[bigjppop]

So I'm getting ready to start on my next project and I'm hoping some of you that are more knowledgeable than I might be able to give me a sanity check.

Basically I'm wanting to build an all-in-one "front end" for my two channel system. I've got a Bottlehead Quickie (battery powered tube preamp) and I want to combine it with a DAC (probably the Gigaworks DAC that's got a HUGE thread at DIYaudio and a smaller thread here) as well as a miniDSP. The rest of the system is a Tubelab Simple SE (roughly 5 watts in SET), some Omega 3i speakers (roughly 93 dB), and some soon to be built Anarchy tapped horn subs (two of them). Oh yeah, a first gen apple TV is my source.

I would like to build the DAC, preamp, and miniDSP into a single chassis. I'm hoping to take the toslink from the apple TV, run it into the DAC section, from the DAC section to the preamp, and from the preamp into the miniDSP where everything above (roughly) 80 hz will go to my Omegas and everything below that will go to the Anarchy subs (this is a strictly music system). To power the subs I'll either be building something like the small Class D audio amp (the subs only need 100 watts, max) or buying something like one of the parts express plate amps, building it into its own enclosure, and running one plate amp to drive the two subs. I'd love to get the Class D into the same single chassis, but at that point my chassis will probably be too big to manage, or I'll have heat issues.

So what do you think? Doable? Have I missed anything glaring? I've read somewhere that the miniDSP doesn't like tubed preamps due to some impedance matching issues but the issue was never really explored/explained (that I could find) so I'm open to thoughts on that issue as well.

Thanks in advance for any insight.

[wushuliu]

Yes, Minidsp has low input impedance (6k I believe), I don't think that rules out tubes but it bears noting ( I dont know the output impedance of the Quickie).

Ambitious, but but may be too many eggs in one basket. Especially when it comes time to troubleshoot 

[JohnR]

What is the gain of your SE amp? miniDSP has 0.9Vrms output, which is fine for most amps but low gain ones will be a problem.

An alternative approach would be to use a miniDIGI card instead of the DAC, doing the volume in the miniDSP, and putting the preamp (buffer/gain only - no volume control) after the miniDSP. You'd need to look at the gain structure to match the sub output. I haven't done this but I think it would be viable

[bigjppop]

Yes, Minidsp has low input impedance (6k I believe), I don't think that rules out tubes but it bears noting ( I dont know the output impedance of the Quickie).

Ambitious, but but may be too many eggs in one basket. Especially when it comes time to troubleshoot 

Bottlehead doesn't list the specs on the Quickie (at least that I could quickly find) but I did find a review that listed the input impedance as 100 KOhms and the output impedance as 1.8 KOhms; I'm not really sure what that actually means (still learning this DIY thing) but do you see any issues?

As for the gain on my amp, I know it was designed to be driven to full output without a preamp; it can be set up with a volume pot and run to full output on the 2V from a standard CD player. I think the .9V out put might be an issue without the preamp, unless I went with the balanced miniDSP which has a 2V output.

I've thought about the miniDIGI but to be honest, I really don't know what it is/does. It appears from the website that it can input digital and output either digital our analog; the digital to analog part I've got, but I'm not really understanding why you'd want digital in, digital out.

I know I could run a miniDIGI/miniDSP combo and greatly simplify my life; you can wire a volume pot straight to the miniDSP and that would eliminate both the DAC and preamp piece from my project. I'm just a little worried on the quality of the miniDIGI as as DAC. I'm guessing I could probably go with a miniDIGI/balanced miniDSP combo and still get the 2V output that my amp wants without a preamp.

Thoughts?

[JohnR]

I've thought about the miniDIGI but to be honest, I really don't know what it is/does. It appears from the website that it can input digital and output either digital our analog; the digital to analog part I've got, but I'm not really understanding why you'd want digital in, digital out.

In your project I'd guess that you would just use it as digital input. The miniDIGI converts from your S/PDIF or Toslink to the digital signal for processing by the miniDSP. It's an alternative to using the analog inputs and the ADC on the miniDSP card; you could also set up a switch between analog and digital inputs - see last page of user manual. It has no analog out - that comes off the miniDSP card.

You could use the digital output capability to generate an S/PDIF signal for the mains to feed to an external DAC (according to the manual -  I'm not saying you'd want to do it but it could be done.) In other setups it's used to daisy-chain the S/PDIF signal across multiple miniDSP boards.

[bigjppop]

Interesting...

It sounds like my original plan might involve a lot of extra steps going from Digital to analog and back again.

I think your (JohnR) idea about going from Apple TV to miniDIGI/miniDSP and then on the amps might be my best bet. I could use my preamp as a tube buffer on the mains, like you suggested, but I'm curious how that would work out with volume control. Would I wire a volume pot to the miniDSP and use that as a master volume for the mains and subs? I'm hoping to avoid individual volume control for each channel. I guess at the end of the day, I already have tubes in my amp, I could probably just pull the preamp altogether... hmmm.

Has anyone used the miniDIGI/miniDSP combo to perform there "DAC" functions? I'm not a big believer in a multi thousand dollar DAC being required for decent sound, but I do have a little fear going from even something like the Gigaworks DAC I mentioned up top (which I know is not SOTA, but I believe great bang for the buck) to the miniDIGI as my soul DAC.

[JohnR]

I don't lose any sleep over the "extra steps." My system is running that way now. Eventually I'll have switchable digital input via miniDIGI as well as analog input but it's a long way from the top of the list. With a digital device in the chain however getting the levels right or at least workable is quite high on the list That's because they have a hard limit. That was my main concern with your initial proposal. With a low power SET I rather suspect that it's going to be driven into clipping a lot more than with say an SS amp and so you are going to in effect have to allow more headroom.

Sorry I'm confused a bit with all of the options myself now! I did like the idea of building in a tube stage into a box with a digital input, I thought that sounded cool. Well, of course you're then more complicated on the power supply too... ah well, choices

[bigjppop]

Decisions, decisions... Now I'm thinking I also need to add a remote volume control option since I found this:

http://us.gear4.com/showcase-unity/

Since I've already got the pre, I guess it would be a shame not to use it. For my current setup I run my Red Wine Audio iMod into my Quickie Pre, then into my Tubelab Simple SE, then to my Omegas (no subs). My main goal is to figure out the best way to integrate two new subs.

I'm a little confused on impedance matching/gain structure stuff so I'm not fully grasping your comment about "getting the levels right." Can you elaborate a little?

[JoshK]

My $.02 is to keep the miniDSP out of the high pass signal circuit by running parrallel outputs from your preamp, one to a line level high pass (simple to design and can be put right into the gain stage of your tube amp) that goes to the tube amp and then one to the miniDSP.  Why convert to digital to convert back if not necessary? 

source > DAC > pre > LLXO > tubelab Simple SE
                           > miniDSP > bass amp

One additional note, if the miniDSP's Zin is indeed 6K, you could add a MOSFET source follower to the low freq output which would keep it out of the high pass signal path but still buffer the preamp. 

[Russtafarian]

I am doing almost exactly what bigjppop wants to do and I'm doing it the way Josh described.  I have three analog sources: phono pre, cd player, and squeezebox.  The sources go to a Bottlehead Quickie preamp built into a chassis and powered by regulated SLA batteries.  The pre out is split with one output going to Quicksilver monos with a passive HPF for the mid-highs.  The other output goes to a miniDSP for woofer and sub XO and EQ.

I think the main difference with my setup is I chose to battery power the cd player and squeezebox and transformer couple their analog outputs rather than go with a DAC. 

Between all the battery supplies and tube amps, it's a lot of power switches to turn on and off.  But when I hear all this cheap gear giving me close to SOTA transparency, dynamics and resolution, it takes me to a happy place.

Russ

[srb]

The pre out is split with one output going to Quicksilver monos with a passive HPF for the mid-highs.  The other output goes to a miniDSP for woofer and sub XO and EQ.

I think that's the way to do it.  If I had digital sources I wouldn't mind applying a bit of EQ to mids and highs while still in the digital domain (if the room required it), but I probably wouldn't want to possibly muck up good analog-sourced mids and highs with entry-level ADC > DAC conversion.
 
Steve

[wushuliu]

My $.02 is to keep the miniDSP out of the high pass signal circuit by running parrallel outputs from your preamp, one to a line level high pass (simple to design and can be put right into the gain stage of your tube amp) that goes to the tube amp and then one to the miniDSP.  Why convert to digital to convert back if not necessary? 

source > DAC > pre > LLXO > tubelab Simple SE
                           > miniDSP > bass amp

One additional note, if the miniDSP's Zin is indeed 6K, you could add a MOSFET source follower to the low freq output which would keep it out of the high pass signal path but still buffer the preamp.

How do you design a LLXO?

[Russtafarian]

How do you design a LLXO?

Here's an image and footnote explanation of a first order line level high pass filter from John Atkinson (via Corey Greenberg's classic 1995 review of the NHT Superzeros).
 

Footnote 7: To calculate the approximate -3dB point for a first-order filter like this, use the formula f = 1/(2PiRC), where Pi = 3.142, C is the value of the series capacitor in farads, and R is the combined resistance in ohms of the amplifier's input resistance and the shunt resistor used. For the example CG gives: a 15k resistor in parallel with a 51k resistor gives a combined resistance of 11.6k ohms; the capacitor has a value of 0.1 x 10E-6 farads; the formula predicts a -3dB point of 137.1Hz.—John Atkinson

[JohnR]

Why convert to digital to convert back if not necessary? 

Well, so he can do DSP on it.... plus he wanted to build the DAC into the unit, but doesn't need to if he used a miniDIGI card. At any rate, it's simple enough for anyone to try it both ways.

[JohnR]

I'm a little confused on impedance matching/gain structure stuff so I'm not fully grasping your comment about "getting the levels right." Can you elaborate a little?

With a digital device (any digital device) in the chain you have a hard limit on the signal level at that point. In the case of the miniDSP that's 0.9v. This is fine if your amplification after the miniDSP has enough gain to get the needed output levels. So that's one thing. The other is that if you have very different gains on one output channel vs another then you will need to attenuate one or the other. This can be done digitally but at some point it would be better to optimize it with some attenuation on the high-sensitivity channel.

I think you'll find it's not a terribly complicated problem once you've tried a few things. It's just something that needs to be kept in mind.

[bigjppop]

I am doing almost exactly what bigjppop wants to do and I'm doing it the way Josh described.  I have three analog sources: phono pre, cd player, and squeezebox.  The sources go to a Bottlehead Quickie preamp built into a chassis and powered by regulated SLA batteries.  The pre out is split with one output going to Quicksilver monos with a passive HPF for the mid-highs.  The other output goes to a miniDSP for woofer and sub XO and EQ.

I think the main difference with my setup is I chose to battery power the cd player and squeezebox and transformer couple their analog outputs rather than go with a DAC. 

Between all the battery supplies and tube amps, it's a lot of power switches to turn on and off.  But when I hear all this cheap gear giving me close to SOTA transparency, dynamics and resolution, it takes me to a happy place.

Russ

A man after my own heart. To get the second output on your Quickie, did you jut parallel a second set of outputs and then put the LLXO on one of them? I've asked about this over at the Bottlehead forum but didn't get a clear answer.

Also, are you doing battery power on your miniDSP as well? If I ended up just going the miniDIGI/miniDSP route and forgoing the DAC I could use batter power for that entire section (mini combo and the Quickie).

[bigjppop]

With a digital device (any digital device) in the chain you have a hard limit on the signal level at that point. In the case of the miniDSP that's 0.9v. This is fine if your amplification after the miniDSP has enough gain to get the needed output levels. So that's one thing. The other is that if you have very different gains on one output channel vs another then you will need to attenuate one or the other. This can be done digitally but at some point it would be better to optimize it with some attenuation on the high-sensitivity channel.

I think you'll find it's not a terribly complicated problem once you've tried a few things. It's just something that needs to be kept in mind.

Thanks for the explanation; I'm assuming that if the .9V output is an issue for my amp I could go with the balanced miniDSP which has a 2V output.

DIY is great because you have so many options... it also is terribly frustrating because you have so many options!

[Russtafarian]

To get the second output on your Quickie, did you jut parallel a second set of outputs and then put the LLXO on one of them?

Yes, I use a Y connector to split the Quickie output to the tube amp and the miniDSP.  I built the cap+resistor HPF into a RCA female to male adaptor that plugs into the tube amp.  Make sure the total resistance of the HPF (shunt resistor paralleled to the amp input impedance) is above 10k or you may load the Quickie's output down too much.

Also, are you doing battery power on your miniDSP as well?

Not at the moment, but I could easily enough.  I'm not as worried about it since the miniDSP is only reproducing below 300hz.  It's not transparent enough to my ear to do full range.

[JohnR]

It's not transparent enough to my ear to do full range.

Could you elaborate? How did you compare it?