Posted 7th February 2015 at 06:47 AM byrjm Updated 14th February 2015 at 09:13 AM byrjm
I recently obtained a pair of AKG K702 headphones to complement my long-standing reference Sennheiser HD 600s. I figured since I'm building headphone amplifiers it would be a good idea to have a reference grade low impedance model as well as the high impedance HD 600s to use for evaluation.
At the same volume position I quickly discovered the HD 600s play slightly louder than the K702s. The datasheet values predict the K702s should be about 3 dB louder, so it seems the sensitivity is off by as much as 6 dB.
K702: 62 ohms, 105 SPL/V ... 93 dB/mW from datasheet, 87~89 dB/mW (99~101 SPL/V) in practice.
HD 600: 300 ohms, 97 dB/mW ... 102 SPL/V.
The K702 requires as much as ten times more power to drive than the HD 600s. The voltage sensitivity is about 3 dB lower than...
Posted 31st January 2015 at 12:28 PM byrjm Updated 18th March 2015 at 01:52 AM byrjm(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.
Posted 14th January 2015 at 06:07 AM byrjm Updated 13th February 2015 at 04:11 AM byrjm
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.
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...
Posted 23rd August 2014 at 11:33 AM byrjm Updated 27th August 2014 at 06:39 AM byrjm
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.
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,...
Posted 11th August 2014 at 12:39 AM byrjm Updated 28th August 2014 at 11:41 PM byrjm
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:
The GeminiPS is another discrete series voltage regulator, with a Zener reference and bipolar pass transistor. It's an old circuit, published in Practical Electronics in 1970-71, and written by D.S. Gibbs and I.M. Shaw. I happen to have a reprint, but there's a nice overview here.
For reference it might be worth checking back to the two transistor regulator. The GeminiPS circuit is related in the sense that it is a more sophisticated take on the same basic principle. With just a handful of components we have a stabilized, 30 W output with soft turn on and short circuit protection. The circuit can be scaled up and down relatively easily, and the complimentary (negative output) version is an easy modification.
The pass transistor (TR2/3, Q2/3) is between the circuit common and the rectifier anodes. This may seem odd, but it was relatively common back in the day when high voltage transistors were both expensive and rare. The...
I've been meaning to take up shunt regulators for some time. I've never got around to building one myself to try, so I'll have to make do by playing in simulation.
Today's circuit is the shunt analog of the Z-reg series regulator: no feedback, Zener reference, single transistor regulation. The output impedance and ripple rejection-characteristics are similar too, with about 40 dB of RR and an output impedance of just a few ohms. It can be built equivalently from either an pnp or pnp transistor. (See attached LTSpice .asc files.)
The difference between shunt and series regulation can best be explained by considering the upstream power supply: In a series regulator an increase in current demand by the load causes the regulator to increase the current to compensate. In a shunt regulator an increase in current demand by the load causes the regulator to decrease the shunt current to balance, so there is no net change in current flowing...
So you have a small handful of parts and want to build a (simple) discrete voltage regulator instead of using an IC. What to do?
For line-level audio circuits, especially op amp based (IC or discrete) preamps with high PSRR, something like the Z-reg is generally sufficient. Robust, works well, has enough ripple rejection to cut power line noise from the preamp output.
If you add just a couple more parts, however, you can add feedback to the Z-reg circuit, a simple error amplifier in the form of an additional transistor Q2, with the output-sampling voltage divider R1,R2.
The ripple rejection is not vastly superior to the circuit without the feedback unless some additional bypass capacitors are added as shown in the first version of the circuit below. The output impedance, however, improves from a few ohms to a few tenths of an ohm as a result of the feedback. Which could, in principle, be of use.
I'm not going to spend too much time on this one. The idea is to increase the input impedance of the pass transistor by buffering it with a jFET so it will support a high-impedance passive CRCRC filter section that generates a low noise reference voltage. The reference is defined not by a Zener or diode stack, but by a simple voltage divider. There is a LM317 pre-regulator on the front, but it is traditionally configured and works independent of the following circuit so it is omited here together with the additional transistor that speeds up the charging of the reference voltage filter capacitors.
The basic problem is that lowering the noise of the reference cannot lower the output noise indefinitely. After a point the output noise is defined by the performance of the pass transistor instead.