jonahn Posted October 29, 2004 Report Share Posted October 29, 2004 I have access to some audio test equipment at work, so I hooked up my MZ-NH700 walkman to it to try a few things. Noticed a few interesting things, but the strangest was this: the headphone output has a frequency response that is far from flat! It is pretty flat from 100Hz to 20kHz (dropping slightly at the top end), but below 100Hz it rises sharply and reaches about +6dB at 20Hz (reference 0dB at 1kHz). This means a couple of things - firstly, if you use the headphone socket as a line-out, you will not get a signal that is true to the recording. Secondly, it means that the maximum safe output level before clipping begins is severely reduced. With a 1kHz sine wave at digital full scale, clipping begins on the headphone output at a volume control setting of about 27. At 20Hz, clipping starts with a volume setting of only 20. This applies when listening with headphones as well, so you will potentially get clipping in music with low-frequency content with the volume control set at anything over 20 - even though there is plenty of headroom left through the rest of the audio band! So unless I've made a mistake with my test, then this seems like a pretty stupid design feature to me! if they were going to put in a bass-boost feature they could have at least made it switchable... (The above test was made with the EQ switched off.) Quote Link to comment Share on other sites More sharing options...
dex Otaku Posted October 29, 2004 Report Share Posted October 29, 2004 This sounds extremely odd. I've digitally transferred sweep tests et al to my NH700 and then recorded them back to my computer through the headphone out [with volume at 100%, 30/30] and gotten flat response from bottom to top with no clipping whatsoever [-12dBfs, -6dBfs, and 0dBfs tone suites]. Very odd. Quote Link to comment Share on other sites More sharing options...
jonahn Posted October 29, 2004 Author Report Share Posted October 29, 2004 Very strange! What was the lowest frequency you used in your sweeps? I'm very surprised you found no clipping even with the 0dBFS sweep... I've found that playing music with the volume turned up to 30/30, the clipping is very audible. Unless you were using the (3V) AC adapter? That might increase the output swing of the headphone amp? I was using the supplied (1.2V) NiMH battery. The method I used was optical out from an Audio Precision box into the NH700, and headphone output back into the AP analogue analyser, with the unit set to paused-record mode. The AP was set to digitally generate sine waves at 0dBFS, and the record level was set to Auto. The results I got were all as expected apart from this low-frequency gain. I double-checked that the EQ was switched off and I've never used the AVLS feature so I'm pretty sure that was switched off too. Is there anything else I could have been doing wrong? Quote Link to comment Share on other sites More sharing options...
dex Otaku Posted October 29, 2004 Report Share Posted October 29, 2004 The sweeps went from 5Hz-40kHz, and showed gentle sloping at 20hZ and around 20kHz. I don't see that you're doing anything wrong, no. I'd double-check the AVLS but otherwise.. not sure. My tests were done with a standard alkaline battery as power. Quote Link to comment Share on other sites More sharing options...
aeriyn Posted October 30, 2004 Report Share Posted October 30, 2004 This is unfamiliar territory to me, but the fact that your NH700 is a European model may have something to do with this; they have reduced output power (3mW I believe as opposed to 5mW). Or you may just have a wonky MD unit. :sleep: Quote Link to comment Share on other sites More sharing options...
jonahn Posted October 31, 2004 Author Report Share Posted October 31, 2004 I've found out what's going on... The low-frequency boost is deliberate... sort of. The headphone amp is a TA2131, is this chip has a "low-pass compensation" feature which allows use of smaller (cheaper) output coupling capacitors. Using the smaller capacitors in combination with the headphone impedance causes a drop in low frequency response, so the amp compensates for this by boosting these frequencies. So with 16 ohm headphones connected, the frequency response should be flat (I haven't checked). But with the output connected to a high impedance load, you will see the compensated signal. BTW I assume I have the non-European version as I bought it outside of Europe... is there any visible way of checking? Quote Link to comment Share on other sites More sharing options...
KJ_Palmer Posted November 1, 2004 Report Share Posted November 1, 2004 Thanks for posting your findings, jonahn, very interesting (and somewhat reassuring). Would you mind checking that 16 ohm headphones give a flat response? I'm using 16 ohm Sennheiser MX550 earphones with my NH600 and NH900 (Euro models). The overall sound certainly seems 'better' than with some higher impedence phones, but it would be nice to know if they really make a difference... Quote Link to comment Share on other sites More sharing options...
skyther Posted November 1, 2004 Report Share Posted November 1, 2004 This forum gets better all the time. :happy: Glad to see we're attracting all the intelligent discussion here. :laugh: Quote Link to comment Share on other sites More sharing options...
dex Otaku Posted November 1, 2004 Report Share Posted November 1, 2004 This doesn't explain why mine measured flat, though. Quote Link to comment Share on other sites More sharing options...
MrToad Posted November 1, 2004 Report Share Posted November 1, 2004 Not so scientific but I have conducted A/B test comparing NZ-700, MZ-E909 against the original CD playing through my domestic Cyrus 8 amp/CD and Wharfdale Pacific 30 speakers with the MD's set to normal and not only did the frequency responses sound identical but the MD's were indistinguishable from the CD. However has anyone noticed a marked difference in volume available from the NZ-700 when changing from normal to equalizer. In normal mode the volume increases progressively from 0-30. With any equalizer selection, even with all bands at maximum, there is little increase in volume above 20 which is surprising and disappointing. Is this typical or is this an individual fault? Quote Link to comment Share on other sites More sharing options...
jadeclaw Posted November 1, 2004 Report Share Posted November 1, 2004 Had found out this too. The Equalizer works on the digital side, so any increase of the volume above a certain level would result in a clipped signal. Since full level is reached when the volume is set around 28/30 on a full scale sine, then there must be a compensation for the +10dB gain of the equalizer. And that one sets in at 20/30, above that, there is a compression effect quite similar to the Optimod used by bad sounding commercial radio stations. How bad it sounds, depends on the eq settings. Btw, I was unable to 'improve' the stock earbuds with it, a simple cowtail tone control had a much more satisfying effect... Quote Link to comment Share on other sites More sharing options...
jonahn Posted November 2, 2004 Author Report Share Posted November 2, 2004 This doesn't explain why mine measured flat, though.One idea... this is a long shot but were your sweeps 180 degrees out of phase between left and right channels? Because the amp's low-pass compensation circuit is shared between both channels, so I suspect it would not work with out-of-phase signals. If not... then I don't know. With the full scale (0dB) sweeps and volume turned up to 30/30, the amp probably would have been saturated across the entire band, so the frequency response wouldn't be meaningful. But it should have been visible on the lower level sweeps. BTW the amp seems to have a soft-clipping effect when it saturates, so the clipping is not necessarily obvious when looking at waveforms. I didn't realise this at first as I was finding the saturation point by looking at THD readings on the AP. KJ_Palmer, I will have another go at checking the response with a 16 ohm load connected... Quote Link to comment Share on other sites More sharing options...
KJ_Palmer Posted November 2, 2004 Report Share Posted November 2, 2004 Thanks jonahn, I look forward to the results. You certainly seem to know your stuff. Quote Link to comment Share on other sites More sharing options...
jonahn Posted November 5, 2004 Author Report Share Posted November 5, 2004 Finally got round to re-checking with a 16 ohm load connected. It does indeed flatten out the response - still not perfect but better. The response still rises initially below 100Hz, peaking around +2dB at 50Hz then drops off below that. The shared compensation circuit causes lots of crosstalk between left and right channels at these low frequencies. The amount of compensation is reduced when left and right inputs are out of phase, but not completely eliminated. Everything I said above about the non-flat respose when using the headphone socket as a line-out, and the higher possibility of output saturation at low frequencies (with or without headphones connected), still applies! Quote Link to comment Share on other sites More sharing options...
KJ_Palmer Posted November 5, 2004 Report Share Posted November 5, 2004 Cheers, once again. Nothing's perfect I guess... I don't suppose you've had a chance to try out an NH900 or NH1 in headphone/line out mode. Or indeed to compare the NH700's results with past models (or different devices).... Sorry, don't want to overburden you, jonahn. Maybe someone else wants to join the fray?... Quote Link to comment Share on other sites More sharing options...
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