It is certainly a little bit different. One might be tempted to say "gilding the lily", but come on, headphone amplifiers are just the right place for these indulgences.
Building your own long tailed pair (LTP) to bolt in front of an IC op amp has fallen out of favor in recent years. I must admit I couldn't see the point then, and still don't.
I've seen a number of headphone amp circuits with 3 paralleled pairs of output devices. I wonder if there any real advantage over simply using one pair at 3x the current, perhaps with slightly larger transistors?
The 5532 and 5534 type op-amps require adequate supply decoupling if they are to remain stable, otherwise they appear to be subject to some sort of internal oscillation that degrades linearity without being visible on a normal oscilloscope. The essential requirement is that the positive and negative rails should be decoupled with a 100 nF capacitor between them, at a distance of not more than a few millimeters from the op-amp; normally one such capacitor is fitted per package as close to it as possible.
He's someone who should know. Anyway, it doesn't take much digging on the internet to confirm beyond reasonable doubt that bypass caps should be as close to the op amp power pins as possible. So thinking about my previous experiments with bypassing the Sapphire, by adding bypass caps around the transistors I also effectively also added a bypass for the op amp, but a rather poor one as the power-pin-to-power-pin round trip loop distance is probably 10...
Posted 12th May 2012 at 01:15 AM byrjm Updated 28th May 2012 at 11:51 PM byrjm
This I have been experimenting - call it a hunch - on the effects of bypassing electrolytic capcitors (Nichicon FW and KW) with 0.1 uF TDK ceramics (Mouser 810-FK28X7S2A104K) with the diamond buffer circuit used in both my B-board preamp and Sapphire headphone amplifier.
This being a mod, I had to solder the caps to the underside of the boards, attached to the leads of the Nichicon 100uF electrolyic capacitors.
I used four ceramics per channel, one per active device in the diamond buffer if you like.
I did several other changes on the B-board at the same time, so it wasn't obvious until I modded the Sapphire in the same way what was the result of the bypassing. Anyway, with both the improvement was immediate and dramatic: any sense of "transistor-like" treble glare is completely quenched. The whole top end takes a step backwards, not in extension, but in prominence.
Posted 8th May 2012 at 12:20 AM byrjm Updated 8th May 2012 at 12:24 AM byrjm
I bought a stepped attenuator (actually, two of them) from ebay seller hanshare-electronics. As always with eBay, I was a little worried about what would show up.
I'm happy to report that this is a nice job. The included resistors are clearly labelled, and the documentation, while spartan, is sufficient.
The only negative is the rotary switch is a bit "clicky". Better quality units I've seen are more damped.
On the positive side, though, the resistor values are very elegantly selected, to give a smooth response of about 4 dB per step from -60 to -40 dB, and 2 dB per step from -40 to -3 dB. Having been-there, done-that, I can say that I'd happily pay the asking price to get those resistors sourced and sorted for me. Next time, I'd also pay to get it assembled. Soldering was more painful than I imagined it would be, and the pre-assembled units much neater-looking than what mine ended up as.
Comparison of the noise baselines, measured at the circuit output using a NI USB-6215 DAQ. Unloaded for the preamps, and with a 6 ohm load for the 0347 amplifier.
B-board: -139 [300Hz-100kHz] 0 dB gain (-139 - 0 = -139 dB, 110 nV sqrtHz input referred). The actual B-board output noise is below this measurement threshold.
0247: -124 dB [300Hz-100kHz] 14 dB gain (-124 - 14 = -139 dB, 110 nV sqrtHz input referred).
0347: -109 dB [300Hz-100kHz] 31 dB gain (-109 - 31 = -140 dB, 100 nV sqrtHz input referred).
See the attached plot for the FFT data. Note the peak at 28 Hz is an artifact of the measurement apparatus.
By way of comparison, a typical audio opamp has an input referred voltage noise figure of 3-8 nV sqrtHz (-170 ~ -160 dB) and can be expected to return this datasheet specification in most well-designed circuits. In other words the output noise is going to be about -160 dB + the circuit gain.
Posted 26th April 2012 at 08:34 AM byrjm Updated 4th May 2012 at 02:30 PM byrjm
Nor the remake, the original. Good film. Story shakes out something like "Twelve Angry Men in the desert": Put together a small, random group of people and pressure them to complete a task. In the case of "The Flight of the Pheonix" this is to make an airworthy plane (this one) from the crashed remains of another (this one).
So. We start with my old red Gainclone case, and a pair of these buffer boards , and a Takman resistor, 24 position stepped attenuator from eBay, unassembled, and start working to transform something old into something new.
Here's my LM3875 gainclone. Served me well, but it is time to bid adieu! (at least to the guts):
Opened up, we see the circuit board, such that it is, and my home-built 11 position attenuator: ...
The original stereo VSPS project is now quite mature. I did a couple of minor tweaks on the last re-spin of the layout, removing the pads for the old Black Gate coupling caps (long gone), adding thermal isolation, and making room for a pair of optional ceramic bypass caps next to the op amp.
The new boards are matt black. Quite cool. I have a small stack if you need any.
Posted 7th April 2012 at 12:15 AM byrjm Updated 9th April 2012 at 03:37 AM byrjm
Once in a while I get emails asking after the X-reg evaluation boards. These are handy little 5x8 cm test boards for the X-reg voltage regulator - or they would be, if I had ever bothered to get a set made.
Since the circuit is built into the VSPS300 and Phonoclone 3, it wasn't really a big priority.
Anyhow, yesterday I re-drew the evaluation boards and I'll be getting a batch made in the near future, along with some of the stereo VSPS that people were also asking after recently. They won't exactly fly off the shelves, but I started to feel bad for those few people who actually wanted them.
Posted 2nd April 2012 at 08:45 AM byrjm Updated 2nd April 2012 at 11:43 AM byrjm
I posted this earlier today, but I think it deserves to be put in the blog - if nothing else so I can find it again next time ... and there always seems to be a next time when it comes to calculating headphone amplifier gains.
Starting at the beginning, the encoded data on a CD goes from 0 to 1 in 2^16 steps, but in a typical CD player or soundcard, the DAC output is -2.8 V to 2.8 V or 2 V rms or 6 dB. Many sources, such as phono stages and portable audio, are lower, perhaps as low as 250 mV.
How loud the sound is depends on the source signal amplitude, the position of the volume control, the circuit gain, and the impedance and sensitivity of the headphones.
As a practical matter, most people would want the volume control at the 9-10 o'clock position for "normal" listening.
For standard "line level" source, the gain required to keep the volume control at a 9-10 o'clock varies depending on the impedance...
Posted 31st March 2012 at 12:20 PM byrjm Updated 31st March 2012 at 12:28 PM byrjm
I did get a couple of responses to my invitation. Well, two, actually.
So congratulations! Free boards will be heading your way in about a month, and I'll throw in most if not all the parts, too.
First was simonov's entry. He's clearly done this several times before. His confident layout ticks all the right buttons: ground plane (check), thermal isolation (check), clean, geometric layout (check). One jumper required, but that's a very minor offence.
While I allowed modifications of the circuit, simonov went and redesigned pretty much the whole thing. CCS replaces the source resistor, current limiter and capacitance multiplier blocks added. The BOM was starting to spiral, and no longer uses my standard parts set so I'd have difficulty supplying kits based on his design. His modifications, although certainly improvements, ended up counting against him....