20db mic pre

[Samuel Groner]

If all you want is a cheap & easy low gain preamp then I'd recommend something based on the typically called "two-opamp intrumentation amplifier". See www.ti.com/lit/an/sloa034/sloa034.pdf, figure 6. Detailed implementation hints on request...

Samuel

 

[zmgwg]

no particular mic. general purpose low gain preamp. liked the idea of all the gain from transformer only

 

[abechap024]

Those old shure mixers have ~1:20 type transformers in them, they aren't the best but they really don't sound too bad either. Could be a good starting point for experimenting or DIYing something.

 

[PRR]

Mikes should be loaded in 2K. Line inputs are often 10K. Impedance ratio 2K:10K or 1:5, voltage ratio 1:2.2, 7db voltage gain.

 

[abbey road d enfer]

Those old shure mixers have ~1:20 type transformers in them, they aren't the best but they really don't sound too bad either. Could be a good starting point for experimenting or DIYing something.

Such a transformer needs to see an impedance of minimum 800 kohms at its secondary.

 

[CJ]

maybe an altec plug-in xfmr,

try a peerless k-241-d.

this is a famous moving coil tranformer because it can be wired for 50 ohms and maybe lower.

very low loss, 1:12 ratio,

or you could design a transformer, find out the power capability of the mic motor,

use low loss 80 perm or mu metal,
figure 1/10 th of the mic motor will be swallowed up as core loss,

find a core with 1/10 th the loss of your mic, this will be small, of course, then wind it up to whatever voltage ratio you need, maybe run it into a fet buffer which needs no input current,

coundn't sound any worse than sonny and cher,

 

[CJ]

you have to prove that you can not do something before you can say that it can not be done, correct?
intuition can be wrong sometimes, especially in physics class,

while searching for info, i came across some usefull reading by none other than our own kev from k and k audio, observe the easy reading>

"To make the theory real, let’s set up a hypothetical example. Imagine an MC cartridge that has a nominal 0.5 mV output and a phono preamplifier that has a 47K input impedance. If we desire to increase the output voltage of the MC cartridge to the “normal” 5 mV level associated with most MM phono cartridges where the phono preamp gives acceptable noise performance, then we would require a 10-fold increase in signal level. To do this, we connect the primary coils on the Lundahl LL9206 in the configuration that results in a gain of 10, or expressed in dB, 20dB, and then wire it up to our preamp and enjoy some tunes. (See the diagram below for an illustration of the hypothetical connection described here) But before we drop the stylus onto the record, we need to also consider the impedance implications of our gain decision. Because the gain we chose was 10, we have an impedance transformation of 10 squared, or 100. So, the cartridge “sees” the 47K load resistor at the input to the preamp through the transformer as being 470 ohms, which is arrived at by dividing the resistor value, 47K, by 100, the impedance ratio. Most MC cartridge manufacturers provide a recommended range for load impedance (or resistance). Because this value is usually substantially lower than 47K, the transformer is performing a valuable service on the impedance front, as well. A typical MC cartridge will sound somewhat “lightweight” when it is loaded too lightly (load impedance is too high) and will sound somewhat dull when the load impedance is too low. So, it’s important to get both of these parameters, the step-up gain and the reflected load impedance, correct to get the best sound. The gain you choose generally has less effect on the sound quality than does the cartridge loading. So it’s important to get the gain in the “ballpark”, but then turn serious attention to adjusting the cartridge loading to get good frequency balance as described above. The discussion below will guide your efforts in this direction.

One feature of the diagram below deserves additional comment. The grounding arrangement around the transformer is appropriate for the vast majority of installations where the phono preamp has unbalanced inputs and the cabling from the turntable to the MC step-up transformer is two conductor wire terminated with an RCA plug. However, with the use of balanced cabling from the turntable, the ground connection between the primary and secondary sides of the transformer can be deleted. Further, for a phono preamp with balanced inputs, neither end of the secondary of the MC transformer would be connected to ground as shown.

You might now ask if when you need a gain of 10, are you then “stuck” with a reflected 470ohm load for your cartridge, even if that’s not optimum? The answer is that you aren’t “stuck”. You can add resistance in parallel with the customary 47K phono input resistor to lower that reflected load impedance. Unhappily, to increase the load impedance above 470 ohms, you either have to remove the 47K resistor and replace it with a higher value resistor or you have to settle for a lower step-up gain. Fortunately, it’s most often true that the MC cartridges that have the lowest output levels also require relatively low values of load impedance, so this is not usually a problem.

For those of you who want to experiment with and optimize your cartridge loading (recommended!), we have provided tables of parallel resistance values that will help you achieve the best sound. As a starting point, I usually chose a load impedance value of 10 times the cartridge impedance (sometimes referred to as resistance in cartridge data sheets). The values given in the tables are referenced to the step-up gains produced by the LL9206 and the LL1678. Note that these values don’t scale precisely with the square of the step-up gain. The primary and secondary resistances of the transformer also figure into the calculation. If you want to calculate loading values yourself, send an e-mail to [email protected] and I will e-mail in return an Excel spreadsheet program that will allow you to do so.

 

[abbey road d enfer]

you have to prove that you can not do something before you can say that it can not be done, correct?

Indeed.

intuition can be wrong sometimes, especially in physics class,

No intuition here, just Ohm's law. Given the constraints, that are: source Z (mic)=200R, load Z (line input) = 20k, gain target 10dB< G <20dB, all calculations show that there's a very narrow range of solutions. Transformers being passive, and considering gain only, power adaptation (matching) gives ratio = sqrt (10k/200) = 7.07. Half the voltage is lost, thus gain = 7.07/2 = 3.535 => 10.97 dB
So, it would be possible to slightly decrease the load on the mic by using a 1:3.5 xfmr, which would provide just 10dB gain and a load of 500 ohms
Personally, I wouldn't expect much sound quality from such a set-up with a dynamic mic (they don't like to be loaded).

while searching for info, i came across some usefull reading by none other than our own kev from k and k audio, observe the easy reading>

You might now ask if when you need a gain of 10, are you then “stuck” with a reflected 470ohm load for your cartridge, even if that’s not optimum? The answer is that you aren’t “stuck”. You can add resistance in parallel with the customary 47K phono input resistor to lower that reflected load impedance. Unhappily, to increase the load impedance above 470 ohms, you either have to remove the 47K resistor and replace it with a higher value resistor or you have to settle for a lower step-up gain.

Exactly. With the basic data we have (200r source, 10k load) there is no latitude to achieve the desired goal of ca. 20dB gain.
There are only two solutions: either decrease the source impedance without losing voltage (tell it to MM. Shure, EV, Audix, AKG, Beyer...), or increase the load impedance; that means modding the line input stages for higher Z.
But wait, if you want 20dB gain, you need at least 1:12 ratio with an input impedance of 150k. A major endeavour...

 

[CJ]

who said our input Z was limited to 10 K?

another option is to rip out the sm 58 transformer in put something in there with a higher step up ratio.

if this is a ribbon mic, then we are in trouble.

 

[CJ]

ok, lets do it from the beginning,

the sm 58 puts out about 15 millivolts when hit with normal speach levels,

but this is music, so we can use 20 millivolts.

what resistance is supporting this 20 mv?

150 to 300 ohms? according to shure,

lets use 200 ohms because it divides into 20 millivolts to give us 0.0001 amps, right?

power is volts times amps,

volts was picked at 0.02 and amps was determined to be 0.0001.

power is therefore 0.002 milliwatts.

so, who knows what to do next?

???

well, we have no flux density problems with a 20 millivolt signal, what about a snare hit?

who cares, saturation sounds cool in a nickel transformer.
helios, studer,

so use the smallest core for low loss, ever seen a beyer core? me has, smallish,

smallest EI lam we can get easy is 094 EI.

0.002 lbs for a square stack.

1/4 inch by 3/8 inch overall, tiny,

what is our loss at 20 millivolts?

depends on the flux level, which depends on the 20 millivolts,

B = 100,000,000 E/4fAN

B = 100,000,000 E/4(50 hertz)(0.0558 cm^2)(Turns)

B = 100,000,000 (0.020)/200(0.06)Turns

B = 2,000,000/12T

B = 167,000/T

so for a 100 turn primary, we would hit the square stack 094 EI super at

B = 167,000/100

B = 1,670 gauss, well below the 7,500 limit.

if we use 1000 turns we will have 167 gauss, which will put us way down on the core loss graph.

a 1:10 might be 750 turns pri and 7500 sec, done on an 94EI,
choose the smallest wire you can wind, as current is not a problem.

wind a simple pri-sec, this is how the beyers are done.

they use a tricky bobbin that gets glued around the core and then wound on a wheel which spins the bobbin while still on the closed path core,

next, compute the input Z at 1,000 hertz.

 

[abbey road d enfer]

who said our input Z was limited to 10 K?

Post 46 from the OP.

 

[abbey road d enfer]

lets use 200 ohms because it divides into 20 millivolts to give us 0.0001 amps, right?

This is the short-circuit current; is this how you want to load the mic? 

well, we have no flux density problems with a 20 millivolt signal, what about a snare hit?

I have a better estimate. The OP says he wants about 20dB gain max, so that infers the signal will be > -20dBu if he wants to hit his line input correctly. 77mV rms

B = 100,000,000 E/4fAN
B = 100,000,000 E/4(50 hertz)(0.0558 cm^2)(Turns)

You're calculating audio xfmrs at 50Hz?

choose the smallest wire you can wind, as current is not a problem.

Sure, current is not a problem, but DCR is. Major cause of degradation of noise performance, HF response too.

a 1:10 might be 750 turns pri and 7500 sec, done

So, you've designed a xfmr without taking into account the impedances and inductances, just voltage based on an hypothetic 50Hz operation, but in the middle of the exercise, you suddenly decide to increase the number of turns by a factor 10!
If you want to "do it from the beginning", you have to define the source level, impedance and lowest frequency and the receiver impedance. Then you work out the necessary minimum impedance of the primary, determine the necessary characteristics of the core and work the number of turns.
I understand that you have a vast practical knowledge of transformers, but this forum is about giving anwwers that are substantiated by some science. Throwing numbers like "a 1:10 might be 750 turns" is not science, it's speculation.
Next time you buy a car, you want to know that the guys who designed the steering and the brakes made it by speculation?

 

[PRR]

> calculating audio xfmrs at 50Hz?

I think he is checking flux and loss. For an admittedly secondary application (all the good mike preamps are in use for more important sounds), and most music source (most portable sound sources) having falling max-output below 100Hz-50Hz, this may be a good point to check.

BBC contract acceptance may require full level at 20Hz.... this isn't the BBC.

My BIG objection is that OP could build dozens of $2 and $10 silicon preamps before he got close to any "094 EI super" or "a wheel which spins the bobbin".

The transistor-radio 2K:10K stocked at Mouser and others will give a good impedance ratio from modern mike to modern line input, and also allow floating the Phantom. Gain is 7db, which IMHO is hardly worth it, but the easy Phantom makes it the dead-simple way to put a hot mike into a Line input. Since we have a 200:2K input mis-match, bass response does not suck as bad as the spec-sheet shows. Any few-db bass error will be boosted in ProTools. I have put several V at 50Hz through these things and nobody complained. Its true musical merit is a pig in a poke..... but the transformer is $2-$5 so the trial is cheap.... cheaper than trying new strings on the gitar or a bottle of wine in the singer, things we try without hesitation.

http://www.mouser.com/Search/Refine.aspx?Keyword=audio+transformer+2K
http://www.mouser.com/catalog/specsheets/XC-600127.pdf
42TM002-RC 200mW $2.30
42TL002-RC  75mW $2
42TU002-RC 460mW $3.20

can: 429-7202-RC $4.50! for 75mW models

 

[CJ]

50 hz = sm58 response
flux is checked at lowest freq since that is where it is highest due to frequency being in the denominator

750:7500 = 1:10 turns ratio

the way you design a transformer is to pick a core , do the math, then re-adjust as necessary.

no phd required to post here, just a bunch of people with a mutual interest trying to learn, mistakes will be made, this is called the learning process,

 

[abbey road d enfer]

50 hz = sm58 response

Then who said it's an SM58? From the OP, I could see that being an RE20 on kick or a Beta 52 on cajon.

flux is checked at lowest freq since that is where it is highest due to frequency being in the denominator

Isn't audio considered 20-20k?

the way you design a transformer is to pick a core , do the math, then re-adjust as necessary.

This is not how I learnt. Starting with source and load parameters, then inductance; the rest follows and there's no trial and error in the design process; there is trial and error in the actual construction. If you ever try to simulate a circuit with transformers in Spice, a transformer starts with two inductances, then a coupling factor.

no phd required to post here, just a bunch of people with a mutual interest trying to learn, mistakes will be made, this is called the learning process,

As a moderator, I think it is essential that the information posted by influential members, such as you, CJ, is as scientifically accurate as possible, and usable by less educated members as a basis for their own tuition and experimentation. It happens to me sometimes, to let something slip in my posts, then I'm happy to be corrected.

 

[CJ]

ok, i did not see the moderator thing, i thought you were slappin people down just for the heck of it, 

ok, 2 choices for experimental transformers right here>


now there is a transformer already in the mic, if you try to replace this with a transformer with ten times the turn ratio, you will probably run into problems, you would save some core loss since you are only exciting one core, but the turns ratio would be too high.

pri z,
try 500 turns on a square stack of 94EI

mag metals catalog says for 94EI,

L = (0.0401*10^-8)KN^2(u-ac),  u-ac = perm = 100,000 for mu metal which we will use for ultra low loss mic signal, K is stacking = 0.9 or abouts for 3 by 3 laceup,

so L = (0.0360 * 10^-8)N^2(100,000)

L = 0.0360 * 10^-3 * N^2

so if N = 500 turns, we have N^2 = 250,000

so L=(0.0360*10^-3)(250,000)

L = 9 Henries for the primary.

so our Z will be reactance for 9 henries at 50 hz,

XL = 2 pi f L = 6.28 * 50 * 9

XL = 6.28 * 450 = 2,826 ohms at 50 hz.

henries for the sec will 10^2 times 9 = 900 henries.

sec Z will be 6.28 * 900 * 50 = 282,600 ohms or about 260 K.

so a high Z input will be needed to make this work without dragging down the signal on the sec.

or, we could drop the pri Z down to 200 ohms, they say that max pwr transfer is when source = load, so if the mic trans has a 200 ohm sec, we will plug it into a 200 ohm pri and see what happens.

ok, so we can reduce turns on the pri (and also sec) if we only need 200 ohms,

but how many turns = 200 ohms?

reverse engineering, 2 pi f L,  200 = 6.28 * 50* L

200 = 314 L,  L=200/314 = about .67 Henries.

so plug 0.67 into the inductance formula, which was simplified to

L = 0.0360 * 10^-3 * N^2

so turnssquared  for 0.67 henries = 0.67/0.000036 = 18,611

take the root and you have your turns, root of 18,611 = 136 turns.

sec will be 1,360 turns, sec z will be 200 * 100 = 20 K, which should be ok.

we already saw that 100 turns would be no problem as far as max flux and saturation,

now if we draw all the power out of the mic motor with a 200 ohm pri z hooked to the mic, what will happen?

me thinks the signal will drop,

so maybe a 94EI with more than 136 turns but less than 500 might work, if plugged into a relatively hi z input,

what is the input Z on a mackie 1604?

1.3 k, wtf, over?


ok, this transformer idea is looking like a waste of time and money,

i suggest a cheap 20 db preamp would be the best option,

if you drop the input Z on the mic, you lose signal.

if you raise the input Z on the sec, you lose signal.

so i guess the dead guys had it right, there is a certain point at which you have to jump from passive to active in order to get the best signal/noise, and it appears that a low z mic motor into a trans that outputs 200 ohms into a amplifier is the best way to go.

however, sometimes experiments prove theory wrong, so it would still be fun to fool around with a 94EI with 250 turns and clip the input resistor across the mackie opamp.

next...

 

[abbey road d enfer]

The OP says he runs out of mic pres, so the question is : what are the line inputs he wishes to use?
If it's the converter's line ins, he has to deliver ca. 0dBu into 10k, then the xfmr option will be someting between PRR's suggestion (2k:10k with only 6dB actual gain) and maximum transfer (matching), i.e. 200:10k with 11dB gain, so that would be sensible for signal levels higher than -20dBu, which would drive the converters at ca. -35-40 dBfs, which is acceptable for 24-bit converters.
If it's a mixer's line inputs, it may be a quite different ballgame, since many "cost-effective" mixers have quite sensitive line ins. Some have up to 40dB gain. In that case, a transformer would not be needed! Mmmm, well, if the line ins are not balanced, a xfmr may be welcome.
There is a case for line inputs with about 15dB max. gain, which would not be enough even for a hot microphone signal, and would benefit from the addition of a 2k:10k xfmr with its 6 dB gain and satisfactory reflected impedance.
As someone said before, knowledge of the environment is essential in providing a documented answer.
I guess the OP is either utterly lost or disgusted...

 

[zmgwg]

well I'm rather lost...
I guess I'll give it a try with a transformer with 10k secondary. It would go to a balanced converter input.

 

[CJ]

a cheap experiment would be to but one of those gadgets for plugging a mic into a guitar amp, i do not know what is inside those things but it has to be a step up  transformer i would imagine

 

[abbey road d enfer]

a cheap experiment would be to but one of those gadgets for plugging a mic into a guitar amp, i do not know what is inside those things but it has to be a step up  transformer i would imagine

these are typically 600:10k xfmrs.