Amp question

dynamo

I scored a good deal on a JBL Synthesis S400 amp. Thing is built like a tank. It is a bridgeable 2ch and is full range but was used as the sub amp in the big JBL home systems. 

I hooked it up and it sounded good for 2ch full range. I haven’t had time for a critical listen but I’d say initial impressions are neutral to a touch hard sound. Played it for an hour at normal levels and it never got even remotely warmer than when it was off (4 ohm load too).

Anywho, on to my question. The amp has a 4/8 ohm switch, to switch voltage rails I assume. It’s rated at 200x2 rms at both 4 and 8 ohms, and 400x1 into 8 ohms bridged.

Why is it rated at the same power at 4 and 8 ohms stereo? Why not the doubling of power? It has enough power for what I plan to use it for, but I’m just curious. It certainly isnt lack of power supply, the torroid is huge with 60k uF power caps, and iirc it is fused at 10A. It says dynamic headroom is 1.8db.

dcibel

My HK6900 is the same, there's a switch in the back that selects the transformer windings for 4 or 8 ohm operation, ultimately choosing the rail voltage. It allows the transistors to operate in their optimal range for either 4 or 8 ohm loads. My HK6900 is a space heater though, very nice sound and also rated 200wpc into 4 or 8 ohms.

dynamo

I used to have an old NAD amp that had the rail switch but it still made different power levels. Interesting. I never ran the NAD hard so I ran it on the 8 ohm setting on 4 ohm speaker. Never hurt it and I liked the sound better at low to normal levels.

dcibel

If the power supply can keep up with your JBL unit, you should get 300Wpc+ if you use 4 ohm speakers with the amp on the 8 ohm setting.

rjj45

Yeah, what is that called? Power supply "stiffness"?  Usually I've seen specs like 100 wpc 8 ohms, and 180 wpc 4 ohms, so not totally double.  Years ago I read a review of a Luxman amp - 150 wpc 8 ohms, 300 wpc 4 ohms and 600 wpc 2 ohms. Christ, that's some beautiful engineering. 

dynamo

dcibel said:

If the power supply can keep up with your JBL unit, you should get 300Wpc+ if you use 4 ohm speakers with the amp on the 8 ohm setting.

That seems right. All these Synthesis amps are either made by ATI or rebadged/made to suit their needs other amps. 

The matching 5ch for the system is the S650, which is literally the guts of five of the THX Marantz skinny monoblocks shoved in one chassis. This appears to clearly be a derivative of an early 90s Parasound, most similar to the HCA 2003 THX high current amp, minus the third channel. Specs are slightly different  including 220w @ 8 vs 200, but that specs 300w @ 4.

A lot of these amps are easily identifiable as to the source of the OE, but this one hasn’t been as clearly identified on the internet. The board color and amp layout are either Rotel or Parasound, but the board layout, big wirewound resistors, and small film caps are all very much Parasound, or the factory that builds them. 

Looks like the newest ones are rebadged Crown custom install amps, which makes the most sense given the corporate ties. Not sure why they didn’t just use Crowns all along..

Based on the lack of any strain at all so far, so long as I’m not running a sub off it, I will plan to just leave it in 8 ohm mode all the time and forget about it like I did with the NAD.