Pad before microphone input transformer

[iampoor1]

Hello

I have some Gates broadcast mic preamp cards with a fixed 45db of gain. I think they sound great, but I want to add some sort of gain control, without modifying the feedback loop, or simply putting an attenuator on the output. They have very low headroom.

The mic preamp input transformer is designed to see a 150 ohm source impedance. I have added phantom power, and am assuming the easiest way to adjust the gain would be to add a fully switchable H-pad at the front of the preamp, using a 4 pole rotary switch to change the values, hence the proper impedance relationship and balancing can be kept. AM I missing any potential pitfalls with this method, or does anyone have a suggestion for an easier method? Lots of offboard wiring with a 4 pole rotary switch on this one...
Thanks!

 

[ruffrecords]

You can do a simple 20dB switched pad with a DPDT switch. Attached pic shows my 4 toggles board which includes phantom, mic/line and phase switching as well. All you need is a couple of 680 ohm resistors, a 150 ohm resistor and a DPDT toggle switch.

Cheers

Ian

 

[emrr]

Ian's drawing is essentially the same as the Jensen input circuitry drawing.  As you said, low headroom, so you will almost always have the input pad on for anything loud, or with high output mics.  There's no value in post-attenuation. 

 

[My3gger]

You could start with 20dB pad as Ian recommended and try 2 positions with 33dB and 40dB pad, or any other combination with 4 pole switch. Some need to be O pads, Uneeda Audio explains it well. This can retain clean sound with low headroom preamps, or drive them a bit. Which Gates model is it, i'm collecting schematics if you maybe have it. Anode to anode feedback like in SA70 is pretty interesting.

 

[buildafriend]

from the orange 86 docs

 

[buildafriend]

maybe you can convince guavatone to sell you his pad PCB as a standalone purchase

 

[emrr]

Jeezus, it's absolutely not hard to wire up a 4 position rotary pad with some resistors.  You don't need a damn pcb. 

 

[abbey road d enfer]

The mic preamp input transformer is designed to see a 150 ohm source impedance. I have added phantom power, and am assuming the easiest way to adjust the gain would be to add a fully switchable H-pad at the front of the preamp, using a 4 pole rotary switch

Not an H-pad; remember that there are two conditions for the pad's impedance: its output should be as close as possible to 150r, but also its input impedance should be as close as possible to 1.5k, in order to satisfy the mic bridging paradigm.
You will find that it's not possible to design a pad that satisfies these two conditions that has less than 20dB attenuation. In that case, the two right-hand resistors are null, which makes for a U-pad.
You don't need a 4-pole switch, just a 2-pole. As Ian suggests.

 

[mjrippe]

Hi Nathan, not sure if I sent you this doc with the preamps.  Sorry if I forgot to.  Anyhow, the pre has a max input level of -40 and max out of +5.  Designed to go to a 600 ohm Daven attenuator and then to a program amp.  A pad on the input and a good output stage would make it much more useful.

 

[QUEEF BAG]

Jeezus, it's absolutely not hard to wire up a 4 position rotary pad with some resistors.  You don't need a damn pcb.

+1

 

[user 37518]

Not an H-pad; remember that there are two conditions for the pad's impedance: its output should be as close as possible to 150r, but also its input impedance should be as close as possible to 1.5k, in order to satisfy the mic bridging paradigm.
You will find that it's not possible to design a pad that satisfies these two conditions that has less than 20dB attenuation. In that case, the two right-hand resistors are null, which makes for a U-pad.
You don't need a 4-pole switch, just a 2-pole. As Ian suggests.

This is great advice.

 

[iampoor1]

Thanks guys, lots to mull over.

I really want to do a 4/5 position pad on this thing. May just have to bite the bullet and have a higher than normal input impedance, with a variable output impedance. Ultimately, I am not after ruler flat frequency response or anything with this preamp...this is DEFINITLY a color piece.

Hi Nathan, not sure if I sent you this doc with the preamps.  Sorry if I forgot to.  Anyhow, the pre has a max input level of -40 and max out of +5.  Designed to go to a 600 ohm Daven attenuator and then to a program amp.  A pad on the input and a good output stage would make it much more useful.

Thank you, I appreciate it!

 

[abbey road d enfer]

I really want to do a 4/5 position pad on this thing.

Then you may still retain the U-pad, but you'll probably need to make the switch a 3-pole.

May just have to bite the bullet and have a higher than normal input impedance,

  Higher than normal input Z is never a hindrance, in fact most dynamic mics love that.

  with a variable output impedance.

That may be an issue, because the noise performance may degrade significantly. That's why I would add a third pole to the switch.

 

[iampoor1]

Then you may still retain the U-pad, but you'll probably need to make the switch a 3-pole.
  Higher than normal input Z is never a hindrance, in fact most dynamic mics love that.
That may be an issue, because the noise performance may degrade significantly. That's why I would add a third pole to the switch.

Ah! Right because if the output impedance of the Pad is larger than 150 ohms, then the EIN will be degraded, correct?

What about using say, two 1.5k resistors on each leg of the U pad, then shunted by an 150 ohm resistor for a ~40db pad. Will both of the 1.5k ohm resistors degraded the EIN figure?

Okay, it took me a few minutes of head scratching but I am 99% certain I see what you mean about using a 3 pole switch. Going to draw up a schematic later, but now its my bedtime.  ;D

 

[ruffrecords]

What about using say, two 1.5k resistors on each leg of the U pad, then shunted by an 150 ohm resistor for a ~40db pad. Will both of the 1.5k ohm resistors degraded the EIN figure?

That will give you 26dB of attenuation not 40dB. For 40dB you need two 7.5K resistors and a 150 for example.

Cheers

Ian

 

[abbey road d enfer]

Ah! Right because if the output impedance of the Pad is larger than 150 ohms, then the EIN will be degraded, correct?

Yes.

What about using say, two 1.5k resistors on each leg of the U pad, then shunted by an 150 ohm resistor for a ~40db pad.

As Ian already noticed, that would be 26dB, not 40.

Will both of the 1.5k ohm resistors degraded the EIN figure?

A pad always degrade the EIN. But the idea is to increase the EIN only by the same amount as the input level is increased, for maintaining a "constant" S/N ratio. How much the EIN is degraded by presenting too high a source impedance depends on the technology (vacuum, BJT, FET). Variations of +100% are generally acceptable.

Okay, it took me a few minutes of head scratching but I am 99% certain I see what you mean about using a 3 pole switch. Going to draw up a schematic later, but now its my bedtime.  ;D

You need the 3rd pole to disconnect the shunt resistor when you need no attenuation. I must say a 2-pole swith can be used for an L-pad, the risk is some degradation of CMRR; some xfmrs are better than others in that respect.

 

[mjrippe]

Here is a three position pad I built using a 3P4T switch.  Values are approximate, but it worked ok in practice on a SpectraSonics 101 preamp. 

 

[abbey road d enfer]

Here is a three position pad I built using a 3P4T switch.  Values are approximate, but it worked ok in practice on a SpectraSonics 601 preamp.

That's what I meant, except the 10dB position loads the mic with ca. 450 ohms, which most dynamic mics would not find adequate.

 

[mjrippe]

That's what I meant, except the 10dB position loads the mic with ca. 450 ohms, which most dynamic mics would not find adequate.

True.  As has been mentioned, anything under 20dB attenuation is a compromise.

 

[emrr]

I used these on a lot of rack-ups.  Standard Lorlin rotary .