The output SOA protection of this amplifier is a typical current limiting design. Based on the resistor values, the 0 voltage current output before limiting is about 3.8 amps. At 75 volts out, the amplifier will not limit current till about 21 amps. This allows the amplifier a drive a load of as low as 3.6 ohms before current limiting starts. They place a .022 uF capacitor on between the base and collector of each limiting transistor to prevent a nasty ringing on the output if limiting occurs. I performed a SOA diagram for this amplifier and under some conditions it may be exceeded. If you are worried about protection I would suggest you change r23/r22 from 2.4K to 3.0K. This puts the outputs into their safe area and still allows for the driving of 4 ohm loads. |
Power Supply: |
The DC power supply is rectified by a bridge rectifier made from a 25 amp bridge diode. From there, the DC power gets a healthy amount of smoothing from a 10,000uF capacitor on each power supply rail. The rail voltage is about 76 volts. In addition, before the DC voltage goes to the amplifiers, a fuse has been placed in-line. For my taste the supplied value of 6amp is a bit high, I would have preferred that Dynaco would have used a 5amp fuse instead. |
Final comments: |
Each amplifier has 19 transistors with in it. When one of these amplifiers came across my bench I just had to run it through its paces to see how good it was. I was quite happy with the results. It has good power output, low distortion, and reasonable output transistor protection. It was a sad day when these amplifiers where not produced anymore and we are left with amplifiers made from STK modules. |