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If I put my notes here, I might be able to find them again later!
The Lab Circuit ideas and innovations, not yet part of a full project.
Old

Headphone amplifier ground. Where to make the chassis connection?

Posted 2nd February 2016 at 07:11 AM by rjm (RJM Audio Blog)
Updated 24th February 2016 at 01:02 AM by rjm

The discussion thread at the headphone forum is here, but I wanted to throw out the problem to the general blog-reading community here at diyaudio to see if anyone can nail this.

The earthed chassis (light blue) must connect to the circuit common i.e. "ground" (pale green). I do not know where the best place on the circuit ground is to tie that connection.

Suggestions please!

(COM and GND are completely equivalent pads on the circuit board, while IN- and OUT- also pads on the board but physically further away on the ground plane.)

****

Answer: as long as it connects at one point only, or the same point of both channels, it doesn't seem to matter at all. I have it connected at the ground tab of the headphone jack and that seems to be as good as anywhere.

****

The noise was in fact magnetic interference emanating from the transformers. Grounding layout changes / electrostatic...
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Old

capacitance measurements with the hp 4192A

Posted 10th June 2015 at 01:10 PM by rjm (RJM Audio Blog)

Work stuff. I was writing Labview vis for an hp 4192A LF impedance analyzer and needed something to measure to check the data acquisition program. So I stuck some of my audio capacitors I happened to have into the 16047A test fixture "just to see".

I have no idea what these measurements are telling me other than yes, the 0.47 uF capacitors are indeed 0.47 uF ... up to about 0.5 MHz anyway. Maybe someone can do some technical analysis. I was struck though by just how quickly the inductance of these big film caps kicks in. As audio coupling caps they are fine, but if you are silly enough to use them as power supply bypass for example...

There are some reproducibility issues I'm still coming to grips with, but the differences shown in the plots is definitely from the capacitors themselves and not the leads or random variations. I've measured them several times over with similiar result.
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Old

Jenson JE-990 discrete op amp

Posted 8th June 2015 at 07:45 AM by rjm (RJM Audio Blog)
Updated 9th June 2015 at 12:09 AM by rjm

LTSpice copy (protection diodes removed) of the original JE-990 circuit. Even with BC327/337 subbed in for all the original transistors the simulation works without further modification.

C1 seems to be critical for stability. C2 and C3 damp overshoot seen on the simulated square wave response, hinted at by the high frequency rise in frequency response shown in the screen grab below.

My impression is that this circuit is of the heavily optimized, no-stone-left-untouched variety.

Sourced from m.nats page and The John Hardy Company.
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Old

AT-HA5000

Posted 21st May 2015 at 01:32 PM by rjm (RJM Audio Blog)
Updated 27th May 2015 at 01:54 AM by rjm

This post, about a push-pull MOSFET output stage for a headphone amp, got me thinking again about the Audio Technica AT-HA5000, which is something of a benchmark in its class. The "basic" signal circuit (not a complete schematic, it's clearly missing some ancillary details) is attached below. Probably out of MJ originally.

I think with any circuit like this, the differences are less about the MOSFETs and the operating points and more about the front end and what tricks you do with the power supply. That, and how you make sure it doesn't go up in a puff of vaporized silicon taking your headphones with it.

The Audio Technica schematic has nice old-school Zener regulators, a discrete JFET front end, a long tailed pair + current mirror for voltage gain and "proper" BJT Vbe multiplier and driver stage. Q7 is presumably in thermal contact with Q10,11 providing overtemp protection, and the output has a protection relay (not shown in detail) for...
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Old

Szekeres 2015

Posted 20th May 2015 at 06:00 AM by rjm (RJM Audio Blog)
Updated 18th June 2015 at 11:26 PM by rjm (added schematic of original version)

The circuit was originally hosted on Headwize, but the site seems to have gone offline.

It was a single stage resistively-loaded MOSFET follower, a unity gain current buffer used to drive headphones.

Some updated versions provided below. As noted in the comments the "Reverso" version with the CCS on the V+ and a p-channel mosfet has better PSRR performance, especially with voltage divider network R6,R7,C4 on the collector of Q2.

So good in fact that I switched around the n-channel version to use a negative voltage rail to obtain the same result!
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Old

JLH-2005 headphone amplifier

Posted 18th May 2015 at 08:15 AM by rjm (RJM Audio Blog)
Updated 27th May 2015 at 01:52 AM by rjm

I was perusing this thread earlier today. Which led me to what I think is the original source, at least as a modern, relatively clean headphone amp version of the original original (by way of ESP).

Some comments from our own Nelson Pass are seemingly relevant.

AC coupled, and simplified to a single supply voltage, the circuit can be run at +5 V operation (USB, etc) with fairly decent performance.

The circuit is optimized for 16 ohm headphones.
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Old

A headphone amplifier gain calculator

Posted 2nd May 2015 at 05:30 AM by rjm (RJM Audio Blog)
Updated 2nd May 2015 at 05:33 AM by rjm

You input the headphone sensitivity and impedance, and it spits out what I think is the ideal amplifier gain.

Even if you disagree (personal preference, difference input levels, etc.), the difference will be consistent regardless of headphone as long as the specified parameters are correct.

The gain value setting is tailored to normal line level input and listening fairly loud with the volume control at 9~10 o'clock. The output series resistance is assumed to be zero ohms.

Adjust as desired, and note that 3~6 dB either way will still be a usable. If your amp has a large output series resistance the gain Av should be scaled up as,

(Routput + headphone Z)/(headphone Z)
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Old

Z-reg II improved simple Zener voltage regulator

Posted 22nd February 2015 at 01:24 AM by rjm (RJM Audio Blog)
Updated 28th February 2015 at 06:17 AM by rjm

I've added an additional RC filter stage (R3, C4 in the schematic below) before the Zener diode, substantially reducing the amount or ripple on the transistor base by cleaning up the voltage applied to the Zener reference. (The original Z-reg is described here.)

Circuit shows C2 with a value of 300 uF. Typically much larger values are used. I kept the filter capacitance to a minimum here to show circuit working with a reasonably high ripple (1 V p-p) on the input. The rectifier diodes used here are of no particular consequence, I just wanted the simulation to generate a realistic sawtooth for the input.

***

OK, this doesn't do as much as I originally thought. The improvement is mostly below 100 Hz, whereas the ripple is mostly in the 100Hz-1kHz band. There's perhaps 3 dB less output ripple, but that's about it. You can verify this yourself in LTSpice, just cut the wire between C4 and the junction or R1-R3 and rerun the sim.
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Old

Refining the Open Loop Diamond Buffer Headphone Driver (RJM Audio Sapphire 3.0)

Posted 31st January 2015 at 12:28 PM by rjm (RJM Audio Blog)
Updated 18th March 2015 at 01:52 AM by rjm (add photo of finished amp)

A couple of years ago I built a standard op amp + diamond buffer headphone amplifier, called the Sapphire.

My original circuit (Sapphire 1.x) was the simple four transistor four resistor diamond buffer of the LH0002. Later small resistors (Sapphire 2.0) were added to the emitters of the driver transistors to boost the output bias current.

In this next go-round (Sapphire 3.0), I've replaced the emitter resistors with current sources. This provides a significant improvement in PSRR, over 20 dB in simulation. The output pair has been reinforced in a Sziklai configuration for lower distortion, and the primary output transistors five-way paralleled for improved thermal stability. The output impedance is 1~2 ohms, limited primarily by the output resistor.

It simulates to <-100 dB harmonics for 0 dB (1 V rms) output into 60 ohms. The total circuit standing current is less than 50 mA per channel.

LTSpice files below. R5,R6 on LTSpice...
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Old

Phonoclone noise measurements

Posted 3rd October 2014 at 12:40 AM by rjm (RJM Audio Blog)
Updated 3rd October 2014 at 09:27 AM by rjm

What we are looking at here is the Fast Fourrier Transform (FFT) of the line output from my b-board buffer recorded at 24 bit, 96 kHz by an Onkyo SE-200PCI sound card. Upstream from the b-board is the Phonoclone 3 MC phono stage, connected to a Denon DL-103. The tonearm is Denon DA-307, and the deck is a Denon DP-2000.

Four recordings, taken 1) with music playing, 2) with the tonearm raised 3) with the phonoclone powered off and 4) with the b-board and all upstream components powered off.

True 24/96 data was obtained, measurements out to 48 kHz are possible, with -130 dB noise floor. (I was using Digionsound 6 to do the recording as Audacity truncates 24 bit recordings to 16 bit in Windows due to licensing issues. The FFT was generated in Audacity however.)

The soundcard's line input may have an impressive-looking low noise floor, but it's still useless for measuring line level audio devices like the b-board because the noise of the preamp/ADC...
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