impedance mismatch

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bluebird

Well-known member
Joined
Jun 11, 2004
Messages
1,070
Location
Los Angeles
So lets say an input TX for a certain preamp input (fet, tube, opamp...) isn't right impedance wise, a little off for the required input of the amp.

does this effect just the amplitude of the signal or does it effect different frequencys of the signal differently?

The reason I ask is, I've just built so much frankenstien stuff, and now I'm actually starting use it all and I just want to get an idea of what to use and not use for the important things... like drum overheads.

if its just effects S/N ratio thats cool but...well you know.
 
Very funny cj.... :green:

Ok then I guess my langevin just ain't as good as my 1272's.

I was recording drums and experimenting with the two pres on kick and snare. The Langevin was darker. I actually liked in on the kick but the 1272 was better on snare. more high end, crisper.

I might have the input to the langevins wired up wrong... I'll produce a schemo later.
 
Ok so heres what I did...

this is close to how it was. I might have not traced the board correctly but you get the picture. Oh, minus the gain pot in the original.


langpre1.JPG




And here is how I have it hooked up...


langpre2.JPG


is this all wrong???
 
I´m missing the emitter resistor of Q2?

I think you need the dc connection via the input secondary from emitter of Q2 to the base of Q1. this stabilizes the operating point and biases Q1

the 2nd schemo shows that Q1 is completely unbiased...the circuit should behave very funky :shock:

ohh..the input tranni must be designed to eat a small amount of dc.

steff
 
Hi Bluebird,

I think you've got a Langevin AM4302a type board- check this schematic to fill in any gaps in your own schem-

Click Me!

As Steff says, there's definitely a resistor missing at Q2's emitter, and the input to Q1 is unbiased. Is the "board fader" actually connected as in the diagram too?

Hope the schem helps!

Mark
 
oh, yes ...the board fader is shorting to ground?

btw. the output imprdance of the circuit may be to high to drive a 600:600 xformer with good low frequency response...if you have too much bass roll off try a 10k:10K instead...

steff
 
In fact, that´s what I think it is:

http://hps.infolink.com.br/rafafredd/AM4103.jpg
http://hps.infolink.com.br/rafafredd/AM4103schem.JPG

There is also another version that has less gain and 10kZ output. I had this one and moded it to the above posted version, so that now it can drive 600 ohms transformers. I´ve also used a film cap at the feedback (5uF) and it sounds very good.

Check that out and if it´s not a 4103, mod it, so it will drive low impedances nicely! It´s not a pain to do it. Just some resistors and caps.
 
I´ve took some photos of my project:

This is the moded board (look, there is a FLYING TRANSISTOR :grin: there):

http://hps.infolink.com.br/rafafredd/board_small.jpg

and this is the guts (You can see two free slots that I´ll be using with some line cards that I have for passive EQ gain make up, so I´ll have two mic preamps and two passive EQs in one box!):

http://hps.infolink.com.br/rafafredd/guts_small.jpg

And my horrible front panel work (including the free space for two simple passive EQ controls):

http://hps.infolink.com.br/rafafredd/panel_small.jpg

Please, masters, forgive my wiring... It´s working like a champ. :wink:

That´s it...
 
Thanks guys... I knew I was doing something a little screwy. yea it looks like I'm missing that bias resistor. and yea I have that fader to ground as a knob.

rafafredd,

thanks I'll mod mine out to that schematic...I have the original input TX's but I thought they might be a little weak frequency wise because of it being in a broadcast board. I think I actually read somewhere they are only flat to 14K or something...they look like the ones on your cards but they might have a different model number.


so what should the secondary on the input transformer be? should this be a high ratio trasformer?

and I should just eleminate the 10K pot then? no post gain control?
 
secondary: i believe the Langevin AM16 (from memory so watch out!) is a 1:2 transformer, so I would say anywhere from 1.2k to 5 k will get you in the ballpark.

Yes, that would be my biggest complaint (constructive criticism here) on the demo. The cymbals, when that guy starts really bashing, do not sound that good. A couple of KM 64's or whatever those great overhead mics are , and a good pre would really help.

My Am16 is naturally kind of "brown" It's great for guitar and vocs, but dosen't have the greatest high end.

Dude, I need the chords for Dance Crazy!
That song rocks!
Kind of a sleeper , but I am just starting tro get into it.
Is that Bm first position?

I want a remix with the guitar solo up front more and the rythem less intrusive.
That fuzz part in the end is pretty wicked, Scott!
cj
 
Thanks chris,

you know we stopped playing that song live for some reason. I think because it does take a bit to get into.

thats my first attempt at mixing anything on a real board.

yea, theres lots of things I would do differently now but we only had 4 days to mix 6 songs....

Next time!!! I'll make sure to run the mixes by you before we commit to 1/2 inch.... :green:
 
I´m almost sure that the transformers on my boards are very high turns ratio, like 1:20 or something like this.

You should use a 180k pot for gain control.

I´m using a bc140-16 as the output transistor instead of the original 2n3053.
 
Mistake: If the Langevin x-former was 1:2, that would make the impedance 2.4 k on the secondary, not 1.2 k.

Sorry!
The impedance is the square of the turns.
:guinness:
 
Ok ya'll I did it up like this:


langpre3.JPG



It already sounds better, not as much distortion. Can't wait to try it out on some drums!!!!

THANKS>>>>>> :sam: :green: [/img]
 
What voltage you running?
Whats with the 33 pf? Stability problems?
Looks like a simulator based drawing? Which simulator?
You could play around with that 1 meg resistor, maybe try 500 k.
cj
 
Cj:

33pF ???dunno... it was just there. I'm running it at 48V
program is circuitmaker.

I changed the TX to 1:10 and it really sounds killer.


Butta:
the 100 is there I just forgot it.

Harmonics? I wouldn't know harmonics from a a hamonica. my brain stopped growing 2 weeks, 4 days, 3 hours and 23 mins ago.

yes, I already have a pad in front.. off to the studio :razz:
 
1:10 !
You dog!
Thats my transfromer!
Jeez, I turn my back for 5 minutes....
:grin:
leave the 1 meg. didn't know you were running that much voltage.
 
You mean the A-10? naw I'm using one of the triads I had you spec. or what do you mean?

I'm still trying to find a mate that for that A-10 so I can do 2 la2a's. do you need it?


cayocosta:

Thanks man!!!!
 

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