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Old 17th December 2002, 01:53 PM   #81
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Hi there.

Adding to the confusion, I'd like to drop my two cents here. Two or three years ago, I made a relay-based volume control for my BoSoZ. It's driven by a uC (AVR), and has a "make before break" feature. For people interested, I posted the schematic here on DIYaudio, along with some infos. It's working faultlessly and more than fine. If you need more info, let me know.
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Old 17th December 2002, 03:42 PM   #82
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Quote:
Originally posted by hifiZen
The 74VHC164 is the SIPO (serial in parallel out) shift register device I used. All that's required is one data line and a clock to shift the data bits in. Quite simple, I think you'll agree, and much less hassle than a micro or I2C chip, although you may need to write a quick little software routine to do the bit-bashing. I don't think this particular chip can be daisy-chained, but you can see how easy it would be to control numerous banks of resistors with a bunch of daisy-chained SIPO registers... Plus, you can write data to this chip at 20HMz, or whatever speed your micro can muster, as opposed to the pokey 100/400kHz clock of I2C.
Well, if it were just a speed issue, then the bit-banging would get me a data rate of about 2.5MHz (internal clock frequency of the PIC I'm using). And that's only going to be 5-6 times faster than I2C (400kHz), max. In practice, bit-banging is going to be maybe 4-5 times faster. And since we're dealing with micro seconds, 4-5 times faster shouldn't be noticable. But I'm also interested in making a conviniently modular solution. And with I2C, I can use the PIC to determine how many passive volume boards are connected. So if you originally had the pic and a single volume board (2 channels), and you wanted to add another two channels, you simply tie an addional volume board to the I2C bus lines, send some kind of EEPROM reset condition to the PIC (through some menu option or jumper setting), and the PIC will recognize the new board and act as if there were always 2 boards (4 channels). Now, I2C isn't a hassel on the PIC side because Microchip built an I2C module into the chip, in order to make I2C more convinient. Price isn't a major issue, as the 16-bit I2C I/O expander I'm looking at is around $2.20 a part (Phillips PCA9555). And the only other thing I have to worry about is the noise from the chip...

You do make really good points for using that SIPO chip though. The best thing I can tell you is that after I get the I2C volume board prototyped, I'll take another look at bit-banged serial.

Anyhow, about the noise concern. The PCA9555 is a 24-pin SOIC part. Would it be sufficient to surround the chip with ground plane, or is there a better way to deal with the noise?

--Jordan
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Old 17th December 2002, 03:46 PM   #83
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Quote:
Originally posted by hifiZen
Regarding relay control ...
Many thanks for the information regarding the drivers and diodes. It was exactly the information I was looking for.

--Jordan
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Old 17th December 2002, 05:40 PM   #84
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Quote:
Mark, can you tell me which switches you used, what were your impressions sound-quality wise?
Hi Jan,

This wasn't really for a mega-serious hifi application - it was for my NICAM tuner - this used standard 4052 CMOS switches to select between NICAM and FM, and do the muting. As there was two spare buttons on the remote, I was planning to have a simple volume control - maybe 2dB steps, and using a simple 4-bit counter that would give 32dB of total range. My hifi wasn't remote-controlled, so this would have given me some control when watching TV...

I never got around to implementing it for a number of reasons - mainly because of the poorly-defined (and rather variable) RDS(ON) of the CMOS switches. That wasn't a problem for the switching mentioned above because they were followed by a buffer with a high input impedance. However, for a constant-impedence attenuator, the resistance variations would cause inacuracy and significant distortion...;(

You could get around that by using high-values resistors, but you'd introduce Johnson noise....

RDS(ON) varies with common-mode voltage

Cheers,

Mark
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Old 17th December 2002, 05:46 PM   #85
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I know that this thread is discussing passive relay control, but I've just had an idea... !

Using an inverting stage, you could make a mix-amp. Each input has a different gain weighting, and by choosing the resistor values carefully, you can make them dB's in powers of two, as shown. This isn't too difficult in practice...

Then, addres the relays using normal binary... It's a crazy plan, but it might work
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Old 17th December 2002, 06:29 PM   #86
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Default Relay etc

Mark,

Thanks. You know of course that there are modern switches with <5 Ohms Ron with very flat resistance. They would be OK used in a virtual ground mode.

You last scheme might work, but note that in the -64dB pos the resistance ratios around the opamp are > 1: 1000. Not easy, you know, noise and all that. An R/2R ladder (available as SIPs from Vishay) with a couple of the above mentioned switched could do the trick.

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Old 18th December 2002, 08:28 AM   #87
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Hi Jan,

Quote:
An R/2R ladder (available as SIPs from Vishay) with a couple of the above mentioned switched could do the trick.
But the steps would be linear!

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Old 18th December 2002, 09:41 AM   #88
sonnya is offline sonnya  Denmark
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It would require a lookup table in a micro to make them log.
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Old 18th December 2002, 10:49 AM   #89
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sonnya's method is one possible solution, and quite an elegant one at that.

The other soution is not bothering at all about logarithmic behaviour. While I admit that there are situations where it is important to have logarithmic readout, it definitely isn't, at least not to me, for the volume setting of an amp.
I usually set the volume so that it is pleasing to my EARs and not to make any kind of readout pleasing to my eyes.

Regards

Charles
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Old 18th December 2002, 11:40 AM   #90
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Default relay etc

Quote:
Originally posted by sonnya
It would require a lookup table in a micro to make them log.
Right. AD had some MDACs that were really 17-bit R/2R ladder DACs with internal conversion through a look-up table so the input dig code would be in equal steps of I believe .375dB for 70dB range. AD7745 or something. Discontinued a couple of years ago because you guys insisted on buying stepper switches ...

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