Significant difference in pot input impedance?

cstop · 1074

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Offline cstop

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on: December 08, 2015, 12:32:30 PM
Just finished building the Crack amp over the weekend and ran the resistance checks yesterday. All resistance measurements seems to be within tolerance (will post the values tonight) except for the pot. I'm measuring a rather significant difference between the two: the left channel measured at ~94 Kohm (okay) while the left measured at ~111 Kohm (not okay).

Would it cause any significant channel imbalance from this? Could a bad solder joint increase the resistance substantially? I have some pictures of the input wiring.

imgur.com/a/oDaPz



Offline cstop

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Reply #1 on: December 08, 2015, 05:38:49 PM
Below are the resistance measurements. Used a NIST calibrated Fluke 787 and used T12 as the ground/reference point per the manual. Test leads measured 0.1 ohm.

TerminalExpected RMeasured R
1*>100k and rising
2*>90k and rising
300.1
4*>100k and rising
5*>130k and rising
62.4k2.488k
72.9k2.927k
800.1
92.9k2.936k
102.4k2.491
1200***
13* will climb slowly towards 270k269.9k**
1400.2
2000.2
2200.1
B32.9k2.928k
B62.9k2.937k
RCA Ground lug (L / R)00.2 / 0.2
RCA Center Pin (L / R)90k - 100k94.1k / 111.2k

Table Notes
** Starts at 10M and drops to ~3.6M before rising from 130k to 269.9k over 10-15 minutes
*** Reference point



Offline Doc B.

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Reply #2 on: December 08, 2015, 08:22:28 PM
The only way to tell if the channel to channel resistance difference you measure will have any influence on level balance is to try it. Measuring across the entire track doesn't tell you what the comparative channel to channel resistance to the wiper is at any other point on the track except the end.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline cstop

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Reply #3 on: December 08, 2015, 11:46:25 PM
Thanks Doc. I'll give it a whirl and see if it's noticeable. I've finished the voltage checks. Should I be worried with the voltages on A1, A6, B1, and B4? Mains voltage was 121.9V.

TerminalExpected VMeasured V
175-9076.3
2170172.7
300.000
4170172.6
575-9077.5
600-0.02
7100106.6
800.001
9100106.6
1000.01
1100.001
1200.0***
13170172.8
140-0.001
15185194
2000.01-0.02
21206215.3
===
A19077.1
A200.000
A400.001
A500.001
A69075.9
A700.000
A900.001
===
B2170171.8
B3100106.2
B49076.9
B5170171.8
B6100106
B700.001
B800.001

Table Notes
*** Reference point




Offline Paul Birkeland

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Reply #4 on: December 09, 2015, 07:14:23 AM
Thanks Doc. I'll give it a whirl and see if it's noticeable. I've finished the voltage checks. Should I be worried with the voltages on A1, A6, B1, and B4? Mains voltage was 121.9V.
None of these voltages are out of spec.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline cstop

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Reply #5 on: December 10, 2015, 10:41:33 PM
Thanks for the responses. It's now up and running. Happy to say that there's no noticeable/audible imbalance between the channels and sounds great.