Channel balance

[Zimmer64]

Hi,

I am a newbie to the DIY community and just finished a Quickie as my first preamp project. I am not an engineer and really just a hobbyist.

I had started with a Gainclone amp and fullrange speakers. Then I needed a pre and since I am fascinated with tubes I stumbled accross bottlehead and really loved the idea of a batterie powered, easy to build kit. ( I am also about to build a CD player in order have a complete Hifi system made by myself, but that is a different story :-).

I really like the sound quality of the Quickie, but I have a noticeable difference in volume between the left and the right channel (right is louder). Is that caused by the pot or might there be another reason?

If it is a function of the pot (I am aware that at the price point of the entire kit, that the pot is probably not a super quality one :-), what kind of upgrade is recommended? Alps? Noble? Or a stepped attenuator? I saw one on the web. Has anyone experience with http://www.8audio-mall.com/servlet/the-382/100K-24-Step-Ladder/Detail? Anything else?

Any help or feedback is really appreciated.

Greetings from Switzerland

Michael

[earwaxxer]

Hi Mike - The easiest thing to do is swap the tubes and see if that is the cause. One tube may be 'newer' NOS or something like that. The tubes are not matched, although I havent noticed any of my tubes being louder than another. If thats not it, then Paul or Granger etc may have an idea.

As far as the pot, there are many good boutique options. Alps is good start. I like my TKD. You can go stepped, etc.

[Paul Birkeland]

Hello Michael,

If you crank the pot up, does the balance even out?

-PB

[Zimmer64]

Thank you for your tips. I have swapped the tubes from left to right, which did not make a difference. Cranking the volume up makes the imbalance a little less noticeable, but it is still obvious. Also, when I turn up the volume from fully down, it takes quite a while until the left channel kicks in. So what do you think? Is it the pot, or is there another check that I can do?

Best regards

Michael

[vetmed]

Verify that your "D" cells are correctly installed, and that the wiring from these cells is identical in both channels. Since the heater is also the cathode battery polarity should effect the cathode bias, and presumably gain as well. Seems to me based on experience that such a pronounced imbalance is unlikely to be caused by the volume pot. If such were the case then you should be able to measure the difference in resistance from channel to channel. Hope this helps.


Robert Lees

[Paul Birkeland]

it takes quite a while until the left channel kicks in.

Well, that's something unusual.  I'd be interested in measuring the DC voltage on terminals 2/4 while twisting on the volume knob, looking for changes.

There's likely a solder joint that's causing a fuss, I would do some careful inspection.

To rule the pot out, temporarily solder a wire between the center lugs of each deck of the pot.  This will make your Quickie into a mono preamp, and each tube will see the same signal from the pot.

-PB

[Zimmer64]

I will try your suggestions tomorrow and report back.

Kind regards and thanks for your help!

Michael

[Zimmer64]

Hi there,

I still need help...  

I have the PJCCS installed and measured the following:

Pot resistance 4K difference between the channels. On low volumes it is for example 96K vs 92k. If I crank the volume up to more than half, the difference goes don to 2k. On full volume 0 ohm both sides.

Voltages on PJCCS:
IA and IB 34.9 volt
OA and OB 2.2 volt difference! OA (right) 23.2 volt, OB (left) 25.4 volt. (strangely enough, the channel having more volt is down on output volume....)
All other voltages at the pins of the tube are the same on both channels (except of course 23 vs 25 as mentioned before).
 
What can one conclude from this? Is there something wrong with the PJCCS? I checked voltages on the PJCCS at all the other point of the PCB and they are the same on both channels. Is one of the transistors faulty?

What shall I do next?

Michael

[4krow]

I will watch this thread with some interest as I have the same problem, but on a smaller scale.

[earwaxxer]

It look like its wired right, cant tell by the pics, on the transistors, are the heat sink sides facing out?

[Zimmer64]

It look like its wired right, cant tell by the pics, on the transistors, are the heat sink sides facing out?

Yes, the metal back plate of them are facing out. Both.

[Paul Birkeland]

Two volts of difference between OA and OB is nothing to worry about, and that voltage should switch sides when you switch tubes.

Did you do the test measuring the voltage on 2/4 while twisting the volume knob?

[Zimmer64]

The voltages on 2 and 4 do not change, when I turn the knob. And yes, when I swap the tubes, also the voltages swap on OA and OB at the PJCCS accordingly.

So, is the 4k difference in resistance of the pot the most likely cause?

Maybe I notice it so clearly because of the Fulfange speaks that I am using (Fostex FX120), which are a bit forward.

What do you think?

Michael

[Paul Joppa]

The imbalance will show up most clearly when measuring the other side of the pot - wiper to ground.

I expect at this point it is the pot that's giving you the most grief. You can confirm this with the above measurement, or follow PB's suggestion of bridging the pot wipers to mono and then seeing if the gains become balanced.

Of course you can do what most of us do, at one time or another. Replace the pot and see if that fixes it. The reason for confirming the diagnosis by measurement first is just to reduce the chances that you might waste time and money fixing the wrong problem. As I said, we've all done that! Most of us have eventually learned to regret it, but not always learned to do better the next time.  :^)

[Zimmer64]

The imbalance will show up most clearly when measuring the other side of the pot - wiper to ground.

I expect at this point it is the pot that's giving you the most grief. You can confirm this with the above measurement, or follow PB's suggestion of bridging the pot wipers to mono and then seeing if the gains become balanced.

Of course you can do what most of us do, at one time or another. Replace the pot and see if that fixes it. The reason for confirming the diagnosis by measurement first is just to reduce the chances that you might waste time and money fixing the wrong problem. As I said, we've all done that! Most of us have eventually learned to regret it, but not always learned to do better the next time.  :^)

I shall try bridging and will will report back.

Thanks to all that have helped so far.

Michael