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Photo's and initial experience of completed DIY projects
I thought I would start off this forum with some photo’s of my lock down project of a Tortuga passive in an old Mission Cyrus 1 Amp. I am very pleased with how the construction went. Fashioning a little metal bracket to mount the OLED in the perfect spot was probably the hardest part.
Earlier this evening, I plugged in and to my disappointment the sound was muffled, constricted and very low. I had other things to do for most of the rest of the evening, so been turnover all the reasons why things are so bad. Impedance mismatch, the fact I have just taken the hot off the XLR to a single board (unbalanced config) thereby halving the effective voltage and reducing what I thought would be 6db. Surely that wouldn’t be it. Cut out the pre and plugged Chord Hugo TT DAC straight into my active crossover and volume and quality was back, so had to be the pre 🙁
Now returned and thought all I can do is try another calibration (I had already done one, but Amp wasn’t wired into the system, so maybe that might make a difference, I had also read, I think, that 2 sweeps on re-cal is recommended?) Anyway nothing to lose.
Great news, everything is now good, volume up, quiet evening listening levels good at 46/100. so plenty of headroom. Nothing fuzzy or constricted in the sound. So very happy
David had reported audible distortion with his LDR preamp which was literally unheard of (no pun intended) from any of our customers to date so David and I had several back and forths over email to try to resolve it.
Cutting to the chase, the distortion was due his Chord DAC running with a very “hot” linestage voltage of several volts vs. the typical linestage level in the range of 1-2 V RMS. As as result of the high input voltage level to the preamp, the preamp had to attenuate a significant amount of energy to achieve reasonable listening volumes. This high V drop across the LDRs drove them into distortion.
It’s a known characteristic of LDRs that they do exhibit higher levels of distortion than most other volume control technologies. And that the distortion level is proportional to the voltage drop across the LDR. Ordinarily this is not an issue because the distortion remains far below the threshold level where humans can hear it. If you happen to be an objectivist audiophile who only believes in what can be measured then this will probably drive you bonkers. On the other hand if you happen to take a more subjective view on things meaning do you like what you hear or not, then this issue simply does not matter.
Because of its high voltage output the Chord DAC just wasn’t a good fit for an LDR volume control. David ended up switching out his Chord DAC with another make/model that operates at typical voltage levels and the audible distortion problem went away. Bottom line is source devices with really “hot” outputs are not a good match for LDR based preamps.