Posted 17th December 2013 at 10:10 PM byrjm Updated 20th December 2013 at 10:14 AM byrjm
Straightforward transplant. Out with the old (anyone want them?) in with the new. Re-used the OPA134 op amp and my dog-eared pair of 0.47uF Multicaps.
On powering up I discovered that with the specified 10 ohms in R9,10 the output bias current was upwards of 200 mA and things were getting a bit toasty. I paralleled a second 10 ohm resistor, dropping the resistance to 5 ohms and dialing back the output bias current to about 70 mA. Latest schematic revision has R9,10 values edited to match.
Currents stable. Heatsink temperatures around 50 C. Output offsets around 15 mV. No noise or hum.
Posted 16th December 2013 at 05:23 AM byfas42 Updated 22nd June 2014 at 06:35 AM byfas42
Bummer!! After mentioning that Brad's (bcarso) units were doing very nicely I swapped CDs while they were set to max. volume, doing a good job on a classical piece. Unfortunately, the new disk was heavily compressed pop, my mind was elsewhere, and I hit the volume control a second too late - everything's OK except the right speaker has dropped about 6dB in volume, :( ...
I suspect that some protection circuitry has tripped, but it's not resetting. Which means, unless anyone has a good idea otherwise, that I'll have to dive inside and sort things out. But, on the other side of the equation that may be an excellent thing to do, it will give me a chance to do some more adventuresome tweaking ...
Posted 10th December 2013 at 03:50 AM byabraxalito Updated 21st December 2013 at 12:48 PM byabraxalito
This one rocks - it plays FLAC, WAV and mp3s. The DAC is WM8740 (not so hot) but the designer has taken the power supply very seriously. Just check out all those caps and TDK noise filter on the PCB on the right Power comes from a lithium battery behind, so its portable but there's no case - there are acrylic front and rear panels screwed on to the PCB spacers. Two 3.5mm jacks provide headphone and line out.
Its using STM32F407 to do the decoding which is a first for ARM in this sphere and the design is configured that there's an I2S link between the two PCBs. So in theory anyone could design a better sounding DAC and replace that bottom PCB whilst keeping the UI. Which I may just do.... Price is 568rmb (just under $100).
Update - I was a little curious about the PSU so I've pulled out all of those SMT 220uF caps to get a closer look. The block filter's a Murata (not TDK) BNX024 which looks like a wrong choice of part primarily because its rated for 15A. Hence...
Posted 9th December 2013 at 07:52 AM byjan.didden Updated 10th December 2013 at 06:02 PM byjan.didden
It’s a recurrent issue: you want to build a preamp, a DAC, a phono stage, anything that needs a nominal supply voltage between 3.3 and 15VDC, positive and/or negative polarity. Sometimes you want several supplies to isolate stages from mutual interference via the power supply. So you want a power supply regulator that approaches an ideal DC voltage source as best as possible within reasonable cost. In your search, you inevitably run into the term ‘superreg’ – so where does the name come from and what is it?
The history of very high performance low-voltage regulators is well documented on Walt Jung’s website (www.waltjung.org – look under Library|Regulators & References). An early design that attracted attention was Mike Sulzer’s, published in 1980 and 1981 in Audio Amateur. I added something to that in 1987, and then I was invited by Walt Jung to work on a further improved version. This was published in a series of four articles in Audio Amateur in 1995 by Walt (part...
Posted 9th December 2013 at 12:00 AM byhbc Updated 14th December 2013 at 05:23 PM byhbc
The EAR 834p, is a marvellous and clever circuit, in its various optimisations and design features that are often not really understood... I acquired my second one a while ago, my first one was tweaked quite successfully to make very acceptable sound by adjusting the 10pf to 5pf and changing output CF to ECC81, and removing some surplus output potential dividers, and careful valve selection. Lowest noise first, and highest gain second.
However, it went away...
The problem I have seen with these is that they sometimes suffer muddy woolly sound in the bass which tends to spoil the over all effect. The new phono box I recently got suffers this way. Now I am suspicious of feedback loops, and how they work. I suspected that the 834p was maybe running a little short of gain in the lower registers. As I enjoy heresy, and after much consideration, I decided that bootstrapping the second stage ought to give me some more gain for the feedback loop to work. See circuit...
Posted 3rd December 2013 at 04:12 AM byabraxalito Updated 18th December 2013 at 01:59 PM byabraxalito
I'd like to be sure that my amp and the passive XOs in my speakers aren't significantly the bottleneck in the sound I'm getting from my DAC prototypes. To get a second reference point I'm starting to build a passive line-level crossover. Here's a pic of it taking shape - its using AD605s because I have plenty to hand and they're cheap and capable of being subjectively transparent with the right power arrangements. The pot core inductors are implementing LR4 XOs in balanced. Using the AD605s with their low input resistance and noise allows manageable inductor sizes for the XO - the largest being 9mH for a 3.5kHz crossover frequency.
Ah - I forgot to mention how I propose to do the transformation from normal (2V) line levels to the 70ohm impedance/20mV paradigm at the AD605 inputs. I shall be winding a pair of ferrite-cored step down transformers with centre-tapped secondaries Lots of turns to look forward to with 0.1mm or perhaps finer wire as I'd like to get at least 5H...
Posted 28th November 2013 at 10:55 PM byabraxalito Updated 12th December 2013 at 06:53 AM byabraxalito
A few weeks ago I built an almost completely passive analog output stage for my DAC/filter front-end and found it sounded noticeably quieter than my active ones which have been using AD605 and AD8129. The only active component in the experimental stage was a single emitter follower buffer run in classA - voltage amplification was achieved by a much higher than usual I/V resistor and passive filter specially designed for the higher impedance.
The comparison left me rather puzzled as to what was clouding the active stages. It turns out it was inadequate power supply decoupling - the fix is an abundance of ceramic capacitors soldered across the top of the chip.
On the AD605, the feedback resistor in low gain mode is a relatively low 820ohms which presents something of a challenge. Also I'm using an inverting stage following with a similar value input resistor. While the low combined values don"t present any significant challenge to the output stage itself (which...
Posted 21st November 2013 at 11:45 PM byrjm Updated 5th January 2014 at 08:17 AM byrjm(update schematic to 20f4)
Update: I've ordered parts for small number of Sapphire 2.0 kits. The normal price will be $125, but as an introductory offer the first batch will be available for $100. Kit includes a set of boards and all the parts for the board. You need to supply the transformers and diodes, as well as a volume control, and the chassis hardware.
November. That time of year for finally getting around to advancing some of my audio projects a little.
The Sapphire has remained in "rev 1+" for some time now, partly because of time constraints, partly because of the lack of popularity, and partly because it was already a re-spin of the beta version and worked just fine.
There were a few housekeeping things I wanted to add though, which have been included in the 2.0 revision.
- added a dedicated ground (GND) pad to connect to chassis...
Posted 10th November 2013 at 11:00 AM byrjm Updated 10th November 2013 at 01:11 PM byrjm
Last one for today.
I was having problems getting this to work, the key seems to be adjusting R6 to null the voltage offset. Works fine now, but this is just a rough reverse engineer of the diagram, the parts values and the type of transistors are essentially placeholders.
The input jFET is shown as a dual package. The output bias current is most likely much lower than the 100 mA I configured, so the class A output power is proportionately smaller. For the rest of the currents and the types of transistors used, your guess is as good as mine.
edit: R7 should most likely be closer to 47 ohms, while another 5p capacitor should go in parallel with the feedback resistor, R17.