It is quite simple; the TI PCM 5102A DAC’s are 2-channel DAC’s. In total you have 11 channels + 2 sub-channels makes 13. Hence 7 DAC’s. The author of this article;
https://www.audionet.com.tw/thread-13948-1-1.html confirms the vertically mounted, in between 2 main, board is for the DAC’s. This is for the Marantz SR7015 which has almost identically lay-out compared to the Denon 3700 and 4700 apart from the AMP-section. DAC-section is similar.
Translated;
The vertical circuit board here is the 11.2-channel digital-to-analog conversion circuit of this machine. It is quite different from most surround amplifiers. The DAC performs multi-channel digital conversion. The upper and lower sides of this circuit board are directly connected to the digital main board and the pre-amplification circuit board, so it is also a "connection board". The design of the vertical digital conversion circuit board can not only reduce the signal path The length can also reduce the number of contacts through which the signal passes, which helps to improve the transmission quality.
Before disassembling the machine, I already guessed where the digital-to-analog conversion circuit board of SR7015 is installed. After disassembling the machine, I took a closer look, oh! It is different from the previous practice: Marantz used "two" Asahi Kasei 32-bit "8-channel" digital conversion chip AK4458VN in the middle and high-priced surround amplifiers of Marantz before, but this time it used "7 pieces of two Channel number conversion chip "TI (Burr-Brown) PCM5102A, is it possible that Marantz really wants to make its own surround amplifier completely compare with two-channel audio equipment? In fact, this is a change design made for "contingency".
If you go to the official website of Marantz to check SR7015, you will see information that it uses two AK4458VNs. This is the status of SR7015 "just launched abroad". However, due to the Asahi Kasei fire and the tight chip manufacturing, Marantz has previously announced that it will adjust some models. The DAC chip used, also said that changing the design will not affect the sound quality. Will it really not change? In terms of pure component performance, the dynamic range and distortion rate of PCM5102A are slightly inferior to AK4458VN. However, in terms of circuit structure, I think that a group of 2 channels (using PCM5012A) will be better than a group of 8 channels (using AK4458VN). Higher channel separation, this is the benefit of "dual-channelization".
There are pictures of the interior of the 3700 on ASR confirming this. This lay-out is applied also in the high-end 8500. The difference being better AKM and later ESS DAC’s having better spec’s, be it outside our human ability to hear the difference. I don’t want to challenge the ASR-die hards who claim they can…. Please read other fora as well.
I can confirm the PCM 4700, subjectively, sounds great. Combined with a Marantz MM7025 power amp it handles everything I throw at it. Both multi-channel movie sound as well as stereo for music. As mentioned above; especially the ability to setup specific setup for two channel sound is great.
Still, if you don’t play Xbox you could try and find a 4700 with AKM DAC’s if that sets you at ease. I really would like to have both versions tested not only on ‘objective‘ measured SINAD figures but also on other criteria like how both machines handle multi channel as well as stereo sound.