Topic: DIY at8538; at8533 (or similar) power modules??

[Scooter]

I'd like to come up with a DIY solution similar to the at8538.  I'm not interested in the impedance matching, just the voltage reduction.  It seems that you would need a voltage regulator in each channel to drop the 48v dc down to 9v.  The problem is that the 7809 and 713 can only handle 35-40v.  Would I need to wire up a voltage divider before the VR to loose some voltage that way??  I was thinking about mounting the electronics in a box with XLR in/outs, and just using reg mic cables.  BTW these will be used to power the Teac version of the Nak 300's.  The mics are already modded to bypass the batts.  I searched a bit, but didn't find what I was looking for...

[Scooter]

Thanks for your responce!

These are Tascam mics, not AT's.  The output impedance used to be 200r, but I have no clue what it is now that I've moded them.  I'm guessing it's whatever the output Z of the tranny is in conjunction with that 1.2k res.

here's what I'm dealing with as far as the mics go...



Is this the schematic of which you speak??



Two questions;
How do I determine the value of R4??

It's a bit hard to tell from the drawing which wires are connected and which jump over other wires, can you elaborate on that??

Is this version correct, as far as the above question?

[Scooter]

Sweetness .

like so?...




I wonder if I should pursue non-elytics for those 10uf caps   Hmmmm. 

[Scooter]

yep, those would be rather large caps .  I'll try the bypass trick.  Thanks for all your help on this man, big +T

[Chuck]

This is definitely a cool little circuit.
I wonder how to modify it to make it a 9v battery box (not run on 48v phantom power) that stays balanced to the Franken-Naks and is also balanced back to the pre-amp?

[Chuck]

I must be getting better at circuit design, because that is exactly what I thought.
Thank you!


edit to add:

Just one more question, if you don't mind...
The only connection I don't understand, is tying Pin3 directly to ground on the microphone end. Though, I realize if the last circuit drawing that Scooter made was right that connection is important.
Won't the battery just drain through that 1.2k resistor?
If so, I need to add a switch.

[Chuck]

Oh, sorry, yeah that was wrong.  I didn't notice at the time    Pin 3 connects to C3 and the 1k2, definitely not ground!

Is this it?

Thanks, again, I really appreciate your input.

[Chuck]

I do have some EEU-FC1J120 (12uf, 63v),  EEU-FC1J101 (100uf, 63v)  and ECE-A1JFS101 (100uf, 63v) Panasonic caps to use for DC blocking. I've been told these are fairly good for audio. I also have some BC 1mf, 63v metallized Polyester film and Seacor .1mf 250v Polypropylene metalized film caps to use to by bypass the electros.

The reason I wanted the circuit to be balanced, is because I'm half thinking about using a pair of transformers that I pulled out of Hosa MIT-176 adapters to output to an iRiver iHP-120 recorder.  The transformers convert a balanced conecction into an unbalanced connection and add some gain along the way. If that works out well, I'll look for some better transformers to replace the cheapy Hosa's.

If I put a switch in it, I can also bypass the transformers and have a simple battery box option too.

[illconditioned]

The same article didn't find a significant difference in distortion when bypassing electros with polys, which was interesting and unexpected.  Of course, since I read that last year, I've lost the link and diligent Googling has failed to uncover it

Can you please explain (again) what happens with an (electrolytic) capacitor is reverse polarized?

I've always wondered what happens with "back to back" caps.  Is one ever reverse biased (and is that bad)?  What about the case when *no* phantom power is there and just an (AC) signal is input (eg.,attenuated line input on a mic channel, or a dynamic mic with phantom off).  Will there be any "crossover" distortion as the voltage on the (pair of) caps goes positive then negative?

  Richard

[Roger Gustavsson]

The output impedance used to be 200r, but I have no clue what it is now that I've moded them.  I'm guessing it's whatever the output Z of the tranny is in conjunction with that 1.2k res.

here's what I'm dealing with as far as the mics go...



Is this the schematic of which you speak??



Two questions;
How do I determine the value of R4??

It's a bit hard to tell from the drawing which wires are connected and which jump over other wires, can you elaborate on that??

Is this version correct, as far as the above question?

Hi Scooter,

I have had some discussion on this with Chuck last month. In order to find out the correct value for R4, you need to measure to output impedance of the circuit inside the Nakamichis. I have measured two of my Nakamichis following this: http://www.jensen-transformers.com/as/as048.pdf   I used a CD-player as a signal generator (just mulitply the readout by that value, 2 Volt is common). My Nakamichis measured 475 and 556 Ohms. If you are running a balanced connection, the impedances must match in order to achieve a good noise cancellation at the mic preamp input. R1 and R2 should be within 0.4% of each other too. "Balanced" = the impedancies must be balanced. If not, most of the point in having a balanced connection is lost. I think R3 can be deleted.

At the micbuilder forum, we had a discussion on the subject too. 

Here are some ways of how to power the Nakamichis. Rather high output impedancies, not suitable for running long cables. C and D are not first choices. Some more components can be added for noise better noise rejection.


This one would be better suited for running long cables. This is for an internal 12V battery (four CR1632 lithium cells). You can place the 2k2-3k9 resistors and the battery elsewhere but you need to add some capactiors then. If you like to use 48V of phantom supply, you need to add a few compontens and increase the voltage rating of the 0.1uF capacitors. For sure, that will take up more space!


I am right in the process of modifying a couple of my Nakamichi CM-300. The microphone bodies have been cut-down. I intend to use them with a internal battery and make the output impedance balanced. I can also power them from my Nakamichi MX-100 mic preamp but then no balanced connection is possible.


Roger


 

[Scooter]

Thanks for the continued discussion fellas, always interesting to follow things like this

[Roger Gustavsson]

I think R3 can be deleted.

Here are some ways of how to power the Nakamichis. Rather high output impedancies, not suitable for running long cables.

I would not remove R3 from that schematic, it will seriously compromise the filtration of the zener noise.  In fact, I would select the 9.1V zener from a batch of 5 or 6, since noise can vary dramatically.

As far as output impedance and long cables, cables longer than 100ft would be problematic.  Anything shorter will be fine.

Schoeps uses a 6.2 V zener for voltage regulation but without that series resistor. Scott Wurcer of Analog Devices also recommended to leave out the series resistor. Yes, zeners can be noise injectors. But if Schopes can come out with a low noise that way, it should be possible. I prefer a local battery supply to a phantom supply through long cables.

Schoeps recommend a maximum cable length of 100 m (300 ft). That is with a buffered output having an output impedance of 35 Ohms. I would be surpriced if you can run Franken-Naks with 30 m (100 ft) long cables without losses from an output impedance of 500 Ohms. 30 m of microphone cables use to have about 3000-4500 pF of capacitance and I doubt the little 2SK118-O inside the Nakamichis are up to such a load. Following the Schoeps "formula" we should stay below 7 m (21 ft) of cable. I am running 12.5 m myself....


Roger

[Roger Gustavsson]

Yes, I have heard that zeners with 6-8 V are the best ones. I think Church Audio said that there is not much to gain with more than 7 V. Is that your opinion too? For the moment, I have no use for a Schoeps-like circuitry. I am more looking into Scott Wurcer's Chargeamp. It would give the gain I need and will let me run my Nakamichis into a line input with very low noise. Here a picture of my cut-down Nakamichis:


Roger

[Chuck]

Roger, those chopped down Naks look good! Congrats!

I wonder how much the output impedance differences of 475 ohms and 556 ohms makes in the real world. 81 ohms is a very small difference in resistance. As a percentage of the numbers 475 and 556 it may be more significant... I don't know.

[Chuck]

Jon, so is this drawing correct for a semi-balanced phantom power adaptor?
See attached: SchoepsPhantomAdaptor2_Final.GIF

I removed the tie between PIN3 and PIN1 on the microphone side. I'm not sure if that's correct, because you said that connection was not correct in the battery box version.

Would adding a larger capacitor value or bypassing capacitor C1 with a smaller metal film capacitor help reduce noise more?