Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Please login or register.

Login with username, password and session length
Advanced search  

News:

SSGuitar has moved to PHP 7.1, enjoy the speed!

collapse
Pages: 1 [2]   Go Down

Author Topic: Biasing opamps in a Marshall Lead 12  (Read 837 times)

dazz

  • Master SSGuitarist
  • ****
  • Chip Points: 0
  • Posts: 85
    • View Profile
Re: Biasing opamps in a Marshall Lead 12
« Reply #15 on: July 10, 2018, 10:12:10 AM »

Err??
It may pass signal but it still not right,
Pin 5 has no bias  :o
R9 needs to go to pin5


Damn it, I put that coupling cap just to screw the bias. Noobs will be noobs.

There are other problems as well which are a bit hard to explain.
Some circuits are hard to convert to single supply and you will have to alter more than R9.
I'll see if I can redraw it for you in the next day or two.

Phil.

I'm not sure I feel comfortable with having you do all the work for me, Phil. You know I appreciate your help immensely, but I think I need to figure it out myself as a part of my learning process. Maybe if you can simply name what those other problems are I can then research them and find solutions I can later post for you to evaluate?

Quote
Opamp Rule of thumb; The positive input has to have a DC path to a reference voltage for single supply or ground if it's split supply.
Phil.

That's something I sort of inferred from the way you biased the Casino 12, and that's the way it was supposed to be, but I messed it up with the wrong placement of C7

This should do the trick, right?
« Last Edit: July 10, 2018, 10:13:24 AM by dazz »
Logged

phatt

  • Legendary
  • ******
  • Chip Points: 217
  • Posts: 1884
    • View Profile
Re: Biasing opamps in a Marshall Lead 12
« Reply #16 on: July 11, 2018, 02:00:34 AM »

Was a late night after a gig,,, but now that you moved R9 it looks like that may work.

Other options;
Delete C5, connect wiper of 22k pot and C6 to "Vref" instead of Ground then you can delete R9 & C7.
The output of IC1A is already at Vref which then deletes the need to isolate IC1B.
I'm fairly sure that will work.
And less parts ;)

I always breadboard circuits before committing to a pcb as there is always some little bug. :grr
Phil.
Logged

dazz

  • Master SSGuitarist
  • ****
  • Chip Points: 0
  • Posts: 85
    • View Profile
Re: Biasing opamps in a Marshall Lead 12
« Reply #17 on: July 11, 2018, 05:36:50 AM »

Was a late night after a gig,,, but now that you moved R9 it looks like that may work.

Other options;
Delete C5, connect wiper of 22k pot and C6 to "Vref" instead of Ground then you can delete R9 & C7.
The output of IC1A is already at Vref which then deletes the need to isolate IC1B.
I'm fairly sure that will work.
And less parts ;)

I always breadboard circuits before committing to a pcb as there is always some little bug. :grr
Phil.

Awesome! thanks Phil.
You know what's weird? That's exactly what I had initially in mind! (see first post attachments) and sure enough, the simulation seems to confirm it works.

So the only problem I had was the reversed opamp inputs in LTspice. unfriggingbelievable.. that crap send got me trying alternatives until I figured it out, then it didn't occur to me the other tweaks might not be necessary. I'm not sure if I should get a sense of accomplishment or feel more stupid. LOL

Re: breadboarding, I tried that for the first stage of the preamp, and it worked, but I wasn't sure how it was supposed to sound so decided to rely on the simulation in what seemed a more "scientific" way of making sure the freq response was equivalent to the original circuit. I've found breadboarding to be impractical for circuits containing more than a few components (I also need to get better in that department) . In fact this project is nothing I plan on building, it was just something that I'm using as a sandbox project to learn LTSpice & Eagle enough to build a proper PCB for your Casino 12, as the veroboard I used ended up being a complete mess with all the modifications, trial and errors, desoldering and resoldering stuff on top of the board... it's fugly as hell and I want to do justice to it, haha

But anyway, there's still something unclear to me. It's the part where you say

Quote
The output of IC1A is already at Vref which then deletes the need to isolate IC1B.

Seems to me the circuit I posted on July 10, 2018, 04:12:10 PM also has it's output at Vref... yet it seemed to need that isolating cap... I'll see what I'm missing now because there must be something different in those circuits cause it now works fine in LTspice without that cap.

EDIT: no! it only needed C1 to isolate IC1A's input, hence having it's output at Vref through R4. I think I get it now

One thing I really need to do is to figure out how to simulate my circuits from Eagle, because copying it to LTSpice and back will certainly introduce errors in the PCB
« Last Edit: July 11, 2018, 05:42:03 AM by dazz »
Logged

phatt

  • Legendary
  • ******
  • Chip Points: 217
  • Posts: 1884
    • View Profile
Re: Biasing opamps in a Marshall Lead 12
« Reply #18 on: July 11, 2018, 10:18:17 PM »

I would just keep bread boarding because you will learn so much faster as you will clearly see (make that hear) where and how noise issues, psu filtering and ground paths can make a big difference to the build.

I use sims as well as real working circuits before I commit to a pcb.
Note the picture of my very messy bread board.  :)
This is a compressor on the front and a cab sim circuit on the back. The small switches are just to by pass so I can hear the difference.
I built this last month for a mate as he was amazed at the presence of my guitar rig.
He noted that I hardly touch the strings but still have lots of punch where he has to belt his to get any volume, The classic muddy sound you get without good EQ.
I explained that a lot of guitar rigs have way too much bandwidth and although this makes a clean strum of open chords sound bigger it will trash a full bore distorted guitar tone.
Distortion only works well if you limit the B'width.

Distortion is the easy part,, getting the right tone shape is where the magic comes from.
Take Santana as example 90% of his tone is all under 1kHz.
But he is using HB pu's so with SC pu's you need a higher roll off.
If you are into HMetal tones you will want a different tone shape but the distortion mechanisms are often much the same.

Re sims and eagle;
I don't know much about transferring Schematic's from Eagle to LT spice but I believe some platforms can transfer spice files.
Too much stuffing around for me I just print out the schematic from Circuit-maker then read off that into KiCad.
This also forces you to actually read and understand the circuit.

Re the DC thing;
Your sims should have a probe setting to read DC at any part of the schematic. If you DC probe the output of U1A it will be very close to Vref. Then switch to AC and you will see that the Zero crossing of the AC signal is centered on the Vref.

Keep going as it does take a while to find your way and if you think you are silly spare a thought for me,, that last build I made,,I messed up in a similar way,,
I linked a resistor to Ground instead of Vref on the pcb which made the volume pot very scratchy. Lucky it was a simple fix but you do feel like banging your head against the wall. :duh
Phil.
Logged

dazz

  • Master SSGuitarist
  • ****
  • Chip Points: 0
  • Posts: 85
    • View Profile
Re: Biasing opamps in a Marshall Lead 12
« Reply #19 on: July 12, 2018, 06:40:52 AM »

I would just keep bread boarding because you will learn so much faster as you will clearly see (make that hear) where and how noise issues, psu filtering and ground paths can make a big difference to the build.

Understood. I'm just about to order a larger breadboard, that should help a bit

I use sims as well as real working circuits before I commit to a pcb.
Note the picture of my very messy bread board.  :)
This is a compressor on the front and a cab sim circuit on the back. The small switches are just to by pass so I can hear the difference.
I built this last month for a mate as he was amazed at the presence of my guitar rig.
He noted that I hardly touch the strings but still have lots of punch where he has to belt his to get any volume, The classic muddy sound you get without good EQ.
I explained that a lot of guitar rigs have way too much bandwidth and although this makes a clean strum of open chords sound bigger it will trash a full bore distorted guitar tone.
Distortion only works well if you limit the B'width.

Distortion is the easy part,, getting the right tone shape is where the magic comes from.
Take Santana as example 90% of his tone is all under 1kHz.
But he is using HB pu's so with SC pu's you need a higher roll off.

I think I know where you're coming from. You mentioned the importance of limiting the bandwidth in the Casino 12 thread too (filter lows to avoid muddy bass with weak power supplies, and of course highs, that I guess also helps with picking up interference) and that Lead 12 preamp is for the most part unfiltered for all I can tell (although it's missing the tonestack). I will definitely keep all that in mind for my future projects though. THis one, I don't care, someone else asked me to prototype it and I simply took the opportunity to cut my teeth on the software, and learn something about biasing opamps. So yeah, my next reading will be on tone shaping networks, filtering techniques, etc. As always, thanks for the info

Re sims and eagle;
I don't know much about transferring Schematic's from Eagle to LT spice but I believe some platforms can transfer spice files.
Too much stuffing around for me I just print out the schematic from Circuit-maker then read off that into KiCad.
This also forces you to actually read and understand the circuit.

Well, it's not so much about transferring schematics, Eagle Autodesk is supposed to have an LTSpice pluggin to simulate the schematic used to build the pcb. It just seems convenient to me to have it all integrated in a single software package. I'm not sure I want to use a third piece of software to transfer schematics from app A to app B which I'll have to review anyway and adds another potential point of failure. So if I can't get the LTSpice integration to work properly in Eagle, I'll do as you do and manually transfer de schematic myself.

Your sims should have a probe setting to read DC at any part of the schematic. If you DC probe the output of U1A it will be very close to Vref. Then switch to AC and you will see that the Zero crossing of the AC signal is centered on the Vref

Yeah, works exactly like that :cheesy:

Keep going as it does take a while to find your way

I will, thanks for your input and encouragement, Phil  :dbtu:
« Last Edit: July 12, 2018, 07:00:22 AM by dazz »
Logged
Pages: 1 [2]   Go Up
 

* User Controls
 
 
Welcome, Guest. Please login or register.
Did you miss your activation email?
* Recent Posts
SWR 550X Bass Amp Trouble by Enzo
[Today at 02:00:52 AM]


Looking for information about Prince guitar amps - make, not artist. by bumblearse
[September 18, 2018, 05:25:53 PM]


Power on LED. by flester
[September 18, 2018, 05:02:35 AM]


Adding Bluetooth to lm386amp by blackcorvo
[September 16, 2018, 12:17:27 AM]


Marshall 5203 by THRobinson
[September 13, 2018, 04:01:39 PM]

* Sponsors