I've finally got around to fixing up a Quad 34 Pre Amp I have had kicking about for many years. Whilst going through the schematic to effect the repair it got me thinking about some of the design decisions. The circuit in itself is nothing special, basic op amp stuff (although the tilt and bass circuit offers some interest).
I design scientific and medical instrumentation for a living, and this is the first time I have actually looked inside some Hi Fi gear! The issues I had all originated in the power supply (specifically a fault TL071 @ IC23) and this is what my questions relate to.
Why does this design have a virtual earth? Rather than just using a dual primary transformer and using the centre tap as an earth point? Why does the amplifier use unequal positive and negative rails? I’m curious to know what the benefits are. In my world we would always take a centre tap and tie that to ground (chassis & mains earth) and always use equal supply positive and negative supply rails.
Thanks for humouring me!
I design scientific and medical instrumentation for a living, and this is the first time I have actually looked inside some Hi Fi gear! The issues I had all originated in the power supply (specifically a fault TL071 @ IC23) and this is what my questions relate to.
Why does this design have a virtual earth? Rather than just using a dual primary transformer and using the centre tap as an earth point? Why does the amplifier use unequal positive and negative rails? I’m curious to know what the benefits are. In my world we would always take a centre tap and tie that to ground (chassis & mains earth) and always use equal supply positive and negative supply rails.
Thanks for humouring me!
I was told that the asymmetric supply provides the best operating point for the analogue switches.
Stuart
Stuart
No. The analogue switches operate on lower symmetrical rails, +/-7.5V.
The purpose of the asymmetry on the audio rails is to overcome a behaviour in the TL071/2s whereby they latch up if they hit the negative rail first. So the negative rail is put slightly lower so that can't happen. It isn't necessary if you upgrade to OPA(2)134 etc., but I've never bothered to equalize it after doing that when servicing.
As for the centre tap, they cost money, more than a TL071 and two resistors. Also the same transformer is used in the FM4 which I believe came first, so it was already available.
The purpose of the asymmetry on the audio rails is to overcome a behaviour in the TL071/2s whereby they latch up if they hit the negative rail first. So the negative rail is put slightly lower so that can't happen. It isn't necessary if you upgrade to OPA(2)134 etc., but I've never bothered to equalize it after doing that when servicing.
As for the centre tap, they cost money, more than a TL071 and two resistors. Also the same transformer is used in the FM4 which I believe came first, so it was already available.