Hello everyone:
My first post here. My name is Alex Crespi, and I am the friend that John Swenson spoke of above. John and I have known each other for about 9 years. I first contacted him for an ahead-of-its-time digital project back in 2005--when Hovland Company, of which I was a co-founder, was still in operation. In fact the DAC which the Bottlehead prototype went up against on Janaury 4th is one of the NOS PCM1704K prototypes with discrete output stage that I inherited when Hovland closed its doors. It has had work done to it since: John put in a WaveIO async-USB>I2S board and better power supplies for the digital side. (If you want to know more about me or my system, I use the same Superdad handle over at ComputerAudiophile.com)
My custom room and system are pretty advanced, and I upsample Redbook to 176.4KHz with Audirvana Plus on a fine-tuned Mac mini booting a very slim OS off an SD card. I have spent a lot of time tweaking all the filter parameters of the iZotope SRC engine that A+ uses, so I have a good ear for what the parameters do. John's visit to my place this month (I'm about a 3 hour drive each way) was not his first, and we have spent time tuning a filter for a PCM5142 (the chip in the forthcoming BH DAC) before. It is amazing what a custom digital filter can do for an otherwise ordinary sounding DAC chip. The filters in most all other DACs are terribly compromised--especially if they are only the ones within the chip as the resources are terribly constrained. But the FPGA designed into this unit you have all been waiting for has plenty of room to load and run a really good filter.
At first we listened to the BH DAC fed by my Mac mini (both with iZotope upsampling and with that off to listen to the filter John already had in the BH FPGA). Power was provided by a prototype of a new PS that John and I will be producing in the next few months. The BH sounded pretty good.
John has techniques to output a list of filter coefficients from the excellent SoX sample rater converter--and to then load those into the FPGA. So what we did then was run SlimServer and SqueezePlayer on a Wandboard under COS (a customized Linux that the folks involved in the Community Squeeze Project have created--John is chief hardware designer for that non-profit project), and used the SoX plug-in to upsample to 352.8/384KHz (that rate turns off the PCM5142's own filter, and John turned off his FPGA filter). The filter already in the FPGA was based on some of the intermediate SoX parameters, but was not critically tuned to this DAC. The parameters are: cut-off, pre-ring/post-ring balance (that's the range of minimum-phase to linear-phase), filter length (that's number of virtual "taps"), steepness (for SoX this is controlled by frequency of cut-off and final cut, if I recall correctly).
Anyway, once we deciphered the numbers and ranges to enter the parameters (on an ugly command line for the SoX plug-in), we were able to spend about 90 minutes with the same four VERY revealing tracks (real instruments, real spaces, very challenging material top to bottom). We went one parameter at a time, bracketing wide, then narrow, until we were making the smallest possible adjustments. It is an iterative process (meaning we sometimes came back to one parameter after tuning another) and somewhat tedious, but John's ears, my system, and my 40 years as an audiophile/music lover (I started when I was 12) made it pretty easy.
In the end we were both grinning like fools. Really that DAC chip (and its built-in opamp output stage) had no right to sound as good as it did. Cymbals, piano, voices, strings, bass, drums--all top of the mark. I have heard a lot of DACs, and I am convinced this Bottlehead DAC will go toe-to-toe with some mega-buck units! Of course John quickly wrote down the numbers for the magic parameters we had settled upon and he's converting them for loading onto the FPGA.
So Doc, I think you'd best mark me down for a Bottlehead DAC too! (I don't mind coming after everyone else here who have waited so patiently.)
Anyway, I just though you might all enjoy a little insight into the final stage of what is going to be a GREAT and very musical DAC. It would be a steal even at twice the price of whatever Doc ends up asking for it. Based on what I heard with my own ears, I don't think ANYBODY will be disappointed.
Best regards,
ALEX
