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Author Topic: Tascam DR-70D 4-channel audio recorder (Part 3)  (Read 109009 times)

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Offline voltronic

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Re: Tascam DR-70D 4-channel audio recorder (Part 3)
« Reply #360 on: July 17, 2015, 12:12:58 PM »
Hi folks, since this concerns the DR-70d, I'd really love help with this: http://taperssection.com/index.php?topic=173946.0

You linked to the FAQ.  What's your question?

funny, that link takes *me* to the thread I started in the "ask the tapers" sub forum about some static on a recording that I hope is as a result of a faulty sd card and not the recorder.

Sorry, my mistake.  I had multiple things open at once.
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Offline voltronic

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Re: Tascam DR-70D 4-channel audio recorder (Part 3)
« Reply #361 on: July 17, 2015, 12:21:56 PM »
I think there was another thread that I can't find now?  Anyway, my comment is plug in the same mics to each recorder in the quietest room you can manage, and stick the mics between a couple of pillows.  The pillows won't stop low frequency noise, but we don't care about that.  Actually it is helpful, because you can use constant LF noise to calibrate gain on the two recorders (because it is either ambient or noise from the mic, which gives us a constant signal source).  Record thirty seconds of silence on maximum gain with each, and post the files.  I will analyze the noise signals and post my results.

Go look at the posts by Phil and me on page 24.  He actually did the tests suggested by both you and Ozpeter.  The problem is that the two recorders have different firmware versions that have different gain levels, so I suggested the test be repeated with matched firmware.
I am hitting my head against the walls, but the walls are giving way.
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Offline Life In Rewind

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Re: Tascam DR-70D 4-channel audio recorder (Part 3)
« Reply #362 on: July 17, 2015, 12:28:07 PM »
All the above begs the question:

Does the mod make the firmware un-upgradable? - or - even if it works - will the upgrade results be unpredictable? (or not the same as an un-modded unit)

Offline voltronic

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Re: Tascam DR-70D 4-channel audio recorder (Part 3)
« Reply #363 on: July 17, 2015, 12:53:59 PM »
All the above begs the question:

Does the mod make the firmware un-upgradable? - or - even if it works - will the upgrade results be unpredictable? (or not the same as an un-modded unit)

It certainly wouldn't prevent you from upgrading the firmware.  That is only affecting the program to the DSP chip, and it isn't touched in the mod.  It could theoretically affect things like gain levels though, as the software is written for the stock opamps.  My unit had the latest firmware update when it went out for the mod.
I am hitting my head against the walls, but the walls are giving way.
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Offline voltronic

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Re: Tascam DR-70D 4-channel audio recorder (Part 3)
« Reply #364 on: July 17, 2015, 01:29:25 PM »
Thanks for that detailed description, Jon.  That jives with the Audio Precision article posted here a few back.  I'm thinking the only way we get a definite answer is we send modded and un-modded units to you for measurement.  Maybe we all buy you a beer and then we can put this to bed. :P

I posted the details of Jim's mod on the 70D FAQ thread.  Short version:  besides the opamp swap, poly caps are added across the existing electrolytics.  Nothing else is changed.
I am hitting my head against the walls, but the walls are giving way.
- Gustav Mahler

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Offline voltronic

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Re: Tascam DR-70D 4-channel audio recorder (Part 3)
« Reply #365 on: July 17, 2015, 04:24:31 PM »
Then gain cannot change from the mod.  It must be because of the difference in firmware.

The poly caps are probably placebos, as Cyril Bateman's very careful measurements (published 10 years ago or so in Electronics World) show that parallel poly and polar electros don't reduce capacitor distortion very much at all, far less than using a nonpolar electro (or a full-sized poly, which is lowest).  The problem being that there probably isn't room inside the case for nonpolar electros because they are slightly larger (or full-size poly, which are a lot larger).

A quieter opamp can improve EIN by reducing input stage noise if the rest of the circuit is designed in a manner to allow for an improvement.  But that might only be realized at gain settings higher than would be used for condenser microphones, especially for amplified concert recording.

Hmm.  Based on what I read in a few posts here, I was under the impression that even these smaller poly caps would help reduce phantom noise since they do that better than electrolytics.  I will try to track down that article you mentioned.

FWIW, I only decided to go for this mod because I record almost 100% acoustic music, and other than marching band season, most of it is relatively quiet (choir, chamber groups) and/or very wide dynamic range (band, orch large choir).  I would expect zero audible improvement with this or any such mod at low gain, recording amplified concerts.
I am hitting my head against the walls, but the walls are giving way.
- Gustav Mahler

Acoustic Recording Techniques
Team Classical
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Offline voltronic

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Re: Tascam DR-70D 4-channel audio recorder (Part 3)
« Reply #366 on: July 17, 2015, 07:34:34 PM »
Jim referred to the stock electrolytics (that the mod adds the poly caps across) as "phantom blocking caps" which is what I was basing that assumption on, and that by adding more / better capacitance at those points, you're reducing the amount of phantom current that could leak into the signal path.  I may have misinterpreted that statement though.

I will try to digest those two articles, though they may be over my head.  Thanks again.
I am hitting my head against the walls, but the walls are giving way.
- Gustav Mahler

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Offline leehookem

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Re: Tascam DR-70D 4-channel audio recorder (Part 3)
« Reply #367 on: July 17, 2015, 10:37:29 PM »
Closing this one down folks.

Part 4 found here
www.texastapers.org


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