PGA2310 and Source Selection, right direction?

[geekysuavo]

hi all,

here's my end-goal:
* 8-source stereo selection
* one pga2310 per source
* microcontroller-based

i've put together and attached a schematic to show a bit more of what i mean. there is only one source (relay and pga) in the diagram now, but essentially the idea is to daisy-chain the pga's together and control them all with an atmel avr micro.

what are your suggestions for making this design better? i'm curious if optical isolation between the analog and digital grounds would improve quality, but how is it doable?

[EDIT:] oh! and do the inputs of the pga's need to be buffered and, if so, would an opa2134 be sufficient?

thanks a bunch!
~ brad.

[geekysuavo]

aha!

well, in a flash of clarity and slight self-deprecation, i realized that there's no benefit to having a separate pga for each source. instead, just assume the design uses 8 relays into one pga, and the volume level of each source is recalled upon switching by the controlling micro.

i feel a bit silly.

still, is there anything you'd recommend to improve the design?

thanks,
~ brad.

[nonoise]

Your idea of using a input buffer is a good one, I would perhaps also add a output buffer using for example a BUF634 + OP634, this will let you drive headphone well too.

[drummer_Dave]

check the data sheet on the PGA if the inputs should be buffered (I know the SSM2160 needs to be, similar VCA idea...)

anyways are you sure you want to use relays for switching your inputs? I really hate relays in the audio path, IMHO you should look into using analog switches/muxs i have used 74HCT4053 muxs with great success, actually i am currently working on an integrated amp/preamp. I should be posting with in a couple weeks. just my $0.02

Isolating grounds great idea, have the analog and digital grounds connect at one point only. You could use opto couplers from the micro to the muxs, for your digital I/O's this would keep everything seperate

Dave

[geekysuavo]

It is important to drive the PGA2310 with a low source impedance. If a source impedance of greater than 600Ω is used, the distortion performance of the PGA2310 will begin to degrade.

i've attached the finished (well, nothing's ever finished...) schematic of my idea. the SN74HC595 controls the outputs of the ULN2064B darlington drivers, which supply 500mA to each relay when on. the PGA2310 is buffered by an OPA2134 opamp and takes its inputs from the relay which is currently on. ten digital I/O lines go back to a controller board, where an AVR will make this thing actually function.

please, any more thoughts?

thanks,
~ brad.

[geekysuavo]

Originally posted by drummer_Dave
are you sure you want to use relays for switching your inputs? I really hate relays in the audio path, IMHO you should look into using analog switches/muxs

any reason why a relay would be unsuitable as compared to an integrated circuit?

~ brad.

[glt]

Someone is doing the same thing here: http://www.arduino.cc/cgi-bin/yabb2/...num=1236803143

[pacificblue]

Originally posted by drummer_Dave
IMHO you should look into using analog switches/muxs i have used 74HCT4053 muxs with great success

Originally posted by geekysuavo
any reason why a relay would be unsuitable as compared to an integrated circuit?

On the contrary. Any integrated circuit, also a mux, has a high "on resistance", because the switching can only be done with semiconductors. You trade the small parasitic inductance and capacitance of a relay (or switch!) contact for the typical non-linearities of transistors and their surrounding components. E. g. the 74HCT4053 is given with 80 Ohms at 4,5 V. The lower the voltage, the higher the resistance and you rarely pass 4,5 V signals, do you?. Relays have contact resistances in the range of 0,1 Ohm and (much) below.

Switches must be very sophisticated and therefore expensive, if they are to be as good as relays in the signal path. Many relays, even cheap ones, come with gold-, silver- or rhodium-plated contacts for low contact and corrosion resistance. If you want a switch with those platings, the manufacturers call it 'high-end' or 'audiophile' and charge you an eye for it. You can get vacuum sealed relays and relays with inert gas filling to improve corrosion resistance and sparking even further. Some models come in metal housings for better electro-magnetic compatibility. On top of that relays are easier to integrate into a PCB and make it easier to maintain the signal path short.

Drawbacks? Yes, relays need a power supply. And you need a switch to control the relays, albeit any cheapo will do.

[zdavesf]

pacificblue i do agree with you... yes due to the larger on resistance of the mux's doing voltage switching can degrade the signal, but have you ever tried using the mux's to switch current instead? by switching the current you wont get the voltage drop across the switch... anyways it sounds better to me... but like i said just my $0.02, the audiophile business is full of controversy

Dave


P.S. I also don't like the clicking

[pacificblue]

Originally posted by zdavesf
by switching the current you wont get the voltage drop across the switch.

Is Ohm's law in Canada different from Ohm's law in the rest of the world?