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Old 20th January 2012, 10:59 AM   #1
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Default 'Strange' bass boost with impedance transformer

I'm trying to wrap my head around a strange bass boost effect in my bass pre-amp. It occured when I connected a impedance transformer (10:1) to the cathode of a DC coupled stage (see schematic). The bass boost at the anode (to rest of pre-amp) was enormous. When replacing the transformer with a 10k resistor, everything went back to normal, so that seems to be the culprit.

It's a cheap 10:1 impedance transformer and I was driving it pretty hard (around 6 or 7Vpp). Could this have anything to do with it? Some kind of low-frequency feedback...?

Is anyone familiar with this effect? I'm just trying to understand what's happening in the amp at a fundamental level .
Thanks!

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Old 20th January 2012, 11:38 AM   #2
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What you did was simply lowering the local degenration in a frequency-dependant manner.

I guess that you'd like to make some DI out - right ?

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Old 20th January 2012, 11:41 AM   #3
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That's exactly what I'm (trying) to achieve. Is there a proper way to get rid of this 'local degenration in a frequency-dependant manner' effect??
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Old 20th January 2012, 11:49 AM   #4
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If you put another smaller resistor in series with the degeneration resistor and then you take the signal from there the influence should be reduced significantly (at the cost of a smaller DI output voltage). And you should also put a dc - blocking cap in series of the primary of the transfomer. It will reduce distortion caused by the magnetic saturation of its core.

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Old 20th January 2012, 12:01 PM   #5
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Like so?:
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I don't get what you mean whit the series DC blocking cap. Isn't the 4.7u doing that already?
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Old 20th January 2012, 12:18 PM   #6
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This fact is by the serias resonance in the primary inductance and the coupling cap. Put a short circuiting jack in secondary of this, so the inductance becomes very small and donīt has any resonance in the audio band. The 10K resistor is to be deleted.

Good luck !!!
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Old 20th January 2012, 01:53 PM   #7
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The output connector will be XLR, so short-circuiting will be difficult. Removing the 10k kind of contradicts what phase_accurate suggested, so I don't really know what to do exactly at this point.
Thanks for the reply though!
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Old 20th January 2012, 01:57 PM   #8
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Canīt you put a connector with pines soldered together, shorting out the secondary of this Xformer?
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Old 20th January 2012, 02:01 PM   #9
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If it's a cheap transformer that you are driving hard, my first guess would be distortion. Overdriven transformers distort, and start doing so in the bass. More harmonics in the bass sounds louder. Well, it is louder. It sometimes sounds pretty cool.

The other answers may be right, but that was my first thought.
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Old 20th January 2012, 02:08 PM   #10
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Canīt you put a connector with pines soldered together, shorting out the secondary of this Xformer?
Well, I could, but it doesn't feel like the proper thing to do. I don't see a lot of amps what such a construction . And what about the times it's connected to a mixer/recorder. Tone will differ with different loads...
So I'm really looking for a way to solve the problem within the given schematic.
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