Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: Colonl_Charisma on July 22, 2021, 09:29:02 PM
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Hi all,
I am experiencing some channel imbalance in my Crack amp again where the left channel is noticeably louder and clearer than the right. This seems to be a recurring issue (or one that I haven’t ever fully resolved). I’ve seen a bunch of posts where a 60 Hz sine wave is played through the Crack and voltages are measured. This is a test that I’d like to carry out because it might help me in diagnosing what is causing my issue. I just don’t know how to perform the test.
I was able to rule out my headphones. I plugged them into another amp recently where they sounded perfectly normal. This is what alerted me to having a channel imbalance. Now, I cannot unhear it haha.
As always, I appreciate any help that is given.
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The first thing to do is to take a 1/8" TRS to RCA cable and download a signal generator onto your phone, then see if your meter is able to measure the 60Hz tone coming out of the RCA cables.
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Thanks, PB! Just to confirm, this is the cable that I would need, right? Also, could I do this from my laptop? I have an iPhone so I would have to pick up an adapter for the cable too haha.
https://www.amazon.com/AmazonBasics-3-5mm-2-Male-Adapter-Stereo/dp/B01D5H8JW0/ref=sr_1_1_sspa?dchild=1&keywords=8th%2Binch%2Bto%2Brca&qid=1627059133&sr=8-1-spons&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExVTFFVFE1WlIwTkNFJmVuY3J5cHRlZElkPUEwNjczNDQ0MlRCMVRWQUtaT1hQUyZlbmNyeXB0ZWRBZElkPUEwNTg4NjY5MVI5WFlWV0VVRUZVOSZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU&th=1
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Sure, a laptop will work. Yes, that is the cable you need.
Your meter will need a 2V AC scale or a mV scale to resolve these readings. If you have a meter that has something like a 200V and 500V AC scale, that won't do the job.
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I have a Crenova MS8233D Digital Multi Meter. I looked up the online manual, and I believe it should be able to resolve those readings.
https://www.crenova.net/Uploads/Product/Manual/5afce34f0db0f.pdf
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Yes that should work.
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Perfect! I may try this tonight. Can I go computer —> dac —> amp (normal setup) or should it be computer —> amp (this using the trs cable to connect the devices).
When I play the 60 Hz sine wave through the amp, is the amp on or off and do I have headphones plugged in?
I appreciate your help!
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That is up to you.
The first tests would be with the amp off and measuring the AC voltage at the center pin of each RCA jack and the center lug of each level of the volume pot with the pot turned up half way.
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Sounds good! Once I’m at my workbench later this evening, I’ll take those measurements and report back.
Would I set my multimeter to the mA setting to record these first measurements?
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Set it to the AC voltage scale, marked "~V" according to the manual.
mA is milli-amperes, which is a current - you want to see voltage for this measurement.
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Thanks, PJ!
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Okay, so here is how I performed my 60 Hz sine wave test:
1) Amp is off.
2) Sine wave is coming from an online generator (Laptop --> DAC --> Crack).
3) I have 6080 and 6SN7 tubes in the amp. I did test whether or not tubes were the issue. I tried different combos, and the channel imbalance was still present.
4) No headphones are plugged into the amp.
5) I'm measuring from 12U
6) Volume knob turned halfway.
Voltages:
RCA left center pin = 1.488 V
RCA right center pin = 1.358 V
Top center lug of volume pot = 0.027 V
Bottom center lug of volume pot = 0.024 V
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You have about 1dB of difference at the RCA jacks; I wouldn't consider that normal!
Your pot is turned almost all the way down, so the output voltages are a bit low, but they are also off by 1dB.
Pull the RCA plugs out of the Crack and check the AC voltage across each RCA plug.
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Yeah, it’s a noticeable difference haha. How do I measure the voltage of the RCA plugs?
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One probe on the pin, one probe on the shell.
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Thanks! This is with the 60 Hz wave playing.
Left channel RCA plug: 1.488 V
Right channel RCA plug: 1.487 V
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OK, this is helpful information. That's 0.005dB.
What DC resistance do you get from center pin to shell on each RCA jack on your Crack with nothing plugged in?
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RCA left = 99.3 k ohms
RCA right = 99.4 k ohms
Nothing was plugged in.
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Something isn't adding up. To get an imbalance when you plug in your RCA cables, you'd need to load down the DAC, which would show as low impedance across the RCA jacks. Consequently I would question the measured AC voltages.
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When I get home, I’ll re-measure everything. I’ll also bend the RCA cables in the angle that they are plugged in at. I have had some issue with this RCA cable in the past with the right channel.
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Quick update! I picked up a new RCA cable, and I took two new readings with a 60 Hz wave playing.
Old RCA cable measurements:
RCA left center pin = 1.487 V
RCA right center pin = 1.368 V
New RCA cable measurements:
RCA left center pin = 1.489 V
RCA right center pin = 1.488 V
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Time to junk that old cable.
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Definitely haha. I'll listen for the rest of the day with the new cable, and I'll see if anything sounds off.
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Hi, PB! I think you can mark this thread as resolved. I've been listening to my Crack for awhile now since switching out RCA cables, and I do not perceive any channel imbalance. What I was experiencing was pretty distinct.
Thanks for your help!
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I'm back with some new measurements. Turns out my channel imbalance seems to still be present. My headphones sound balanced when just plugging them straight into my computer, but the right channel sounds quieter than my left channel when plugged into my Crack.
I followed the measurement instructions on this thread https://forum.bottlehead.com/index.php?topic=7788.15
My RCA jack measurements, middle lugs on potentiometer, and A2 and A7 were more or less equal. However, I seem to have a discrepancy when measuring AC voltage on terminals 1 and 5.
AC voltage with amp on and volume pot turned all the way up.
Terminal 1: 6.51
Terminal 5: 6.92
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That's half a dB, which is audible but not horrible. What is the DCR of each headphone driver?
You may find that tube rolling could yield more equal balance, or you could fit a balance control if you prefer.
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I agree. It isn't as bad as what I was experiencing with the bad RCA cable, but I still notice it.
What does DCR mean? I am unfamiliar with the acronym haha.
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DC resistance. You can use your meter to measure the resistance of each driver, though at 60Hz that measurement may not be so useful.
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I can take the measurement now. Do I need to disassemble my HD600 to get to the driver?
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No, you can measure across the headphone plug. The tip is L+, the ring is R+, and the sleeve is common.
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Thanks for your help, PB! I am unable to post pictures (site not allowing it). Do I measure across the plugs that connect to the left and right sides of the HD600 while the headphone is plugged into the Crack with the 60hz sine wave playing?
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No, you unplug the headphones from the Crack and measure across the plug on the headphones.
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My bad haha. Got those measurements for you!
L headphone DCR = 0.303 K ohms
R headphone DCR= 0.302 K ohms
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Yeah, I think you could try some different tube combinations to tighten up the channel balance.
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Thanks, will do! The channel imbalance does seem less noticeable when using a 12AU7 tube compared to a 6SN7. I have a new problem now though haha. I opened up a new thread.
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That's half a dB, which is audible but not horrible
It's audible by a few with a lot of ear training, but not many.
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Doc brings up a good point, I'm not sure I would readily pick up on half a dB myself!
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Right on! Doc B and Paul, it could easily be my mind playing tricks on me haha.
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That's half a dB, which is audible but not horrible. What is the DCR of each headphone driver?
You may find that tube rolling could yield more equal balance, or you could fit a balance control if you prefer.
PB, you mentioned that I could try fitting a balance control. I am all about modding my Crack. How much space would be required to add one? I believe space to be my limiting factor.
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You'd need to fit the 100K linear version of the volume pot we supply, and you'd want it close to the volume pot itself.
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Cool! Do you have the dimensions by any chance? I can take a look at my amp to see if it would be worth the trouble. It would be a fun addition to my build.
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You have the dimensions, it's exactly the same as the stock pot but just has a different part number.
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Oh haha. I'll look into this on my end, and see if I can implement this into my amp.
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Hi, PB! So, after looking at my amp, I think I will be able to install another potentiometer for balance control. Can I order the pot off of DigiKey or Mouser?
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Mouser has it. We provide RV24BF-10-15R1-B100K-LA as a balance pot with our kits. You'd need a pair of 33K resistors as well.
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Thanks! What kind of wires do you use for the Crack? I would like to pick up some extra to have on deck.
I’ll get those ordered, and I guess we can come back to the wiring once everything arrives.
edit:
Are these 100K resistors fine?
https://www.mouser.com/ProductDetail/Vishay-Dale/CCF02100KJKE36?qs=sGAEpiMZZMsPqMdJzcrNwncUUYJvQsyPlLH02FNSk8g%3D
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You want 20-22 gauge solid core tinned copper wire. Be sure it's tinned and be sure it's not stranded.
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Thanks for the info, PB. I will be sure to order that type of wire.
How do those resistors look? Does the power rating or tolerance matter for what I will be doing?
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Sorry, you want 33K resistors.
I would not use 2W resistors in that position, that's overkill and just too big in general.
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All good! I actually have two 33K resistors that are 1/8 W. I used these on my volume pot to pad it.
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If you're ordering a pot, I would order more. It helps if they are a bit longer.
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Will do! I'll get all this ordered tonight.
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I should have everything in on Monday. How close should the balance pot be to the volume pot? I am going to go ahead and drill a hole in the chassis for the new pot.
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2"-3" is about enough.
I've made a crude image that shows the proper perspective if you're looking from the back of the chassis. Be sure this is adequately clear before drilling holes or anything.
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Thanks, PB! Should I desolder all of the old wire, or could I just clip the wires that I currently have and reattach them to the balance pot?
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That is up to you. I've never done what you're doing, so you'll have to slog through it and figure some of this out on your own.
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No problem. This will be an interesting experiment haha. I do have my extra wire (and plenty of it), so if I need to redo my wiring it shouldn’t be a problem.
As far as the actual wiring goes, left and right rca jacks to the balance pot, and the balance pot to the volume pot, what is that looking like? Or is that something we will need to work through?
I really appreciate your help.
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Ross,
If space is tight and you haven't drilled yet, I was able to get the CtoC on the Vol/Bal pots down to 1.75" as per the photo Paul attached and 1.5" sitting side by side with the terminals pointing to the same side.
Another simple fix for the imbalance is dual-mono volume pots.
Good luck on your mods,
Karl
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Hi Karl,
I have not done any drilling yet. Would you mind posting a pic of your build? I'd like to see a longview of the wiring. The closeup view that Paul provided helps a lot, but I'd like to see the full view (I also realize that Paul's pic is not a Crack so a larger view of that one would probably not help me out much).
I appreciate the kind words! I believe this mod will go smoothly once I get going.
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I got my holes drilled and my new potentiometer place inside my Crack. So far, so good. I am going to solder the connections as they are shown in the above image. I'll also try to post some pics later tonight of how it is looking after I solder the connections.
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Okay, I believe that I got the wiring figured out! Let me know what you think PB.
Looking from the back of the chassis here are the connections that I made to the balance pot.
RCA to balance pot:
Left RCA --> top left lug balance pot
Right RCA --> bottom right lug balance pot
Right RCA ground -- > top right lug balance pot
Left RCA ground --> bottom left lug balance pot
Balance pot to volume pot (I just went off the picture for these connections):
Ground wire --> between bottom left lug of balance pot and bottom right of volume pot.
Ground wire --> between top right lug of balance pot and top right of volume pot.
Left input --> between top middle lug of balance pot and top left of volume pot.
Right input --> between bottom middle lug of balance pot and bottom left of volume pot.
33K resistors between top left of balance pot and top left of volume pot + bottom right of balance pot and bottom left of volume pot.
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Let me know what you think PB.
Does it work?
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I'm letting some standoffs settle into epoxy under the chassis. I had to raise one of my film caps up to get it out of the way. Once the standoffs get settled and I solder the film cap connections, I will turn the amp on and see if everything works.
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Hi PB! I confirm that the balance pot works, and my Crack amp is up and running just fine. I've been listening for about 30 minutes with no issues.
The balance pot clearly changes the prominence between the left and right channels. For instance, when I crank the balance pot all the way to the right, the right channel becomes more prominent with sound still coming from the left channel. My volume pot is working just fine too.
I threw on some mono tracks to test everything out.
If any issues arise, I will post them here. I really appreciate all of your help! This is a very cool mod.
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I have noticed a couple semi-issues.
1) When I turn my amp on with my volume knob not in the zero position, I get really loud buzzing coming from my headphones. It only lasts for like a minute or so before going away. However, semi-issue #2...
2) When I turn my volume knob there are occasional loud scratching and popping noises. Much louder than what I have heard pots generate. Loud, as in I need to take my headphones off to turn the volume knob.
None of these issues occur with the turning of the balance pot. Other than these two things, everything sound great!
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You can spray some electronics cleaner into the pot if you'd like, as the pots we provide are open and debris can get in there.
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I have a PEC pot that is closed. I would get some scratching before, but never any pops when turning the volume pot. The scratching is also super loud. Do you think I can do anything about it?
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Just be sure that all your solder joints are solid.
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Thanks, PB, and I’ll double check all my solder joints. The buzzing at start up is weird. No idea why that is occurring.