Have had my Crack for about two years and loving it with headphones. I wanted to see what tubes would sound like on my loudspeaker system so used it as a preamp. Worked great and sounded really good (convinced me to order a Moreplay). However at random, sometimes after a few minutes and sometimes after 1-2 hours, it causes the protection circuit on my McIntosh power amp to go int protection mode. This does not happen when I use a Schiit Sys (simple switch and pot) instead of a preamp, and happens to both channels. I believe it is a problem with the Crack, perhaps a voltage spike? Could this be a bad joint? I redid all the connections to solve an earlier problem, but some of them were burned when I built the Crack. This was my first DIY project. Thanks for your help.
Crack voltage spikes
- Thread starter arugula
- Start date
I would imagine that your McIntosh amp is DC coupled, and these circuits are extremely sensitive to anything even remotely resembling DC appearing at the input jacks. In the Crack, the coupling caps are rather large to work well with headphones of relatively low impedances (compared to the input impedance of a power amp), and those 2.49K resistors on the headphone jack are sized to not eat up much of the output current but to also keep the output of the Crack relatively close to 0V.
If the cathode emission of the 6080 isn't perfectly constant over time, a little bit of DC voltage can appear at the outputs. In a DC coupled solid state amp, that DC would actually be amplified and passed to your speakers, which isn't desirable. Probably the easiest solution in this case is to use something like a Harrison Labs or Rothwell attenuator in the -10dB to -12dB range at the inputs of the McIntosh, as this will cut the appearance of this near DC content and make it less likely the protection circuit will trip. Somewhat by accident, the Moreplay architecture kind of does this on its own.
(Another solution is to just not to use DC coupled amplifiers)
-PB
If the cathode emission of the 6080 isn't perfectly constant over time, a little bit of DC voltage can appear at the outputs. In a DC coupled solid state amp, that DC would actually be amplified and passed to your speakers, which isn't desirable. Probably the easiest solution in this case is to use something like a Harrison Labs or Rothwell attenuator in the -10dB to -12dB range at the inputs of the McIntosh, as this will cut the appearance of this near DC content and make it less likely the protection circuit will trip. Somewhat by accident, the Moreplay architecture kind of does this on its own.
(Another solution is to just not to use DC coupled amplifiers)
-PB
Hi Paul - Thanks a lot for your help. I have two further questions:
1) When the amplifier turned off, the lights indicating "Power Guard" circuit went on. This is a specific type of protection. Manual says "The Power Guard circuit is a waveform comparator, monitoring both the input and output waveforms...If the amplifier is overdriven, there will be a difference.... When the difference exceeds 0.3% the Power Guard activates the PG light and a dynamic attenuator at the amplifier input reduces the input just enough to prevent further distortion... There are no audible effects". I heard a loud series of tones - whoop whoop whoop - of decreasing volume. Then the whole thing turned off. Is this consistent with the DC current issue you described? I guess the Crack is doing something that first trips the Power Guard but also turns off the amp through another protection. The only other protection mentioned in the manual is an 'Output Transistor Protection Circuit"
2) Rather than experiment with the Crack further, I will build the Moreplay. If I understand you correctly, the DC current problem you describe is not an issue with the Moreplay. The suggestion to use an amplifier that is not DC coupled will not work for me- bought this fancy amp and would be tough to switch now. I googled "DC coupled and Mc462" (my amp). Some posts talked about the fact that the amp uses an auto former rather than being DC coupled, but I don't think that is relevant here?
Sorry for the detailed further questions. Will the Moreplay actually be OK with this amp?
1) When the amplifier turned off, the lights indicating "Power Guard" circuit went on. This is a specific type of protection. Manual says "The Power Guard circuit is a waveform comparator, monitoring both the input and output waveforms...If the amplifier is overdriven, there will be a difference.... When the difference exceeds 0.3% the Power Guard activates the PG light and a dynamic attenuator at the amplifier input reduces the input just enough to prevent further distortion... There are no audible effects". I heard a loud series of tones - whoop whoop whoop - of decreasing volume. Then the whole thing turned off. Is this consistent with the DC current issue you described? I guess the Crack is doing something that first trips the Power Guard but also turns off the amp through another protection. The only other protection mentioned in the manual is an 'Output Transistor Protection Circuit"
2) Rather than experiment with the Crack further, I will build the Moreplay. If I understand you correctly, the DC current problem you describe is not an issue with the Moreplay. The suggestion to use an amplifier that is not DC coupled will not work for me- bought this fancy amp and would be tough to switch now. I googled "DC coupled and Mc462" (my amp). Some posts talked about the fact that the amp uses an auto former rather than being DC coupled, but I don't think that is relevant here?
Sorry for the detailed further questions. Will the Moreplay actually be OK with this amp?
The MC462 looks to be a DC coupled amp with an autoformer at the output of each channel, so the same issues can crop up that a normal DC coupled amp would have.
The Moreplay is better setup for this situation, and actually the BeePre 2 with the second upgrade installed is really our product that's most optimized for this situation specifically.
The Moreplay is better setup for this situation, and actually the BeePre 2 with the second upgrade installed is really our product that's most optimized for this situation specifically.
