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Author Topic: Serious Blocking Distortion  (Read 62476 times)

Littlewyan

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Re: Serious Blocking Distortion
« Reply #105 on: August 07, 2013, 06:09:46 PM »
I have just changed the jfet source cap to .68uF which has made the amp sound better, as for the distortion I'll have to take it to my next band practice so I can crank it. May try the MPF102 as I'm sure I read somewhere that its a bit more warm sounding, then again different circuits will sound different etc. I'm having fun experimenting though!

Littlewyan

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Re: Serious Blocking Distortion
« Reply #106 on: August 07, 2013, 06:13:21 PM »
Also has anyone ever done what this guy has done with the two zeners? He says its to protect the JFET from blowing.

http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp3.htm

Roly

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Re: Serious Blocking Distortion
« Reply #107 on: August 08, 2013, 12:25:43 PM »
Looking at the emitter of the second stage you're starting to get some serious tweekology going there.

I use MPF102's because I've got them in stock, they are cheap, available, and come in a tasteful black ... but not for their thermal qualities, which I would frankly take with a fairly large grain of NaCl.

Back-to-back zeners across the input will certainly give the FET some protection, however they will only do so if there is some resistance in series with the input.  An equally effective way of protecting a FET input is with a couple of common-or-garden signal diodes such as 1N914's.  One is connected between ground and gate with the cathode (band) towards the gate, and the other between the gate and the supply, again with the diode pointing northwards with its cathode connected to the supply.

In either case there must be a resistor in series with the input, say 10k to 68k, to limit any surge current through the protection diodes or you will simply have a shorted protection diode instead of a shorted FET, but it will still stop the show.
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 #108 on: August 08, 2013, 01:07:19 PM »
I'll probably use the zener option as I know I can definitely get hold of a few 5.1V Zeners.


Roly

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Re: Serious Blocking Distortion
« Reply #109 on: August 09, 2013, 10:41:21 AM »
Fine, but be mindful of what I say about including a resistor in series with the input to limit any overdrive current through the zeners.
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 #110 on: August 09, 2013, 12:58:55 PM »
I've already got  a 33K resistor in series with the input before the Zener Diodes so should be ok. I ran it through LTSpice to make sure, great program that is!

Littlewyan

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Re: Serious Blocking Distortion
« Reply #111 on: August 13, 2013, 05:44:00 PM »
Here is the amp as it stands. I have added the two zener diodes to protect the JFET and increased the voltage from the PSU to 30volts to increase the volume. I may change the JFET source resistor to 5K6 to lower the gain slightly, which may also give me a fuller tone. Do you think I will benefit at all from changing the 10K Pot to a 100K Pot? I should see an increase in volume shouldn't I? Actually I think the preamp will just be stupidly overdriven.
« Last Edit: August 13, 2013, 05:58:24 PM by Littlewyan »

Kaz Kylheku

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Re: Serious Blocking Distortion
« Reply #112 on: August 13, 2013, 11:38:05 PM »
Here is the amp as it stands. I have added the two zener diodes to protect the JFET and increased the voltage from the PSU to 30volts to increase the volume. I may change the JFET source resistor to 5K6 to lower the gain slightly, which may also give me a fuller tone. Do you think I will benefit at all from changing the 10K Pot to a 100K Pot? I should see an increase in volume shouldn't I? Actually I think the preamp will just be stupidly overdriven.

I would say that the thing to do is to revise the impedances so as not to have to use these ridiculous 22 uF capacitors in the signal path.

They are actually already too large.  You have a 22 kOhm impedance into the TDA stage input. If the coupling capacitor is 22 uF, the -3dB frequency is 0.33 Hz, which means you have nearly flat frequency response down to 3.3 Hz (10X the -3dB frequency: this is a rule of thumb).

That is ridiculous even for hi-fi, let alone guitar.
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Littlewyan

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Re: Serious Blocking Distortion
« Reply #113 on: August 14, 2013, 03:17:31 AM »
Good point Kaz, I didn't realise they affected the tone that much, I thought as the Source and Emitter capacitors were 0.68uF it shouldn't matter. I'll change these tomorrow night for 0.68uF capacitors as I have plenty of them. Unless you think 1uF would be better?

J M Fahey

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Re: Serious Blocking Distortion
« Reply #114 on: August 14, 2013, 06:01:44 AM »
Quote
I would say that the thing to do is to revise the impedances so as not to have to use these ridiculous 22 uF capacitors in the signal path.
Agree.
I suggested you realistic capacitor values some 10 posts back, yet you keep those ridiculously large ones ... then complain about blocking distortion  ::)

FWIW "Magic" has no place in Electronics.
Using ".68uF because Marshall uses them" is nonsense, the proper parameter is the RC value.
If you concentrate on C but ignore R, you will never solve the problem, because alone it's meaningless.
All value corrections I posted cut useless bass below, say, 60Hz or thereabouts.
Having 0.33Hz bandwidth, as Kaz Kylheku pointed, is musically useless and can only lead to trouble.

Littlewyan

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Re: Serious Blocking Distortion
« Reply #115 on: August 14, 2013, 06:46:57 AM »
You did JM but I was too naive to use them. I'm still learning  ::)

Also I'm not using 0.68uF because Marshall uses them, I went to 0.68uF as 1uF was still too much and as I have about 20 of these capacitors now I may as well use some as coupling capacitors.

Apologies JM I thought I remembered you used 0.1uF as the coupling capacitors in your modded version of my pre amp. I shall use 1uF capacitors instead of 0.68uF. What I shall do is first change the coupling capacitors and then move onto the others you've specified. I like to hear the tonal differences as I change these things :)

Again also I agree there is no magic in electronics.

I shall make changes to the circuit tomorrow night and will post the results. Once I'm happy with the circuit I'll see if I can create some audio clips and take some photos.
« Last Edit: August 14, 2013, 02:53:24 PM by Littlewyan »

Littlewyan

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Re: Serious Blocking Distortion
« Reply #116 on: August 14, 2013, 02:42:34 PM »
On LTSpice when I run a simulation using the AC Analysis, I get these two lines. The solid line I believe shows how the volume increases with the frequency, however I cannot work out what the dotted line is?

Kaz Kylheku

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Re: Serious Blocking Distortion
« Reply #117 on: August 14, 2013, 04:58:54 PM »
On LTSpice when I run a simulation using the AC Analysis, I get these two lines. The solid line I believe shows how the volume increases with the frequency, however I cannot work out what the dotted line is?

Note how the vertical units on the far right of the LTSpice plot are degrees.

The dotted line gives the phase shift.   Frequency response is a complex number: it is two dimensional. Not only does it have amplitude, but it has phase.

Signal processing devices can change the amplitude in a frequency dependent way, but also shift the phase in a frequency dependent way, and the two are inter-related.

So frequency response has two dimensions, and this is why the output of the Fourier transform is in the two-dimensional doman of the complex numbers (even though the input is a real-valued time-domain signal).

« Last Edit: August 14, 2013, 06:22:02 PM by Kaz Kylheku »
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Roly

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Re: Serious Blocking Distortion
« Reply #118 on: August 14, 2013, 10:38:50 PM »
{ooooh boy, here we go ...}   ;)
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 #119 on: August 15, 2013, 02:24:21 AM »
Haha Roly :P

Ok i've done some reading and am i right in saying the Phase Shift is basically a change in the audio signal? Like a delay perhaps? And that its for mixing two signals together? Which for a guitar would be the original note that was picked and then the harmonics that come after?
« Last Edit: August 15, 2013, 02:33:58 AM by Littlewyan »