The second change that must be made to completely cure the noise issue in these tubes is to elevate the heaters above ground.
The hum is present on some input tubes, not all, due to heater-to-cathode leakage. This improves the hum but does not eradicate it, I suspect due to significant primary-to-secondary leakage currents in the transformer which is of poor quality. A virtual center-tap can be made using two 100ohm 1/2W resistors. The heaters in this amplifier are not center-tapped, the designer has grounded one leg of the heater at the power LED ground point. The widely reported hum issues in this amplifier can be corrected by making two circuit changes. I'll post something in the DIY thread to show it off when it is done, but wanted to point something out here. These are not hocus pocus modifications, actual circuit design changes based on sound tube amplifier design.
The Crack is still pretty much my favorite amp for the HD800s at the moment.I am doing a total circuit overhaul of this amplifier to improve the design and also correct many of the mistakes of the original designer, just a fun DIY project for someone on Head-Fi. But having a good solid state amp like the Beta, it's nice to have the Crack which offers a greater contrast in sound quality against the Beta.
If I only had one amp, I would want the Little Dot over the Crack because, to me, it sounded a bit more "accurate" - less euphonious coloration added. The Little Dot MK III, in a sense, fell between the Beta and the Crack. So, for me, to able to choose from the romantic sound of the Crack and the "precise" sound of the Beta-22 is a really nice option. I would have to say I preferred the Crack overall if I want tighter bass than the Crack I can go to my Beta 22 which is fantastic in this regard with very clean highs. I liked the tighter bass from the Little Dot, but the overall sound from the Crack added a lush quality that I liked. There was also a slightly "smoother" quality of the highs on the Crack, maybe what I'm hearing here is the sound of film caps in the Crack vs. As you may recall, I found both the Little Dot and the Crack to "sweeten" the harmonic balance of the highs a bit compared to the Beta-22, and this comparison seems to show that the Crack adds a little more treble sugar than the Little Dot. Overall, the Crack had a more "tubey" sound. The Crack sounded like it had a little bit of rolloff at the highest frequencies compared to the Little Dot This actually was OK on the Sennheisers which are a little hot in the highs. The Crack added a little more harmonic sweetness, while the Little Dot seemed a bit more transparent in the highs.
The sound of the treble range differed between these two amps. Separation / imaging sounded about the same with maybe a trifle better sense of space from some recordings produced by the Crack. However, the lowest bass notes seemed better served by the Little Dot- this gave the Little Dot a tighter bass sound, overall.īecause of the midbass "bump" from the Crack, mids seemed to have a little more "body" than with the Little Dot, although not by a big margin. As with all the tube amps I own, these amps both added a little euphonious signature to the sound, but each amp had a different character.Ĭompared to the Little Dot Mk III, the Crack added a little midbass which made the music sound a bit warmer and a little "richer" in a certain way. I carefully matched the volume levels of the two amps using pink noise & a 'scopeĪs the Crack is not intended for low impedance phones and won't drive my LCD-2's well at all, I only used my Sennheiser HD-800's for this test.