8 sources preamp

[otherside]

Hey guys,

Trying to find the best way for a preamp with multiple inputs (8).

My expertize lies with microcontrollers, digital stuff etc.

So far i had 4 ideas, but i'm not sure if these will work and what's the best.

out = (LM4780 input)

1. Input->Buffer->Relay (controlled by micro)->Tie all output from relays together->PGA2311->out

2. Input->Buffer->Clickless Audio switch (MAX4910)->Tie all outputs from switches->PGA2311->out

3. Input->OP-AMP Controlled gain from digital pot->Tie all outputs from opamps ->PGA2311->out

4. Input->Buffer->PGA2311->Tie all outputs from PGAs together ->PGA2311 (master control)-> out
(this can act as a mixer as well when needed :-)

Which solution is best ? or if all of these are not good a suggestion ?

I'm in particular interested in the solution 4. I don't really mind the cost since i'm looking for two of these pre-amps and i have already the ICs.


Note: Tie all together -> i mean seperatelly the L-R channel of course.

Also using the PGAs, by controlling the two (L-R) seperatelly i beleive you can implement balance control ? is that right or i'm missing something ?

[jaycee]

Purists will tell you to use the relays. Theres a cool IC from TI (TPIC6C595 if memory serves) which is a power octal shift register designed for driving relays. That saves you having 8 switching transistors etc...

Another way would simply be to use a multiplex IC like the DG507, or an improved version from Analog Devices - ADG407. Dallas Maxim also make an improved version - MAX337. All of these would be suitable.

Digital pot's are usually crap as they only run on 5v. Maxim do have dual supply ones with log tapers, but they are in ultra-small packages that you can forget soldering. Go with the PGA2311. One placed after the multiplexer will do. You may want to have a buffer before the PGA2311 though.

For more ideas see Mark Hennessy's pre-amp here -> http://www.mhennessy.f9.co.uk/preamp

[otherside]

Purists will tell you to use the relays.

It's an option but i prefer non mechanical parts.

Another way would simply be to use a multiplex IC like the DG507, or an improved version from Analog Devices - ADG407. Dallas Maxim also make an improved version - MAX337. All of these would be suitable.

Has enyone used them for Audio? These are pretty bulky, i've used them a lot before for other projects, even the CD4067 from TI which is cheaper than these is better. The on resistance is quite large and frankly it varies a lot depeding on the signal so my guess is that this will just add noise.
Not sure guys, you're the experts, but based on my knowledge i'd never use a analog mux for good quality audio.

Digital pot's are usually **** as they only run on 5v. Maxim do have dual supply ones with log tapers, but they are in ultra-small packages that you can forget soldering.

I could use different supplies... Anyway you'll ve surprized with what i can solder, i just soldered a TQFP 100 with .5 pitch ;-).

For more ideas see Mark Hennessy's pre-amp here

Excellent projects!!!.

Still will solutions 2 or 4 work well?

[jaycee]

Originally posted by otherside

Has enyone used them for Audio? These are pretty bulky, i've used them a lot before for other projects, even the CD4067 from TI which is cheaper than these is better. The on resistance is quite large and frankly it varies a lot depeding on the signal so my guess is that this will just add noise.
Not sure guys, you're the experts, but based on my knowledge i'd never use a analog mux for good quality audio.

The ADG407 is used in quite a lot of audio amps, it is an improved device from Analog which has a significantly lower on resistance of 50 ohms versus the normal 400 ohms of the standard DG407 device. It is designed for high quality audio use. They're available in PLCC format if board size is a concern.

Originally posted by otherside
I could use different supplies... Anyway you'll ve surprized with what i can solder, i just soldered a TQFP 100 with .5 pitch ;-).

Well if you can solder that small you might well find the Dallas Maxim dual rail volume pot's usable

Method 2 sounds usable if you can solder TQFP. Method 4, i doubt the PGA2311 would provide good enough isolation in MUTE mode, plus it's rather overkill.

I would say if quality is the paramount issue, go with the relays, and if you don't like relays, use either the ADG407 or the MAX4910.

[otherside]

Thanks for the comments,

One issue is availability, i'll try to see what i can get and test.

[jaycee]

Analog, Maxim and TI have great sample programmes

[otherside]

Programming them is the only thing (for me) which is not a issue :-). If you ever need any help with programming these things let me know.