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!
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!
An externally hosted image should be here but it was not working when we last tested it.
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.
Regards
Charles
Regards
Charles
Like so?:
I don't get what you mean whit the series DC blocking cap. Isn't the 4.7u doing that already?
An externally hosted image should be here but it was not working when we last tested it.
I don't get what you mean whit the series DC blocking cap. Isn't the 4.7u doing that already?
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.
The other answers may be right, but that was my first thought.
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.
Take both outputs from the cathode.
Or add a resistor in series with the transformer primary.
Or use two resistors for the cathode, but with the lower value one at the ground end, and put the transformer connection on the junction.
Better still, decide exactly what you want to do then design or find a circuit which does it.
Or add a resistor in series with the transformer primary.
Or use two resistors for the cathode, but with the lower value one at the ground end, and put the transformer connection on the junction.
Better still, decide exactly what you want to do then design or find a circuit which does it.
Transformer saturates at low frequencies and its impedance drops, which increases the gain of the stage at the plate. Move the output to the cathode and remove the plate resistor or bypass the plate with a large cap (at least a few uF). What's the transformer for anyway? Is it REALLY suitable for that purpose? You can't expect a 12AX7 to drive less than 100K, even as a cathode follower. That implies that the transformer should have several hundred Henries of inductance in the primary.
What I meant is almost what you posted but different: The transformer accross the SMALLER resistor of both.
Even if it is the effect described by Osvaldo (=series resonance) then it should also be minimised by my suggestion. And yes even some Exciter effect as described by Pano should be reduced by it.
Regards
Charles
Edit: Haven't seen that newer posts appeared in the meantime.
Even if it is the effect described by Osvaldo (=series resonance) then it should also be minimised by my suggestion. And yes even some Exciter effect as described by Pano should be reduced by it.
Regards
Charles
Edit: Haven't seen that newer posts appeared in the meantime.
I don't see a load on the secondary of the transformer. If the mixer has a high impedance input, and the transformer is expecting to drive a low impedance, you will get a bass boost because with no load its acting more like a capacitor (and the inter-winding capacitance is how it does that) than a transformer.
Try loading it with a 1000 ohm resistor at the output. I think you will find problem solved at that point.
Try loading it with a 1000 ohm resistor at the output. I think you will find problem solved at that point.
Unlikely. With no load the transformer will act as an inductance, with some parasitic capacitance in parallel. The capacitance will create an HF resonance, but the main effect will be frequency dependent degeneration, as has already been said. Bass boost at the anode is the outcome, exactly as seen. There is no mystery about what is happening; the difficulty is deciding the best way to rescue the situation while retaining essentially the same circuit.
I built a similar circuit and made the same mistake. When I loaded the transformer the difference in bandwidth was instantly visible on the scope. IOW I'm not spouting theory, instead experience. Ask any transformer manufacturer and they will tell you the same thing.
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