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The Lab Circuit ideas and innovations, not yet part of a full project.
Old

AT-HA5000

Posted 21st May 2015 at 01:32 PM by rjm
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
Updated 28th May 2015 at 02:13 AM by rjm

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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File Type: asc szekeres 2015 reverse split CCS.asc (3.8 KB, 23 views)
File Type: asc szekeres 2015 negative.asc (3.3 KB, 21 views)
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Old

JLH-2005 headphone amplifier

Posted 18th May 2015 at 08:15 AM by rjm
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
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
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
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
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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Old

Windows volume control settings in dB.

Posted 23rd August 2014 at 11:33 AM by rjm
Updated 27th August 2014 at 06:39 AM by rjm

I suppose everyone has at one point or another adjusted the volume sliders in Windows. The ones that go from 0-100, and you are never quite sure what whether its a boost, or an attenuation, or what.

Some years ago I measured the outputs and inputs using a fixed amplitude .wav file created in audacity and played back through the Onkyo SE-200PCI. I've taken another look at the worksheet I made and I've noticed that the volume settings correspond to very logical, even steps, namely:

100 0 dB
90 -1 dB
80 -2 dB
70 -3 dB
60 -4.5 dB
50 -6 dB
40 -8 dB
30 -10 dB
20 -14 dB
10 -20 dB

or for the mathematically inclined: 20*log(volume/100)

This scale is the same for both the output master volume and the line input, so its probably maintained throughout the operating system.

So now you know.
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Old

Technics SU-9070

Posted 20th May 2014 at 12:01 AM by rjm
Updated 20th May 2014 at 02:15 AM by rjm

The Technics SU-9070 is a 2U rack mount stereo preamplifier from 1977, matched to the SE-9060 amplifier. The preamp was sold as the SU-9070II in Japan.

The circuit is shown below, for educational purposes.

The power supply regulation is quite elegant, I hope to get to that in a future post. Here, just note that there are separate regulated lines for the MC stage, the input/VAS sections of the MM and line amps, and the output sections of the MM and line amps.
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Old

Technics SE-9060

Posted 19th May 2014 at 12:04 AM by rjm
Updated 19th May 2014 at 11:58 PM by rjm

There was a series of relatively slim (2U chassis rather than 4U), upmarket audio separates put out by Technics in 1977: the ST-9030T tuner, SU-9070 preamplifier, and SE-9060 amplifier. They are two degrees of separation from the top of the line models at the time, the range went 9600>9200>90x0.

I recently acquired the tuner (more on that some other time) and have the others in my sights.

For educational purposes, the amplifier schematic for the SE-9060 is shown below. The input stage and voltage amplifier was driven from regulated 55 V supplies (Va+ Va-), while the driver stage was powered directly by the rectified DC at about 50 V (Vb+ Vb-) filtered with 18,000 uF per rail per channel.

Note that there were 9060 and 9060II as well as 9070 and 9070II models in Japan, but the export model of the preamp which sold as the 9070 was actually the 9070II rather than the 9070 domestic version. Likewise the SE-9060 shown below (from a European...
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