There are a few tweaks that you can experiment with on the SkeletonDAC. Of course the two of the greatest "tweaks" you can accomplish is by constructing a SkeletonCableDAC or a SkeletonHeadphoneDAC.
Several connections are elminated in the CableDAC version of the SkeletonDAC. The result is a slightly quieter and much more convenient-to-use DAC. Simply replace the mini-USB jack with a pigtail USB cable, then wire in some leads on the output that terminate in RCA plugs. Details are available for either the BantamCableDAC or GrubCableDAC - the construction is similar (identical for the GrubCableDAC).
This is perhaps one of the most exciting features of the SkeletonDAC - a HeadphoneDAC. The Texas Instruments PCM2704/5 chip has a built-in headphone amplifier. It can't power full-size audiophile headphones, but can do a great job with most earbuds and a pretty good job with something like Koss KSC-75's. Details can be found on the menu selection at left for "SkeletonHeadphoneDAC."
Another semi-tweak is to use the SkeletonDAC as a USB-to-SPDIF convertor. Many top-of-the-line audiophile DACs take only a SPDIF input. This is because the SPDIF protocol has superior bandwidth and sampling rates compared to USB. However, in the current age of computers, etc., it limits the versatility of a high-end DAC. A SPDIF convertor will let you connect such a DAC to a typical USB PC output, allowing you to use the high-end DAC with your PC/laptop/netbook/etc with an almost un-noticeably slight degradation in performance. For many, the convenience may outweigh that last 0.1% of performance. That said, USB-to-SPDIF convertors are somewhat scarce, sort of inconvenient, and a DIY-version does not exist, as far as this author knows - until the SkeletonDAC. Again, simplicity itself: simply install the optional T1 transformer and wire up your preferred SPDIF cable to the SPDIF outputs on the SkeletonDAC PCB.
Of the other tweaks, they all consist of capacitor changes. These changes are divided into two categories:
I. Power Capacitors
This is the primary power capacitor for the DAC. Due to size and height limitations, a super-low ESR capacitor will provide the most benefit here, something like an Os-con or other super-low ESR organic polymer electrolytic is a good upgrade. Anything up to about 560uf will work well, but the pads are limited to 6.3mm diameter and 2.5mm lead spacing. Note also that height is limited to 10mm in order to fit under the lid of the Hammond 1551 HTBU plastic case.
Mouser has a very good "off-brand" alternate for an Os-Con, a United Chemcon sized at 220uf and 6.3V: 661-PSA6.3VB220M This capacitor is also available at Beezar.com
DigiKey has the United Chemcon 560uf, 6.3V capacitor used on the GrubDAC - 565-3255-ND. It's also a good choice, as its height is low enough to fit into the Hammond case.
II. Signal-Coupling Output Capacitors
Of all the tweaks you can perform on the SkeletonDAC, the signal-output coupling capacitors - CL and CR - are the most signficant. Since a USB/PC power supply is +5VDC, the analog output of the DAC is offset at +2.5VDC (this allows a signal swing of +2.5VDC and -2.5VDC in theory). Because of that, coupling caps are needed to block this DC offset. The result is that the coupling caps pass every bit of the signal. That means the audio quality of the coupling caps is very important. It also means that the coupling caps form an RC filter with the resistance of the connected load (usually an amp). So, unless the caps are sized for a minimum capacitance, bass frequencies will be filtered out. Generally speaking, 1.0uf is sufficient to guarantee all audio bass frequencies for most amps. 0.66uf is probably OK in many circumstances, but with a 10K pot (input impedance) on some amps, 0.66uf might be a little low.
Still, 1.0uf and the pads on the SkeletonDAC (5mm or 10mm - also length) allow a lot of good choices for CL and CR:
- Wima MKS2 - 1.0uf 50V. This cap is the standard capacitor for the SkeletonDAC. Again, a metalized polyester film cap, these look very similar to the MKP10's used on many amps. Initially they will sound a bit thick and bassy, but eventually break in to some of the best and strongest mids available on the SkeletonDAC. These capacitors are provided by default in SkeletonDAC kits. They are also available at Mouser.
- Vishay-Roederstein MKT1817510064 - 1.0uf, 63V. A light green box 11.5mm tall with 5mm lead spacing, this cap is also the default film cap in the power supplies for the Millett MAX/MiniMAX headphone amplifiers. The construction is metalized polyester. These caps are very neutral, with good bass and sparkling highs. They are very fast. However, some consider them a tad lean and the mids are perhaps recessed slightly. Those qualities serve them in good stead when combined with a tube amp such as the MAX/MiniMAX, which has ample mids, bass and presence. These capacitors are available at Mouser and at Beezar.com. One important note: These caps are too tall for the Hammond 1551 HTBU case.
- Black Gates NX HiQ - 47uf 6.3V. The default selection for the AlienDAC, these caps perform superbly in the SkeletonDAC. Give them a chance to break-in: it won't take as long as the larger caps used on MAXes and such, but allow them a couple of nights and days of running before deciding whether you like them or not. Unbelievably transparent for an electrolytic cap, they differ from the film caps above by some of the punchiest, dynamic bass available with a SkeletonDAC. Highs are not quite as transparent or sparkly as with the Vishay-Roedersteins, but only by a tad. Highly recommended for a full-bodied, dynamic sound signature. These capacitors are available at Soniccraft and at Parts Connexion.
- Elna ROA Cerafine - 47uf 10V. At 10V rating, these caps are still only 6.3mm dia and 11mm high. The traditional description for Elna Cerafines has been "same as Black Gates, but without the break-in requirements." These capacitors are available at Beezar.com. One important note: Mounted vertically, these caps are too tall for the Hammond 1551 HTBU case.
- Elna RFS Silmic II - 47uf 6.3V. These caps are 5mm dia and 11mm high. They are Elna's replacement caps (new and improved?) for the Cerafines. Some people have expressed a preference for these caps over any other on the AlienDAC. As the SkeletonDAC is very similar, they should perform equally as well. These capacitors are available at Beezar.com. One important note: Mounted vertically, these caps are too tall for the Hammond 1551 HTBU case.