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Old 30th May 2008, 01:37 PM   #11
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You can still go expensive on the AD stuff.
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Old 30th May 2008, 02:57 PM   #12
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AndrewT, you are correct in that for a single load on the buffer stage described the amp would be seeing 150 ohms. You need to generate a signal of twice the voltage you want as half of it is lost in the source impedance defining resistor and the other half arrives at the far end of the cable. The source resistor is called a back terminating resistor I think. This type of system results in a high "return loss" both for forward and reflected signals. This type of system with matched source/cable/load impedances has very limited application in audio outside of telecommunications.

In the real world of audio, reflections due to what ever cause are going to arrive back at the source much sooner than the period of the highest frequency signals and thus can be ignored. This reality is why source impedances tend to be low and load impedances high and the cable characteristic impedance does not matter. It goes by the name of "voltage matching" and has been in use for 30 odd years.

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Old 30th May 2008, 03:05 PM   #13
AndrewT is online now AndrewT  Scotland
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Thanks Keith,
but does the buffer see only the 150r as a load, or does it see 150r//cable capacitance, or something else?

I'm thinking about very long twisted pairs (CAT5) travelling throughout the house. Well, very long in terms of domestic interconnects.
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Old 30th May 2008, 03:11 PM   #14
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From today's TI newsletter...

DRV601

"DirectPathÖ Stereo Line Driver with Variable Input Gain: The DRV601 is a cost-effective, integrated stereo audio line driver designed to drive 600┘ or high-line impedances. An integrated charge pump enables operation from a single external supply voltage and the device allows input gain to be set by external resistors.
SNR: 109dB
2VRMS/Ch output voltage into 600┘ at 3.3-V supply
Applications: Set-top boxes, CD/DVD players, DVD receivers, HTIB, PDP/LCD TVs
DRV601 Price (1ku): $0.75"
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Old 30th May 2008, 03:18 PM   #15
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Andrew, you must have been at your computer when I posted! Yes all cables will have capacitance which will be in parallel with the load. It is not the value of the capacitance alone that counts but the value in relation to the source impedance of the generator (the device driving the cable). With these two elements we have a low pass filter. This is explained in the Rane notes mentioned above.
Keith
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Old 30th May 2008, 03:26 PM   #16
AndrewT is online now AndrewT  Scotland
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that gives me F-3dB of 193kHz for 110ohm CAT5 that are about 50m long and 150pF/m.

I can live with that, but obviously 75ohm or 50ohm would allow higher bandwidth.

I had a feeling that terminated cables didn't behave like that.
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Old 5th June 2008, 01:17 PM   #17
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Redrabbit, if you are still following this thread there is a thread on a forum at http://picocompressorforum.com/forum...c.php?=7&t=123 dealing with this exact topic. Maybe it's another incarnation of you? Cheers
Keith
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Old 5th June 2008, 01:45 PM   #18
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Quote:
Originally posted by Keith Taylor
Maybe it's another incarnation of you? Cheers
Keith
Yes, that's my other ego.
I went over there because I knew they (mediatechnology, etc) have a lot of experience with the "THAT" audio products.
Anyone wishing to see my block diagram/schematic should visit the link Keith gave above.
I've already built it, so far, so good. Very simple. Testing it this week.

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Old 11th July 2008, 07:34 PM   #19
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Redrabbit, the first op amp is inverting because the person that drew it made a mistake and transposed the inputs
Correct. Based on the typography it looks like it came from TI. They probably ought to fix that one.
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Old 12th July 2008, 03:38 AM   #20
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Quote:
Originally posted by mediatechnology


Correct. Based on the typography it looks like it came from TI. They probably ought to fix that one.
(sarcastic giggle)...As far back as 1999 (or longer??) this mistake has been present. The current TL08x datasheet still wrong.
http://focus.ti.com/lit/ds/symlink/tl081.pdf

But the TL07x has it correct (shows the same circuit application).

Datasheets too often have mistakes. I don't think they pass thru the lab for approval.

Can you find a mistake in this Analog Devices schematic?

Click the image to open in full size.

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