KOSS A1220 - Vintage Tube Headphone Amp - Diagnose and Mod questions...

[the fuhz]

Hello all,
I am new here, and I hope that I am posting this in the right forum.

As for Intros, I live in the Detroit area. Like to tinker with tube audio as a hobby.

I own 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 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.

My first goal is to find a way to adjust the EQ circuit and get more low end. *note that neither of these units have tone controls.
From what I have read online, you can sometimes adjust filter caps to get different tone, correct?

Here is the schematic for the single tube version. Does anyone have any suggestions on what I can do to get a better sound?

Many thanks.

[Pano]

Wow! That's about as simple as it gets.
My first guess would be C2, the mutipart electrolytic cap. They can go bad with age.
Someone else here may be able to point out other things.

Welcome to the forum.

[the fuhz]

All three caps within the C2 group have been replaced.

My initial thought was that C1 could be upped from .01uf to maybe .047uf but I don't know how to read the circuit enough to know if it will even effect the tone. I think it may just be there for ground hum purposes.

[Pano]

The C2+R5 group is the HV power supply. And a very simple one at that. Changing those (within reason) should not effect tone.

I see that the volume pot contains the power switch. Wonder if that adds to the hum?
Another thing might be a leaking power transformer.

[stratus46]

Originally Posted by the fuhz

All three caps within the C2 group have been replaced.

My initial thought was that C1 could be upped from .01uf to maybe .047uf but I don't know how to read the circuit enough to know if it will even effect the tone. I think it may just be there for ground hum purposes.

Don't change C1 unless it's to remove it. It ties one side of the AC line to chassis ground. This may be of some value when the other units have similar caps and you would reverse the power cord to possibly reduce hum.

I was going to say nobody does that anymore but in fact, the NON medical power line filters with IEC cords do just that. Those filters can cause all sorts of headaches in large systems (commercial dub house).

G²

[Eli Duttman]

For good reason, C1 is known as a death capacitor. It must go and a proper, 3 wire/safety grounded, power cord installed.

The CHEAP 1/2 wave rectified B+ supply is highly suspect. Install a full wave bridge made from 4X UF4007 low noise diodes.

Recapping was mentioned. I suspect a multi-section can part. Were the original values retained? Additional capacitance can and (IMO) should be added to the CRC PSU filter.

If, as may be the case, you can't separate the filament winding from the rectifier winding, it will have to be abandoned to install the necessary full wave bridge B+ rectifier. That's not much of a problem as either a DC supply or an AC supply biased off B+ for the 12AU7 heater may be the final piece of the hum control puzzle.

BTW, if the heater supply has to be reworked, make it more competent than OEM. Then, a 12BH7, which is very much better than a 12AU7, can be installed as the tube.

[the fuhz]

Hey, thanks for the info so far guys, I appreciate it!

To answer a few questions....

I replaced all electrolytics on each of the units. On the single tube amp (the one with schematic you are seeing) I had to use caps that had a higher value than what was called for. This was because I was unable to find ones that would properly fit within the chassis. The 40/30/20 group is now rated at 80/40/37.
However, with the three tube version, I was able to find much closer replacements.

Regardless, each amp hums the exact same. I am thinking now that this is maybe due to their 2 prong power cords. ...or do these circuits simply have a natural hum in them? After all, they aren't audiophile grade and were designed 50+ years ago.

Anyway, my big concern still is trying to shape/mod the tone circuit. I have tried two different pairs of headphones: Grado 225i (32imp) and vintage Panasonic SE-SL01 (imp unknown). And both suffer from the same problem with no low end present.


Photo of the single tube version:

[Eli Duttman]

It's hard to see the power trafo in the photo. By any chance, does it strongly resemble this Allied Electronics item? That Allied item can energize a 12BH7 heater.

The resistors in the photo don't match the schematic values.

IMO, the layout sucks. Along with the previously mentioned on off switch on a volume control, the power cord enters next to the I/P jacks. They did use shielded cable between the I/P jacks and the volume controls, but the shields are carrying signal. Better is 2 conductors plus a shield. Then, the shield is attached to the chassis at only 1 end and does not carry signal.

Fuhz, are you prepared to tear into this thing? Low hum requires separation of signal and chassis ground and the signal grounding scheme should be either a "star" or a bus.

We'll keep the AC mains away from the signal handling stuff by mounting a power switch where the I/P RCA jacks are currently located. The I/P jacks will move to the front panel and a ganged, 2 deck, volume control installed. The previously mentioned full wave bridge rectified B+ PSU is essential.

[Don Hills]

The hum might be coupling from the power transformer to the output transformers. Is the hum still present if you run the amp with the tube unplugged?

[TubeHead Johnny]

The power supply resistor R5 is 1.5K on the schematic, but in the picture it's 4.7K. That might be part of the bass problem. Although bringing up the voltage might also bring up the hum.