vintage KOSS 1220

Two KOSS A-1220 headphone amplifiers share the same model number but have entirely different internal configurations. One amplifier utilizes a single 12AU7 tube, while the other incorporates a 12AT7 along with a pair of 6AQ5A tubes. Despite searching various forums, there is limited information addressing the specific issues encountered. Both amplifiers have been recapped and produce acceptable sound quality; however, they exhibit minimal bass response and a slight background hum. This hum remains constant regardless of the volume level and persists even when the preamp tube is removed. The impedance of the headphones in use may be a contributing factor, as many older headphone circuits were designed for high-impedance loads. The presence of audible hum may indicate that the headphone impedance is too low for optimal performance with these circuits.

Electrolytic and DC filter capacitors are often the first components to consider for replacement, as they can degrade over time. Although recapping has already been performed, increasing the capacitance may yield improvements. The voltage output from the power transformer should be measured, as degradation over time is a possibility. An upgrade from a 19V/0.75A power supply to a 20V/2.5A military surplus supply has been noted to improve sound quality, making it more effortless and relaxed without significantly altering the overall tone. The tubes may run slightly warmer with this modification, necessitating caution when handling them.

It is essential to consider whether output transformers may degrade over time and whether the original transformers are suitable for the headphone impedance being used. Resistor swaps may not significantly impact the tonal balance unless they are intentionally limiting the tube's plate voltage. It is recommended to verify resistor values to ensure they are within specification. Additionally, the grounding of the circuit should be examined for safety, particularly if high-voltage components are present. Modifications to high-voltage circuits should be approached with caution to avoid safety hazards.

In summary, addressing the issues with the KOSS A-1220 headphone amplifiers involves evaluating the headphone impedance, considering upgrades to capacitors and power supplies, and ensuring that the circuit is properly grounded and safe for operation. Careful attention to these components can enhance performance and reduce unwanted noise.Two KOSS A-1220 headphone amps. Each share the same model number, yet have completely different internals: One uses just a single 12AU7, while the other has a 12AT7 and a pair of 6AQ5A`s. I have searched the forums and found one or two threads about this amp, but nothing that addresses my problem.

I have recapped each of them and they sound`ok`. The problem I am having is that there is little to no bass present (in each unit). And there is a slight hum in the background. The hum does not raise when the volume knob, and is still present when the preamp tube is removed. What headphones are you using A lot of really old headphone circuits from back then were designed for use with high impedance load cans. The fact that you are getting an audible hum in the background, "can be" an indication that the headphone impedance is too low for optimal use with that circuit Electrolytic caps and DC filter caps would be the first thing I`d go for, because those can degrade over time.

Looks like you`ve already got a handle on that, although you should consider increasing mfd capacitance. There could be some improvements there to be made. I don`t know enough though to steer you towards the right exact values. let someone else reply with that detail. Have you metered the voltage coming out of the power transformer circuit I don`t know if step down transformers "degrade" with time, but I guess anything s possible.

My Earmax amp OEM power supply was spec`d at 19V /. 750A, I bumped that up to a military surplus 20V / 2. 5A. It didn`t severely alter the overall tone of the circuit, but it opened it up the sound more. Sounds became more effortless and relaxed. The amp doesn`t "gasp for air" as much as it used to. It doesn`t strain/struggle as much, and it definitely got louder with lower impedance headphones. I think the tubes get a "little" warmer to the touch now, they are just barely too hot to comfortably touch. When I tube roll, I now have to use a glove to comfortably pull them. Do output transformers degrade with time Are the OEM ones "right" for the impedance of the headphones you are using You should look into this as well.

I dont think resistor swaps will gain you much, again in terms of the overall tonal balance of the circuit. Unless they are intentionally starving the tube of plate voltage. A lot of guitar preamp manufacturers will do that to increase / extend tube life and keep temperatures down.

You should meter them out just to make sure they are correct value. Side note. is the circuit earth grounded, hard to tell from the schematic thats kind of scary if its not. If they have spec`d C1 at 600V (!), thats some spicy mustard there. I would look into that just as a safety precaution / mod. I just finished a simple mod to my darkvoice 337 and it has some big can ~500V caps that were right next to some chassis screws. My heart raced. but I HAD to do it. I put sheets of layered electrical tape around everything and the screwdriver. Managed to get through the mod without giving myself a heart attack or arc-welding any of my tools! 🔗 External reference