DAC-Burr Brown PCM1792

ben62670

Well you might not be any more. I swapped in a couple 134's, and the results were awesome. We'll see.
http://www.polkaudio.com/forums/showthread.php?t=71222

Systems

ben62670 wrote: »

Well you might not be any more.

Ok

http://www.polkaudio.com/forums/showthread.php?t=71222

not sure what Im supposed to see in this thread.

ben62670

Sorry bro bad link. Here it is.
http://www.polkaudio.com/forums/showpost.php?p=1005497&postcount=70

ben62670

OK so I am studying the 4.5v output circuit. Seeing that the output opamp sums the two IV opamps there is no gain. I like the less components in the circuit. The sound stage, and detail is better on the 4.5v section than the 2.5 section, but the 4.5v output is too high:( Is there an easy way to drop the output voltage without having to add another output stage?There is an 820r on the IV stage in ua1-4 that could be changed to decrease the voltage, but my concern is that it would effect the filter slope:( Another option maybe change RA2,3,6,7 to say 1k to drop the voltage without sacrificing SQ?
Thanks
Ben

Systems

Your pre amp should be adjusting gain, or is it that the high output level only allows for the first quarter or so of rotation in the the volume control?If thats the case then the simple fix would be to add a voltage divider with 2 resistors to the DAC's inputs on the pre to knock it's level down.

ben62670 wrote: »

The sound stage, and detail is better on the 4.5v section than the 2.5 section

This is a mystery to me,mainly because even if both sections were to be level matched the change in filter frequency is far outside the audible range.Also thats counter intuitive as the 2v outputs filter section is optimized for redbook CD.In theory the 4.5v outputs will have higher levels of HF noise (with standard cd)above the audible range because the filters corner frequency is substantially higher.

but my concern is that it would effect the filter slope

Changing the resistors will alter the frequency of the filter.

I have not done a comparo of the 2 vs 4.5 v outputs but might when I get mine back together,I stole the regulators to use in my active xover.At some point I plan to install better regulators and output buffer stages that should take its performance up a couple of notches.If the output buffers work as planned you might recieve a pair for evaluation in yours.;)

Systems

What is the input impedance of your preamp?If it is something like 49k ,then a voltage divider made up of 2 10k resistors will attenuate the output down to approx 2 volts.If you want less attenuation and want to experiment with different voltage divider values then try this.;)http://hyperphysics.phy-astr.gsu.edu/hbase/electric/vdivac.html#c1


Hope that helps.

ShinAce

GV: If he's going to experiment, why not use a pot(linear taper, or stamped and take the slider as the output?

Get it dialed in and then swap it for a hardwired metal film resistor divider.

Definitely confirm your pre-amp too. Is it the tube buffer?

ben62670

My pre is a passive at the moment with a 100k pot. I have lots of Vishey metal films here:) I may go a little higher than recommended for more accurate tracking at lower volumes. I have an Alps pot, but I am not overly impressed with the performance at lower levels. I would rather have a couple DPDT switches to set the desired cut depending on mood;)

ben62670

OK I am going to bypass the second stage of OpAmps on the DAC board. I am using the first stage of the board I sent you omiting the second stage on the DAC board(I know I kinda said it twice). The Problem I am having is I don't have any 22pf caps. I have .001uf, and .003uf. What should I do? Second problem is I have 681r, or 182r for the 320r's in the second stage:( I want to get this together, and I am hoping that the offset r with work well with the offset c.

ShinAce

ben62670 wrote: »

I don't have any 22pf caps. I have .001uf, and .003uf. What should I do?

Buy some 22pF ceramics. They're ultra common.

0.001 uF = 1 nF = 1 000 pF

ben62670

If you could help that would be awesome. On page 21 I am going to pull the signal from JP-1, and JP-2. I am using an OPA-2134 instead of the NE55XX. I am pretty sure that CA14, and CA15 can be omitted with the new OpAmp. CA18 is the filter cap? I am trying to simplify the analog section. If I could change RA2,3,4,5 to 681r I would be very pleased.
http://focus.tij.co.jp/jp/lit/ug/sleu037/sleu037.pdf

ben62670

Looking at the diagram over and over again it seems that it is filtered in the IV stage, and the output stage. Could all the caps be omitted in the second stage? It appears to me that the buffer stage is more of a summing stage than anything?

Systems

ben62670 wrote: »

I am using an OPA-2134 instead of the NE55XX. I am pretty sure that CA14, and CA15 can be omitted with the new OpAmp.

No they are part of the LPF.

CA18 is the filter cap?

No it is a compensation cap.The 5534 is not unity gain stable so needs this cap to prevent it from oscillating.The134,2134 's do not need this compensation,as they are unity gain stable.

I am trying to simplify the analog section.

It is already fairly simple.

If I could change RA2,3,4,5 to 681r I would be very pleased.

Doing so will change the corner frequency of the filter.

ben62670 wrote: »

Looking at the diagram over and over again it seems that it is filtered in the IV stage, and the output stage.

CF1,2 RF1,2 in the I/V section are feedback components not part of the LPF.

Could all the caps be omitted in the second stage?

No,unless you want to eliminate the LPF entirely.

It appears to me that the buffer stage is more of a summing stage than anything?

It is a differential to single ended converter (or summer),plus it isolates( buffers)the filter components from the output.

ben62670

OK two last questions before I go and burn another finger:o If I completely eliminate the LPF would I be asking for trouble? I have the active XO after the DAC. Also If the I eliminated the LPF would the 681r's be OK?

Systems

ben62670 wrote: »

If I completely eliminate the LPF would I be asking for trouble?

Not likely,the noise will be above the audible range.The likely issue will only be of the measurable kind,without the filter to remove the HF noise the measurable S/N ratio will take a hit.

Also If I eliminated the LPF would the 681r's be OK?

I'm guessing it will be fine.


Oh and your welcome.

ben62670

Thanks Bro. I will get this done very soon. With our Pre/phono cards I am hoping that this will work nicely. I don't know how you feel about stuffing that card in your DAC, but keeping the two in the same case eliminates an extra piece of gear, and the extra PS. Also being able to use a nice dual channel OpAmp instead of being limited to a pair of single channel 8pin dips should help us both out:) Sorry about being a pimple on your rear:o I am trying to get this done before the fellas drop in this weekend. I wish you were closer.
Take care.
Ben

ShinAce

ben62670 wrote: »

Thanks Bro. I will get this done very soon. With our Pre/phono cards I am hoping that this will work nicely. I don't know how you feel about stuffing that card in your DAC, but keeping the two in the same case eliminates an extra piece of gear, and the extra PS. Also being able to use a nice dual channel OpAmp instead of being limited to a pair of single channel 8pin dips should help us both out:) Sorry about being a pimple on your rear:o I am trying to get this done before the fellas drop in this weekend. I wish you were closer.
Take care.
Ben

p.s. the NE5534 does not have internal compensation and it's mono.

The NE5532 is internally compensated for unity gain, dual channel, and 8 pin dip.

I don't know which circuit board you plan on using, so I'm just tossing it out there.

ben62670

I know the 5534 is mono;) The dual channel chip is going to be mounted externally. We were looking for good single channel dip chips, and the selection is very limited unless you use surface mounted. What I wanted for the output buffer was a stereo chip 8 pin dip. There are a bunch of those, and they are nearly the same price as the mono chips:) I am running the OPA2134's in the rest of my setup in my pre, and LR 24db HP/LP active XO. As you know these are also stereo chips in the 8 pin dip package. I am just trying to get everything uniform, and keep the cost of upgrading the OpAmps down. I am going to replace the NE5534's with the OPA134's. I'll post some pics later. I am very excited about getting this done. IF I stick with the 681r's would I half the caps, or double them
Thanks
Ben

Systems

ben62670 wrote: »

IF I stick with the 681r's would I half the caps, or double them
Thanks
Ben

To keep the same filter freq, caps = half the value.You could use the 2700pf's for the time being, in fact moving the filters corner freq down to 100k should have no audible effect.:)(Actually in theory 100k would be better for redbook replay).

zingo

Throwback bump:

I received one of these 1792 DAC boards a while back, but I put it aside after chatting with Fred a bit as I got busy. Now, I've rebooted my efforts, and here is the power supply I just got in that should power it quite well. This is a low noise power supply that was destined for an Oppo by Rotel, and will provide me all the power I need for the 1792 DAC board. Sound quality would probably benefit if I get a second power supply for two of the 5V sections, but at least it will get me listening in a high quality fashion.

I will be using the DAC as a dedicated unit for my Squeezebox Touch, and it will be always on with a simple digital coax in and stereo RCA out.

Thank you Fred and I appreciate all your help and expertise!

FTGV

Great you got it.Thats a good lookin PS for the money.(I like that shielded toroid).Add a couple of LM7805's for the 5volt analog and digital sections and you'll be up and running.
Do you have a line on a chassis yet?
Interesting reading this old thread.You have the board that I originally sent Ben.

SCompRacer

FTGV wrote: »

Great you got it.Thats a good lookin PS for the money.(I like that shielded toroid).

+1 to that.

Nice DAC project. Fred rulz!

zingo

I tested the PS tonight and all the voltages measure as claimed. I will be using an IC 7805 regulator initially for the 9.5V and feed both 5V using it. I haven't tested it with the PS yet, but it's rated for the current.

Fred, I do not have a chassis yet if you have any suggestions?

zingo

EndersShadow

zingo wrote: »

Fred, I do not have a chassis yet if you have any suggestions?

Shoebox

zingo

Just thick card board to keep it all together and work on placement until I can find the right chassis.

SCompRacer

What's your chassis budget? The diyaudio store sells some of the Italian HiFi2000 chassis line. Like $79.

http://www.diyaudio.com/store/

I used a HiFi2000 chassis for my DAC and had modushop machine the front panel. Not many anodize after machining but they do.

http://www.modushop.biz/ecommerce/index_l2.php

FrontPanel Express offers both chassis and machining with free software to use for design.

http://www.frontpanelexpress.com/

Hammond Enclosures.

http://www.hammondmfg.com/scpg.htm

Bud Industries

http://www.budind.com/

I've seen some folks gut an old CD/SACD player and install their DAC's in it. You can retain the transport for the old disc method of play along with your other inputs on rear. Sometimes that requires making a new rear panel.

strider

I put mine in an old CD changer chassis I gutted. In addition to Rich's links, circuitspecialists.com has a few different option.

http://www.circuitspecialists.com/metal-cases

FTGV

This is nice but may not be quite big enough.Searching ebay stores might turn up something suitable.

http://www.ebay.ca/itm/2806-Aluminum-Preamplifier-enclosure-DAC-case-amplifier-chassis-AMP-BOX-/111147701555?pt=UK_Sound_Vision_Amplifier_Parts&hash=item19e0eb5533#ht_3320wt_1124

If looks are not real important the basic aluminum Hammonds would do in a pinch. http://www.hammondmfg.com/dwg21.htm

zingo

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