Double Bass XLR to 1/4" - Adapter Versus Line Transformer

[Steve Freides]

I need guidance.

I have been using something here (description below) to plug a Shure SM-57 into various heads and amps that I usually play electric bass through. The amps/heads have 1/4" jacks, and I have microphone cords with XLR on each end because I also use an Acoustic Imagine Ten2, which accommodates an XLR input very nicely.

The "something" I've been using doesn't have a brand or model number (only says "China") but it does say, "500 Ohm to 50 KOhm Line Transformer" on it. I'm without a clue as to where or when I bought it. I believe it's passive - have never put a battery in it.

Sooooooooo ....

If I purchase an "adapter" like

Amazon.com: LIQUN XLR 3-pin Female To 1/4" 6.35mm Mono Male Plug Audio Cable Mic Adapter: Home Audio & Theater

am I worsening any aspect of what I'm doing, e.g., am I creating more opportunities for interference, altering frequency response, or anything like that? Or perhaps the line transformer is already altering frequency response and an adapter would be an improvement?

At the moment, I have two mics, need to use them both in 1/4" inputs on guitar/bass heads, and only have my one "line transformer", so my choices are to buy

• another line transformer - I cannot even find one online, anyone have a link?
• 
  
• an adapter, XLR to 1/4".
• 
  
• another cord that does what I need, so XLR on the mic end and 1/4" on the other.

Thanks in advance.

-S-

 

[neddyrow]

is this what you are looking for?

Amazon.com: CBI Ultimate Series Female XLR to 1/4 Inch TRS Cable - 6 Foot: Musical Instruments

i have used both...i dont notice any difference. we used it to connect the output of a phantom power box to go into a preamp.

Mandolin > AKG 411 > phantom > CBI cable > LR Baggs Venue DI > FOH

i prefer the cable because its all-in-one - one less connection to worry about.

all my XLRs are CBI - they have never let me down for 15+ years of gigging

and american made!

 

[Francois Blais]

Depends if impedance matching or ground-isolation is critical.
But I guess in normal (most) situations, it's not, so the straight, xfo-less adaptor would be okay.

 

[okcrum]

The frequency response of the mic gets a bit peakier in the high end and the noise comes up some without a transformer, but gain, EQ, and a quality short cable can take care of that.

You'll get more output with the transformer, though. It steps up both impedance and voltage.

In a 500-50k transformer the impedance ratio is 100 (50k/500) and the voltage ratio is 10 (the square root of the impedance ratio). Think of it as ~20 dB of free voltage gain, with the bonus that the mic response is flatter.

 

[Steve Freides]

I went to the local Sam Ash today, explained my need, and the guy pointed me to the same thing I already had, so I bought a second transformer. It did what I needed today so I'm happy.

@okcrum, what you explained is near the limits of my understanding of electricity - can you put all the relevant variables into an equation so I can get my math geek on? Thx.

-S-

 

[okcrum]

OK - to keep it simple let's stick to the ideal transformer case with no losses, and perfect impedance matches:

Zs/Zp = (Es/Ep)**2 = (Ts/Tp)**2

Zs, Zp secondary and primary impedances
Es, Ep secondary and primary AC voltages, whose ratio is the same as the turns ratio, Ts/Tp
Ts, Tp number of turns of wire on the secondary and primary transformer windings

20*log(Es/Ep) is the voltage gain of the transformer in dB. Note that there's no free energy; the secondary current falls in the same ratio that the voltage rises, so the power gain is one ideally - no loss or gain.

 

[Steve Freides]

You mention "free voltage gain" - does this mean any change in volume? Later, you say "no loss or gain" in power - which one matters to the volume I hear at the other end?

Thanks.

-S-

 

[DoubleMIDI]

You get higher voltage, but cannot load (with a low impedance) as much. Factor 100: 100 times voltage, but also 100 times output impedance or 1/100 of current available at the output.
But since a normal studio mic has 600 Ohms, going up to 60 KOhms works with an instrument input on every amp. 60 KOhms would be a bit high for line inputs which are often 10 to 20 KOhms, so use an instrument input, not an effects return or a mixer line input.
As long as the output impedance of your mic-transformer-combination is lower than the input impedance of the amp input following you get 100 times the voltage in your case. But since mics have a low voltage output you might need that for something audible. And you get less in practice (half of it?) because of the losses in the transformer.

 

[seamonkey]

The SM57 has a high impedance mic cartridge and a built in transformer to match to a low impedance mic mixer.
There are some who modify the SM57 to remove the transformer to work better with high impedance 1/4 input on a regular bass amp.
Search around for transformer-less SM57 mod

 

[Steve Freides]

Search around for transformer-less SM57 mod

Maybe better to use a different mic in this situation?

-S-

 

[okcrum]

You mention "free voltage gain" - does this mean any change in volume? Later, you say "no loss or gain" in power - which one matters to the volume I hear at the other end?

Thanks.

-S-

Sorry I missed this. The line inputs on boards vary but have input imedances well above those of microphones - typically they are greater than 20k ohms. You'll get more volume with the transformer than without. The channel level on the board should show you that in a quick A/B test.

 

[Don Hergert]

... The "something" I've been using doesn't have a brand or model number (only says "China") but it does say, "500 Ohm to 50 KOhm Line Transformer" on it. I'm without a clue as to where or when I bought it. I believe it's passive - have never put a battery in it. ...

When you plug a mic into a FOH mixer, most mixers have built-in internal impedance transformers that will sense and accommodate the variations between low impedance mics and high impedance mics. Typical instrument amplifiers do not have those transformers built in -- some amps do have a MIC button you can toggle that turns on an internal impedance transformer, but many amps do not have that feature.

If you are plugging a mic into an instrument amplifier and the volume headroom is poor, meaning you can just barely hear the mic through the amp no matter how high you turn the volume, you probably have an impedance mismatch and need an impedance matching transformer. The inline variety of these transformers look like a longer than normal female-XLR-to-male-phono(1/4") plug adapter and are usually labeled "Impedance Matching Transformer" or something like that; you simply plug your mic cable into the transformer and the transformer into the amp socket and your volume headroom problem is solved. They are passive and do not require a battery. Here are a couple of common examples of these: Shure A85F Transformer and Audio-Technica CP8201 Microphone Impedance Matching Transformer.

On the other hand, if volume headroom is fine when plugging your mic into an amp, you probably either have a built in impedance transformer in your amp, or your mic is a high impedance mic (or possibly both).

It isn't unusual to find people using inline impedance matching transformers when they really just need a female-XLR-to-male-phono(1/4") plug adapter. Installed properly, this doesn't hurt anything beyond normal use wear-and-tear, but plain adapters cost about 1/2 as much, and they are probably more solid for use as an adapter.

 

[DoubleMIDI]

If the mic input is too low it is because of low voltage, not the impedance, which is rather low on mics (200@ Ohms to 2 KOhms).
The transformer gets you higher voltage at the expense of higher impedance (which means less current).

 

[Steve Freides]

Sorry I missed this.

You missed it by a few days shy of 4 years. I see that some people find the topic still of interest.

-S-

 

[Ric Vice]

I am constantly amazed by the level of intelligence and knowledge among the Talkbass community. It astounds me on a daily basis.