Simple amp - symmetric or not?

[JensRasmussen]

Hi all,

A friend of mine needs amps for his 5.1 system and has asked me to find out the best solution looking at spec vs price.

The amp schould be arround 100 - 120 W pr. channel to have some headroom and to make the amps powerfull enough to drive his system at high levels.

For the sub I'm thinking a bridged amp?

I'd like to make a descrete amp, the problem is the VAS stage.

If I go for a symetric configuration I need to double the number of transistors in the input stage but are symetrics designs better sounding? - is the money well spend?

The result will be avalible for free download at my website

\Jens

[subwo1]

I like the symmetric especially if the outputs are complementary. It can provide twice the drive current. I know the dilemma about needing to work with more parts. The cost is often not the main thing, for me it is the extra complexity and effort. For the sub, bridging is good because it allows more output power and some distortion cancels, so the circuits may be able to be simpler.

[JensRasmussen]

Hi,

If I remember correctly D. Self only has non symmetrical designs in his book about power amps, and Slone has only symmetrical in his book – why is this?

Thinking about the operation of the symmetrical design it must be superior to the other type because both the positive and negative signal has the same signal path (Assuming of cause that NPN and PNP transistors behave the same)

But since there are several nice non symmetrical amps out there (P3A I have heard and ASKA witch I regrettably haven’t heard – among others) are these as good as a well build symmetrical amp all other things being equal?

In the end, is the choice made to satisfy the eye or the ear?


\Jens

[subwo1]

Hi,
could it be to tickle the desire for both of those aspects or just to have something unique to set it apart from the rest. The main things I would strive for are the best performance for with the least cost and complexity regardless what the actual topology is.

Honestly, being more of a pure tinkerer, I have never tried any pre-designed amps other than the Tiger ages ago. In constructing quite a few amps for one project like a 5.1 system, I would even consider audio chip use in some of them. But I can't say I know much of anything about home theater systems.

[mcp]

Hello Jens

Symmetrical topologies have their strengths but they are also not without their share of problems.

If one of your design objectives is for a wide-bandwidth, high slew rate power amplifier, I would suggest this type of topology.

[JensRasmussen]

HI,

My design objectives are to make a simple and powerful amp with a minimum of parts.

I want to use a configuration that is proven, but design the amp from the bottom for my own education.

\Jens

[subwo1]

Hi,
those are good criteria to me. One thought that comes to mind for a powerful class AB sub amp is not even symmetrical, but uses only N-channel mosfets. It is the schematic X-pro posted not very long ago. It is an output stage with constant current setting the bias. X-pro's output stage
You drive it with a class A or AB driver stage of your choice and can include it in the gobal feedback loop.

[mcp]

Hi Jens

Pass Mosfet Citation 12 would be a good starting point. You probably won't get 100 watts, but once you are familiar with it, it is not difficult to modify for higher power.

[djk]

Over thirty years ago I built my first SWTPC Tiger .01, symmetrical from input to output.

For many years I felt that this was the only topology that sounded good. The big Luxman, Accuphase, Bryston, SAE, GAS, all had the same basic topology.

The amplifiers with single diff pairs didn't sound as good.

The amplifiers with quasi output stages didn't sound as good.

Though similar, the Leach seemed to sound the best.

Why?

Several reasons, the first being capacitors. The Leach had power supply bypass caps, signal path bypass caps, and no ceramics. Leach didn't like an active current source for the input pairs, he uses resistor to a regulated voltage.

SWTPC came out with a lower cost version of the .01, gone were the meter, input level conrol, active current sources, and pre-drivers. While it didn't measure the same, it sounded the same.

McIntosh went from a quasi output stage to a symmetrical type output stage driven with a single diff pair and a single VAS. No CCS, just resistors to a regulated supply. Stock, the amplifier sounded very soft, lacking in detail, and muddy in the bass. Everyone always blames the output transformer and the barely out of the single digit damping factor for this sound.

Dead wrong!

The McIntosh has no power supply bypass caps, no signal path bypass caps. Adding these changed the sound so much that the frequency response was measured! Dead flat!

What does this tell me WRT home theater?

I would build a small supply for the front end for all five channels, run it a few volts higher than the outputs.

Front end?

The McIntosh is sure attractive in its simplicity. A diff pair plus the VAS, it makes the AKSA look complicated. But keep in mind that the original Leach symmetrical front end is only three more transistors that add less than $5 to the cost of all five channels. You may have to flip a coin on this one.

Output stage?

Emitter follower with MJ21193/21194 outputs, one pair for each 125W of output required. The QSC USA1310 does 1KW with eight pairs. You want to use 30Mhz plastic, fine, just use twice as many. 100W at 4 ohms is 7A collector current. At 2A the plastic devices can do 50Mhz, what do they look like at 7A or higher? The MJ21193/21194 are only 4Mhz, but they can do it at 7A+, they are faster and have higher gain at high currents. Plastic doesn't handle the power as well, but if you use twice as many you can get the current down and the speed up too. I use metal.

http://homelf.kimo.com.tw/skychutw/a...pzilla_sch.jpg

The SON is 150W/4R, 250W/2R and bridges to 300W/8R or 500W/4R. The MJ21193/21194 parts are much heavier than the original outputs, you could probably lose the current limiting.


The MJ21193/21194 have such good SOA at ±100V that it makes no sense to bridge for high power, just raise the voltages and parallel enough outputs.

[JensRasmussen]

Hello,

Thanks for a great reply

>
The McIntosh has no power supply bypass caps, no signal path bypass caps. Adding these changed the sound so much that the frequency response was measured! Dead flat!
>
Are the supply caps you refer to the small caps across the bigger caps?

Are the signal bypass caps the one across the VBE thermal transistor in the output stage?

I have alvays used 1µF film caps across the big eletrolytic caps, and 1 µF across the VBE transistor in the ouputstage for better speed.

BTW can you please check the link? I can't get it to work

\Jens