summing mixer with monitor control

[matthias]

hello,

now I have some time and can go on with my summing mixer project...

I changed the concept and will use the input transformers of a Neumann 475 0-ohm summing card..

the features:

-> 16 balanced inputs 8 of them switchable from 0db to -30db
-> 4 unbalanced inputs
-> stereo master insert with bypass switch
-> master level switch from -10 db to +10 db
-> switchable mono circuit for monitoring section
-> 1 stereo output for external peak meter or analyzer
-> 2 bbc ppm peak meters
-> 2 stereo monitor outputs

I'm nearly finished with the schematic.. I tried to keep it all as simple as possible...

summing-box.pdf

the schematic shows the left channel, the right channel will be identical...

before I will go on with the pcb layout... there are still some things where I'm not really sure about...

1) can I connect the pots for monitor level and master level in parallel as shown in the schematic or do I have to separate them with another opamp..?
2) is it ok to put the master level switch in the feedback loop, or would it be better in front of the opamp input?
3) is that the best solution to use noninverting opamp-configuration after the monitor and headphone level pots or is an inverting configuration better...


beside that... do you still see some things that needs to be improved..?

thank you in advance...

matthias

[mediatechnology]

Thanks for posting this matthias.

The design does look very simple, clean and useful. One thing I would examine is R76 and the 300R source impedance (at LF) that is presented to the 1646 input. Ideally you'd like to see something driving a 1646 that's <<100R and ideally directly from the output of an op amp. Are you sure that you need that RC network?

Other than that, that's all I saw. In order to get "the big picture" of signal flow I'll print the pdf.

Wayne

[matthias]

Hi Wayne, thanks for your help...

regarding the 300R resistor and the 6,8n cap
this RC-filter is adapted from the neumann card schematic
it's a -6db lowpass filter at 78 khz ... honestly I don't see any reason why not to omit these components.

[mediatechnology]

OK, good. Removing the 300R in front of the 1646 should improve both it's noise floor and output balance.

[JR.]

From a quick browse, it looks like you will get clicks from several switches. 553x opamps have significant input bias current that will cause DC offsets at the impedances used. One technique is to add resistor in series with + input to balance out DC, but most just use DC blocking caps in series with bleed resistors to ground to make switches silent.

I don't know enough about the THAT chips to comment on them... Perhaps a newer, more precision and lower input current opamp would be simpler to help de-click the 553x opamps, since adding DC compensation resistors adds (a small amount) noise too.

For personal use if you don't mind the clicks, never mind...

JR

[mediatechnology]

JR. makes a good point about Ibias.

WRT the 1646 and 120X I don't recall what the typical bias current is. For 124X line receivers (FWIW PNP input) I recall it to be about 900 nA max so it would be high enough - in some cases - to click when switched.

For the 1646, to maintain LF balance, I wouldn't use DC blocks anywhere other than the sense caps unless one had to. If the 1646 input is switched and a cap is required, I'd make it larger than usual to maintain LF output balance. The 1646 Rin is 5K nominal but can be as low as 4K. 47uF would be the minimum value I'd use there. That puts the corner at less than 1 Hz which might seem excessively low but the reactance realtive to the 5K Rin - and the rest of the internal OutSmarts bridge - is kept low.

[matthias]

that's no problem for me... if there are some klicks I think I can live with it...
I'm not sue if I will use NE553x opamps in the end.. I still have some opa134, ad797 and opa2132 here and will try them out...

regarding the datasheet of the 1646 the output gets inreased by 6db. so I will have to decrease the level before the 1646 to get unity gain.
I think the best way to do this is to lower the outpu of the first opamp after then input transformer and then change the that1206 (-6db) to the that1200 (0 db)

[matthias]

here's an updated schematic including the headphone amp
it's based on the ti tpa6120a2 using the schematic from Per-Anders Sjöström

I also made a frontpanel layout...

[radiance]

So, how's your mixer project going? I really like the frontpanel lay out.

[matthias]

Hi Roland,

I've finished all the metalwork
pcb design is finished ~70%
had redesigned the switching logic for the illuminated buttons

here are some pics...


please note: the meter illumination is white (not blue)