SEX 2.1 problem

[Todd R]

Noticing some problems with my SEX 2.1.
Always thought it had a slight channel imbalance (yes I swapped tubes, cables, sources, headphones, it's the amp)

I didn't have the 120 ohm resistors that were shown in the schematic installed on the headphone jack because there were no instructions on where they went, but Doc e mailed that to me and I put them in.

Once I got the resistors installed and the channel imbalance became worse (sound is off center to the right). Took them out & jumpered the connections and it was better, but still a little off.

I assumed it was due to bad resistors, so I ordered some Vishay replacements, which I installed today.
Same problem, off center to the right.

My amp is wired for 32 ohm, and I can hear a slight buzz in the right channel at low volume when using my Senn HD-650, but when I tried it with my 18 ohm impedance IEM's, the buzz was very loud and the imbalance was hard to the right.

I checked the resistance readings again and this is what I found to be off from the manual:

7 - 2 ohms, 17 - 0 ohms
14 & 24 - 91 Kohms
A4, B4 - 92 Kohms
A7, B7 - 772 ohms
 
I did on voltage check off the speaker taps while running a steady test tone through the amp and at about half volume had .564 volts on the right channel and .525 volts on the left.

I can get more voltage checks if you need them.

Any ideas?

[Paul Joppa]

Since you seem to be getting very a close match between channels at the speaker outputs, it should be between there and the headphone jack. There's not much there, so I would first suspect a bad solder connection on the jack itself, and second a possible bad connection inside the jack (poor contact with the plug). The three wires jammed into terminal 23L, which is also screwed to the chassis, seems like a place where it may take more heat than usual to get the solder to flow over everything.

[Todd R]

Paul,
23L is solid.
Any other thoughts?

[howardnair]

todd--either reheat and touch with a little solder the places that paul pointed out to you--or take a solder sucker and suck them quickly clean and then resolder--even though a joint may be or look solid sometimes it is still " so to speak" not a good joint

[Todd R]

Just got done checking & re-soldering some joints.
Still have an imbalance. Wonder if my volume pot is not tracking evenly?
That would explain the imbalance, but not the low level buzz.

[Paul Joppa]

The buzz also bothers me. Could be the ground at T7, which you measured to be 2 ohms - not close enough to zero, so there must be an iffy connection between there and the chassis ground at T13.

You measured the channel balance at one volume control setting, and it was good. I previously made the assumption that the balance was good at other settings, which would mean that the balance problem is downstream of the speaker outputs. But as you suggest, it might be the volume control is not well balanced at other settings. Should be easy to check, since you have a test tone source.

[Todd R]

Paul,
Funny thing is I checked T7 today and it's a fraction of an ohm now, but the low level buzz in the RH channel is still there?
I'll let you know once i get a chance to do more measurements.

[Todd R]

Checked all the joints again and re soldered the ones I suspected.
Replaced resistors in the signal path with Vishay's, 120 ohm, 1.27, and 249K. (couldn't find all of them at Digikey)
Replaced .1uf & 1.5uf caps.
Replaced the wires from transformers (connecting the wires before installing the chokes & transformers stressed the connections from all the movement).

Still have the same problem.
Channel imbalance with the HD-650's, singer is over my right eye instead of being centered and a slight right channel buzz.
Severe channel imbalance with my JH Audio (16 ohm) or Sennheiser HD-238 (32 ohm), almost all the sound is in my right ear and very noticeable buzz in the right channel. 

Volume pot measures pretty close between left & right.

Paul, I don't know what else to do apart from start replacing components in the power supply?

[Doc B.]

It's a long shot but it might be that you got one OT-2 output transformer and one OT-3 output transformer by mistake. Try measuring the DC resistance of the secondary of each transformer to see if they are very different.

[Todd R]

Doc,
They both say OT-2 and both measure 195 ohms (terminal 10 to 5)

[Paul Joppa]

The OT-2 and OT-3 have identical primary windings, which is the one you measured. Check the secondary - that's the DC resistance at the speaker terminals. It will be low - an ohm or two - and most meters will be inaccurate, but it should be repeatable.

One more idea - the 100uF/35v cathode bypass caps. Really if that is bad or has a bad solder joint, it should only affect the bass, but I'm running out of other ideas.

You have already confirmed that the measured levels at the speaker terminals are well matched. I assume that is unloaded (no speakers, no headphones) - might be worth re-checking that with the low-impedance headphones plugged in. At this point, I think we really must find some measurable problem to match the perceived problem!

If you have some clip-leads and some patience, you can clip the headphone to the speaker output, bypassing the phone jack entirely. If the measured voltage is matched, you should hear that. If you still hear an unbalance, do the voltage test again with headphone connected.

Incidentally, you said the potentiometer was matched; you did not say whether you measured that at the speaker output or just the potentiometer resistances. Every scrap of information may potentially have the clue!

I keep coming back to the jack because there is nothing else between the measured-correct signal balance at the speaker outputs, and the incorrect headphone loudness. I don't have one of the new jacks on hand - Doc had to change them at the last minute because the originals are no longer available, and we have not located a substitute anywhere - so I can't independently confirm what the correct connections should be.

[Todd R]

Paul,
You right, I did measure the primary...whoops!

Ok, measuring resistance at the secondary, terminals 7 & 8 I got 1.1 ohm on both channels A & B.
So far so good.

Measuring where the wires meet the headphone jack I get the same.

Measuring resistance at the speaker terminals I get get 1.3 ohm on the left and 126 ohms on the right! (I have the 120 ohm resistors on the headphone jack installed. (I had only previously measured the voltage there)

Plugging something into the jack, I get nothing at the speaker terminals as I should.

Plugging the cord from my Sennheisers into the headphone jack (without the phones connected) I measured the pins that connect to the headphone.
Right channel 2.6 ohms, left channel 124 ohms.

Think you nailed it....bad headphone jack?
I'll run over to "The Shack" this afternoon and see if they have something else I can put in there. Since I'm not running speakers a regular jack should hold me for a while.

[Todd R]

Success!
I changed the headphone jack. All good now.
Thanks everybody.

[Paul Joppa]

It's been a frustration - the only jacks available that can cut out the speakers, do not allow the 120 ohm resistors to be used with the headphones. We put those resistors in when the prototype showed the enormous loudness difference between high and low impedance phones. Some sensitive low-impedance phones make the amp's self-noise audible, and we went to a LOT of trouble to make this amp quiet enough for headphones. Maybe we'll have to go to an external switch in version 2.2! (No, no such beast is being planned.)

[Todd R]

Paul,
I ordered one of these

Mouser #:   550-NMJ6HCS
Mfr. #:   NMJ6HC-S
Desc.:   Phone Connectors STEREO SWITCHED JACK WITH HARDWARE

It should work, right?

http://www.mouser.com/Search/ProductDetail.aspx?R=NMJ6HC-Svirtualkey56810000virtualkey550-NMJ6HCS

I'm not running speakers with this amp, but if I was, the switching feature wouldn't be that big of a deal to me.