Posted 21st May 2015 at 01:32 PM byrjm (RJM Audio Blog)
Updated 27th May 2015 at 01:54 AM byrjm
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...
Posted 20th May 2015 at 06:00 AM byrjm (RJM Audio Blog)
Updated 18th June 2015 at 11:26 PM byrjm(added schematic of original version)
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!
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 12:24 PM byrjm (RJM Audio Blog)
Updated 25th February 2015 at 11:41 PM 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 01:24 AM byrjm (RJM Audio Blog)
Updated 28th February 2015 at 06:17 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 11:47 AM byrjm (RJM Audio Blog)
Updated 12th April 2015 at 02:59 AM byrjm
Signal Input : 2x XLR female , balance
2x RCA, unbalanced
Maximum input level : +21 dBu, impedance 10kΩ
Input impedance : XLR: 10kΩ, RCA: 68kΩ
Input Sensitivity : +6 dBu
Main amplifier gain : +8 dB
Main amplifier gain adjustment range : -4 / +2 / +8 / +14 / +20 dB
Frequency response : 0-55kHz (-0.5dB)
Damping Factor :> 400 @ 50Ω
Dynamic range :> 128dB (A -weighted )
THD + N (1kHz 1W @ 100Ω): <0.00035%
THD + N (1kHz 0.5W @ 32Ω): <0.0007%
Crosstalk :-110db (1kHz)
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...
Posted 7th February 2015 at 06:47 AM byrjm (RJM Audio Blog)
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...