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Old 3rd January 2004, 07:17 AM   #1
Benjlv is offline Benjlv  United States
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Default USB powered HIFI headphone amp/DAC

http://chartermi.net/~benjlv/zcarforum/usbhead.gif

This circuit is (click above link to view) my attempt at a USB powered sound card that will drive one or two sets of headphones. Its based off the Burr Brown PCM2702 USB DAC. That output goes to a OPA604 with low pass filter which is then buffered by a BUF634 which in turn drives the headphones.

The rest of the circuit is the power supply. For those who are not familiar with USB specification I’ll sum it up real quick. The USB port can provide up to 500ma of nasty power at 4-6.5 volts. To limit inrush current I have implemented a slow start circuit for the power supply based around Q2 in the schematic. In rush current must be limited to that of a 10uf cap. There is also a fuse…just in case although not necessary as I understand the USB port is current limited internally. But when dealing with a $1400 laptop I like to be overly cautious USB specification also requires a shutdown mode in which current draw is near nothing. In reality I’ve read that this can be up to 10ma or so, but tests with no device connected to the USB port have allowed me to draw 500ma with no problems. In case though I do have Q3 in the schematic to turn the MAX743 off since it has no shutdown input…but does have a soft start circuit built in.

In order to obtain a solid 5volts from the typical 4.5-4.9volts I have found on the USB port there is a switching power supply based off the MAX731. This provides power only for the analog stage of the PCM2702. Which I am curious to know whether or not it needs to be powered in order for the PCM2702 to go into standby and come out of… Would anyone who has a up and running PCM2702 test this for me?

There is also another supply based off the MAX743 which supplys +-12volts or +-15volts for the opamp output. Like I said before I have this setup to be turned off by Q3 in standby. When turned back on is this going to cause a loud noise on the headphone output?

Also what do you all think of the filtering on the outputs of the powersupplys? How much noise is accepatble? Granted it will be around 170-200khz..?

Another question I have regards the output stage. In my simulation software I have found it to have a gain of about 1. Which means there is a full 2.5v p-p at the output. Which is about 24mw RMS. I plan to add more gain to the circuit. The part that I cannot decide on/figure out is how to implement the volume control. I want to run the computers mixer at full output so it doesn’t resample. So it will be necessary to implement a analog control. In doing the output stage I referenced these designs
http://home.swipnet.se/~w-50719/hifi/qrv04/index.html
http://www.keces.com.tw/~korping/diy...cm2702_sch.pdf

What I am confused about is obviously how to do the volume control…
I have thought of several ways, but do not know which would sound best.

Any thoughts on the type of opamp? I picked the opa604 since it had the fastest slew rate in the budget range.

If I should have R15 and R7 in the circuit (the BUF634 datasheet suggested to)…
How to set the output impedance of the circuit. I understand that R10,R11,R2 and R3 play a large part in this but do not understand why/what their values should be.


Anyways the design will be done on two separate boards. The power supply except for the 3.3v LDO will be through hole. And the other board will be SMD to safe space.

I realize that many of these questions should be answered by prototyping, but I wish to get this circuit up and running since the quality of the sound card in this computer is terrible! Any input is highly appreciated!!!!!! thanks guys

PCM2702
http://www-s.ti.com/sc/ds/pcm2702.pdf

OPA604
http://www-s.ti.com/sc/ds/opa604.pdf

BUF634
http://www-s.ti.com/sc/ds/buf634.pdf

MAX731 (+5v)
http://pdfserv.maxim-ic.com/en/ds/MAX731-MAX752.pdf

MAX743 (+-12 or 15)
http://pdfserv.maxim-ic.com/en/ds/MAX743.pdf

MAX1598 (3.3v LDO)
http://pdfserv.maxim-ic.com/en/ds/MAX1598.pdf
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Old 4th January 2004, 10:36 PM   #2
Benjlv is offline Benjlv  United States
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no takers?
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Old 5th January 2004, 04:30 AM   #3
alvaius is offline alvaius  Canada
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Technically you can only draw 100mA off USB until the device has been enumerated at which point it can draw 500mA if it is allowed to by the host.

Alvaius
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Old 5th January 2004, 12:05 PM   #4
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IIRC WW/EW mag had just this a while back.

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Old 5th January 2004, 03:43 PM   #5
Benjlv is offline Benjlv  United States
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I tried 4 different computers...
A pentium 4 with intel motherboard
a Pentium 3
A pentium 2
and my Dell inspiron 5100 laptop.

Each one of them allowed me to draw 500ma with no communication on the USB port.

Perhaps if the device is enumberated for 100ma the port will impose a limit? Hmmm sounds like time to take about my USB mouse and see.

thanks for the replys BTW
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Old 5th January 2004, 07:30 PM   #6
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If you want to be shure that you can at least consume 0.5A, just use a external powerd hub.

I don't think you can limit or control the power consumption, the only thing that is easaly controled is the data-bandwith of each USB port. (you can view the power consumption at COMPUTER MANAGMENT/USB CONTROLLERS)


de BB opa 132/134 sound a bit better than the opa 604 (i've tried them in a rotel cdplayer) and has about the same price.
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Old 6th January 2004, 01:23 AM   #7
jcx is offline jcx  United States
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why don't you just use the PCM2702 internal volume control? as a delta-sigma converter it is likely that the effective bit resolution of the ouput is way better than 16 bits; considering oversampling and inherent dithering from the delta sigma process i would bet on the internal digital volume control being better than most analog solutions - a unavoidable source of distortion is the analog output satge of the 2702, any way of reducing the signal level there will likely yeild distortion benefits

if you are only using 32 Ohm headphones then +/- 15 V supplies are unnecessary, 5 Vpp is 100 mW and with typical high 90s dB/mW sensitivities (and today's insane 6 dB dymanic range in pop recordings) this is quite deafening

i would consider a sw-cap doubler and linear 5 V reg with one of Maxim's bridge headphone driver chips (actually i would roll my my own amp with AD823 and discrete transistor buffers inside the feedback loop)
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Old 6th January 2004, 01:18 PM   #8
alvaius is offline alvaius  Canada
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The USB Host does not limit the amount of power that devices draw. Heck, on a desktop PC, I would not be surprised if you could not draw well over 500mA. However, what the host does, assuming all the devices connected are truly USB compliant, is ensure the system is properly functional. Hence, if a device requests 500mA and there is not 500mA available, it will tell it that it can not draw that much. There is nothing stopping the device from still drawing 500mA, but if it is USB compliant it won't. However, drawing excessive power may cause your other USB devices to fail due to excessive voltage drop on the power.

Alvaius
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Old 6th January 2004, 02:35 PM   #9
tool49 is offline tool49  Canada
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Hullo,

I've been working on a similar thingy, but I hadn't figured out how to get ± 15V from the usb port; I like your idea! I might suggest adding a regulator to ±12V (such as the lm317/lm337 or better) right after the chip that does ±15v just to insure minimal noise in your amplification stage.

For the volume control you have two choices:
1- You could had a potentiometer between Vout of the DAC and the OPA
2- You could use the provided switches on the PCM2902E (which should be nearly pin compatible with your PCM2702) EDIT: They are far from pin compatible, and the 2702 has slightly better specs but a tougher implementation to my view; The SPDIF IN/OUT of the 2902 outbalance the loss in SNR/DR in my opinion (might be a personnal choice).

For the current limiting feature, why not use another chip from maxim; the max1607 is designed specifically to limit the current load on a usb device.
http://www.maxim-ic.com/quick_view2.cfm?qv_pk=2093

Hope this helps!
Sébastien
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Old 7th January 2004, 06:59 AM   #10
Benjlv is offline Benjlv  United States
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First i apologize for not getting back to this topic for a few days. I have been pretty busy with life.

In order to have +-15volt supplys there must be two switching power supplys to keep power requirements below 500ma.

But like jcx said there is no need for such large voltages. Although I do wonder if there is a reduced sound quality...does anyone have any insight on this?

I was unable to find a cap charge pump that met the current demands of the circuit though. Although maxim does have have several inductor based solutions that'll work nicely with a linear regulator.

So with no negative rail available i changed the analog stage to look like this. The idea is that the out put of the OPA134 should be based around 1/2 the supply voltagle. then the buffered out put would be coupled to the headphones.

would this work?

My largest concern is if the reduced supply voltage would degrade the sound quality. Like i said before i had been told that opamps sound better at the typical -+12 or 15

Tool49-
look at the MAX631...its got a inductor based 5,12,or 15volt output then a built in negative capacitor charge pump which can be set at -vout or 2x -vout both have a 1.2volt drop from a set of diodes... Which has a circuit resistance of about 30ohms the datasheet says. That pretty much means you should be able to get a good amount of current out of it, but it will require a linear regulator.

Originally I set out to build a USB dac based on the PCM2706 and use its I2C output to drive a PCM1730 DAC...I then added a SPDIF reciever in front of the DAC to use its reclocking abilities. But when all was said and done the power requirements of the circuit where too great and much to complicated. I mean you need 6 opamps plus 2 current buffers Click the image to open in full size.

to do the analog stage. I think my design was up to nearly 15 chips for the power supply and everything else. Just to much stuff and too much $$$ for me. However I believe that if the voltage is left low enough on the analog stage the power supply requirements could be low enough to be powered by a USB port. Assuming you could use a single 5volt supply for the thing.
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