Comments from Sandy Bird
For the most part, my feelings echo Brian's in their entirety. To summarize, Yamaha has once again released a stellar price-oriented product. What I love about Yamaha is they make receivers that cover everyone's budget. They just add features or a little power and you end up with a receiver that exactly matches your shopping needs. The 600 series from Yamaha is what I consider to be the perfect balance of price vs. performance (and features). The receivers below the 600 series lack some of the features I consider essential, such as pre-outs, real binding posts, and power for all channels. The models above the 600 start to add feature like more power and fancy remotes which have less impact on performance in most environments.
As Brian mentions, I did find the 640 to be a little edgy or as I would call it bright. This is nothing new for Yamaha receivers, they are bright, but some people like that and many others don't care. So take that comment with a grain of salt if you a looking for a unit in this price category. I will say the unit seemed to clip a little too soon for being rated at 85 watts per channel. It also clips hard, jumping from 0.1% THD+N to double digit percentage in just one attenuation step on the volume control.
In my fairly large room (12 x 20 feet) the RX-V640 was not quite powerful enough. It got the job done, but it was struggling with some of the dynamic explosions of 5.1 movie soundtracks. In a smaller room or if you listen to your wife when she tells you to 'turn it down', you will not run into this issue.
On the Bench
All pre-out measurements wee taken at 1 volt in and 1 volt out. Speaker output measurements were taken at 55 watts into 8 ohms. The unit clipped before the rated 85 watts.
Using the S/PDIF input and the pre-outs we found a respectable 0.01% THD+N. The RX-V640 would make an excellent processor to be used with an outboard multi-channel amp. You can also see how clean the upper spectrum is with only a slight first and second harmonic making any noticeable impact. There are some excellent electronics under the cover of this unit.
Again using the S/PDIF input and the pre-outs, IMD measured 0.002% using 5 kHz and 6 kHz test tones.
We found an almost identical THD+N figure using the analog CD input and pre-outs.
IMD too remained basically unchanged using the analog inputs.
Frequency response via the pre-outs was flat from 20 Hz to 20 kHz and was just starting to roll off at the extremes.
Switching to the speaker output we saw 0.08% THD+N. This is slightly unfair as the unit was getting very close to clip at this point. Reducing the volume a couple of attenuation points reduced the THD+N values to more closely match the pre-out measurements.
IMD also showed the Yamaha approaching its limits. While it measured a respectable 0.0030%, looking at the upper frequency spectrum, you can see some nasty harmonics starting to take over.
As with the pre-out measurements, speaker level outputs continued to have a flat consistent frequency response.
As I mentioned, the speaker level measurements could have had better results had I backed off the volume a little, but I wanted to point out the difference in purchasing a $500 receiver vs. a $3000 receiver (the RX-V1 for example). In all cases you get what you pay for. The RX-V640 is a great unit for the price. If you are using it in a small room or will be upgrading to an external power amp you will be very happy with the RX-V640.
On the whole, we very much like this little receiver. We'd have no reservation in recommending it to someone on a tight budget with a relatively small room.