Yesterday’s post covered output transformers on tube power amplifiers and why they were needed. It also covered the fact that IMHO transformers are the last thing I would add to the output stage of a solid state power amplifier because output transformers like this contribute so much to the sound quality of an amp – and not in a way that would make better sound, again IMHO.
Several of you asked “if that’s true, why then do some McIntosh solid state power amplifiers have transformers?” My first thought was they must have added it to get a tube sound since the venerable McIntosh product line was tube based forever and a day.
I did a bit of research and found this not to be the case – it seems they were trying to solve a classic engineering/marketing problem with power amps – by using an output transformer – not something I would do but it’s worthy to note what they did and why.
What you want to accomplish when driving a loudspeaker is to develop a voltage across the loudspeaker terminals that is large enough to move the speaker drivers and make sound. That voltage should be an exact, but larger, version of the musical signal fed into the amp.
In order to deliver voltage into a loudspeaker it takes current (power) and the combination of volts and current is expressed in watts.
Loudspeakers are tough to drive and take lots of power, as we’ve been discussing in these many posts. They need this power because they’re using it to build a magnetic field in the speaker driver – a field big enough to move a heavy cone and push the air in the room around. We express this need for power in Ohms: the lower the Ohm number the more power we need to move the cones and air.
As the Ohm number goes down, the need for power goes up proportionally. So the power needed to make an 8 Ohm speaker produce sound in the room needs to double when the Ohms cut in half to 4 – and double again for 2 and so on. Lower Ohms, higher power to achieve the same sound pressure.
As an amplifier manufacturer, we need to design power amplifiers to handle all practical loads a loudspeaker might present to us – and we never know what speaker our customers are going to try and power with our amplifiers. Speaker load impedances vary from manufacturer to manufacturer and when they operate, they vary from their specified Ohm rating with the music as well!
This means a good power amp must be able to produce the same voltage regardless of the Ohm rating or the actual Ohm load of any loudspeaker. In practical terms this can be daunting to a power amp manufacturer. For example, our new power amp we hope will be shipping this summer produces 350 watts into 8 Ohms, 525 watts into 6 Ohms, 700 watts into 4 Ohms, 1050 into 3 Ohms and 1250 into 2 Ohms. To be perfect, our amp should do 1400 watts into 2 Ohms and 2800 watts into 1 Ohm etc. The numbers become astronomical at some point drawing far more power than even the wall AC power can supply.
From a practical standpoint, an amp the size we are going to produce should cover 95% of the loudspeakers, rooms and music types out there.
So back to the McIntosh solution. The problem for any amplifier manufacturer is two fold: how do you rate the amplifier and what average power level do you design it for?
Most amps are rated at 8 Ohms, but most speakers are 4 Ohms. From a marketing and practical standpoint I want to say our power amp is a 700 watt product – yet I will be held to the 350 watt rating even though that number is antiquated and less descriptive of the amp’s true use case.
What will be the average power used on this amp? The FTC developed a rating system years ago that said whatever wattage level you wish to rate your amp at – the amp must be able to operate at 1/3 the rated power for 1 hour without shutting down – which ours will easily do at 1/3 700 watts. Unfortunately, the magazines will still use the 8 Ohm rating.
Interesting issues – and what McIntosh did is designed their power amplifier to always run at the highest power rating it was capable of regardless of speaker load – through the use of an output transformer to couple the speaker. This is exactly the opposite of what tube designers do – couple low impedance loudspeakers with high impedance tubes.
Using this method allowed McIntosh to run and rate their amps at one wattage level regardless of what impedance speaker is attached and so the amp always drove a very low load – and they were able to rate their amp at a very high wattage and get away with it because it did that same wattage for any impedance speaker. Clever.
They solved one problem but created many others – through the sound quality degradation of an output transformer – as we’ve discovered over this series which brings me to my final point: if you put all your resources into solving a “problem” that your marketing department has, at the expense of your product’s sonic performance, you’ve probably done a bit of disservice to your customers and their musical enjoyment.
McIntosh has a very loyal following and one that is certainly deserved – they are a cool old company that has made wonderful products throughout the years.
I don’t agree with all their engineering decisions and I am sure they don’t agree with all of mine.
Paul McGowan – PS Audio, Intl