That1580/1510 mini mixer

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weroflu
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That1580/1510 mini mixer

Post by weroflu » Thu Apr 20, 2017 2:46 am

Hello:

I have been noobing on a diy 6x2 THAT1580/1510 mini altoids tin sized mini mixer. I'm not sure whether to split this up into two threads since I keep waffling between which one I want to finish.

specs:

-fit everything into altoids tin
-6 mic preamps/ 3.5mm jacks
-stereo summing, unbalanced output interfacing with 5k-20kOhm portable recorders/adc.
-LCR pan
-phantom power
-phase reverse
-modular cards
-1510 based physical switched gain
-1580 based bluetooth>>rfduino gain control

What's working:
That1510 mic pre cards
LT8330/LT3012 phantom power card
2 x LTC3265 +-15V rail card
backplane
rfduino card

(P/S card for the 1580 has everything on board, P/S for the 1510 is 3 smaller boards)

P/S card for the 1580/5171:
LTM8049 micromodule dc converter >> +-17V
TPS7A4701 regulator >> +15V
LT3015-15 >>-15V
LT8330/LT3012>>P48
LT1763-3.3>>3.3V logic
(noise issues aside, it works. needs filtering and grounding work, but that's tabled for now)

Not working:
1580-/5171 preamp card. It's passing signal but distorts too easily. DC offset coming off the 1580 is about -.5V, trying to track down problem with the servo section?
Summing card schematic was wrong, fixed it and most likely a trivial fix but for now not working. I'm using a barebones arduino sketch to load 30db of gain on startup since I don't have spi variable gain control/interface working yet.

https://1580mixer.shutterfly.com/pictures/8

The first version was THAT1510 based. I got very close to finishing it, but I didn't think through the gain switch well enough. The only pots available were alpha 9mm-sized and that created some physical constraint headaches (more later) and the lack of exact channel matching led me to table that version and try to get a 1580/5171 version working.

All cards are modular: backplane, preamp cards, summing card, p/s card, rfduino breakout board card (1571 version only).

I think I solved how to make the 1510 gain switch work (fit) with a combination of the grayhill 75ap36-01-1-10n screwdriver actuated switch
(http://eu.mouser.com/ProductDetail/Gray ... DzgrKUYw==) and a mini slide switch.

screwdriver actuated for two reasons: there is no room for fingers to get around a knob with 1.5mm clearance on either side, and I thought it would be more compact keeping as many controls inside the box as possible. 10 way switch for 5db gain increments on the preamp, then a 5 way miniature slide switch for post gain 1db increment attenuation. Apem makes a mini slider that I think will fit.

Phantom, phase, and pan are all on mini slide switches. Phantom and pan are not ideal since the switches are non-shorting, but I can live with this. Similar issue with the input jacks being 3.5mm and phantom, need to be careful not to hotplug.

I am aware that since I have zero design experience that the 1580 version could very well just not work due to noise issues, or other unforeseen problems. Mixed signal routing is many levels over my paygrade, but signal passed, and gain registers were set so I'll keep plugging away on it.

Current problems to tackle on the 1580 mixer are DC offset, distorted signal with higher input level, some high frequency noise even when using two batteries for supply rails bypassing the switchmode p/s, some motorboating from the rfduino card, fix summing card (new board arrival this week), then start chipping away at all the noise issues and bluetooth interface. I originally planned for +-15V rails, but this will likely change soon since it's unnecessary when interfacing with prosumer unbalanced line level gear. Probably will tone it down to something like +-8 or 10.
Attachments
1580SUPPLY.pdf
4 rail power supply
(58.67 KiB) Downloaded 68 times
1580master.pdf
summing card
(18.86 KiB) Downloaded 66 times
1580card.pdf
preamp card
(51.04 KiB) Downloaded 67 times

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mediatechnology
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Re: That1580/1510 mini mixer

Post by mediatechnology » Thu Apr 20, 2017 11:17 am

Thank you for joining us and for your post!
1580-/5171 preamp card. It's passing signal but distorts too easily. DC offset coming off the 1580 is about -.5V, trying to track down problem with the servo section?
I had to stop at that statement to reply and admit to not having yet looked everything over.

It is normal for a THAT1580 to have a -Vbe offset at the outputs.
The servo that I think you might be referring to is a differential servo which only removes offset between the outputs.
You either need a common mode servo to remove the approximate -0.64V DC, AC-couple the outputs or have a downstream common mode rejection stage remove it.
See page 11 of the 1580 data sheet.
You really need a downstream CMR stage so that would be my recommendation.

It's a very ambitious project and it sounds like you got a lot of the key pieces ready.
So is this for DSLR audio capture?
I'll look over your attachments and drop back in.

EDIT: Wow, this is cool: https://1580mixer.shutterfly.com/pictures/20#17
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mediatechnology
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Re: That1580/1510 mini mixer

Post by mediatechnology » Thu Apr 20, 2017 11:33 am

OK.

I don't see anything obviously wrong with the 1580/5171 though I didn't trace every wire.
Some of the problems you are seeing may be layout/routing related.
I haven't looked at that.

I don't see an advantage to using a 1206 as a CMR stage.
InGenius® doesn't provide any advantage since the source impedance feeding it is low and internal to the box. (The 1580 output.)
You could use a 1246 there and it would be a little quieter.
Plus you'd save Cbootstrap.

The 1286 panner circuit is clever but you do need to be aware that the internal resistor tolerance for the 1286 is about 40% IIRC.
Since you are using external resistors with it the overall gain could vary greatly from device-to-device or lot-to-lot.

I'll study your drawings some more.

What level does the preamp start to distort at?

EDIT: Check to make sure that your differential servo isn't resonant: http://www.thatcorp.com/datashts/AES919 ... ifiers.pdf
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weroflu
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Re: That1580/1510 mini mixer

Post by weroflu » Fri Apr 21, 2017 1:59 am

Ok, I think the priority now is to pass a good signal on the 1580/5171 preamp circuit.

Thank you very much for taking the time to look into these problems, it is very much appreciated.

Re: THAT1206 vs. 1246, is it correct that even though there is no advantage there is also no disadvantage to using the 1206? I will change over to the 1246 on the next board run.

re 1286: I originally got that circuit from here...

http://www.analog.com/en/analog-dialogu ... ifier.html

So it looks like I will return to using the AD8273 due to better tolerances. AD recommends 10uf tantalums on the rails as well. Do you think these are necessary? Not a big deal to fit them on I suppose.

I'm not sure if I described the signal problem properly. I just today got a scope, so it will be a day or two before I can post some measurements. Using a 57 for crude testing, signal sounds thin and light on bass. So it might not just be distorting at higher input levels. I'm hearing distortion on the output even when the signal is below clipping.

Edit: found cold joint on that 0402 R. Now the challenge to to tack a resistor lead onto the side of a 0402 without dislodging it.

Maybe this is not the best testing method, but on the non-working card, I took the 5171 off and tried to tack on 2 0402 5k resistors across pins 3-4 and 5-6 on the 5171 footprint. I was going to add a 200r to set a fixed gain in the hopes of eliminating all of the digital control circuitry as the problem. But, I measured 5k across pins 3-4, and 19Mohm across 5-6, that one doesn't seem right.

Will check for resonance later today.

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mediatechnology
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Re: That1580/1510 mini mixer

Post by mediatechnology » Fri Apr 21, 2017 9:02 am

1) The 1206 may be a little noisier than the 1246.
So it looks like I will return to using the AD8273 due to better tolerances
2) If you look carefully at the AD8273 datasheet you will see in note 5 that the absolute tolerance is also ±20%.
For simple LCR switching I think you can come up with something simpler.

3) Tantalums as bypass are OK.

4) Soldering the 5171 by hand is a PIA. Amazed it works at all and am impressed with your SMT soldering skills.
Based on your resistance measurements its sounds like the RB gain leg is open.

5) What DC potential is the 5171 thermal pad referenced to?

6) You're really going to need a test oscillator, coupling transformer and pad to measure a mic input.

7) BTW maybe its just me but on the switcher board you might want to put ground in between the 48V and 3V3 rails. Having the 48V that close to 3V3 with those small clearances looks to be an opportunity for leakage/creep or "opportunistic" shorts between the highest voltage and lowest voltage rails. 48V to 3V3 is the path of greatest destruction.
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weroflu
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Re: That1580/1510 mini mixer

Post by weroflu » Fri Apr 21, 2017 11:04 am

I think there was a miscommunication on my part. I have two 1580 preamp cards soldered. The first one was not passing signal, or maybe extremely weak. I could not find the problem so I decided to try soldering the second one, which is working but has the possible low bass problems (Im not even sure at this point).

In an effort to salvage the first one, and also test whether the problem was caused by the 5171 i removed the 5171, and attached fixed gain resistors. This turned out to be a bust. So I think this card will be retired. Though I am looking at the costs adding up, and wondering if I should try another 1580 chip on there. Im always paranoid about too many reflows/heat damage, but also cheap and trying to save as many parts as I can which is probably not a smart idea when testing.

I tested the 2nd preamp card again with the new summing card (that seems to work), and the signal sounds actually pretty good. At this point it's too hard to trust my ears so a proper testing jig is high on the list. Noise was not bad either, just some very faint high frequency that could have been ambient, or radiated who knows. The motorboating from the rfduino was very much attenuated compared to last time, barely audible. I have no idea what's improving things.

I'm going to try for a better LCR panning circuit.

For the smt parts, the supposed hardest parts are the easiest if you get it just right. For the passives I'm using paste and hot air mainly, graduated from tormenting myself with an iron. I tried 3 different paste syringes and could not get any of them to work. The last one, a small edsysn syringe, I tried again and it worked well. It turns out it was a simple fix. I had been letting the end dry out too much, even with a cap placed. Just a few primer squeezes and the paste came out more liquid and easier to control the dose. The more liquid paste seems best to me.

This https://www.indiegogo.com/projects/pcbi ... -holder--4#/
helped tremendously. I was using a cheap alligator clip vise and it was terrible.

The bga initially I did with hot air, even though it did supposedly work (after 3 attempts) since I got good outputs on the ltm8049 I wasn't happy. Got a 10$ hotplate, did a few thermocouple measurements around the perimeter and tried another 8049. It sat down perfectly, slid it off onto a larger pcb, and that turned out the easiest of them all. Alignment was very easy since there was no air or movement while the reflow took place.

Soic, Sop's are pretty simple to do with paste, just a thin line and some hot air. Then touch up with an iron.

The qfn's were a little tricky, and I'm still sort of working that out. For the 5171 I tinned the pads, cleaned, preheated the board with hot air, then applied tacky gel flux, then placed the part with one hand and used the other for hot air, it reflowed fairly quickly and self-centered very well. 5171 Thermal pad was left floating, I guess next rev. I will connect to Vee?

I ASSumed the same would be true for the 1580, but no! I guess due to the pcb thermal pad underneath it dissipated too much heat and it would not self-center, or just barely so (just enough to plant a seed of doubt in your head as to whether it actually worked). I switched methods: tinned the underside thermal pad, tinned the top pads, cleaned, fluxed and then heated the thermad pad from below with a blob-tinned iron tip for better conduction. It self centers fairly quickly like this. I meant to ask you if this is dangerous and is more likely to damage the part. My past failures with qfn were all due to not enough heat. Depending on your answer to this I might switch over to the hotplate for both the 1580 and 5171. I'm leaning toward this anyway because both hands will be free and it's easier to poke the chip and test for centering.

I have a signal generator on the oscilloscope, just have to get a transformer and make the correct pad.

The ltm8049's are not cheap. I got two as samples, and from here on it looks like it's out of pocket, or maybe I'll try the lt rep again. The preamp cards as well are not so cheap as they are 4 layer, so I need to be a little more careful with revisions. I think for the p/s board so long as it's working I'm inclined to wait for something to blow up before I make a change. For noise issues though I would like to see if I can fit some more filtering on board.

I need to do another run on those as I messed up the summin card connecter pad spacing. I moved the two large caps on the p/s board to the chassis which allows larger values, maybe 1000uF for the bulk input and 330uF for the phantom cap - I know it doesn't do much, and need to revisit phantom for a possible rc/other filter there, but first the preamp has to work properly. There is space on the chassis around the rfduino board but not sure what I can do there. I also meant to ask you about this alternate power scheme. I updated the shutterfly page with some more images. You can see the chiclet sized ltc3265 card there, small and almost no depth to it. At one point I considered using 6 of these as plug in sub-boards on the the preamp cards for the bipolar rails. It would mean bussing Input supply which is why i didn't do it, but I suppose if the dc supply was filtered well, bussing it would not be so bad. That would make the only radiated noise source the phantom supply.

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mediatechnology
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Re: That1580/1510 mini mixer

Post by mediatechnology » Mon Apr 24, 2017 4:15 pm

indiegogo is a cool site. I got lost in there.

I don't have much experience soldering the QFNs.
I prototype with DIP adapters.

One thing I have become aware of is rising LF THD when there is a thermal void under the QFN.
The way you pre-tin it and heat from below should reduce the potential for a void.
I'm not sure that there is much harm from the thermal pad electrically floating but I wanted to make sure that it wasn't grounded since it is tied to Vee.
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weroflu
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Re: That1580/1510 mini mixer

Post by weroflu » Tue Apr 25, 2017 5:34 am

For simple LCR switching I think you can come up with something simpler.
The only ideas I came up with are a dual op amp with discrete resistors as in 1206/8273, or 4 resistors off of the sp3t switch, two center resistors adjusted to whatever pan law I want. The latter foregoes a buffer amp but I think I'd have to switch over to virtual earth summing from the current passive mix bus plus gain.

Is there an advantage to having the center channel -3.5db down as opposed to 3db as this provides .5db in gain step resolution?

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mediatechnology
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Re: That1580/1510 mini mixer

Post by mediatechnology » Tue Apr 25, 2017 8:49 am

The only ideas I came up with are a dual op amp with discrete resistors as in 1206/8273, or 4 resistors off of the sp3t switch, two center resistors adjusted to whatever pan law I want. The latter foregoes a buffer amp but I think I'd have to switch over to virtual earth summing from the current passive mix bus plus gain.
That sounds like a good plan. I would go with current summing.
That AD circuit saves a lot of parts but the resistor absolute tolerance of 40% is just too much.
Is there an advantage to having the center channel -3.5db down as opposed to 3db as this provides .5db in gain step resolution?
JR and ricardo can shed a lot of light on this subject.

My take on it is that constant voltage panning has a -6 dB dip in the center; constant power -3 dB.
Both have their merits.
A compromise is -4.5 dB.

When I worked for SSL -4.5 dB was what center was trimmed to.
That makes the constant voltage center bump +1.5 dB and the constant power dip -1.5 dB.
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Re: That1580/1510 mini mixer

Post by JR. » Tue Apr 25, 2017 9:16 am

mediatechnology wrote:
The only ideas I came up with are a dual op amp with discrete resistors as in 1206/8273, or 4 resistors off of the sp3t switch, two center resistors adjusted to whatever pan law I want. The latter foregoes a buffer amp but I think I'd have to switch over to virtual earth summing from the current passive mix bus plus gain.
That sounds like a good plan. I would go with current summing.
not sure I follow,,, do you mean virtual earth?
That AD circuit saves a lot of parts but the resistor absolute tolerance of 40% is just too much.
Is there an advantage to having the center channel -3.5db down as opposed to 3db as this provides .5db in gain step resolution?
JR and ricardo can shed a lot of light on this subject.

My take on it is that constant voltage panning has a -6 dB dip in the center; constant power -3 dB.
Both have their merits.
A compromise is -4.5 dB.

When I worked for SSL -4.5 dB was what center was trimmed to.
That makes the constant voltage center bump +1.5 dB and the constant power dip -1.5 dB.
ASSuming the bulk of the audience is listening in stereo -3dB pan law will sound more constant loudness while panning.. I have never embraced the -4.5dB compromise as being wrong for both scenarios.

JR

PS: I never tried BGA but have successfully prototyped processors in QFN package. The tiny class D chip I use in my tuner has a heat sink pad on the bottom, a little solder paste and hot air does the trick. You can always touch an iron to vias in the bottom pad to make sure solder on top has melted.

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