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

the fuhz

Member
2008-02-06 6:49 pm
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.

[IMGDEAD]https://i9.photobucket.com/albums/a60/darmen/Koss1schematic_zpscf357a61.jpeg[/IMGDEAD]
 

the fuhz

Member
2008-02-06 6:49 pm
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.
 
Last edited:

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
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.
 
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).

 
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

Member
2008-02-06 6:49 pm
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:
[IMGDEAD]https://i9.photobucket.com/albums/a60/darmen/Koss1_zps12a58527.jpeg[/IMGDEAD]
 
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.
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
I agree with Eli, best would be to tear it apart and rewire it better. It's a small, simple amp, so not a huge chore. Worthwhile as a learning exercise, too. Adding a mains ground to the chassis should certainly help. It makes a big difference to my vintage 2 prong tube amps.

I do worry about the full bridge rectifier. Isn't that going to double the B+ voltage?
What is the B+ voltage now? Have you measured it?
 

the fuhz

Member
2008-02-06 6:49 pm
Switching to full wave rectification doubles the ripple freq., not the nominal rail voltage. Remember, it's a cap. I/P filter. Rail regulation and ripple level improve, tremendously.

Look at the OEM parts list. The filter caps. are shown as 150 WVDC. The OP did well to use higher rated stuff for replacement.

Switching to full wave rectification doubles the ripple freq., not the nominal rail voltage. Remember, it's a cap. I/P filter. Rail regulation and ripple level improve, tremendously.

Look at the OEM parts list. The filter caps. are shown as 150 WVDC. The OP did well to use higher rated stuff for replacement.


Thanks for all of the suggestions, however I am little intimidated because I don't know nearly half of the changes you are talking about. I'm new to this field. :eek:

First things first. How do you recommend properly upgrading to a three prong plug? I assume that I clip out the 'death cap' and connect the new ground wire to the chassis? Should the positive lead then go to the on/off switch? and the neg go to the power transformer?
 
Thanks for all of the suggestions, however I am little intimidated because I don't know nearly half of the changes you are talking about. I'm new to this field.

If you "go under the hood", a knowledge of how things work is important. Google NEETS, which is U.S. Navy training material and available gratis on the WWW. ;) Tubes or SS, some grounding in theory is needed to be an effective tweaker, let alone designing from scratch.
 
Yes, the hum is still present without the tube. Is that a sure sign of anything?

It's a pretty good sign that the magnetic field of the mains transformer is coupling to the output transformers. Headphones matching the amp in age tended to have poor bass response compared with modern phones, so it probably wasn't a problem back then. It's hard to tell from the picture but it looks like the mains transformer is on the top of the amp. Is it aligned the same as the output transformers? Look at the lines formed by the metal cores. The output transformer cores are aligned "front to back". If the mains transformer is aligned the same way, you can try turning it 90 degrees it so that its core is at right angles to those of the output transformers. You may be able to remove the mounting screws and carefully turn it to see if it makes a difference.

But certainly do as the others suggest - FIRST, remove the "death cap" and replace the mains cable with a 3-core one before you make ANY other changes.
 
Huh? Oh yeah, you're right. I couldn't see it under the other components. Thanks.
Looks like a case for installing some humbucking coils... the transformers are unshielded, hopefully there's enough room between the windings and the cores for another layer of winding.

Humbuckers were quite common back in the days of electro-dynamic (field coil) speakers when the field coil was also used as the choke in the B+ filter. The humbucking winding was on the same bobbin as the field coil. It picked up some of the ripple signal and was connected in series with the voice coil, so that it opposed the ripple signal.
In this case, if the hum is definitely coming via magnetic coupling from the mains transformer, I'd try a few turns around the power transformer feeding a few turns around each output transformer. Definitely a "suck it and see" situation, though.