LXH2 "marshall" typre amp+cab sim

[krille2]

I have tried my hands on the LXH2 "marshall" type preamp cab simulator circutry and ended up with a handwired design on a lab board. I just can´t  make this thing work like I want to. There is a powerful "squeel" note (just a high pitched solid note) whe I turn up the output volume. The guitar signal is very low and weak and it seam to be unaffected from the second vol pot and tone stack - though I clearly can hear slight noise shift when I turn these potentiometers. If I connect op-amp 3 out (TL084) to GND my signal is clear and loud without any "squeel" at all, strange I thing. I have layouts / schematics and photos to provide if anybody can help me!

[krille2]

Anyone, any ideas?
FYI - If I make a GND connection from op-amp 3 out there is a really strong signal coming out without any squeel at at all even if i turn the vol. 100% up. But the output signal is so extremly strong in the treble, almost painful. String separation is fantastic though - like a real amp!

[MartyMart]

A link to a schematic /layout would be helpful, as i dont have this circuit to look at .....
Sounds like a mis-wire somewhere ...?

MM.

[krille2]

My layouts (only preamp is scanned so far - but the circutry includes the cab sim as well) can be found on this page:
http://hem.passagen.se/amps

The schematic could be found here:
http://home3.netcarrier.com/~lxh2/marsh.html

My thoughts is just the same, some kind of miswire - but I just can´t seam to find where 

[Steben]

Wow, quite some opamps there!
I suggest you try to split the schematic in input/tone/cab sim.

[MartyMart]

 

That is what I call a circuit !!
Good luck finding the "error"  !!   

[krille2]

Well, I´m preatty aware of the complexity, sure  . But I do believe that the promlem is somewhere around the first 15 op-amps, thats why I´ve incuded only that part in the layouts provided on the first link in an earlier post. I thought there may be standard ways of diagnosing stuff like this. 
Based on the problems I have it couldn´t be impossible.
* The first 1x100 pot (after the 2x100 dual gang pot) isn´t working - at least not audiable.
* Strong signal when op-amp 3 out soldered to GND (otherewise VERY weak signal plus high pitched squeel at +80% output volume)
* Very little soundcoloration from the "marshall" tone stack (between op-amp 7 and

Any hints?

[MartyMart]

Of course, it can be "fixed"
Building an "audio probe" is great advice BTW !
OK, how is this powered ? +/-15 v or more .... less ?
Check all power and VB or "half" power lines ( 0 volts on a dual supply )
Tracing where they may "dissapear", I did this recently with a TS808 with
some help from RG, 4.5 volt line was "wrong" and it pointed to the op amp.
On your photo's it looks a little "cramped" here and there, any shorts ?
... etc etc 
MM

[krille2]

It's powered by a +-15v dual supply which I have taken down to approx +-10v. The circuit calls for +-9v -> +-12v I've been told. I have measuring all DC voltages around the circuit and it looks fine to me, so far. My plan is to listen to the output of each and every op amp to see where it fails...

[krille2]

Ok, the cab sim part from op 9 to op 25 is perfectly ok. The problem is around the first 8 op amps. So I have reasolderd and replaced the 2nd op amp, no difference. Must be in ic 1 then or in some component. There is a parallel diode pair, have can I tell if it´s ok?

[krille2]

Can anybody tell how to properly test a diode pair?
I will resolder ic1 too just to check if there is an error around it...

[MartyMart]

You can check two ways, either "out of circuit" the same as a resistor, using DMM to check
for forward current, or if they are clipping diodes in a working circuit, but you have little/no
clipping from the gain stage, lift the earthed end to see if there's any difference.
AS far as i know, you can't just test them when they are "in circuit" without the rest of
the circuit working around them.
Anyone else ... ?

MM.

[krille2]

Well, I don't really know what the problem was in the preamp but I succeeded in making it work yesterday. I rewired (and changed some components) the first 3 ICs and everything works now!

Cool thing, LXH2 "Marshall" type preamp and cab sim! Will post some mp3 demos on my website soon if time permits. 

[MartyMart]

That's great, I'm glad you got it running 
Clips would be nice ....

MM.

[krille2]

Soundclips are coming, I will post here to notify you!

[kusi]

hi there,

how sound these LXH2-preamp-circuits sounds before an amp as stompbox?

any experiences?

thx & mfg kusi

[krille2]

I have updated my website with som quick sound clips! These were very quickly recorded so don't expect too much. But I will put more efforts into the sound clip section after x-mas!

[krille2]

...and of course the look would be nice, too 

http://hem.passagen.se/amps/

[MartyMart]

I think you put enough effort in to those  !!
Great, it sounds just fantastic ...

Marty.

[Joust]

all links on this thread are dead.
I made this in the past and its awesome.
does anyone have a complete schematic of this circuit?
the closest one i found is http://www.astersart.net/preamp/simulators/marshall_type_amp_simulator.htm
but it is not complete. it does not show the filter.
if the standard filter is used and plugged in to points A and B there will be no difference between a dip and a peak. That's a pretty important difference.