Questions about the Marshall Lead 12 preamp

[Rutger]

Hi,

I'm planning on building my first small ss-guitaramp. I've bought a LM1875 kit on ebay and want to build a Marshall Lead 12 preamp myself. There is however one thing that confuses me in the Lead 12 schematic, probably due to my limited knowledge.

Here is the schematic: http://music-electronics-forum.com/a...38675-3005.gif

There is a connection from pin 5 of IC1B back to the ring of the low-input. What does it do, is it a feedbackloop? What confuses me is that actually it's just connected to ground (via the ring). Does this mean that I can just make a connection to ground? Are does it work differently?

Second question is: according to the schematic the MC1458 opamp runs at +/-16V, will it be safe to run it at +/- 18V? That is it's maximum volt rating so I don't know if that's okay.

[Steve Conner]

If you ground pin 5 of IC1B, there will be no sound. So, that is probably a switch setup that mutes the amp when there are no plugs in the input jacks.

Like most op-amps, the 1458 has an absolute max voltage rating of +/-18V. You can only run it there if you're sure that your supply rails will never rise above 18V under any circumstances. If they are unregulated, as shown in the Lead 12 schematic, then that's not a safe assumption. You should reduce the voltage to 15 or 16 with a regulator, or some zener diodes or dropper resistors or something.

Disclaimer: Maybe the unregulated supplies in the Lead 12 are part of its tone. So use the dropper resistors like in the Marshall schematic.

[Rutger]

Thanks!

Thing is, I actually forgot to order these components (2x 1k and 22u)...

I run the LM1875 on +/- 18V so I thought that maybe it wouldn't harm the 1458. But I guess it is, so now I need to order these components anyway.

Now I see about the input, there is a little arrow in the schematic that says "hey, I'm switchable"!
Well that's a good thing because I want to add a compressor before the preamp, that means that I don't need to add this 'loop' to my build.

[Rutger]

Still have one question though... How "unregulated" is an unregulated powersupply?
The elco's I use in mine are quite a bit larger (4x 4700uF) than that of the Lead 12. How much is the chance that the railvoltages will exceed the +/-18V?

Is there a way to calculate/guess those variations in such a powersupply?

Also, I wouldn't mind a little extra headroom in the preamp, in my experience it makes things sound just a little beefier/more tight.

[J M Fahey]

The capacitors of any size will charge to the peak of the AC voltage, or 1.4142 times its AC value, minus almost a volt per diode junction in its path.
Consider 1.5 or 2 V loss and you are done.
You will lose useful DC voltage for 3 causes:
1) Resistive loss in copper
2) Magnetic loss in the core
3) Ripple voltage.
For a practical, seat_of_the_pants estimation, you'll lose between 20% for a good power supply to 30% for a poor one, in the real world.
Anyway, at idle (no music),it will go to the peak value and that can damage your Op Amps.
Use a 1K 1/2W resistor + 100uFx25V capacitor + 15V 1/2W Zener per rail and sleep like a baby
Post some pictures and MP3 along the construction.

[Rutger]

Hi,

I finished the amp a couple of weeks ago and it works! It's very nice indeed, even the cleans are very acceptable but it really shines when distorted.

Playing with it, you start to wonder how to make the amp a little more versatile. I would like to make it sound just a little more bright when paying clean, to get a little more 'snap' in the highs. Therefore a bright-switch would be usefull. But I wonder, normally this brightcap is placed on the gainpot, but I don't think that's possible in this circuit.
Is it usefull to place a brightcap on the mastervolume pot? And how can I calculate which capvalue I need? Normally I like something like 100pF or lower.

Another mod I would like to try out is a groundlift on the midpot. Looking at the schematic, there's also a little cap (C10) next to the midpot to the ground. What does this cap do, does it roll of some highs? And how do I calculate which frequencies it rolls of?

[loudthud]

C10 controls the highs that don't go to the Bass pot. The tone stack is one interactive mess, no simple calculation will tell you much. Checkout Duncan's Tone Stack Calculator, available free: TSC

A bright cap at the Master Volume control will add brightness. The best way to get the value is the tried and true method called "Cut and Try It". Something between 100pF and 270pF should work.

[Rutger]

Thanks,

I'll try some caps on the MV pot and stick with the one I like

I've allready simulated the tonestack with the Duncan tool, but you can't add stuff like this cap. That's why I wonder what it exactly does and which frequencies it cuts.
I wonder if it is necesarry that, if I lift the midpot from ground, I should lift this cap as well. Or maybe I can use this cab as a bright cab by lifting it from ground seperatly?

[loudthud]

If you want to see what lifting the Mid pot from ground does, change the value to 100Megohms. Want to see what lifting the cap C10 does, change the value to 1pF.

I just took another look at the schematic. It's not the same as the one in Duncan's TSC. Notice how Duncan's C2 is replaced with a wire? Still, operation isn't that much different from the Marshall model except for the Treble pot's effect on the mids.

[Rutger]

Thanks for the explanation of the tonestack!

Yeah, the treblepot is doing an awful lot with the mids, I noticed that when playing with the EQ. Funny to see how the diagram really explains what you've already heard. Basspot doesn't do a lot actually.
And it IS an interactive mess. But hey, it's part of the amps tone isn't it?

Lifting C10 would attenuate the bass and boost the highs, and that's not'what I want to achieve. My plan for lifting the midpot was to actualy get a flat freq-response. But with the duncantool I figured out I can get a decent flat response with the the EQ itself (except for everything below 100Hz, but you do want to get these low frequencies dimed), and get a nice overall attenuation so that the preamp doesn't get distorted right away.

So I might leave the whole groundlifting and stick with the brightcap switch.

[loudthud]

There is a little bit of tone shaping going on in the preamp prior to the tone stack. C1/R5 is giving a high boost and C3/C4/C5 are forming some kind of bandpass having to do the with Gain control. The easiest way to look at what is going on is with a modeling program or just fiddle with the caps to see if they do anything usefull.

[Rutger]

Hey,

Old thread I know, but it keeps the forum clean

Still using this little amp a lot. Never got to experimenting with mods though, allthough I recently replaced the cheap speaker I used for an Emi Legend 1015... wow, now this little fellow has become a little monster, it rocks hard and loud! Really, really nice Not gigged with it though, the poweramp can put out 20W easilly, so I wouldn't be surprised if it could hold its own.

One thing though, and I wonder what it is and if it can be fixed. When the preamp starts to break up the overdrive starts as a funny fizzy crisp. The same thing when the notes fade out: it ends with a funny fizzy crisp. I think this crisp stays when you drive the amp harder, then it sounds more like overdriven highs which can be nice. But it can be annoying when I play with a little OD, blues for instance, than it's quite noticable.
When playing clean it's not there.

I wonder if it is a characteristic of the chip the preamp uses? It has a LM1458 in it. Can this be fixed by using another (type of) chip?

First I thought it was the cheap speaker, but it's the same thing with the Emi, allthough the Emi is not fully broken in yet (played with it a couple of hours).

[J M Fahey]

Well, *it is* an SS amp, after all.
Will do the heavy stuff very well, but Blues requires smoother overdrive, not its forte.
You may add a "reverse bright" capacitor at the Master Volume.
Add a (switchable) 1000pF cap , wiper to ground.
Experiment with its value but do not go overboard with it or you'll get into Mesa Boogie mud real quick.

[Steve Conner]

Well, *it is* an SS amp, after all.

I agree. I think the "fizzy crisp" is just the sound of an overdriven op-amp. If it were me, I'd try adding a diode clipper to the circuit, as found in the Tube Screamer.

Diodes clip softer and more progressively than op-amps, especially if you use oversized high voltage ones like the 1N4007, and the result is smoother distortion. But maybe too smooth for the heavy stuff...

The speaker may be emphasising it because it's new and not broken in. Every speaker I've bought, I found that the high end became softer with break-in.

[Rutger]

I was afraid I would get an answer like this. Well, it is a rockamp after all, it might be a little too much to ask. Luckilly it is not that noticable when playing loud.
First I thought "But I don't have that crisp with the Vox Pathfinder!" but just saw that that preamp uses LED clipping


So I need to start modding after all. Thanks for the tips!
Or just put a nice OD pedal in front of it, good excuse for a new project
Or I might as well go wild and build an extra preamp and make it footswitchable.