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Azimuth meter

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Fatmangolf View Drop Down
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    Posted: 22 Apr 2013 at 9:35pm
Just wanted to share a bit of DIY: I am building a stereo crosstalk analyser using the AD8302 RF/IF chip. I based the schematic on the datasheet and adapted it to work at audio frequencies not Mega Hertz.

In short I've breadboarded a device that will be in a box later this week as follows:

    Battery powered by 3 AA batteries
    2 phono sockets to connect my turntable via the phono preamp
    Analogue meters for left channel separation (LED comes on), right channel separation (different LED comes on), and phase difference.
    The range is +/- 30dB (adjusted to 20 or 40dB with a switched attenuator on one channel only) for measuring crosstalk and 0-180 degrees on phase difference.


If anyone's looking at the datasheet I wired the left meter from Vmag to Ground and the right from Vref to Vmag. The 1.6V low current LEDS block the reverse current flow to protect the meters and bring them in from about 20dB when the diode conducts. And they look pretty. The phase meter is wired from Vref to Vph. The AD8302 is 14 pin SMD needing few external parts, I used 22uF caps for the input and offset decoupling, giving about 100Hz as the corner frequency.

After construction and calibration for +30dB FSD on each meter I plan to use the meter to fine tune the azimuth on my turntables using a test LP that has test tracks with 1kHz on only the left and right, respectively. I will note the changes to maximise and equalise the L-R and R-L separation, whilst minimising and matching the readings for phase difference. I will also be listening carefully to ensure aligning the coils coincides with aligning the stylus.

If anyone is interested I will post a schematic and some photos?
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 2013 at 10:46pm
Sounds interesting. A novel application that could prove really useful. Cartridge set up is something akin to a black art, this should counter some of the apparent magic (ie guesswork!).
More info would be good, I look forward to seeing the development and your results.

Ian.

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tg [RIP] View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tg [RIP] Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2013 at 7:33am

A sort of DIY Fosgometer then.

Bit beyond my DIY skills (or electronic knowledge FTM) but certainly interesting.

Piccies are always good :)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2013 at 10:21pm
A bit like it but the phase measurement is different. Balancing that as well should improve the stereo.image with stable positioning and width.

I'll draw a schematic and photograph the pcb.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JamesD Quote  Post ReplyReply Direct Link To This Post Posted: 23 Apr 2013 at 10:40pm
Sounds interesting, will have a look at your schematics if you post them. Always good to have some means of confirming what you think your ears are telling you!
Aren't ears brilliant
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 24 Apr 2013 at 10:05pm
This is the datasheet:
AD8302 datasheet

On page 16 you'll see the schematic. For 1kHz test LP tracks the input caps C1/4/5/6 are all 22uF polarised and I used a spare 470uF for C2.

For the left signal separation I wired a 250uA meter in series with a 1.6V LED and a 1k preset pot, connected from Vmag to 0V. For right the same but wired from Vref to Vmag. Please note that +30dB (approx. 200mV ac on L/6mV ac on right) and -30(6mV and 200mV, both ac) give 1.8V and 0.03V. I adjusted the presets to full meter deflection at +/- 30dB. +35dB or more overshoots the 1.8V stated limit on Vmag to give about +1.95V.

It's the phase difference on crosstalk (i.e. from the active left channel to the almost silent right channel, or vice-versa) that adds another dimension to azimuth adjustment. If you've done this with a dual channel oscilloscope (or software equivalent) you'll know why!

I will draw the schematic I've built and photograph the circuits at the weekend. This is simply using a production IC and schematic but for a different use. Once I spotted the potential application I did search online to see if someone had already done the fine tuning.


Edited by Fatmangolf - 24 Apr 2013 at 10:24pm
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 29 Apr 2013 at 9:25pm
The unit does work but the metering on L and R channel separation is less linear than I would like. In reality connecting one digital voltmeter to the Vmag (channel level difference) and another to Vph for the phase difference is more precise.

After trying various resistors in series with and in parallel to the level meters, I am reconsidering a larger meter with the midpoint. The aim was to design a self-contained unit for level and phase so I will persevere when I next have time.

I can confirm that the IC works fine at audio frequencies and works with only a few extra parts. I ran a 22k resistor from each phono socket to the dc blocking capacitor on that IC input. That took the 300mV coming from the phono preamp down into the log detector's midband, see the datasheet for more details.
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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