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

[LeeYoo]

Hi outrecording.
I sort of follow this thread because I have recently designed a 4-mic preamp board (3"x4")
Same three-chip design per channel as the eden preamps, but with full input/output protection and onboard phantom supply reg.
90% completed picture of a test board attached.
Those FOUR mic preamps draw only 70mA (measured).
I think that 80mA each is what a channel COULD draw when is has to drive a long XLR lead.
I think 200mA for four preamps is enough.
Leo..

P.S.
Had questions about the Goldpoint attenuator, but I found the answers in another post.

 

[ricardo]

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.

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.

No.  Regardless of whether the Eden boards have RFI, you need the 100p ceramics AT the XLR.  Use the shortest possible leads and wire p1 DIRECTLY to chassis.  Ideally the ceramics decouple p2 & 3 directly to chassis.

Even 2" of wire between the XLR and the ceramics makes them useless for RFI.
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By doing this however, you are making the circuit connection to CHASSIS at the input.  This means your star earth point also has to be at the input.  Bring the grounds from your supplies to this star point and also the Clean & Dirty earths from your Eden boards.

The supplies themselves should have been well filtered AT their modules.  Do any extra RC or LC there.  The 48V +ve & -ve leads should NOT have any of this decoupling current.
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But then your output ground isn't connected to CHASSIS and will conduct RFI into the box.  The cure is 100n ceramic at each output socket ground directly to chassis.  100p from tip & ring to chassis is good too.
_________________

LeeYoo, the Goldpoint 'attenuator' is used as the gain setting feedback resistor(s)
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Bruce, that's an excellent THAT lecture.  Outrecording, worth downloading all the THAT DNs & articles it mentions.

 

[outrecording]

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)

I'm making my schematics now. Nearly done with the PSU schematic, but I'm having difficulty determining the values of my LC filter for the 15V rails. I've been trying several calculators and I think I'm putting the wrong numbers in.

The murata has a switching frequency of 330KHz. So corner frequency would be 1/10th of that...33KHz.

Using this calculator: http://www-users.cs.york.ac.uk/~fisher/lcfilter/

I input Butterworth....Lowpass
Order: 2
Corner Freq: 33000 (Hz)
Characteristic Impedance: ??
Series

The impedance is what I'm probably getting wrong. I initially thought it was asking for 75ohms (using ohms law; 200mA draw from Edens at 15V) but that gives me some odd values. Maybe I'm not understanding what value they're looking for here.

Where did I go wrong?

- For the LC/RC filter caps on the 15V rails, since the Murata is floating, do I just connect them to chassis?

 

[outrecording]

Hi outrecording.
I sort of follow this thread because I have recently designed a 4-mic preamp board (3"x4")
Same three-chip design per channel as the eden preamps, but with full input/output protection and onboard phantom supply reg.
90% completed picture of a test board attached.
Those FOUR mic preamps draw only 70mA (measured).
I think that 80mA each is what a channel COULD draw when is has to drive a long XLR lead.
I think 200mA for four preamps is enough.
Leo..

Wow, that's a heck of a small footprint for all that's in it! I'd love to hear how it sounds when you get it finished. Is that just for a personal project?

 

[outrecording]

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.

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.

No.  Regardless of whether the Eden boards have RFI, you need the 100p ceramics AT the XLR.  Use the shortest possible leads and wire p1 DIRECTLY to chassis.  Ideally the ceramics decouple p2 & 3 directly to chassis.

Even 2" of wire between the XLR and the ceramics makes them useless for RFI.
_________________

By doing this however, you are making the circuit connection to CHASSIS at the input.  This means your star earth point also has to be at the input.  Bring the grounds from your supplies to this star point and also the Clean & Dirty earths from your Eden boards.

The supplies themselves should have been well filtered AT their modules.  Do any extra RC or LC there.  The 48V +ve & -ve leads should NOT have any of this decoupling current.
_________________

But then your output ground isn't connected to CHASSIS and will conduct RFI into the box.  The cure is 100n ceramic at each output socket ground directly to chassis.  100p from tip & ring to chassis is good too.
_________________

LeeYoo, the Goldpoint 'attenuator' is used as the gain setting feedback resistor(s)
_________________

Bruce, that's an excellent THAT lecture.  Outrecording, worth downloading all the THAT DNs & articles it mentions.

Thank you Ricardo. That makes sense and I'll do that. And yes, the THAT DN article was well worth the read and explained everything well.

When you say the Eden's "clean & dirty" earths, what do you mean? (the clean & dirty part I mean)

 

[ricardo]

When you say the Eden's "clean & dirty" earths, what do you mean? (the clean & dirty part I mean)

I always define 3 earths; Clean, Dirty and Chassis.

Consider carefully what you are doing when you connect anything to an earth.  If you connect supply decoupling capacitors to an earth line, you make it Dirty.

Clean earth is used ONLY for input / output earths and feedback components.

Dirty earth carries our supply decoupling currents.  The 3 earths are joined ONLY at one point, at the input in your circuit.  Output current does not upset the Clean input and feedback paths.

Some of you will recognise this as similar to "Star Point" earthing in power amplifiers.

I don't know how Eden is laid out but any Low Noise circuit should have such an arrangement.

 

[bruce0]

The impedance is what I'm probably getting wrong. I initially thought it was asking for 75ohms (using ohms law; 200mA draw from Edens at 15V) but that gives me some odd values. Maybe I'm not understanding what value they're looking for here.

The impedance should be low, because it is power, 75ohms is too high. Anything lower than 10 ohms is ok probably, the lower the better because why waste power in a battery powered unit heating up the filter (and causing a voltage drop across the filter).


You don't need to be at 1/10th of the switching frequency.  Lower frequencies are fine. 


If you want to learn about spice, spice will model the filters for you and put out nice graphs of the frequency response, which generally goes out to the corner frequency, has a resonant peak and falls rapidly, which is what you want. 


Easy, you can just use the same filter on the 15V as you do on the 48V but the current on the 15V rails is much higher so you should use higher current rated inductors, and you can use lower voltage caps which are smaller.

So figure out what your 4 mic preamps draw for current (this is on the spec sheet probably), and engineer for 20% more than that (traditionally).

The cap's don't care about the current in an LC filter like this, the inductors do.  Low ESR caps do a better job, or you can sister them with little ceramics.  There are little RF inductors that only handle a few milliamps, inductors that handle higher current are larger and heavier (but at this inductance and current level, you are still talking about something the size of a thimble or smaller, and weighing as much as a coin).  So just find an appropriate rated inductor, a small one, and a low Q factor (which implies the resistance) is fine because it tends to reduce the resonant peak at the corner frequency.  When choosing inductors take a look at the self resonant frequency and make sure it is not anywhere near the switching frequency (this is unnecessary advice because your switching frequencies are low and the self resonant frequency (SRF) is usually megahertz in these little inductors.  You don't need a shielded inductor, those are used in the switching PSU however use what you have it won't help but it won't hurt.

As ricardo points out, filter placed near the switching supply so the power running around the box on wires and traces is clean and not radiating 100K plus AC pulses near the amplifiers (which... amplify whatever they get).

 

[Rochey]

I don't know how Eden is laid out but any Low Noise circuit should have such an arrangement.

I'm partially following this thread, looking for eden specific questions.

Eden uses a single plane of GND, with ground return paths being managed by pcb layout partitioning.
48V is not generated on the board, and shouldn't be brought into the board without AC coupling caps.

/r

 

[bruce0]

I don't know how Eden is laid out but any Low Noise circuit should have such an arrangement.

I'm partially following this thread, looking for eden specific questions.

Eden uses a single plane of GND, with ground return paths being managed by pcb layout partitioning.
48V is not generated on the board, and shouldn't be brought into the board without AC coupling caps.

/r

Rochey thanks for the post.  OP is about ambisonic recording which combines the noise of several mics to produce a mono or stereo signal, so the poster is concerned about minimizing noise for the battery powered portable rig he is building.

I think some of the questions you could help with are:

1) Does the Eden have an RFI filter cap on it's inputs like the right hand red one on page 20 of this PDF?http://www.thatcorp.com/datashts/AES129_Designing_Mic_Preamps.pdf

2) We understand that space for coupling caps is provided on the eden board, correct?

3) We understand that the audio ground and PSU decoupling ground paths both go to the ground plane, so there is one ground connection to the PSU for the Eden, correct?

4) What does a single eden preamp draw?  This is for sizing the PSU filters on the SMPS DC-DC converter, and for sizing the fuse.

5) Are the Eden’s balanced or unbalanced out?

6) What PSU filtering exists on the board, just 100nf opamp caps, or more?  We have been discussing what type of filtering to put on the SMPS rails, and don’t want to duplicate what you have on board.

7) Is there phantom discharge/short/asymmetric short protection on the board (such as those described in the Phantom Menace series on Phantom Power protection for mic preamps)?  If not do you have a suggested Phantom switch circuit for the Eden?

 

[Rochey]

Okay, rolling my sleeves up...

1) Does the Eden have an RFI filter cap on it's inputs like the right hand red one on page 20 of this PDF?http://www.thatcorp.com/datashts/AES129_Designing_Mic_Preamps.pdf
Try this: http://expataudio.com/diy/edenmicpre/ina163pre-layout2%20-%20PG2.1-sch.pdf
2) We understand that space for coupling caps is provided on the eden board, correct?
Yes, there's a little space.
3) We understand that the audio ground and PSU decoupling ground paths both go to the ground plane, so there is one ground connection to the PSU for the Eden, correct?
Yes... but from what I've read recntly, Audio should only coming in hot and cold. no gnd. Shouldn't that signal go directly to the star?
4) What does a single eden preamp draw?  This is for sizing the PSU filters on the SMPS DC-DC converter, and for sizing the fuse.
Mousefarts of power. I think I calculated around 30mA or 40mA each? less if you remove the power LED's.
5) Are the Eden’s balanced or unbalanced out?
Balanced.
6) What PSU filtering exists on the board, just 100nf opamp caps, or more?  We have been discussing what type of filtering to put on the SMPS rails, and don’t want to duplicate what you have on board.
10uF electrolyitic per rail, plus a few 0.1uF ceramics.
7) Is there phantom discharge/short/asymmetric short protection on the board (such as those described in the Phantom Menace series on Phantom Power protection for mic preamps)?  If not do you have a suggested Phantom switch circuit for the Eden?
Phantom power implementation is left as an exercise for the reader however, there are external ESD protection diodes on the inputs, to protect from such surges. (having been burning by 48V surges)/

 

[bruce0]

Awesome... Thank you for the reply. 
And especially thanks for the schematic, in my earlier post I stated that I didn't know if it was available.

That all sounds good.

Regarding your question about ground will run from your 4 boards to the star.  The issue comes when looking at other grounds ... ones that should be clean, like when someone uses a TS connector on output, and ones that are full of noise, like those that filter the spikes out of the SMPS, and how to arrange them.

 

[ricardo]

Rochey, thanks for the Eden schematic.

There appear to be 3 separate earth connections to the PCB .. input, output & the +/-15V supply.

Which if these is closest (physically)  to the 2x10u?

Do you have a recommended earthing scheme for Eden?  Where do you suggest we put the Chassis connection?  What do we do at the other end of the PCB ?

 

[Rochey]

PSU

 

[outrecording]

(comments on Eden's lack of grounding at bottom of post)

I wanted to put these schematics I've drawn up out there for your opinions. Apologies if they're too large. Any smaller and I worry you'll be reaching for your magnifying glasses. 

My caveat: this is my first set of schematics. I tried my best.

Heading out to make the wife happy (shopping); I'll comment on some of the earlier posts later. Thanks!

One thing to note:

I don't have a chassis ground on the Eden yet. There are technically four grounding points (after putting the 100k to Post-Cap ground; although according to the Eden schematic, the input and Post-Cap grounds look to be connected?)

I'm unsure if I should just be grounding from one place or all of them to chassis?

 

[bruce0]

Looking at values, you need to be concerned about the RC filters and voltages.

There are some large drops across the Resistors R2, R3 and R4.

The corner frequency for 150R/220R is about 5hz, so that is ok, well out of the audible spectrum.

Phantom:

But if you draw 56ma (14*4) through R2 you will drop 8.4 Volts.  Of course you will never draw that much except if you short all the phantom power to ground.  Typical is 4-7ma per mic or less, more if there is an LED or for some DI's.  So engineering for 24mA should be fine, and the voltage drop would only be 3.6V.

You can solve the issue in various ways.
0) ignore it, because you are still getting 44V which is fine for all phantom power mics
1) increase the voltage output by the FAB1248 to 53V or 54V
2) reduce the value of the resistor increase the value of the cap.  For instance use a 1000uF cap with a 33R resistor.  Same corner frequency, much lower voltage drop.
3) put separate filters on each mic connection.  This can keep things low profile (because the components are smaller) but probably takes more space overall.  It depends where you have space.

Most phantom power mics will run on a broad range of voltages, 52V is usually the max spec'd but some will run on very low voltages down to 12V or 18V etc.  However as the voltage falls the maximum signal level handling of the mic head preamp so if you intend very high SPL usage then use a low current mics or modify the filter for less voltage drop.

I would choose quiet stable phantom rail over maximum voltage.

I rarely mike loud guitar amps, and in an ambisonic mic you would never close mic anything to my understanding (they are for free field kind of use right?).  So lower phantom voltage is not a big deal.

You can also do less filtering, especially if this will always be run on battery then the likelyhood of low frequency (50hz or 60hz) noise is much reduced.  So you have good filtering on the HF end (where the SMPS is making the noise) and you could raise the corner frequency.

As for R3 and R4, I think there the voltage drop is a big issue.  You are getting a load of 120ma, and across a 150R resistor that is too much drop (150 * .120 = 18V drop).  So you need a bigger cap and a lower value resistor.  But you can use low voltage caps which are smaller, so this should not present a size problem.  So for instance you could output 16V and engineer for a 1V drop 1/.120 = 8.3R... so use a 10 ohm resistor with 3300uF or LESS (because eden and other opamp based amps are good at common mode noise rejection they DON't NEED power to be as quiet).  There are several, they are not that big.  Here is a 3300uf search in mouser

http://www.mouser.com/Passive-Components/Capacitors/Aluminum-Electrolytic-Capacitors/Aluminum-Electrolytic-Capacitors-Leaded/_/N-75hqw?P=1z0x6edZ1z0wrk0


You may have to experiment with fuse value depending upon how the DC-DC converters behave as they charge the caps, or slow blow fuse.

That on off control looks unconnected.


On the channel

You do have the opportunity to make the switched resistor in a different style.  This way at high resistances you have many resistors in series.  This tends to make a larger path (loop of wire) and each resistor has some noise. 

This will be totally fine, and is very common and I have done it that way too. 

But you should be aware that in your case you can also just use a single resistor in each switch position.  To a common conductor that goes to the gain connection on the eden (the other goes to the switching side of the switch).


Also, I am not sure you want that Ground near R1 on the switch, it should be open, (actually as drawn it is shown as open , but it says ground so I thought I would mention it). 

The other R1 and R2 (6K81) should be selected carefully to be matched as close as you can achieve.  Using a DVM gets you much closer than 1%.


The schematic is great thank you, and allows very detailed comments (as you can see) I hope they are helpful.

One important thing on a schematic.  Each page should only use a named designator once.  Better in a whole design each designator should be used only once (for instance on sheet 1, resistor 1 should be R101 and on sheet 2 resistor one should be R201).  Keep on that way till you design something with 10,000 components in it, and then you can solve it another way.

That way when I say R1, I am referring only to a single resistor.  In this schematic I am referring to at least 3!

b

 

[ricardo]

I don't have a chassis ground on the Eden yet. There are technically four grounding points (after putting the 100k to Post-Cap ground; although according to the Eden schematic, the input and Post-Cap grounds look to be connected?)

I'm unsure if I should just be grounding from one place or all of them to chassis?

Recommendations attached.  This is a PHYSICAL wiring recommendation too.

Not ideal but forced by Eden's ground plane.

Think about restricting all SMPS currents to the PSU. 
Hence the wiring of the filter caps directly to the SMPS pins and all wires twisted.
Don't duplicate your switch to the SMPS cos that makes the LOOPs bigger.

P48V is earthed to the XLRs so its current never appears on Eden earths.
+/-15V to Eden's GND cos that pin is closest to Eden's 10u caps

You need to be very specific about naming your earths and where they are connected.

 

[ricardo]

the other bit

 

[Rochey]

Hey Ricardo, thanks for stepping in on this one

/R

 

[ricardo]

Hey Ricardo, thanks for stepping in on this one

Rochey, I'm really interested in YOUR comments too.  Maybe something you should include some similar grounding recommendations in the Eden book of words.

 

[Rochey]

My thoughts? I would have thought that not connecting to the input GND from the XLR is important... as you don't want stray current from the shield and from the case straying into your circuit. It's already differential,hot is referenced to cold.

As the 48V is handled offboard, I'm struggling to get my head around the current flow of the phantom power.

I'm assuming that your phantom power source will be intensively filtered offboard? in which case, the filtered noise on your phantom power supply should be starred back to the same place as your eden psu-ground. That should stop the filtered noise current flowing through your eden board to the PSU-GND.