Tascam DR-70D 4-channel audio recorder (Part 3)

[voltronic]

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.

[voltronic]

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.

[Life In Rewind]

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)

[voltronic]

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.

[voltronic]

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.

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.

[voltronic]

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.

[voltronic]

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.

[leehookem]

Closing this one down folks.

Part 4 found here