Wanting to Build A Very simple SS circuit of about 30W

[vmd1]

Hi folks,

New to this forum, and I have a question. I'm wanting to build a very simple SS guitar amp head of about 30 watts. I want it to have ONLY a single volume knob. I don't care about a pre-amp stage for distortion/overdrive at all. Essentially, that is the purpose of this project, to build a "hi-fi" head. I just want it to be an amplifier in its most simple form, zero coloration/modification of the input signal at all. This may seem like a silly or even pointless task to some... but its what I'm wanting to do.

More specifically my question pertains to matching the output impedance of an electric guitar to the input impedance of an amp kit specifically this one:

https://www.amazon.com/dp/B01F8RXXZI/ref=olp_product_details?_encoding=UTF8&me=

Can someone explain to me exactly how I would make the input stage for a guitar, and have around a 1Meg ohm input impedance, with a simple pot for the volume control. Exactly how would you design a simple input stage to be used with this power amp, for the purpose of plugging a electric guitar into it?


If someone has a better idea for what i'm trying to accomplish in general please share it with me, as i don't have to use a class A SS hi-fi amp, I just thought it would be a really cool unique project.

Thank you and looking forward to making some friends on here! 

[tonyharker]

First of all your're going to need a humongous heatsink,  then I would use a fet preamp to give you the high impedance needed. Something like the tillman preamp here http://www.till.com/articles/GuitarPreamp/index.html . change the 51k resistor for a 47k pot for the volume control.
BTW welcome to the forum.

[vmd1]

thanks!!! this type of pre-amp circuit is exactly what i've been wandering around the internet looking for! 

[tonyharker]

Instead of that class a amp you might like to look at a chip solution eg. LM1875, TDA2030, LM3875, LM3886.  There's a lot of information about these on the 'net also check out their datasheets - google. Kits and bare PCBs are available on Amazon and even eBay if you want to risk it. The LM1875 although listed at 20w will put out more at higher voltages into 8 ohm speakers.  The boards will be OK even though the IC may be suspect but they are cheap and a good starting place.

[galaxiex]

Cool project. 

May I suggest that you will need some kind of "tone shaping" in that amp.
Guitars just don't sound good played thru a "HiFi flat response" type of amp.

If you have a home stereo, try playing your guitar thru it,
possibly getting some adapter and using the phono input if it has one.

Of course the speaker will have a huge impact on the sound.
If you use a "real" guitar speaker that will help.

I have been playing around with this circuit...

http://www.muzique.com/lab/notch.htm

You can add this circuit inside and not even use a "tone" control.
It will give you a "notched mid" response which helps prevent the tone from being muddy.
A very useful circuit, I have added it to a couple of amps and played around with the values.
Made some crappy sounding amps sound great!

Cheers!

[vmd1]

Cool project. 

May I suggest that you will need some kind of "tone shaping" in that amp.
Guitars just don't sound good played thru a "HiFi flat response" type of amp.

If you have a home stereo, try playing your guitar thru it,
possibly getting some adapter and using the phono input if it has one.

Of course the speaker will have a huge impact on the sound.
If you use a "real" guitar speaker that will help.

I have been playing around with this circuit...

http://www.muzique.com/lab/notch.htm

You can add this circuit inside and not even use a "tone" control.
It will give you a "notched mid" response which helps prevent the tone from being muddy.
A very useful circuit, I have added it to a couple of amps and played around with the values.
Made some crappy sounding amps sound great!

Cheers!

I'll add this circuit, it seems to be a logical thing to do...

Thanks!

[vmd1]

More thinking about the topology of this circuit has led to a few more questions:

1.Optimally, Where should the tone circuit stack or "notched mid" EQ circuit be... before or after the FET preamp stage?

2.Could a selector toggle switch be used to either apply the tone stack for a "notched" response in one position, and the other position, bypasses it and couples the FET preamp directly to the power stage?

[J M Fahey]

A guitar pickup can only drive the crudest tone control: a capacitor to ground killing highs, so for anything more sophisticated, you need some active electronics.
Meaning it should be *after*  the first gain stage.
As of making it switchable, be my guest

[vmd1]

So to remove this "notch" tone shaping circuit, Do I just have to put a spst switch right after the 4n7 capacitor that goes to ground?

by lifting the circuit from ground, will this allow the signal to pass through the circuit without being affected by it?

[J M Fahey]

Yes, between 4n7 and ground.
You will have a huge volume difference.
You might use a "big Muff"  type control, which gives you huge range with a single knob, check the TSC tone stack simulator for suggestions and adjustments.

[vmd1]

Well... I have another question about the volume control of this pre-amp. If I use a pot in place of the 51k resistor in this pre-amp schematic, should I wire the pot with all the lugs as a voltage divider, or should I wire the pot with just lug 1 and the wiper, as a rheostat? for the purpose of volume control. schematic is pictured below.

[galaxiex]

As a voltage divider.
Lug 2 would be the output.

[vmd1]

Would lug 1 go to ground?

[galaxiex]

Yes lug 1 to ground.

[solderdripper]

I bought this kit https://www.amazon.com/gp/aw/d/B00BKN1YTM/ref=mp_s_a_1_6?ie=UTF8&qid=1506789408&sr=8-6&pi=AC_SX236_SY340_FMwebp_QL65&keywords=nightfire+amplifier. I built it and connect guitar to input and speaker to output and it works well enough.  Now I have a base to build and test preamps against. I was working on a Phabbtone front end, hopefully I can get back to that soon.

Was wondering why you said you wanted 30w and then I saw the kit you linked. Isn't that a stereo amplifier? 30w might mean 15 on each channel.


Sent from my SM-G935P using Tapatalk