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AT-HA5000

Posted 21st May 2015 at 01:32 PM by rjm
Updated 22nd May 2015 at 12:00 AM by rjm

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 overcurrent...
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Old

Szekeres 2015

Posted 20th May 2015 at 06:00 AM by rjm
Updated 22nd May 2015 at 12:02 AM by rjm

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.

If I was going to built it today, I would build it as the attached schematic. It's little changed really, just an active current source replacing the source resistor. It runs about 2 watts per channel. Distortion figures aren't great, but the operating point is reasonably optimized for 16-300 ohm loads. The transistor parts are just placeholders for the simulation. In practice you'd use something like BD135.

Mostly though the LTSpice sim is just to serve as a reminder of how poor this circuit topology is, and how well it was designed originally. Even on a "best effort" basis there's little more to eke out from it than was already in the original.
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Old

JLH-2005 headphone amplifier

Posted 18th May 2015 at 08:15 AM by rjm
Updated Today at 12:46 AM by rjm

I was perusing this thread earlier today. Which led me to what I think is the original source, at least as a modern, relatively clean headphone amp version of the original original (by way of ESP).

Some comments from our own Nelson Pass are seemingly relevant.

The rough sim, below (LTSpice .zip attached), shows that the circuit performs well enough.** The linearity of the circuit is commendable, but the bias currents and DC offset stability (before the coupling cap, which I added to the sim - the original JLH-2005 circuit is DC coupled!) are most definitely not.

** except PSRR, which is beyond terrible. I'm working to fix that!

In my humble opinion single-ended output stages should be AC (transformer or capacitor) coupled and the biasing circuit should be designed to have reasonable operating point stability.

AC coupled, and simplified to a single supply voltage, the circuit is of interest primarily for its ability to run...
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Old

Standard Resistor Values for RJM Audio Projects

Posted 11th May 2015 at 09:15 AM by rjm
Updated 14th May 2015 at 10:40 PM by rjm

From now on every effort will be made to consolidate to the following values, 1/4W metal film:

1, 4.75, 10, 47.5, 100, 150, 221, 475, 1000, 1500, 2210, 4750, 10000, 22100, 47500, and 100000.

Also, the 1/4W 47 ohm and 68k carbon comp. resistor is widely used as damping and bleeder functions, respectively.

Exceptions will be made for the RIAA eq. of the Phonoclone and VSPS circuits, and the business end of the X-reg, where specific, non-standard resistance values are required.

*****

On resistor selection-

I honestly don't know if one resistor sounds better than another. I do know, however, that one resistor can be more expensive than another. At Mouser your basic 1/4W metal film resistor can run between 5 cents and 2 dollars in 100 unit quantities. Setting aside sound quality, it's not at all clear that you are getting any kind of material benefit at all by getting the expensive parts. Instead, it really...
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Old

Commentary on the TDK Life on Record A33 Wireless Weatherproof Speaker

Posted 10th May 2015 at 03:23 AM by rjm
Updated 11th May 2015 at 04:58 AM by rjm

There is something freakish about a brick-sized block that sits there and plays room-filling music ... with no wires attached whatsoever and no obvious moving parts. It gave me the same "I'm living in the future!" sense of wonder I got buying my first 1 TB hard drive.

It doesn't take too much searching the internet to discover that among wireless portable (bluetooth) speakers, the TDK A33 is highly recommended for its exceptionally good sound quality. That comes with a massive caveat, however: Most of the people writing these reviews only have Bose, Beats, and the internal speaker of their iPhone as references for comparison.

So does the A33 sound good in a hifi context?

Read on to find out...

No, okay don't bother. The answer is "no".

But it doesn't sound bad. I'm listening to it now as I type this, hooked up via the AUX input to my Onkyo PCI-200SE sound card, with the A33 tucked under the...
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Old

A headphone amplifier gain calculator

Posted 2nd May 2015 at 05:30 AM by rjm
Updated 2nd May 2015 at 05:33 AM by rjm

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,

(Routput + headphone Z)/(headphone Z)
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Old

I've been cloned!

Posted 29th March 2015 at 12:26 PM by rjm
Updated 31st March 2015 at 09:31 AM by rjm

Back at the dawn of time one of the first audio circuits I worked on was the Gainclone, followed closely by The Dac of the Klones (Oh my, the nostalgia!) and of course the Phonoclone.

The VSPS was a side-project that grew out of the Phonoclone, and actually ended up first out of the gate as a working circuit.

Apart from the general design philosophy (low parts count, simplicity, careful layout and grounding) it has no particular link to 47 Labs. While the concept of a non-inverting op amp active phonostage is nothing original the circuit is mine, particularly the configuration and values of the RIAA filter which I calculated and simulated myself. The rest is an amalgam from a dozen or so different sources, textbooks, datasheets and application notes &c. All the values are quite carefully chosen.

That said I've always put the circuits and everything else on the internet, with source attribution as I felt necessary. The boards and kits came...
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Old

Doin' a "Gilmore" : a discrete transistor headphone amplifier

Posted 10th March 2015 at 12:17 AM by rjm
Updated 13th March 2015 at 10:42 PM by rjm

Recently I spent some time updating the diamond buffer of Sapphire headphone amp circuit. Later I stumbled on Kevin Gilmore's headphone amp circuit. Well, I'd read it before, but it had slipped my mind.

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...
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Old

Which is better, Sennheiser HD600 or AKG K702?

Posted 25th February 2015 at 12:24 PM by rjm
Updated 25th February 2015 at 11:41 PM by rjm

The HD600s.

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...
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Old

Z-reg II improved simple Zener voltage regulator

Posted 22nd February 2015 at 01:24 AM by rjm
Updated 28th February 2015 at 06:17 AM by rjm

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.
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