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Old 23rd February 2017, 03:42 AM   #1
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Question Cmoy amp - unbalanced ground ?

Hi everyone,

After a huge disappointment from what could have been an awesome audio project on a crowdfunded website, I've decided to get my hands dirty and to built my own home tube amp. Despite having some background in electronics (from a few years ago...), I don't feel comfortable going on such a big task right away of course, so I'm starting with the CMoy amp for TangentSoft, to catch-up with general electronics knowledge and soldering before getting into tubes.

I have built the entire amp, but of course I couldn't make it work the first time

So here are the symptoms : From my computer as audio source, I end up with a really (really!) low output from the amp, with lots of noise (fun fact : following TangentSoft troubleshooting page, with a 1kHz sin input, I end up with a much more audible output from the amp, with almost no noise at all)

I have already investigated a few hints, and so far here are my remarks :
  • I couldn't find any short from any horizontal conductive strip to the virtual ground (please forgot my English, non-native speaker here) except the ones connected to it on purpose, meaning the two resistors and capacitors on the power section (see pictures below, I differed a little from the original lay-out as I couldn't find the proper board. The two vertical stripes are my virtual ground and I soldered the components on the horizontal stripes)
  • I do have +9 and -9V on V+ and V- on the op-amp (the design includes a 2x9V battery) but only with the chip off the board. If I place the chip on its socket, I end up with a -2.6V on V- and 16.2V on V+
    • Is the chip supposed to unbalanced the voltage on V+ & V- once active ?
  • following the step-by-step process to find the problem on TS website, I can say that :
    • AC Voltage on input look ok : 1.37V on both channels
    • same goes for the wipers of the volume control, from 1.37V with the knob on full left to 0V on full right
    • Things get weird once measuring on IN+ and IN- where I respectively get +0.61V AC and +0.88V AC (my theory on op-amp is clearly rusty, but from the process, it looks like I should get almost "same voltage as at the volume controlís wiper")
Below the mentioned pictures. Not sure if I should clean the board with alcohol, I used "no-clean rosin core solder". Please forgive my first soldering in like 8 years
Attached Images
File Type: jpg 20170222_144650_HDR.jpg (1.01 MB, 129 views)
File Type: jpg 20170222_144651_HDR.jpg (385.0 KB, 133 views)
File Type: jpg 20170222_144652_HDR.jpg (309.8 KB, 127 views)
File Type: jpg 20170222_144653_HDR.jpg (521.3 KB, 124 views)
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Old 23rd February 2017, 06:47 PM   #2
MooCow is offline MooCow  Germany
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Have you measured DC at the output?
It might be that the opamp is faulty.
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Old 23rd February 2017, 06:57 PM   #3
Mooly is offline Mooly  United Kingdom
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Is it this version ?

https://tangentsoft.net/audio/cmoy/

If so then place your meter black lead on the 'ground' symbol and measure and report the voltages on all 8 pins of the IC. Do NOT connect headphones for this.

Copy and paste with the voltage

Pin1=
Pin2=
Pin3=
Pin4=
Pin5=
Pin6=
Pin7=
Pin8=
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Old 23rd February 2017, 08:52 PM   #4
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Quote:
Originally Posted by MooCow View Post
Have you measured DC at the output?
It might be that the opamp is faulty.
@MooCow : -1.97V DC on right and 1.34V DC on left

Quote:
Originally Posted by Mooly View Post
Have you measured DC at the output?
It might be that the opamp is faulty.
@Mooly : yes, that's the version I was going for
Here are the measures :
Pin1= 16.26V DC
Pin2= -1.97V DC
Pin3= -1.69V DC
Pin4= -2.44V DC
Pin5= -2.54V DC
Pin6= -2.11V DC
Pin7= -1.20V DC
Pin8= -1.54V DC

I have a silly question though : I was a bit hasty when soldering the chip socket and and ended up with the notch reversed (the chip is in the right orientation though). Can you confirm that if the chip and the socket notches are not matching, this has no incidence on the chip performance ?
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Old 23rd February 2017, 09:18 PM   #5
MooCow is offline MooCow  Germany
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Quote:
Originally Posted by bromatofiel View Post
@MooCow : -1.97V DC on right and 1.34V DC on left
That's massive, don't connect headphones.
It should be, depending on the opamp only be in the single digit mV range(if at all).
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Old 23rd February 2017, 10:15 PM   #6
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Quote:
Originally Posted by MooCow View Post
That's massive, don't connect headphones.
It should be, depending on the opamp only be in the single digit mV range(if at all).
You made me doubt, but after a second try, I confirm this is indeed DC output... any idea on the root cause ?

Last edited by bromatofiel; 23rd February 2017 at 10:44 PM. Reason: typo
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Old 24th February 2017, 06:49 AM   #7
Mooly is offline Mooly  United Kingdom
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Those voltages suggest a major construction error and appear almost as if you have the IC fitted or wired incorrectly.

Pin 8 should be the most positive (+ supply) and pin 4 the most negative (- supply).

You have pin 1 and pin 5 with those voltages ! Almost as if you are looking at the IC from the wrong side of the board.

Remove the battery and check that the battery plus terminal goes to pin 8 and the battery negative to pin 4.

Looking down on the IC from above and it looks like this (picture below).

With the IC removed from the socket you should be able to measure +9 volts on pin 8 and -9 on pin 4. All the other pins of the empty socket should be at zero.

Quote:
I have a silly question though : I was a bit hasty when soldering the chip socket and and ended up with the notch reversed (the chip is in the right orientation though). Can you confirm that if the chip and the socket notches are not matching, this has no incidence on the chip performance ?
Doesn't matter which way round the socket is fitted (they are only metal pins at the end of the day) as long as you fit the IC correctly to match the board below.
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Old 24th February 2017, 05:25 PM   #8
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Quote:
Originally Posted by Mooly View Post
Those voltages suggest a major construction error and appear almost as if you have the IC fitted or wired incorrectly.

Pin 8 should be the most positive (+ supply) and pin 4 the most negative (- supply).

You have pin 1 and pin 5 with those voltages ! Almost as if you are looking at the IC from the wrong side of the board.
Indeed, and mea culpa, I read the specs as down view and not top view. Here are the updated measures. I do have pin 8 as the most positive and pin 4 as the most negative. Moreover, my layout checks out with TangentSoft's one.

Pin1= -1.47V DC
Pin2= -1.13V DC
Pin3= -1.39V DC
Pin4= -2.58V DC
Pin5= -2.47V DC
Pin6= -1.72V DC
Pin7= -2.01V DC
Pin8= 16.08V DC

Sorry for the mistake.

Quote:
Originally Posted by Mooly View Post
Remove the battery and check that the battery plus terminal goes to pin 8 and the battery negative to pin 4.
Confirmed, they do.

Quote:
Originally Posted by Mooly View Post
With the IC removed from the socket you should be able to measure +9 volts on pin 8 and -9 on pin 4. All the other pins of the empty socket should be at zero.
With op-amp removed :

Pin1= 0V DC
Pin2= 0V DC
Pin3= 0V DC
Pin4= -9.33V DC
Pin5= 0V DC
Pin6= 0V DC
Pin7= 0V DC
Pin8= 9.33V DC

Quote:
Originally Posted by Mooly View Post
Doesn't matter which way round the socket is fitted (they are only metal pins at the end of the day) as long as you fit the IC correctly to match the board below.
Thanks for the confirmation.
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Old 24th February 2017, 05:38 PM   #9
Mooly is offline Mooly  United Kingdom
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no problem.

Have to say then, that at this point you should try another chip. The voltages with the chip removed from the socket are fine.

What chip are you using ?

You can try pretty much any common dual opamp such as TL072, TL082, 4558 etc to confirm it all works. The voltages you are seeing are way off and don't even mirror one channel vs the other.

The circuit is a classic text book design and will work if constructed correctly.
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Old 24th February 2017, 05:48 PM   #10
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Quote:
Originally Posted by Mooly View Post
What chip are you using ?
I have the OPA2132PA-ND from Digikey. Ok then, I'll order another one along the TL072.
Let's hope it's just about faulty component.
Any "chance" I just damaged it while mounting the beast ? I don't know how sensitive are those op-amps.
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