A POINT OF VIEW: Rolling it off

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Paul McGowan writes .....

Recently I explained the real problem in using a direct coupled product like a power amp or preamp is not the DC they produce, which is minimal at best, but how these products handle DC from OTHER products.  If the designer has built the classic “DC to light” amplifier,  you risk big problems if you input even a little DC, which is why such claims are usually not entirely accurate.

If DC is amplified the same amount as the musical signals, which are AC, then any little bit of DC entering the unit will be amplifier by 10 to 30 times and that can spell big trouble for connected equipment – in particular loudspeaker woofers.  What this all means is that 99.9% of all amps and preamps aren’t really flat at the bottom end – because if they were they’d amplify DC – so by default these amps and preamps are rolled off on the bottom end.

This is an area where great differences in sonics can occur.  Ever heard a product that reviewers and yourself exclaim has “thunderous bass”, while others have “ok bass”?  When you look at the specifications of these two products no doubt they are ruler flat to below 20Hz and therefore it’s a mystery why they have such good bass.  Well, let me unravel some of that mystery for you.

When we design a product like a preamp, power amp, DAC etc. we roll off the bass starting at about 2Hz and below.  The point the designer chooses to begin the rolloff has EVERYTHING to do with how “thunderous” and “solid” the bass you hear on your music.  I have helped a number of younger audio designers just getting into the field with this knowledge because conventional engineering would tell you that as long as you’re ruler flat to 20Hz, you should be fine.

There are two ways to roll off the bottom end: capacitors and servos.  Let’s focus on capacitors and then we’ll move on to servos.

I explained earlier that placing a capacitor directly in the path of the audio is something to be avoided if at all possible; on a solid state design it’s pretty easy to do that.  But if we don’t use capacitors in the signal path to roll off the bass at low frequencies, where can we place them?  In the gain setting elements – where they are still technically in the signal path – but not directly.

Using a capacitor in the right place and out of the direct signal path, allows designers to have the product’s gain variable with frequency – DC has a gain of 1 and all the music has a gain of 30 – thus we can pass DC unimpeded yet unamplified and still have great bass response.

But capacitors are the easy way out and if you REALLY want the best bass impact and slam without any hesitation, you need a servo.

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