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Author Topic: Tascam DR-70D - modification discussion  (Read 21381 times)

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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #45 on: July 28, 2015, 10:19:02 PM »
Hard to say exactly what that artifact could have been.  It could have been an interference signal that was present during the original recording.  Both the stock and modified opamps would have sufficiently high PSRR, and the caps added don't do anything about power supply noise or interference.  So either the source of the artifact is not related to the mod, or there was likely another change involved in the mod.

The mention of input transistors could change the noise analysis materially.  Discrete transistors are frequently used as a first-stage in front of an opamp for improved noise performance.  It takes more space though, so you might not see it as often in portable gear.  But it's a cheaper approach than paying for an expensive low-noise opamp (and solves a related problem of input topology).  Since there is gain in the first stage, the noise of the opamp becomes relatively lesser important as first-stage gain increases.

So if the 70D uses that type of input circuit, then opamp noise might not matter except at very low gain where you don't care anyway.

There is definitely nothing else done beyond swapping all of the opamps and adding the poly caps.  I took the instructions from Jim, bought the parts myself and sent them off to the SMT shop.  Again, the full details with a parts list are on the FAQ page for those who haven't seen this yet.

Investigating further into this spike I'm seeing: Most, but interestingly not all pre-mod recordings made with the 70D at 48kHz sampling rate have a spike at 15,734 Hz, with the left channel peaking about 10-12 dB higher than the right.  The levels of those peaks I'm seeing are around -63 to -74 dB for the left; -73 to -83 for the right.  This is with HIGH gain setting, typically with the gain 1/3 to 2/3 up.

96 kHz recordings (the very few I have) move that spike up to 21,903 Hz or 21,904 Hz.  I had done a test at this sampling rate recording with my CM3s at max gain for each of the 4 levels, just sitting open on the carpet in a quiet room.  The peak reached as high as -63.5 dB at the HIGH+ gain level, with each of the lower 3 gain levels proportionally lower according to their maximum gain setting.

This pops up in a variety of recording venues.  It's also not the mics, as I analyzed a couple 4-channel recordings that also used the X-Qs and I got the same exact thing, just higher in level.

All recordings I have made with the 70D post-mod do not have this spike at either sample rate.

However, I did find a very small number of recordings with my FP24 > M10 chain also (again using different mics), suggesting either there is something going on with either of those devices which similar to what I'm seeing in the pre-mod 70D, or it's somehow random and environmental as you suggest.

I can't find anything interesting about those two frequencies above 21kHz, but 15,734 Hz is the NTSC horizontal refresh rate.  It would make sense to get that spike if I was ripping the sound from old TV shows...

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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #46 on: July 29, 2015, 05:22:38 AM »
I suspected the video source pretty quickly when you mentioned 16kHz.  If there is a CRT TV on anywhere nearby, it can cause that.

21kHz could be from a lamp ballast or something.  It's odd that the different sample rates are associated with different interference signals though.

No CRTs anywhere I've recorded recently, and the 96 kHz recordings at home where I got the 21903 Hz spike were all made during the middle of the day with lights off.  Maybe it's just one of those weird random things.
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Offline phil_er_up

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Re: Tascam DR-70D - modification discussion
« Reply #47 on: July 29, 2015, 09:46:34 AM »
I suspected the video source pretty quickly when you mentioned 16kHz.  If there is a CRT TV on anywhere nearby, it can cause that.

21kHz could be from a lamp ballast or something.  It's odd that the different sample rates are associated with different interference signals though.

No CRTs anywhere I've recorded recently, and the 96 kHz recordings at home where I got the 21903 Hz spike were all made during the middle of the day with lights off.  Maybe it's just one of those weird random things.

Had those spikes too many times. Not just with this recorder and with different mics. Think it is either a battery, cell phone, remote control or some device that has a signal at that rate. It happens at home and at the venues though not all the time. Record at 24/96 and the spike is always over 20000 Hz though did not measure the exact frequency. Run 6 channels sometimes and its only on one set of mics. The same mics do not spike each time. It is like whatever mic/preamp is closest to the source picks it up and the other sets of mics don't. It was random and sometimes I go weeks without it happening then 3-4 show in a row it shows up. No pattern I could ever figure out to what caused it.
« Last Edit: July 29, 2015, 09:56:23 AM by phil_er_up »
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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #48 on: August 04, 2015, 04:15:11 PM »
Thanks to 2manyrocks, I can now share before / after "pillow test" files and analysis using Line Audio CM3s.  What we each did was stuff our mics in between some pillows / blankets, and record at maximum level for each of the 4 gain settings at 24/96.  I then trimmed each file to the quietest 15 seconds and analyzed with Izotope RX4.

What was very apparent is that while my home appeared quiet on the day I did this, there was a ton of low frequency content being picked up.  In contrast, 2manyrocks had a nice quiet background throughout, with only a slight bass rumble.  You'll see this in the waveform stats and spectrum screenshots. 

When I saw this, I thought we had just wasted our time and the test was invalidated by the drastically different noise floors in our two environments.  But looking closer, the differences were only in the bass.  So in addition to analyzing the files as we recorded them, I also sent everything through a steep 1kHz high-pass filter to level the playing field, and I analyzed those files as well (see the "1kHz HPF" folder).  That was the cutoff where I determined the before/after mod samples sounded the same.

https://www.dropbox.com/sh/wv262wtsca8xhbh/AAC1-wEt-7E5TDTtLIM2JNiga?dl=0

I'm sure some of you will want to examine these files in more detail.  For those that don't, I'll cut to the chase: above 1kHz, the JW-mod unit did not appear to measure any differently than the stock unit for this test.

Thanks to Jon for proposing this idea.
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Offline 2manyrocks

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Re: Tascam DR-70D - modification discussion
« Reply #49 on: August 04, 2015, 06:20:52 PM »
I appreciate your work on this.

I've having a little difficulty remembering how Jim Williams described the effects of his mod, but my hazy recollection is along the lines of improved responsiveness rather than a lowering of the noise floor?  If that's the case, we wouldn't expect to measure a lower noise floor after the mod. 

But how could we test for "improvement" in recording quality?  For example, I have an Adams tuning fork pitch C256.  Would it be possible to record a standard pitch like that and see a difference in the waveform between the modded unit and the stock unit? 

To my hearing, there is a subtle difference between the piano recorded on the stock unit before and after the mod, but how would you measure it?

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Re: Tascam DR-70D - modification discussion
« Reply #50 on: August 04, 2015, 07:06:57 PM »
I appreciate your work on this.

I've having a little difficulty remembering how Jim Williams described the effects of his mod, but my hazy recollection is along the lines of improved responsiveness rather than a lowering of the noise floor?  If that's the case, we wouldn't expect to measure a lower noise floor after the mod. 

But how could we test for "improvement" in recording quality?  For example, I have an Adams tuning fork pitch C256.  Would it be possible to record a standard pitch like that and see a difference in the waveform between the modded unit and the stock unit? 

To my hearing, there is a subtle difference between the piano recorded on the stock unit before and after the mod, but how would you measure it?

Well it would be difficult to measure subjective sound quality differences.  I suppose you could take a stock and modified unit and then make a recording with two very closely matched microphones mounted coincidentally, making a dual-mono recording - one to each recorder, and then compare them like is sometimes done with microphone shootouts.

A tuning fork wouldn't show you all that much.  Maybe something with more pronounced harmonic content and a long decay time, like a large Tibetan prayer bowl, would be a better choice.

Either way, if there are differences, different people may subjectively prefer one or the other.
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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #51 on: August 04, 2015, 10:30:59 PM »
It's possible the mod is slightly quieter, maybe 0.5dB, with those mics.  I am not sure that 1kHz is an exact point of adjustment, hence the uncertainty.  It could be nothing, or it could be slight.

Again, that is entirely dependent on the mics in question.  Mics with a lower absolute noise floor may show an improvement.

As for other tests, really the best thing you can use are test signals.  There are plenty of sets of test signals available; you can look at squarewaves (make sure those are formed properly for the sample rate used), intermodulation tests that use pairs of HF sine waves, and simple sine waves that look at distortion.  You can also test bursts (impulse response), but those are more interesting for filters, and there are no filters here in question.

Anytime you introduce a real-world complex source the analysis gets so complicated that a listening test will only first determine audibility and second preference.  Those might be interesting, but a measured preference on one source won't necessarily be preferred on another.  The easy-to-saturate transformers I stuck in my boxes are a pretty good example--and there will be no difference in this modification anywhere near as large as that example.

I went by ear on the 1kHz cutoff - it's possible I was too aggressive in setting the filter that high.  The original files are there on that dropbox link if you'd like to try playing with them.
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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #52 on: August 04, 2015, 11:04:32 PM »
I did analyze the dropbox files using very high-res FFT.

I wasn't questioning your analysis - I meant if you'd like to try a different/better cutoff than the 1kHz I went with.  It probably doesn't matter though, as far as the results for this test are concerned.  As you said, we'd need quieter mics and a more controlled test to find anything further.
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Offline 2manyrocks

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Re: Tascam DR-70D - modification discussion
« Reply #53 on: August 04, 2015, 11:47:04 PM »
Isn't there a way to generate a test signal in audacity or reaper and feed it from a PC into the preamps via a line in?  Should be able to compare the test signal versus the recorded test tone I would think?

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Re: Tascam DR-70D - modification discussion
« Reply #54 on: August 05, 2015, 07:25:16 AM »
Isn't there a way to generate a test signal in audacity or reaper and feed it from a PC into the preamps via a line in?  Should be able to compare the test signal versus the recorded test tone I would think?

Sure, but then you're just testing the line in, which you'd have to set at a pretty low level to not slam the input.
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Re: Tascam DR-70D - modification discussion
« Reply #55 on: August 05, 2015, 09:34:15 PM »
You would be limited to the resolution (THD) of your PC's output.

If you want to test the mic input at high gain, you'd have to pad the output of the PC as well, since it is likely to have lowest THD (and certainly THD+N) near maximum output.

I actually tried this with my stock unit, running from one of the TRS line outputs of my Focusrite interface.  I wasn't trying to measure noise, but was trying to determine the minimum values of the gain pots for each level setting, since that is not in the Tascam specs.  I had to set the monitor volume quite low to avoid brickwalling the 70D inputs when set to MIC, and as a result the recorded signal was quite noisy as Jon indicates.  But it didn't matter for what I was doing - I was able to take the known maximum of each of the 70D gain settings, and lowering the pot slowly until the cutoff point, was able to do some simple math to get the minimum values, which you see on the FAQ page.
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Re: Tascam DR-70D - modification discussion
« Reply #56 on: August 05, 2015, 10:44:54 PM »
Since we've established that this mod doesn't change the self-noise in any significant way, at least with the mics I use, I decided to take the piano comparison recordings and put them both through RX4's Denoise to remove the hum in the background.  I tend to do this with most things I record anyway, as I'm usually recording quiet music in an auditorium with loud HVAC.  Denoise settings were the same for both files; obviously I used different noise profiles but those were nearly identical.

The difference I hear between the stock and modified unit is not changed by the noise reduction, at least not to my ears.  YMMV.  If you didn't hear a difference before, you probably won't hear one now.  Either way, it's at least a bit more pleasant to listen to now.

https://www.dropbox.com/sh/w1d5wujkdhb5fpf/AAAQc7W2XZe1E_4EiphDwMHMa?dl=0
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Offline Ozpeter

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Re: Tascam DR-70D - modification discussion
« Reply #57 on: August 07, 2015, 09:01:29 PM »
It seems to me that this mod is coming down to the question - "would you prefer to have a modded unit or a non-modded unit?" - which allows for all personal preferences and prejudices and reasons for satisfaction, logical or not, and can't be argued with.  I admire the tenacity, ingenuity and hard work put in by those testing and comparing, however!  It could well have thrown up evidence of major differences, had they been present.

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Re: Tascam DR-70D - modification discussion
« Reply #58 on: August 07, 2015, 10:03:57 PM »
It seems to me that this mod is coming down to the question - "would you prefer to have a modded unit or a non-modded unit?" - which allows for all personal preferences and prejudices and reasons for satisfaction, logical or not, and can't be argued with.  I admire the tenacity, ingenuity and hard work put in by those testing and comparing, however!  It could well have thrown up evidence of major differences, had they been present.

That's a fair assessment.  I think the sample size is far too small and the testing far too limited and compromised to make any kind of definitive judgement though.
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Offline MakersMarc

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Re: Tascam DR-70D - modification discussion
« Reply #59 on: August 09, 2015, 12:28:40 PM »
It seems to me that this mod is coming down to the question - "would you prefer to have a modded unit or a non-modded unit?" - which allows for all personal preferences and prejudices and reasons for satisfaction, logical or not, and can't be argued with.  I admire the tenacity, ingenuity and hard work put in by those testing and comparing, however!  It could well have thrown up evidence of major differences, had they been present.

That's a fair assessment.  I think the sample size is far too small and the testing far too limited and compromised to make any kind of definitive judgement though.

Having owned a ton of Oade modified gear, I have to say that if your ears like it, it was worth it, specs notwithstanding. Pleasing audio doesn't always relate to specs. You can go nuts trying to justify your mod. I'm pretty sure, for example, that I trade some detail for warmth. I could go with the flow and run a v2 behind my 4022s to bring out the detail, but I tried it and didn't like it for anything for bluegrass. I have zero doubt that the v2 out specs the 148, no offense to Doug. Specs weren't the point to his build I don't think. Same deal with an ad2k vs. a mods m1. Just liked the sound, damn the specs.
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