vintage KOSS 1220

Posted on Feb 4, 2014

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

vintage KOSS 1220
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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...

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