Measuring voltages while playing a 60 Hz sine wave. [resolved]

[Colonl_Charisma]

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.

 

[Paul Birkeland]

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.

 

[Colonl_Charisma]

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

 

[Paul Birkeland]

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.

 

[Colonl_Charisma]

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

 

[Paul Birkeland]

Yes that should work.

 

[Colonl_Charisma]

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!

 

[Paul Birkeland]

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.

 

[Colonl_Charisma]

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?

 

[Paul Joppa]

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.

 

[Colonl_Charisma]

Thanks, PJ!

 

[Colonl_Charisma]

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

 

[Paul Birkeland]

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.

 

[Colonl_Charisma]

Yeah, it’s a noticeable difference haha. How do I measure the voltage of the RCA plugs?

 

[Paul Birkeland]

One probe on the pin, one probe on the shell.

 

[Colonl_Charisma]

Thanks! This is with the 60 Hz wave playing.

Left channel RCA plug: 1.488 V
Right channel RCA plug: 1.487 V

 

[Paul Birkeland]

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?

 

[Colonl_Charisma]

RCA left = 99.3 k ohms
RCA right = 99.4 k ohms

Nothing was plugged in.

 

[Paul Birkeland]

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.

 

[Colonl_Charisma]

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.