4 Channel Portable Ganged Gain Mic Pre with Eden Preamps for Ambisonic Recording

The links are helpful. Thank you.

I went with the Fab filter FAB1248: http://www.fabmodules.com/shop/index.php?route=product/product&product_id=50

From what I can tell, it seems it won't be a noisemaker? At any rate, it shouldn't be difficult to add filtering after the fact.

bruce0 said:

I don't know how the Expat guys support phantom switching etc, is it on the board?  Do they have a suggested schematic?

There is a GREAT set of articles on the topic from thatcorp start with DN-140 (http://www.thatcorp.com/datashts/AES7909_48V_Phantom_Menace_Returns.pdf http://www.thatcorp.com/datashts/dn140.pdf ) if the eden folks don't have a schematic for you.  DN-140 deals with lots of phantom switching approaches, but not really filtering.  What supply are you using, let us know and we can take a look at the spec and make a recommendation but I suspect you can use something like the schematic mentioned below for filtering.

Any complete schematic of a phantom supply will have filtering caps on it (in combination with the supply resistors the stabilze the supply) but they don't necessarily have to be on each line (since you are tight on space, and connecting this to a "known" type of mic. ) or mounted directly to the XLR.  Also a filter arrangement would have a resistor before the cap, as you will see in most phantom supplies.

As an example look at page one of this schematic http://www.soundskulptor.com/pdf/mp66-schematic.pdf  The phantom power has a RC network to filter it (R7, C3) and another (r6, C1, and C2) and C1/C2 caps also act as a reservoir like the one you show in the picture.

I use the soundskulptor schematic as an example because this particular one has a switched mode supply in it (it is a tube preamp and has one for 250V SMPS supply is shown on the schematic. Incidentally you can see an LC and and RC filter are in use on the 250V supply before it hits the vacuum tube.  This type of LC/RC combination is typical (for low noise rails) and  I am sure will be the case in whatever SMPS (switched mode power supply) you choose)

Click to expand...

outrecording said:

From what I can tell, it seems it won't be a noisemaker? At any rate, it shouldn't be difficult to add filtering after the fact.

Click to expand...

Exactly!  Now you designing!  Design is compromise and planning.

The link says the ripple is higher (.68% to .95% - (I.E. up to .4V) ) at low loads.  And battery powered you don't want to add load.

But you are in good shape because they say ripple is at 164KHz so just leave yourself a spot for a (5mm?) ceramic cap and a (5mm?) inductor and you should be able to stomp on any high frequency noise IF it is a problem.

Hi outrecording.
Already decided what main supply have you are going to use.
Why not use rechargeable battery packs.
With 2x3, or 2x4 Li-ion cells (18650)
That will give you 2x12, or 2x16volt.
Those boards wouldn't have a problem with 2x 8v4 either.
A lower supply voltage just decreases the headroom.
"Dealextreme" has pleny of them.
Look under "battery pack", "bycicle battery", "helicopter pack" etc.
Best to use a "protected" pack.
Some come with chargers.
Leo..

Thanks Leo.  Yes, now I'm pretty set on what my setup is going to be. I've got a "massive" 36000 mAh brick that I've been using. It puts out an almost perfect 12V. I've been running it for the past year or so with my (reportedly finicky) Tascam DR-680 with no problems. It's WAY more than I need, but I'm a subscriber to the "have more than you'll ever need" philosophy, just in case. Prob a habit picked up from my military days.

Here's a product page from the local manufacturer (a one man band) if anyone is interested. About $150:

http://www.hdb-tech.com/ENGLISH/V3-e.htm


This one says 24000 mAh, but the one I got is an older version:

http://goods.ruten.com.tw/item/show?21108107231499


Anyhow, I hope it plays nicely with my preamp. We'll see...

Sounds great. I've got the last of the parts coming in this week. (minus the case) Time to get down to some testing.

I guess at this point I should start a new build thread. The title of this thread and what's now contained within is kind of confusing I think.

I still have some questions about piecing this together. And no idea if my design layout "makes sense" to more experienced hands. That thread coming soon...

bruce0 said:

outrecording said:

From what I can tell, it seems it won't be a noisemaker? At any rate, it shouldn't be difficult to add filtering after the fact.

Click to expand...

Exactly!  Now you designing!  Design is compromise and planning.

The link says the ripple is higher (.68% to .95% - (I.E. up to .4V) ) at low loads.  And battery powered you don't want to add load.

But you are in good shape because they say ripple is at 164KHz so just leave yourself a spot for a (5mm?) ceramic cap and a (5mm?) inductor and you should be able to stomp on any high frequency noise IF it is a problem.

Click to expand...

You can if you want.i think you gan just change the subject.
Usually Build threads are for if you are going to make the boards available (are you?)

I don't think if I change it it changes, but if you do it does because you started the thread (you  may have to modify the first post or something).

I think an appropriate next step would be to make some sort of schematic (at least a block diagram).  This will help you, and help others to understand what you are doing.

I initially put together a photoshop diagram to help me plan. It's what I'm semi-proficient in. So thought I could post this as a first step. It's probably not what you're used to looking at, but the layout/design is simple enough, so hopefully it isn't confusing.

Everything is "wired up" in the pic except grounding. That would make it a picture of spaghetti for sure!

For the actual layout, this is what I came up with based on my requirements and what seemed logical at the time. Being completely new to this, there are probably some things I'm doing totally wrong and others I could do better. Hopefully I'm on the right track and can get some feedback to refine things.

My requirements for the portable preamp:

It will be sitting in a recording bag upright, so...

- nothing can be on the side that will be facing down (top side of the pic)

- inputs on the right, outputs on the left (makes sense with the way I arrange my bag

- XLR inputs a must

- smallest size I can possibly get away with (currently about the size of a Tascam DR-680 = 7.95" W x 2.12" H x 6.93" D)


Here's my idea for the layout



I'll attach Panasonic FC 47uf 63v caps on the Edens to block phantom power


Based on this, I have a few questions/thoughts:

1. I don't have much room on the right side. So I thought to put the 6k81 resisters on the stripboard where there's a lot of room. Does that make sense? I'll have 8 wires going to the XLR inputs. It seems a little excessive, but maybe not?

2. Do I need a fuse for this? (there's room)

3. How would you approach grounding? Star ground everything to the center of the case?

4. I would like to test this before ordering the case. Is it possible to assemble everything without the case and test? I'm at a loss as to where to ground everything if there's no case.

5. How did I do? 


Thanks for your input. I think I'm leaving something out. It's been one of those days! I need to drink a few cold ones and vegetate a bit.

Subject officially changed. Thanks for the idea.  ;D

bruce0 said:

You can if you want.i think you gan just change the subject.
Usually Build threads are for if you are going to make the boards available (are you?)

I don't think if I change it it changes, but if you do it does because you started the thread (you  may have to modify the first post or something).

I think an appropriate next step would be to make some sort of schematic (at least a block diagram).  This will help you, and help others to understand what you are doing.

Click to expand...

looks reasonable.

thoughts
you can move the 6.81k resistors out to sit on each xlr (better for noise and space)
you can put little 100pf c0g or npo ceramic caps between pin 1 and each sig leg (rf filtering) tradition and space say solder them right to the XLR.
you currently switch 48v for all 4 inputs together, with no indicator led.  is this ok?  (good for power usage and space)

you currently have no power indicator or power switch.  I would encourage you to add at least the switch because it would save battery if you could turn this thing off, but I also think a power indicator of some kind might be worth it if you can afford the amps (little red or yellow led, in a bag, would be easily visible with 10ma) because it might remind you to turn it off.

you will want to think about filter caps on the +/-15V and the 48V.  dont know what is on the eden boards locally they very well may filter the power, 

If you have trouble with noise you might want to put an LC filter on each BEI15 rail, but op amps have great common mode PSU noise rejection so not sure you need it.

but for the 48V it is a different story.  Phantom powered things have LOUSY PSU noise rejection so you probably want 150R in followed by a 220uf 63v cap minimum for the whole rail.

As i mentioned before an LC filter before that RC filter to filter HF switching noise is not a bad idea.  That filter should be near the fab1248 (so as not to carry the HF around inside the box).  Rule of thumb the filter should have a corner frequency that is 1/10th the switching frequency.  So you have a 164K Hz switching frequency so filter 16K hz or lower. 

The current you run through the 48V is very low (MAX into 4 short circuits is 56mA) so you can use a pretty small inductor.  A 100uH inductor followed by a 10uF cap to ground would filter with a corner frequency of about 16K, and if you follwed that with the 150R/220uF RC filter you would stomp the HF noise and have a nice stable phantom supply.  To give you more choice in inductors larger Henries is fine..up to 1000uH is fine and would filter "better", but I am not sure you could ever tell, because these filters are steep.  Don't worry too much about Q factor on the inductors (if you are selecting) that just indicates the inductor has resistance and lower q factor will actually reduce the resonant peak that is inherent in these filters.  Pretty much any little inductor that will handle 56mA is fine.  ( If it has too much resistance you have to be aware of the voltage drop across it, but it is going to be in series with that 150R resistor... you can always just lower that value if the inductor has too much drop)

When 48V is on you can't put 48V on the eden inputs so you will want coupling caps there (47uf max?) but look at the eden schematic EDEN may have these onboard

as far as grounding is it balanced out? assuming yes, pick a ground point on your (metal) chassis, 18 - 22 gauge solid wire ground connects 4 sleeves (shield) on output to each other then chas, 4 pin 1's on input to each other then chas, preamps use same chas gnd (assuming balanced out), filter caps use same gnd. 

the grounding of the dc to dc converters is different, the bei15 is isolated and the dc- of it's input should not be connected to the ground I think.  See it's data sheet.

As far as the F'ing fab board, they don't provide a schematic, I have no idea, and DIY without schematics just makes me fume... I fume almost like I am smoking... like a device without a fuse that has a wiring problem....and that's another thing! ;D . 

Back to the cursed Fab board... Is it isolated? Who the hell knows, they don't tell you nuthin... (don't get me going... the schematic... it is a big secret... copy of like 10,000 other boost converters.  Of course that doesn't explain why they are on rev 2 and still have significant ripple on the output.  I think they better get better at copying....)... I would start out with the assumption that it was, ( or call the guys that make it maybe they will put a datasheet online.  I sent them an email and got no reply) and wire it up the way the BEI15 is wired.

The reason that you want to keep the grounds of the 12V supply separate is that you really don't want your car cigarette lighter or the switching supply current draw, polluting the ground of your preamp (ever listened to an music player or walkman (Walkman!  I just found my old walkman, it had all these motors and touch buttons, a beautiful device... lovingly constructed of things that turned... I digress) plugged into a car lighter...bzzzzzzz). ( In a battery powered situation neither of the battery leads is a safety ground so I don't think they need to be connected for safety (wiser folks might chime in, but the BEI15 datasheet tells you to hook it up isolated and I would try that for both of them))

Your drawing is GREAT and a necessary part of designing a device, thinking through the details (like the way the 48V wire runs a long way after those 6.8K resistors makes for a high impedance loop of non-trivial size that looks very antenna like.  This fact is VERY obvious on your drawing, but I never imagined that it would be wired that way.  Usually the resistors are near the XLR's but that is hard to do when the XLR's are not board mounted, so ... your drawing exposed this and now it is easy to address).

But... a schematic will help you think through and is also needed.

But you don't have to draw all the channels. 

Tradition would have you draw two schematics

1) A single channel of the input/preamp/output (showing only a single deck of the attenuator).  You could "black box" the EDEN and reference it's schematic (if they provide one), or copy it's schematic into yours and it would be annotated (Channel 1 of 4... or Channel 0 of 4 if you are a computer guy).

2) A PSU schematic.  This would show the DC in jack, power switch, FUSE (Hey... Yeah... what about a fuse!), and DC-DC converter (possibly black boxes).  It would also show the power rail filtering and input reservoirs if any.

Schematics make a lot of this stuff easier to see, and as a next step to your diagram are appropriate.  If you tried to add all this stuff to your diagram it would become a mess of detail, the schematic can hold that detail without presenting a mess. 

There are lots of schematic drawing software packages, Eagle works really easy on both PC and Mac OSX.... And for small boards (100x80mm?) it is free.  You don't even want a board maybe, but if you did, you could self etch or OSHAPark it with eagle pretty easy at that size.

Kicad is open source, free and powerful and unlimited but a little harder to get a handle on and really runs best on UNIX and Windows.  It is terrible, but barely functional, on OSX. 

If you are a mac guy, go eagle, if you are PC guy... and might ever want to do anything with large boards, learn Kicad because eagle is like $700 a seat or more for large boards.  I am a mac and linux guy, with 200 schematics in Eagle, and I wish like crazy that I started in Kicad... but I still love to use Eagle.


And lastly... I think this is a VERY cool project.
It makes me want to try Ambisonic recording.
And you are moving forward and doing it... which is very cool and proves that you understand that

Perfect is the enemy of good.

The most sensitive parts of your circuit are the connections from XLR & Crosspoin to Eden.  The leads should be AS SHORT AS POSSIBLE AND TWISTED.  Don't hang ANYTHING not required to these .. especially long lengths of wires.  So ..

bruce0 said:

you can move the 6.81k resistors out to sit on each xlr (better for noise and space)

Click to expand...

Do this.

you can put little 100pf c0g or npo ceramic caps between pin 1 and each sig leg (rf filtering) tradition and space say solder them right to the XLR.

Click to expand...

These caps MUST be soldered on the XLR and CHASSIS.  XLR p1 DIRECTLY to CHASSIS

Look at the THAT datasheets & application notes.

you will want to think about filter caps on the +/-15V and the 48V.  dont know what is on the eden boards locally they very well may filter the power, 

If you have trouble with noise you might want to put an LC filter on each BEI15 rail, but op amps have great common mode PSU noise rejection so not sure you need it.

Click to expand...

DO IT on all 3 supplies AT the Murata & FAB.
_________

Is the FAB isolated from the 12V input?

Is Eden balanced out?

Thanks a lot of the detailed reply Bruce and Ricardo. Rather than trying to put everything in one post, let's take it one part at a time.

Starting with the RF filtering

Yes, the Edens are balanced out. If I'm reading you right, then...

Put two 100pf ceramic capacitors, c0g or npo, connecting pins 1 & 2, and pins 1 & 3; and pin 1 to chassis. All pin 1's should be connected by 18 - 22 gauge solid wire.

- 50v caps should be fine?

- Can I use stranded wire for the Pin 1's to chassis?


Basically as outlined on page 20 of this datasheet:  http://www.thatcorp.com/datashts/AES129_Designing_Mic_Preamps.pdf

Looking at that page, it shows a 470 pf cap at amplifier input pins to reduce differential high frequencies from both internal and external sources.

Would you advise putting this in?

Power Switches

That part I didn't make clear enough. Apologies.

I'm planning to use a single DPDT switch to (ON - OFF - ON) to handle both phantom power and preamp power.

One question on this. Is there a generally accepted place to add the switch?

By this I mean, my 12V input is split so I thought to have it switch on each leg of the 12V before hitting the two converters. In all the schematics I see, it shows the phantom power switching at the 48V.

I think I'll hold off on an LED for now. I've been pretty good at shutting everything down before hopping to the next recording place. The Tascam is all lit up, which reminds me to shut it off before heading out. As a matter of habit, I turn off the battery as well. I'll add it later if I find I'm forgetful.

RC Filter and LC Filter

Put 150R followed by a 220uf 63v cap on my +48V and +15V and -15V (RC Filter)

So the RC filter is needed for sure, right?

And possibly put an LC filter (100uH inductor followed by a 10uF cap to ground) on phantom power if I'm getting some noise. And same for my 15V rails if needed.

So maybe don't include an LC filter for now, but leave some space in case I come up with some noise.

Am I following you correctly?

Misc

- I'll move 6k81 resistors over to the inputs. I was initially worried space would be tight. But I can move the Edens and the attenuator over a bit to accommodate them (and the 100pf caps) if necessary.

- The Edens have a space to put the phantom blocking caps. I'm planning to use 47uf 63V caps there.

- I was advised by Expat Audio to put 100KOhms to GND across the inputs to avoid (possible) oscillation and shutting down if no cable is connected. Some people have had issues with this I guess.

- Oh! And a fuse. I'll put one on the stripboard before the 12V splits. Would that be the "ideal" location? The Murata recommends a 4A fuse before it. That single 4A fuse would probably be enough to cover the FAB as well?

Let me read over all this again and see if I'm missing anything. I'll try my best to come up with a proper schematic(s).

outrecording said:

Thanks a lot of the detailed reply Bruce and Ricardo. Rather than trying to put everything in one post, let's take it one part at a time.

Starting with the RF filtering

Yes, the Edens are balanced out. If I'm reading you right, then...

Click to expand...

Good, then grounding advice for the outputs is correct.  If you do unbalanced out, then the grounding of the output connector changes a bit.

outrecording said:

Put two 100pf ceramic capacitors, c0g or npo, connecting pins 1 & 2, and pins 1 & 3; and pin 1 to chassis. All pin 1's should be connected by 18 - 22 gauge solid wire.

Click to expand...

Yes wire should run along the xlrs, daisychain style, short and fat.
Solid is best, and tradition as well, stranded is OK, and will work.
Fatter is best, getting a good solid ground is important in controlling ground noise, the fat wire "bus bar" is part of that concept.
[/quote]

outrecording said:

- 50v caps should be fine?

Click to expand...

Signal filter caps.

Yes, but you will find that more electrolytics are available at 63V.  The main thing is that 48V is between pin 1 and the sig pins, and between the inputs and the amps, and those caps should be rated to handle it.

Power filter caps, no.  It will probably be fine, but if you want to stay in spec, the cap you use for the 48V rail LC filter should be Low ESR and rated 63V because the "ripple" that you see on there from the 164k inductor might have peaks well above 50V. You might put a little ceramic cap in parallel, or even do it with a ceramic.  Depending upon how bad filtering is it might be much higher.  63V is fine.

outrecording said:

- Can I use stranded wire for the Pin 1's to chassis?

Basically as outlined on page 20 of this datasheet:  http://www.thatcorp.com/datashts/AES129_Designing_Mic_Preamps.pdf


Looking at that page, it shows a 470 pf cap at amplifier input pins to reduce differential high frequencies from both internal and external sources.

Would you advise putting this in?

Click to expand...

As far as the RF filter on the front of the preamp (internal) you should check the eden, that may already exist on the board.  The phantom power switch you have already decided will do all 4 channels, the 33uf caps protect the input but might be on the board, but do take a look at the phantom power "drain" on that sheet (the C1 and C2 get charged when you turn on phantom.  If you turn it off, and there is not a phantom mic connected, it STAYS CHARGED. Thus the ground on the phantom power switch.  This is why I asked for a schematic from you, these details get sorted out, and there are lots of ways to do it.


I am sorry to say I don't know why solid wire on XLR pin 1's but it is tradition, specified in every spec I have ever respected for constructing boxes, and consoles all use solid bus bar to make the ground.  The issue is that ground values (the voltage from some theoretical perfect reference of the wire that we call ground (to keep myself out of trouble with the ground zealots) vary because ground has resistance.  The goal is to make that resistance very low.  But practically in a tiny little solid metal box with 4 inputs, keeping grounds the same is probably easily achieved. 

What do I think the key noise issues you will be addressing in your box?  1) getting quiet 48 out of a switching supply. 2) keeping switching supply noise from effecting things inside that little box.

Thus, the tightly twist advice above is great advice, and very relevant for your situation.

To (mis) quote whitlock tightly twist means tightly, 5 twists an inch, balanced signal pair only don't include the ground in the twisted pair.

outrecording said:

Power Switches

That part I didn't make clear enough. Apologies.

I'm planning to use a single DPDT switch to (ON - OFF - ON) to handle both phantom power and preamp power.

One question on this. Is there a generally accepted place to add the switch?

By this I mean, my 12V input is split so I thought to have it switch on each leg of the 12V before hitting the two converters. In all the schematics I see, it shows the phantom power switching at the 48V.

I think I'll hold off on an LED for now. I've been pretty good at shutting everything down before hopping to the next recording place. The Tascam is all lit up, which reminds me to shut it off before heading out. As a matter of habit, I turn off the battery as well. I'll add it later if I find I'm forgetful.

Click to expand...

Ok, so the power will always be on for phantom. That is fine, but I wonder about draining the 48V, please do a schematic.

You only have one 12V supply, you only need 1 spst switch, as shown on the bei15 datasheet (a very clear schematic of how to switch and ground it exists there).  Then split the wire (AFTER) the switch, to power both converters. 

Put a FUSE in there where the schematic shows.


Lets talk about 48V.  Is your plan NEVER to use this preamp with a non phantom powered mic?

outrecording said:

RC Filter and LC Filter

Put 150R followed by a 220uf 63v cap on my +48V and +15V and -15V (RC Filter)

So the RC filter is needed for sure, right?

And possibly put an LC filter (100uH inductor followed by a 10uF cap to ground) on phantom power if I'm getting some noise. And same for my 15V rails if needed.

So maybe don't include an LC filter for now, but leave some space in case I come up with some noise.

Am I following you correctly?

Click to expand...

Ricardo wants filtering on all the rails, I agree, it is what I would do.  But you might peak at the converters to see what they have on them.

They are all HF converters, minimum I would put LC and RC filter on 48V and and RC filter at 15V+ and 15V-.  They are tiny, and cheap, and easy, I would do them all.

outrecording said:

Misc

- I'll move 6k81 resistors over to the inputs. I was initially worried space would be tight. But I can move the Edens and the attenuator over a bit to accommodate them (and the 100pf caps) if necessary.

- The Edens have a space to put the phantom blocking caps. I'm planning to use 47uf 63V caps there.

- I was advised by Expat Audio to put 100KOhms to GND across the inputs to avoid (possible) oscillation and shutting down if no cable is connected. Some people have had issues with this I guess.

- Oh! And a fuse. I'll put one on the stripboard before the 12V splits. Would that be the "ideal" location? The Murata recommends a 4A fuse before it. That single 4A fuse would probably be enough to cover the FAB as well?

Let me read over all this again and see if I'm missing anything. I'll try my best to come up with a proper schematic(s).

Click to expand...

The fuse on the murata will cover the whole thing if the switch wiring is correct.  You are just trying to stop it from burning up your bag if something goes short or bad.  4A is probably too much (holy cow too much), but it depends upon how the converters draw and the size of the cap you put on the front of the converters (you see that on the BEI schematic?).  Probably the amperage of the fuse should be (eden mA draw times 4 plus 56mA ) times 2 or I don't know 500ma?

100pf caps are the size of a grain of rice, and are pretty insensitive to solder heat, so just slap them between the pins on the back of the xlr, they take no additional room.

If the 100K is across the inputs beyond the caps, and you don't have a phantom switch, then the Phantom power 6.8k resistors are plenty of pin-to-pin connection (I.E. you already have a 13.2K resistor there.  If they mean inside the blocking caps... well then maybe you do need it.  But did I mention that a schematic would help

bruce0 said:

Lets talk about 48V.  Is your plan NEVER to use this preamp with a non phantom powered mic?

Click to expand...

Getting late here, but wanted to reply on this one. Save the rest for tomorrow. 

I'm going to try and set it up so the DPDT switch behaves like this:

(Preamp ON) - (OFF) - (Preamp & Phantom ON)

My Summit preamp has one switch that works this way. Real convenient and saves a little space. I opened it up and found a DPDT doing the job.

I tried it out with directions how to wire it (reminds me I gotta say thanks to the poster who helped with that) and it's working ok - according to my DMM at least.


Schematic coming soon!    Promise