On Dec 19, 2022, at 10:24 AM, Michael Schuster <schuster.ma@...> wrote:

[Edited Message Follows]

Sadly, my SSB 2 has the noticeable hiss at 0 volume on both speaker and headphone as mentioned by Todderbert @ 14:20 in his comparison between the SSB and SSB 2,

Hmm, sounds familiar. See analysis of XHDATA D-808 from

https://www.radiomuseum.org/forum/xhdata_d_808_non_documented_or_overlooked_behaviors.html

• Audio stage Background noise floor

  • Some owners and testers reports about a permanent background noise, audible when the volume knob is at minimum, but it becomes hidden when increasing the volume to 5 or 10% of the rotation knob.

  • This audio stage background noise is present no matter what RF band and settings are selected. It is present to the speaker or earphones having the analog volume potentiometer at the minimum position.

  • This audio stage noise is more audible thru the speaker than on earphones, what seems to make no sense. But when looking to the circuit we see the reason. The earphones are driven thru a series RC network with 100uF and 100 Ohm. Depending on the earphones impedance, the noise will be stronger for higher impedance units. However the common earphones would present 32 Ohm, a low value when compared with the series 100 Ohm resistor.

  • Initially I thought that this behavior was related to a defective component, but now that I have my unit (new version 2021), I am not so sure. As the volume is a pure analog potentiometer, the noise must be generated inside the CS4863 (LM4863 equivalent) audio power amplifier IC.

  • After analysing the LM4863 datasheet (the CS4863 datasheet is in Chinese but looks identical in content) and looking into the D-808 circuit design, I believe I found the root cause for the background noise.

    • ​As the audio output from the SiLabs Si4735-D60 DSP is around 80mVrms, a audio pre-amplifier should have been included in the D-808 design to rise the audio signal to around 2Vrms in order to be able to run the CS4863 audio power amp at unity gain where it shows the lowest and inaudible background noise.

    • The design choice used on the D-808 was to save costs on the audio pre-amp and simply configured the CS4863's OpAmp1A and OpAmp2A  with a very high bridged mode differential gain of 77 (!)

      • ​Calculation: AVD = 2 × (Rf / Ri)  where D-808 use ​Rf = 150K and Ri = 3.9K

      • A typical AVD would be between 1 and 5, maybe a mximum of 10, to contain the IC noise floor under inaudible levels. But doing so would result in very low volume levels because of the missing audio pre-amplifier stage.

   You paid something like $200 for that thing. You should not be bothered by a hiss that they could not have missed when it was still at the early testing phase. There is no excuse for shipping the product before fixing a known, and obvious, problem. Please use the warranty. Send it back or demand a replacement. If customers let this slide CCrane will continue to send out defective, or poorly designed products.
  Sorry if i sound a bit sharp on this but it is because my only previous (first and last) experience with a CCrane product was an internet radio, that was a very poor design to begin with, which then became completely unusable because the internet portal CCrane had designed the radio to use ceased operation. The radio's user interface was borderline useless, requiring a ridiculous number of button presses to get anything done. And that was made all the more frustrating because it came with a tiny remote (about 2 by 3 inches) with very small membrane buttons that became painful to use in minutes. It should never have been sold. I wish now that I had taken my own advise and sent it back, but I am in Canada and it would have been way more trouble than just throwing it out.

Regards,
Gord Seifert