Crack+Speedball, distortion after upgrade and resistance issues [resolved]

[jmacinnes]

Hi there,
After a summer of enjoying my stock Crack, I decided to go for the Speedball upgrade. I gave it a listen after installing the small circuit board and everything sounded superb. Really clean, no background, and great staging. However, once I added the large circuit board, things took a turn for the worse. The mids and bass are distorted, and the background noise (which vanished after the small circuit board install) is back and even louder than before (I think?). Not sure what I messed up.

The Speedball resistance checks passed. With the Speedball voltage checks, I measured 110V at both OA and OB, which is right at the limit of the listed 75-100V +/- 10% values. All LEDs light up on both circuit boards and the 12AU7 tube.

I went back and tried to reflow any joints that looked suspect, and then went back to the original voltage and resistance checks from the Crack manual. All of the voltage checks are within range. Problems are showing up in the resistance checks -- resistance at terminals 7, 9, B3, and B6 is showing as 1 (open circuit) on my DMM (in the manual, each of those should read 2.9K).

I'd really appreciate any suggestions for where to go next. After having that brief glimpse of how amazing the Speedball upgrade can sound, I'm eager to get this working properly.


Many thanks,
Jeff MacInnes

[Paul Birkeland]

The resistance checks for 7 and 9 are no longer valid once the Speedball is installed.

How are the OA/OB voltages on the small board?

Generally issues like these come down to bad solder joints, but do keep in mind that while installing the Speedball, you may have disturbed a pre-existing connection in the original amplifier.

[jmacinnes]

Thanks for the fast response! I have 183V at both OA/OB on the small board. I'll go over all of the joints again tonight to see if I can track down a bad connection or two. 

[Paul Birkeland]

Thanks for the fast response! I have 183V at both OA/OB on the small board.

Your small board is not working if your OA/OB voltage there are that high.  Are the LEDs on the 9 pin socket lit up?

[jmacinnes]

Apologies, my mistake, those were the readings from 1A/1B. I have 74V at OA and 78V at OB

[Paul Birkeland]

Posting some detailed photos of your build could be helpful in this scenario.

-PB

[jmacinnes]

I went over the connections again and reflowed most of them. The distortion improved (but didn't disappear) for a bit, but after ~1hr it was noticeable again. I took the large board off and am planning on touching up those connections, and maybe even replacing the wires connecting the board to the terminal strips (didn't have strippers small enough for that wire, and worried that I might have damaged/weakened some of the wires while stripping with the cutters. Picked up a new, smaller gauge wirestripper today). In the meantime, I've attached some detailed pics that may offer some clues (the extra red and black wires coming out of the 9pin socket are for a 6V indicator lamp I added so that I'd stop forgetting to turn the amp off).  Really appreciate the help on this!

[Doc B.]

We really need to see the underside in some fully assembled form, either with the small board and cathode resistors on the big tube, or with both boards installed. Also please disconnect any mods to the stock circuits like the lamp. We are able to triage this stuff online only if the circuit looks like what the finished circuit looks like in the manual. Otherwise there are too many deviations to be able to say what might be out of spec.

[jmacinnes]

Update and fixed (I think). I replaced the wires connecting the large board to the terminal strips, but still noticed some distortion, most prominent in the mids and bass. I realized however, that the problem disappears if I drop the volume on the input source (laptop running into a Wavelength Proton DAC) down to ~90%, and then make up for it by turning the Crack volume up a bit. The signal stays clean even after turning the Crack volume up beyond any comfortable listening levels. I hadn't thought to check this before since with the stock Crack I ran the input at 100% and never experienced this problem. I did some more digging on the forum and saw an older post mentioning an additional ~15% gain with the Speedball upgrade. Still pretty new at this audio and circuitry stuff...could that additional gain have lead to the distortion I was experiencing. Going forward, I'm happy to keep the input volume scaled back a bit if that's the best solution. Really impressed with how it's sounding so far.

Thanks again for all of the assistance tracking down this issue!

[LolNole]

The problem is with how you are using the Wavelength Audio Proton DAC. The solution is basically what you have discovered. If you are using the Proton DAC's line output, adjust the volume on the connected computer to 90% or 91% of maximum. Here is the explanation. The Proton DAC has its own built-in headphone amplifier and analog volume control. If you connect headphones to the device, you can adjust the volume through the connected computer all the way up to 100%, where the last ~10% controls the gain from the Proton DAC's amplifier. However in your current setup, you want to use the Crack as your headphone amplifier. By setting the volume to 100%, you are using both of them as amps which results in distortion and clipping. By doing what you have already done (adjust to ~90%), you are using the Proton DAC as you intended: a DAC.

[jmacinnes]

Ah, gotcha, that makes a lot of sense. Thanks for the helpful info on the Proton DAC, LolNole. I got it as a hand-me-down and hadn't learned much about its inner workings yet. It's reassuring to hear the solution I stumbled on is the recommended practice and not just a hacky fix.