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The front view of the QSX Amplifier.
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The printed circuit board of the QSX Amplifier
Why is it so important with the phono amplifier in order to get good sound quality?
A real difference
I thought, honestly speaking (back in 1986), that I couldn't hear any real difference between different kinds of phono amps. Boy, was I wrong! The difference can be small but noticeable to huge! I'm not joking here. A good vinyl, played with a good turntable and good RIAA correction amp can sound really, really good, but I don't say that vinyl is better than digital sound. Because we have a huge vinyl collection we don't have to play the with bad gear. The vinyls are worth to be played with good hifi stuff.
A fragile link
Today the amplifier and the CD are the strongest links in the hifi chain. When the vinyl was popular there were very often deficienies in the phono amp department and surprisingly often nowadays if you buy a 100-300 dollar unit!
Nowadays you can buy rather cheap separate amps with very poor sound quality (of course there are better ones too). I made 'reverse engineering' of one amp in the price region of 50 dollars. I was horrified, to be short! Junk! I have also looked at more expensive ones and I'm not impressed.
What makes the phono amp good?
The main design problem is to make an amplifier with an output stage which can deliver rather much current into a highly capacitive load, the RIAA correction filter. The correction filter may have an impedance of 300 ohms purely capacitive which means that the output stage must be able to deliver 10 - 20 mA with a phase angle of +90°. This is a difficult task to do together with low distortion and a stable signal shape. Many amps can't deliver square wave into a capacitive load without overshoots. The other considerations are low noise and low distortion, not so hard to achieve.
One forgotten thing is the input impedance. If you have too low input capacitance (for your the chosen pickup) you get very often a very sharp resonance peak at 10-20 kHz. This is sometimes a positive effect to a tired old amp with an equally tired cassette deck but is a bad thing in a modern stereo and good loudspeakers. Especially MM-pickups are sensitive. My Ortofon FF15 needs around 400 pF in order to sound good. MC-pickups are not that sensitive but they still need correct load.
How does it sound when the output capability is low?
The sound is without life, like seeing through dirty glasses, like an old Dolby-C recording. You can't put finger on what's wrong. The high tones are especially effected. If you can compare with a better amp, you are surprised how big the difference can be! The clicks and noise sounds also different. A slow (or bad) amp can't handle click equally well as a high performance amp. With a good amp you will get much more listening pleasure, sounds nicer, the clicks aren't that irritating
How good does vinyl sound really?
We (me and my friend) amused ourselves one evening with comparing CD and the same album on vinyl. Test albums were (don't forget it was 1986): Dire Straits, Brothers in arms, Benny Andersson & Björn Ulvæus (ABBA), Chess, Roger Waters, Pros and cons of hitch hiking and David Bowie, Let's dance. These albums were recorded with digital technology. With normal listening levels (quite high actually) there were only minor differences between CD and vinyl. But when the volume was very high, of course, you could hear noise, clicks and rumble from the vinyls. You must play at quite high volume in order to determine which sound source is CD and vinyl.
With this I want to say that my phono amp can measure itself with very expensive amps if you use it for MM-pickups. The amp in MC-mode is also high performing but you can make some enhancements if you replace the transistors in the input stage.
The original design used only low price transistors but my brand new professional pcb has been equipped with the possibility to use ultra low noise matched transistor pairs.
How does it sound nowadays?
I have now listened to the remake of the amp. I'm very pleased to be short. It's was so exciting to apply voltage for the first time and connect my headphones directly to the output of the amp. The first albums were Anabel Lamb, the first and second and Donna Summer, "Living in America". I notice that I have forgotten how good vinyl can sound. It was now some time since I listened to vinyl with critical ears. It's fun!