what significant differences between AU6600 vs AU6900, AU7700 vs AU-7900?

[ignoramustot]

Hello again Sansui brains trust

An occasional temptation of mine (when not enjoying the sweet sounds of my recently recapped and upgraded AU-555A) is to keep an eye open for an opportunity to upgrade from my trusty AU-4900 to an AU-6900 or AU-7900, if I can find a good one that is not too expensive (hah! ).. (And before anyone asks, the AU-5900 doesn't seem to offer enough above an AU-4900 to make the switch.)

I notice that the AU-6600 and AU-7700 seem very similar in design and features to their corresponding models in following series, AU-6900 and AU-7900, albeit with slightly better performance specs quoted when new for the later models (higher RMS wattage; lower distortion; etc.).

Are there any significant differences or improvements in the latter AU-x900 models that make them worth waiting for or pursuing over their equivalent siblings from the immediately preceding generation?

For example, and with reference to the recent entertaining parallel discussion in this forum about Sansui heat sinks, I notice the rear heatsinks on the AU-6900/7900 appear a bit more substantial or exposed than those on the AU-6600/7700. Does that have much significant difference on reliability?
e.g. compare http://www.sansui.us/images/AU7700_Series/au6600_6.jpg with http://www.sansui.us/images/AU7900_Series/au6900_2.jpg

(And being conscious to rate physical condition of the amp and known history of the seller as more important determinants when choosing between comparable available amps, in most cases...)

Thanks,
Rob

 

[bolero]

Good question: probably studying the schematics would reveal the answers.

I did read the AU-7700 has twice as many output transistors than it needed, so it runs easier & cooler apparently. I think the AU-7700 was the prototype for that whole generation of professional series that followed. The AU-6600 is a lighter version of the 7700: a lot of the boards are the same but less inputs, less eq, less output power.

Re heatsinks: I keep the amp above the tuner, so those rear heatsinks get good airflow. They extend back & clear the chassis of the matching tuners completely, so there's decent space back there above & below. Being elevated is better than sitting down at the bottom in a corner, IMO.

It's also easy to access all the jacks & switches at the back.

The AU-7900 has more power, and beefier heatsinks. I'm not sure about circuit improvements but they were advancing all the time: I wonder if the preamps changed much between x700/x900/ series? Was it just power amp design that evolved?

And why wasn't there an AU-x800 series?

 

[quadmatrix82]

I have owned a au-7900 for 15 years and have restored the 7700 6900 and most recently a 6600 -the tonal signature is all very similar each higher model just has that bit more power and punch and few extra features but the tone is almost the same

 

[Zekeman]

Agree. Tone is very similar. Sometimes my brain tells me that it likes the tone just a wee bit more with the 7700 series.