Source Selection IC

[Mooly]

I know what I would do ! Find which pins on the IC are the CD input. Unsolder them and link them across on the print. That way the switch is really out of circuit and you can give it a long listen.
It's so easy to think something sounds better ( or worse ) when you change it. You have to listen long and hard.

[mikesnowdon]

Good idea. Then I can hear if theres a difference and how much the ic is degrading the sound. Good one.

[mbates14]

I think there was an all-in-one digital TDA7XXX IC out there somewhere.

had 4 selectable inputs, digital volume control, tone, and bass. along with gain. There was example code on the web that worked this IC also. I2C bus.

[Mooly]

Hi Mike,
You need to unsolder pins 3, 12, 17 and 26. Then connect the print of pin 3 to print of pin 12. Same for print of 17 and 26. That 100% removes the IC from the audio chain and couple the CD input directly to the other stages.

[mikesnowdon]

Hi Mooly

I've asked CA for a 340A 'SE' service manual as I want to study the areas of the circuit not shown in my non-SE manual. The thing Im most interested in is the i-Pod connection. There's a front panel jack input which activates a relay when you insert a plug. When doing so the input switches to 'AUX' and the rear 'AUX' phonos are switched out by the ic. So when listening with i-Pod the signal is only passing a relay and not through the ic (I think).

Im thinking about re-jigging this for the CDP input so while using the CDP the signal isnt passing the selector ic. The realy could be activated by the 'direct' switch - switching out the 'aux' and switching in the CDP input phono (I dont use the tone controls) direct to the preamp section. Do you get the idea? It seems easily do-able to me. I need that manual though so I can see whats going on (and a little help ). If its a goer I could even use a relay with better contacts (Gold plated or silver perhaps). It may even be possible to rig this so it works off the front panel push buttons and remote.

What do you reckon?

Mike.

[Mooly]

Did you try 100% removing the IC from the signal chain as in the other post.
That's the only way to see if the IC is causing any audible degradation.
It's possible the IC can, by adding it's own distortion products to the signal, actually make the whole thing sound "better". The only way you will know for sure is to link it out leaving everything else unchanged.

[mikesnowdon]

No I havent tried it yet but definately will.

Theres a schematic of the i-Pod circuit and relay here:

http://www.diyaudio.com/forums/showt...13#post1667213

I dont know if the relay is activated by a pulse or a constant. I need to know to determine if I need a latching switch or push button to activate it. I'll start working on the rewiring for CD soon.

By the way the realy does switch the selector ic completely out of the signal path.

Mike.

[TheSeekerr]

Hi Mike,

Relays come in two varieties - normal relays, and "latching" relays. What you want is a latching relay, but they're by far less common, since they require additional mechanical complexity (a tiny little mechanism with a solenoid, ratchet and cam, traditionally, although there are other ways of achieving the same thing).

If your relay is of the normal variety, you'll either need to use a latching switch instead, or replace the relay with a good latching relay.

Checking it is easy...just work out which pins on the relay are the control pins, and which are the switching pins, and measure the voltage across the control pins while switching it on or off. If you see a voltage spike, it's latching, and if you see a jump to a new constant voltage, usually 5V or 12V, it's normal.

[mikesnowdon]

Thanks.

In the current set-up the relay is automatically activated when an i-Pod is connected via the front panel jack socket (schematics in above link). Im a novice so I havent figured it out yet exactly how this works, though I understand the basic concept.

I can easily re-wire the switching pins of the relay to the CDP phono inputs and pop an empty jack plug in to select CDP. Alternatively there is an available latching push button switch on the front panel (source direct) which I can use (I plan to bypass the tone contols intenally giving a shorter signal path).

Ideally though I'd like the relay to be activated by a pulse so that the existing CD push button (nono-latching) can be used. When an i-Pod is connected the 'aux' front panel LED comes on so I can re-route this to the cd button. Lots of figuiring out and experimenting to do!

Im so pleased I found this mod! I can eliminate the switching ic from the signal path with the CD input and probably get better sound as a result. Thats the idea anyway.