Residual white noise on DP24SD headphone output

Mitch Gallant

Mitch Gallant

Member
Joined
Oct 12, 2020
Messages
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From
Carleton Place, Ont Canada
Website
www.jensign.com
Gear owned
DP-008 DP-24SD DR-40
Has anyone noticed a faint but noticeable white noise on the headphone output? It is independent of the monitor output level. I noticed this and compared to the headphone output on my older DP008 and the DP008 doesn't have audible white noise even at full headphone level.
 
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Interesting comparison Mitch thanks... and welcome to the forum :)
Have just looked at some of the headphone amp specs in Tascam's manuals.
DP-008 is 20+20mW at 1% distortion
DP-008EX is 20+20mW at 0.1% (not sure if the DP-008 distortion is a typo?)
DP-32SD is 70+70mW at 0.1% distortion.
All values are with a 32 ohm load.
It would appear Tascam have used a more powerful amp for the dp32, and maybe this has contributed to the background noise. The other thread mentioned by MJ has a lengthy discussion on this.
 
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Thanks Phil. I wish I had joined this group 2 years ago when I got my DP24SD (after using my DP008 for the2 years before that). Lots of catching up on reading here lol

Regarding the specs higher output headphone of DP24SD, 70mW into 32ohm, higher power output stage by itself doesn't necessarily mean that the ambient noise must be higher.
For example look at my Stereo Out measurements (which in schematic is before the internal headphone amps) of DP24SD which includes an output FiiO E17 amp rated at 220 mW/ch at 32ohm loaded.
The noise measured there at max output gain was similar (slightly higher) to the excellent DP-008 noise but still much lower that the DP24SD output noise through the DP24SD headphone out. Would love to see the DP24SD output schematic for the analog amps section.
 
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Haha yes it kept me busy when I first got mine :)
Agreed re. power vs ambient noise. I probably should have said a 'different' amp as opposed to 'more powerful'. The schematics for both models are available on the internet. Give me a shout if you can't find them.

By the way, can you do a similar measurement for the monitor out jacks at the back of the machine? We've been suggesting that an external headphone amp on these sockets would be one solution to the 'hiss' problem, so would be nice to have more measurements for comparison (Mark already made some in his post). I guess you'd need to convert the balanced L/R outputs into a single stereo plug for the FiiO E17 input, unless it has separate L/R inputs.
 
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Sorry Mitch, must have missed that. Wow that's quite a difference. Yes, if the block diagram is an accurate reflection of the circuitry, we'd expect the monitor path to be slightly worse than the stereo path with that additional amp stage, but they're both well below the headphone amp so either are acceptable for an external headphone amp (imho at least).
Interesting articles on your website!
 
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I've always used an external monitor controller, hence any use of headphones with my DP-32 (or anything else for that matter) comes off the controller, which in turn gets fed from the DP-32 Monitor outs.
 
Phil, the Monitor output noise level is considerably higher than the Stereo out by about 5x .. not surprising as the Monitor out first passes through an analog gainstage (OM1). Maybe you misread what I wrote or I have a typo lol.

I've just added a short listening test section in my article at end on adding the FiiO E17 headphone amp to the DP-24SD and comparing level/noise to headphone out.
 
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I saw it, but didn't think it was an issue as it's still quieter than the headphone output. The question is can you hear it? You say the stereo output noise is inaudible but did you try the monitor output? I don't hear anything when the monitor outputs go to my active speakers, but I've not done a critical listening test with a headphone amp. Maybe Mark can shed some light on it. Just noticed the 2 to 1 difference on your measurements between C1 and C2 in the headphone test. Is that left & right?
 
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Yes my DP24SD has a difference in the headphone out L vs R channels (C1 C2).
I just checked the monitor out with same configuration and I can hear the hiss noise but its lower than headphone out as expected from the scope measurements. Also just checked the Effect Sends out and they are dead quiet, like the Stereo Outs .

I have made over 150 recordings with my DP-24SD in the past 2 years and have used the headphone output always so it isn't a big deal for me .. just noticeable with phones on in quiet sections. But since others have noticed it, I was curious to know why the hiss is noticeable. Using the Monitor out might not be noticeable with active speakers as you are not quite as close to the sound as with headphones.
 
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Phil ... could you point me to the DP-24SD schematic? Thanks.
I'll be updating my article on the noise in the next day. Some of those noise measurements I reported are too high because they were integrated scope measurements and didn't restrict the measurement BW to the audio 20kHz range. The new table of measurements still show very low Stereo out, higher Monitor out, and very high headphone out noise.
 
It's an interesting pursuit, these noise measurements. My recordings are clean and I've never noticed the noise floor.
 
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I agree. The noise I discussed is only present/generated in the analog output stages. I've never noticed any issues in noise within the recorded tracks on my DP24 and certainly my recorded 16/44.1 and 24/96 wav files when exported and examined or played back through other low noise systems are quiet in the background. The DP24 Stereo Out noise is very low and never noticeable. The Monitor out is very slightly noticeable, and the Headphone out is noticeable (not surprising as that Headphone output stage has a gain of 5 after the Monitor out and the noise before it is always gained as the Monitor level control is digital and only adjusts the digital signal.
 
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Yes I recall recording some drum tracks with headphones on using my DP-008 and had to crank the headphone level to max to hear other backtracks over the drums I was recording, so I see why DP-24 upped the gain of phones circuit.

At any rate for what it's worth to those bothered by the fixed level hiss, here's a very simple passive DYI external mod using a voltage divider at the output:

The DP-24/32SD headphone output impedance is about 54 ohm (two internal 27 ohm series resistors at output of the headphone driver amp).
These guarantee that the amp is loaded by at least 54 ohm even if the headphone out is shorted.
For typical headphones such as my Senns at about 50 ohm, the noise reducing workaround is simply put a low value resistor in parallel with your phones. In my case, using a 33 ohm resistor will lower the output voltage level across the phones by about -5dB (by the simple voltage divider network with internal 54 ohms) and this will lower the headphone hiss sufficiently, (which is at a fixed level, independent of the monitor gain adjustment), so it won't be noticeable. Just compensate the volume drop of playback by turning up the Monitor level since there is abundant gain from that output driver chip (by design). Just put the resistor in a small external box with 2 jacks and possibly a SPST toggle switch to enable or disable the noise reduction divider. No batteries or power. One resistor, 2 jacks and a small box. Heck mount them in a Keen's Mustard box:
NoiseReducer.jpg
 
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I love the mustard powder housing! But I also like these types of hacks anyway, great stuff!
 
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