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Author Topic: the K15  (Read 11673 times)

teemuk

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Re: the K15
« Reply #15 on: September 24, 2007, 07:15:17 PM »
Let's see if I can answer some of your questions...

1. The input stage: Some likely needs for bandpass here are reducing RF interference and AC coupling the input in case the source happens to have any DC offset. Rolling off some bass and high-freq outside the frequency range of guitar is also very usable.
2. I suppose the value was chosen mainly to suppress oscillations. Lower corner frequency undoubtedly works as well.
3. Yes, the switches are on at the same time. It's a dual switch.
4. In non-inverting opamp configuration the DC offset is lowest when DC resistance from non-inverting input to ground equals the resistance of feedback resistor (R12=R13). However, without the capacitor there is an alternative DC path to ground through the tone control circuit. This is in parallel with R12 and since the tone control circuit's impedance is rather low it imposes a considerable effect. A capacitor can break this path. If you wish to get rid of the output cap then adding this coupling cap may become a considerable tweak.

The unity gain issue and C3 & R5. No, this can't be modified but you can add a 100K series resistor to VR1, which limits the gain drop to unity when VR1 is turned to zero ohms. However, unlike the drive channel (which already has this limiter) the clean channel has no individual volume control. I suppose you could (similarly to drive channel's arangement) add one to "upper" signal path that goes to SW1b.

Unfortunately I can't take credit for the idea of different color LEDs. It' just the inherent nature of the dual LED package or "bi-color LED". The following is a direct quote from wikipedia:

"Bicolor LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red and green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity."

It is a pretty nice invention because a digital control signal (either 0V or 5V) can easily control the color of a "single" LED (see attachment). Nice way to tell when things work (green) and when they don't (red) using a single indicator. The schematic displays the component a bit strangely (it should have three pins) but anyway, I suppose you got the idea.

benzer

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Re: the K15
« Reply #16 on: September 25, 2007, 08:15:30 AM »
Quote
However, unlike the drive channel (which already has this limiter) the clean channel has no individual volume control. I suppose you could (similarly to drive channel's arangement) add one to "upper" signal path that goes to SW1b.
isnt that VR1 is supposed to do? its a gain modifier for the clean sound so ???

Quote
The schematic displays the component a bit strangely (it should have three pins)
well its 3pins but 2 of em are connected together and to ground,,right?

onething more is that the tone control itself ac couple its input with its output,there is just no dc path . so is it the output cap put there for the dc offset caused by the power chip?
and also wuts the cool thing about having R14 R16?some idea behind having the C12 inside the feedback loop.!!?
Good OL' BeN

teemuk

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Re: the K15
« Reply #17 on: September 25, 2007, 09:28:33 AM »
isnt that VR1 is supposed to do? its a gain modifier for the clean sound so ???

Exactly, and turning that potentiometer to less than 100K resistance (with "clean" channel selected) turns that stage into an attenuator (VR1 < R5). Since it's logarithmic any (clean channel) volume setting less than "5" runs the stage as an attenuator. Anyway, I think this issue getting a bit too much attention. You can try that configuration and if it oscillates then just try it with a different opamp (which may cope better with this circuit). The bottom line is that you should get it to work but personally I avoid this kind of configuration since alternatives exist.

Quote
and also wuts the cool thing about having R14 R16?some idea behind having the C12 inside the feedback loop.!!?

I already mentioned it. The circuit is explained here:

http://sound.westhost.com/project56.htm

benzer

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Re: the K15
« Reply #18 on: September 25, 2007, 06:22:45 PM »
some important q :
how the heck can we calculate the powerchip voltage gain?
its a lil complicated thing to do.
ill try to get its transfer function n plot it by matlab but it doesnt look easy

and is c11 there to attenuate high freq signals?
Good OL' BeN

J M Fahey

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Re: the K15
« Reply #19 on: June 06, 2008, 01:07:53 AM »
K-15´s are great little amps. They sound specially good when coupled with a somewhat larger 10" or even 12" guitar speaker. What saves the distortion, which "should" be buzzy, is that it has no Master control, so when you distort it you´re also clipping the output stage, which clips the buzzy peaks; besides you can´t distort without putting out at least 8/10 watts.
Some answers :
1)R1 suppresses RF interference by a)working with the OpAmp´s input capacitance and b)"Killing" or at least degrading the input junction diode  which would have demodulated parasite RF.
2)473=47+"three zeros"=47000 pF=.047 uF
22/16= 22uFx16V
3)Clean gain= pot value/33k or approx. 250/33=8x  max.
Dirty gain: 250/1.5=166 ; besides, because of the diodes, the clipping peak voltage lowers from almost 15v to around 700 mV .
4)Log pots preferred (same as Fender uses)
5)I think C19 is a drawing error, maybe carried over from an earlier single supply version. It iss not necessary and can actually harm the sound.
6)R9/10 and C10/11 lower slightly and filter the preamp supply.
Good luck with your project.
PS: the distortion footswith is very crude but actually works; remember to isolate it from ground and use a screened cable, with the screen connected to C5.
PPS:I almost forgot: the gain of the output chip is =(5100/180)+1= around 30. Part of the magic is that the distorted signal (which is attenuated by the tone control) barely drives the power amp; the mix of preamp (buzzy) and power amp (flat and boring) distortions gets interesting: smoother than one and sharper than the other.
« Last Edit: June 06, 2008, 01:15:38 AM by J M Fahey »