Why are power amps so difficult to design?

September 27, 2017
 by Paul McGowan

One comment on “Why are power amps so difficult to design?”

  1. Over the years of my interest in audio (over 50 now) I have found the various protection circuits designers employ to protect their amps from smoking when given a difficult load to drive affects the sound and the overall performance of the unit. Many of the amps I have liked over the years were designed with enough guts to handle a 2 ohm load and used a simple fuse to protect them from a dead short or a load lower than 2 ohms. As a volunteer audio nut sales person (I would take too long to explain how that happened.) I demoed a set of speakers to a customer who liked what he heard and wanted to buy a pair. These speakers dropped down to 2 ohms right in the midrange where there was a lot going on so I asked what amp he was using to drive the speakers. The amp he had was rated at something like 400 watts a channel but I knew that unit did not like to see a load below 4 ohms. Sadly, I told the guy I would not expect that amp to drive the speakers he wanted to purchase. With this, he got very mad thinking I was trying to sell him a power amp. I had an amp there rated at only 40 watts that would handle a 2 ohm ohm and knew that it would actually do a better job. I suggested he bring his amp in so we could hook the thing up to the speakers he wanted. The next week he came in with his power amp. We fired up the amp, put on some music , turned it up a bit and within a couple of minutes the amp shut down! Next, I hooked up the rather inexpensive unit rated at 40 watts a channel into 8 ohms and double that into 4. We fired that one up using the customers music and it played and played and played at rather high SPL without any issues. The amp had meters and they would laying down completely indicating a bit more than “full” output. The customers faced dropped. He could not believe what he was hearing and seeing! He could not get his head around why his “monster” power amp would not drive those speakers. In the end, he bought 2 sets of the speakers and a couple of power amps. He bought the little amp for a small room he had and a much more powerful but very stable amp for a much larger listening room in his house. He called in the next week to say he had never heard his music sound so good so in the end, everybody was a winner

    With other amps using lots of protection circuits I have found they simply don’t sound very good. I have a rather large high output live sound system amp from a leading manufacturer and several very good home power amps. I have A-B tested the PA amp with one of my much smaller output home amps. The little amp sounded far better until driven into clipping and that level was much louder than I would normally want in that room. In this case, the big amp did play louder and it did not shut down. However, it sounded harsh on the top and did not perform all that well with strong bass passages. No warm sweet midrange either. It’s sound was not at all what I would want to listen to. Today, things are much more complicated regarding power amps with all the different designs and most areas have no stores with adequate setups where a customer can A-B various components. As far as “high end” stores only exist in large population areas. I would have to drive 4 hours to find a descent store and even those do not have very good listening setups to A-B components. So then, how could I make a good, informed decision regarding any component? How could I really know what I have in a power amp? Answer? You have no idea! You are buying a “pig in a poke.” Most folks just pick up something from the “Big Box’ store or order something online they have never heard.

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