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September 27, 2021, 07:27:39 AM

Author Topic: Measuring voltages while playing a 60 Hz sine wave. [resolved]  (Read 1083 times)

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Offline Colonl_Charisma

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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.
« Last Edit: July 26, 2021, 03:26:50 PM by Paul Birkeland »
Ross

Online Paul Birkeland

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #1 on: July 23, 2021, 05:29:50 AM »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Online Paul Birkeland

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #3 on: July 23, 2021, 07:07:01 AM »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Colonl_Charisma

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #4 on: July 23, 2021, 07:53:39 AM »
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
Ross

Online Paul Birkeland

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #5 on: July 23, 2021, 09:38:46 AM »
Yes that should work.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Colonl_Charisma

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #6 on: July 23, 2021, 10:32:18 AM »
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!
Ross

Online Paul Birkeland

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #7 on: July 23, 2021, 10:44:14 AM »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Colonl_Charisma

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #8 on: July 23, 2021, 11:13:33 AM »
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?
Ross

Offline Paul Joppa

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #9 on: July 23, 2021, 11:39:02 AM »
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.
Paul Joppa

Offline Colonl_Charisma

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #10 on: July 23, 2021, 11:52:48 AM »
Thanks, PJ!
Ross

Offline Colonl_Charisma

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #11 on: July 23, 2021, 02:06:41 PM »
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
Ross

Online Paul Birkeland

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #12 on: July 23, 2021, 02:29:14 PM »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Colonl_Charisma

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #13 on: July 23, 2021, 03:55:28 PM »
Yeah, it’s a noticeable difference haha. How do I measure the voltage of the RCA plugs?
Ross

Online Paul Birkeland

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Re: Measuring voltages while playing a 60 Hz sine wave.
« Reply #14 on: July 23, 2021, 04:07:38 PM »
One probe on the pin, one probe on the shell.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man