Passive preamp

[jishnudg]

http://beavishifi.com/projects/Passive_IPOD_Preamplifier/Passive-iPod-Preamplifier-Schematic.jpg

Could anyone explain to me how the above circuit works....can it be modified and used as a passive buffer of some sort,or DI?
Thanks,
J.

[Johan]

They appear to use only the primary of a small push pull transformer as an auto transformer to increase the signal voltage. Works fine as long a the impedances allow for it. Nothing is free thou, and you probably have to experiment a lot before you get what you want

[artifus]

* beep! *** paging prr *** paging prr *** diystompboxes.com needs you (again) *** paging prr *** paging prr *** beep! *

http://beavishifi.com/projects/Passive_IPOD_Preamplifier/

the relevant forum thread appears to have disappeared - hopefully the original author will be along shortly to explain it for you far better than i could    

*please imagine that the following pic has been photoshopped for your own personal amusement:



*

[PRR]

It's for iPod, not guitar.

iPod can drive 32 ohms. Guitar strains to drive 10,000 ohms.

Different world.

[Thecomedian]

I don't think there's any such thing as a passive pre-amp, is there? I mean, the idea of a buffer or pre-amp is a transistor or op-amp performing work to Transform the signal from high impedance to low impedance, or from small current/volt to larger current/volt.

You don't get something for nothing. Am I missing something?

[Gurner]

I don't think there's any such thing as a passive pre-amp, is there? I mean, the idea of a buffer or pre-amp is a transistor or op-amp performing work to Transform the signal from high impedance to low impedance, or from small current/volt to larger current/volt.

You don't get something for nothing. Am I missing something?

Well with a transformer, you can get a higher voltage out than in ....& analyzing the word 'amplifier' ....IMHO I reckon it means to make something larger...therefore if your original signal voltage is small, a transformer can make it larger (albeit at the expense of current...as you say, no something for nothing here!).....which I guess fits the template....and it's all passive, therefore a transformer is a passive amplifier! Though I still prefer the snazziier name ...a transformer.

[Hemmel]

You don't get something for nothing.

Laurence Krauss begs to differ 

[jishnudg]

Okay....what if I stuck a simple opamp buffer behind it (like a Jack Orman circuit snippet)....would I then be able to use any old transformer, or have to stick to studio quality specs?

[Johan]

Depending on what transformer you chose, You need something that can drive the transformer, possibly a 386. Then whatever comes after the transformer needs to have input z high enough to keep the transformer happy, or the transformer won't do what you expect of it.  PRR pointed ot it was specifically designed for Ipod driving a line input, so a headphone amp(386) and a10k load (with that particular transformer) would probably work....with all the supporting electronics needed to go from guitar to transformer for voltage gain, it's hardly passive anymore...nothings free...

[PRR]

> what if I stuck a simple opamp buffer behind it

Then why would you need a transformer??

There may be reasons. But you don't say what you *really* want to do.

If you are trying to tenderly take signal from a guitar, *start* with a tube, FET, opamp, or BJT.