Channel imbalance issues

arkrazor · 6297

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Offline arkrazor

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on: April 06, 2015, 07:00:39 PM
Hey everyone, just finished a crack+speedball build not too long ago and everything works perfectly for the most part. The only problem that I'm having with my crack is that there's a very slight channel imbalance, with the left being more prominent than the right (or the right being weaker than the left).

All of my measurements are within acceptable ranges and I'm using the volume pot past the 11 mark. The only thing I think is worth noting though is that the volume pot's right channel is about 7k Ohms higher than the left channel and that Terminal's 1 and 5 have a roughly 7V difference. Other than that, everything measures in the green.

I've tried touching up the solder joints and swapped out the RCA jacks, anything else I should try out? I was thinking of swapping out the volume pot next, but I figured I should ask the experts before throwing down more dosh.

Thanks in advance!



Offline JamieMcC

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Reply #1 on: April 06, 2015, 10:01:30 PM
Hey everyone, just finished a crack+speedball build not too long ago and everything works perfectly for the most part. The only problem that I'm having with my crack is that there's a very slight channel imbalance, with the left being more prominent than the right (or the right being weaker than the left).

All of my measurements are within acceptable ranges and I'm using the volume pot past the 11 mark. The only thing I think is worth noting though is that the volume pot's right channel is about 7k Ohms higher than the left channel and that Terminal's 1 and 5 have a roughly 7V difference. Other than that, everything measures in the green.

I've tried touching up the solder joints and swapped out the RCA jacks, anything else I should try out? I was thinking of swapping out the volume pot next, but I figured I should ask the experts before throwing down more dosh.

Thanks in advance!

In the sticky section of the Crack there is a FAQ where a solution for this is given.

http://bottlehead.com/smf/index.php?board=24.0


Shoot for the moon if you miss you will still be amongst the stars!


Offline Grainger49

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Reply #2 on: April 07, 2015, 02:09:44 AM
If it is an imbalance throughout the volume range it is probably a tube. 

If you can not use most of the volume range try what Jamie suggests.  It is always much better being able to use all the volume control.  It gives you finer adjustments.



Offline arkrazor

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Reply #3 on: April 07, 2015, 07:20:03 AM
Hey everyone, thanks for the input.

I forgot to also mention that I already did the resistor mod as detailed in the crack FAQ. The channel imbalance persists throughout the entire range of the volume pot. I also thought that it might've been a tube problem so I ordered a used set off of eBay. No luck with that either. Could the seller have given me a bad tube or is it something with my circuit? Should I post my voltage readings just in case?



Offline Paul Joppa

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Reply #4 on: April 07, 2015, 10:54:36 AM
Was the channel balance problem there before the resistor mod? If not, then it's worth checking that the right resistors are in the right places, and that the various resistances (pot output, pot input, RCA jack) are matched between channels. Set gain to the usual, I think that was halfway up?

I suggest the above because since the tube swap had no effect, the input wiring becomes a suspect.  :^)

Paul Joppa


Offline arkrazor

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Reply #5 on: April 07, 2015, 01:05:43 PM
Hey Paul, appreciate the input.

I double checked and the resistance imbalance is most likely originating from the volume pot. Here are the readings for what they're worth.

R RCA jack: 204K Ohms
L RCA jack: 197K Ohms-198K Ohms
R Input Pot: 97K Ohms
L Input Pot: 90.3K Ohms
R Output Pot at half gain: 12.3K Ohms
L Output Pot at half gain: 15K Ohms

At full gain, the output resistance matches the input resistance.

Is a 3K-7K Ohm difference enough to make a noticeable channel imbalance?



Offline Paul Birkeland

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Reply #6 on: April 07, 2015, 02:41:34 PM

Is a 3K-7K Ohm difference enough to make a noticeable channel imbalance?
It's a far more complicated question than that.  Based on your numbers, you just have 100K padding resistors before the pot? 

At full volume, the right half will have -6.45dB of attenuation, while the left half will be -6.77.  It's safe to say that this is inaudible.

Can you download a 60Hz sine wave and play it into your amp with your phone?  If so, let me know, and we can trace signal through your amp and see where it's getting lost.

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline arkrazor

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Reply #7 on: April 07, 2015, 06:56:06 PM
Hey Paul,

Correct, there is roughly 100k padding resistors before the pot. I have a 75k and a 33k resistor installed in each channel, as per the crack FAQ.

Unfortunately, I don't have a 3.5mm to RCA adapter, so I don't think I can use my phone to play the 60hz sine wave. Is using a computer out of the question?



Offline Paul Birkeland

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Reply #8 on: April 08, 2015, 08:40:23 AM
A computer will work just fine. If you have the 33K and 75K resistors, you shouldn't have 200K from center lug to ground on the RCA jacks (it should still be 100K).  If you have the 33K resistors across the pot, your R/L input at the pot should show 24K.

If you'd like, you can post a picture of what you have wired.  Based on this information, I would say that the resistors are not correctly installed.

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline arkrazor

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Reply #9 on: April 10, 2015, 06:54:40 AM
Hey Paul,

Unfortunately, I am out of town at the moment, so I can't take a picture of my setup. I do remember how it was wired though. I had the two resistors put in between the wires leading from the RCA jacks to the volume pot itself. I'm guessing this configuration is incorrect. Do you, by any chance, happen to have a picture of the correct configuration?



Offline kekoukela

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Reply #10 on: April 22, 2015, 02:44:52 PM
I would like some pictures of the correct setup as well



Offline arkrazor

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Reply #11 on: June 05, 2015, 10:48:37 AM
Hey everyone, sorry for the late reply. Been busy with some irl priorities and whatnot.

Anyway, I took a look at my crack again and tried to see if I could fix it. I read through the instructions again, and idk if it's just my problem, but they seem really ambiguous. I want to take a picture, but all I have is my phone camera, which is absolute rubbish at taking close up photos. I'll try to describe my setup in better detail.

Right now, I have a 33k resistor between the input and output lugs on the volume pot, for both levels. Should there be another resistor connected to the ground? I also have the 75k resistors in between the RCA jack and the volume pot, but based on Paul's previous information, I'm assuming this is incorrect. Should the 75k resistors be between the output wires and the volume pot?

Here's a quick, poorly drawn diagram I whipped up for further clarification:
(http://i.imgur.com/YgMlZbW.png)

Is this the correct setup? The picture applies to both levels.



Offline Paul Birkeland

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Reply #12 on: June 05, 2015, 04:41:34 PM
The 33K resistors go between input and ground on the pot.  The 75K resistors go from the input lugs on the pot to the wires coming from the RCA jacks.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline arkrazor

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Reply #13 on: June 16, 2015, 09:25:46 AM
Thanks for the input, Paul. I think I have everything in its correct place. What should I do with the 60hz sine wave now?



Offline Paul Birkeland

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Reply #14 on: June 16, 2015, 11:17:05 AM
Thanks for the input, Paul. I think I have everything in its correct place. What should I do with the 60hz sine wave now?

Do you still have a channel imbalance?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man