Biased towards right channel after Speedball upgrade

[Linus1330]

Hello!

This is my first post at this forum. I purchased and built the basic Crack OTL about six months ago and have been enjoying it since then. Last week, I decided to add the Speedball upgrade. I thought the sound was awesome before, but it just got even more so!

However, adding the speedball introduced a channel imbalance that was not there before. It is very noticeably louder in the right channel. It is not the volume control (potentiometer?), listening on very low levels it is instead biased to the left. Listening on normal and too loud levels, it is biased to the right. Also, when not feeding anything into the RCA inputs, and when I crank the volume up > 50%, there is noise in the right channel. I have tried lowering the volume of the source and instead cranking up the volume in the crack, but it makes no difference. I do not think it's the tubes as it worked fine prior to the speedball.

Now, I am very new to soldering and electrical physics in general, so I do not know how to even begin troubleshooting this. One observation, which might not be relevant, was that I did not have a wire stripper suitable for the small wires shipped with the speedball, so I went ahead and used a pair of scissors instead. I was careful to try and avoid cutting into the metal, but I might've done so by mistake. Is it possible that a damaged wire might cause this? Also, when I was to make the final voltage check, the crack did not power up at all. I then unplugged the power switch, looked everything over (didn't see anything), and then just tried again. The second time it powered up fine and the voltage check was alright. I didn't think much about it at the time, but now I'm wondering if it could be connected to the imbalance.

Going by an older thread, http://bottlehead.com/smf/index.php?topic=9615.msg89694#msg89694, I guess that would leave that I have a bad solder joint somewhere?

Thanks in advance!

[Linus1330]

Sorry for doubleposting but I just noticed something that is probably important that I hadn't before. Just when starting, when it has been cold, there is no channel imbalance. It seems to only appear when it has warmed up after a couple of minutes.

[Paul Birkeland]

Can you post your voltages for OA and OB on all the Speedball boards? (just to be sure)

Then, can you download a 60Hz tone and play it on repeat into your Crack?

Start with the volume control all the way up, put your meter on the 2V AC scale (if it isn't autoranging), then measure the AC voltage between ground and each center pin of each RCA jack, then the middle lug of even level of the pot with the pot turned half way up.  Post those voltages also, then we can go from there.

-PB

[Linus1330]

Alright, thanks, will get to that as time allows later in the week.

However, it has now been powered on for over 30 mins and it is currently still perfectly balanced(?)! So I might've been talking out of my ass about it being correlated to it being "warmed up". There is some other mysterious factor to this. Also, it is now consistently biased to the right no matter the volume level (it being completely in the right channel at very low volumes), while previously it was biased to the left at very low volumes.

I'ts placed on my rather rackety desk, so it does get slightly rattled every now and then... This may also be related to it not powering up at first, but doing so after I had turned it around and removed and remounted the tubes.

Anyway, I will post an update later in the week. Thanks again.

[Linus1330]

So, for the past week the balance has been great, up until Saturday when i noticed biased towards the right channel again. Just now, I was going to go ahead and do the suggested voltage readings, but the Crack didn't even power on once I flipped it upside down and was ready with the multimeter. I then went ahead and performed the resistance checks of the original Crack build.

I got a zero reading from terminals 7 and 9. I also got zero at B3 and B6. The other terminals were alright. This seems to suggest that there is a problem in the larger speedball PCB, right?

[Paul Birkeland]

We don't really include resistance readings in the Speedball manual because it's more important to have them down solidly in the original build.  If you want to check the operation of a Crack with Speedball, the voltage measurements are the place to start, and just the outputs of the 4 PC boards ("O" terminals) will tell you everything you need to know.

-PB

[Linus1330]

Alright,

Good news: The Crack is operating!

Bad news: The bias is still there.

So, this mostly demonstrates how much of an amateur at this I am, but the reason for it not starting up was due to the power switch no longer connecting properly. I already knew that I had messed it up since it used to be in a permanently on state, but I guess it shifted into a permanently off state instead when moving it around.

I performed the suggested readings regarding the bias:

For the small Speedball board:

OA: 80.6V
OB: 80.4V

Large Speedball board:

OA: 106.6V
OB: 106.4V

AC voltage check on RCA jacks with 40hz tone, volume all the way up
Left: 1.23
Right: 1.22-1.23

AC voltage check on pot, 40hz tone, volume halfway up:
Left: 0.72
Right: 0.72

This seems to indicate that there is no imbalance, right? But I can still definitely hear the right channel being louder when listening to music. When I listen to a 60hz test tone, I do not notice a difference. It seems like the bias is pronounced only on higher frequencies. It is very noticeable just listening to a normal song. It is slightly noticeable listening to a 1600hz tone.

I also tried measuring the AC voltage on the RCA jacks and the pot using 440hz and 1600hz. The channels had identical readings.

[Deluk]

Going sideways a bit, are you able to swap your headphone cables over at the ear ends? Does the bias swap over? Or try another set? Wondering if something is breaking down, maybe the cables.

[Linus1330]

Yes. I have considered:

1. Swapping RCA input cables, to rule out imbalance of the source.
2. Turning the headphones around on my head to rule out my own hearing.
3. Trying the headphones into another source to rule out the headphones.
4. Turning the source down and the pot up on the Crack to rule out imbalance on lower volume levels.

Swapping the cable at the ear ends does lead to the bias swapping over.

The strange thing is, again, that the imbalance has not been constant but varying from perfectly balanced to bias towards right. It does also seem to be affecting high frequencies more. Listening to the start of this song


the regular beat seems more in balance but the drawn out high-pitched effect is considerably biased towards the right. So it's sounds like a muffled-like filter effect on the left channel.

[Tom-s]

Try other headphones with Crack.

[Linus1330]

As I mentioned, I've tried the headphones with other amplifiers, so I don't think it's the phones.

I finally got some readings to back me up! I started doubt my sanity for a while. They seem to confirm my observation about higher frequenices.

I measured the AC voltage on the headphone output instead:

Using 60hz:
Left: 12.44V
Right: 13.07V

Using 1600hz:
Left: 2.98V
Right: 3.96V

So that is a huge difference... and more pronounced the higher the frequency.

Any suggestions on where to go from here?

[Paul Birkeland]

12. 4 vs 13.07 is half a dB. Your measurements at 1.6kHz are only valid if your meter is capable of measuring AC up that high (generally you would use a scope for that).

If you want to check up a little higher, your meter is likely still OK at 150Hz.

[Linus1330]

I guess that resets me to my default idiot position  . When I said "huge difference" I was referring to the reading of the 1600hz signal, which I guess is worthless. So, I tried using 150hz instead:

Left: 5.10
Right: 5.34

Which is almost the same proportion as the 60hz signal.

I don't know what's more disconcerting; that I am that receptible to suggestions made by myself or that there is some untroubleshootable problem with the amp. Most likely it's the former. Maybe it's a transient problem somewhere else in my setup after all (or I'm just having auditory hallucinations).

I think I'll just relax this a bit and invite some people to try and, without telling them first, see if they notice an imbalance. Anyway, sorry for polluting your forum with this sillyness!

[Paul Birkeland]

You can also try measuring the AC voltage at the ends of your headphone cable if it disconnects from the headphones.

-PB

[Paul Joppa]

...Anyway, sorry for polluting your forum with this sillyness!

Not a problem. The point of the forum is sharing, also known as teaching and learning. I call it "each one teach one." It succeeds when it's a safe place to be ignorant and ask ignorant questions.