Audio level range and input impedance

[nocturnal_transmission]

Hello:

I was unable to find an answer on this.  I need to send receiver discriminator audio over the RRC-1258 MKII using the audio input; my discriminator output impedance is likely 5k to 10k ohms, ~200mv.  However, the 50-ohm input impedance of the 1258 is loading it way down, and I cannot get an adequate level out the far end.  The CODEC input gain does not help enough,  but I am also finding that the 1258's noise level rises with the gain, so the S/N remains unchanged.

There does not  seem to be a schematic for the 1258 for general use (although there are interface instructions for specific use cases - which is great). 

I figured there would be a simple jumper for HI/LOW input impedance , but I may have to remove resistors instead?  Is there an jumper setting for this?

Where to I find the specifications for input/output level ranges?

Surely the analog and digital input and output circuits schems are available?

Update 1: Schematics found in the back of manual.  The Tip to sleeve DC impedance reads 50 ohms; but the ring to sleeve is very high - so possibly the second channel is high-Z.  There are two larger 100 ohm surface mount resistors in parallel that provide the 50 ohms on the tip line.  Those appear easy to remove if needed.   

Update 2 (final): Mine is NOT an (s) (dual channel version).  I am hosed as I need both channels (the other for squelched audio).  But this is a good page to determine your model:

http://www.remoterig.com/wp/?page_id=1395

As was noted in a another post, R82 and R137, R102 and R103 can be removed (these existing only in the S versions) to raise the "speaker" input resistance to about 1K, instead of the original 50 ohms.   This might be enough, but 1k is still  low IMHO.  I may have to increase the remaining voltage divider resistors (which total 1K ohm) with another higher value set.

It would be nice to have a jumper setting for 50 ohm / ~10k ohm, for each input channel on the (s) version.   
   
Also - still looking for a product like this one with audio input level controls (on the board), selectable de-emph settings (and impedance).     

Thanks!

Greg

[VE3ZI]

I think the schematics show the audio input circuitry.

I was faced with a similar problem, and although it would have been easy enough to remove the resistors I decided to build a small audio amplifier instead Has been working fine for a a long while now.

73 Roger
VE3ZI

[nocturnal_transmission]

Ho Roger:

I stand corrected!  Found the schematics way in the rear of manual; both inputs should have 50 ohms Z, but one is either open or high-Z, and I never found the second set of 100 ohm resistors on the board.  Investigating further. 

Greg

[sm2o]

There are only 4 big 100 ohms resistor on the pcb. If you only have 2 resistors you have an earlier version with a single channel codec. The audio interface is built to match 99% of the needs but as written in the earlier posts you can by removing the resistors incrase the imput impedance, if you remove the decoupling cap on the input and put a new one direct to your source you get very high input imepdance.

73 de mike

[nocturnal_transmission]

Hi Mike:

Excellent idea - I can just open C82 and run the audio right into the amplifier's input (with a new DC-blocking cap), bypassing the input resistors.

Is there a capacitor on the ring input (schematic does not show this)?

Greg   

[sm2o]

yes C132

/mike