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- Jan 15, 2018
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Hey ASR,
Here is another amplifier I designed. The 6AS7G family OTL headphone amp is everywhere these days. I have a large collection of these tubes, thought I'd do my own version of the circuit, it's now something I build on a select basis for Head-Fi friends, they seem to like it. This type of amplifier is typically paired with high-impedance headphones given the relatively high output impedance of the circuit (e.g., Sennheiser, Beyerdynamic, ZMF).
Nothing innovative, but tried to make what I felt was the best version of this common topology, here is an overview:
-Dual mono power supply, dual secondary toroidal mains, HEXFRED diode SS rectified, properly snubbed
-Time delay circuitry at startup, 30 second delay before application of B+, allowing the tube heaters to fully warm before power is applied and avoids excessive grid-to-cathode voltages on the power tubes (which are direct-coupled)
-Separate Maida style HV regulators for left and right channels with built-in soft start functionality
-50VDC elevation of the heater supply, avoids noise associated with heater-to-cathode leakage and obeys the maximum heater-to-cathode ratings of the Tung-Sol 5998
-Gyrator loaded 6J5 inputs, unbypassed cathode resistors, direct-coupled to output tube grids
-6AS7G / 5998 outputs, sections wired as parallel cathode followers with active loading
-265uF film output caps
-Output bypass circuitry at shutdown, giving the output caps a low DCR path to ground to discharge, as opposed to discharging through the headphones (these DC transients manifest as unpleasant pops at shutdown and can damage the drivers)
-Properly rated fuses and safety ground included (Sorry, couldn't help but poke fun given the current Carver Crimson 275 debacle).
That's the design at a high level, here are some photos of the most recent one I built. This particular build used dual mono V47 Goldpoint stepped attenuators as the owner has a hearing imbalance due to working in the trades.
With the Russian 6J5 and 6AS7G I have been including with the amp.
Here it is in operation with some fancy tubes, GEC L63 and 6AS7G. The amp is dead silent, no hum or noise to be heard even with relatively sensitive headphones.
Here are some measurements with two different output tubes.
Russian 6AS7G
Output Z - 50ohm
1mW into 300ohm - 0.11% THD
1mW into 120ohm - 0.22% THD
1mW into 80ohm - 0.31% THD
1mW into 32ohm - 0.67% THD
Tung-Sol 5998
Output Z - 30ohm
1mW into 300ohm - 0.032% THD
1mW into 120ohm - 0.094% THD
1mW into 80ohm - 0.15% THD
1mW into 32ohm - 0.38% THD
Frequency response at the two extremes.
300ohm
32ohm
1mW into 300ohm square waves.
1kHz
10kHz
100Hz
1kHz sine wave into 300ohm dummy load just before the onset of hard clipping. So just about 1W out.
Think that's it. It's a good sounding headphone amp, I actually won't have one for myself (I have too much tube gear already), but others have been very happy with the sound. Not running a business, just building for friends
Here is another amplifier I designed. The 6AS7G family OTL headphone amp is everywhere these days. I have a large collection of these tubes, thought I'd do my own version of the circuit, it's now something I build on a select basis for Head-Fi friends, they seem to like it. This type of amplifier is typically paired with high-impedance headphones given the relatively high output impedance of the circuit (e.g., Sennheiser, Beyerdynamic, ZMF).
Nothing innovative, but tried to make what I felt was the best version of this common topology, here is an overview:
-Dual mono power supply, dual secondary toroidal mains, HEXFRED diode SS rectified, properly snubbed
-Time delay circuitry at startup, 30 second delay before application of B+, allowing the tube heaters to fully warm before power is applied and avoids excessive grid-to-cathode voltages on the power tubes (which are direct-coupled)
-Separate Maida style HV regulators for left and right channels with built-in soft start functionality
-50VDC elevation of the heater supply, avoids noise associated with heater-to-cathode leakage and obeys the maximum heater-to-cathode ratings of the Tung-Sol 5998
-Gyrator loaded 6J5 inputs, unbypassed cathode resistors, direct-coupled to output tube grids
-6AS7G / 5998 outputs, sections wired as parallel cathode followers with active loading
-265uF film output caps
-Output bypass circuitry at shutdown, giving the output caps a low DCR path to ground to discharge, as opposed to discharging through the headphones (these DC transients manifest as unpleasant pops at shutdown and can damage the drivers)
-Properly rated fuses and safety ground included (Sorry, couldn't help but poke fun given the current Carver Crimson 275 debacle).
That's the design at a high level, here are some photos of the most recent one I built. This particular build used dual mono V47 Goldpoint stepped attenuators as the owner has a hearing imbalance due to working in the trades.
With the Russian 6J5 and 6AS7G I have been including with the amp.
Here it is in operation with some fancy tubes, GEC L63 and 6AS7G. The amp is dead silent, no hum or noise to be heard even with relatively sensitive headphones.
Here are some measurements with two different output tubes.
Russian 6AS7G
Output Z - 50ohm
1mW into 300ohm - 0.11% THD
1mW into 120ohm - 0.22% THD
1mW into 80ohm - 0.31% THD
1mW into 32ohm - 0.67% THD
Tung-Sol 5998
Output Z - 30ohm
1mW into 300ohm - 0.032% THD
1mW into 120ohm - 0.094% THD
1mW into 80ohm - 0.15% THD
1mW into 32ohm - 0.38% THD
Frequency response at the two extremes.
300ohm
32ohm
1mW into 300ohm square waves.
1kHz
10kHz
100Hz
1kHz sine wave into 300ohm dummy load just before the onset of hard clipping. So just about 1W out.
Think that's it. It's a good sounding headphone amp, I actually won't have one for myself (I have too much tube gear already), but others have been very happy with the sound. Not running a business, just building for friends