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Author Topic: "J20" (K-20X Preamp, only OD and with 071 instead of 072)  (Read 26945 times)

phatt

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #15 on: December 09, 2010, 06:39:48 AM »
Hi folks,
I found this led voltages listed down the page. :tu:

http://www.kpsec.freeuk.com/components/led.htm

I used little Green Leds for testing Joes circuit but I doubt it makes a lot of difference.

I tend to find the debate about the pros and cons of small details like this is hardly ever going to be noticed in live situation.

Quite frankly a lot of these opamp type OD tricks are much the same,,, in the end whether you limit the swing with a pair (or more) of diodes or let the signal slam into the supply rails it often sounds much the same.

It's more circuit design and other tone shaping tricks that tend to make the difference.

I even tried some very high voltage diodes once but I think the fv drop was likely much bigger that the 9 Volt battery the circuit ran from.  :duh
But it was all good fun to experiment, even though I was a clueless noob at the time. :lmao:
Phil.



joecool85

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #16 on: December 09, 2010, 09:14:22 AM »
Excellent info as always fellas.  I appreciate it.  I'll probably test my LED in my K20-X to see what colors it is (apply voltage to that part of the board and watch them light up).  I'd like to keep this is original as possible but don't want to have to go to a special vendor to get parts.
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joecool85

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #17 on: December 21, 2010, 09:40:47 AM »
In the interest of keeping parts count low, I might try Juan's solution of using the Dist+ as the base circuit, swapping in a TL071, using "K20-X" style clipping diodes/caps/resistors etc at the tail, and then a BMP style tone control.
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joecool85

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #18 on: December 29, 2010, 11:19:09 AM »
So yes it could work as a pedal if you change it to a single supply and add a reference voltage.

I'm interested in doing this so I can run the circuit as a stock full fledged K20-X preamp on 9v or maybe 12v single supply.  How would I go about changing it to a single supply with a reference voltage?  Is it as simple as putting th V- pin to ground and V+ to 9-12v DC positive?  What exactly is a "reference voltage"?
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teemuk

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #19 on: December 29, 2010, 11:57:22 AM »
It's pretty simple. really. To convert from bipolar power supply to a single-sided one all you need to do is to provide the OpAmps a reference voltage that's half of the rail-to-rail voltage.

I know you've seen this setup before numerous times.


Added components are circled in red. Basically the only additions you'll only need are the voltage divider and filter for generatingthe reference voltage and one coupling cap.

Essentially, your biggest problem is not the conversion from bipolar to single-ended power supply, but that you try to decrease the rail-to-rail voltage from about 30V to only 9V. Basically, doing that will cut about 2/3 rd's of the headroom. Since the clipping of this particular circuit relies on the somewhat high forward voltage of the LEDs the result may end up being that at high levels of overdrive the OpAmps also start clipping.

I think you might have to add in some kind of DC-DC converter that generates 30VDC from a 9VDC battery supply.

joecool85

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #20 on: December 29, 2010, 12:06:53 PM »
Thanks for the info Teemu, and you are right, I have seen that before but just didn't "get it".

If I was to keep a dual power supply, say two 9v batteries, could I keep the schematic the same as the original and just have ground be the intersection of the two batteries, V+ the postive and V- the negative?  If I am correct this would get rid of the need for a VREF as well as increase headroom substantially over using just a single 9v.

V+(+9v/-9v) GROUND (+9v/-9v+) V-
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teemuk

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #21 on: December 29, 2010, 03:45:51 PM »
I didn't really understand how you meant to connect the batteries, but yes, series connection of two of them, with the node where the terminals connect being the ground would work perfectly in that application.

VCC +[:::::] GND +[:::::] VEE

I hope you understand the crude "diagram" above.

You have 9+9=18 volts affecting between the positive and negative terminals of the battery string. Since the mid point is tied to ground reference (zero volts) the potential read from positive terminal to ground is +9V and the potential read from negative terminal to ground is
-9V. So, you have a bipolar supply.
« Last Edit: December 29, 2010, 03:53:09 PM by teemuk »

joecool85

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #22 on: December 29, 2010, 04:05:00 PM »
I didn't really understand how you meant to connect the batteries, but yes, series connection of two of them, with the node where the terminals connect being the ground would work perfectly in that application.

VCC +[:::::] GND +[:::::] VEE

I hope you understand the crude "diagram" above.

You have 9+9=18 volts affecting between the positive and negative terminals of the battery string. Since the mid point is tied to ground reference (zero volts) the potential read from positive terminal to ground is +9V and the potential read from negative terminal to ground is
-9V. So, you have a bipolar supply.

Your crude drawing depicts the same thing as mine, albeit a bit easier to read.  I think I'll try it this way.  I don't mind using two batteries in it.
Life is what you make it.
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joecool85

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #23 on: January 05, 2011, 10:40:54 AM »
About the Vref:

I understand the 10k resistors are electrically in series, and the point between makes half the original supply voltage - that makes sense.

Two questions though:

Wouldn't it make sense to use 100k resistors in stead of 10k?  If I'm correct, with 10k we are bleeding out 4mw of power whereas with 100k we would only bleed out .4mw of power.

Second, I'm assuming the 22uF cap was added as a supply line filter.  Why is this necessary if we already have a filter at the 9v + line and how is this 22uF value calculated?
Life is what you make it.
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J M Fahey

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #24 on: January 05, 2011, 03:41:24 PM »
Just convenience.
You could use 1M if necessary (as in Jerry GarcĂ­a's guitar preamp), but probably any current "eaten" by Op Amps or whatever you reference to that point will make it change, (usually going lower) and losing its symmetric status.
10K is a low enough value as to ignore any sensible load.
Remember this has been in use for ages, before Fet input Op Amps.
22uF is a generic value; 100uF would be better and 1000 or 2200 uF much more so, only they start being on the bulky side.
Remember that your problem is not only having no ripple there, but being an *audio ground* reference for many gain circuits. In fact some people use an "as large as possible" electrolytic, paralleled with a .1 ceramic or similar.
No, it does not need to be a SoZo, Mustard, Orange Drop or Silver Mica one.
No Mojo here.

joecool85

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #25 on: January 05, 2011, 03:57:55 PM »
Remember that your problem is not only having no ripple there, but being an *audio ground* reference for many gain circuits. In fact some people use an "as large as possible" electrolytic, paralleled with a .1 ceramic or similar.

Could you elaborate on this "audio ground"?  Also, why would having a small .1 or so ceramic cap help?  I've seen it a lot in pedal board PSU designs but never knew why.

Thanks for the excellent explanations.  I've been doing this hobby for 5 years now and know how to do stuff, but not always why.
Life is what you make it.
Still rockin' the Dean Markley K-20X
"New" amp: Fender Frontman 25 DSP (FM25DSP)
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phatt

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #26 on: January 07, 2011, 08:27:32 AM »
Remember that your problem is not only having no ripple there, but being an *audio ground* reference for many gain circuits. In fact some people use an "as large as possible" electrolytic, paralleled with a .1 ceramic or similar.

Could you elaborate on this "audio ground"?  Also, why would having a small .1 or so ceramic cap help?  I've seen it a lot in pedal board PSU designs but never knew why.

Thanks for the excellent explanations.  I've been doing this hobby for 5 years now and know how to do stuff, but not always why.

Hi Joe,
In a nut shell, you are lifting your AC signal *Common* above the DC Ground. you are Floating it at a bias point higher than ground,,  it's biased at a different quisient point.

The really tecky part (Why Capacitor on the bias voltage) I'll leave for the experts to explain but what helped me big time was *Simulation* as you get to see stuff that, done in real time would cost a small fortune in scopes, sig gens and half a dozen DMMs just to take all the readings.

For a hobbist it really opens up a whole new world that you would not see outside of working in a repair shop.

Sims will show you just how the AC signal is moved up and down on the DC line (the Q point).
Quite frankly I would still be messing around with 2 or 3 transistor circuits and wondering why it does not work if it was not for Sims.

Heck the MaxiVerb, the PhAbbTone, The DDC circuit. None of these would not have been possible without the help of simulation software.

I use Circuitmaker (student edition) which is free up to 50 commponents.
I now have 100's of sim files I can call up if I need to reference something.

Now sims are not perfect and Absolutely nothing beats years of experience from qualified teck folk. (You know,,those smarty guys that brag about building 10,000 Amplifiers and make there own speakers) :loco ;)
We  hobby types will never be able to reach such dizzying heights,,  :'(
But as a hobby tool sims will save you 100's of hours of building waste of time circuits.
Just think of the solder you will save. LOL.
----
Ed,,, whoops Did I  mention Circuitmaker student is *FREE* ?
http://my.ece.ucsb.edu/bobsclass/2C/Simulation/circuit_maker.htm

I think this still works if not just google it I'm quite sure it's floating about just waiting to save you from hard labour.  Oh yes it even has some Valves to play around with. :tu:
Cheers, Phil.

« Last Edit: January 07, 2011, 08:35:36 AM by phatt »

DJPhil

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #27 on: January 07, 2011, 01:30:19 PM »
Could you elaborate on this "audio ground"?  Also, why would having a small .1 or so ceramic cap help?  I've seen it a lot in pedal board PSU designs but never knew why.
Audio ground means the same as 'virtual ground'. In a single supply system it sets a point near or at the middle of the supply for the AC audio signal to ride on. Because it will have to source and sink (a little) current you want it to have a very low impedance. If it were high impedance then the ground would follow the signal as it moved up and down, anywhere from slightly (causing frequency dependent attenuation) to horribly (almost no signal and badly distorted). The lower the impedance the better the virtual ground's compliance as a (sort of) regulator.
The choice of resistors is a balance between getting that low impedance and wasting lots of power on smoking hot ten ohm resistors. 10k is a nice middle ground (sorry for the pun) and is often used. The 22uF capacitor and it's 100nF friend help keep noise off of the virtual ground by providing it a low impedance path to ground. They serve a similar purpose to the bypass capacitors in a voltage regulator, and help insure that the virtual ground is as flat and stable a DC value as possible. The extra smaller cap helps with higher frequency noise and RF interference in a manner I can't easily describe. This is where joecool85's simulations come in very handy, otherwise you find yourself doing the hard math.
This all becomes academic if you choose to use a 2x9V battery setup, as you can leave the circuit as it was originally. The virtual ground link above has some additional details on various methods that go beyond a simple voltage divider if you like.

Regarding simulation, I'm a fan of LTSpice (also free), though it took me a few weeks of tinkering to get used to. I'd recommend playing around with a few of the free ones out there and seeing what works for you. Simulators shouldn't be trusted for everything, but they are indispensable for quickly working out simple things like the above. Their ability to graph output gives you access to the sort of information you might not have the equipment to see in practice (oscope, spectrum analyzer, etc.).

I hope that helps some. As usual, always double check my work against the pros to be sure. :)

joecool85

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #28 on: January 07, 2011, 02:16:45 PM »
You guys always have the best answers.  I will probably build a full K-20X preamp with clean and OD and run it off a "normal" transformer/PSU board situation, maybe only as little as +/- 6v though depending on what I can find for a transformer cheap.  Think it would sound ok with such little voltage?

Eventually I will get around to playing with the circuit and building a TL071 Distortion Plus with K-20X style clipping and maybe some other mods.

**edit**
Also, I'm thinking I may eventually build this as a pedal with two 9v batteries and have two stomp switches.  One for bypass and one for clean/OD circuits.  It'd be pretty cool I think.  I might try it with a single 9v at some point though, it is a strong point running on just 1 battery.
« Last Edit: January 08, 2011, 08:55:48 PM by joecool85 »
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"New" amp: Fender Frontman 25 DSP (FM25DSP)
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joecool85

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Re: "J20" (K-20X Preamp, only OD and with 071 instead of 072)
« Reply #29 on: January 11, 2011, 08:17:22 AM »
Quick question.  On the schematic it shows C19's value as "223".  Does this mean 223pF?  223nF?  And what about C7's value of 333M?  I know it can't mean 333mF because that would be a huge electrolytic.

All the capacitors in question are brown film-type caps and regardless of the M or not they are all about the same physical size.
Life is what you make it.
Still rockin' the Dean Markley K-20X
"New" amp: Fender Frontman 25 DSP (FM25DSP)
thatraymond.com

 

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