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Author Topic: Tascam DR-70D - modification discussion  (Read 19777 times)

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Offline voltronic

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Tascam DR-70D - modification discussion
« on: July 23, 2015, 07:59:43 PM »
So as to not crowd the main 70D thread with mod discussions, I'm starting this topic.  It would be interesting to hear recordings of modified units for those who have had them done.  This may help people decide whether or not such mods are worth their while.  So far, the only known mods are from Jim Williams (described on the FAQ) and Busman Audio.

My JWmod 70D arrived today, and now I have before and after recordings to share of me playing a short piano excerpt.  As I said in the main thread, this is not really a valid comparison as the ambient noise in my apartment is somewhat high (DVR box and computer in same room, appliances, etc.) and the recordings are made more than a week apart.  I did my best to keep my playing in the second recording somewhat consistent with the first recording, and other than the tempo I think I got pretty close.

For those that care, this is a Yamaha U1 upright, up against an interior wall and definitely overdue for a tuning.  I'm playing very softly, although it doesn't seem so on the recording because I was using the HIGH gain setting.

Jim established that there wouldn't really be much reduction in self-noise, but there should be an improvement in detail / transparency in the mid and high ranges.  These recordings are probably not the best to judge those things, but I had to go with something I could reasonably repeat most of the factors with.

The recording setup was exactly the same both times: CM3s in NOS, 5 feet up, 6ft away and at 3:30 position behind and to the right of keyboard.  I did this to sound like you're sitting in a chair reading the music over the pianist's shoulder.  This was direct into the 70D, running at 24/96, HIGH gain, pots at 10:30.  The ambient noise before I started playing was reading about -45 to -50 dB for both recordings.

No editing was performed on these files, other than trimming the start and end and converting to FLAC.

The files are labelled A and B, but I'm curious to see if others can tell which one is the stock unit and which one is modified.  I'll withhold my thoughts on how this turned out until we have some guesses as to which file is which.

EDIT: No more mystery - they are just labeled as what they are.  I also uploaded a noise-reduced version of both files to remove the background hum from my DVR box and fridge.
https://www.dropbox.com/sh/adfu4jc5z1r1qx9/AABMBw5LWl7rrekjqKhgQOCXa?dl=0
« Last Edit: February 06, 2016, 04:30:06 PM by voltronic »
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Offline Colin Liston

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Re: Tascam DR-70D - modification discussion
« Reply #1 on: July 25, 2015, 08:09:34 AM »
How much is the mod?
Occasionally....music mics record

Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #2 on: July 25, 2015, 08:57:03 AM »
How much is the mod?

Jim Williams does not do the mod himself - he modified his own 70D, and then made the details of it free for anyone who wants to do it themselves.  I have posted this on the FAQ page.

You need to find a surface-mount shop to do this for you, so the price will vary.  My shop asked that I source the parts myself as well.  Parts came to $27.06 shipped, and that included one extra opamp and two extra caps just in case.  USPS was $17.90, labor and return shipping $91.00.  All together it ran me $135.96.

Other shops may charge a bit more for labor.  The shop I used said they could charge me a bit less based on what they had learned from doing one of these before.  They don't want to be publicly named, as they don't really have time to do lots of these, sorry.

I think at least one person here has gotten the Busman mod.  I don't know anything about that, as he doesn't advertise it on his website.
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Offline Life In Rewind

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Re: Tascam DR-70D - modification discussion
« Reply #3 on: July 25, 2015, 09:40:12 AM »
How much is the mod?

Jim Williams does not do the mod himself - he modified his own 70D, and then made the details of it free for anyone who wants to do it themselves.  I have posted this on the FAQ page.

You need to find a surface-mount shop to do this for you, so the price will vary.  My shop asked that I source the parts myself as well.  Parts came to $27.06 shipped, and that included one extra opamp and two extra caps just in case.  USPS was $17.90, labor and return shipping $91.00.  All together it ran me $135.96.

Other shops may charge a bit more for labor.  The shop I used said they could charge me a bit less based on what they had learned from doing one of these before.  They don't want to be publicly named, as they don't really have time to do lots of these, sorry.

I think at least one person here has gotten the Busman mod.  I don't know anything about that, as he doesn't advertise it on his website.

Only True Believers need apply! ;D
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Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #4 on: July 25, 2015, 10:06:11 AM »
How much is the mod?

Jim Williams does not do the mod himself - he modified his own 70D, and then made the details of it free for anyone who wants to do it themselves.  I have posted this on the FAQ page.

You need to find a surface-mount shop to do this for you, so the price will vary.  My shop asked that I source the parts myself as well.  Parts came to $27.06 shipped, and that included one extra opamp and two extra caps just in case.  USPS was $17.90, labor and return shipping $91.00.  All together it ran me $135.96.

Other shops may charge a bit more for labor.  The shop I used said they could charge me a bit less based on what they had learned from doing one of these before.  They don't want to be publicly named, as they don't really have time to do lots of these, sorry.

I think at least one person here has gotten the Busman mod.  I don't know anything about that, as he doesn't advertise it on his website.

Only True Believers need apply! ;D

I'm not sure where your snark is directed - at the mod I got, at Busman, or at all of this in general.
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Offline groovon

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Re: Tascam DR-70D - modification discussion
« Reply #5 on: July 25, 2015, 11:19:38 AM »
How much is the mod?

Jim Williams does not do the mod himself - he modified his own 70D, and then made the details of it free for anyone who wants to do it themselves.  I have posted this on the FAQ page.

You need to find a surface-mount shop to do this for you, so the price will vary.  My shop asked that I source the parts myself as well.  Parts came to $27.06 shipped, and that included one extra opamp and two extra caps just in case.  USPS was $17.90, labor and return shipping $91.00.  All together it ran me $135.96.

Other shops may charge a bit more for labor.  The shop I used said they could charge me a bit less based on what they had learned from doing one of these before.  They don't want to be publicly named, as they don't really have time to do lots of these, sorry.

I think at least one person here has gotten the Busman mod.  I don't know anything about that, as he doesn't advertise it on his website.

Has anyone seen a schematic of the input circuit? I'm very curious, as just two dual (NE5532) opamps to cover four balanced inputs seems a bit low on the 'parts count' (not talking about the 4580 output amps here, just the 4x mic input stage). Any ideas?

Dave

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Re: Tascam DR-70D - modification discussion
« Reply #6 on: July 25, 2015, 11:36:17 AM »
How much is the mod?

Jim Williams does not do the mod himself - he modified his own 70D, and then made the details of it free for anyone who wants to do it themselves.  I have posted this on the FAQ page.

You need to find a surface-mount shop to do this for you, so the price will vary.  My shop asked that I source the parts myself as well.  Parts came to $27.06 shipped, and that included one extra opamp and two extra caps just in case.  USPS was $17.90, labor and return shipping $91.00.  All together it ran me $135.96.

Other shops may charge a bit more for labor.  The shop I used said they could charge me a bit less based on what they had learned from doing one of these before.  They don't want to be publicly named, as they don't really have time to do lots of these, sorry.

I think at least one person here has gotten the Busman mod.  I don't know anything about that, as he doesn't advertise it on his website.

Has anyone seen a schematic of the input circuit? I'm very curious, as just two dual (NE5532) opamps to cover four balanced inputs seems a bit low on the 'parts count' (not talking about the 4580 output amps here, just the 4x mic input stage). Any ideas?

Dave

Well as you said they're dual opamps, so each one handles 2 channels.  I have the datasheets linked on the FAQ.  If you look at the pin assignments, there are separate pins for 1IN-, 1IN+, 2IN-, and 2IN+ (+ and - indicating positive and inverted polarity).  So there's your two balanced inputs for each chip.

The data sheet for the LME49270MA (used in the JW mod) shows this much more clearly - see Page 2: http://www.alldatasheet.com/datasheet-pdf/pdf/174239/NSC/LME49720MA.html
« Last Edit: July 25, 2015, 11:38:21 AM by voltronic »
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Offline groovon

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Re: Tascam DR-70D - modification discussion
« Reply #7 on: July 25, 2015, 11:45:11 AM »
How much is the mod?

Jim Williams does not do the mod himself - he modified his own 70D, and then made the details of it free for anyone who wants to do it themselves.  I have posted this on the FAQ page.

You need to find a surface-mount shop to do this for you, so the price will vary.  My shop asked that I source the parts myself as well.  Parts came to $27.06 shipped, and that included one extra opamp and two extra caps just in case.  USPS was $17.90, labor and return shipping $91.00.  All together it ran me $135.96.

Other shops may charge a bit more for labor.  The shop I used said they could charge me a bit less based on what they had learned from doing one of these before.  They don't want to be publicly named, as they don't really have time to do lots of these, sorry.

I think at least one person here has gotten the Busman mod.  I don't know anything about that, as he doesn't advertise it on his website.

Has anyone seen a schematic of the input circuit? I'm very curious, as just two dual (NE5532) opamps to cover four balanced inputs seems a bit low on the 'parts count' (not talking about the 4580 output amps here, just the 4x mic input stage). Any ideas?

Dave

Well as you said they're dual opamps, so each one handles 2 channels.  I have the datasheets linked on the FAQ.  If you look at the pin assignments, there are separate pins for 1IN-, 1IN+, 2IN-, and 2IN+ (+ and - indicating positive and inverted polarity).  So there's your two balanced inputs.

The data sheet for the LME49270MA (used in the JW mod) shows this much more clearly - see Page 2: http://www.alldatasheet.com/datasheet-pdf/pdf/174239/NSC/LME49720MA.html

What you're referring to is just the standard opamp input. All opamps have a differential input (i.e. both an ' inverting' and a 'non-inverting' terminal) but that has nothing necessarily to do with the design of a balanced-input mic (or line) preamp, the better examples of which often use two or more opamps per input in order to improve performance.

(edit) In the case of a design (as here) being limited to one opamp per input, then the opamp is most likely being used in it's most basic form--which is why I'm so curious, given the apparently excellent performance.

Dave
« Last Edit: July 25, 2015, 12:34:44 PM by groovon »

Offline groovon

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Re: Tascam DR-70D - modification discussion
« Reply #8 on: July 25, 2015, 12:28:11 PM »
The problem with using a single opamp stage as a differential input is that it is a trade-off of noise performance vs. input impedance, which has a couple of effects, mainly that it requires larger input capacitors.  If input impedance is increased to allow use of smaller capacitors, then the thermal noise of the input resistors may dominate, potentially reducing or eliminating any benefit from a lower-noise opamp.

Thanks Jon, I was hoping you might be around to fill in some of the finer points. That's pretty much what I was getting at when I referred to the apparently 'minimal' design (of the DR-70 mic pres).

Cases like this make me wonder all over again how much of a part the circuit plays in what we are actually hearing, if even the simplest/compromised(?) design can give such 'unobjectionable' results.

Dave
« Last Edit: July 25, 2015, 12:57:05 PM by groovon »

Offline voltronic

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Re: Tascam DR-70D - modification discussion
« Reply #9 on: July 25, 2015, 12:30:46 PM »
The problem with using a single opamp stage as a differential input is that it is a trade-off of noise performance vs. input impedance, which has a couple of effects, mainly that it requires larger input capacitors.  If input impedance is increased to allow use of smaller capacitors, then the thermal noise of the input resistors may dominate, potentially reducing or eliminating any benefit from a lower-noise opamp.

Would the fact that this unit has digital level control affect this at all?  In a traditional mic input with an analog level control, wouldn't you have a second opamp following the gain pot?  In this case, the opamp is going to a DAC and then DSP for level adjustment.

FWIW, the input impedance of the 70D mic inputs is 2k ohms.  That doesn't seem all that high, but I doubt the input caps are very large in something this small.  I looked up a few others for comparison: the Shure FP24 / MixPre is also 2k, the SD 788 is 3.3k, and the Zoom H6 is way up at 6.8k.
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Offline groovon

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Re: Tascam DR-70D - modification discussion
« Reply #10 on: July 25, 2015, 12:39:01 PM »
The problem with using a single opamp stage as a differential input is that it is a trade-off of noise performance vs. input impedance, which has a couple of effects, mainly that it requires larger input capacitors.  If input impedance is increased to allow use of smaller capacitors, then the thermal noise of the input resistors may dominate, potentially reducing or eliminating any benefit from a lower-noise opamp.

Would the fact that this unit has digital level control affect this at all?  In a traditional mic input with an analog level control, wouldn't you have a second opamp following the gain pot?  In this case, the opamp is going to a DAC and then DSP for level adjustment.

FWIW, the input impedance of the 70D mic inputs is 2k ohms.  That doesn't seem all that high, but I doubt the input caps are very large in something this small.  I looked up a few others for comparison: the Shure FP24 / MixPre is also 2k, the SD 788 is 3.3k, and the Zoom H6 is way up at 6.8k.

~2k is pretty standard for pro mic inputs. IMO, the question should be, 'how does the actual circuit look', so as we might have a chance of knowing what a mod will likely do, before 'we' do it  ;D

Dave
« Last Edit: July 25, 2015, 12:54:18 PM by groovon »

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Re: Tascam DR-70D - modification discussion
« Reply #11 on: July 25, 2015, 12:54:25 PM »
The problem with using a single opamp stage as a differential input is that it is a trade-off of noise performance vs. input impedance, which has a couple of effects, mainly that it requires larger input capacitors.  If input impedance is increased to allow use of smaller capacitors, then the thermal noise of the input resistors may dominate, potentially reducing or eliminating any benefit from a lower-noise opamp.

Would the fact that this unit has digital level control affect this at all?  In a traditional mic input with an analog level control, wouldn't you have a second opamp following the gain pot?  In this case, the opamp is going to a DAC and then DSP for level adjustment.

FWIW, the input impedance of the 70D mic inputs is 2k ohms.  That doesn't seem all that high, but I doubt the input caps are very large in something this small.  I looked up a few others for comparison: the Shure FP24 / MixPre is also 2k, the SD 788 is 3.3k, and the Zoom H6 is way up at 6.8k.

~2k is pretty standard for pro inputs. IMO, the question should be, 'how does the actual circuit look', so as we might have a chance of knowing what a mod will likely do, before 'we' do it  ;D

Dave

I doubt you're going to get your hands on a schematic, but you could at least take your unit apart and see what you could get from that.  Cheesecadet posted a couple of pictures of his unit, but didn't show the other side of the main board where the opamps for the mic inputs are located:

http://taperssection.com/index.php?topic=172109.msg2131985#msg2131985
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Offline groovon

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Re: Tascam DR-70D - modification discussion
« Reply #12 on: July 25, 2015, 01:07:26 PM »
The problem with using a single opamp stage as a differential input is that it is a trade-off of noise performance vs. input impedance, which has a couple of effects, mainly that it requires larger input capacitors.  If input impedance is increased to allow use of smaller capacitors, then the thermal noise of the input resistors may dominate, potentially reducing or eliminating any benefit from a lower-noise opamp.

Would the fact that this unit has digital level control affect this at all?  In a traditional mic input with an analog level control, wouldn't you have a second opamp following the gain pot?  In this case, the opamp is going to a DAC and then DSP for level adjustment.

FWIW, the input impedance of the 70D mic inputs is 2k ohms.  That doesn't seem all that high, but I doubt the input caps are very large in something this small.  I looked up a few others for comparison: the Shure FP24 / MixPre is also 2k, the SD 788 is 3.3k, and the Zoom H6 is way up at 6.8k.

~2k is pretty standard for pro inputs. IMO, the question should be, 'how does the actual circuit look', so as we might have a chance of knowing what a mod will likely do, before 'we' do it  ;D

Dave

I doubt you're going to get your hands on a schematic, but you could at least take your unit apart and see what you could get from that.  Cheesecadet posted a couple of pictures of his unit, but didn't show the other side of the main board where the opamps for the mic inputs are located:

http://taperssection.com/index.php?topic=172109.msg2131985#msg2131985

I've had the bottom cover off mine a number of times. I may even go further than that now, just out of curiosity. Much can be surmised by following circuit traces--however nothing beats having the actual schematic of the PCB you're looking at.

And yes, I'll bet you're right, that we'll probably never see a proper schematic--as much as I love Tascam for their excellent and (otherwise) highly accessible products :D

That said, I would love it if Tom would prove us both wrong!

Dave
« Last Edit: July 25, 2015, 01:10:20 PM by groovon »

Offline 2manyrocks

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Re: Tascam DR-70D - modification discussion
« Reply #13 on: July 25, 2015, 01:15:53 PM »
How much is the mod?

I believe there was a post in the other thread that Busman would do his mod for $150 for TS members.  Although we know the specific parts used in the JW mod, I don't believe the parts used in the Busman mod have been described in any detail here. 

I haven't opened up my 70d to look inside.  Wonder how hard it would be to solder the JW mod parts? 

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Re: Tascam DR-70D - modification discussion
« Reply #14 on: July 25, 2015, 02:22:50 PM »
How much is the mod?

I believe there was a post in the other thread that Busman would do his mod for $150 for TS members.  Although we know the specific parts used in the JW mod, I don't believe the parts used in the Busman mod have been described in any detail here. 

I haven't opened up my 70d to look inside.  Wonder how hard it would be to solder the JW mod parts?

Cheesecadet had posted saying that Chris (Busman) mentioned Analog Devices opamps, and that he thinks his mod would sound better, that's about it.  He's selling it, so you can't reasonably expect him to give you all the details.

The opamps are surface-mount, which is much harder to do by hand than regular through-hole components.  I wouldn't even think of attempting it yourself unless you've got lots of experience with such things.  Jon has said he works with surface-mount, he can give you more insight as to why you'd want to have a professional do this kind of work.
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