Stepped Attenuator for Mic Preamp

[dukeofearl]

Is it possible to use one of these balanced stepped attenuators (or something like it) before and/or after a mic preamp? As a variable pad/trim?

https://www.ebay.com/itm/Dale-23-Step-Attenuator-Stero-audio-amplifier-Volume-potentiometer-10k-50K-100K/122497225669?ssPageName=STRK%3AMEBIDX%3AIT&var=422942992986&_trksid=p2060353.m1438.l2649

or

https://www.ebay.com/itm/DACT-Type-21-Stepped-Attenuator-Potentiometer-20K-D-Shape-Shaft-HiFi-grade/192105859271?hash=item2cba66c8c7:g:ylYAAOSwawpXwVD~

 

[JohnRoberts]

For a pad perhaps but gain is better managed as a variable R between gain nodes. 

Which can be done nicely with a stepped switch.

JR

 

[dukeofearl]

Yeah this would essentially be a pad. 10k?

 

[scott2000]

Yeah this would essentially be a pad. 10k?

I'm not sure of the terminology but there are different versions of "pads" and I'm also not sure of the interactions between the different types.  There's always the impedance thing you have to think about . Like what's feeding the switch and what's the switch feeding..or however that is supposed to be said....... But if you are good there, it should work really well.....

I know there are some threads here somewhere that get into this a bit but, it always seemed foggy to me even after.....



I'm interested in this as well and hope to get more info about it. Using gain properly is in my opinion one of the  most useful things available.

For a pad perhaps but gain is better managed as a variable R between gain nodes. 

Which can be done nicely with a stepped switch.

JR

Like doing gain inside the circuit as opposed to using  gain or/// attenuation// externally between devices???? 

I know for selecting -10 or +4 levels, it's just a simple resistor in some circuits I have so, a switch/pot would give more options in the same circuit....???

 

[ruffrecords]

Headroom, noise and gain are inextricably linked. In the very old days when everything was made with tubes, mixers consisted of fixed gain amps with attenuators before and in between them to set the overall gain (think V72 and EMI REDD consoles). A couple a V72 amps cascaded will give you 68dB of gain - good enough for a speaking voice from a ribbon. But, the noise at the output will be around -50dBu. If you have a 30dB louder source, say a condenser mic and a singer, the input signal will be a lot higher so the output will be a lot higher - the second amp might even run out of headroom and clip. To bring the output down to normal line level you have a few alternatives. You can add a pad to the input. This will drop the signal level and avoid clipping but the noise at the output still remains at -50dBu. Or you can put a pad between the two amps. This will reduce the gain by the same amount but also reduce the noise from the first amp that is being amplified by the second so the noise at the output could drop by as much as 30dB as well. If you have an even louder source, say a condenser mic in front of a kick drum, there is a danger you may even overload the first amplifier. In this case you would need to add a second pad at the first input. The gain is now reduced by the value of the two pads so the headroom is improved but the noise at the output is unchanged.

In modern mic pres, the gain of the pre is varied instead of using a pad between stages. This means both the noise and the headroom are always the best they can be. For really loud inputs, a pad at the input is still used.

Cheers

Ian

 

[dukeofearl]

thanks for the information. impedance is something i've read a bunch about and am still doing my best to wrap my head around it.

the application i mainly intend to use it for is a 1960s wsw mic pre that has already been racked but has no input pad or output fader. it has very high gain and is difficult to use on anything remotely loud. i have been using an inline pad but would like to control the amount of attenuation. also, the pre sounds great when pushed, so would also like a variable output. the pre has a switch for three different input impedances. the output is 600 ohms.

i was thinking i could rack a few of these attenuators and connect it to my patchbay, so i could use it on the wsw and any other application where attenuation is needed.

any advice on which of the above options would be best would be appreciated!

 

[ruffrecords]

As it is 600 out the simplest thing would be to make some 600 ohm balanced switched pads. You could then use them at the input and the output.

Cheers

Ian

 

[dukeofearl]

yes that would probably be best. i just get confused with resistor values.

 

[ruffrecords]

Here are a couple of simple balanced attenuators you can make with regular pots. To make them stepped just replace the pot(s) with a stepped pot of the same value.

Cheers

Ian

 

[abbey road d enfer]

the application i mainly intend to use it for is a 1960s wsw mic pre that has already been racked but has no input pad or output fader.

More info is needed; there are many types of WSW preamps. Some seem to have a quite low input impedance.
Input pads are quite difficult. Most microphones want to see a rather high impedance, typically 1-2k. For a 200 ohm rated input, the result is that  pads between 0 and 15dB are impossible to make.

 

[dukeofearl]

thanks, it's a wsw 811403. it has a switchable input impedance for 180, 600 and 1900. i see now the manual says output is <30 ohms.

 

[abbey road d enfer]

thanks, it's a wsw 811403. it has a switchable input impedance for 180, 600 and 1900. i see now the manual says output is <30 ohms.

OK, so 1900 is what will get you near modern bridging mode; it may not matter as much with transformerless condenser and active ribbon mics, but for dynamics and passive ribbon (except the very low impedance types such as Coles), the other positions may leave to be desired. I would recommend you make U-pads of no less than 15 dB, which would put you where you need for most types of proximity recordings.
These may also be used for the output, although the resulting attenuation would be significantly different. Finally, the schematic I have indicates the output transformer is under global NFB, which should make it pretty insensitive to the load impedance; that may not be the case with other preamps, that may need an additional termination resistor.
And lastly, note that many web calculators are for constant-impedance types, typically 600ohms in to 600ohms out. In your case, you want 1.5-2k in to 200 out for the input pad. I would recommend you use a simulation program, such as LTspice (free) to optimize your calculations and assess the behaviour when used as an output attenuator.

 

[ruffrecords]

If you want a simple continuously variable pad all you need is a dual 1K log pot and a 68 ohm resistor (see attached diagram).

Cheers

Ian

 

[abbey road d enfer]

thanks, it's a wsw 811403. it has a switchable input impedance for 180, 600 and 1900. i see now the manual says output is <30 ohms.

After re-reading the gain chart I wonder why you'd need an attenuator, since the 30dB gain position should be capable of handling most of the needs in proximity recording.