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 (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.
+/- 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.
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.
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.
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.
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.
Quote from: Rutger on February 05, 2011, 06:55:17 PM
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
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.
Quote from: J M Fahey on February 07, 2011, 10:09:07 AM
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.
Quote from: Rutger on February 04, 2011, 04:02:19 AM
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.
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.
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!
Quote from: phatt on February 07, 2011, 06:48:47 PM
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
(http://jjsbar.dyndns.org/51/lead20.jpg)
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.
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.
Quote from: phatt on February 08, 2011, 07:21:30 AM
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.
It is *very* stupid and *very* dangerous to mount the 120/*240V in the same board as the audio circuits.
You are repairing it *out* of the chassis, over your desktop, or even worse, unmounted but still in the chassis, because you need it still connected to the power transformer, and a small unexpected movement (your attention is naturally focused on some other point you are repairing) and the full chassis becomes live.
Are they *CRAZY*. All I can think is S.O.B. :trouble :grr
I *always* pull those fuseholder clips, *destroy" the tracks so it can't be re-mounted and add a "flying type" fuseholder, the kind sometimes used in cars, plus adding a glued note that *now* that's the main fuse, no substitutions.
Hi,
I've 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 playing 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?
Haven't you asked this exact same question somewhere else?
I answered it earlier.
Well, I've started a similar topic at musicelectronics. But apparently more people read the some fora as I do :)
But not at ssguitar, no.
Juan, it doesn't count if you answered it on a different site haha.
To be fair, I didn't even remember *where* I had answered it.
It might very well have been a double posting inside SSGuitar itself (has happened before)
Sometimes anxious musicians (is there any other kind? ;D) post something and not receiving answers after an incredibly long hour :loco either bump it up or simply post a new topic.
Oh well.
Quote from: J M Fahey on April 14, 2011, 10:23:03 AM
To be fair, I didn't even remember *where* I had answered it.
It might very well have been a double posting inside SSGuitar itself (has happened before)
Sometimes anxious musicians (is there any other kind? ;D) post something and not receiving answers after an incredibly long hour :loco either bump it up or simply post a new topic.
Oh well.
It's true, I've seen that here before. Just figured I'd give you a friendly jibe.
Yea, I got it ;D
Quote from: Rutger on April 13, 2011, 09:55:29 AM
Hi,
I've 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 playing 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?
That mid lift trick only works on old fender circuits, I very much doubt it will work well here but try it and find out.
As to the bright problem ?? Hard to say.
If I get time I'll try a simulation and see what the tone curves look like.
C10 Q,, google *fender tone stacks* or try *tone stack types*
just click on stuff that looks like it might be explaining how they work.
BTW at a glance,, the one you have is not ever going to be a Rolls Royce.
Phil.
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 I 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).
You might try just about any dual opamp. Install a socket so it is easy to swap parts. The 4558 is popular and of course the TL072 or 5532. Report back on how they sound and if the fizz goes away.