Poor Man's Monitor Controller/Headphone Amp

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michal_k

Well-known member
Joined
Nov 12, 2008
Messages
370
Location
France
Hello everybody,

I'm in the process of designing a headphone amp that could be also configured as a simple monitor controller. It will consist of a relay attenuator, optional buffering, optional input/output selector and all that will be controlled by msp430 series processor.  I am currently writting firmware for the uC, in the meantime please comment on the analog section (attached schematic).

I was thinking about making a basic pcb with attenuator, buffer/hpa and uC controlled by encoder with a pushbutton and then add extra features like input/output selector or display on separate boards(so that the most basic configuration is cheap and easy to make). Any thoughts? What features besides volume control + mute/dim with a pushbutton do you suggest? Should it be fully balanced or have bal->se->attenuator->bal configuration to make attenuator two times smaller?
 

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What are we calling "poor mans"? I like the overall concept. My guess would be that se attenuation would be cheaper, using fewer relays and resistors. A simple that chip in/outbalancing scheme would work fine. I've also seen the THAT1646 used as a headphone driver.(with some other parts added for current drive of course.)
 
I'm thinking about it as poor man's solution because I wan't to use msp430 chip. Thing is that with that kind of project, a lot of money go to somebody that sources and programs processors. With msp430 everybody can buy an evaluation board for $4.30 shipped and do it on it's own. Moreover, in the most basic version, that is only volume control and buffer(unbalanced), you're gonna need one mcu($4.30 since you can use one from evaluation board), one relay driver(less than a dolar), six/seven relays (few bucks), some fancy opamps(well, you decide, but I'll try to be universal with footprint), some cheap passive components and... that's about it. Add some connectors and psu and you have a relay based, 1 or 0.5db precision attenuator that can be expanded with display, input/out selector, whatever(I'm going to provide pads for all mcu ports for future developement).

Now the best part, mcu comes in dip20 package so you can socket it and  reprogram whenever you want/some new firmware becomes available.

I'd like to show schematic of the final device, but it is still work in progress, just wanted to get some input on possible features for now.
 
michal_k said:
Now the best part, mcu comes in dip20 package so you can socket it and  reprogram whenever you want/some new firmware becomes available.

Replace a socketed device when you want to update the firmware?

'round these parts, we use in-system-programming with JTAG. I haven't used a socketed device (for firmware store or anything else) in ages.

I haven't looked to deeply at the TI MSP430 parts (I really like the Silabs 8051s, so shoot me), but I can't believe that they don't support ISP with JTAG.

-a
 
michal_k said:
...and you have a relay based, 1 or 0.5db precision attenuator ...
running the numbers most often ~0.5dB, but 8x ~0.25dB, 4x ~2.45dB (step 8,24,40,56) and the more extreme jumps ~6.1dB (step 16,48) and ~8.38dB (step 32). Seems, a resistor value is a little off.

-Harpo

edit: excel datasheet updated. Fu...d up math was assuming the upper drawn line at gnd and junction R19/K8-P1 taken to R1 instead. Still missing the load of attenuator R1 / R3.
Numbers from schematic as drawn (only varying the series resistor element) don't look that promising. (min.loss 20.76dB, 30dB attenuation range, stepwidth all over the place).
 
headphone amp part: if you see any major f*ck ups please raise your hand.

PCB:
http://cl.ly/1X1b10342f1K0o2L0l1E

Schematic:
http://cl.ly/1K43340s2b3X2E3s3k1s
 
buildafriend said:
Did this ever get finished? I am in the market for a monitor controller.

heh, wrapping up PCB with attenuator right now.

Question: Do people need input selector? I like to be able to switch between different monitors, but I've never used any other input than my interface.
 
michal_k said:
Question: Do people need input selector? I like to be able to switch between different monitors, but I've never used any other input than my interface.

I can imagine multiple uses for input selector but that is up to everybodys rig and needs. If it is easy to implement why not? Then one could listen the occasional iPod / whatever, headphone mixes on tracking day, 2 track return etc. ;)
 
Relay board:
http://cl.ly/3A080r0z291y0L3C1Q0y

I don't have resistor values yet so no schematic. In the final version I'll put an option for through hole resistors. I still have PSU and control boards to do, I hope to finish the whole thing till the end of the week. Keep your fingers crossed.

About input selection, since everybody will have different needs I'll try to do this kind of stuff on separate, optional boards.
 
Like all my other projects, I have one of these half done (including the msp430 part!)

However, I did the attenuation using a pga2311.

/R
 
I'm thinking about digital pot as well cause I can't get the resistor values right. Steps always end up at least 0.5db off.
 
michal_k said:
Relay board:
http://cl.ly/3A080r0z291y0L3C1Q0y

I don't have resistor values yet so no schematic. In the final version I'll put an option for through hole resistors.
Maybe useful, routing the series resistor pads R110/R111, R210/R211, R113/R114, R213/R214 closer together, that a 90° rotated single resistor could be placed instead.
Opinions vary, but some molex style connector footprints for input, output and supply might help easier servicing (or just being the reason for servicing needed).
Dunno, if flyback diodes are already on chip or better be placed across the relais coils.
 
Thanks again Harpo! I'll change what you suggested in my layout. In the meantime I found some interesting eagle files at diyaudio.com and after some cosmetic changes we have nice, all-smd, i2c controlled attenuator board(props to max williams):

http://cl.ly/2z2M191c1J430F332o38

I'll make a balanced version of it later.

EDIT:
for future reference, I've found a nice calculator:
http://www.amb.org/audio/delta1/
 
Ok, here's my version of the i2c attenuator. As usual, all tips most welcomed.

m.

http://cl.ly/2F0N0C2q0Q3f3K1Q192N
 

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