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Stepped Attenuator

Posted 8th May 2012 at 12:20 AM by rjm
Updated 8th May 2012 at 12:24 AM by rjm

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.
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RJM B-board vs. 47 Labs 0247

Posted 5th May 2012 at 12:29 PM by rjm
Updated 6th May 2012 at 06:16 AM by rjm

B-board vs. 0247.

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.

The 0247...
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Old

The Media Room

Posted 1st May 2012 at 01:28 PM by DrDyna

Well, I still need another coat of paint and some crown molding / baseboard, but it's coming along nicely.

The front wall is out about 25" from the foundation, I plan to install 2 Ficar IB318's in there. The EP2000 should be here tomorrow!
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DACalito - transversal modification of DAC-AH Lite

Posted 30th April 2012 at 06:46 AM by abraxalito

Plenty of audio DIYers have discovered what an excellent modding base the DAC-AH Lite is. I'm a latecomer to the party, but better late than never here's my contribution to the art of DAC-AH-alikes.

Most of the mods I've seen focus on the output stage, some upgrade the PSUs. Here's yet another variant - change the DACs. The TDA1545 is pin compatible with the TDA1543 so I swapped out 8 TDA1543s and replaced them with 8 DAC-stacks made up of 5 TDA1545s each. 32 DACs in parallel produce the normal NOS output, the additional 8 DACs are fed from a two-stage delay line (16 74HC595s) to correct for the zero-order hold. I/V conversion is done passively and differential - 1.3ohms in each polarity. The post I/V stage is a pair of AD605s giving a true differential output. In between the two is a passive filter made up of TDK ferrite beads and NP0 capacitors.

How does it all sound? - well of course one is always biassed when describing one's own offspring, but in a word...
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Old

General biking update...

Posted 29th April 2012 at 03:44 PM by chris661

After many months of riding and not cleaning my bike, I cleaned it.

It was considerably lighter after such a treatment, which was great when we (me and the college people) went up to Gisburn Forest for a ride around...

Note the pictures below aren't mine, but show where we went quite well...

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Click the image to open in full size.Click the image to open in full size.

Click the image to open in full size.

and, of course, here's a vid of someone else trying the black run.

hully gully (rollercoaster) gisburn 05-06-10 - YouTube

Some great riding there - if you're reading this, and happen...
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Old

Using Audacity to do NOS droop correction

Posted 29th April 2012 at 03:36 AM by abraxalito

If you have a NOS DAC (like a Metrum) and are curious to hear how it sounds with the zero-order hold corrected, here's a fairly simple way to try out my 3-tap filter using Audacity.

Load up the file you'd like to process and make two duplicates (select original and use Ctrl-D) - we'll call them A and B. We need to apply delay and gain to A and B which we do with Audacity's time-shift button (in the group of 6 tool buttons to the right of the transport buttons) and the 'Effects - Amplify' feature. A is shifted one sample to the right, and B, two samples right. I've shown how this looks with a mono chirp signal in the first image.

The gain is input into Audacity in dB so 0.15 becomes -16.5dB and 0.026 is -31.7dB. These are the gains for A and B, respectively. A needs to be inverted too - use 'Effects - Invert'. The second screen grab shows how it looks after these operations (I hid the device toolbar to give more space for waveforms).

Having processed...
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Old

First measurement of NOS droop correction

Posted 27th April 2012 at 06:13 AM by abraxalito

Here's a ten discrete tone test waveform played back into my Sony PCM-M10 then FFT'd in Audacity with 512point FFT.

Audacity reports lower freq tones at -18.6dB and the highest (17.3kHz) at -18.9dB - a droop of 0.3dB. This might be in part my passive (LC) reconstruction filter which I have yet to characterize separately. So it appears to work
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Old

Hello world

Posted 26th April 2012 at 02:13 PM by ARIYAHOOR (های-فای و سیستم های استریو)

I'll start posting articles in Persian language, in hope of having Persian visitors
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Old

Flight of the Pheonix

Posted 26th April 2012 at 08:34 AM by rjm
Updated 4th May 2012 at 02:30 PM by rjm

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):
Click the image to open in full size.

Opened up, we see the circuit board, such that it is, and my home-built 11 position attenuator:
Click the image to open in full size....
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Old

Stereo VSPS rev 50c

Posted 25th April 2012 at 12:48 AM by rjm

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.

All the info you need to build one is attached.
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