Current-Amp to drive bass guitar cab? - diyAudio
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Old 16th February 2011, 03:38 PM   #1
gruni is offline gruni  Germany
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Default Current-Amp to drive bass guitar cab?

Hi!

I have a 4x10" Ashdown ABM bass cab driven by a Hughes&Kettner bass top. I just had the idea of modifying the bass top to act as a current amplifier with 8 ohms output imepdance. Has anyone tried this so far? Obviously we're not talking about HiFi here!
I once had the cab driven by a Tube Amps 8Ohm output and although i didnt hear that much difference as i expected, i think, the sound was a bit less harsh even with the amp not driven into saturation.
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Old 16th February 2011, 07:24 PM   #2
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Sorry, but that sounds a completely silly idea - the speakers won't be damped correctly, and you're throwing away most of your power.

If you REALLY want to do it?, just feed the speaker via a LARGE 8 ohm resistor.

The valve amp sounded less harsh because of it's limited anf unlevel frequency response.
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Old 16th February 2011, 08:01 PM   #3
gruni is offline gruni  Germany
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Originally Posted by Nigel Goodwin View Post
The valve amp sounded less harsh because of it's limited anf unlevel frequency response.
honestly, i don't think so. even when cutting all the top end on my amp, it doesnt sound as soft as the tube amp.

is running the cab through an 8 ohm resistor not wasting much more power than driving the cab by current with having only a resistor of about half an ohm in the signal path?
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Old 16th February 2011, 08:16 PM   #4
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Originally Posted by gruni View Post
honestly, i don't think so. even when cutting all the top end on my amp, it doesnt sound as soft as the tube amp.

is running the cab through an 8 ohm resistor not wasting much more power than driving the cab by current with having only a resistor of about half an ohm in the signal path?
It's exactly the same, running from an 8 ohm output impedance wastes at least 50% of the power in the feed (in this case the resistor) - in a valve amp it's wasted in the valves, the transformer etc.

If you want to run it from half an ohm, place a half ohm resistor in series, but the same reasons apply (just less).
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Old 16th February 2011, 09:00 PM   #5
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Before you go further you might calculate the speaker QTS using a seriese resistance
of 100 ohms to simulate a current source amp. I expect you are going to end up with
a high QTS and a big bass peak.
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Old 17th February 2011, 01:16 PM   #6
teemuk is offline teemuk  Finland
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Chances are, the H&K amp already implements a current feedback scheme that increases output impedance. Those are extremely popular in instrument amps.

What H&K amp model is it?
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Old 17th February 2011, 04:30 PM   #7
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Chances are, the H&K amp already implements a current feedback scheme that increases output impedance. Those are extremely popular in instrument amps.
Can you give an example of such an amp?, I've never seen one or heard of one?.
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Old 17th February 2011, 06:55 PM   #8
teemuk is offline teemuk  Finland
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You surely must have seen this scheme in numerous musical instrument amps. It has been pretty much a standard feature since early 1990's, though earliest examples of its implementation in SS guitar amps date to mid 1960's. That's how known it is.

Anyway, load current is sensed across a low-ohm resistor in series with the speaker. The resulting signal is fed back (in negative phase with the input), which subsequently increases the output impedance and naturally skews the amplifier's frequency response when its driving a reactive load such as a loudspeaker. Unlike with the usual only-voltage-feedback setup, the amp's voltage gain now becomes affected by the load impedance, like it does in tube amps with low amounts of NFB and inherently high output Z.

The technique is basically just an inversion of the positive current feedback scheme that was developed in 1950's to linearize response of a tube amplifier, which suffered from the effects of high output impedance. Basically, instead of positive current feedback, which decreases output impedance, you use negative current feedback, which has an opposite effect.

The schematic example, I believe, is from some Fender amp. You'll find a similar setup from hundreds of other musical instrument amps as well.
Click the image to open in full size.

Last edited by teemuk; 17th February 2011 at 06:59 PM.
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Old 17th February 2011, 07:32 PM   #9
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That's used in a VERY small number of amplifiers, I have seen it very occasionally and I suspect it's more to do with overload protection than anything else.

But thanks for the circuit.
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Old 17th February 2011, 08:17 PM   #10
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Quote:
Originally Posted by Nigel Goodwin View Post
That's used in a VERY small number of amplifiers, I have seen it very occasionally and I suspect it's more to do with overload protection than anything else.

But thanks for the circuit.
Absolutely not. Particularly in guitar amps it's a common solution for emulating the low damping factor resulting from rather small amounts of NFB present in a lot of tube amps. You might say it allows the "character" of the speaker to develop fully.
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