Welcome, Guest. Please login or register.

July 30, 2021, 04:13:05 AM

Login with username, password and session length

Recent Posts

 

Author Topic: Converting jFet overdrive circuit from 9 to 18V  (Read 16941 times)

Alexius II

  • Master
  • ***
  • Posts: 72
  • Chip Points: 6
    • View Profile
Converting jFet overdrive circuit from 9 to 18V
« on: March 21, 2011, 09:41:08 AM »
Hi,

I'm building a small amp (head) powered by LM3886. I will be using "chipamp.com" PCBs for the power amp and power supply. However, for the preamp I want to use a "marshall flavoured" circuit from a stompbox which I really like: Thor (from runoffgroove.com). I've built a few of them, and they are great (at 9V that is). Between the preamp and power amp I would like to have a full (bass/middle/treble) tone stack.

I would really like to run the preamp at 18V to get more voltage gain out of it - so that the tone stack loss wouldn't be such a problem... and this would hopefully get me a bit further from the noise floor. :) The 18V would be regulated from about 25V (the amps power supply). The preamp schematic is here:



I do understand most of this circuit, but I still need a bit of help, if anyone would be so kind.

The first stage is a common source amplifier (looks like a fetzer valve). Second stage is again a common source amp. With double voltage, I would have to re-set both of their drain resistors/trimpots. If that much gain would result in too much distortion, I would use lower gain jFets and source/drain resistors would be replaced accordingly.

So far so good... but now comes the part I don't understand: the Mu amp  ???
I've searched the internet and couldn't find any understandable (for me) explanation on how is this thing biased and how are some values chosen (for example 390r on source). Is this thing supposed to work with higher voltages without modifications, or do i have to change anything? I suppose higher voltage would give me greater headroom?

After that you can see an opamp biased at half the supply. I doubt this part of the circuit needs any changes. Maybe I could use the other half of the opamp as an buffer, to help drive the tone stach that will follow?

Any help would be much appreciated! :tu:

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #1 on: March 21, 2011, 03:38:31 PM »
Don't worry, the mu stage and the Op Amp are correctly biased from the voltage divider (that's real biasing); the "fet-tubes"are not biased (you already know that's what I don't like from the ROG/GGG approach) and so the absolutely unpredictable drain voltage *must* be individually trimmed.
Oh well.

Alexius II

  • Master
  • ***
  • Posts: 72
  • Chip Points: 6
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #2 on: March 21, 2011, 04:55:00 PM »
So the only thing I have to change is to re-set the drain voltage of first two stages? That is great to hear!  :D

I'm away from home until friday, so I won't be able to try it out until then. I will report back on how it sounds  :tu:

By the way, how would one change (lower) the gain of the mu stage, if lower gain would be required? I found the "original" mu-amp here, but this does not help my understanding of this stage. I can see that ROG added 390R source resistors, somewhat changed the "bias" resistors (two 10k and a 1M instead of two 10M) and the input resistor is changed from 10M to 220k. Does someone have a simple explanation of this configuration?

mensur

  • Master
  • ***
  • Posts: 94
  • Chip Points: 5
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #3 on: March 21, 2011, 07:45:20 PM »
So the only thing I have to change is to re-set the drain voltage of first two stages? That is great to hear!  :D

I'm away from home until friday, so I won't be able to try it out until then. I will report back on how it sounds  :tu:

By the way, how would one change (lower) the gain of the mu stage, if lower gain would be required? I found the "original" mu-amp here, but this does not help my understanding of this stage. I can see that ROG added 390R source resistors, somewhat changed the "bias" resistors (two 10k and a 1M instead of two 10M) and the input resistor is changed from 10M to 220k. Does someone have a simple explanation of this configuration?
Actually it's a pretty simple stage, you see 390R is stage biasing resistor, the upper fet is current source, meaning(active load), varying potential divider i.e. 10K's you will send less dc voltage on the gate of the upper one, hence, smaller current you will get on the source,
100nF cap is there to determine low freq response of the stage. Here, low freq response is 1.59Hz - 3dB's, which is pretty low, but you can try 22nF, or 2.2nF,just to keep bass tight.
Here's an example, If we have a JFET that has VGS 7V, and maximum current of 12mA, then, we will have 4.5VDC on the gate of the upper FET(9VDC-10K-10K potencial divider gives 4.5VDC), so current on the source will be around 9mA, continuous, so that is our maximum current on the stage, or you can set 1K resistor directly form a supply(9VDC/1K=9mA).

Cheers

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #4 on: March 22, 2011, 12:41:38 AM »
A mu-stage is, by definition, the way to extract *maximum* gain from a gain stage, whatever value it has.

Alexius II

  • Master
  • ***
  • Posts: 72
  • Chip Points: 6
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #5 on: March 22, 2011, 02:30:19 PM »
Thank you both very much!  :tu:

So the 100nF forms a filter with the 1M?
I will try some lower values and see if it changes for the better ;)

Just one more question:
Can anyone help me understanding the "bass boost" circuit? I could not find any similar circuit with at least some explanation. I would really like to have the bass boost engaged at all times, but only if I could somehow lower the intensity. I like the boost, but I would like to reduce it to around half. Is there a simple solution for this?

Alexius II

  • Master
  • ***
  • Posts: 72
  • Chip Points: 6
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #6 on: March 27, 2011, 01:22:34 PM »
Hello again,

I've had some time today to tinker with this "preamp" powered by 18V. I used the original transistors and adjusted the two trimpots accordingly (first one to 2/3 and second one to 1/2 of 18V). The distortion has increased, as predicted. My previous sweet-spot was around 2 o'clock, now It's around 'noon' (log. gain pot). At this setting I did also notice a tiny bit less bass, which I suppose comes from the new position of gain pot (more treble or less bass, I can't realy tell). I like it!  :tu:

Now that I've tested it with my TDA2003, I will build an enclosure and a LM3886 amp to provide some more 'juice' :D

I might also add another input "low gain stage" using MPF102, and make the original and this lower gain switchable (A/B). I could even use double inputs (high/low) as an homage to JCM800 :)

I'll report back!  ;)

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #7 on: March 27, 2011, 01:32:47 PM »
Way to go :tu:
That's the spirit of this Forum: experiment, test, (hopefully) improve, post, share.
Congratulations.

Alexius II

  • Master
  • ***
  • Posts: 72
  • Chip Points: 6
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #8 on: April 10, 2011, 10:09:24 AM »
Thanx!  :)

The amp is coming together VERY slowly. I'm currently ordering LM3886 parts and I haven't even got time to think about enclosure yet. It will probably take me more than a month to finish this thing  ::)  But - while tinkering with the thor circuit for the purpuse of my (pre)amp, I *did* finish another one in the usual form: stompbox.

I recorded a quick sample.
Keep in mind, this is guitar > pedal (gain around 11'o clock) > TDA2003 (no tone control whatsoever) > openback 1x12 (CL80) > center mic (e606) > soundcard. There are two rhytm guitars (bridge humbucker) panned hard left/right. The only added "effect" is a slight echo on the solo guitar (neck humbucker) in the center. Please don't be offended by my sloppy playing  ;D

And this is how it looks:

(Maršal is pronounced same as Marshall in our language :tu:)

I think it already sounds pretty balanced (without any tone control), but I still intend to make mine a bit more flexible with a B/M/T(/P) tonestack.

joecool85

  • SSGuitar Admin
  • Legendary
  • ******
  • Posts: 3409
  • Chip Points: 993
  • SSG Creator
    • View Profile
    • thatraymond.com
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #9 on: April 12, 2011, 08:01:33 AM »
That sounds excellent!  "Quick sample" ??  Makes my recordings look pretty bad lol.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Alexius II

  • Master
  • ***
  • Posts: 72
  • Chip Points: 6
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #10 on: April 13, 2011, 05:44:40 AM »
Hehe, I consider all my samples that start as an idea, and end as a mp3 less than half an hour later, quick  ;D

Now this makes me wonder how would a BSIAB (with the full tone stack) work as a preamp at 18V..
I could easily make this a 2 channel amp if it is as good as thor (probably as a "lead" channel).

I think it is time for me to build a "browny"... and experiment further  :tu:

joecool85

  • SSGuitar Admin
  • Legendary
  • ******
  • Posts: 3409
  • Chip Points: 993
  • SSG Creator
    • View Profile
    • thatraymond.com
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #11 on: April 13, 2011, 03:40:23 PM »
Hehe, I consider all my samples that start as an idea, and end as a mp3 less than half an hour later, quick  ;D

Now this makes me wonder how would a BSIAB (with the full tone stack) work as a preamp at 18V..
I could easily make this a 2 channel amp if it is as good as thor (probably as a "lead" channel).

I think it is time for me to build a "browny"... and experiment further  :tu:

Sounds like a good idea to me.  I'm a big fan of the BSIAB tone I've heard, though I haven't built one myself.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Alexius II

  • Master
  • ***
  • Posts: 72
  • Chip Points: 6
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #12 on: April 24, 2011, 05:51:52 PM »
A small update:
I haven't yet started on a BSIAB... but instead I tried to modify my Dr.Boogey (which I really like) for higher voltages. I'm a metalhead, doh  ;D

For now I didn't change any resistors/caps, but instead played with different types of transistors and biasing. Although I do like it, I will rebuild it using sockets (for resistors/caps) at some points, where I think I could improve the sound (especially source resistors and bypass caps) and experiment further.

Here is another short sample: Dr.Boogey at 15V using a combination of j201, mpf102 and 2n5457 transistors. I did some slight post eq-ing, because I recorded on an open back 1x12 CL80... (this beast loves my vlosed back 4x12 V30)

Until next time..  :tu:

Alexius II

  • Master
  • ***
  • Posts: 72
  • Chip Points: 6
    • View Profile
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #13 on: May 08, 2011, 07:16:16 AM »
Hello again! ("spamming" my own thread :D)

I had a few free days which I used mostly to practice guitar and to play with two of my favourite preamps: Dr.Boogey and Thor. I was just playing around with different voltages, when I had an idea. The Dr.Boogey is basically the preamp section of the real preamp... but the Thor is emulating preamp (common source stages) and the amp itself (mu-amp stage), kind of. So, what I first tried is to connect Thor at the output of the Boogey, but this created just too much distortion. Then I took boogey output and plugged it at the input of the mu-amp stage... and this resulted in a big smile on my mouth  ;D

Now what I have is Dr.Boogey's distortion and tone stack + Thor's mu-amp and output filtering.
This enables me to use less preamp distortion and use a bit of that "mu" juice after the tone stack, and it sounds really good. The tone controls behave much more like a real (hard-driven) amp now. Much less fizz and more agressive treble, plus the thor's "bass boost" does wonders for that deep metal sound. It's currently running at 15V. I'll do a sample next weekend, if anyone is interested  :tu:

This is the combined schematic, which I'm using at the moment: (from gaussmarkov schem.)

joecool85

  • SSGuitar Admin
  • Legendary
  • ******
  • Posts: 3409
  • Chip Points: 993
  • SSG Creator
    • View Profile
    • thatraymond.com
Re: Converting jFet overdrive circuit from 9 to 18V
« Reply #14 on: May 09, 2011, 08:25:37 AM »
Hello again! ("spamming" my own thread :D)

No worries, I like seeing the updates  :tu:
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com