Bottlehead Forum

Bottlehead Kits => Crack => Topic started by: cstop on December 08, 2015, 12:32:30 PM

Title: Significant difference in pot input impedance?
Post by: cstop 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

Title: Re: Significant difference in pot input impedance?
Post by: cstop 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.

Terminal	Expected R	Measured R
1	*	>100k and rising
2	*	>90k and rising
3	0	0.1
4	*	>100k and rising
5	*	>130k and rising
6	2.4k	2.488k
7	2.9k	2.927k
8	0	0.1
9	2.9k	2.936k
10	2.4k	2.491
12	0	0***
13	* will climb slowly towards 270k	269.9k**
14	0	0.2
20	0	0.2
22	0	0.1
B3	2.9k	2.928k
B6	2.9k	2.937k
RCA Ground lug (L / R)	0	0.2 / 0.2
RCA Center Pin (L / R)	90k - 100k	94.1k / 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

Title: Re: Significant difference in pot input impedance?
Post by: Doc B. 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.

Title: Re: Significant difference in pot input impedance?
Post by: cstop 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.

Terminal	Expected V	Measured V
1	75-90	76.3
2	170	172.7
3	0	0.000
4	170	172.6
5	75-90	77.5
6	0	0-0.02
7	100	106.6
8	0	0.001
9	100	106.6
10	0	0.01
11	0	0.001
12	0	0.0***
13	170	172.8
14	0	-0.001
15	185	194
20	0	0.01-0.02
21	206	215.3
=	=	=
A1	90	77.1
A2	0	0.000
A4	0	0.001
A5	0	0.001
A6	90	75.9
A7	0	0.000
A9	0	0.001
=	=	=
B2	170	171.8
B3	100	106.2
B4	90	76.9
B5	170	171.8
B6	100	106
B7	0	0.001
B8	0	0.001

Table Notes
*** Reference point

Title: Re: Significant difference in pot input impedance?
Post by: Paul Birkeland on December 09, 2015, 07:14:23 AM

Quote from: cstop 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.

None of these voltages are out of spec.

Title: Re: Significant difference in pot input impedance?
Post by: cstop 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.