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Old 21st June 2002, 03:28 PM   #11
tvi is offline tvi  Australia
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<center><a href="http://164.195.100.11/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1='3983421'.WKU.&OS=PN/3983421&RS=PN/3983421">US Patent 3,983,421
Yogore September 28, 1976
Remote audio attenuator</a></center>

Abstract
A remote audio attenuator in which a volume control variable resistor of the attenuator regulates a volume control variable resistor of a distant audio system with complete isolation between the circuit of the two volume control resistors, and with rapid response between the two control units. The attenuator employs a lamp controlled by a differential operational amplifier, with the lamp set to equally illuminate slave photoresistors, one or more of which serve as the volume control units of the distant audio system, and one of which is connected to a resistance bridge to provide feedback to the differential operational amplifier, the output of which is also controlled by the remote control variable resistor which is also connected to the resistance bridge.


James
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Old 21st June 2002, 03:41 PM   #12
dorkus is offline dorkus  United States
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Default Re: Attenuator placement

aurora, you probably always want the attenuator followed by a buffer stage of some sort to keep output impedance low. so yeah, it should be at the input. however, some people even prefer to put the attenuator between stages, so it is completely isolated, as this give you better impedance characteristics and thus lower noise. i think putting a fine attenuator (0db to -9dB) at the input and the coarse attenuator inter-stage is a good compromise.
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Old 21st June 2002, 03:44 PM   #13
dorkus is offline dorkus  United States
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tvi, my denon surround preamp uses a japanese volume control IC, i forget who makes it... it's not toshiba or sanyo but another one of the big semi companies there... maybe NPC or something? anyway it is surrounded by tons of opamps (BB OPA2604 i believe) but to be honest, considering how "dirty" the signal path it, this thing sounds pretty good. it can't compare to my passive preamp though. all those ICs seem to be buffered internally, probably with an opamp of some sort, so many think that will be a limiting factor.

the Maxim part looks ok, but it does not have 1dB resolution throughout its range, which is a requirement for me.
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Old 22nd June 2002, 05:39 PM   #14
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How about this topology? Related post on active circuitry is here

Son of Dork: Active Circuitry

So what is good about this?

1. It is balanced
2. It uses minimal number of components
3. As shown it is a shunt

Any switching method can be used to set the volume -- from semiconductor switches to relays. My personal favourite would be a set of mercury wetted relays, perhaps 12 relays in sets of say 8, 2, 2. With a delay on the switching + some form of Gray code (one changing position at a time), one could get very long life, minimal "clapping" and serious range.

Of course, a regular pot is simpler to implement and cheaper.

JFET switches are also cheap and probably good enough, especially since this is fundamentally shunt operation with minimal output loading + it is acceptable to set them up in pure binary mode since there are no moving parts. 10 such sets yield 1024 levels.

What do you think?

Petter
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Old 22nd June 2002, 11:33 PM   #15
Helix is offline Helix  United Kingdom
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I would like to go with who ever suggested this idea, just have about 8,9 or even 10 FETS and have the signal biased at about 30V (ideal to dangle on the end of my single ended pre-amp and then put the output cap after it).

would it work? one would have to build 2 for each channnel if you want balanced.
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Old 23rd June 2002, 12:37 AM   #16
dorkus is offline dorkus  United States
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it's a neat idea, another great way to use the dc offset of a SE design to your advantage. my only issue with this is that it would have to use a buffer after it to keep the output impedance constant and the attenuation consistent. so, if i need a 2-stage attenuator, i would need 3 active stages total... 2 SE stages with this attenuator, followed by another buffer. it would be nice if i could get rid of one attenuator stage but i don't know of a simple way to get the range and resolution of attenuation i need w/a single stage. i was toying with a design that could switch arbitrary combinations of shunt resistors in parallel but it will take a lot of calculation to figure out the required values and combinations need... i should probalby just write a program to calculate it for me. =p
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Old 23rd June 2002, 03:03 AM   #17
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I start holidays in 2 weeks, I'm going to put in some serious time on Wayne's secret attenuator which would be perfect for this and would fit between the 2 stages.
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Old 23rd June 2002, 01:27 PM   #18
Helix is offline Helix  United Kingdom
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thats the idea, you have to control the FET's from a micro or whatever you choose. Then you can arange the R values so that you have a MSB (most attenuation) and a LSB (least attenuation) you can get the values of R's from the Pass preamp i think. that way with 8 FETs you have 255 diffrent level settings, and just drive the FETs with bineary. One has to be careful that ALWAYS at least one FET on! or otherwise no attenuation and the full preamp gain might be a bit high for the amp.
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Old 23rd June 2002, 02:12 PM   #19
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So what is needed is pull-up resistors to ensure that FET's are on unless specifically switched off.

I believe BJT's can be just as good as FET's -- and JFET's are possibly better still. Perhaps choosing a device which shuts itself off more or less completely is as good idea as one that has low on-resistance/noise/forward drop

One thing I don't like is that unless you attenuate the signal against a significantly lower voltage, you will not have symmetrical load for positive and negative swing. However if you do load down against a significantly lower voltage you stand the risk of messing with gain stages be they previous or next.

One way to avoid the asymmetric loading would be to pull up as well as down, but I still favour direct attenuation across phases instead of single attenuation to ground (or significant negative voltage).

It would probably also be useful to use capacitors to slow down the device switching somewhat.

Petter
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Old 23rd June 2002, 05:35 PM   #20
dorkus is offline dorkus  United States
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you can get 255 combinations with 8 switches in a ladder config, but i'm not sure if it will be possible to optimize the values to get the desired consistent 1dB or .5dB steps, at least in a variable-shunt-only config.
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