I've been meaning to get around to updating this by folding in the improvements to the diamond buffer stage made during development of the Sapphire 3 headphone amplifier. Here is the first look of the bboard v2 under LTSpice.
I've gone back to simple emitter resistors on the input, running under much lower current to keep the input impedance high. The output is simplified to a basic Sziklai compound transistor pair with the bulk of the bias current running in the second transistor.
In terms of distortion, for line level output level, CCS loaded input has no advantage. I'll have to double-check PSRR and a few other things before signing off on this version though.
Posted 20th October 2015 at 07:49 AM byabraxalito Updated 26th October 2015 at 01:42 PM byabraxalito
Nowadays with discrete transistors as affordable as they are, the most cost-effective solution for a particular audio application may well be a discrete one when SQ (rather than numbers) is uppermost. Audiophile faddishness about discretes aside.
Here's a case in point - my pic shows a headphone buffer where the design aims were lowest cost, smallest size and lowest battery drain, while maintaining acceptable SQ. There are 28 transistors which go for 0.04rmb each on Taobao. That's 1.12rmb. OK so you can also buy 2 NJM4556s for that, but how do they sound? In my experience of building an O2-alike, not so great. They're also going to take 15mA at 7V whereas this design takes 6.5mA at 3.6V input. So an integrated design will be more than 4X as power hungry. With a 2600mAh single cell LiIon this could run for 400hrs - over two weeks continuous if played at low level.
The power supply is created by an LM2662 which inverts the 3.6V positive input for a -3.6V rail. It...
I have had the chance to listen to Q-cap Polyprop vs PIO. This is a very interesting pair of caps indeed. There is a difference. The Polyprops sound modern in a good sense, very good bottom and the upper freqs ar not extremely in the foreground. In an open sounding cirwquit, this makes sense. A very good capacitor, indeed.
But in my ears, the PIO are even a little better, little more open in a very very musical way, but not sharp. Wonderfull flowing and livelike, natural and dynamic.
The choice always depends on the rest of the chain and personal taste , But these are my favourites now.
It´s fun to listen with both of them and I´m really looking foreward to the time, when they are burned in...
Here's the power supply I've lashed up to feed the balanced SE classA amp.
Its fed from a 5VA EI transformer with a 65VAC secondary. One 390uF cap follows the rectifier, then there's a 30mH choke, two 390uF caps beyond that.
A series regulator is made from a string of 3 TL431s as reference (the max from a single one is 36V - I've gone for a total of 78V) and that's followed by a 2SK213 simply because I had no other high voltage transistor to hand. There's an RC filter feeding the gate of the MOSFET to reduce the output noise from the shunts (68k,200nF).
Output ripple isn't visible on my scope but I plan to feed the output into my AC millivoltmeter and see what its giving out in terms of noise.
One thing I hear from objective audiophiles is that amps should strive to be as neutral as possible because you should only hear what the record producer intended for you to hear.
Philosophically I do agree that I should hear what the artist meant for me to hear. However how do you know that the album was recorded while listening to a similarly neutral amp? Recording to a master tape usually doesn't include the amp powering the monitors so whatever they heard through their amp and speakers isn't captured on the tape. In order to really hear what was in the studio, you would need the same amp and the same set to speakers to listen to what they listened to. So if my home setup has a miracle amp and speaker that truly is ruler flat from 1hz-1MHz and 0.00001% THD, I probably still am hearing things differently then the artist because they probably used an average monitor that has -10db at 30Hz.
So how important is it really that an amp be neutral?
Posted 10th October 2015 at 01:55 AM byabraxalito Updated 16th October 2015 at 05:36 AM byabraxalito
Headphone amps aren't any different from speaker amps in principle - what you hear (apart from a bigger version of the input signal) is the power supply's noise coupled through the inadequate PSRR of the electronics.
SE classA operation is a way to minimize the generation of power supply noise by arranging the current flow to be constant to a first order so that any remaining ripple on the supply is the result of the finite output impedance of the follower's loading current source and those of the driving stages. But how significant are these 2nd order effects? This design is an attempt to find out - by reducing them as far as practicable.
The idea is to run SE classA at a much higher voltage than is needed to drive the 'phones (balanced, with 80V supplies giving 144V peak-peak) then step down the output voltage with a custom-wound output transformer. This has the effect of increasing the PSRR of the amp's output stage. I'm not worried overmuch about the PSRR...