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/attachments/2552d1212538675-3005.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.

[joecool85]

+/- 18v is the maximum voltage for the MC1458 according to the datasheet: http://www.datasheetcatalog.org/datasheet/philips/MC1458.pdf

I don't have time to look at the schematic right now to let you know about the pin 5 thing.

[tonyharker]

Pin 5 is connected to ground via switches on both input jacks to reduce noise.  When a plug is inserted into a jack this connection is broken.

[Rutger]

Thanks!

Now I see, 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.

About the 1458, I simply forgot to order the components (2x 1k and 22u) that brings the voltage down to 16V...  :loco
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.

[joecool85]

My K20-X has a TL072 based preamp in it and it's running at maximum voltage no problems.  While not recommended, it should be fine.

[Rutger]

Thanks Joe! Do you use a regulated powersupply/ zener diodes in your K20X, or an unregulated one like in the Lead 12? Actually I use 4x 4700uF elco's in my powersupply.
As you might noticed that's what makes me a little insecure about feeding it with it's max. supplyvoltage, I don't know what the chances are that 18V will be exceeded with such a unregulated powersupply...

But I wouldn't mind a little extra headroom, in my experience it makes things sound a little beefier.

[joecool85]

Thanks Joe! Do you use a regulated powersupply/ zener diodes in your K20X, or an unregulated one like in the Lead 12? Actually I use 4x 4700uF elco's in my powersupply.
As you might noticed that's what makes me a little insecure about feeding it with it's max. supplyvoltage, I don't know what the chances are that 18V will be exceeded with such a unregulated powersupply...

But I wouldn't mind a little extra headroom, in my experience it makes things sound a little beefier.

It's unregulated, no zeners or anything.  Here is the schematic: http://deanmarkley.com/Info/LegacyAmps/Schematics/D1515.pdf

[J M Fahey]

Please remember that the K20 is probably running on no more than +/-18V, and those 150r on each rail drop a couple volts each.
But if you connect that preamp, as-is, to, say, an LM3886 amp or similar, kaboom!
That "no zener" trick is used on many low-cost starter amps, where every cent counts.

[joecool85]

Please remember that the K20 is probably running on no more than +/-18V, and those 150r on each rail drop a couple volts each.
But if you connect that preamp, as-is, to, say, an LM3886 amp or similar, kaboom!
That "no zener" trick is used on many low-cost starter amps, where every cent counts.

Juan, I measured it at V+ and V- and got +18.1v and -18.1v.  I didn't measure before the 150r resistors, I'd imagine it's probably a volt or two higher there.  I wouldn't recommend it, but I think the little TL072 can handle more voltage than they say it can.

On another over voltaging note, I built a Little Gem for my brother in law a year ago.  He wanted it to be louder and also didn't want a battery anymore.  The only transformer I had kicking around was a 12v unregulated PSU, it runs at 18v when driving the Little Gem.  The LM386 in it was the cheapie one from Radio Shack.  Pretty sure it is suppose to be a 12v max supply, but it has yet to release it's magic smoke.  I'm sure it's life has been shortened, but this is just a toy for him so no big deal.

[phatt]

Thanks!

Now I see, 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.
About the 1458, I simply forgot to order the components (2x 1k and 22u) that brings the voltage down to 16V...  :loco
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.

Just in case you missed it,, That Input trick is not an *Effects Loop* !
It's simply a high or low input setup.
Phil.

[J M Fahey]

Hi Joe.
Yes, "official" *is* 36V rail to rail and it's *good* Engineering practice not to surpass it.
Yet, desperate times call for desperate measures.
I have personally used TL071 and plain old humble 741 in Fender power stages, which use a "high voltage op amp" rated to +/-22V driving directly 2 or 4 TIP142/147.
Not all stand it, but a few do.
I have also done the far more dangerous swap of using 741 instead of LM391 (rated 80/100V) in those Polytone MiniBrutes (80/120W with +/-42V rails).
In that case I fed variable, tracking power rails to the 741, because, as I said, the rating refers to "rail to rail" voltage and not to absolute value.
When the poor little thing was going "up", the + rail increased as needed and the - rail lowered to -4 V keeping the abuse "not_that_high".
AFAIK that amp worked for almost a year, until its owner could get somebody to buy an LM391 for him in USA. (absolutely unknown in Argentina in those days).
Oh well.

[Rutger]

Okay, I'll better be on the safe side then. I want to use this amp pretty much so no time for burned IC's!

Just in case you missed it,, That Input trick is not an *Effects Loop* !
It's simply a high or low input setup.
Phil.

Didn't miss that

I actually only wanted to use the high-input, and instead of using a compressor stompbox, I wanted to put it in front of it right in the amp itself. I allways miss some compression in ss-amps, so I wonder if I can solve this issue by using a (optical) compression circuit.

But I gues I changed my mind, I will go for the lo/hi input annd place the compressor in a stompbox, so i can use the compressor in other setups as well.

Actually.... which input socket should i use with this switching option? On this guts shot it just looks ike a stereo input

[phatt]

Hi Rutger,
             pulling the jack out halfway engages the second circuit.
Note one switch is N/C the other O/C.

The circuit is mono,, just a hi low input all in one socket. An old trick to save space and cost, just by using a stereo socket like you have there.
Phil.

[phatt]

Oh I'll probably get labeled padantic,, but I just Grrr up when I see a naked mains fuse right next to the input socket of a guitar amp.

OK as is it is safe but I would not be able to sleep at night if I had built that Amp.

All to save a chassis mounted fuse holder.

They are so mean and cheap they don't even bother to cover the mains fuse.

Rant over.
Phil.

[joecool85]

Oh I'll probably get labeled padantic,, but I just Grrr up when I see a naked mains fuse right next to the input socket of a guitar amp.

OK as is it is safe but I would not be able to sleep at night if I had built that Amp.

All to save a chassis mounted fuse holder.

They are so mean and cheap they don't even bother to cover the mains fuse.

Rant over.
Phil.

I'm the same way and would never mount a fuse that way.  Also, it just makes more sense to have the fuse accessible from the back of the amp in a chassis mounted holder.  But, alas, cheap amps aren't meant to make sense, just to make dollars.