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Volume Control use Dual 8-BIT/12-BIT DAC - I don't understand how it works
Volume Control use Dual 8-BIT/12-BIT DAC - I don't understand how it works
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Old 30th July 2016, 04:32 PM   #1
tiefbassuebertr is offline tiefbassuebertr  Germany
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Default Volume Control use Dual 8-BIT/12-BIT DAC - I don't understand how it works

A friend of me use since many years Linn's KAIRN as preamp. Last weekend there are suddenly no signal at the output (at all inputs and at both channels).
After a closer look, I realized that at the pins 13 and 19 of the volume control IC (V-REF channel "A" and Channel "B" - here used as signal input) the signal is still present.
But at the output pins 15 and 17 (= I-out) the signal is no longer present - regardless of the volume control position number in the display.

The main question is how I can prove whether the IC is defective or whether the data from control PCB are incorrect. Therefore I want to understand, what is the main difference between a Dual 8-BIT/12-BIT DAC for use as stepped attenuator and typical RC volume control ICs like PGA2310 ? - go to post #1, C) under
http://www.diyaudio.com/forums/analo...ew-wanted.html
for more such typical volume control ICs.

The Linn KAIRN uses this DAC for volume control:
AD7549JN (AD7549)
http://www.analog.com/media/en/techn...ets/AD7549.pdf

Unfortunately I haven't the schematic of model "KAIRN" from Linn, but in the predecessor, the Linn LK1 is similar DAC for volume control in use:
AD7528JN (AD7528)
http://www.analog.com/media/en/techn...ets/AD7528.pdf
Schematic of LK-1, arround the AD 7528 JN you will find on
page 9, second pdf and page 6, first pdf from post #1 under
Question arround the Power Supply of Linn's Preamp LK-1

Who can help me to find out the malfunction ?
Thanks in advance.

Last edited by tiefbassuebertr; 30th July 2016 at 04:49 PM.
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Old 30th July 2016, 06:52 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Have you traced the signal with a circuit diagramme? How far did it go?
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Old 1st August 2016, 08:41 AM   #3
tiefbassuebertr is offline tiefbassuebertr  Germany
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Quote:
Originally Posted by JonSnell Electronic View Post
Have you traced the signal with a circuit diagramme? How far did it go?
check the top of post #1 and the datasheet under
http://www.analog.com/media/en/techn...ets/AD7549.pdf
The Vref pins 13 (one channel) and 19 (second channel) serves as audio signal input.
Obviously is not typical of the use of this IC at this point (volume control and not a DAC).
Unfortunately I haven't the genuine KAIRN schematic, but those from LK1 (as previous mentioned) must actually help to understand basically operation.
But for exact understand all functions, particularly at the bus control pins, actually a detailled schematic is necessary, include processor (HD6303RF), EPROM, PCF8574 (I/O Expander for I2S) and SAA1064 LED Driver, here used as source select driver for DG309.
I will try to get a second device of this model to changing front-PCB and EPROM for fix the reason.

Last edited by tiefbassuebertr; 1st August 2016 at 08:55 AM.
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Old 1st August 2016, 05:30 PM   #4
Monte McGuire is offline Monte McGuire
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Volume Control use Dual 8-BIT/12-BIT DAC - I don't understand how it works
The 'I-out' pins are a current output from the DAC, meant to be loaded into a virtual ground. You are not supposed to see voltage at that node, unless the current to voltage amp is somehow blown up. The DAC needs this pin to be held at virtual ground - only signal current passes through that node - no (or extremely little) signal voltage develops when it's working properly.

Follow that I-out node to the amplifier that this pin is loaded into, presumably an inverting input terminal, and then look at the output of that amplifier, probably by following the feedback resistor to the amplifier output. You should see signal there.

Semiconductors are pretty reliable these days. I would suspect more mundane problems such as soldering failures, connector problems, or user errors.

It's also useful to verify that each chip has the proper power supply voltages present. Warning: please be careful when you do this - a slip of a probe that shorts two adjacent pins together _will_ generally blow up an IC, where there was no problem before.

Best of luck!
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Old 3rd January 2018, 11:20 AM   #5
tiefbassuebertr is offline tiefbassuebertr  Germany
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Default Sound Quality Differences between Volume Controls by DAC and By Attenuator Networks

The Linn LK1 had realized a volume control by the use of the DAC - datasheet under
http://www.analog.com/media/en/techn...ets/AD7528.pdf
and for schematic go to page 9 from the second PDF in post #1 under
Question arround the Power Supply of Linn's Preamp LK-1

PIN 18 is the input, PIN 19 the feedback pin and PIN 20 the output.

The LM1972 uses a resistor network instead a DAC - go to
http://www.ti.com/lit/ds/symlink/lm1972.pdf
Also the PGA2311 - go to
http://www.ti.com/lit/ds/symlink/pga2311.pdf

What are the pros and cons between Linn's approach and the today's usual approach for the volume control ?

Thank you for advices.
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Old 3rd January 2018, 11:50 AM   #6
scottjoplin is offline scottjoplin  Wales
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I've only recently been made aware of the approach used by Linn, that is, adjusting the DAC's reference voltage. It appears to have advantages over analogue attenuation, mainly it would seem, no additional circuitry or pots etc. However, I wonder if the DAC output was any appreciable distance from the amplifier, whether it wouldn't be better to attenuate from a higher signal just before it.

Is today's usual approach the latter?
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