Microphone Impedance Matching

As far as I had understood in the past, a microphone will give it's best performance when matched with a preamp of the same impedance.

I was reading about putting a low impedance mic into a high impedance input and the benefits of getting a maximum voltage from the mic.

Now what I am wondering is how would EXACTLY matching impedances affect the sound.

I've heard that the Groove Tubes Vipre has an impedance matching circuit and that it can improve the sound when properly used.

I'm new at this but I'm planning on designing an impedance matching circuit for a final project in my college program.

[quote author="christopherleary"]As far as I had understood in the past, a microphone will give it's best performance when matched with a preamp of the same impedance.[/quote]

I thnk you may be thinking of matching a speaker to an output for maximum power transfer.

I was reading about putting a low impedance mic into a high impedance input and the benefits of getting a maximum voltage from the mic.

Click to expand...

This is fairly standard. You tend to aim for a load of say, roughly 10X the source impedance.

Now what I am wondering is how would EXACTLY matching impedances affect the sound.

Click to expand...

It will affect voltage transfer and therefore volume and similarly current transfer. You aim for a compromise. Tone can be affected too - bass loss can result if the mic is not loaded properly for instance.

It can be looked at as simply as Ohm's law. Draw three simple circuits - each has a signal generator with internal impedance of 200 Ohms. Then draw a load resistor in series. Do one with a 50 Ohm load, one with a 200 ohm load and one with a 2K Ohms load. Now calculate the voltages dropped on the load, the current flowing into the load and the power transferred to the load in each instance.

I've heard that the Groove Tubes Vipre has an impedance matching circuit and that it can improve the sound when properly used.

Click to expand...

I've not used one, but it may partly be hype. Condenser mics are less fussy, by the way.

I'm new at this but I'm planning on designing an impedance matching circuit for a final project in my college program.

Click to expand...

Cool. There is tonnes of info here. There was also a similar question to this earlier on - have a look.

As the preamp load that the mic "sees" drops, it increases damping on the diaphragm of dynamic mics. This tends to dull the mic because it isn't as sensistive to transients. As mentioned above, 10X or more is accepted practice for mic loading, I believe 50X or 100X can be better in some cases. Of course, there are those that will experiment with loading to affect the tone, but I believe it's better to get everything the mic can provide onto tape (or recording medium of choice) and then boost, cut, chop etc. later.

One example..

I have seen more than one person suggest that 57's prefer a slightly lower input Z than is normal these days , to dampen ringing and other nasty stuff in the top end.

[
This is fairly standard. You tend to aim for a load of say, roughly 10X the source impedance.

Now what I am wondering is how would EXACTLY matching impedances affect the sound.

Click to expand...

It will affect voltage transfer and therefore volume and similarly current transfer. You aim for a compromise. Tone can be affected too - bass loss can result if the mic is not loaded properly for instance.

It can be looked at as simply as Ohm's law. Draw three simple circuits - each has a signal generator with internal impedance of 200 Ohms. Then draw a load resistor in series. Do one with a 50 Ohm load, one with a 200 ohm load and one with a 2K Ohms load. Now calculate the voltages dropped on the load, the current flowing into the load and the power transferred to the load in each instance.

Nice example.

It would seem, that the greater the resistive load (i.e impedance as reflected from the secondary), gives us the biggest voltage drop (E = IXR). Equals more voltage drop (bigger signal) to be amplified.

Click to expand...

[quote author="caps"]One example..

I have seen more than one person suggest that 57's prefer a slightly lower input Z than is normal these days , to dampen ringing and other nasty stuff in the top end.[/quote]


As a side note to the 57 thing mentioned above. I had a tube pre that had nom. input Z of 600 Ohms. 57's sounded particularly good with this unit. Maybe this was why.

Shure's own mic mixers--dating from around the time the 57 was introduced--have a low-ish input impedance (around 800 Ohms, IIRC). It could be coincidence, or it could be that this value was chosen to sound good with their mics. I can't say for sure. About 20 years ago, Shure even made a very basic little mixer (the M68FCA) with a 300-Ohm(!) input impedance.

But that's looking at input impedance in simple resistive terms, possibly oversimplifying the matter. Since presumably your tube pre (like the Shure mixers) uses an input transformer, it could be that the reactances of the transformer happen to complement those of the 57 in such a way as to smooth-out or otherwise subjectively improve the response.

So tell me...

Is there any point to matching impedances between a microphone and a preamp or the higher the input impedance the preamp has, the better?

[quote author="christopherleary"]So tell me...

Is there any point to matching impedances between a microphone and a preamp...

Click to expand...

Yes, of course! You must have a load within a suitable range in order to work / sound at its best.

...or the higher the input impedance the preamp has, the better?..

Click to expand...

No. You have to reach a compromise. There are different ways of looking at it. You do want a decent amount of voltage to fall onto the load (ie. the majority of the source voltage). But, If you make the load impedance too high, you will strangle the mic and it will have trouble pushing any current into the load. I remember reading about this a while ago. Basically, it's a compromise of getting a good voltage transfer whilst also getting a reasonable power transfer too.

As I said earlier, a load of 10X the source impedance is a bit of a thumb-rule, eg. 2K Ohms.

I don't know what you've been reading, but power transfer from microphones has been a non-requirement for about a hundred years (since the invention of the triode tube). We want maximum output voltage from the microphone, we don't care about power. You want the load impedance to be as high as possible, except in cases where the mic needs to work into a lower impedance in order to damp out peaks in its response. So, it's true that sometimes the load impedance needs to be somewhere between open-circuit and "power matched", but not for the reason you have in mind.

[quote author="NewYorkDave"]I don't know what you've been reading, but power transfer from microphones has been a non-requirement for about a hundred years (since the invention of the triode tube).

So, it's true that sometimes the load impedance needs to be somewhere between open-circuit and "power matched", but not for the reason you have in mind.[/quote]

I stand corrected. Sorry.

I can't remember where I read that, but it was not suggesting that power transfer was a necessity - it suggested that the best compromise was between max. voltage transfer and max. power transfer which I suppose is not far from the truth.

I didn't know about some mics utilising a lower impedance load to damp resonances - I've never thought about that.

So...

How does one actually calculate the load impedance of say, a tube stage, to be connected to the secondary? Cant find any hard and fast info on this despite alot of searching and reading...

a tube grid has a very high impedance, many megohms. If you use grid leak bias, the impedance will be determined by the resistor from the grid to ground. BUT, the transformer is a compromise - Obviously it isn't a practical task to wind a transformer with a 150 ohm primary with a secondary of X megohms, the losses created within the transformer would degrade performance. Instead we settle for 20,000 to 60,000 ohm secondary, giving us a reasonable high impedance secondary, but still within the realm of a workable transformer.

This is a more complex subject than meets the eye (or ear).
In the early days of sound recording it seemed like a sensible idea to achieve maximum power from the microphone so that the noise would be minimised. In the 1950s amplifiers had improved to the extent that high impedance inputs were achieving noise levels that were comparable to the thermal noise in a resistor. We all did a shift to 'voltage matching' where the idea is to load the mic with a high impedance; its own low impedance source will then shunt the noise at the input of the amp.... and this works fine.
So for conventional microphone amplifiers, the approved convention is to have an input impedance of 10 times the mic source impedance or more; and if it's 100 times more it doesn't matter one jot (until you unplug the mic!) :razz:
But mics do sound different when loaded with different impedances, especially dynamics and ribbons; I'm not at all happy about comments like 'low impedance damps the highs'; it's not necessarily true, A dynamic mic produces an output CURRENT that is proportional to the deflection of the diaphragm... NOT voltage. And the same goes for a ribbon, so I could crow about how perfect my own 'current mode' mic amps are... but that again would be incorrect because capacitor and other active mics would not be at all happy driving into a short circuit, so I have to introduce an artificial 'impedance' to strike a compromise.
The bottom line is that the sound of most microphones IS affected by the impedance to which they are connected; Capacitors and some dynamics (say AKG D202 etc) sound best into higher impedances, many dynamics and ribbons don't mind driving into low loads, and sound lovely doing it.
As with so many things in the (professional) music business, try it... if it sounds right, then do it. :grin:

Ted, there's no disputing that a dynamic mic is essentially a "motor" that converts a mechanical displacement into an electrical current. But as you know, a conventional voltage amplifier can only act on a voltage difference between its input terminals, so an impedance needs to be provided between those terminals to provide the voltage drop. And the higher the impedance, the greater the voltage drop, and the higher the output signal from the amplifier.

As for the load impedance affecting the flatness of response, isn't it a case like any other transformer-coupled generator, with all the "vile resonances" involved? Most of us have encountered this when looking at the peaky response of a transformer-coupled output run into a load impedance much higher than nominal.

Current-mode input does seem a clever way to avoid these problems, at least with dynamics and ribbons.

Yes Dave no dispute at all. If it's a 'voltage' amp that you have, then it's sensible to have as high an impedance as possible at its input; but is that really the case?
You could consider all input amplifiers as 'power' (voltage X current) amplifiers; they just vary according to the leaning towards either voltage or current :roll:
An old U47 still sounds good into an old matching tube amplifier. :wink:

Here is what Sound On Sound magazine say:

[quote author="Hugh Robjohns"]In the early days of microphone development, with ribbon and moving-coil designs being the only high-quality devices available, most microphone and preamplifier systems were designed with impedance-matched interfaces ? typically operating at 300(omega), although other standards did exist. Later on, with the introduction of capacitor microphones and their internal impedance converting head amplifiers, the idea of voltage matching was adopted and is retained to this day for all microphone types. There are a few microphone preamplifiers available which are designed specifically for use with vintage ribbon microphones and still include impedance-matched interfaces. However, these are rather specialised devices and are of little practical concern to most of us.[/quote]

[quote author="Hugh Robjohns"]Reducing the input impedance places greater demands on the microphone to supply current, and this can cause all sorts of odd effects with some mics. In general, dynamic mics will respond to a lower input impedance by producing a more uneven frequency response, as resonances in the electro-mechanical system become more emphasised. This can be thought of as a kind of 'free EQ', though its effects are rather unpredictable and not always useful![/quote]

The first bit's fine... the second is pure conjecture..... and mostly wrong. :?
I think Hugh would admit that it was speculation and adequate for the time; but there are a significant number of mics that respond well to lower loads.... I don't have many ribbons, but they are all quite happy and sound nice driving into a near matching load, and the old faithful SM57/58 family sound happiest into 600 to 800 ohms.

As many probably know, I use transformerless Valley People design preamps,; these have an input impedance of 100K. Dynamic mics sound great through these preamps, as well as offering lots of very low noise gain. The 100K may be a little on the "overkill" side of things, but still an admirable preamp. These things are basically flat from DC to daylight, and I find that many "average" dynamics sound very respectable. They take on some of the characteristics of a good condenser mic as far as transient response that a dynamic usually doesn't have a lot of... . Discuss it all you want, but give it a try and make your own observations.

I take it thats 100K as seen from the secondary bill ?