Posted 2nd May 2015 at 05:30 AM byrjm (RJM Audio Blog)
Updated 2nd May 2015 at 05:33 AM byrjm
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,
On seeing the Gilmore circuit again the thought process re. a Sapphire+Gilmore went something as follows,
"Toss out op amp, convert the Gilmore dual-LTP front end to bipolar, bolt the Sapphire3 buffer stage to the back, and substitute in the Sapphire3 current sources. Wrap in a mild feedback loop."
The result is shown attached. The Vbe multiplier is still a simple resistor (R33) ... that may need to be refined to add thermal throttling. The offset servo is not shown, but the action is shown as Vadj. Alternatively a trim pot would be placed between R30 and R32 to provide a small measure of offset adjustment. Most of the open loop gain is controlled by R14,R15 ... it seems to me that some work could still be done in that area. Despite going...
Posted 25th February 2015 at 01:24 PM byrjm (RJM Audio Blog)
Updated 26th February 2015 at 12:41 AM byrjm
The HD600s.
Ok, so why don’t you like the K702s?
I didn’t say I didn’t like them. Just that I think the HD600s are better.
It’s pretty simple really:
The K702s have a strident, hard upper-midrange "bump" that I find disagreeable. Yes, it makes tracks sound more “live”, but it’s 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 31st January 2015 at 01:28 PM byrjm (RJM Audio Blog)
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 2013 at 12:29 PM byrjm (RJM Audio Blog)
Updated 27th December 2016 at 02:38 AM byrjm
I couldn't even build it myself for the price they are selling it at, $50 on ebay. It caught my eye for the linear, split power supply. I love the chrome 3-pin power connector, too.
As is usual when you buy cheap ultra-Chinese audio gear via eBay some adjustments are needed, however.
The basic problem seems to be a little mix up with the input coupling capacitors. It came with polar electrolytic capacitors, following the markings on the circuit board. If you lstudy the schematic I sketched up below you will notice the input signal and DC offset can swing in both positive and negative directions relative to the op amp inputs - polar capacitors in this position are a bad idea.
I recommend anyone buying this to replace the input caps with Nichicon Muse ES or similar 4.7 uF or 10 uF bipolar electrolytic caps. (I have some extra I can mail out. pm me if interested.)
Other than that it's a pretty solid circuit as far as I can see....
