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Author Topic: Serious Blocking Distortion  (Read 66766 times)

Littlewyan

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Re: Serious Blocking Distortion
« Reply #135 on: August 21, 2013, 09:48:05 AM »
No i mean i simulated the tone control LtSpice and didnt get as much drop, obviously at the moment i don't really get any signal drop as its doing sod all at the moment. I might remove the Tone Control and put a switch in it's place to switch between different capacitors on the JFET Source. Need to do a few tests first to see if it'll do what i want it to though.

Littlewyan

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Re: Serious Blocking Distortion
« Reply #136 on: August 21, 2013, 02:16:45 PM »
Ok here is what I propose to do. I'm going to take out the tone control completely and in it's place on the chassis I shall install a SPDT that has three positions. The switch will basically change the JFET Source Capacitor to give me three different 'Channels' if you will. I've attached a picture showing what I plan to do. Not sure about the clean channel at the moment though, wondering whether its best to have that part of the switch not connected to anything to clean up the sound as I imagine the 220uF will make the amp VERY bassy. Also I may need a resistor in series with the input as when I switch to the 0.68uF Cap I imagine it will pop as the capacitor charges up.
« Last Edit: August 22, 2013, 05:20:04 PM by Littlewyan »

stormbringer

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Re: Serious Blocking Distortion
« Reply #137 on: August 23, 2013, 05:01:53 PM »
if you place the tone stack between your jfet and bjt and increase the bjt gain you have your recovery stage done.

Littlewyan

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Re: Serious Blocking Distortion
« Reply #138 on: August 23, 2013, 05:53:31 PM »
I think i want to remove the Tone Control Stormbringer. Do you think what i proposed will work?

stormbringer

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Re: Serious Blocking Distortion
« Reply #139 on: August 24, 2013, 10:20:10 AM »
I think it would work more as a treble/prescense control than gain, you could switch between 2 different source resistors instead and probably get a more satisfying result. The source bypass capacitor helps with the gain as it has a low impedance path to ground, but the size affects frequency rather than gain.

Littlewyan

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Re: Serious Blocking Distortion
« Reply #140 on: August 25, 2013, 06:16:29 PM »
I tried this in LTSpice, starting from 6K2 on the JFET Source and then lowering the resistance as a Pot would and this just seemed to increase distortion. I think I may just put the BJT back how it was minus the Emitter Bypass Capacitor and just keep the tone control where it is. As in this configuration I lose very little gain and the tone control works well. I've attached the circuit I used in LTSpice to test this.

Roly

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Re: Serious Blocking Distortion
« Reply #141 on: August 26, 2013, 12:19:40 PM »
Have a look at the preamp circuit here;

http://www.ssguitar.com/index.php?topic=2912.msg22358#msg22358

Fust point; have a look at the arrangement on the Source of the first FET.  A variation is to use a pot for the Source resistor and connect the cap to the tapping.  These are two ways to alter the AC conditions without changing the DC conditions.

Secnt point; notice the pot declarations;

Rtot=<VAL> wiper=<VARIABLE>

and the parameter value lists

.step param <VARIABLE> list <LISTITEMS>

When you do the simulation run it takes each of the list items in turn and uses them as the variable, building a nest of curves for each list item value, in this case pot position.  You can use all ten 0 0.1 0.2 ... 0.9 1.0 but it gets a bit busy so I normally use just five or six; 0 0.2 0.4 ...
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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Re: Serious Blocking Distortion
« Reply #142 on: August 26, 2013, 01:15:00 PM »
Ah i see now. Shouldn't i use the configuration hes using on his JFET source? I see what you mean about replacing the JFET Source with a pot which would work, until the pot is turned all the way and the cap is effectively shorted. I'm on holiday this week so can't try anything until i'm back on Saturday but will definitely give this a go. Also i've done a bit of reading about valve sounding JFET preamps and it seems the best way to achieve this is by using multiple JFET stages all softly clipping the waveform.

Roly

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Re: Serious Blocking Distortion
« Reply #143 on: August 26, 2013, 01:39:26 PM »
That "he" is actually me, that's my design.

Quote from: Littlewyan
valve sounding JFET preamps

Yes, well, there's a lot of writing out there, and some of it even matches up to reality.  The characteristic slope of a triode valve and a FET are similar but not identical so the distortion they will produce is similar but not identical.

IIRC the slope of a FET is proportional to x2 while the slope of a triode is proportional to x2/3.  In both cases this results in a sinewave becoming slightly peakier on one side and slightly flattened on the other.

When it comes to guitars and distortion, an awful lot has to do with personal taste.
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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Re: Serious Blocking Distortion
« Reply #144 on: August 26, 2013, 02:23:30 PM »
O right sorry didn't see that!

This is very true. When it comes to overdrive/distortion no one can really describe it as it'll be an opinion not necessarily shared by others.

Littlewyan

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Re: Serious Blocking Distortion
« Reply #145 on: September 01, 2013, 06:03:46 PM »
I believe I finally have the amp the way I want it. I tried the gain control Roly but didn't like the way the sound went very bassy when I reduced the gain. So I used a switch to go between two capacitors. I'll attach a schematic to this post. Basically one capacitor gives me less bass and in turn less distortion, whilst the other gives me pretty high distortion. Also the switch I used had a middle position and this just leaves the source resistor by itself without a bypass capacitor.

Roly

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Re: Serious Blocking Distortion
« Reply #146 on: September 02, 2013, 09:12:19 AM »
Cool; you have to try stuff to see how possibilities work out in practice - that's what rolling your own is all about.   ;)

I would make one little suggestion 'tho, if you are getting clicks when you change the switch position it's because the two caps are not staying charged to the same voltage that is on the FET source (and why should they if they aren't connected?)

The fix is quite simple, a couple of high value resistors, say 100k to 1Meg, but at least x10 the Source resistor value, connected across the switch between the Source and each cap.  This will keep them charged to the same voltage even when they are disconnected by the switch, so little or no click on switching.  This little de-clicking trick is worth pasting in your hat for similar situations.

Now you have got there, how about some sound samples?   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

Littlewyan

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Re: Serious Blocking Distortion
« Reply #147 on: September 02, 2013, 01:18:20 PM »
Ah good idea Roly. I was going to ask about the clicking, I thought it may of been due to the capacitors quickly charging up and could be solved by having a small resistor in series with the source and switch, but your solution sounds like it'll actually work :P. I'll implement this fix and will get the sound samples soon as possible. Unfortunately as I still live with my parents and they aren't big fans of loud guitar so I'll have to wait until they're out.

Littlewyan

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Re: Serious Blocking Distortion
« Reply #148 on: September 05, 2013, 02:40:16 PM »
Well guys I created a few sound clips, unfortunately they were a bit rushed and recorded using my iphone. But they should hopefully give you an idea of how the amp sounds. These recordings are only of the .68uF capacitor setting.

Also it no longer pops or clicks when I switch between capacitors, added a 100K resistor between the source and .68uF Cap and a 820k between the source and .22uF cap. Reason for the different resistors is the 820K worked for the .22uF but wasn't enough for the .68uF, so I just changed the .68uF's cap and left the .22uF's as is.
« Last Edit: September 05, 2013, 02:42:06 PM by Littlewyan »

Roly

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Re: Serious Blocking Distortion
« Reply #149 on: September 06, 2013, 11:03:54 AM »
hmmmm ... could it be that your 0.68uF is a bit leaky?  New, or ex-equipt?
If you say theory and practice don't agree you haven't applied enough theory.