Posted 25th February 2015 at 01:24 PM byrjm Updated 26th February 2015 at 12:41 AM byrjm
Ok, so why donít you like the K702s?
I didnít say I didnít like them. Just that I think the HD600s are better.
Itís pretty simple really:
The K702s have a strident, hard upper-midrange "bump" that I find disagreeable. Yes, it makes tracks sound more ďliveĒ, but itís also fatiguing and a bit clinical, and - as many others before have noted - makes the sound overall somewhat thin. In direct comparison the HD600s seem full the point of boominess, but I'm willing to accept that midbass plumpness for the Sennheiser's warmer, luxurious midrange. In imaging, the K702s trend to a wide, distant, airy soundstage while the HD600s run towards a closed in, intimate presentation. In that sense the K702 are more like listening to speakers, and I can certainly see people being attracted to that.
These are both top-shelf headphones at the top of their game, I don't mean to imply that the AKGs are bad. The two...
Posted 22nd February 2015 at 02:24 AM byrjm Updated Yesterday at 07:17 AM byrjm
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.
Posted 14th February 2015 at 12:47 PM byrjm Updated Today at 01:26 AM byrjm
Rhymes with "custard"...
Each channel has a BB OPA134PA - socketed - for voltage amplification and an eight transistor discrete buffer with 2 ea. 2SA1837. 2SC4793, C546B, C556B. Dual mono layout (more or less ... the circuit board itself is shared and not completely symmetric).
There's a pair of NE5532s at back for balanced-unbalanced input conversion, and a pair of transistors (TIP122 TIP127) with trim pots and circuitry between the input and the main amplifier which looks a bit like voltage regulators to me but I can't see where they fit in to the scheme of things.
Pretty decent quality for the price. (I paid $350/delivered)
First impressions of the sound are quite positive, plenty of depth and decent smoothness. Letting it cook overnight now and will put it in the main system tomorrow.
Comparison notes vs. Sapphire (with Sennheiser HD600s)
Posted 7th February 2015 at 07:47 AM byrjm Updated 14th February 2015 at 10:13 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 01:28 PM byrjm Updated 27th February 2015 at 05:40 AM byrjm(update schematic to final)
A couple of years ago I built a standard op amp + diamond buffer headphone amplifier, called the Sapphire.
It's a fairly common circuit, from eBay on up its not hard to find similar designs. The Sapphire is distinguished by its full dual mono design, Zener voltage regulators (Z-regs), and by keeping the buffer circuit outside of the op amp feedback loop.
Regarding the feedback loop, there are two schools of thought:
1. Put the buffer inside the feedback loop, so the op amp does active error correction on the output signal, and the buffer provides the high speed currents required to make that correction.
2. Put the buffer outside the feedback loop to isolate the load from the op amp. The load currents are provided by the buffer, but the output signal relies entirely on the low output impedance of the buffer for accuracy.
The explanation is too long to bother with here, but I am strongly in favor of using a buffer...
Posted 14th January 2015 at 07:07 AM byrjm Updated 13th February 2015 at 05:11 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 3rd October 2014 at 01:40 AM byrjm Updated 3rd October 2014 at 10:27 AM byrjm
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...
Posted 23rd August 2014 at 12:33 PM byrjm Updated 27th August 2014 at 07:39 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.