6x2 mini* mixer

Updated information:

case is very small (mint tin)

output will feed portable unbalanced recorders ~10k-20k input impedance

6 balanced mic pres using that1580/5171 chipset with rfduino spi control and android bluetooth phone app for the control interface

Left,Center,Right dedicated physical switched pans on each channel card.

external dc transformer  wall wart p/s nominal  ~ 9-15Vin

onboard +-15v supply: ltm8049 micromodule regulated with tps7a47 and lt3015-15

48v supply: lt8330 regulated with lt3012, switchable phantom on each channel

3.3v digital logic supply: lt1763-3.3 regulated from Vin

stereo master card with  3 way switch for gain control: 6,12,18dB, unblanced stereo out.

Onto the power supply, preamp  and summing cards are supposedly done but I will post schematics before final order for boards.

What is the best strategy of the following for +-15v/48v:

The objective is small size ( mint tin for the whole thing) otherwise I would build a transformer/linear supply and not bother anyone.

1 
~17vDC transformer wall wart-->>lmz34002(or other) buck/boost-->tps7a33/similar ldo --> -15v
~17vDC transformer wall wart-->tps7a470/similar ldo --> +15v

2
~32vdc transformer wall wart-->tps7a470 regulator-->30v-->local tle2426 splitters on each channel for virtual ground. 

3
same as 2 but array of tle2426's on a single p/s card

4
~17vDC transformer wall wart-->>ltc3265charge pump  inverter--> -17v --> tps7a33/similar ldo --> -15v
~17vDC transformer wall wart-->tps7a470/similar ldo --> +15v

5
~17vDC transformer wall wart-->> 2 x (ltc3265) charge pump  inverter--> +-15v


48v supply
~17vDC transformer --> lt8330 boost --> ~50v --> lt3012 ldo --> 48v

mouse said:

One of the things I'm unclear about with passive summing is how to handle unused channels. Is it a dealbreaker having a variable number of inputs connected?

Click to expand...

What do you mean by an 'unused' channel? Also I am not clear which signals you are feeding to the mix bus. Are you trying to mix from the balanced outs of each channel or somewhere earlier in the chain.?

Cheers

ian

The 6 unbalanced preamp outputs are switched and fed to the master input.

Attached master channel schematic to first post, channel schematic to second post, that should clear up the question.

Rough mint tin-size birdseye layout of everything  attached here. Two leftmost footprints are the bipolar suppy and phantom supply.

OK, the schematics make things clearer. I assume there are no channel faders and each OUT is connected to an IN. In that case the mute switch is correctly wired. Each mix bus has four sources so the loss will be 12dB. I think you are basically there.

Cheers

Ian

thanks for looking over the schematic.

Did you see the questions about the power supply?

mouse said:

thanks for looking over the schematic.

Did you see the questions about the power supply?

Click to expand...

You seem to have several options that include switchers about which I know nothing so I am not the best person to advise you.

Cheers

Ian

Revised summing schematic

Revised channel schematic

Mostly things are the same except for added switch in the summing, changed to bridge rectifier in preamp to save space.

I took out C7 from the original schematic on first post,  looking to save space and not use large electros. The thinking was DC is already driven down in the preamp circuit.  Is this ok?

Is C5 necessary?

In the channel schematic in this post, does it matter if signal ground on the output of the 1510 is taken from ref or is common ground used?

mouse said:

Revised channel schematic

Mostly things are the same except for added switch in the summing, changed to bridge rectifier in preamp to save space.

I took out C7 from the original schematic on first post,  looking to save space and not use large electros. The thinking was DC is already driven down in the preamp circuit.  Is this ok?

Click to expand...

You can. But you will have better stability with it, particularly against low frequency ground borne noise.

Is C5 necessary?

Click to expand...

It is as it blocks +48V (as does C4).

In the channel schematic in this post, does it matter if signal ground on the output of the 1510 is taken from ref or is common ground used?

Click to expand...

I don't exactly understand the relevance of the question at this stage but the grounding scheme will depend on how you lay down the PCB and the overall grounding scheme you will adopt in the final product.

Thanks for the help.  Sorry, I didn't explain properly.

In the schematic on post #1 C7 and C12 I think are to drive dc offset down.
In post #7 I removed C7 and just have the 10uF on the output of the amp.

The summing bus is fed by standard 1510/ina217 datasheet preamp with the control loop to drive dc offset down to zero.

So I was wondering about how much needs to be done to keep dc down in the summing amp since dc is already low.  Maximum gain on the summing amp is 18db - roughly gain of 8.

Also I'm curious about opinions on  using aluminum polymer vs. aluminum  for both phantom blocking and the 10uF output caps.

mouse said:

Thanks for the help.  Sorry, I didn't explain properly.

In the schematic on post #1 C7 and C12 I think are to drive dc offset down.
In post #7 I removed C7 and just have the 10uF on the output of the amp.

Click to expand...

The purpose of C7 and C12 is to block DC.

If you remove C12 you will be passing DC to the next stage.

If you remove C7 there are two points you have to consider.

1). Now the DC offset at the output will also divide. Therefore attenuating the DC feedback applied to the inverting input. Whereas with C7 you'll have full DC feedback.

2).  You have gain select switch on the low side. You'll have awful clicks. If this was a potentiometer you'd have awful scratches.

Also I'm curious about opinions on  using aluminum polymer vs. aluminum  for both phantom blocking and the 10uF output caps.

Click to expand...

I would pay attention to ESR. Aluminium also has low ESR types.

Changed  the resistor scheme on gain switch to eliminate potential pops.

Boards being sent in a few weeks, parts are here already.

I'm  wondering about some sort of alternate preamp card aside from that1510 that can be compacted down to 50 x 19mm board size. I would like to do a miniature green but it looks like there are too many large-ish caps to fit.  Anyone have a suggestion for something that sounds good?

Boards came, started populating and testing.

(refer to schematic from post #8)

No signal on output, found a few micro-tombstones, corrected, still no signal.
Then found continuity between pins 5-7 and 5-4 on the that1510. desoldered it, still had continuity on those pads
Desoldered the OPA137 and continuity went away.
The pads on the OPA137 looked good so I don't think it was a soldering error, maybe fried opa?

Getting close to finishing. Phantom supply works, bipolar supply working, preamp cards working, getting signal from the summing card but both channels are outputting signal no matter what - need to find the problem. The last (known) problem is I used the wrong pots and unfortunately the design is very physical size dependent. I could not find BI Technology c.p. reverse log pots P08x series stocked anywhere.

So I need to use a larger pot, or somehow figure a way to cram in both superexpensive grayhill 75 series 10 way for 5db gain increments and also a small 5 way switch for post gain/pre summing trim.

Pot suspects are alpha 9mm, which are 11.35mm in height, bourns ptd90 carbon

I could lower the channel count to 5 and maybe find a 1/2" conductive plastic pot, but is there any audible difference between carbon and c.p.?  Same question for either pots vs. switched gain.

You could try the smaller/cheaper Grayhill 56 series switches.

Cheers

Ian

I have 11mm in between channel cards, so I was looking at the 75 series (~$25 a piece).  Those will fit, but I can't figure out how to fit in a fine gain trim or switch in addition so it looks like it's going to be alpha pots by process of elimination.

At one point I was considering making this a 6 channel preamp box with no mixer. At a later time after some listening tests if I hear a noticeable difference between switched gain and the alphas then I might revise things and turn it into a preamp box.

But I'm already working on the digitally controlled gain version using 1580 and 5171/bluetooth android/ rfduino.

I am not sure I have this right but are you saying is channel strip is only 11mm wide?

Cheers

Ian

correct

mouse said:

correct

Click to expand...

I am sure you have good reasons for making the channel 11mm wide but that seems awfully small to me. The closest spacing I have ever had between controls on a panel is 12mm. That is between toggle switches on my 4toggles PCB and even that feels a bit close to me. what diameter knobs are you using?

Cheers

Ian