A New Take on the Classic Pass Labs D1 with an ESS Dac

[ichiban]

Originally Posted by relder

Given the DAC output is around ground potential you'll have DC above the MOSFET, you have to get rid of it, which is most easily done with a cap, sorry.

How about if the dc blocking cap were moved to the input of the B1 circuit and using Salas's symetric B1. Then a smaller(affordable) high quality cap could be used instead. I don't know if the input cap at that circuit point loads it too much?

[qusp]

Originally Posted by opc

......Anyone feel like helping with the digital side of the DAC?

Cheers,
Owen

awesome!!!! this is exactly the sort of thing I had in mind as one of the options. I had transformer coupling in mind too as another 'module' but I have been eyeing off the B1 buffer (what can I say; I like nice caps niot always, but I have a weakness) sio yes I think this and the acko dac would make great partners and I would be interested for sure; I think it easier to keep a wider pitch on the pin headers though, to keep it more flexible.

Originally Posted by jkeny

Owen, I have an idea to make a modular design between your pcb & ACKO's Sabre DAC board - here's my post from the Buffalo tweakers thread in response to qusp wishing that the tweakers board had something to plug into it by which I assumed he meant an output stage:

I have an idea about that - basically to allow output boards to be plugged in from the top into the tweakers board using standard (low impedance) pin connectors i.e a 3D design. I reckon this would give the shortest path to the output stage (probably shorter than the Buff with on-board o/p stage?) & would be stable with 8-10 pins on each side PLUS would make it modular - just plug in another IV stage or whatever as long as it has the correct standard socket I'll be floating this one with Acko & Owen

What do you think?

absolutely!!!

Originally Posted by opc

Hi jkeny,

That would probably be the best way to go if people want to be able to swap various stages. We would just need to come up with a standard pinout for the connector, and I think that should be optimized based on the DAC outputs. With good layout, you could probably keep those connections to less than half an inch which would be about as good as having it on the same board.

yep, I think its just as good as on the same board and much more flexible. that way you could have preregs and regs below and feed them from there, with the ability to try different regs without desoldering anything. seems ideal

I'm guessing the standard 0.1" pitch inline connectors would be the best bet, but I'm open to suggestions. I have seem much more expensive and finer pitch connectors for video applications, but that might just complicate things for people.

yeah stay with 0.1" IMO, keeps more options open, less fiddly and as you say its cheaper.

Maybe another two connectors one either side of the IV stage could be used for swapping regs for the +/- 18V. I'll look into it.

I should have the layout mostly finished by the end of this weekend so I'll post a preliminary version for comments.

Cheers,
Owen

cool will keep a look out for it; things are coming together. I like it

[jean-paul]

Please add output series resistors just like the B1 and the Hypnotize/Mezemerize have. They are needed. When the B1 part would be fed symmetrically the output cap can be omitted. Even a ( very slow ) servo + "muting at power on" circuit can be added to adjust output offset.

[opc]

Hi Guys,

Jean Paul,

You’re absolutely correct; the circuit does need that 1k series resistor. I have it on my prototype, but it wasn’t in my simulation, which is what I posted. There are a few other discrepancies, namely that the two resistors to set the voltage at the gate of the fets are not as shown. I’ll double check their values and re-post a proper schematic. The lower resistor (gate to ground) is actually a pot, but it’s easier to just use a resistor for simulation. Also, the 1k on the output is a load, and should be the same 221k as the B1.

Ichiban,

As for the capacitor questions, it’s not possible to do away with the output cap entirely since, as relder pointed out, the drain of each of the fets is at about 9VDC. The cap could be moved to the input, if the lower fet was tied to –18V instead of ground, but as Jean Paul mentioned, you’d likely need some form of pop/click suppression. Overall, I much prefer the cap on the output, but the other way could work as well.

By the way, the 22uF is just a suggested value. I’m using 4.7uF right now for testing, and it doesn’t roll off by any meaningful amount in the bottom end. Take a look at the input impedance of what you’re driving and calculate it from there. If you have a 10K input impedance on the next stage, you can get away with a 1uF cap and your turnover frequency will be 16Hz.

I still haven’t had a chance to fire up the AP and measure the single-ended performance, but I’ll hopefully get around to it this weekend. I’m still working on the layout, but it’s not quite there yet.

I’ll post an update with a final schematic (identical to my prototype) and the layout when I get some spare time.

Can someone post a direct link to Acko's most up to date schematic?

Cheers,
Owen

[jkeny]

Hi Owen,
Acko isn't going to post a schematic but I believe he emailed you to collaborate on the plug & socket arrangement between boards. Drop him an email!

[acko]

Hi Owen,

I have sent a PM to you. Please contact me for details of the ackoDAC interface.
Hope to hear from you soon. Regards, Acko

[qusp]

Originally Posted by opc

Hi Guys,

Jean Paul,

You’re absolutely correct; the circuit does need that 1k series resistor. I have it on my prototype, but it wasn’t in my simulation, which is what I posted. There are a few other discrepancies, namely that the two resistors to set the voltage at the gate of the fets are not as shown. I’ll double check their values and re-post a proper schematic. The lower resistor (gate to ground) is actually a pot, but it’s easier to just use a resistor for simulation. Also, the 1k on the output is a load, and should be the same 221k as the B1.

Ichiban,

As for the capacitor questions, it’s not possible to do away with the output cap entirely since, as relder pointed out, the drain of each of the fets is at about 9VDC. The cap could be moved to the input, if the lower fet was tied to –18V instead of ground, but as Jean Paul mentioned, you’d likely need some form of pop/click suppression. Overall, I much prefer the cap on the output, but the other way could work as well.

By the way, the 22uF is just a suggested value. I’m using 4.7uF right now for testing, and it doesn’t roll off by any meaningful amount in the bottom end. Take a look at the input impedance of what you’re driving and calculate it from there. If you have a 10K input impedance on the next stage, you can get away with a 1uF cap and your turnover frequency will be 16Hz.

I still haven’t had a chance to fire up the AP and measure the single-ended performance, but I’ll hopefully get around to it this weekend. I’m still working on the layout, but it’s not quite there yet.

I’ll post an update with a final schematic (identical to my prototype) and the layout when I get some spare time.

Can someone post a direct link to Acko's most up to date schematic?

Cheers,
Owen

only problm with that is if 1k is added in series with the sabre output, it will no-longer operate in current output, will become voltage. the sabre needs to see around 680-700R output impedance or under to remain a current output dac. now whether that is at all meaningful now, with the possible implementation of the D1B1 is another thing. I just prefer current output dacs thats all

[opc]

Hi qusp,

The 1k resistor goes on the output of the I/V stage. The input impedance to the IV stage is very very low. With 30mA p-p of input current, I see less than 18.5mV p-p of swing at the DAC output. That’s an effective impedance of less than 1 ohm. It’s part of the reason this circuit is so nice. It really lets the DAC operate at its best.

Hope this helps.

Cheers,
Owen

[qusp]

well that sounds perfect, sorry I had thought you were talking about the entrance to this circuit, not the output of this; input to the next amp stage. NICE!!

also, with the possible implementation of a digital volume control with the micro, will the buffer have enough drive to feed 300ohm balanced headphones directly like I can/could with my buffalo32 if I wish?? and also lower Z cans with the addition of an optional load resistor on the output of the buffer?? this is how I had by buffalo set up, I used an otto (relay) to switch TX2575 650R resistors into the signal for low z cans and remove them for less efficient cans and driving my active monitors and amp directly. I really liked this flexibilty. no biggie if not, but would be cool if that was still an option.

[opc]

Hi Guys,

I started a discussion with Acko, and it looks like everything should be a go! Once I get details on how the output section of his DAC is setup (layout, size, pinouts, etc...) then I can lay out the I/V stage and get boards made that will fit like a glove with the DAC.

In the meantime I'll be tweaking the circuit a little more, and hopefully I can get some SE measurements this weekend.

qusp:

I ran the headphone option through the simulator, and it doesn't look like it's going to work. I'm not sure what the headphones need for voltage to get decent listening levels, but the output voltage has dropped quite a bit by the time you get a 300 ohm load across the output. You'd be down to under 2V p-p which may or may not be enough. I don't know how much voltage a pair of headphones would need.

What I can do is add a connector at the output to allow a separate headphone amp to piggy-back on the outputs. If it were me, I'd probably build a scaled-down SOZ designed to drive headphones, but an op-amp could work well too. You could also use a multitude of other headphone amps designed specifically for the purpose and get very good performance. I often listen to music on my headphones at work, so I could be easily persuaded to do a headphone amp option.

Cheers,
Owen