Moreplay Channel Imbalance

[jminassi]

Hi Folks,

Ever since I built The Moreplay I have had a channel imbalance.  I have to rotate the balance control 25 percent of the way to the right because the right channel is lower than the left in volume.  I switched the tubes, but the right channel remains lower in volume.  It does this regardless of the amplifier it's driving.  I never investigated it, because I find the sound of it intoxicating.

Any thoughts on this?  Has anyone else experienced this, and what was the cause?  What should I be checking?

As always I appreciate the help,


John

John

[Paul Birkeland]

Easy things that come to mind which would cause a channel imbalance but wouldn't appear in the DC voltage checks would be:

1.  The 150uF cathode bypass cap on the quieter side isn't well soldered.
2.  The balance potentiometer has a 33K resistor that's not well connected.
3.  Other miscellaneous cold solder joint.

To narrow down what's going on, find an old phone/laptop/tablet with a headphone jack and use it to play a 60Hz tone through an 1/8" to RCA cable into the amp.  You can set your meter to AC volts and check for AC voltage at these points to see how well balanced things are:

1.  Output pins of the selector switch (the two that aren't black will show the AC signal voltage).
2.  Middle lugs of balance pots.
3.  Rear lugs of volume pot where red/white wires attach.

The channel imbalance will appear at one or all of those nodes, and that can be used to narrow down what's going on.

[jminassi]

Thank you Paul.  I will try to narrow it down as you suggest.  I have actually built two Moreplay preamps.  The unit I built for my friend is fine.

I'll let you know what I discover.

John

[jminassi]

Ok, I am using a 1 kHz oscillator to input a signal to the Moreplay.  The left channel output of the oscilator is 2.82 VAC and the right channel is 2.90 VAC.  I have the ground lead of my DMM attached to the ground lug closest to the channel selector.  With the volume pot set to twelve o'clock I get a reading of 0.210 VAC where the red wire is attached and 0.185 VAC where the white wire is attached.

I get no meaningful voltage readings from the middle lugs of the balance pot or the output pins (the two that are not black) of the selector switch.  I checked the solder joints at the bypass caps and the 33K resistors on the balance pot, and they appear to be good to me.

I'm not sure what this means....perhaps I misinterpreted Paul's instructions.  When the preamp is in a system it sounds great, but I need to turn the balance pot 25% of the way to favor the right channel in order to get it balanced. 

Any further guidance would be appreciated.  See picture of test rig below.

John

[Paul Birkeland]

You'll need to set that meter to the mV setting and toggle it to AC (usually denoted with a ~).

You can do the selector switch and balance pot tests with the preamp off, so take some time to get those sorted before moving on to the others. 

For audio ground, I would use terminal 2.  The chassis and the audio ground are not at the same potential in the Moreplay.

[jminassi]

Moreplay Channel Imbalance Test 2

DMM is set to mV AC.  Terminal 2 is where the black lead of the DMM is attached.

Preamp is off.  Selector:  White wire reads 0.37mV and black wire reads 0.33mV.

Balance pot:  center terminal farthest from the chassis plates reads 0.45mV and center terminal closest to the chassis reads 0.16mV.

Preamp is on.  Volume pot white wire (furthest from chassis plate) reads 464.2mV and the red wire reads 514.5mV.

Does this make sense?


Thanks,


John

[Paul Birkeland]

Balance pot:  center terminal farthest from the chassis plates reads 0.45mV and center terminal closest to the chassis reads 0.16mV.

Are you 100% positive that you have the 100K linear pot (100KB) in the balance position and the 10K log (10KA) pot in the volume control position?

Your balance pot is off by about 9dB, which could happen if the log pot got stuck in there by accident.  You'll have to unbolt the pot and push it back from the chassis plate a little bit to read the part number off it.

[jminassi]

Yes, I'm 100 percent certain that the 10K pot is in the volume position and that the 100K pot is in the balance control position between the volume pot on the left and the selector switch on the right.  I just pulled both pots and they are clearly labeled.  The 10K pot has BJ3 printed on the side of it.

What else can I check?

Many thanks for your patience,

John

[Paul Birkeland]

That would be a production code if it's in black ink on the side.  An easy way to double check is to just measure between the outer lugs on the top deck of the volume pot to be sure it's 10K.  It's also worth double checking that the printing on the face of the 100K balance pot says 100KB rather than 100KA since we stock both parts and there's a small chance that you got sent a log pot on accident.

Beyond that, you'd want to center the balance pot and take resistance measurements between the wiper (center lug) and the front/back lugs on either side of it for each level and then post them here. 

[jminassi]

Yes, BJ3 is printed in black ink on the side of the volume pot.  On the face it has A10K, and it measures 10.2kohms between the outer lugs.

The balance pot has B100K printed on its face.  On the lugs furthest from the chassis plate I read 46.60kohms between the lug with a black wire attached and the center lug.  I read 20.66kohms between the lug with the white wire attached and the center lug.  On the lugs closest to the chassis I read 19.65kohms between the lug with the red wire attached and the center lug.  I read 56.6kohms between the lug with the black wires attached and the center lug.  The balance pot is centered.

Let me know if this makes sense.

Thanks,


John

[Paul Birkeland]

The balance pot resistance measurements and the balance pot voltages measurements don't agree.  What this most likely means is a loose connection or cold solder joint around the balance pot wiring (including the wires leaving the balance pot and going to the 6V6 sockets).

[jminassi]

Thanks Paul.  Let me check this out thoroughly.  I promise to report my findings although it may take me a few days to circle back to this project.  Hopefully I am able to correct the imbalance, and other builders can learn from it.

Thanks for your help,

John

[Paul Birkeland]

I suspect you'll find that there's some AC signal voltage present at the balance pot on a lug with a black wire, which would help confirm that there's a ground that's not 100% solid.

[jminassi]

Spot on Paul.  I lit up the Moreplay and measured the voltage at each of the lugs.  I had about 400mVAC on the middle lug closest to the chassis with black wires attached to the balance pot.  I turned off the preamp, undid the nut/washer, and removed the balance pot from the chassis.  Then I re-flowed the solder on that lug and at every point any black wire attached to that lug terminated.  I turned the preamp back on.  When I measured AC voltage on the middle lug closest to the chassis, it now read zero.  I reattached the pot and hooked the preamp up to an Akitika power amp on my bench.  I fed the signal from a CD into the Moreplay and it is now perfectly balanced.

So while I've completed 21 electronics builds over the last three years, I apparently still don't know how to solder....


I'm currently using the Moreplay to drive a diyaudio First Watt F4 clone power buffer.  The F4 has tons of current, but it needs to get its voltage swing from the Moreplay.  The F4 has little sonic signature of its own so what you hear is the beautiful sound of The Moreplay.  I've got an Oppo BD-95 spinning discs into a Denafrip Aries II DAC.  The DAC and a VIP Classic Turntable feeding a tubes4hifi PH-16 phono stage are plugged into the Moreplay.  Speakers are the Arendal 1723 Towers.  It sounds amazing.

I don't think I ever posted pictures of my finished Moreplay on the Forum.  See below.  A friend in Montana built me a solid walnut base with round wooden feet that have cork bottoms.  I had the chassis plate powder coated in a blue sparkle metallic.  I think it looks cool.

Paul, thanks for the help in troubleshooting this.

John

[hmbscott]

Love the walnut, and the blue. I think it looks cool too.