using an impedance adapter to make an external DAC drive 32Ω headphones? - Page 3 - diyAudio
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Old 27th April 2010, 10:33 AM   #21
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ok, the manufacturer of the NanoPatch+ just told me:
Quote:
the impedance load on the DAC will increase and the voltage at the headphone will decrease. The value of the pot is 5KOhms.
do we have a deal? thanks!
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Old 27th April 2010, 10:43 AM   #22
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In my opinion, the nano patch contains a stereo pot with 5k resistance.
This seems far too high for your application; my advice is to build the electronic part on your own (using four fixed resistors or a standard stereo pot of 2x 100..470ohms)

You may chosse where to mount it; in general it would be possible to mount this stuff into a small plug or even into the DAC housing itself. You may also take a NanoPatch and replace the pot inside by one with lower resistance.

But in general I will not give comments on actual devices which i don't know.
So feel free to try.. anything that dosn't cost tto much money.
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Old 27th April 2010, 11:08 AM   #23
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ok, thanks for the reply!

"far too high" as in what exactly? because when the phone is plugged directly to the DAC output, I can't go over 10% volume on the computer...well, the housing looks neat, so indeed I could install RCA inputs and change the pot if needed
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Old 27th April 2010, 11:25 AM   #24
Gordy is offline Gordy  United Kingdom
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Quote:
Originally Posted by leeperry View Post

...my Sony CD1000 headphones are 32Ω/104dB...

...headphones are VERY sensitive(104dB) so they're very easy to please...
Hi, I've not read the whole thread, so don't know if this has been mentioned already...

Your issue is not primarily about impedance match, output impedance or voltage swing... it's first and foremost about current drive capability!

First thing to do is to convince yourself that the DAC can output the required current. The DAC is surely designed to drive 10,000 R not 32 R. Driving 10,000R will require very little current, but 32 R is totally different!

Your quoted 104 dB is most likely at 1 mW, which (across 32 R) requires a peak current of 7.9 mA. For realistic music peaks you will need to look at approximately 110 dB (some might argue higher!). If you aim for 110 dB you will require 4 mW which is a peak of 15.8 mA. Can your DAC supply that?

However if you listen at lower levels (say 70 dB), then your peaks might not go past 95 dB, which will require just 1/8 mW (i.e. 0.125 mW), which is a peak of 2.8 mA.

So best to check the DAC output capability before painting yourself into a corner!
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Old 27th April 2010, 11:44 AM   #25
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hi Gordy,

The service manual only says 104dB, but prolly at 1mW indeed.

I've put 2*LT1028ACN8 as PCM1793 LPF in the DAC, I think they should even be able to drive 32Ω tbh...but the manufacturer says that the DAC output is 100Ω, so they prolly added some components to force it this way.

I only listen at very low volume, and I get a perfectly balanced FR with the Shure L-Pad attenuator, LT1028 is a banger and my cd1k is dead easy to drive...also, I cannot go >10% volume on the computer when I remove the attenuator, so it means that I'm killing the actual bit-depth from the computer and that I have a hell lot of headroom for attenuation?

Last edited by leeperry; 27th April 2010 at 12:06 PM.
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Old 27th April 2010, 11:53 AM   #26
paulb is offline paulb  Canada
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This would be an impedance adapter
http://headwize.com/projects/showfil...szeke1_prj.htm
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Old 27th April 2010, 12:33 PM   #27
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very nice! but I'd rather keep it passive
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Old 27th April 2010, 09:33 PM   #28
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Quote:
Originally Posted by bocka View Post
Well, a nasty idea: As many people like passive i/v converters (simple resistor) wouldn't it be worth a try to direct connect the headphone to the dac? The tradeoff is the volume control has to be done by the dac and the output is current driven. But as many power amps only use a series resistor to make a headphone output this would the most simple solution for a headphone dac
the less components between the LT1028ACN8 LPF and my phone = me happier

that's what Firestone Audio's tech support told me:
Quote:
There is no problem to use 32 Ohms headphone; you do not need to add a serial electric resistance to the connection.

Most of our customers use the headphone connected to the RCA out of our Fubar2, the sound power is actual enough.
that's the fubar2: USB Audio - USB DAC - Fubar II USB DAC

but I find the sound rather harsh and bright, volume loudness is not the issue....impedance mismatch is..

I also can't go past 10% volume on the computer, so I'm killing a lot of dynamics here...OTOH outputting 100% to a passive pot wouldn't get any better probably

SM Pro Audio: Nano Patch+ - Passive Volume Controller
Quote:
Reducing levels from your software only reduces the audio bit depth. It is much more appropriate to keep your software masters at unity and attenuate the audio to your active monitors with a passive controller like the Nano Patch+. This way you can maintain maximum audio resolution and attenuate your monitoring volume with the confidence that the sonic integrity of your audio is not compromised.
OTOH, my current media player does volume attenuation in 64bit float
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Old 28th April 2010, 12:05 AM   #29
Gordy is offline Gordy  United Kingdom
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Ok, take your DAC output into a 1K potentiometer. Take the output (i.e. wiper) of the potentiometer to the primary of a Hammond 119DA transformer. Connect your headphones to the secondary of the transformer.

Use the 600:8 tap and you’ll get a 8.7:1 ratio which at 2V rms input will net you a staggering 1.6 mW output into 32 R. That will be approx 105 dB in your headphones. (Actually maybe a little less because there are small transformer losses and the DC resistance of the windings to factor in. You might need to crank your volume knob up a bit, but that’s not necessarily a bad thing).

The Hammond 119DA seems cost effective and is available from Antique Electronic Supply (USA) or BlueBellAudio.com (UK).

: )
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