I found the parts to be quite decent quality on the tiny jlh 1969 headphone amp kit. The audio performance is pretty good, and can be very good - I'll be doing some more measurements soon.
......
One problem I had was mounting the pcb - the pot doesn't mount flush with the edge of the pcb at the front - meaning it sits back from a front panel .. just enough so that the small amount of pot thread isn't able to secure it strongly. I use a froont panel thickness of 1.5mm.
I had to grind off some of the pcb front edge to make it work. Also, I had to use a couple of brackets to securely mount the pcb to the fron panel. A couple of nut-heads protruding on the front panel face.
Best to do before soldering up the jack and pot
Apart from that - took me a while to figure the output devices mounting on the underside of the pcb - no documentation of course
Get it the wrong way and you'll know it quick enough!
And of course, those itsy bitsy tiny smt resistors are a royal pia for me - I can barely see them let alone solder them. I used some tweezers and so on. What normally takes 15seconds takes a few minutes
Progress, right?
The upside (apart from allowing the small form factor) is they are supposedly tight tolerance which makes for excellent matching -- I verified very close performance, left and right.
That is pretty important to me, because I find the pcb really excels at driving a load 'balanced mono bridged mode' where symmetrical performance across the left and right is a plus.
ie. feed the pcb module unbalanced inputs which are the left and right phases of a balanced signal, say from an XLR connector. Then take the 'left' and 'right' outputs and connect the load across them.
You now have a 'mono' amp with balanced XLR input and balanced XLR output, capable of (ideally) 4x the power to drive a load (somewhat less in practice) and with super low THD, resulting from cancellation effects in the load.
The load can be anything from 8ohms to 10K. THD+N is super super low
and the resulting sound is pretty damn fine ).
Can be used as a powerful line amp/distribution amp, summing amp, high performance headphone amp capable of driving several phones etc.
At present, I'm doing a couple of 'recycled' builds, in the shells of what once were my first couple of pieces .....
I'm just setting up to really do some detailed measurements etc comparing with regular opamps and such
Since then I have also bought a few more JLH class a based modules
- a headphone amp but in a larger form factor, onboard heatsinks, bipolar supplies (no output cap) and some of other '1996 updates'
- low power amp (5W) tiny pcb version of the '1969 version' for use as a line /distro amp
- high power amp version using T-O3 output devices and the '1996 updates' (30Wrms of audio power, 100W of heat, per channel!)
- small capacity and large capacity regulated psu modules giving superior ripple performance over standard lm series 3 terminal regulators
.....
So, I'll be measuring up and posting the spectra and findings in my other thread .. soon
I think it's fair to say I've been pleasantly surprised by the audio performance from such a simple circuit.
But the 'mono balanced bridge mode' performance has been the real eye opener, particularly when driving a good output transformer - where the 'balanced mode' beats down the 'para-feed' operation by a good margin in the low THD stakes.
The net result is that I am now looking to integrate some JLH circuitry with some single-ended tube circuitry
Had to happen
Basically looking at a cheap and simple way of getting the wonderful spectra of say a 6922 tube's plate , to a speaker - with some power, and without losing the h2 and adding as little h3+ as possible
Nothing revolutionary there - as usual plenty, plenty others have trod this path afore me!
Push-pull tube amps have a fair bit of THD and a good amount of it h3 and h3+.
Class B or AB transistor amps have very low THD but still a fair amount of h3 and h3+
Class A tube, without nfb, has a lot of THD but most all h2. (da good stuff!)
Preserving the simple, most all h2 spectra all the way to decent power ie. 30Wrms or so, is not so easy.
So far, from my own builds, only my 'bigSE amp' has gotten me there.
I think I can better it AND in a slightly more convenient package!
My 'bigSE' amp weighs in at 25 kg for mono 22Wrms
and has a 250VA plus psu traffo