Basically my discussion with bigguyca has been about the reasons for the increased distortions at higher preamp output voltages, in addition to the one we already know, that is, the effects of the power amp clipping.
To me I can think of two main reasons as follow:
1) The volume control IC in the D+M AVRs has a wide spread in its tolerance for THD+N so logically speaking, it is possible that you can measure two identical model units and one would have lower THD+N lower than the other.
2) The volume control IC is rated for about 4.3 V, but the datasheet does not include the graph of THD vs Output voltage for various load impedance, except it specified the test load is 47 kOhms.
If you read all the posts, you would know that I openly asked for comments on 1) above, and have absolutely no response from bigguyca or Amir. So I have to assume neither one has the answer that they feel comfortable with.
Regarding 2), bigguyca is of the opinion that because the test load used for the specs of the IC D+M used was 47 kOhm, the distortions would/or could, be higher if they had used with an external power amp that could have impedance of 10 kOhm or even lower, especially if the internal power amp is not disconnected. He cited the IC used in Yamaha's are more robust because the test load used was 10 kOhm. I don't know if we can just based on the test load value to determine which IC is more "robust", but we do know what Gene has said about the pre out performance of Denon, Marantz vs Yamaha's in the past. All I can say is that bigguyca and I are both guessing, because there are not enough details in the available/downloadable datasheets for either ICs. The fact is, Yamaha's Rohm chip is an 80 pin LSI chip with much more parts jammed into 1 and Denon's NJU chip is a MSI chip for volume control only with much fewer parts in it, so logically speaking, I would think it should be at least equally if not more "robust". Again, we don't know so
ignore what we said (me anyway) on this.. Based on available block diagrams in some service manuals, it does seem that Denon's have no buffer amps outside of the volume control IC, while Yamaha appears to have. But we don't know for sure if Denon's service manuals show every details either.
Now, about your question on buffer amps, basically a buffer amp is one that offers very high input impedance and low output impedance and and often have gain = 1 so in that case it could be the final stage of a preamp, to make sure it can be paired with a power amp that may have relatively low input impedance.
It is easier for me to provide links as follow.
First, wiki's definition, that sounds good to me:
en.wikipedia.org
.
"A voltage buffer amplifier is used to transfer a voltage from a first circuit, having a high output impedance level, to a second circuit with a low input impedance level. The interposed buffer amplifier prevents the second circuit from loading the first circuit unacceptably and interfering with its desired operation. In the ideal voltage buffer in the diagram, the input resistance is infinite and the output resistance zero (output impedance of an ideal voltage source is zero). Other properties of the ideal buffer are: perfect linearity, regardless of signal amplitudes; and instant output response, regardless of the speed of the input signal.
If the voltage is transferred unchanged (the voltage gain Av is 1), the amplifier is a unity gain buffer; also known as a voltage follower because the output voltage follows or tracks the input voltage. Although the voltage gain of a voltage buffer amplifier may be (approximately) unity, it usually provides considerable current gain and thus power gain. However, it is commonplace to say that it has a gain of 1 (or the equivalent 0 dB), referring to the voltage gain."
And if you don't mind reading more:
The first one below explains more about what a buffer amp is and the second one explains in more detailed and simpler terms when/why you need it. One of them should give you a good enough understanding but it does not hurt to read both if you have time. Feel free to ask questions after, I am sure someone will answer.
This article explains what a unity gain buffer (or unity gain amplifier) is and what its purpose is in a circuit.
www.learningaboutelectronics.com
In this article, we explain when a circuit needs buffering.
www.learningaboutelectronics.com
