Help with design for a balanced stepped attenuator

[crazytooguy]

I want to add a stepped attenuator to my monitor control box, but all of the designs I can find are for unbalanced signals. So, if I use a series design, I'll need a four deck switch, with positive and negative legs for each channel. What should the total resistance be? I'm using a +4 dBU level for feeding my monitors. Thanks for the help!

 

[NewYorkDave]

Just use an unbalanced stepped pot design of your choosing, halve the resistor values and duplicate the pot in both "legs" of the balanced circuit.

For example, if you wanted a 10K balanced stepped pot, you'd essentially build two 5K stepped pots.

http://www.goldpt.com/schm_ser.html
http://www.goldpt.com/r_series.html

Your question is too vague to allow a more specific answer than that. For instance, you don't specify source and load impedances.

 

[Boswell]

You didn't say whether the attenuator was for use as a volume control for the monitors or whether it was simply for level matching. You can go with the full-blown balanced attenuator, or do a simplified version if you don't want too much attenuation range.

The simple version would work for an attenuation range of about 30 - 40dB, and consists of a pair of series input resistors and a switched pad resistor. This needs only one switch wafer per channel.

If R is the value of each of the input resistors and P the value of the pad resistor, the attenuation for each switch position is given by

attenuation in dB = 20 * log( P/(P+2R))

You could build this either as switched individual resistors or as an incremental chain. Either way, create a spreadsheet and plug in some values. The value of R should not be more than 10K and much less than that if you don't need high attenuations. There's no minimum for P, so the end position could be a short circuit (P = 0) for muting.

 

[crazytooguy]

Thanks for the replies!

The source is my Fireface 800 DAW interface, and the balanced line outs have an output impedance of 75 ohms. The input impedance of the active monitors isn't specified in the manual, but they are DynAudio BM5 monitors. I do want to use the attenuator as a monitor volume control. The DAW software has a monitor volume, of course, but I want to set it to maximum and use the outboard monitor control box, placed in my recording electronics rack, to control the monitor volume. I may just experiment with some extra pots I have laying around to get a good idea of what resistances I need. Any other advice is appreciated! Thanks!

 

[kruz]

take a look at mcs's (a member here) kits here:
http://electronics.dantimax.dk/
i'm doing a box like yours for a friend of mine with his beautiful relay kits

 

[Rochey]

it's not ideal, but you could always convert to unbalanced,

Do your volume control etc

and then convert back using either a transformer, or alternatively an INA134/7 and DRV134/5

remaining completely matched on a balanced input really would be tough.

 

[Boswell]

Have you worked out what range of attenuations you need? Muting is a special case, but will you need an unattenuated position, i.e. monitors get full balanced signal?

 

[ruairioflaherty]

[quote author="kruz"]take a look at mcs's (a member here) kits here:
http://electronics.dantimax.dk/
i'm doing a box like yours for a friend of mine with his beautiful relay kits[/quote]

Hey Kruz,

I had never seen those kits before. Any idea how they sound (or don't sound). I use a Benchmark DAC-1 feeding a pair of Adam S3As and I intend to build a stepped attenuator of some sort in the coming months to use instead of the poor internal pot.

I do a lot of mastering so I'd like whatever I make to be as clean as possible - up to now I had only considered switches with resistors,

cheers,
Ruairi

 

[kruz]

Hi Ruairi,
these kits are basically just switches with resistors..
the switching is made with relais that are controlled digitally with a little microprocessor
So, the audio path runs still just into resistors.. nothing else

 

[ruairioflaherty]

Thanks Kruz,

excuse my lack of knowledge but is there any reason to avoid relays in an attenuator circuit. I know the Cranesong Avocet uses relays to switch in a similar manner but I know little about the implementation.

I have seen mention of gas filled relays which avoid oxidisation of the contacts over time - not sure if the dantimax kits uses these. I'm interested in these kits for several reasons including the value and what I see as the easy possibilty to add functions like dim, mute and speaker on/off switching.

I will spend some time looking at the Dantimax site but I'd love to hear any feedback when you build your attenuator. I guess the question is - for absolute purity should I stick with a switched attenuator for any reason or could this relay solution be just as good?

cheers,
Ruairi