stepped t attenuator...again.

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bluebird

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Jun 11, 2004
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I know this has been discussed but I cannot find what I'm looking for.

I'm trying to make a 600 ohm t attenuator with an dual 11 pos. rotary switich.

does anyone have any charts? I think NYDave had a link a while ago...

thanks
 
That calculator gives you values for a T-pad, which you can't implement with a two-pole switch. You'll need to make a bridged-T. Here's an easy-to-use design chart:
58kB GIF

In switchable attenuators, the bridged-T is almost always preferred over the 'regular' T since it requires fewer switch contacts.
 
thanks guys,

dave thats what I was talking about.

as far as the db scale... For an 1176 with a 600:600 input transformer how far should I go? 30 db? 40 bd?

I mean:


I assume I position one end of the ruler at 600 ohms Z, starting with the other end on 1000 for R3. then go up to ?
 
Hi,

I have a question regarding attenuators for a particular application that I could use some help with, being that I am pretty much a newbie for all intents and purposes. :oops:

I have an Ampex 351 mic pre I was hoping to mod. There's a document from Greg Norman (Albini's tech) that gives some details as to what he did to modify Electrical Audio's 351s.

Modification document (PDF)
Electrical Audio web page describing the mod

Basically, his document says that he used a Daven attenuator on the output after the o/p. However, the only specifics he gives regards the number of sections (three) and that "the Daven o/p attenuator is not an off-the-shelf item anymore. What you need to get is a o/p attenuator that will have an input and output impedance of 600 Ohms through its rotation. You might be able to find this at a hi fi audio builder site." The part number he used for his mod would have been nice to have... :sad:

I emailed him about it but he has not replied. I know he's real busy.

So basically, I have no idea if the attenuator is supposed to be bridged, ladder, straight, "L", "T", or "H", etc. I also don't know how much attenuation should be provided or how many steps would be practical. I am not so sure his schematic gives me any clue to this, but I am a newbie, so therein could lie my problem. :shock:

Could anyone help me learn how to determine/understand these factors?

I guess I could always build an attenuator using a rotary switch and some resistors using Dave's monograph, right?

Any help/insight is greatly appreciated.

Thanks,
Chris
 
How much attenuation is necessary? If you can answer the following questions, then the answer to the first question becomes easy.

For input attenuators: what's the maximum input the device can handle, and what's the maximum signal that's ever likely to be thrown at it in normal use?

For output attenuators: what's the maximum unclipped signal the device can produce, and what's the overload point of the devices you're likely to connect to it?

Balanced inputs and outputs should really use balanced attenuators, which unfortunately require at least three sets of switch contacts to implement. You can use an unbalanced attenuator at an input, if it goes between the input transformer and the first amplifier stage, but this will do nothing to protect the input transformer from overload.
 
> What you need to get is a o/p attenuator that will have an input and output impedance of 600 Ohms through its rotation.

Oh, bosh. There is no need (any more) for a constant 600 output impedance, only a "low" impedance.

Put a 1K linear pot on the output. That is close enough to 600 to keep the output transformer resonance loaded. The impedance seen by the next box in the chain will vary from 0 to 300 to 60 ohms, which is plenty "low".

> as far as the db scale... For an 1176 with a 600:600 input transformer how far should I go? 30 db? 40 bd?

You haven't even said why you want a pad, if it is input or output, what your studio level is (in theory and in reality),,,,,, how are we supposed to know?

Put a pot in there. Diddle it until you are happy. Remeber the knob settings. Take it out and measure the attenuations you like, and then design your switched attenuator.

But do not get hung up on this notion of Constant Impedance. It does NOT "sound better". It just makes system gain structure calculations simpler. Most folks here don't calculate gain structure, and use whatever levels "work".
 
chris-

ragarding the 351- I have been using a 351 since well before it was chic to do so and really the best thing to do is just put a switch on the VU meter so you can shut it off, or just disconnect it completely. You dont need to go through the labor of a variable attenuator on the output unless you REALLY need to drive the thing like mad in which case you can just build a pad approximating your average usage level (like, if you always use it on guitars, it should be hard to figure how much you'd need to pad the output) and just go with that. People do lots of work on these, Ive never felt the stock unit was lacking, the only real thing is disconnecting the meter, but the gain control works very well as is and stock you'll get that LA Woman sound every time. Worked well enough for the doors, right?

dave
 
NY Dave, I’m not so sure I know what the maximum unclipped signal the 351 can produce. I am not sure how I would be able to determine this, could I use an oscilloscope?

Paul, my desire for an output pad was based on Mr. Norman’s suggestion that I could amplify the signal to distortion and then trim the output back to a useful level. I figured it might be good to have since he proclaims “it has been a handy feature.” And you know, I am not sure what my studio level is as I have just started building it. I was hoping there was some default configuration I could use to start with and as my studio builds, I could tweak it to what I want/need. Actually, my plan was to get my 351 pre up and running with a LA-2A and 1176 clone and try to interface these with a PC based recording system, possibly giving tape a shot if the funds and time allow. I am not entirely sure if this is what you were getting at with “what my studio level is.”

Soundguy Dave, the mod includes a meter switch, which I think is good as I hear that some distortion can get introduced with the meter. But I think I may just go with Paul’s advice for now and when I can actually put the pre through its paces I’ll have a better idea of what I want/need.

Thanks for the advice, guys! :green:
 
PRR,Dave,.

I simply want to emulate the T atten. on the original 1176

I have some incorrect value T attens in there right now. they work but I would like them to be correct and stepped.

1176.JPG


So I'm trying to scale R1A and R1B for 11 steps with the chart put up by NYDave. I just want to know how high on the chart I should go?
 
Here's why I think it's better to use a balanced attenuator on balanced inputs and outputs. Consider the simple equivalent circuit of the ideal impedances of a balanced output driving a balanced input:
cmrr1.gif

Rs is the balanced source impedance, Rdiff is the differential input impedance, and Rcom is the common-mode input impedance. The attenuation in both "legs" of the balanced line is equal.

It may have been more accurate, in an absolute sense, to model Rcom as a single resistor to ground off a centertap of Rdiff, but the result is going to be the same for this particular analysis, anyway.

Here's what happens when you introduce an unbalanced attenuator (such as a pot or an unbalanced bridged-T):
cmrr2.gif

The attenuation in the two legs is now unequal and CMRR suffers. If Rcom is very high, then it may not suffer enough to be very noticeable. That's why high common-mode input impedance is important to CMRR, and transformers tend to excel in this regard. But we can't always guarantee that we're always going to hook up devices with a high common-mode input Z and excellent CMRR to the output of our device; we may very well need all the help we can get.

Here's the case with a balanced attenuator, shown here as a simple "u" pad:
cmrr3.gif

Balance and CMRR are preserved.

If you have an output which needs to see something not-too-much-more-or-less-than 600 ohms, and a high-Z input that is not sensitive to variations in source impedance, and if you can tolerate a 3dB minimum loss, then this simple variable "u" attenuator might be the ticket:
volcontrol.gif


I've had an idea for a "universal" 600-ohm bridged-H attenuator, 0 to -50dB in 2dB steps plus "off", that could be build cheaply in a little Bud box. It looks like it's time to flesh out that idea. I'll post a schematic soon, depending on how much goof-off time my day job allows this week ;)
 
chris-

I think the distortion from the meter is stuff to look at on a scope not listen to on a speaker, but that debate aside, the REAL reason IMO to take the meter out of the circuit is not to break it. To get the usable gain out of the pre that you are looking for, you'll be pinning the meter, so its good to have it on a switch so you can flip it out of the circuit so you dont bust the thing...

dave
 
By the way, here's a design chart for a switchable bridged-H attenuator
bridgedh.gif


It's drawn with an individual resistor for each step to simplify calculations, but a more typical implementation would use resistors in a ladder arrangement, which involves a little more simple math since the values add in series.
 
Uggg I don't have a three pole. I see the advantages to symmetry in the H pad though.

I figured out the resistances for the bridged T. it will attenuate to 30 db's I figure thats enough for the input of an 1176.
 

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