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Chip Amps Amplifiers based on integrated circuits

Make a preout
Make a preout
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Old 26th November 2004, 07:34 PM   #1
Thror is offline Thror  Norway
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Make a preout
Default Make a preout

Hi, I wonder if it's possible (and pretty easy) to make a preout on your gainclone? I would like to use my sub with the amp (when I'm finished..).

-Torsa
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Old 26th November 2004, 08:38 PM   #2
Nuuk is offline Nuuk  United Kingdom
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I presume that you are talking about a Gainclone with a volume control!

You must take the output from the output of the volume control. I do that to feed my (sub)woofers. Try and keep the interconnects to the sub short if you can.
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Old 26th November 2004, 09:23 PM   #3
Thror is offline Thror  Norway
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I was hoping it was possible to split the signal, doing as you say will make my signalpath a least 3 meters longer.

-Torsa
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Old 26th November 2004, 09:31 PM   #4
Nuuk is offline Nuuk  United Kingdom
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I was hoping it was possible to split the signal, doing as you say will make my signalpath a least 3 meters longer.
I can't see why? Where were you going to split the signal from?

Perhaps you could give a brief description of your system and set up.
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Old 26th November 2004, 09:43 PM   #5
Thror is offline Thror  Norway
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Maybe I didn't understand you right, you meant to take the signal from the pot to the sub amp then back again to the gainclone? How do they then do it in preamps you buy that has 2-3 pair of preouts?

I'm trying to plan a bit until I recieve my order :>

Torsa
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Old 26th November 2004, 09:47 PM   #6
xplod1236 is offline xplod1236  United States
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Take the signal from the pot to the gc and also from the pot to the sub
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Old 26th November 2004, 09:56 PM   #7
Thror is offline Thror  Norway
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Oooh, thought I couldn't do that
Thanks a lot

-Torsa
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Old 26th November 2004, 11:01 PM   #8
Swedish Chef is offline Swedish Chef  Sweden
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You definitely want to use a buffer/line driver before tapping off the signal to the sub. I have no idea as to whether this is included in the design you mention but anyway.

/Magnus
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Old 27th November 2004, 09:27 AM   #9
Nuuk is offline Nuuk  United Kingdom
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You definitely want to use a buffer/line driver before tapping off the signal to the sub. I have no idea as to whether this is included in the design you mention but anyway.
I don't have a buffer before the take-off point for my subs but I keep the interconnect to the sub crossovers to less than a metre. It works well but I have one further comment.

I use stereo subs. Previously I tried with a mono sub and couldn't get the thing set up without it affecting the sound of the main speakers quite badly. When the sub was connected, it seemed to suck the life out of the main speakers.

I tried taking the feed for the sub from the volume control and after a buffer but it made no difference.
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Old 27th November 2004, 12:43 PM   #10
Swedish Chef is offline Swedish Chef  Sweden
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Nuuk,

that is exactly the kind of behaviour one could expect (among other bad things). Let's look at it:

Say you have a 50k pot in the GC. The input impedance of the sub amp/crossover is probably in the 10-100k range, typically 22k or 47k. This means that if you have the pot in mid setting (GND->25k->OUT->25k->IN) and thus have a -6 dB attenuation and then connect the sub with a 22k input impedance the level suddenly drops to -10 dB!

But this is simply the beginning of the nightmare...
A typical shielded cable for audio has a connector->shield capacitance of about 100 pF/m. So a quite short sub cable of only 3m thus have a capacitance of 300 pF. Together with the pot this will form a textbook lowpass filter thus rolling off the highs. Let's again assume the pot is in mid setting. The corner frequency (-3 dB) will thus with a 50k pot be:

fc= 1/(2*pi*R*C) where R=12.5 kohm and C=300 pF (R will actually be dependant on the input impedance of the sub too)

fc=42 kHz which is out of the audio band but one can still expect a very slight rolloff in the top octave. Using a 100k pot and 5m of cable the corner frequency will be well into the audio band. But things get even worse! We have so far only considered the pot in mid setting. In reality the corner frequency will depend on the pot setting being very high near the extremes and dropping considerably when approaching mid level. We have created an EQ that is dependant on volume setting!

The cable will also be a perfect antenna picking up and injecting hum and all kinds of RFI under the stars into the high-impedance node (input of the GC). None of the GC designs I have seen so far even has an RFI filter on the (low impedance) input and this approach is just begging for trouble!

The conclusion is that this is a very bad way of doing things. Cables for analog audio should always be driven by a low-impedance output (unless 20 km long or so). The best way by far is to use a balanced interconnection as done in all pro sound gear.

On the other hand you might enjoy the "high end" approach where you can tune the response of the system by using different cables, orienting the components differently or reorienting the planets.

Feel free to choose your path!

Cheers
/Magnus
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