Originally Posted by StratocasterMan
How much voltage does the iDSD output through the RCA jacks when it is turned to full volume? I can't seem to find this spec. It really should have been designed with the option of outputting a fixed voltage through the RCA outputs so that the volume control would only affect the headphone output while providing a line-level out via RCA at the same time for speaker amps. The RCA output should have had a fixed option, so that folks would only have to use their speaker amplifier volume control to control the volume of their speakers.
Since the volume control seems to affect the RCA outputs, it creates a problem of not overdriving speaker amplifiers that are accepting an RCA input. When a person uses the volume control to turn up the volume in their headphones plugged into the iDSD, they apparently have also just changed the voltage being output through the RCA outputs to their speaker amplifier. Any changes made to the volume control for headphone listening apparently will also have an effect on the voltage being sent out of the RCA outputs to the speaker amplifier. This creates a problem of overdriving the speaker amplifier when one only wants to change headphone volume.
Since the volume control affects the RCA outputs, what is the range? What is the variable voltage output range of the RCA outputs? With the volume control at max, what is the RCA output voltage? I would really like to know that particular spec...
Please see below reply from Thorsten Loesch, Head of R&D (AMR/iFi):
> How much voltage does the iDSD output through the RCA jacks when it is turned to full volume?
> It really should have been designed to output a fixed voltage through the RCA outputs so that the volume control
> would only affect the headphone output while providing a line-level out via RCA at the same time for speaker amps.
> The RCA output should have been fixed, not variable.
We have received quite a number of iDAC customers who complained that the line outputs (which was designed following indeed this concept) were fixed and the volume control worked only on the Headphones, which forced them to use adapters from the headphone outputs to get volume control on the line levels.
This was presumably due to the use with active speakers or in other kinds of systems lacking a volume control.
As a result we decided to offer on all future iFi Products with Volume control so that it operates on both line and headphone outputs, with minimal compromise to the sound quality of the line out. In the iDSD nano we use modern, highly-integrated circuits in the stage.
The actual Volume control Potentiometer is not part of the signal path, but instead varies a DC voltage applied to an AD converter which in turn sends the position of the pot, derived from DC voltage to a Microprocessor. The Microprocessor then controls a 64 Step attenuator which form an integral part of the output stage.
The complete output stage combines a differential input and lowpass filter, a 64 step stepped attenuator and a Output Amplifier/Driver. It allows both inputs from the DAC and outputs to be direct coupled (no coupling capacitors).
Due to the high output current capacity designed into the output amplifier it can drive headphones directly with very low distortion. It can drive 700mV into 16 Ohm with 0.005% Distortion, at higher load impedances, especially at line output loads of a few KOhm or more, distortion is vanishingly low.
DAC Chip -> DC coupled to -> Filter -> Stepped Attenuator -> Amplifier DC coupled to Headphone Jack or RCA Jack
If the Volume control is turned to the maximum this is switched to:
DAC Chip -> DC coupled to -> Filter -> Amplifier DC coupled to Headphone Jack or RCA Jack
This is about as minimal a circuit as possible and has as little impact on the sound quality from the DAC as possible and when the stepped attenuator is advanced to maximum the signal path no longer includes it.
To add a fixed output would have meant to add a lot more circuitry in the audio-path which could at best not have improved the line out over what is in place and made the headphone one worse.
Or we could have "faked" a fixed output by detecting the switch-over between line and headphone and then setting the output automatically to maximum and depriving anyone who actually wants a variable line out of that feature.
So we feel our choice offers the best compromise between features and sound quality, at the cost of the user having to turn the volume control to the maximum if requiring a "fixed" line output.