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

Posted 31st January 2015 at 01:28 PM by rjm
Updated 18th March 2015 at 02:52 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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MUSES premium audio semiconductors from NJR

Posted 14th January 2015 at 12:29 PM by rjm

Have I just been living under a rock or why the hell haven't I heard about this before now!?
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Portable headphone amplifiers

Posted 14th January 2015 at 07:07 AM by rjm
Updated 13th February 2015 at 05:11 AM by rjm

The shift of the center of gravity of the high end from component, rack systems to portable continues. Exhibit A

I've also noticed that over the last couple of years the basic blueprint for a portable headphone amplifier as defined by the Sony PHA-1 has now been taken up by all of the major Japanese audio companies.

Sony PHA-1, PHA-2, PHA-3
Denon DA-10
Onkyo DAC-HA200 and TEAC HA-P50 (variants of the same basic unit)
JVC SU-AX7 (report) (seems to be mostly Kenwood DNA)
Audio Technica AT-PHA100
and Fostex hp-p1

For all the above you are looking at a battery powered, slim-cased DAC + headphone amp typically with some sort of guard around the controls. They all feature a good variety of analog and digital inputs, offer switchable gains, and are priced over a range from $200 to nearly $1000.

You are looking at the convenience of having the DAC built in, the small size, and the rechargeable lithium...
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Denon DL-103

Posted 14th October 2014 at 01:55 PM by rjm
Updated 14th October 2014 at 01:57 PM by rjm

Eye candy.

My fourth. Each one comes packaged the same, with a serial number and individual calibration sheet.

This little guy is no. 2096, .39/.40 mV. Signed off by Mr. Nemoto (根本さん、ご苦労様!)
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Phonoclone noise measurements

Posted 3rd October 2014 at 01:40 AM by rjm
Updated 3rd October 2014 at 10:27 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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Windows volume control settings in dB.

Posted 23rd August 2014 at 12:33 PM by rjm
Updated 27th August 2014 at 07:39 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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Onkyo SE-300PCIe sound card review part II.

Posted 20th August 2014 at 03:18 PM by rjm
Updated 23rd August 2014 at 07:21 AM by rjm

Part I is here.

Setup notes are in part I. Listening system downstream is the Sapphire headphone amplifier and Sennheiser HD-600 headphones. As the SE-300's line output routes though the Windows sound mixer, while the SE-200's bypasses it, it was not possible to keep the headphone amplifier volume at a constant setting between cards. Since I found the built-in headphone amplifier of the SE-300 to be good but not at the level of the Sapphire, only the stereo RCA output is being reviewed here.

Let me begin by saying that Windows is fundamentally an anti-audiophile proverbial dog's breakfast of setting and driver layers (quick, what's the difference between the DirectX and WaveOut sound modules?), and most soundcards are also anti-audiophile in that they cater to gamers and casual listening with a full barrel of virtualization, equalization, and reverb features enabled by default.* No surprise then that both cards require careful setup to sound their best, or,...
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Onkyo SE-300PCIe sound card review part I.

Posted 11th August 2014 at 01:39 AM by rjm
Updated 29th August 2014 at 12:41 AM by rjm

When I upgraded my computer recently I accidentally bought a motherboard with no PCI slots which meant I could no longer use my SE-200PCI card, my main reference source now for some years. Rather than switch motherboards again I figured I'd try Onkyo's latest version which has been out for a while now, the SE-300PCIe. I picked up a used "R2" model for $200.

I mention the price up front because the cost of this thing outside of Japan is astronomical. I've seen asking prices of $450 US! In Japan the retail price is about $300 in most stores. That's still very expensive. Despite the good things I have to say about it, the cost/performance must be taken into account based on the particular price you are looking at paying.

This is a Japan-only product, so the web site is Japanese:

main page (gallery)

The basic specs for the stereo line output is 120 dB S/N A weighted, 24/192 capable, 0.3-88kHz -3dB for the 2 channel stereo...
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How Beats Conquered The World (via The Verge)

Posted 13th June 2014 at 01:04 AM by rjm
Updated 13th June 2014 at 01:44 PM by rjm

Original article, by Ben Popper

~ my spin ~

There was the iPod. It was cool, and the distinctive while earbuds that came with it showed people you had an iPod, so they were cool, too. A few audiophiles invested in better IEMs, but they tended to be expensive and discreet and anyway were only ever a niche thing.

Meanwhile the Japanese headphone makers - JVC-Kenwood, Sony, Audio Technica - tried competing with the iPod, and they came up with a spectrum of earbuds and headphones of every shape, price, and color. They sold as commodities, but none developed any real kind of identity or reputation. Certainly there was little effort at building a brand.

On the other side, Grado, AKG, and Sennheiser continued doing pretty much what they always did, making nice, expensive headphones for home/studio/DJ use.

Neither group addressed the obvious hole in the market: non-audiophiles wanting "good" sounding over-ear...
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Technics SU-9070

Posted 20th May 2014 at 01:01 AM by rjm
Updated 20th May 2014 at 03:15 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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