Here's the rundown on the DAC

[tasar]

I'm not quite sure what you are asking, the filter is an oversampling filter whose output frequency is 352.8 or 384. The "X" number (2X 4X etc) is determined by the input sample rate. If the input is 44.1 or 48 it is 8X, if the input is 88.2 or 96, it is 4X, if the input i s 176.4 or 192 it is a 2X, if the input is 352.8 or 384 no filter is used at all.

I hope that covers your question, let me know if that wasn't what you were after.

John S.

FIR tap refers to a quantifiable array of memory based delay filters, intrinsically minimal with off shelf chips. Basically the more taps, the better the source wave is reconstructed in memory. This too, is a component of jitter, beyond the more basic timing issues. So my question reworded is, are you relying on the DAC chip's organic wave reconstruct, or is their added tap filtering going on ?

[John Swenson]

FIR tap refers to a quantifiable array of memory based delay filters, intrinsically minimal with off shelf chips. Basically the more taps, the better the source wave is reconstructed in memory. This too, is a component of jitter, beyond the more basic timing issues. So my question reworded is, are you relying on the DAC chip's organic wave reconstruct, or is their added tap filtering going on ?

I am not using the reconstruction filter in the DAC chip. I've implemented my own reconstruction filter as an FIR with about 1200 taps. At 44.1 (or 48) input it is an 8X upsampling filter. 

The FIR is implemented with a single 52 bit MAC running a loop going through the coefficients and intermediate results. This loop runs at 96MHz no matter what the sample rate.

The filter itself is an intermediate phase, it has a little bit of pre-ringing and some post ringing. This sounds significantly better than either straight linear phase or minimal phase.

That's about it for the basics of the filter.

John S.

[tasar]

I am not using the reconstruction filter in the DAC chip. I've implemented my own reconstruction filter as an FIR with about 1200 taps. At 44.1 (or 48) input it is an 8X upsampling filter. 

The FIR is implemented with a single 52 bit MAC running a loop going through the coefficients and intermediate results. This loop runs at 96MHz no matter what the sample rate.

The filter itself is an intermediate phase, it has a little bit of pre-ringing and some post ringing. This sounds significantly better than either straight linear phase or minimal phase.

That's about it for the basics of the filter.

Ok, what benchmark or constraints did you apply to your filter ?  Basically why only 1200 taps ? Was the reconstruct measured with a time domain limit and might the ringing side effect be mitigated with further filtering ? I ask, as researching other implementation of FPGA, tap rates are now 10x the order of magnitude of 1200.

[John Swenson]

Ok, what benchmark or constraints did you apply to your filter ?  Basically why only 1200 taps ? Was the reconstruct measured with a time domain limit and might the ringing side effect be mitigated with further filtering ? I ask, as researching other implementation of FPGA, tap rates are now 10x the order of magnitude of 1200.

I did a LOT of listening to a large range of filter parameters and found that the long filters do not sound as good as short filters. The long filters let you get very low alias attenuation (better than 130db) which a lot of people focus on, but my listening indicates that these long filters don't sound as good. By relaxing the alias attenuation to 80db I was able to get much better sound. Relaxing it even more starts letting enough aliasing through that sound quality starts going down. 80db seems to be a good compromise. The result of the relaxed alias attenuation is a much shorter filter. That is most likely significant.

Other parameters of the filter were worked out with my friend Alex and I spending hours tweaking parameters and listening to the results with a wide range of source material. This was very definitely a tuned by ear design rather than meet some mathematical constraint.

John S.

[kumasan]

Hi John,

1) Your DAC has no output stage. My experience says an output stage can change the sound from detailed and airy to a more robust and weighty sound. Not just in the macro sense but more by giving for example a piano more body and energy transmission in each tone even when played subtle. A superior output stage can do this and still keep the details and airiness much like a superior preamp can.

How are your views on these observations and if you agree did you manage to keep the weightiness even without the output stage?

Did you experience any disadvantages at all by not using an output stage or did you find it better in general but not necessarily in every aspect?

2) Owning a NOS DAC without digital filtering I'm interested to know if I can emulate the sound your DAC is providing by running a test on some converted files using your filter settings.

I know you've played around with the SOX upsampler and like to know if you can expose the SOX command line to convert a CD format to eg. a 24 bit 4x44.1=176.4Khz file offline where the parameters follow your filter settings and will this emulate the sound of your DAC?

I don't understand the SOX in more than general terms so I don't know how to apply a general description on filtering to a command.

Even if I can emulate the sound by converting files offline I could still be interested in supporting the project for the following reasons

# I don't have to spend 100+ hours to convert my music collection
# I don't have to spend another 100+ hours to convert my music collection if you find a better filter setting
# I don't have to spend $ on two extra hard drives to keep my converted collection together with the original rip
# I don't have to spend $- on power running two extra hard drives for years
# I don't have to spend $ every year on more more cloud backup storage for the converted files
# I don't have to spend $$ on a planned power cable for my DAC as I'll opt for the battery instead and I don't have to worry if a $$$ power cable might be better than a $$ power cable
# Using only one shelf in the rack section my DAC is at I can skip a complete section of my rack and free space for an art sculpture or another eye pleaser as your DAC can fit in the first rack section

Tx for your effort on improving digital playback.

[Doc B.]

This DAC may very well upset some conventional notions. I don't think that you will see John or me the least bit upset if it does. As I explain in the description we had assumed - when the DAC project was initiated with a very different design using a very different DAC chip - that there would be a tube output stage.  When this design was put together it was put up against earlier tube output designs and it was a clear winner in terms of weight, dynamics, extension, noise floor and imaging.

A few other commercial DACs have been brought by during the listening stages of this project and there was some disparity in quality levels that made it difficult to relate the two. My hope is to head to our studio in the next couple of weeks so that we can compare against a Pacific Microsonics Model2 and hopefully some other studio grade gear.

I would offer that there may be many other things that are different between your DAC and this one besides the filter. Even if there was a way to emulate the filter that is just one component of the design and I don't think you could really get a full sense of what this DAC might sound like by emulating that one element.

We are hoping to do some demos of the DAC in the near future. I don't know where or when yet, but we will certainly announce any opportunity here on the forum.

[tsingle999]

Theres a lot of questions flying around and rightly so at the size of the investment but i also know if Doc et al spent 4 years developing it that it will sound amazing and still be a bargain! Taran

[Doc B.]

Thanks Taran, and thanks for being one of the first to commit. Thank you too Dave, your fearless support of us the minute we release a new product is really special to us! You guys rock!
I've been absolutely buried trying to get everything lined up today, so I'll just post here before I get to the official post,

the crowdfund has started

here's a link -

http://bottlehead.com/product/bottlehead-dac/

[Paul Birkeland]

Did you experience any disadvantages at all by not using an output stage or did you find it better in general but not necessarily in every aspect?

Having built several of the prototypes (including the last one with a tube output stage), I can say that there was a distinct disadvantage in the case of this DAC when adding any of the tube output stages we tried. 

This was not the case with the earliest prototypes that required an output stage of some sort to function. 

Owning a NOS DAC without digital filtering I'm interested to know if I can emulate the sound your DAC is providing by running a test on some converted files using your filter.

I can't speak for Doc and John, but this isn't an open source project, and a request like this comes across strangely. 

[Rob181]

I am in Australia...sending the DAC back for unsoldering one chip & upgrading or inserting a replacement would be an expensive, inconvenient & the least preferred option.

Is the a Plan B for OS sales...Thanks

[Paul Birkeland]

I am in Australia...sending the DAC back for unsoldering one chip & upgrading or inserting a replacement would be an expensive, inconvenient & the least preferred option.

It's about $60 each way to ship.  That is expensive, but not incredibly so.   

This still boils down to wanting to maintain your warranty.  If something happens and your DAC board is damaged in the upgrade process, you really want that to be our responsibility, not yours! 

[JamieMcC]

It's about $60 each way to ship.  That is expensive, but not incredibly so.   

This still boils down to wanting to maintain your warranty.  If something happens and your DAC board is damaged in the upgrade process, you really want that to be our responsibility, not yours!

I would be nervous about getting stung for a second lot of import duty on it return! Not sure how that works if anyone can clarify the procedure of shipping a purchased item from the UK for warranty/service/upgrade work for customs and receiving it back it would be appreciated.

[Paul Birkeland]

Yeah, we can slip an invoice in with the shipping paperwork.  This has come up when we've retubed and serviced our Tape Repro amps internationally, and so far there haven't been any problems. 

-PB

[mcandmar]

I got hit with that before when i sent a DAC back to a manufacturer in California.  The return shipping label clearly said "returned from warranty repair", $0 value, and $500 insured value so some wise ass in customs charged me duty on the $500 + shipping + handling charge + vat, and i had to go the depot to collect it myself.  I had a colorful vocabulary that day.

I got a refund in the end after submitting a claim, had to get a letter from the manufacturer to verify it was a warranty repair with a copy of the original invoice to prove i owned it. Two months later i got a cheque in the post refunding the cost - the handling charge

The worst part is every single package i have received from the US since then has been sezied and had duty charged on it. As the postman in my local depot put it "your on somebodys shit list!"

[Paul Birkeland]

The worst part is every single package i have received from the US since then has been sezied and had duty charged on it. As the postman in my local depot put it "your on somebodys shit list!"

Maybe we can mail it to your neighbor instead?  (Or to your work address?)