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Old 16th August 2012, 11:13 AM   #1
tsiros is offline tsiros  Greece
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Default looking for USB/SPDIF DAC with volume control

The source will be my computer, feeding the digital data into the DAC which will drive an LM4780 power amplifier directly.

what i need:
  • volume control on the DAC but no preamp (no gain)
  • spdif or usb, don't care much, unless you convince me otherwise
  • line out, not headphone out. Don't know if it matters any anyway.
  • max price $200.
related but possibly off topic: i do not want the computer to attenuate the signal... that is, the flac decoder produces a stream of sample values. Until they reach the DAC i do not want any kind of volume control, I want these bytes unaffected, because i do not trust winamp/windows/soundcard drivers to leave them alone even if i set the volume at maximum (or disable "volume control" in winamp's settings)


thank you for your attention and your time.
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Old 16th August 2012, 04:37 PM   #2
TheGimp is online now TheGimp  United States
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CS4398 DAC has a digital attenuator, but it is accessed via SPI and the boards I've seen don't support an SPI interface.

Another consideration is the 26dB gain of the LM4780 which dictates a 1vrms input for full output.

Many of the DACs don't put out 1Vrms at full scale if powered from the USB port as they only have a 5V supply available.

Vo =(5V/2)/sqrt2=0.88Vrms.

The DAC will have to use power supplies greater than just the USB +5V.

This looks interesting as it uses a microprocessor and it might be possible to control the gain with it.

Since it uses a transformer and is not USB powered it may have a higher output level.

CS8416 + Dual CS4398 Parallel DAC Board with VFD Display & Remote 24Bit/192KHZ | eBay

Above your price point, but it looks like what you are looking for.

http://www.ebay.com/itm/Assembled-Du...item19cb22cee6

Last edited by TheGimp; 16th August 2012 at 04:40 PM.
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Old 16th August 2012, 05:23 PM   #3
tsiros is offline tsiros  Greece
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Quote:
Originally Posted by TheGimp View Post
Vo =(5V/2)/sqrt2=0.88Vrms.
you sure?
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Old 17th August 2012, 02:17 AM   #4
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The formula's low by a factor of two, but the answer's roughly correct for single ended outputs anyway unless the output buffer op amps are RROs---the exact headroom depends on the op amp, supply ripple, and what you consider acceptable distortion. Going for differential outputs will get you another factor of two.

DAC output swing requirements are a function of required output power. The majority of folks need 5V or less into the speakers. So, statistically speaking, there's likely benefit to fronting the 4780 with an attenuating stage to get better bit depth utilization in the DAC (and to mitigate unbalanced interconnects' 0dB CMRR if you're going that way). Everyone's end to end gain budget is different but usually a DAC+output buffer can manage 2-4V differential, in which case the end to end gain in the power amp often falls in the 0 to 6dB range.

If you sent the gain budget properly the value of in DAC volume control is often significantly reduced, meaning it can be fine to send a divided down 24 or even 16 bit stream to the DAC. In other words, you may be able to solve the volume management issue by adding two cents worth of resistors to your power amp. If you're not locked into the 4780, consider using a different power amp which does not require 20+dB gain in its feedback loop for stability. The 3886 THD is on the order of -50dB at typical output powers so I wouldn't expect the 4780 to be all that great either (it's essentially a pair of 3886s)---it's perhaps kind of silly to be worrying about DAC bit depth when the amp is likely worse than the phase modulation distortion floor of the drivers in a point source speaker.
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Old 17th August 2012, 09:36 AM   #5
tsiros is offline tsiros  Greece
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in short, you can't find me something like what i asked?

a usb dac with an output attenuator?
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Old 17th August 2012, 04:02 PM   #6
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My power amps run at unity gain from a DAC which outputs 4V differential. So I've never found the need to look for a DAC with output attenuation. Please pardon my suggestion of an alternative solution.
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Old 21st August 2012, 10:15 AM   #7
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Default This is my old toy

Quote:
Originally Posted by tsiros View Post
The source will be my computer, feeding the digital data into the DAC which will drive an LM4780 power amplifier directly.

what i need:
  • volume control on the DAC but no preamp (no gain)
  • spdif or usb, don't care much, unless you convince me otherwise
  • line out, not headphone out. Don't know if it matters any anyway.
  • max price $200.
related but possibly off topic: i do not want the computer to attenuate the signal... that is, the flac decoder produces a stream of sample values. Until they reach the DAC i do not want any kind of volume control, I want these bytes unaffected, because i do not trust winamp/windows/soundcard drivers to leave them alone even if i set the volume at maximum (or disable "volume control" in winamp's settings)


thank you for your attention and your time.
TE7022 as usb I/O, and has Line out, also has a analog input. And optical I/O too. Use a rotary switch to control the volume(digital way). And a Mute function by key. 1.7Vrms for Line-out.
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Old 22nd August 2012, 04:29 PM   #8
tsiros is offline tsiros  Greece
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Quote:
Originally Posted by hifimediy View Post
TE7022 as usb I/O, and has Line out, also has a analog input. And optical I/O too. Use a rotary switch to control the volume(digital way). And a Mute function by key. 1.7Vrms for Line-out.
thanks, will look into it! if it had RCA it would be perfect

Quote:
Originally Posted by Cyan Lo View Post
Dear Friend.

I know the unit.that is tonemax P12.it also support usb 192k.
CS8416+Ad1955 dac+CS3310(volume control)

the unit you can but it form http://lotsee.taobao.com.
more infomation please goto www.tonemax.com.cn
thanks, but it looks a bit too much...

Last edited by tsiros; 22nd August 2012 at 04:31 PM.
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