Q17 is on GitHub under GPL3
Measurements
How to order your transformer
How to order your own PCB from JLCPCB, or click & order your Q17 with dual output pair PCB @  Purchase parts for one channel with one click from mouser.
Purchase parts for active power supply from mouser. Does not include synchronous rectifiers and optional decoupling MKP.

Introduction

Another QUAD405-maniak ? No, just an old one.

Yes, I must admit that, I am. I consider myself an audiophile too. You know … cables and other stuff … :-D

In the last 35 years I did a lot of modification based on this brilliant principle named Feed-Forward Error Correction.

Why ? Because it works very well and the potential behind was barely exploited. 

Please, don’t get me wrong. QUAD405 was an exceptionally designed amplifier, but highly limited by the parts available at the time.

With today technology and innovation in opamps, mosfets and power supply, the design can be pushed to sound fantastic. I mean, really fantastic.

Usually I do not write so much, because I’m an engineer :-D. On other side, I also appreciate well documented projects. So, this is my effort to explain, in simple words and some basic arithmetics, amplifier topology, operation and some pellicular sonic decisions.

As I already mentioned, I have performed a lot of modifications over initial QUAD405 schematic. The result was a very revealing amplifier, with very low distortion. The sound was analytic, for some people too analytic, but it was missing something very important, the synergy. At low listening levels sound was flat and boring. Otherwise, at high level, the amplifier was OK, but still missing dynamics in entire bandwidth. Unfortunately this is same with original QUAD405 and even recent releases as QUAD606/707/909. Then I fall on the dark side of the audiophile force and started to change operational amplifiers. Some sound better than others, each have his own sound signature, but they do not help too much.

Finally I decided to go further and change the design completely, taking an audiophile approach. Don’t put distortion as forefront rule and instead make it sound great. In fact there are so many amplifiers that measure 1% THD and sound amazingly good. This does not mean I do not base my decisions on simulation and measurement, it mean that I’ll trust my ears more than my oscilloscope. :-D

Here are my initial technical requirements:

- stable operation. Amplifier must be stable in any operation condition. No oscillation at any frequency.

- 100W in 8ohm at 1kHz. This should be enough for an audiophile as long his “impossible to drive” speakers are moving.

- frequency response cutoff 10Hz - 100kHz. This will allow a decent slew-rate as well.

- IMD inter modulation distortion under 0.01>#/p###

- THD total harmonic distortion under 0.01% @ 1W&full power

- design must be affordable, with relatively easy to find components and equivalents THT or SMD.

- cost effective and easy to build. High-end audio must be affordable.

- open source. QUAD405 is a great design, who inspired people like me, to learn and want more. Let’s keep it that way and offer even more, schematic, simulation, source pcb files and bill of materials.

- designed by using LTspice simulation with final touch by extensive bench measurements and testing. Starting with KiCad 6.0.2 ngspice simulation is possible and files have been made available by Holger Vogt 

https://forum.kicad.info/t/simulation-examples-for-icad-eeschema-ngspice/34443

- final design decision are always my platinum ears. ;-) This mean Q17 is designed by comparing different topologies and final decision is taken, not by measurement, but by listening. Q17 is indeed designed by ear. :-D

And here are on-ear requirements:

Sound is paramount. It must sound very good, at any level. There must be no listening fatigue. Sound must be vivid, full body, plenty of harmonics with long decay,...