ZinAmp at Nov 9 th and 10 th 2019 – San Francisco, CA
ZinAmp .. in a Nutshell Hybrid Tube / Lateral-FET Integrated • Tube Phono and Pre-amps, ‘loosely’ based on Marantz 7 • Power Amp – Class A/B Lateral FETs with Class A BJTs - in parallel • Inspired by the GEM amp • • GEM created by Graham Maynard Output for additional power amp • Because you never know… • Auto-power off.. • ..and on again .. even with a vinyl record! • weird, but fun to demonstrate • EQ Toggle Switch • Flat and Dynamic settings • Similar to a loudness curve, but quite subtle/gentle • 120W Per Channel – probably! •
Lid off !
..and on again!
Block Diagram
Why Bother? • Modern mainstream amps in 2016 sounded underwhelming to me Got interested in vintage equipment – 70s & 80s Technics et al • • Research led me to a “dream combo” of a Hafler DH200 and a Marantz 7 Couldn’t find these affordably in the UK • Wanted a Tube Phono Integrated with a Class A output – didn’t exist! • • I was going to have to build this amp! • Found a really good mentor to help me
My Mentor Henry Dulat – UK “Top Repairman”!
Acknowledgements Rod Elliot - ESP Doug Self https://sound-au.com
In memory of… Graham Maynard • UK DIY Audio and Radio enthusiast • Creator of the GEM Amp • Respected contributor to diyaudio.com • Passed away in October 2014 • www.bovan.net
The GEM Amp • Conventional design with … • Class A/B stage • Single Ended Class A stage • Two local feedback loops • Quiescent supply of 600mA • Carlos from Brazil built one
Graham’s Mission • To achieve the sound of John Lindsley-Hood’s 1969 Class A design • 100-200W power – needs Class A/B • Crossover distortion in A/B made worse by back EMF from loudspeakers Not visible when testing with steady sine waves into resistors! • but is present with real music signals into speakers, due to transient back-EMF • • Parallel Class A stage mitigates effects of back EMF Smoothing the crossover region • Similar idea to Quad 405 current-dumping design from 1978 • • More info at www.bovan.net
John Linsley-Hood - 1969
My Challenge • To get the GEM amp to work in my ZinAmp integrated box • I have respectable heatsinks, but not like mono-blocks! • Limited space for filter capacitors • Avoiding noise and hum is a key constraint… • ….followed by space inside the box
Keep it Simple (Stupid)
Design Decisions Push-pull Class A would be more efficient • Suggested by my mentor, Henry Dulat • Should requires less quiescent current • Cooler running than the GEM single-ended • Lateral Mosfet A/B Stage • Interest in Hafler led me down this path .. prototypes already developed • Requires fewer components than BJTs • More thermally stable etc. • Regulated Power Supply • Smaller capacitors but a little more dissipated heat • Remove one of Graham’s LFB loops • Feeding back into –ve rail • John Broskie (tubecad.com) suggested this would be noisy at high-volumes • Broskie also suggested a regulated supply •
Power Amp - schematic Push/Pull Class A BJT Output • - in parallel with FET Class A/B Output • Local Feedback to VAS • Bootstrapping for VAS • Additional Bootstrap driving +ve side • of Class A stage
Driving the Push-pull Class A • VAS with parallel followers VAS Follower 1 • VAS Follower 2 • • Collector of each follower drives one side of output • Both followers are bootstrapped
Power Amp - PCB
Regulated Supply - schematic Conventional Design • Rod Elliot / ESP • +/- Rails • Load Limiter • Big Power Transistors • - Darlingtons - 10-15A rating FB transistors also need to be fairly • robust Dissipates a little heat • Does the job! •
Regulated Supply – PCB
Setup and Biasing • Aim: to maximise the amount of class A current without overheating! Using double-die FETs so twice the A/B bias current • • Class A stage will not ”mop-up” c/o distortion from under-biased FETS FET bias current of < 70mA can sound ’chalky’ (Exicon FETS) • Sweet Spot • Class AB biased to 75mA – or double-die FETs to 150mA • Class A biased to 200mA • Sounds nice and doesn’t run too hot • Class A stage generates most of the heat though • Tempted to try mono-blocks with bigger heat-sinks!! • Plenty of scope for experimentation … and a bit more current! •
Setup and Biasing cont.. • Voltage across R bias = 700mV • V GS = 350mV per device • underbiased • Voltage across R bias = 900mV • V GS = 450mV per device • Sweet spot - sufficient • Voltage across R bias = 1200mV • V GS = 600mV per device • Ok with single-die FET • Double-die FETs run hotter
Trade-offs • Rails can’t go beyond 48v without encountering heat issues Tried 58v - sweaty – approx 160 ° F • Power output intrinsically limited to about 120W pc music-power • How much power do you need? • Push-pull Class-A vs Graham’s Single Ended • Better Rail Rejection – although regulated supply mitigates this • Lower current, less heat • Lower second order harmonics perhaps no longer predominant as with single-ended – although • class A stage is only providing 1 watt or so. • This is a work in progress! Subjective listening is good…. • …though full measurement/analysis is required • Bob Cordell measured a THD of approx 0.07% yesterday, mostly lower 2 nd 😏 •
How does it sound? • Really good … well, I would say that! • Definite step forward from the Class A/B amp I had built previously I wouldn’t go back • • Gives a sense of space between notes and sounds – no smear Like the blacks in the picture on a good plasma-screen TV • • Vocals and drums sound particularly life-like Anything that’s acoustically mic’d • • My wife likes this amp – and she has better hearing than I do 👃
Please come and listen Questions welcome – thanks!
Why not just over-bias the FETS? Over-bias FETs into Class A: Class A/B in parallel with Class A: Moderate heat dissipation • Moderate heat dissipation • Class A at moderate volume • Class A at moderate volume • | At the expense of | Retaining headroom • Slight loss of headroom • | At the expense of | Loss of Class A performance at higher volume • Loss of Class A performance at higher volume • Either way, as you crank up, the speaker back-EMF increases and the Class-A effect diminishes | Although with the parallel class A stage, you get a little more headroom | …but the GEM amp was simply too interesting not to try!
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