Having had an opportunity to test Denon AVR-S640H vs Denon AVR-A1H, I’d like to share my impression here to facilitate your choice of a new AV receiver.
The number of channels for receivers is different - 5.2 versus 15.4, respectively, for devices. In terms of power, there are differences, the AVR-S640H has such a ratio W/Ohm - 75/8, 100/6, while the AVR-A1H has a power of 150/8, 190/6. The value of total harmonic distortion is also different, and for the AVR-S640H it is 0.08%, but for the AVR-A1H it is 0,05%.
Competitive models use the same DAC. The Bi-amping function is implemented only in the Denon AVR-A1H. Both Denon AVR-S640H and Denon AVR-A1H can transmit an audio signal directly to the amplifier and bypasses any DSP processing. Each of the receivers has a Bluetooth connector. Both devices can work with such Apple Music services: AirPlay, AirPlay 2. Access to the Spotify service is on each of the competitors. The number of available streaming services varies for receivers, the Denon AVR-S640H works with Deezer, TIDAL, Pandora, Napster, SiriusXM, TuneIn Radio, iHeart Radio, Sound Cloud, but the Denon AVR-A1H works with Deezer, TIDAL, Pandora, SiriusXM, Napster, SoundCloud, Amazon Music, Amazon Music HD, MOOD:Mix. Signaling over HDMI in standby mode can provide the AVR-S640H and the AVR-A1H. Video scaling is only possible with the AVR-A1H. Each of the participants of the comparison has a Dolby Vision.
The number of HDMI inputs/outputs varies depending on the receiver, the AVR-S640H has 5/1, and the AVR-A1H has 7/3. HDMI Audio Return Channel is implemented in both devices. HDMI eARC is only supported by the AVR-A1H. When comparing receivers, it becomes clear that each of the rivals can work with HDMI CEC. The AVR-S640H has support for the standard HDCP 2.2, compared to the AVR-A1H, which supports the standard HDCP 2.3. Like most receivers, our models have a built-in phono stage for connecting a vinyl player. Voice control is performed using Amazon Alexa, Google Assistant, Siri, Josh.AI in both models. There is an ECO mode in either AV receiver. The Denon AVR-S640H and the Denon AVR-A1H can be properly configured using the setup assistant.
A Dolby Atmos multichannel audio format is available only in the AVR-A1H. Only in the AVR-A1H realized a DTS:X surround technology.