3/20/2021 0 Comments Darkvoice 336Se Schematic
Once the headphone plug is inserted, the switched contacts open, effectively removing the 10-ohm resistors from the circuit.Well, at least this weeks circuit continues the thread from my last post on cathode-follower output stages, but differs by being an OTL.
The reader, Arief, who sent me the schematic had no knowledge of its source, but he heard that it was a headphone amplifier capable of delivering 1W into 32-ohm headphones. By the way, I looked up the meaning of his name, Arief, which means in Indonesian: one who is clever and intelligent. How did I know Just divide the B voltage of 150Vdc by 3.3k ohms and you must get some current flow less than 30mA, which could never deliver the needed peak 0.25A current swing needed for 1W into 32 ohms. SPICE simulations confirmed my evaluation, as only 25mA flowed through the output triode and cathode resistor. Furthermore, negative output voltage swings into 32 ohms didnt exceed 0.8Vpk, which only yields 10mW into 32-ohm loads, not 1W. I concluded that the DC coupling between the input stage and the cathode follower was a bad idea, as a coupling capacitor would allow the output triode to see a much larger cathode-to-plate voltage, which would allow much great current flow. Why Why not use two grid-stoppers and larger valued ones Its not that they cost a fortune. ![]() Grid-stopper resistors are a beneficial idea in grounded-cathode amplifier but essential in a cathode follower. I was stunned not to see grid-stopper resistors, as the designer was extremely capable, not someone who just sprinkled fancy boutique parts on circuits from the 1950s. When asked why no grid-stopper resistors, he replied, Theyre there. All the electrical benefit, with none of the audionervosa anxiety. Brilliant. Imagine invisible seatbelts for those who fear that seatbelts somehow slow a cars performance down. Google brought up the headphone amplifiers name, Darkvoice 336SE, and an alternative circuit. I also like the 30k plate resistor for the 6SN7. Many of the schematics and photos showed three 10F coupling capacitors in parallel, which is far too low in value to work with 32-ohm headphones.). If it were primarily 60Hz, I would look into improving the grounding scheme and replacing the existing heater wires with tightly twisted wires, not the long parallel stretches of stiff wire I saw in the photos. On the other hand, if the hums frequency was 120Hz, then the power-supply noise was leaking out the output. The obvious workarounds are to beef up the power supply, maybe add a bigger power-supply capacitor or a choke or a regulator. The headphones ground is shared between channels, but it does not have to actually terminate into ground. Post 367 reveals a devious way to just sidestep the leaked power-supply noise. The SRPPs PSRR is a poor -6dB (i.e. AC voltage divider. Thus, we assume that a 10 AC voltage divider will be needed for this headphone amplifier. That is not what SPICE simulations revealed, however, as something closer to 30 yielded the best PSRR enhancement. A tiny amount of current leakage, say 0.1mA, against a 10k resistor equals a DC offset of 1V. Of course, when we plug in the 32-ohm headphones, the DC will fall to 3.2mVbut plugging in the headphones will create a popping sound, as the DC offset discharges to the lower level. One workaround is to use a switching headphone jack, so when the headphone plug is removed, the switches shunt the left and right outputs with a 10-ohm resistor to ground.
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