I am currently working on a fun project to use an LM3875 to be the PA section of a small 10 to 20 watt guitar amplifier.
So far I have designed a simple 2 transistor preamp stage utilizing a "fender tone stack" circuit between which is really nothing more than a simple 3 band passive EQ between the output of the inverting input transistor and the input of the second inverting buffer transistor stage which then couples into the chipamp.
I thought about using an OPAmp to do the preamplification but honestly I think the simplicity of a two transistor stage outperforms anything on a chip for noise.
Not absolutely sure yet if I will be able to pull this off with only 3 active parts (LM3875 and 2 transistor preamp) but if I can that is incredible considering the tiny amount of space required.
The tone switches and volume and EQ pots take up more space than the perfboard I am building this whole thing onto!
Sorry no pictures yet, but I promise I am working on this bench project enough that the pics and circuit ideas will come soon as time permits.
I am just curious if anyone else here has tried something similar and if you could share opinions and ideas before I decide on finalizing a design only to later regret not trying other things.
Thanks.
So far I have designed a simple 2 transistor preamp stage utilizing a "fender tone stack" circuit between which is really nothing more than a simple 3 band passive EQ between the output of the inverting input transistor and the input of the second inverting buffer transistor stage which then couples into the chipamp.
I thought about using an OPAmp to do the preamplification but honestly I think the simplicity of a two transistor stage outperforms anything on a chip for noise.
Not absolutely sure yet if I will be able to pull this off with only 3 active parts (LM3875 and 2 transistor preamp) but if I can that is incredible considering the tiny amount of space required.
The tone switches and volume and EQ pots take up more space than the perfboard I am building this whole thing onto!
Sorry no pictures yet, but I promise I am working on this bench project enough that the pics and circuit ideas will come soon as time permits.
I am just curious if anyone else here has tried something similar and if you could share opinions and ideas before I decide on finalizing a design only to later regret not trying other things.
Thanks.
If you are in to rock/heavy metal you might want to consider a soft limiter front end.
This gives a valve type sound using a couple of op-amps.
This gives a valve type sound using a couple of op-amps.
An externally hosted image should be here but it was not working when we last tested it.
Interesting schematic.
I plan on making this amp use all outboard effects outside of the basic controls like vol/tone/mid/bass and maybe a few toggles for a jazz on/off sound or maybe add in some diode clipping with a metal on/off toggle.
You did make me think about how harshly realistic these chipamps can be. They are like a scalpel to audio and I know as a guitarist I love the more toned sound of imperfection so I will have to play around with a few tonal circuit ideas.
Generally I go with outboard effects though. That is not to say I can't always design other ideas *as* outboard 🙂
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Tossing in just a penny's worth, the amp will be fine, and of course you can shape the tone as you wish. Your main concern is the power supply.
Remember you're getting RMS watts out of the thing, not fictional "music power." When your bass and keyboard players see what's what they're going to make a beeline for what you thought was your amp.
The 5th and 6th strings on a guitar are hard enough on power supplies, but when the bass plugs in, or the keyboard starts playing chords (when do they not?), there's going to be serious power involved.
The rule of thumb for hi-fi listening is a power supply at least 1.5 times the audio amp watt rating, and double is much preferred. For your use...well...how good do you want to sound?
Unfortunately, the main expense with power supplies is the transformer, an expense you really can't work around. But I suggest you stretch your budget to the limit and be glad you did.
FYI, in case you don't know, amps are rated in watts, power transformers are rated in VA. For all practical purposes these are the same thing.
watts = volts x amps = VA
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Tossing in just a penny's worth, the amp will be fine, and of course you can shape the tone as you wish. Your main concern is the power supply.
Remember you're getting RMS watts out of the thing, not fictional "music power." When your bass and keyboard players see what's what they're going to make a beeline for what you thought was your amp.
The 5th and 6th strings on a guitar are hard enough on power supplies, but when the bass plugs in, or the keyboard starts playing chords (when do they not?), there's going to be serious power involved.
The rule of thumb for hi-fi listening is a power supply at least 1.5 times the audio amp watt rating, and double is much preferred. For your use...well...how good do you want to sound?
Unfortunately, the main expense with power supplies is the transformer, an expense you really can't work around. But I suggest you stretch your budget to the limit and be glad you did.
FYI, in case you don't know, amps are rated in watts, power transformers are rated in VA. For all practical purposes these are the same thing.
watts = volts x amps = VA
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Said power factor being...?
Right, you don't know. So you need some kind of "rule of thumb" which will give you a workable solution without getting into engineering. Some rule such as...
watts = volts x amps = VA
...which will serve you every time without making life more complicated than it needs to be.
On the other hand, some people like making things complicated. So why should I argue, enjoy.
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Bentsnake, Google is your friend, I think if you take the PF to be .7 then you will be safe. It can even be higher . You will have to have the specs. Ask guy's like AJT, he will give you exact numbers. VA value is allway's larger then the watt rating. (sorry Tony)
Take an example. If you have a 400watt transformer and you convert it to VA with a power factor of 0.8 which is viable. 400watt/0.8= 500 VA. It actually does make a difference in the way that you plan a power scheme and monetary as well. SMPS's are much higher than this.
Take an example. If you have a 400watt transformer and you convert it to VA with a power factor of 0.8 which is viable. 400watt/0.8= 500 VA. It actually does make a difference in the way that you plan a power scheme and monetary as well. SMPS's are much higher than this.
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If you use 0.7 for a power rating, then apparently you design toasters for a living. Transformers are in excess of 95%, more like 99%.
Those who like to be confused, and confuse others, go here:
Watts to VA conversion calculator
Everybody else just take it that watts and VA are the same thing, and you'll be fine.
I'll be fine too, since I'm out of this. I have actual work to do.
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If you use 0.7 for a power rating, then apparently you design toasters for a living. Transformers are in excess of 95%, more like 99%.
Those who like to be confused, and confuse others, go here:
Watts to VA conversion calculator
Everybody else just take it that watts and VA are the same thing, and you'll be fine.
I'll be fine too, since I'm out of this. I have actual work to do.
.
Okay a few questions here...
I have the preamp designed now and am looking at either a single or dual input jack like most other guitar amps have.
It will have a headphone output jack, along with a speaker output jack, and banana connections for a wired speaker instead of one that uses the 1/4" jack style cables so that the unit will be versatile.
What I am wondering is the grounding scheme.
I think the jacks ground should be insulated from the metal chassis ground right?
So I gather there should be a star ground on the amplifier circuit itself, with the preamp having its own star ground going to the main star grounding of the PA amp section, then the input and speaker jacks insulated from chassis ground.
Then the chassis is to be grounded to the middle ground earth prong of the AC plug, and a resistor/capacitor combo from the chassis to the PA ground star.
Is this correct or am I overthinking things?
I ask because old guitar amps have the signal jacks ground going directly to chassis ground right from the jacks nut. Sometimes with the AC earth ground going to chassis, and sometimes not.
Also this unit is running off of a power adapter that has a two prong AC plug, but my chassis is metal. I gather that to be in compliance with electrical codes this means I MUST use a three prong AC plug since the chassis is exposed metal, but is this really necessary?
Thanks.
This grounding stuff is really driving me nuts.
By the way, little bit of a rant..
I never have had to deal with such finicky amplifier designs when it comes to grounding and precise star grounding.
What I find odd is that I have never had to really think about this when designing small 5-10 watt guitar tube amps, but for some reason, well, maybe it is just me but these chipamps really are sensitive to grounding and the slightest screwup in design can really make them go bonkers compared to other types of amplifiers in my opinion. Am I wrong?
I have the preamp designed now and am looking at either a single or dual input jack like most other guitar amps have.
It will have a headphone output jack, along with a speaker output jack, and banana connections for a wired speaker instead of one that uses the 1/4" jack style cables so that the unit will be versatile.
What I am wondering is the grounding scheme.
I think the jacks ground should be insulated from the metal chassis ground right?
So I gather there should be a star ground on the amplifier circuit itself, with the preamp having its own star ground going to the main star grounding of the PA amp section, then the input and speaker jacks insulated from chassis ground.
Then the chassis is to be grounded to the middle ground earth prong of the AC plug, and a resistor/capacitor combo from the chassis to the PA ground star.
Is this correct or am I overthinking things?
I ask because old guitar amps have the signal jacks ground going directly to chassis ground right from the jacks nut. Sometimes with the AC earth ground going to chassis, and sometimes not.
Also this unit is running off of a power adapter that has a two prong AC plug, but my chassis is metal. I gather that to be in compliance with electrical codes this means I MUST use a three prong AC plug since the chassis is exposed metal, but is this really necessary?
Thanks.
This grounding stuff is really driving me nuts.
By the way, little bit of a rant..
I never have had to deal with such finicky amplifier designs when it comes to grounding and precise star grounding.
What I find odd is that I have never had to really think about this when designing small 5-10 watt guitar tube amps, but for some reason, well, maybe it is just me but these chipamps really are sensitive to grounding and the slightest screwup in design can really make them go bonkers compared to other types of amplifiers in my opinion. Am I wrong?
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Everybody else just take it that watts and VA are the same thing, and you'll be fine.
You can use different power converter tools, including watts, horsepower and BTU etc.
Power Conversion
You can use different power converter tools, including watts, horsepower and BTU etc.
Power Conversion
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