[DESIGN] A general-purpose 600-ohm balanced attenuator

There seems to be a lot of call for something like this. Build a couple of these in small metal BUD-boxes with XLR or 1/4" TRS connectors and keep 'em handy.
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Note that this is suitable for line-level devices that want to work into a 600-ohm load. It is NOT for padding mic inputs.

Although not specified on the schematic, the metal chassis of the attenuator should be connected to pin 1 of the XLR jacks or the sleeve of the 1/4" TRS jacks.

Attenuation is 0-50dB in 2dB steps, plus "off", accurate to within 0.1dB, limited by the tolerance of the resistors.

Have fun!

cool dave!

why did you add the 600 ohm termination?

dave

[quote author="NewYorkDave"]
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[/quote]
Nice, but some problems with do couple.
It uses much of different resistors.
It is uneconomical to buy resistor in two pieces of value.

I mean, that this symmetric attenuator can be done as cascade
of attenuators of the some value., i.e. 10 X 1 dB and 4 X 10 dB

You will not need too many different valued resistors.

xvlk

Dave:

The switchable termination is for when you have an output that wants to see 600 ohms (a transformer-coupled output, for example) and you're running the output of the attenuator into a high-Z input.

xvlk:
I'm sorry, but I don't get most of what you're saying.

so you would not use the termination if you are running into another 600 ohm input, right?

correct!

COOL, NY Dave! Thanks!

I plan on experimening with this on my Ampex 351 (despite PRR's suggestion in the earlier thread to use a 1k pot; I just wanna try it for myself for my own education).

I have a question, though, I hope it's not dumb. Instead of 2 rotary switches, could I wire it for just atten. 2 and ignore the atten. 1 section of the schematic? Would the values on the schem for the atten. 2 section change if I tried it this way? I can't see where it would, but I just wanted to confirm. I know attenuation may not be as flexible with atten. 2 only, but I don't know know if I have the real estate for two rotary switches.

I guess I could also wire in a 3PnT (n = number of steps) rotary and determine all the values myself for total attenuation and how many dBs/step... :?:

Again, thanks for giving me a clue! MUCH appreciated!

:guinness:

Chris

Oh, I had another question...

What would be the advantages/disadvantages of buying something like, say, a Daven or Shallco attenuator compared to building an attenuator using a rotary switch? Is the Daven or Shallco (or equivalent) better than building your own?

It's just two switchable attenuators in cascade. You can build one, or the other, or both.

Be aware that if you go with Atten 2 only, 10dB steps are EXTREMELY coarse. Each 10dB increase is about twice as loud!

A Daven or Shallco attenuator is nothing more than a rotary switch and resistors, but usually with no click-stops between steps. Typically, they're 21 steps, -2dB/step, with the last few steps tapered to "off." A balanced Daven or Shallco is hard to find and expensive. The types you usually see on evilBay are unbalanced ladders and bridged-Ts. Balanced ladders occasionally turn up, but they have a 6dB minimum loss.

OK, cool. One problem...

Where the hell does one get a 3P6T shorting rotary switch? I can't find anyone who sells them! :sad:

I don't know if I have the real estate for two rotary switches.

Click to expand...

What real estate? You build it in a little BUD box and patch it in where needed.

Where the hell does one get a 3P6T shorting rotary switch? I can't find anyone who sells them!

Click to expand...

Use a 4P6T and just ignore one set of contacts.
http://www.mouser.com/catalog/specsheets/011103.pdf
$3.17 each... The price must have shot up recently from $1.57

[quote author="NewYorkDave"]What real estate? You build it in a little BUD box and patch it in where needed.[/quote]

Dave, I was thinking of putting the rotary inside my 351.

[quote author="NewYorkDave"]Use a 4P6T and just ignore one set of contacts.[/quote]

OK, that what I was thinking, though I wasn't sure I could do that.

Thanks a million, NY Dave! :green:

NY Dave, is that an Alpha switch? I've been trying to get hold of those in the UK with no luck.

The UK distributors for Alpha (TM Electronics) initially bombarded me with e-mails, phone calls, catalogues etc then lost interest when I told them I couldn't meet their minimum order requirement of 1000 pieces/type!

Anyone know a UK supplier?

Steve

Yes, it's an Alpha. Perhaps Mouser will ship to the UK. Even with the cost of shipping, it would be cheaper than having to meet a 1000-piece minimum. Failing that, I'm sure some Lab members here in the States would be happy to help.

Come to think of it, since buying a couple of each value of resistor is not very cost-effective, as xvlk pointed out, this project seems ideal for a group buy IF there's enough interest. Unfortunately, I cannot administer this myself, since I'm way too busy with my "day job" and other stuff right now.

Come to think of it, if switches with PCB pins, rather than solder lugs, were used, this circuit would be ideal for a PCB. The whole thing could mount to the chassis of the builder's choice with only two holes for the switches. Again, there's just not enough hours in the day...

[quote author="SteveG"]NY Dave, is that an Alpha switch? I've been trying to get hold of those in the UK with no luck.

The UK distributors for Alpha (TM Electronics) initially bombarded me with e-mails, phone calls, catalogues etc then lost interest when I told them I couldn't meet their minimum order requirement of 1000 pieces/type!

Anyone know a UK supplier?

Steve[/quote]
No, but there is an alternative way to construct the switch with parts that are available from RS. They have a range called the 'Rotary Switch Kit', and if you look at the wafers available for it, you'll find mbbs available, and also on the standard size switch kits at least, there are dummy wafers available to mount the resistors to. I'm not aware of any other mbb multipole switches that are easily available in the UK at all. But I would have thought that you could fabricate quite a neat version of NY Dave's attenuator using these ones.

I found RS's page regarding the rotary switch kit. What a cool product! I wish that were available here in the States.

Cheers for the replies.

I'm familiar with the switch in the RS catalogue, but the Alpha switches are so nice. Great 'feel', and vintage look. Miles better than the 'Lorlin' rotary.

A company called CPC used to have some Alphas, but they discontinued them. They were only 60-odd pence each! I wish I'd stocked up now, while I had the chance. (That always seems to happen, BTW.)

It may be worth dealing with a US company if there's no other way.

Steve

[quote author="SteveG"]
I'm familiar with the switch in the RS catalogue, but the Alpha switches are so nice. Great 'feel', and vintage look. Miles better than the 'Lorlin' rotary.
[/quote]
Yes, Lorlin switches aren't exactly wonderful - but the RS Switch kits are actually made by NSF Controls, and are rather better than Lorlins - I have a few, and even the miniature ones have a good 'feel' to them - it has a lot to do with those two springs you can see in the pictures, and a couple of good-sized balls...

Dave, I was thinking of putting the rotary inside my 351.

Click to expand...

I'm sure you've probably seen this. But this may help.

http://www.electrical.com/phpBB2/viewtopic.php?t=498

Nice pdf in there to download and the thread itself has some tips. I'm slooooowly working on a pair myself. Unfortunately one has the old phenolic boards. I need input trafos too. Anywho...

Miko,

I am aware of Greg's mod. In fact, I referenced that link in another thread. :grin:

I had Brian at Sowter blueprint an original 351 input tranny I borrowed from Dave Dintenfass. Pricy, yes, but original 351 input trannies aren't exactly falling off trees...

Sowter 9735