Posted 26th April 2012 at 08:34 AM byrjm (RJM Audio Blog)
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: ...
Posted 25th April 2012 at 12:48 AM byrjm (RJM Audio Blog)
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.
Having been a fan of NOS DACs now for something over a year I've decided it was high time for sorting out their most serious drawback - the roll off in the HF. This is an unavoidable result of using a zero-order hold function to reconstruct the original (impulse) samples. Droop exists even with oversampled DACs, its just at its most severe in NOS.
One of the most popular ways to flatten the response is to add on some kind of analog filter with a rising response (to 3.16dB @ 20kHz). A first order shelving filter can't quite cut it though so an LC tank circuit has been employed by a few. This needs to have a moderately high Q to achieve the correction.
I've played around with inductors to achieve this and haven't much liked the resulting sound. Whether this was due to the particular inductors I was using I didn't experiment. Admittedly they were very cheap ones. In general though when I've used high Q circuits in crossovers I haven't much liked the colouration...
Posted 7th April 2012 at 12:15 AM byrjm (RJM Audio Blog)
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.
It doesn't sound tooo bad,really. It's not the best thing ever,but for a couple hours of tinkering with junk that was on the bench, I'm pleased.
It's got more than enough power for 'phones,some care needs to be taken to prevent any accidents.
So yea,yesterday was more or less; Troubleshoot monitor,discover...
Well, it has been a few years since I built my last digital crossover - like six I think.
It is not that I was planning to do this - but a conversation with someone about my seemingly modest choice of the AD1941 DSP chip for my old crossover made me look at what else Analog Devices are offering.
The ADAU1442 is on the surface a very similar chip to the AD1940, but it has a lot more integrated into it, and offers significantly greater capacity.
So I set about designing a new crossover that used this, and also addressed a few of the shortcomings of my original design.
The goals were broadly:
- A modular DSP based crossover
- That provides a standard interface for the ADC
- That provides a standard interface for the DAC
- That includes SPDIF in and out
- That anybody can design digitisers and dacs for - no code level drivers built in, though the interface does ALLOW for SPI control of these
- That is controlled from a simple...
Posted 2nd April 2012 at 08:45 AM byrjm (RJM Audio Blog)
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 (RJM Audio Blog)
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....
Posted 29th March 2012 at 04:35 PM byrjm (RJM Audio Blog)
I tell people: "Buy a nice, heavy power transformer. It will sound better."
They are skeptical, because the circuit only draws a couple of watts, and less than 100 mA current.
The image below shows how the power transformer, and rectifier diodes, actually work much harder than you would estimate from looking at the output power.
It shows a zener regulated supply with a load drawing 100 mA at 20 V. That's 2 W.
As a result of the capacitor input filter directly after the diodes, however, the diodes and transformer do not conduct current all the time, but instead for just a couple of milliseconds twice every cycle of the AC wave. They have to supply all the output current in just that short space of time. As you can see in the simulation, the diodes are pushing peak currents well in excess of 1A or 10x the output current. This is a typical "normal" power supply with a initial ripple ratio of a modest 1/40, things...