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Old 29th April 2013, 09:41 PM   #1
ATAUDIO is offline ATAUDIO  Austria
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Default Pre-mix for boombox

Hi all,
I am building a boombox for "street artist" use.

I need line input for MP3 player and a dynamic mic input.
So I need some kind of circuit for:
1)raising the MP3 reader input of a factor of 3 (to get close to 2 Vrms that the power amp would like)
2)raising the mic input 0 to 16 db adjustable.
3) mixing the things

I have designed the following, as the most simple solution possible, with the following assumptions:
a)the line output level can be cut using the player volume.
b)the power amp is AC coupled , and 20k input .
c)mic is non balance short cable dynamic cartridge.
d)inverted output phase is not a problem
I am not sure that the voltage splitter is really necessary, though.
Maybe a simple 2 x 10k resistors would do fine.
Do you see any gross mistake?
Thanks in advance
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Last edited by ATAUDIO; 29th April 2013 at 09:51 PM.
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Old 30th April 2013, 12:25 AM   #2
DUG is offline DUG  Canada
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IMO:

You don't need D1, T1, R11.

You should buffer/amplify your dynamic mike so you can set the gain easier and review (ie change) the way you use R9 (pot)

See jpg for a similar idea on this. (dual supply config)
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Old 30th April 2013, 03:34 PM   #3
ATAUDIO is offline ATAUDIO  Austria
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I concour that the power splitter transistor can be eliminated (see modified schematics).

But the rest of your circuit is exaxctly like mine . R9 just lets you set the gain of the mic input between circa 0 and 16 dB, without altering the gain of the line input. R9 can be seen as an extension of R7 and R8. And BTW, I need some 4-6 dB gain on the line in as well.
Do you mean it won't work like that ? Can you explain me why?
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Old 30th April 2013, 06:35 PM   #4
ATAUDIO is offline ATAUDIO  Austria
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Anybody thinks this way is much better?
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Last edited by ATAUDIO; 30th April 2013 at 06:38 PM.
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Old 1st May 2013, 02:09 AM   #5
DUG is offline DUG  Canada
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I do.

I see someone who knows how to use a bjt and bias it properly.

Hfe > 100 I hope.

Input Z will be about 5K

Looks good to me.

Build time.
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Old 1st May 2013, 08:47 AM   #6
ATAUDIO is offline ATAUDIO  Austria
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Thanks for the nice worrds.
Yes hfe >100 (at 5 mA Ic it should be around 300 for the quoted type). With the quoted values you could probably direct couple the BJT to the op-amp, but you probably would need to put a trimmer in the input divider to fine adjust the DC offset, and I do not think it is worth.
Note that the Re is not shunted by capacitor so the gain is reasonably independent from hfe and set around 10.
But my question holds: what are the real advantages of this more complicate circuit?
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Old 1st May 2013, 11:20 AM   #7
DUG is offline DUG  Canada
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"...Note that the Re is not shunted by capacitor so the gain is reasonably independent from hfe and set around 10...."

Yes, and is more linear.

Post # 4 has constant input impedance on the mike line...

Post # 3 has input impedance that varies with gain setting...I just don't like this as a variable on a dynamic mike input.

Now that I think about it, what input Z does your dynamic mike need?

You might need to increase the resistor values in the bias chain.

Just a thought.

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Old 1st May 2013, 11:56 AM   #8
ATAUDIO is offline ATAUDIO  Austria
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Quote:
Originally Posted by DUG View Post
"...Note that the Re is not shunted by capacitor so the gain is reasonably independent from hfe and set around 10...."

Yes, and is more linear.

Post # 4 has constant input impedance on the mike line...

Post # 3 has input impedance that varies with gain setting...I just don't like this as a variable on a dynamic mike input.

Now that I think about it, what input Z does your dynamic mike need?

You might need to increase the resistor values in the bias chain.

Just a thought.

Impedance should be ok. Mikes like the Shure M57 have 2-300 Ohm impedance. If my hfe is around 300 I should be around 7k, so I do not see problems with dynamic mikes using the BJT input.
With the other simpler circuit, I might get some plateau effect at the extreme end of the regulation, (i.e. when the pot i completely short) but not really something to be afraid of, IMO.
On the other hand, the circuit with the BJT is more clipping-prone (you can get something like 1 V RMS out of a good mike, if the rocker is really screaming ).
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