In keeping with another current project (Stratocaster build) I'm building a guitar amp.
Many of the other aspects of it's construction have been decided and I have started by selecting the drivers (two Eminence Legend 1258) and building the box to house it all. Details on this to follow but first the issues that need attention: preamp design and power amp choice.
First, the preamp that I have put together with inspiration from Rod Elliott's design:
View attachment GUITAR PREAMP 1.pdf
Basically buffer, gain, tone, gain, buffer.
Room for improvement here? I'm new to this type of thing and any input would be greatly appreciated.
Many of the other aspects of it's construction have been decided and I have started by selecting the drivers (two Eminence Legend 1258) and building the box to house it all. Details on this to follow but first the issues that need attention: preamp design and power amp choice.
First, the preamp that I have put together with inspiration from Rod Elliott's design:
View attachment GUITAR PREAMP 1.pdf
Basically buffer, gain, tone, gain, buffer.
Room for improvement here? I'm new to this type of thing and any input would be greatly appreciated.
The input buffer seems pointless and has a too low input impedance to begin with. You want input Z to be closer to 1 Megaohm, like in the second stage.
Overall, I don't like the Elliot's preamp design and there are several reasons for this. The clipping diodes are a joke because due to the large bandwidth of the preamplifier they will only develop annoying buzz and fuzz. The only decent tones you'll get from this preamp are clean ones. I would seriously consider ditching this design and taking a look at the preamplifier circuits in some of the guitar amplifiers you are fond of. Study them for a moment and you'll begin to realise how awful the Rod's design actually is for this kind of application.
Overall, I don't like the Elliot's preamp design and there are several reasons for this. The clipping diodes are a joke because due to the large bandwidth of the preamplifier they will only develop annoying buzz and fuzz. The only decent tones you'll get from this preamp are clean ones. I would seriously consider ditching this design and taking a look at the preamplifier circuits in some of the guitar amplifiers you are fond of. Study them for a moment and you'll begin to realise how awful the Rod's design actually is for this kind of application.
If your looking for harmonics in the design then consider a soft limiter.
This will give a range of tones similar to a valve amp in overdrive.
This will give a range of tones similar to a valve amp in overdrive.
An externally hosted image should be here but it was not working when we last tested it.
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OK
Made me look at these one more time
http://www.guitarmusician.com/traditional-floor-desktop-pre-amp-bass-guitar-amps.php
Made me look at these one more time
http://www.guitarmusician.com/traditional-floor-desktop-pre-amp-bass-guitar-amps.php
Hi John; if you design for a lead guitar you need a harmonic enhancer that makes sound brighter when louder, and softer when it decay.
Here is the main idea: Si diodes in feedback (Log. converter), Ge diodes after (Exp. converter). The pot is a pedal ("Expression"): push the pedal and get more drive.
Here is the main idea: Si diodes in feedback (Log. converter), Ge diodes after (Exp. converter). The pot is a pedal ("Expression"): push the pedal and get more drive.
I would forget the IC topology and do an all tube design.
If your preference is for heavily overdriven sounds then look at some of
the early Marshall master volume model schematics. If you are more of a
clean twang sort of guy then a Fender front end is the way to go.
I also recommend a spring reverb unit, these sound really great and if you
can stretch it a tremolo circuit as per early Fenders.
Good luck
T
Thanks Nigel, Thanks Wavebourn! 
I'm digesting these ideas as I capture the G100A in Multisim.
Hi Terry,
Glassware is out - I'm not serious enough for that luxury (besides the fact that I can't play a chord!)
I'm working with the Gibson G100A shown above. My original plan was to have reverb and I have space inside the box for the tank.
I'm not sure how I want it to sound, really. I want to make everything as modular as possible, so that if I want to experiment with a different preamp, it's just a matter of pulling the old and plugging in the new.
I'm digesting these ideas as I capture the G100A in Multisim.
Hi Terry,
Glassware is out - I'm not serious enough for that luxury (besides the fact that I can't play a chord!)
I'm working with the Gibson G100A shown above. My original plan was to have reverb and I have space inside the box for the tank.
I'm not sure how I want it to sound, really. I want to make everything as modular as possible, so that if I want to experiment with a different preamp, it's just a matter of pulling the old and plugging in the new.
New amp scheme:
View attachment GUITAR PREAMP 2.pdf
A near carbon copy of the Gibson. Some device changes but nothing that's not appropriate.
Look it over please, see if there are any issues.
A near carbon copy of the Gibson. Some device changes but nothing that's not appropriate.
Look it over please, see if there are any issues.
Hey John,
You might wanna take a peek at http://runoffgroove.com/articles.html , They have quite a few JFET-"copies" of tube amps, and at least some people say they sound quite faithful to the originals. Or, worst case, you'll have a few "ready-made" preamp ideas to choose from
You might wanna take a peek at http://runoffgroove.com/articles.html , They have quite a few JFET-"copies" of tube amps, and at least some people say they sound quite faithful to the originals. Or, worst case, you'll have a few "ready-made" preamp ideas to choose from
Hi John,
you are on good way,
only discrete designs!
Do not forget about good star ground.
Try Peavey transtube preamp,
it is worth
regards
P
Hi
please share your entire project with us,
a schematic at least...
regards
P
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Looks similar to my hand-made pre-amps, except the op-amp. Q3 is a buffer stage, trying to present a high-impedance to the guitar input, because the guitar coils are connected to 500K pots, so you need something equal or (10 times) better. Of course if you chose 10x, eg 5MB you'd be picking up noise from Mars, so a compromise is found.
I use a jFET, 2n5459, look it up it is quite cool for presenting a high impedance buffer, low noise and all.
The source is given as 100mV peak - rubbish. I strum on all three guitars we have here and the scope easily peaks 400mV. 100mV is like picking one string at a time. I think the input resistors are for protecting the device from plugging in something horrible rather than for attenuation.
But the supply is huge - 35 Volts - nothing can possibly clip there, I run my pre-amps at 12 Volts.
There is no provision for hum suppression (except the reverb post-amp) and unless you run it off chemical batteries, or a super dupper regulated power supply you will be in trouble.
By the way if you look at Fender's Princeton Chorus schematic, uses op-amps but it is very much similar to this one.
You also need "Send" and "Return" jacks, invaluable in my opinion.
I am attaching a part of my design, version 2, you will see the general idea is the same.
Attachments
Thanks Khron,
They have a lot of good looking projects there! I'm like a kid in a candy store
Thanks padamiecki
That was an arbitrary number, just to test the circuit in simulation. As I said, the circuit is nearly a carbon copy of the Gibson G50A preamp.
I forgot the RCRC filter on the rail - it drops the voltage down to ~24VDC. See the new schematic: View attachment GUITAR PREAMP 2.pdf
There are a few other changes, mostly stuff I accidentally left out in the previous one.
Thanks for tip on the jacks. Where should they be located, between the preamp and power amp?
Added the RCRC filter as shown in the original schematic.
The supplies are the next topic of discussion. The +/-12VDC supply will be regulated and the +35VDC will be tapped from the power amp supply. Should I regulate the +35 as well? Or just load it with smoothing caps or maybe a cap multiplier?
Thoughts? Opinions? Bring 'em!
Send/Return is used typically after the pre-amp, also called "effects loop". Basically a "line out" and "line in". Running effects off that "send" output and then returning to the "return" allows for less noise than if you had plugged it at the guitar input. Also allows all other amp effects (tone controls etc) to take place, in case.
But reverb is an exception and it comes as the ultimate "effect" before the port amp.
In your Gibson schematic you'd hook the Send/Return jacks at C5 but you'd need some buffering after the tone controls, so another transistor.
I would also strongly recommend a peak-detector (or more), calibrated to show you when you are clipping, very useful, for example when you are a distance from the amp on stage and meddling with stomp boxes. I attach my (noddy ) design. As it stands, the RED LED will flash at 1.25 V RMS which is when my post-amps clip.
But reverb is an exception and it comes as the ultimate "effect" before the port amp.
In your Gibson schematic you'd hook the Send/Return jacks at C5 but you'd need some buffering after the tone controls, so another transistor.
I would also strongly recommend a peak-detector (or more), calibrated to show you when you are clipping, very useful, for example when you are a distance from the amp on stage and meddling with stomp boxes. I attach my (noddy ) design. As it stands, the RED LED will flash at 1.25 V RMS which is when my post-amps clip.
Attachments
Yes, that would be handy (although I'll probably (no, definitely) won't be on a stage with this!). Thanks again.
I have worked up a layout for the preamp itself:
A good starting point. Everything else gets added to this on separate boards.
What it will look like:
Yeah, I couldn't resist
(love the 3D view)I have all of the parts for this on hand except the pots. I've picked these from Digikey.
Just another idea for you. Done many times before, by many people, try it... you might be surprised.
http://bas.elitesecurity.org/leadstd.pdf
http://bas.elitesecurity.org/leadpro.pdf
http://bas.elitesecurity.org/leadstd.pdf
http://bas.elitesecurity.org/leadpro.pdf
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