Can a transformer be added to a transformerless mic?

[LDT2]

Hi all,

I'm pretty much covered with tube mics at this point with a V77, 960 and modded 460 and 1050. I have some transformerless FET mics, but don't have a FET with a tranny in it, except for a C3000B which doesn't sound good at all next to these other mics.

I'm thinking of getting an Apex 415 and adding an output tranny.

Is this possible?

I'm going after a U87ish mod with maybe a Peluso 67 capsule and some electronics updates and such. The 415 looks the most like an 87 to me and has 3 polar patterns, too.

What do you think?

Next can of worms opened.

Lucio

 

[PhilGood]

My opinion is that there are drawbacks to doing it either way. A good FET impedance converter will show off the capsule with no coloration as long as the circuit is well designed. That can be very useful in some situations, but may not offer that thick, lush sound we like.

Transformers can add a pleasing coloration and some warmth, depending on the core material and how well they are built. Not all transformers are equal, nor do all transformers do well depending on capsule selection, output caps, etc. Nor do they necessarily out perform an FET stage in regards to noise, SPL etc.

You may be opening a can you're not inclined to eat from.

 

[mshilarious]

It's not really a question FET output vs. transformer, since most transformerless FET mics have BJT outputs. I say most, because I'm not a huge fan of BJTs. Instead, I've done a few mics with FET outputs, but I don't think most manufacturers do that. Anyway, that is typically not the same FET as the impedance converter (although it can be) as the FETs that are ideal for that job are still somewhat high impedance to be really good as a line driver.

My preference for FET output has to do with the distortion signature of FETs vs. BJTs. The BJT circuits I've tried have higher third harmonic distortion. The FETs tend to run a higher distortion figure overall, but mostly second harmonic. I also find it easier to deal with biasing the FETs in low voltage conditions, and it takes fewer parts. Stated another way, that can probably be overcome with much more complicated designs than you'd see in a typical mic (at least the Chinese mics. Shure and AT condensers seem to have fairly involved circuits, but I lack the patience to trace SMT). So I'm working on an opamp output mic, since in the battle of TI vs. me, TI wins every time . . .

The big advantage of the transformer to me is it doesn't cost you any current from the supply, so you can run a low output impedance mic on less than 1mA. That is useful for electret capsules where you don't need a high polarization voltage, giving you a low-voltage, low-current mic. That's what I've done in my latest design, which in the great scheme of microphone circuits, is still really simple.

 

[LDT2]

Thanks for the replys.

OK. I guess I need to understand some of the terminology here.

Does FET only apply to a transformerless circuit?

I know that FET is a "field effect transistor", but what does BJT and SMT stand for?

Does this mean that you can't use a FET based circuit with a transformer output?

If so then I would have to create a different circuit to accomplish this, correct?

Maybe I should start with a transistor balanced mic from the start, like a cheap MXL or something.

As always I appreciate any guidence that you can offer.

Thanks,

Lucio

 

[Robert D]

Another way to get a good sounding tranny (besides calling a 976 number) into the signal path is to get or build a preamp with a good input transformer. Then you can use it on any of your mics. Just a thought.

 

[craigmorris74]

Another way to get a good sounding tranny (besides calling a 976 number) into the signal path is to get or build a preamp with a good input transformer.

Can he only have one? My signals some times see five (or more!) before getting to my computer! If you didn't have an gear with transformers, I see what you're saying though.
Craig

 

[mshilarious]

Does FET only apply to a transformerless circuit?

No. All condenser mics have either a FET or a tube. Either type may have a transformer (tubes usually do).

I know that FET is a "field effect transistor", but what does BJT and SMT stand for?

Bipolar junction transistor, and surface mount technology.

Does this mean that you can't use a FET based circuit with a transformer output?

Not at all.

If so then I would have to create a different circuit to accomplish this, correct?

Possibly. First you'd need to find a schematic, an existing mod, or trace the circuit.

 

[LDT2]

Robert D,

That's a great option, because unfortunately, I don't have any pre's with transformers, yet.

Which brings up another question about adding external trannys infront of transformerless pre's. I was thinking about using something like the Ebtech "hum eliminators" as a base line unit, because it's TRS in/out and just swap the trannys with Jensons or Cinimags, but I don't know if that would work the same way as internal trannys do.

mshilarious,

As usual, thanks for the explainations. I guess when I get the mic I can make a schematic up and post it, because I have searched for anything on this mic in regards to pics and such, but can't find anything but the odd mention of the mic. Apex no longer has any schematics on their site, so I'm glad I downloaded what I did for the 460.

I was so hoping it would somehow be possible to just wire a tranny inside ahead of the mic output, but I wouldn't know what tranny or ratio to use and that can't be decided until we know what the circuit is, sooo....

again thanks a bunch,

Lucio

 

[mshilarious]

I was so hoping it would somehow be possible to just wire a tranny inline ahead of the mic output, but I wouldn't know what tranny to use and that can't be decided until we know what the circuit is, sooo....

That would block phantom power. Same problem with putting a trafo in front of a mic preamp. Some transformer-output mics are designed to let the phantom flow through a center tap, which is then used to power the rest of the circuit. But a transformerless mic would not be expecting that, so the circuit would be very different. Many of them use PNP outputs, which means that the transistors are intended to direct couple to the mic's output (avoiding the need for capacitors).

That's why I said you'd have to know what the circuit was, and possibly replace most or all of it, to add a transformer.

 

[LDT2]

OK gotcha. I figured that I need an external phantom supply with the pre, but I never thought about it in the mic. Duh.

As soon as I get that info I'll post it, because I'll definitely need some guidance with it.

I found some U87 schematics to study, they're definitely more complicated than the tube mics were. If this experience turns out anything like modding the tube mics did, I'm gonna need to open up the mic before anything will make any sense. For some reason if I can see it in person I can process the info better than trying to visualize the schematic. After I see the circuit, then the schematic makes sense to me.

To be continued......

 

[mshilarious]

I found some U87 schematics to study, they're definitely more complicated than the tube mics were.

Actually the U87 was reasonably simple, but there are some bells and whistles that you might not be concerned about. A lot of the stuff around the capsules is the multipattern circuit, pad and bass rolloff switch. Then there is the internal battery switch, you can leave that out. There is just the single FET feeding the transformer. The power rail is created in front of the transformer with the 2k2 resistors. It is split two directions, for the capsule and a lower, regulated voltage for the FET.

Actually it's a pretty good example of what I was talking about earlier--the schematic (I think I am looking at the original U87) lists consumption as a mere 0.4mA. That's low current; with an electret instead of an externally biased condenser, you wouldn't need the high voltage either.

 

[LDT2]

The schematic I have is for the U87,87p,87iand 87ip which I believe are the original 87's.

Man, I wish I could separate the schematic like that. This is what I'm seeing from the U87/U87p upper section:

I can see where all the switches are to the left of the symbols that kinda look like this: -) only the dash is thicker. Is that what you're talking about as bells and whistles?

The internal battery switch would be to the right where it has EXT. INT. on top of each other, right?

Would the single FET be the S 2436 with the G, D and S connections with D connecting to C7?

I see the 2k2 resistors at R18 and R19. Not sure about where the split is for capsule and FET though.

I see where itm says 48v 0,4mA right between R18 and 19. Is that the consumption you're talking about?

I wish that the text on the document wasn't in German.

Man I sure do appreciate this.

 

[mshilarious]

I can see where all the switches are to the left of the symbols that kinda look like this: -) only the dash is thicker. Is that what you're talking about as bells and whistles?

Some of it. I believe those are pin connections inside the mic, not sure as I've never seen one! But the three switches are to the left of that (pattern, pad, and rolloff)

The internal battery switch would be to the right where it has EXT. INT. on top of each other, right?

Yes.

Would the single FET be the S 2436 with the G, D and S connections with D connecting to C7?

Yes. Gate, Drain, and Source; those are the parts to a FET. On many FETs, drain and source are interchangeable, but the pin where the positive voltage is applied will be called drain.

I see the 2k2 resistors at R18 and R19. Not sure about where the split is for capsule and FET though.

From the R18 R19 junction, the power rail goes down to the battery switch. From there, it goes up to R16 (the capsule supply) and over to R22. Gr1 is a 24V zener diode that regulates the voltage to the FET. That is primarily for protection against overvoltage from the phantom supply, as the 56K R17, with 0.4mA across it, will drop 22V. That is to prevent the FET from being damaged. The capsule can be left unregulated, since small variations in capsule voltage aren't too critical.

I see where itm says 48v 0,4mA right between R18 and 19. Is that the consumption you're talking about?

Yes.

 

[LDT2]

Man, this is helping alot!!! Thanks. So far I'm with you.

I just noticed that there is english under the German text. I wish there was a way to blow these up without losing the detail.

On the tube schematics it usually said what the tranny ratio was, but I don't see any reference here. Is there a way to determine what it should be from the schematic?

I can't wait to dig into this project. Just waiting for the tax return to come so I can get the mic.

Thanks again,

Lucio

 

[LDT2]

OK. There are a couple things I can't figure out with this schematic.

R10, R20, R21 and R22 have an E after the numbers. What would the E stand for?

R11 has an *) which means SELECT. But there's no value given so what do you put in there?

Thanks.