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Old 29th May 2011, 09:50 PM   #1
claas is offline claas  Germany
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Join Date: Apr 2011
Default Modular Balanced Preamp

Hi there,

Im planing my "little" modular Preamp and im looking for a little bit of usefull critic and input.

Heres my "plan":

-dual mono balanced Preamp with mirrored PCBs

-swappable Amp stages, so i can try different modules in the future (maybe discret, pass? or whatever comes to my mind)

-swappable Phono stage

-swappable DAC-Boards (only the DAC itself and the I/V stage will be on this one, the receiver, SCR, clock etc. will be on a separate shielded Controllboard)

-input and output switching with latching relays

-two outputs with volume attenuation per side (additional active X-overboard for Satellites and subwoofer-->switchable stereo/mono)

-two unregulated outputs for kitchen or bedroom extender amps (just one input-->pga2310-->and a cheap integrated poweramp for example)

-all balanced input with a switchable +6dB boost for unbalanced signals

-volume control via r2r with 8-latching relays or shunt attenuator with i dont know how many steps with 8:1 muxes

-nearly everything in SMT


so here are my question :
if i would use a shunt attenuator how many steps do i really need?
what i need are 1dB steps because the attenuation of the Main speaker and the Subwoofer has to be flawless at any volume position.

does anybody has any input on my instrumentation amp stage?
i build it quickly two month ago with thru hole components and got really pretty slick results how i think (measurements below). no input filter...nothing.. only a input resistor before the OPA1642 and after it the OPA1632.

what do you guys think about the input- and outputboard PCBs? The Input PCB will be mounted on top of the Output PCB.

Can anybody recommend a nice balanced Phono stage?

thanks and happy bashing

claas
Attached Images
File Type: png Input Board PCB.png (115.4 KB, 316 views)
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Old 29th May 2011, 10:29 PM   #2
claas is offline claas  Germany
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sorry, struggeling a little bit with the resolutions.
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File Type: png Ouputboard PCB.png (116.5 KB, 297 views)
File Type: png Outputboard Schematic.png (60.4 KB, 288 views)
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Old 29th May 2011, 10:32 PM   #3
claas is offline claas  Germany
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here both Ampstages until now.
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File Type: png Inputstage schematic.png (25.6 KB, 283 views)
File Type: png Output Stage Schematic.png (28.7 KB, 274 views)
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Old 29th May 2011, 10:36 PM   #4
claas is offline claas  Germany
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and last but not least the mentioned Inputstage, measured with a RME audio converter.
no lowpass filter on the input and dead quiet.
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File Type: png THD.png (22.7 KB, 118 views)
File Type: png NOISE.png (24.9 KB, 27 views)
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Old 29th May 2011, 10:58 PM   #5
claas is offline claas  Germany
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Join Date: Apr 2011
ha, it finally worked with a different resolution. last pic.

the input board.
i build in a small RF Filter (2x47pF,1x470pF) directly under the XLR connector like suggested in a THAT corp. paper. This ground/shield is not connected to circuit GND. it is directly connected to the enclosure.

as you can see the output of the Inputboard is switchable between the five inputs (incl. phono) and a direct signal from the DAC board. so the DAC out don`t have to run through the inputstage.

and i forgot one thing, signal GND and supply GND is always separated.

So, what does this forum think. did i make any mistakes?


thank you very much.

claas
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File Type: png Input board sch.png (131.7 KB, 124 views)
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Old 31st May 2011, 08:34 PM   #6
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hmm, 110 views and no input. thats kinda sad.
anyone...., some critique on the circuit or PCB?
or should i count that as a yes? its ok anyway.....
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Old 16th June 2011, 03:39 PM   #7
zdavesf is offline zdavesf  Canada
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Location: alberta canada
Looks really good. It appears you have thought most of it through. Unfortunately I don't see anything to comment on (but that doesn't mean much, I have been out of audio design for a couple years now ). As you noted you are making this modular so if your first input card design doesn't work as you want its a simple plug and play to try another one (yes i know it will cost some money).

I am actually just starting up a very similar modular design pluggable input, volume, dac, and output cards... I am working out right now the best way to do the case/cards. MY intent was to plug them in from the back and have the card header connectors plug into a vertical backplane pcb that has all the signal routing. Hold it all in with an aluminum plate on the xlr/rca connectors screwed to the rear panel top and bottom. Have you started thinking this through yet?

Not sure who or how you are doing PCB's but i have found that Silver Circuits prototype pricing is pretty reasonable. I make sure i design my pcbs so that I can utilize the most of the 6.3" x 4" boards (usually split it up into a couple smaller pcbs).

As far as grounding I Highly recomend reading Dave Davenports article. I am sorry as i mentioned its been a while since I properly looked at this stuff so you may have it all covered already. Drawing out a ground schematic is just as important as drawings out the signal schematic.

All the best, I look forward to seeing your progress.

Dave
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