Son of Dork: Attenuator

The Son of Dork (SOD) project is the result of the diyAudio.com multi-channel preamp initiative (http://www.diyaudio.com/forums/showthread.php?threadid=3517). The project has been split into threads dedicated to discussion of the preamp's different functional units:

- General Discussion (project status, functional requirements)
- Active Circuitry
- Attenuator
- Power Supply
- Connections and Layout (including input switching)
- Chassis and Construction
- Hardware Logic, Controls, Display
- Microprocessor (programming, implementation)

I will be managing the project as a whole, as well as initially overseeing individual threads. For a background on the project, please see the original thread referenced above. A website dedicated to the project will be located at http://dorkus.org/diy/sod.

Let the fun begin!
 
the WM8816... i know some don't like it because it requires an opamp, but was wondering how it sounds in combination with a "good" IC, e.g. AD825 or what not... if the chip is capable i may try it with a discrete opamp design.

Absoluted no need for an op amp whatsoever, discrete or otherwise. All that is needed is a inverting amp, you could do it with a plate output triode if you so desired, a single transistor...

Sonny,

Have you tried the ad8610 with the wolfson yet?
 
No Paul ... i have not been able to find time for this right now... But in this summer i will have 4 weeks vacation so there should be plenty of time...

For the inverting and no feedback...

The performance of the WM8816 is based on a feedback design... But if the output of the WM8816 is held onto virtuel ground then this should be possible!?

Nice idear Paul ... Worth a try! ... With the right circuit the output will also be in phase with the input.

Drawback ... The resistor network inside the WM8816 will change a lot from unit to unit .. ~30% So you have to compensate for this gain variation

Sonny
 
well...

i'm running out of attenuator ideas...

other than the wolfson, i have maybe one other scheme. we could use a dual-stage circuit, as i saw posted on some german website... there would be a 10-position attenuator at the input of the preamp (0dB to -9dB in 1dB), followed by the gain stage, then followed by a 8-position attenuator (0db to -70dB in 10dB steps), then followed by a buffer or another gain stage. this would effectively give us 0dB to -79dB attenuation in 1dB steps. suggested gain stages are Borbely balanced SE JFET, Borberly "super buffer", maybe even a simple single-ended FET buffer for the 2nd gain stage. the actual attenuators would be fixed-series, variable shunt using solid state switches to select the shunt resistor (e.g. AD SSM2404). since one end of the switch will be connected to ground, an FET switch should be fairly linear here, with low on-resistance.

thoughts?
 

mlloyd1

Member
Paid Member
2001-02-25 7:10 pm
Northern Iliinois
Re: has anyone tried the Wolfson chip?

The Dallas/Maxim DS1808 attenuator is also an "op amp less" part. It might be worth investigating. I haven't been successful yet at getting any samples though. So, I wonder if it's a real part or vaporware.:confused:
mlloyd1

dorkus said:
the WM8816... i know some don't like it because it requires an opamp, ....
 
There are Japanese manufacturers of Volume Control ICs, alot include bass, treble and loudness functions, the following don't:
<a href="http://www.semicon.toshiba.co.jp/eng/prd/mpsig/ft_mpsig.html">Tohiba Electronic Volume Control ICs</a>
<a href="http://www.semicon.toshiba.co.jp/en/bucat_2/bucat_3/bucat_14/td_7/TD.pdf">TC4959</a> is 91 step 0 to -89dB
<a href="http://www.semicon.toshiba.co.jp/en/bucat_2/bucat_3/bucat_14/td_8/TD.pdf">TC9482</a> is 97 Steps 0 to -95dB, 6 channels in one chip.

<a href="http://service.semic.sanyo.co.jp/semi/ds_pdf_e/LC7536M.pdf">Sanyo LC7536 </a></body> is 81 step 0 to -79dB

As to sound ????


James
 
<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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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.
 
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.
 
How about this topology? Related post on active circuitry is here

http://www.diyaudio.com/forums/showthread.php?threadid=3625&perpage=15&pagenumber=5

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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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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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
 
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.
 
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