I've already suspected the power supply section, since I've thought of a similar scheme, but fahey told me that it's wrong. Using it as an amp's preamp with ±9v would be ok, I guess.

Found this schematic while browsing. Apparently it was made by a jazz player named Mark Smart (http://www.marksmart.net/gearhack/fenderizer/fenderizer.htm).
He said err.. wrote that it's a Fender Stage 160 preamp in a pedal. No distortion. Just Fender clean sound.
Thought I'd just share it here. Any comments would be appreciated.

Note: I haven't build it yet. So please don't ask me how it sounds.

It was Little Rebel. Only 2W amp, so I didn't expect much noise from it.
But surprisingly it was so noisy before mounting it in a chassis. And surprisingly quiet after chassis work.

I'm very interested, Phill. I followed your link but was unable to find the pdf link. A direct link would be appreciated.
Thanks before.

Just sharing my newbish "experience".
I also had noise problem when first building my own amp.
But it was BEFORE I put it in a chassis. After putting the whole thing into a chassis and connecting input ground to chassis, it was noiseless.
As far as I know, you should connect only ONE ground point to chassis. Please correct me if I'm wrong.

Wow.. Thanks alot again, Fahey. Your replies worth to read for noobs like me who try to optimize their budgets without risking quality and long term performance. I would suggest pinning this thread for future (newbie) members.
+1 again for you. :dbthumbsup

Thanks alot, Fahey. That was great. Too bad chip point is limited to only 1 point per hour. :-)

Does "power waste" in your explanation mean "power dissipation" in datasheet? If yes, I'm reading
@Vs=+/-20V & Rload=8ohm
Po=15W
Pdissipation=11W

Is it correct?

Thank you.
Anyone can explain regarding my last question (minimum current needed for a chipamp)?

Salam kenal juga. Saya dari markasnya Pak Jokowi: Solo, Jawa Tengah. :-)

Translations:
Nice to meet you too. I'm from Solo, Central Java. :-)

Thanks for the quick reply.

One more general question. You said more current capability is better for transformer. How to determine the MINIMUM required current from the datasheet? Or simply (for example) I=P/V=25W/25V=1A ?
I'm taking the numbers from LM1875's power output vs supply voltage graph. Just for example, not that I will use 25V instead of 20V that you recommend.

Awaiting you to visit Indonesia. ;-)

Wow, thanks. That clears my hesitations right away. :-)
I'm from Indonesia. On the world map you'll find it just above Phill's continent. (I think he's Australian. Am I correct?)

I am planning to build 3 separate boards: preamp, power amp, and psu; hoping that I can reuse the boards in future designs. Will I get problem with this? Particularly with noise and ground loops. I've read about ground loops but still don't quite understand.

Having built low wattage amps (Ruby & Little Rebel) with some degree of success, I am now looking to build bigger watt amps. I've downloaded Marshall MG15 schematic and intend to build the power amp section. The problem is that I can't figure the needed transformer's rating.

So here are the noobish questions:
What is the proper transformer's rating for this amp?
LM1875 datasheet says P out is 25W at +/-25V. Is it ok to use +/-25V? Will this amp actually deliver 25W instead of 15W if I use +/-25V?
I have a transformer with "1A" mark on it. It has dual 12V, 15V, and 18V outputs. Can I use it?
What happens if transformer's rating is below needed voltage and amperage? What happends if it is over/beyond?
How to decide the proper amperage rating from a chip's datasheet? I'm guessing I should not exceed max supply voltage, but cannot determine the needed amperage.

Sorry for the annoyingly confusing questions. I don't speak English very well and I AM a newbie in electronics. Any help would be VERY APPRECIATED.

Note: In my country, transformers are not rated in VA. They only marked 1A, 2A, 3A, 4A, 5A, and so on.

Hi, I'm interested in building this preamp for my next amp. Do you have noise problem at high gain setting? How do you filter your power supply?
Thanks before.

Just found this. The pic is not complete but seems familiar. :cheesy:
http://www.guitar-pedals-effects.com/LinkedDocuments/GV-2.gif
http://www.guitar-pedals-effects.com/MarshallGuv%27norGV-2.html

Quote from: J M Fahey on August 12, 2012, 03:09:01 PM
It can be done, of course, but I fear it will introduce new variables into what we are trying to keep very simple.

Oops I didn't think of that. I guess I will upset the low pass and high pass filters and such.

Quote from: J M Fahey on August 10, 2012, 10:48:11 AM
It's basically a ripoff of the Marshall "Guv'nor" family pedals,
http://www.freeinfosociety.com/electronics/schematics/audio/pictures/marshallguvnor.gif
with a bass boost added (since it will typically drive very small speakers) , straight driving a flat LM386 amp.

Btw Fahey, when I looked at Little Rebel and Guvnor, I realized that they're virtually identical. The difference is that in Little Rebel the bass control is made "fixed" with 1k5 and 100k resistors, and it uses (as you mentioned) a variable bass boost instead.

So here's my mod:

• Replace 1k5 and 100k resistors with a 100k pot and use it as bass control.
  
• Replace the 1M bass pot with a jumper. Or better yet, rename it to "depth" or "bottom" control, I think it's more appropriate. But my front panel is too small and crowded, so I just jumper it.

Viola... The tone control now have better and more "normal" freq response, especially bass freqs. :dbtu: <3)

Thanks for pointing out. :dbtu: :dbtu: