Gear / Technical Help > Recording Gear
Tascam DR-70D - modification discussion
groovon:
--- Quote from: voltronic on July 25, 2015, 08:57:03 AM ---
--- Quote from: Colin Liston on July 25, 2015, 08:09:34 AM ---How much is the mod?
--- End quote ---
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.
--- End quote ---
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
voltronic:
--- Quote from: groovon on July 25, 2015, 11:19:38 AM ---
--- Quote from: voltronic on July 25, 2015, 08:57:03 AM ---
--- Quote from: Colin Liston on July 25, 2015, 08:09:34 AM ---How much is the mod?
--- End quote ---
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.
--- End quote ---
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
--- End quote ---
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
groovon:
--- Quote from: voltronic on July 25, 2015, 11:36:17 AM ---
--- Quote from: groovon on July 25, 2015, 11:19:38 AM ---
--- Quote from: voltronic on July 25, 2015, 08:57:03 AM ---
--- Quote from: Colin Liston on July 25, 2015, 08:09:34 AM ---How much is the mod?
--- End quote ---
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.
--- End quote ---
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
--- End quote ---
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
--- End quote ---
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
groovon:
--- Quote from: ~Jon Stoppable on July 25, 2015, 12:12:19 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.
--- End quote ---
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
voltronic:
--- Quote from: ~Jon Stoppable on July 25, 2015, 12:12:19 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.
--- End quote ---
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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