Possible Channel Imbalance[Solved]

[XforceVesa2]

Hello Community,

I've Build my Crackatwoa a few weeks ago and everything seems fine , except one thing.

I was the entire time unsure if my Crackatwoa actually has an channel Imbalance or not because I could swear that im Hearing the Left channel a little bit louder than the right channel and thought it was just my imagination.

But today I took measuring from my Headphone Cable (HD 600 the Cable is Detachable) and got the following measurements:

At my "normal" listening Volume I've got:

L = 0,22V

R = 0,21V

So it equals to (if I'm not mistaking) an Channel Imbalance of 0,4 dB.

I've took another measuremnt, this time with the Volume upt to 0dB(I have the TwoQuiet Stepped attenuator) and got these measurements:

L = 12,05V

R = 11,29V

So the channel Imbalance is 0,6 dB.

I hope you can help me with that Problem.

[Paul Birkeland]

Can you repeat the test but measure Q7 and Q9 instead (the amp can be off) just to ensure that the channel balance is OK coming out of the attenuator?

[XforceVesa2]

I've measured Q7 and Q9 with the Volume set to max(0dB) and got the Following Measurements:

Q7 = 1,06V

Q9 = 1,08V

Edit: I've used for all my measurements a 60 HZ Test Tone.

[Paul Birkeland]

The attenuator balance looks good. 

A problem like this is usually either a solder joint that isn't 100%, or just a bit of tube imbalance (in the 12AU7).

-PB

[XforceVesa2]

Good to Know

Are there Input tubes out theres that have a little better Channel balance?

[Paul Birkeland]

It's a bit of a roll of the dice unless you pay a tube vendor for one that has been manually checked.  You may also find that the imbalance fades a bit once the tube has been run in for 100 hours. This of course presumes that the tube is the issue and not a flaky solder joint.  If it's a bad joint, then all 12AU7s will appear to have the same imbalance.

How are the OA/OB voltages on your center PC board?

[XforceVesa2]

I've got at OA = 70,3 V and at OB = 73,2 V


Edit: I don't know if it could have to do with that problem but If I look at my Input tube it seems like that on the bottom part of that tube one side glow a little bit brighter than the other?

[grufti]

12AU7 tubes are relatively cheap, even some NOS ones. There are NOS Sylvania made for Baldwin organs 12UA7 tubes on sale for $12.00 right here for example:

http://www.audiotubes.com/12au7.htm

Much further down on the same page they offer GE organ stock tubes for $9.00. Current production 12AU7 tubes cost roughly the same.

[Paul Birkeland]

The glow won't necessarily be even, and you have healthy voltages at OA and OB, so the tube is working nicely.

[XforceVesa2]

I can swear that I can hear the Channel Imbalance(Left is a little bit Louder).

Are there any contacts in the Amp that I can measure too to find the "Possible" Problem?

[Paul Birkeland]

When the 12AU7 has amplified the signal, there is DC in the presence of the AC voltage.  You could try measuring the AC voltage at pins 1 and 4 on the octal socket, but your meter may not resolve the AC on top of the DC all that well.

[XforceVesa2]

I would measure this next Time.

Should I check all Solder joints in the Amp or only these near the Input and Power Tube?

During the Build I've checked almost all Solder Joint with my eyes and my multimeter(checked the connections from one solder joint to another with the Impedance check that its always shows 0.00 Ohms).

In the Input socket both leds are lit bright if the amp is On, only on the Larger PC Board I've noticed that the Leds on it doesn't lit so brightly like the Others Leds did.

[grufti]

There is another way you can measure before you do a whole lot of work. This is going to be short, because I'm in a rush.

Do you have a soundcard in a computer? If, yes, then:

find online or create yourself a 1kHz signal at least 30 sec long / loop if shorter
play it back through soundcard and measure with AC rms meter - adjust -  0.77V or 0.5V are both good
install voltage divider in eros input - go for something like -40dBV or a bit more attenuation
play signal from soundcard into one channel of Eros at a time and use the same output on the soundcard for both Eros channels

that way you can be sure that what goes in is the same for both Eros channels

measure AC V rms at the output

[Paul Birkeland]

That is a good plan for the Eros, but this is a Crack-a-two-a. 

[XforceVesa2]

Could the Channel Imbalance have something to do with the Large PC Board as stated above?

What Channel Imbalance does the Crackatwoa normally have?

Are these Values that I have measured at the begin at this Thread a imbalance that normally Occur at this Amp?