Preamp input impedance question

[stormymondays]

I have three preamps that offer variable input impedance: Tree Audio The Branch II, Warm Audio Tone Beast and GAP Pre 73 MkIII. I am still a bit confused about their impedance switches despite all the reading I've done on the subject.

A little bit about the units:

The Tree Audio has a variable switch that "Controls input impedance of input transformer, selectable between 37.5 Ohms, 150 Ohms or 600 Ohms, nominal input for line input is 150 Ohms".

The Warm Audio has a Tone switch that drops input impedance from 600 Ohm to 150 Ohm, and strangely enough the manual recommends the 150 Ohm setting for ribbon mics. There's a long explanation about what it does in the manual.

The Golden Age has a Low-z switch that drops from 1200 to 300 Ohm.

My questions:

-Aren't most of these input impedances a bit on the low side, especially for ribbons?
-What does "nominal impedance" mean for the Tree Audio? 150 is mentioned as "nominal" in the manual for all uses: line, mic, DI.
-When/why would you use the 37.5 and 600 settings on the Tree Audio then?
-Why dropping the impedance increases gain? I know it has to do with transformer windings or ratio, but I don't really understand it technically.
-And lastly, to add to the confusion, I did ask Warm Audio about these a while back, and their reply was "The true impedances of the mic pre are 10 times the transformer impedance, so the TB12 truly is 1500 and 6000 ohms, not 150 and 600." So, does that reply apply to the other pres as well?

Mostly everything sounds good on any setting, I just want to understand the science/electronics behind it!

I hope some of this can be material for a future SOS article/question...

[Hugh Robjohns]

Aren't most of these input impedances a bit on the low side, especially for ribbons?

Highly debatable! The tonality of active ribbons won't change (significantly) with different preamp impedances, of course, but the loading will change the tonality of passives to some degree, as well as their sensitivity. Maximum sensitivity and HF is obtained with a high impedance impedance, and many ribbon-specific preamps offer input impedances anywhere from 10k to 30k Ohms, for example.

However, vintage ribbons were often designed to work in a matched-impedance environment and so 'expect' to see a nominal 50 Ohms or 300 Ohms or whatever was popular at the time... which is why some preamp manufacturers still like to offer these input impedances.

You can't damage the ribbon (or any other mic) by changing the input impedance -- it just affects the tonality and sensitivity a bit... so think of it as a kind of pre-tone control! Adjust to taste -- there is no right or wrong, just as there is no right or wrong mic choice or placement (within reason!). If you like the way it sounds, that's all that matters.

What does "nominal impedance" mean for the Tree Audio?

Input impedance is rarely constant, especially when a transformer is involved, and will change to some extent with input frequency. So they pick the figure at 1kHz, or something like that.

When/why would you use the 37.5 and 600 settings on the Tree Audio then?

To get a different sound from your passive mic!

Why dropping the impedance increases gain? I know it has to do with transformer windings or ratio, but I don't really understand it technically.

It depends how they have configured the transformer, but typically you would have several separate input (primary) windings and wire them in different series/parallel combinations to achieve different input impedances. But in doing so you will also be changing the number of turns int he complete primary side, and that obviously changes the ratio between the primary and secondary (output) windings, and thus the voltage gain across the transformer.

And lastly, to add to the confusion, I did ask Warm Audio about these a while back, and their reply was "The true impedances of the mic pre are 10 times the transformer impedance, so the TB12 truly is 1500 and 6000 ohms, not 150 and 600." So, does that reply apply to the other pres as well?

The input impedance of the active electronics in the preamp is 'seen' by the mic through the transformer, but it is multiplied by the square of the turns ratio of that transformer. Hence the 'real' input impedance being considerably higher than the impedance value of the transformer's (unloaded) primary winding on its own.

H

[stormymondays]

Wow, this forum never fails to impress! I'm proud to be a long-time SOS subscriber. Thanks Hugh!

Of course if it sounds right it is right, and that's how I've been using these units but since switching impedance causes a gain boost, it makes it harder to judge the differences in tonality, it's never a straight A/B.

The impedance switch in the Tree Audio also affects gain for the line input. I wonder why they chose the 150 Ohm as the default. I will shoot them a question.

I'm still confused about the last paragraph. So, the "real" input impedance is then higher than the transformer impedance then? This would be good news for modern passive ribbon mics that like to see a very high input impedance, for use in a bridging environment. However, that would make it very hard to use mics that work in a matched impedance environment.

So which one is it?

[Hugh Robjohns]

... but since switching impedance causes a gain boost, it makes it harder to judge the differences in tonality, it's never a straight A/B.

True enough, the sensitivity will vary slightly as you adjust the impedance up and down -- getting louder with higher impedance settings, and vice versa. But also other aspects change, including HF response, transient response, perceived room tone and so on... so just adjust until you like the way it sounds and go with that.

The impedance switch in the Tree Audio also affects gain for the line input. I wonder why they chose the 150 Ohm as the default. I will shoot them a question.

Probably just the nominal impedance of the chosen transformer. From what you say, it would seem that they pad the line signal down and route it through the mic transformer too.

This would be good news for modern passive ribbon mics that like to see a very high input impedance, for use in a bridging environment. However, that would make it very hard to use mics that work in a matched impedance environment.

Not really. The physics and electronics haven't changed. Vintage mics never really worked in a matched impedance environment either...

H

[stormymondays]

True enough, the sensitivity will vary slightly as you adjust the impedance up and down -- getting louder with higher impedance settings, and vice versa.

Actually in the case of the Warm Audio the lower impedance increases gain by 6dB due to the change in transformer turns. I'm not at the studio right now so I don't remember what happens with the other two units, but I believe they also increase gain when dropping the impedance.

But also other aspects change, including HF response, transient response, perceived room tone and so on... so just adjust until you like the way it sounds and go with that.

Yes, I think Warm Audio did the right thing here by labelling the switch "Tone"

I've been when using the high impedance as the default, and the lower impedance for its effect on tone, following the lead of the Warm Audio manual, or if I absolutely need the extra gain.

How about line level sources? I suppose they will be hardly affected by the impedance change?

Of course I need to test all that with my ears, so if you can suggest stuff to try, I'll do it! I'm thinking that I need to test at least one dynamic, a few ribbons, one condenser, and a line-level source, and check for the differences in response by ear. Still, not sure how to go about it...

[Hugh Robjohns]

Actually in the case of the Warm Audio the lower impedance increases gain by 6dB due to the change in transformer turns. I'm not at the studio right now so I don't remember what happens with the other two units, but I believe they also increase gain when dropping the impedance.

Yes, it depends how its done, and you have the added complication of two changes simultaneously -- the actual impedance and the transformer turns ratio.

So as far as the mic is concerned, a higher impedance means less load and so a higher output from the mic itself, with better and more accurate transients and greater HF extension.

However, if that higher impedance is achieved by re-connecting the transformer primaries in series, the preamp itself will have reduced gain. So the mic tonality will improve, but the gain will appear to go down.

Going the other way, wiring the primaries in parallel will reduce the input impedance, so degrade transients, reduce HF and load the mic more heavily so that's its actual sensitivity decreases, but the preamp gain will increase because of the altered turns ratio.

How about line level sources? I suppose they will be hardly affected by the impedance change?

Yes, the active output electronics of a line-level device should be able to cope with more or less any impedance, although devices with transformer outputs may well react with slightly altered level and HF extension. And devices with feeble active outputs might suffer some transient distortion at low impedance settings.

Of course I need to test all that with my ears, so if you can suggest stuff to try, I'll do it! I'm thinking that I need to test at least one dynamic, a few ribbons, one condenser, and a line-level source, and check for the differences in response by ear. Still, not sure how to go about it...

Listen for transients and HF extension -- so recording hand-percussion should reveal differences clearly, but also listen for the room tone. Passive ribbons and moving-coil dynamics will tend to show changes more easily. Capacitor mics with straight active outputs (most modern capacitor mics) usually aren't affected at all, but those with transformer-coupled outputs might be.

H

[stormymondays]

Ok, it's tambourine time then!!!

I will test with an SM57, Royer 101 and a transformerless condenser mic to begin with.

I just got me a pair of KM84, so it's worth testing acoustic guitar with them and see if their response changes, being transformer-coupled.

And then, I'll test the rest of my ribbons: Stager SR-2N, Xaudia Beeb, Avantone CR-14. Fun times!

I will make sure to report back with my findings.

[Hugh Robjohns]

[ManFromGlass]

Following this with interest even if the tech is a bit beyond me.
But what I'm getting so far is that - if it sounds good it is good so don't be afraid to experiment.
Not having an electronics background my concern has always been about blowing something up. Teenage memories about connecting guitar heads to speakers that went poof surfacing now!
But it sounds like with mics and pres that, except in rare instances of some older mics not being able to handle phantom power, experimentation is part of the fun.

[Hugh Robjohns]

Yep.

[Guest]

Isn't impedance for matching, you must be thinking of voltage.

[TheMiller]

... and there was me thinking that in order to improve the tone of a tambourine player, you just removed the tambourine

[Martin Walker]

... and there was me thinking that in order to improve the tone of a tambourine player, you just removed the tambourine

Welcome to the SOS Forums TheMiller!

And with in-depth technical comments like that, you'll soon make lots of friends here

Martin