Background infoMy father purchased the Bottlehead Crack 1.1 kit with the Speedball 1.1 upgrade many years ago. He built the kit, completed all the soldering, and made all the connections, but never used it much. I inherited the amp in January 2025, and when I connected everything for the first time and tested the sound, I didn’t get the experience I had hoped for.
There was only sound on one side, and when I adjusted the headphone jack up and down, I suddenly got sound on both sides. I asked my father if he had ever noticed this issue before or if he might have rushed the build and caused an error, but he insists that he has never experienced any problems and that the kit worked fine. (I should mention that he is very stubborn, haha.)
I might be more sensitive to sound than he is, and I quickly noticed that something was off, given that this amp is supposed to deliver an “audiophile” and magical experience—which I haven’t gotten yet.
Unfortunately, I suspect that my father built and used this amp (along with an expensive DAC and headphones) without ever realizing that he has only been listening in mono and that the headphone jack position has created a "false two-channel effect."
I really want to get to the bottom of this issue and unlock the amp’s full potential.
My Setup🎧 Headphones: Massdrop x Sennheiser HD 6XX
🎛 DAC: Hegel HD 11
🎚 AMP: Crack 1.1 OTL Headphone Amplifier Kit + Speedball 1.1 Upgrade
The IssueFrom the very first time I plugged in my headphones and tested the amp, I noticed that audio was only playing on one side when the headphone jack was fully inserted. When I slightly pulled out the headphone jack, audio played on both sides, but stereo separation was missing—I only get mono sound.
🔹(Headphone jack fully inserted) → Sound plays on the right side, while the left side has static noise and almost no sound.
🔹(Headphone jack slightly pulled out) → Sound plays on both sides, but it's mono.
(Headphone jack slightly pulled out) → Sound plays on both sides, but it's mono.
The audio balance changes depending on how far the headphone jack is inserted into the amp.
Troubleshooting Steps Taken🔹 I have double-checked all connections against the Bottlehead Crack 1.1 manual, and everything is correctly wired.
🔹 The only exception is the power wiring method on page 28 instead of page 27:
The one used is: "IF your voltage is GREATER than 235V AC and LESS than 245V AC" instead of "IF your voltage is GREATER than 225V AC and LESS than 235V AC".
The only difference is that the jumper between Terminal 15 and Terminal 16 should have been between Terminal 14 and Terminal 16.
Could this cause my issue, or is it unrelated?🔹 I have also checked all steps and wiring for the Speedball 1.1 upgrade, and everything appears correct.
🔹 However, I noticed a difference in the wiring of the headphone jack resistors between two manual images:
Page 33 in Speedball 1.1 manual: Both 2.49KΩ resistors are connected to the lower lug of the headphone jack.
Page 46 in Crack 1.1 manual: The 2.49KΩ resistors are attached on each side of each "lug".
Could this be causing my issue? Which wiring is correct?Suspicious Observation - LED IssueDuring troubleshooting, I noticed that one LED is not lighting up:
🔹The HLMP-6000 LED located between the center pin of the 9-pin socket and Terminal A3 is not illuminated.
🔹Upon inspection, I found that the LED is mounted in the opposite direction from the image in the manual.
Could this reversed LED placement be causing my issue?Resistance & Voltage Measurements📏 Equipment used:
📌 True RMS Digital Multimeter (UNI-T UT161D)
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Resistance Check - Bottlehead Crack 1.1 Manual vs. My ResultsTerminal Manual Resistance My Results
1 * .ol kΩ
2 * .ol kΩ
3 0Ω 0.0 kΩ
4 * .ol kΩ
5 * .ol kΩ
6 0Ω 0.0 kΩ
7 2.9KΩ .ol kΩ (Speedball 1.1 installed)
8 0Ω 0.0 kΩ
9 2.9KΩ .ol kΩ (Speedball 1.1 installed)
10 0Ω 0.0 kΩ
12 0Ω 0.0 kΩ
13 * .ol kΩ
14 0Ω 0.0 kΩ
20 0Ω 0.0 kΩ
22 0Ω 0.0 kΩ
B3 2.9KΩ .ol kΩ
B6 2.9KΩ .ol kΩ
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RCA Jack Resistance CheckRCA Jack Manual Resistance My Results
White Wire - 12U to white wire, 94.7kΩ
Red Wire - 12U to red wire, 94kΩ
Center Pin (Left Input) 90KΩ-115KΩ 94.7kΩ
Center Pin (Right Output) 90KΩ-115KΩ 94kΩ
Ground Tab 0Ω 0.0 kΩ
potentiometer
left L 94,2kΩ , U94,8kΩ
middle L 94,2kΩ , U94,8kΩ
right (ground) 0,0 kΩ
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Voltage Measurements - Bottlehead Crack 1.1Terminal Manual Voltage (DC) My Results
1 50-100V 79.1V
2 170V 176.2V
3 0V 0V
4 170V 176.3V
5 50-100V 173V
6 0V 0V
7 90-115V 107.2V
8 0V 0V
9 90-115V 171.5V
10 0V 0V
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🛠 Speedball 1.1 Voltage Check (Small Board)Terminal Manual Voltage (DC) My Results
oa 60-90V 79.2V
ia 170-270V 175.4V
b-A/b 0V 0V
ib 170-270V 175.2V
ob 60-90V 171.9V
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🛠 Speedball 1.1 Voltage Check (Large Board)Terminal Manual Voltage (DC) My Results
OA 75-100V 171.1V
OB 75-100V 106.9V
G 0V 0V
B+ 170-195V 175.9V
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💡 Questions for the Forum:- Could the headphone jack resistor wiring difference be causing my mono/stereo issue?
- Is the reversed LED at Terminal A3 a critical mistake?
- Does the different power wiring (Terminal 15→16 instead of 14→16) impact this issue?
- Do any of my resistance or voltage readings seem incorrect?
Uploading pictures below! Any help would be greatly appreciated! 🙏🎧🔧
