Laney GH100TI preamp

[traktop]

<quote author="Sensei Tim">
when the boss is away...

I *think* this is correct.  I used the JCM800 layout from Mark as a starting point.

NOTE: i am not sure if the trimmer on Q4 should be 100k or 200k-ish based on the schematic

(edit)
Awesome!
So you´ve jut emulated both pi triodes and the presence control, right?

Correct me if I´m wrong, but looking at the schematics, I think that volume 3 and one side of the 100k resistor should be one row below so it connects with both Q6 drains maybe? and the presence pot should be wired 3, (above), and 2&1, (to 100n cap to ground)

Another thing should be clearing Treble2-Master3 on notes, and 25k pot for presence knob.

The rest seemed ok to me.

[Sensei Tim]

from the schematic, v4a and v4b sources and drains connect to the presence pot and then to the 100k and then to the volume.

note that "volume" is not the same as "Master Volume"

[Travis]

I noticed while looking at that JCM800 emulation layout that Q5 and Q6 are not an LTP. The schematics at FSB are gone because of photobucket but somebody linked this ROG schem which is similar

https://s3.postimg.org/soiam5qhd/thunderchief.png?dl=1

Notice they kinda simulate the biasing of 1/2 of the PI with Q5, then use Q6 as another gain stage presumably to simulate the power amp. They added the presence network between Q5 and Q6 but without a pot.

[Sensei Tim]

Well i've got the parts on order for this. Time to see if it'll work.  I'll report back in a bit with results.

[Sensei Tim]

So i built this up tonight and  it's  sort of verified.

It works. it sounds nice when everything is dimed (at least thru my practice amp. note i haven't wired up the switches yet.

I didn't include the volume at the last stage.

Master volume seems to only hit > unity near the end of its travel.  I'm probably going to just skip this master and  just bypass the pot and connect Treble 2 directly to the master 2 connection on the vero.

q6 sounds best at max voltage. i don't think there's a point in having a trimmer there
the trimmer has an impact on the overall tone for Q7. i'll need to play with this thru a proper amp when it's not so late at night.

The cool thing is that adding the phase inverter seems to have a pleasant effect on the overall tone.

now some of the negatives:

there is a big volume loss as the signal goes thru q6 and q7. i'm not sure why this is. Maybe lower those two 1M resistors connected to the gates?

The tone stack.  this isn't operating properly.  when i zero the mids it cuts the signal completely. it doesn't seem to adjust any frequencies, it just lowers the level. Treble does something, but it doesn't sound like it's doing it properly.

should mid 1 really be grounded?  That might explain losing volume if it's just bleeding everything to ground?

[Travis]

Do you have a layout? The JCM800 layout only has 6 JFETs

[Sensei Tim]

layout and schematic are on page 1.  i used the 2nd layout with the phase inverter.

[Travis]

Where you connect the tone stack pots to each other you don’t have the caps in between so that’s why instead of filtering it is acting like a volume control

Gotta take a deeper look at how you set up the phase inverter but I’m afraid that may be messed up as you wouldn’t want to actually copy the phase inverter exactly but rather replace it with a similar gain stage like the JCM800 emulator

[Sensei Tim]

You mean c22 and c24?  22n and 220n are connected as far as I can see?

[Travis]

The outputs from the phase inverter would come from the drains rather than the gates which probably explains the volume loss. One of them will be out of phase so you don’t want to combine the outputs or you will have phase cancellation

If you take a look at the JCM800 emulation they worked around this. The easiest way I can suggest to make your idea work is to copy the tail end of the JCM800 emulation (which was a smart idea for you to reference btw)

[Travis]

Sensei Tim wrote

You mean c22 and c24?  22n and 220n are connected as far as I can see?

Sorry I was mixed up there you are right. But now I am even more confused. If the wiper of the treble pot isn’t connected then how is the treble pot supposed to do anything?

[Sensei Tim]

I forgot to put it in the comments but T2 goes to master 3

The treble does something but it didn’t sound right.  I’ll need to try it again just to make sure.  It’s the mids control that would cut the output completely when set to 0

[Travis]

How does it sound if you put an audio probe at Q6 or Q7 drain?

[ibanez48]

I bread boarded the laney aor 50 schematic about a year ago i used j201 and trimmers. I didn't do the phase inverter or the presence control just the preamp and tone stack. It worked perfect the jfets biased perfect the tone stacked worked pefrect. I even did the middle boost switch sounded great. I had to order some 10m resistors for the mid boost. Other than that preamp sounded great it had alot of crunch. I just run the treble 2 out a volume control and then to my stereo power amp to my 4x12 cabinet. I think i might bread board it again this time add the phase inverter like u did and see how it will sound.

[glasspak73]

I been trying to understand how to make a jfet emulation of a phase inverter.I'm not sure if any of this info will help but it does look very interesting.

http://www.amplimos.it/phase%20splitter.htm

I hope this can help out in some possible way?

[Travis]

Since you’re not using a push pull amplifier there is no reason to split the phase in a stompbox as one of the outputs would be unused. What you’d want to do is replace it with a single gain stage with a similar response

[Sensei Tim]

I might have some time tomorrow or Monday to mess with this.

The audio probe on master 2 was loud. On q7 its quiet(er)

I’ll double check the tone stack and I’ll try bypassing one of the two phase inverters.  Maybe this is why the tone stack seems to be weird - phase cancellation happening with q6 and q7

Will report back

[Sensei Tim]

so for whatever reason i made a note in the layout to jumper Treble 3 to Mid 2 as well as through the caps on the board.

as soon as i removed the jumper everything worked as expected and... it sounds freaking amazing through my tiny THR-5

I can't wait to plug this into a big beast.

Bass switch works.  resonance switch works (woohoo)  i can't hear a difference with the bright switch tho. could be the tiny amp. could be something else.  will be checking this more later in the week.

consider this one to be verified!

[Travis]

I think that’s what I was looking at first but I started tripping out after flipping between the layout and schematic a million times haha

Nice job getting it working

[ibanez48]

I was looking at your layout and the schematic i don't see treble 2 or master 3. looking at the schematic treble 2 should go to master 3 right unless i'm missing something.