Transistor help

[Dr.EM]

I want to build this circuit, "Better electret microphone phantom powering circuit", from this site:

http://www.epanorama.net/circuits/microphone_powering.html

It specifies the use of a PNP transistor, and gives the example BC479. I found a datasheet on this, which said it was an ultra low noise transistor. It also says that the VCE can be 36v. Now, I would naturally just use that BC479, but typically, it isn't stocked where I buy my parts. I'm looking through this list:

**broken link removed**

And can see several that will handle the voltage, but i'm not sure which ones are ultra low noise, I imagine they need to be as they are dealing with the very small signals from a microphone. I only have experience with the 2N3906, which I know to be an audio transistor and is low noise, but is this suitable? Its a different series and case type all together? I know some of you people have far more knowledge of transistors, so could anyone advise me as to which of these might be most suitable.

 

[audioguru]

Hi Dr. EM,
The BC479 is pretty old, maybe not made anymore. Its noise spec ranges from 1dB (very good) to 4dB (like an ordinary transistor). The 2N3906 also has a max noise spec of 4dB so if you test a bunch of them, you'll probably find some very low noise ones just like with the BC479.

The BC549 is the same as a 2N3906 with a max noise spec of 4dB and in the modern plastic package, but its leads have a reversed pinout. People have found extremely low-noise ones. :lol:

 

[Dr.EM]

Ah,. cheers. I have a few 2N3906s, in my collection, how would I test them for noise and matched gain. I'm pretty sure that I actually don't have the equiptment to do it, but anyhow :lol:

 

[audioguru]

Hi Dr. EM,
He, he. So it's noise that you want. I know how to make noise with a transistor. Just bias it with its base-emitter junction reverse-biased with a resistor in series to limit the current and a supply of about 9V or more. The junction will have avalanche breakdown at about 7V like a zener diode and you'll have so much noise that you won't know what to do with it all! :lol: :lol:

Seriously, To test a transistor for noise it is best to use the circuit it will be used in, because noise level depends on collector current and source impedance. Use a 5.6k resistor to simulate the impedance of an electret microphone. You could temporarily use 3 of the pin-sockets from an IC socket. Connect the circuit to a high gain amplifier and listen for the hiss. Measure the voltage of the collector and match transistors to have the same. :lol:

 

[Dr.EM]

A noise generator might be a useful addition to my synth, for wind sounds and such. resonant filter, whooooosh, :lol:

I should try the circuit out, in use but without the element, on breadboard. Then, just put the volume on the mixer right up, and pop different ones in. I bought mine in 2 batches, and will order some more along with the parts, so I have a few to choose from.

I will be trying to build this on a piece of stripboard just 6 tracks accross, with a maximum component height of just a few mm, so that it will slide into the tube for the mic. I think its possible, I have at least 13cm length to use. Whilst i'm asking, it says that the element should be connected with shield cable. Now, if this goes to plan, the element will be less than 1 inch from the circuit, so I doubt this is nessasary. However, the mic will be encased in a metal tube, would it be sensible to ground that tube, effectively shielding the whole circuit, or would that mean that the mic will make noise when I hold it (though it could be in a stand anyhow)?

 

[Nigel Goodwin]

However, the mic will be encased in a metal tube, would it be sensible to ground that tube, effectively shielding the whole circuit, or would that mean that the mic will make noise when I hold it (though it could be in a stand anyhow)?

You should connect the metal tube to the screen of the mike lead - that's why mikes have metal bodies!. If you DON'T do that the mike will pickup hum and interference.

 

[Dr.EM]

Apart from that cheap dynamic one I have :lol:

Cheers, i'll make sure I do that then.

 

[Dr.EM]

Hi again. Looking at which actual element to use, there are only two I can get hold of, one i've used before, and another 1/4 of the price :shock:

This is the element which I used before:

**broken link removed**

I would say it's given pleasing results, although the specs on the sheet don't show it to be all that great. I think that they vary quite a bit, perhaps I was lucky with mine :lol:

This is the much cheaper alternative:

**broken link removed**

Looking at the spec sheet, it looks almost identical (bear in mind i'm not worried about its size, they are all tiny :lol: ) it does however cost a lot less. A little put off but the suggested applications though, telecoms, answering machines, sound activated toys :shock:

The element i'd like to use is a Panasonic one, which has a flat frequency response up to 20khz, but its very difficult to get hold of here in the UK.
Any ideas which is better, are they just the same and Maplin have some seriously bloated prices?

 

[audioguru]

Hi Dr. EM,
Both mics are cheap crap that are made for toys. My Canadian Newarkinone (Farnell) and Digikey catalogs list about 30 excellent ones each, and they aren't expensive.
You can EQ a cheap mic to make it sound better, but if you like it the way it comes then use it. :lol:

 

[Dr.EM]

I have normally EQ'd the 1.4khz band up a bit using it. Your lucky to live out there, parts are much easier to get. All of the ones on digikey seem to be at least £8 though, it's not a huge amount, but is still quite a bit more than 40p (I don't want to spend too much, I just want them in case I need them sort of, but they need to be reasonable). I'd go for that EMKAY condenser and FET one from Farnell, but they have a minimum order value of £20, so i'd need to buy a lot of them :lol:

Also, in the circuit itself, I can't see how exactly the mic is going to get the correct voltage. They have a maximum supply of around 10v, and the zener diode is a 12v, how does it get the correct voltage from the 48v phantom supply? AND, those resistors should really be metal film, 1% should'nt they, but I suppose if i'm only using the cheap electrets, it won't make much difference?

 

[Nigel Goodwin]

What are you actually wanting the microphone for?.

 

[Dr.EM]

More or less, multi purpose. I don't often use the mics, but sometimes I like to record an acoustic guitar, a vocal, or most of the time, random sounds of hitting spanners or something :lol: . I have used one of those cheap Maplin ones running on batteries for a vocal before, hear the result on my Dorothy collaboration track, link in my signiture.

 

[Nigel Goodwin]

More or less, multi purpose. I don't often use the mics, but sometimes I like to record an acoustic guitar, a vocal, or most of the time, random sounds of hitting spanners or something :lol: . I have used one of those cheap Maplin ones running on batteries for a vocal before, hear the result on my Dorothy collaboration track, link in my signiture.

Behringer sell a pack of three dynamic 600 ohm mikes, which come complete in a plastic case - you can get them for £18 - really nice solid microphones!, and only £6 each!. I bought a pack a few months ago, for my daughters band - the plan was to use them for now, and buy a Shure SM58 for lead vocals when funds allow (about £100).

Anyway, after using the Behringer mikes I've given up the idea of buying a Shure - the Behringers sound just as good to me!, certainly good enough not to spend twenty times the price for a Shure one!.

I would suggest you try those?, rather than playing with 'toy' electret ones.

 

[audioguru]

Philips used to make some excellent 1/4W low noise, low price metal film resistors. They sold that part of their business to BC Components which might be called something else now. You don't need 1%.

There is a surplus electronic parts joint in the US that people say have very good prices. The word, "surplus" is in very fine print and doesn't show on your screen. A guy bought 4 Panasonic electret mics there and they had excellent specs, for only about 40p each. Needless to say, they didn't work. Panasonic will say that they aren't a dealer of theirs and maybe the mics are a cheap copy or are counterfeit. You usually get what you pay for. :lol:

There is a surplus electonic parts and products place near me. When I see a skid of hundreds of power supplies or something with a name-brand on it, and last year's date stamped on it, I go for their very low price and end up very happy. :lol:

 

[Dr.EM]

I have seen the mics you speak of, and expect they are good quality, they looked it for the price. They are however dynamic type, and i've never had much luck using them, always needed to be placed very close to the sound source, might just be my cheap one, dunno. Either way, more fun to do a DIY job :lol:

I can get individual quantity metal film 5% from Maplin, they might be worth using then. It is typical that the two places I can easily buy electronic components from don't have much selection for electret capsules :x

 

[eblc1388]

There is a surplus electonic parts and products place near me. When I see a skid of hundreds of power supplies or something with a name-brand on it, and last year's date stamped on it, I go for their very low price and end up very happy. :lol:

I used to buy my electronics toys from a UK company called "Greenweld" until the management changed hand and they no longer has great surplus bargain.

Anyone know of any surplus shops sort of like the old GreenWeld in the UK?