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Old 21st November 2009, 04:31 AM   #21
qusp is offline qusp  Australia
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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.
excellent!!! now this is what i'm talkin' about!!

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In the meantime I'll be tweaking the circuit a little more, and hopefully I can get some SE measurements this weekend.
i'm sure there will be some happy people right there.

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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.
well, I have a variety of headphones, actually my primary ones are only 28ohms, I use 680R in series to get just under 1vrms for them and that drives them to bleeding levels, the 300R headphones work fine without any resistors, I guess using the impedance of the headphones themselves to create the voltage needed. a simple unity gain diamond buffer is enough to drive most of my headphones

Quote:
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.
oh yeah I know, there are plenty of other options for amping them, just the direct nature of driving direct from the DAC appeals, interested if you come up with something, but it would have to be balanced; all my headphones are recabled to balanced terminations. 2 of them have removeable cables that I could use an SE cale with, but I would prefer not to. I reckon we just see how it shapes up as is, but sure if you can think oif a good optional add-on, great. I already have a couple of separate headphone amps, but they are all single ended. i'm playing with some balanced designs of my own with the discrete buffers as output, so looks like i'll be sorted either way. there is a tube amp for them on the horizion too in the near future.

anyway mate, looking forward to this project even more now, I reckon we've got a serious contender here.

cheers

Jeremy
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Old 21st November 2009, 02:16 PM   #22
qusp is offline qusp  Australia
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one thing; when I made my order from sumR for my iron, I totally didnt see this coming, so I only have 20va primary, with dual 15v secondaries. I might be able to eek that out to 18v DC in the regs, but it would be touch and go. is 15v DC enough to run the rails on the B1?? or is 18V the minimum?? obviously I could put the secondaries in series, but 12v is a lot to burn off in the regs, would need some serious sinks, could burn some off in a prereg I guess, but then I lose 2 secondaries. unfortunately its too late to change my order for the iron, they should be nearly finished by now; last update I got tyhey were up to enamelling.
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Old 21st November 2009, 04:10 PM   #23
opc is offline opc  Canada
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Hi qusp,

I have the exact same problem. The blue potted transformer I'm using now has 15V dual secondaries, and it's too close for comfort. I'm right on the edge of the regulator's dropout voltage. It does work though, but I wouldn't suggest it in the long term.

I ran a quick simulation with +/- 15V and it works just fine. You'll have less voltage swing capability, but I don't see why anyone would want 10VRMS out of a DAC anyway.

All you need to do is adjust the top resistors to get about 1/2 Vcc (7.5V) at the drain of each mosfet. In my sim, it was 180 ohms in place of the 200.

The adjustable gain feature is pretty slick, since all you need to do is increase the 200 and 400 ohm resistors. I personally don't like a lot of gain, since most systems have far too much as it is, and it results in more noise and distortion for no reason. With this circuit, you get to balance gain with the amount of voltage swing you're willing to accept at the output of the DAC. Less gain = less voltage swing at the DAC. I think a nice 2VRMS at FS is a good way to go, but if someone wants more, it's easily done.

One of the things I want to try is dropping the rails down as low as possible, and seeing at what point it negatively affects the sound and the performance. If you only want 2VRMS output, then the rails should be able to go much much lower which would result in less dissipation on the parts, and more flexibility in the power supply. I kept them high in order to allow people to get a lot of gain, but maybe I'll make it an option.

Anyone out there with an opinion on gain? Anyone who wants more than 2VRMS at 0dBFS? If so, can you tell me why, and how much you would want?

Cheers,
Owen
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Old 21st November 2009, 06:40 PM   #24
qusp is offline qusp  Australia
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Originally Posted by opc View Post
Hi qusp,
Hi Owen

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I have the exact same problem. The blue potted transformer I'm using now has 15V dual secondaries, and it's too close for comfort. I'm right on the edge of the regulator's dropout voltage. It does work though, but I wouldn't suggest it in the long term.
ahh I see, well at least it will work initially. so are you saying you are driving the D1B1 with the one potted trannie with 2 x 15v secondaries?? or just mentioning the spec as I was. I have 2 x fully encapsulated cans with a single 240v/20va primary and 2 x 15v secondaries for the VA section and 1 x 20va/2 x 10v for digital does this improve matters?? likely not I guess, still the same output, but not as strained as if it were just the one.

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I ran a quick simulation with +/- 15V and it works just fine. You'll have less voltage swing capability, but I don't see why anyone would want 10VRMS out of a DAC anyway.
nah I dont need more than 2vrms for any reason I can think of. as I said the only things i'm wanting to drive are headphones and biamped powered monitors. only thing I can see in that future is clipping.

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All you need to do is adjust the top resistors to get about 1/2 Vcc (7.5V) at the drain of each mosfet. In my sim, it was 180 ohms in place of the 200.
ok, nice one, looks like I have another reason to make another order with texas, perhaps a GB would be in order for TX2575 for the gain setting resistors?? but thats something to talk about later. I cant fill the B1 with them $$$$, but if you can mention any other specific positions that you have experienced particular benefit from really tight tolerances and matching, i'll grab some for those slots in 0.005% love the texas components R's

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The adjustable gain feature is pretty slick, since all you need to do is increase the 200 and 400 ohm resistors. I personally don't like a lot of gain, since most systems have far too much as it is, and it results in more noise and distortion for no reason. With this circuit, you get to balance gain with the amount of voltage swing you're willing to accept at the output of the DAC. Less gain = less voltage swing at the DAC. I think a nice 2VRMS at FS is a good way to go, but if someone wants more, it's easily done.
that is pretty sweet and at any time to be able to adjust it with a coupe of parts is great. but yeah the standard line level 2vrms is good by me, leaves it flexible what gear it can be used with without a pre/. dont hold me to that just at the minute, but at this stage I cant see any reason for me going higher. i'll have a think on it. plus if others pipe up with reasons why, i'm all ears

Quote:
One of the things I want to try is dropping the rails down as low as possible, and seeing at what point it negatively affects the sound and the performance. If you only want 2VRMS output, then the rails should be able to go much much lower which would result in less dissipation on the parts, and more flexibility in the power supply. I kept them high in order to allow people to get a lot of gain, but maybe I'll make it an option.

Anyone out there with an opinion on gain? Anyone who wants more than 2VRMS at 0dBFS? If so, can you tell me why, and how much you would want?

Cheers,
Owen
yeah I mean we arent talking DNR here, if so then I would be all for it, but more gain in this circuit means you pay somewhere. I would definitely like it as an option, in the meantime I will keep researchin a partner for this dac in the next stage. speakers other than my monitors are out of the frame, so it would only be if I were to get better performance from my amp with more gain. but so far it seems llike it will suit my needs pretty well without one for the moment.
thanks for the numbers re the gain setting resistors. great dialogue so far. keen as mustard

cheers

Jeremy

Last edited by qusp; 21st November 2009 at 06:44 PM.
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Old 22nd December 2009, 06:03 AM   #25
regal is offline regal  United States
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I thought the DAC chip like all S-Deltas had 1/2Vcc DC offset and the D1 stage was designed for R2R DAC with 0 offset, I don't see this being dealt with in the circuit, something to check.

Also we really do need to have a nice discrete balanced to unbalanced mixer to reduce common mode noise and let the DAC chip show its full potential.

Very nice project btw.
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Old 22nd December 2009, 03:31 PM   #26
opc is offline opc  Canada
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Hi Regal,

I've addressed this a few times, and I think I mentioned it in the first post. The normal D1 uses trimpots to set the gate voltage on the mosfets. This in turn is used to set the voltage at the source to 0. In my circuit, the same thing is done, except the voltage at the source of the mosfets is set to 1.65VDC which is 1/2 VCC, exactly what the DAC wants to see there.

The top two resistors feeding the drains of the fets are then adjusted to get about 9V at the drain of the mosfets, which is what the B1 wants to see at it's input. The final current through the mosfets is about 50mA, which is much much higher than the D1.

I have the exact values for all the resistors in the working circuit, but I'm out of town for the holidays and I won't get a chance to work on this again until I get back on the 29th. From what I can remember, you need 400R on the sources, 180R on the drains and adjust for 1.65V at the source.

You are correct about the balanced to single ended converter. This circuit works with a single ended output, but you lose all the advantages of the balanced DAC and the balanced nature of the D1B1. Performance definitely suffers for it.

I personally don't care to have SE outputs since the entire rest of my system is balanced, but I can understand the need for a good SE output.

Anyone with any suggestions? This might be a good spot for a single UG op-amp with good CMRR. A transformer would also work very well here.

Happy Holidays!

Owen
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Old 22nd December 2009, 04:02 PM   #27
jkeny is offline jkeny  Ireland
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Any further collaboration with Acko's DAC?

A suggested op-amp might be OPA365 50MHz, Low-Distortion, High CMRR, RRI/O, Single-Supply, UG op-amp?
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Old 25th December 2009, 06:04 AM   #28
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balance to unbalance
DESYMETRYZATOR do kazdego DAC'a z napieciem 2.5V - Do It Yourself - Forum Audio - Audiostereo.pl
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Old 25th December 2009, 07:39 AM   #29
regal is offline regal  United States
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Hi Regal,

I've addressed this a few times, and I think I mentioned it in the first post. The normal D1 uses trimpots to set the gate voltage on the mosfets. This in turn is used to set the voltage at the source to 0. In my circuit, the same thing is done, except the voltage at the source of the mosfets is set to 1.65VDC which is 1/2 VCC, exactly what the DAC wants to see there.

The top two resistors feeding the drains of the fets are then adjusted to get about 9V at the drain of the mosfets, which is what the B1 wants to see at it's input. The final current through the mosfets is about 50mA, which is much much higher than the D1.

I

Thanks, I just don't see how you can come up with the 1.65V at the Mofset Source from the datasheet, running 50 mA and all the charts are in amps. I guess you need Spice or something.


It would be very useful for a lot of us with 5V DAC's to know how to change R1&R3 to get 2.5V at the source.
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Old 16th January 2010, 01:07 PM   #30
regal is offline regal  United States
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It would be very useful for a lot of us with 5V DAC's to know how to change R1&R3 to get 2.5V at the source.
I guess what I'm asking is how do you change the 1.65Volts at the mofset source to 2.5V which is very common for the Burr Brown DAC's (179x) whould be useful for people building the EZDAC etc.
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