OP: I’m afraid that I can’t directly answer your question about an external DAC that will decode HDCD. However, below I discuss universal players that decode HDCD.
I know you said that you only want CDs. If you like classical music, why are you not interested in newer formats (i.e., compared with the 40-year-old Redbook CD format)? There are MANY classical recordings available on SACD, and Blu-ray variants, and hi-res downloads. There are many hi-res recordings of modern classical performances. And, there are some remastered older recordings (e.g., historically significant performances) that are available on SACD or Pure Audio Blu-ray or hi-res download.
You mentioned “The Planets”. The following hybrid SACD from Reference Recordings of a modern performance of Holst’s “The Planets” features an SACD layer, and a CD layer which features HDCD.
FWIW, I’d be more concerned with being able to play the SACD layer of this hybrid disc, vs. being able to decode the HDCD on the CD layer.
There are also some classical recordings on Pure Audio Blu-ray, including this remastered older 4 track recording of “The Planets”:
FWIW, I’d be more concerned with being able to play the Pure Audio Blu-ray disc vs. the CD.
I don’t own either of the recordings listed above. Here’s the SACD that I own that includes “The Planets”:
If you get a universal player, you will have available a wealth of modern hi-res classical recordings, which IME offer superior audio quality. If you’re interested in more discussion about this:
Relevance of Hi-Res Recordings for Classical Music
FYI, there are limitations associated with coax and TOSLINK based DACS, discussed in the later paragraphs of this post:
Examples of Hi-Res Classical Recordings & Discussion of DACs.
The Oppo BDP-105 and BDP-95 will play everything via their internal “audiophile-grade” DACs (including decoding HDCD), except these older Oppo machines won’t play Ultra HD Blu-ray. (As has been pointed out, the Oppo UDP-205 will play HDCDs, but as a regular CD.)
What is your budget?
If you only need stereo analog audio connections, you can often buy a Sony UBP-X1000ES (1 generation old universal player) on eBay for $300 including shipping. Or, you can spend $500 for the newer UBP-X1100ES. These may not decode HDCD, but they will play the wealth of classical recordings that are available in modern hi-res formats, including discs and downloads. FWIW, as a classical music lover, because of the wealth of hi-res classical recordings, I’d rather spend my money on a universal player that will play all modern formats, vs. on an outboard DAC that won’t play SACD or Blu-ray.
If you want a universal player that decodes HDCD, then I’d get a used Oppo BDP-105 or BDP-95. Following is verbiage from Oppo’s Knowledge base about the BDP-105:
HDCD (High Definition Compatible Digital) discs can offer expanded dynamic range and improved audio resolution over traditional compact discs.
By default, HDCD decoding is disabled. You can enable HDCD Decoding from the Audio Format Setup menu:
1. Go to the Setup Menu and then Audio Format Setup.
2. Select HDCD Decoding and set it to On.
If you are using analog outputs for audio, you will want to ensure that HDCD Decoding is set to On. If you are using any of the digital audio outputs (HDMI, optical, or coaxial) whether or not you enable HDCD Decoding will depend on whether your receiver is capable of performing HDCD decoding on its own.
If your receiver cannot decode HDCD audio, set HDCD Decoding to On in the player. If your receiver can decode HDCD audio, set HDCD Decoding to Off in the player. In some cases, having HDCD Decoding enabled in both the player and receiver may result in occasional popping or crackling sounds due to HDCD being decoded twice.
Bottom line, I suggest that you consider newer formats such as SACD and Blu-ray, in addition to HDCD. And, I suggest that you beware the limitations of many external DACs (particularly those employing coax and TOSLINK) in terms of supporting
without compromise SACDs (i.e., playing the SACD layer, not the CD layer) and Pure Audio Blu-ray (without down-grading the audio).