Topic: Input impedance of the analog inputs of the ADI-2 PRO FS

I'm considering using my ADI2PROFS in my home audio system to digitize the signal from the RECORD output of my tube preamp.
The tube preamp spec. states that loads lower than 10k are not acceptable for this output, and I note that the ADI2PRO input resistance spec is 9k unbalanced and 18k balanced.
The preamp produces a single ended phono output, and I am concerned as to whether the preamp and the ADI2PRO are incompatible without the addition of a padding resistor.
Two questions. Firstly, can I use the balanced input in conjunction with a suitable phono to XLR or TRS adapter so that the perceived load impedance is large enough?
Secondly, does the input impedance remain the same as the input level control is adjusted?
The maximum output level from the preamp is about +11dBu.
Thanks.
Wyn Palmer

2

Re: Input impedance of the analog inputs of the ADI-2 PRO FS

1. You can solder a serial resistor of 10 kOhms into a TS connection. This will raise the input impedance to 18 kOhms and attenuate the input signal by 6 dB.

2. Yes.

3. I doubt that you will notice any difference. IMHO connecting the Pro to the tube amp will just work without a special cable.

Regards
Matthias Carstens
RME

3 (edited by Jas0_0 2019-05-19 14:03:42)

Re: Input impedance of the analog inputs of the ADI-2 PRO FS

Hi, I have a similar question.  I am thinking of buying a Puresound P10 phono preamplifier, and want to run the signal from this through the ADI-2 Pro FS. 

However I understand the Puresound requires the unit receiving its (unbalanced) signal to have an input impedance of 47kOhm or more.

Does this mean it is simply not possible to use the Puresound with the ADI-2 Pro?

Thanks,

James

4

Re: Input impedance of the analog inputs of the ADI-2 PRO FS

That sounds more like the unit's input impedance. I found two sources that say output impedance is <5 kOhm, so it will work with the ADI. To be sure ask them. Then you can also ask them why they aren't able to put such information on their own website.

Regards
Matthias Carstens
RME

Re: Input impedance of the analog inputs of the ADI-2 PRO FS

47k is standard for mm inputs.

Re: Input impedance of the analog inputs of the ADI-2 PRO FS

Thanks Matthias and Joachim.  I was told on another forum that the input impedance of a unit should be 10x the output impedance of the unit feeding it.  So with the RME's unbalanced input of 9kOhms, the output of the phono stage should be no more than 900Ohms.  Is that not correct?

7

Re: Input impedance of the analog inputs of the ADI-2 PRO FS

That usually relates to headphones with varying impedance over their frequency range. On an amp that doesn't make sense. If both amps have the same impedance (out to in) you just have 6 dB level drop.

Regards
Matthias Carstens
RME

Re: Input impedance of the analog inputs of the ADI-2 PRO FS

MC wrote:

That usually relates to headphones with varying impedance over their frequency range. On an amp that doesn't make sense. If both amps have the same impedance (out to in) you just have 6 dB level drop.

Ok that’s good, thanks.  So there are no other ill effects aside from gain drop?  Is there a way of calculating the gain drop from 5kOhms to 9kOhms?

9

Re: Input impedance of the analog inputs of the ADI-2 PRO FS

Sure. -3.84 dB.

Regards
Matthias Carstens
RME

Re: Input impedance of the analog inputs of the ADI-2 PRO FS

MC wrote:

Sure. -3.84 dB.

Great, thanks