Quote from: TomHealey on October 07, 2012, 05:37:20 AM
You can consider different connections as an error, but it is not in term of workability In other words, it works good and it is simplification. Tone is exactly the same (as the tonal characteristic stays, look at the attachments), the only difference is - if You turn down middle, treble and bass - you will shout down the signal, there will be no flow. In Marshall style tone stack, it is not possible because middle pot cannot be shout to the ground by the wiper, as there is no connection between wiper and bass potentiometer.

If it comes to the sound, I like it as it is, but of course anyone can do any modification they like  :dbtu:

Thanks man for the explanation.. maybe i should say : "i Think there is an error .." sorry 'bout my poor english I`m from Peru.   :dbtu:
If you did the schematics like that on porpouse .. then there is no error.. 
by the way one question..    mids bottom goes to ground right? and mids lug connected to bottom? .. thanks in advance..  ohh and very good idea to put a buffer at the end.  :dbtu:
 

Hello everyone here.. i`m writing from Peru.. Cool project by the way but i just found an error in th schematic and in the layout. 
In the Original schematics of a Marshall amp in the tonestack the 22n is conected to the wiper of the Mid Pot.      I'm i right ? 

i made first the Layout posted here without mods.. and comparing with the moded tone stack with the marshall specs. i found that the first one sounded Fenderish an the second almost like a real Marshall .  :duh
what dou you think? 

Quote from: Loudthud on September 20, 2012, 11:47:01 AM
C32 is a fairly high value limiting the high frequency open loop gain of the Randall amp.

I'm confused about C32 .. 

Quote from: Loudthud on September 18, 2012, 01:04:23 PM
Break the connection of C1 to ground and connect it to the top of R12. If you want to experiment, connect a 25 ohm wirewound pot (1 or 2W) across R12 and connect C1 to the wiper. That way you can adjust between voltage mode and mixed mode feedback. There will be some gain change as you adjust the pot. R12 can be 0.22 5W (or higher). JM basically gave you the answer in post #21.

Cool thanks Loudthud , so C1 and R3 should instead of C5 and R9? yes i think i need to read more about Mixed mode feedback.. concerning to the pot instead of R12 i think i'll keep R12 at 0.22 Ohms .. (mixed mode feedback) because i`m trying to get close to the Randall RG100es Power amp:

by the way i see in Randall's schematics R44 and C33 why are they there? 
Would be a good idea to put the same values of the Randall in my Darlington power amp (i mean only R44, C33,C36, R52 and R63)? thanks for all your help ..you're so nice people..

Any help please??? i need the values of the new components..  .

I think this will be my last try in power amp section then i`ll post mp3 demos  :tu:

Quote from: J M Fahey on September 15, 2012, 12:36:59 AM
Basically right, but please repost the schematic labelling every component , such as R1/R2 ....C1/C2.... Q1... etc.
Also label input/output and +/- on the speaker.
I *hate* explaining "the resistor to the left of the transistor which is upside down between ..... "
Also the added feedback resistor goes to the lower end of the speaker, not straight to ground.
Good luck.

Whoops i made a mistake in the drawing  like you said. Now here is the corrected schematic:


thank you ... :dbtu:

Quote from: J M Fahey on May 01, 2012, 12:03:02 AM
No problem.
I have posted it earlier myself, but don't remember the link, so repost.

EDIT: I saw the project you found.
Don't like it very much: it uses the obsolete single supply configuration, has no short circuit protection, the feedbark network is unnecessarily complex (and probably wrong) and it's not easy to modify it into a mixed feedback system.
Study this:
http://www.diyaudiotr.com/islecler/tammetin.php?metinid=6&durum=e#sonyorum
It is *very* crude, simplified to the maximum, but it works and can be built.
The PCB is easy to homebuild and works too.
And is very easy to modify to mixed feedback.
It's in Turkish but google can translate the page easily.
http://translate.google.com/translate?sl=tr&tl=es&js=n&prev=_t&hl=en&ie=UTF-8&layout=2&eotf=1&u=http%3A%2F%2Fwww.diyaudiotr.com%2Fislecler%2Ftammetin.php%3Fmetinid%3D6%26durum%3De%23sonyorum
Some small corrections:
1) forget the 150W they claim and even 100W, it can provide honest and loud 70W into an 8 ohm speaker.
2) the supply voltage suggested is not too safe.
I suggest (by experience) not passing +/- 40V rails, which demand a transformer of no more than 30+30VAC (or 60VCT).
3) there is a separate .pdf with the ready to print PCB.
www.diyaudiotr.com/resim/proje/kapibara/pcb.pdf
I recommend the iron on laser thermal transfer process, but it requires some experience.
Meaning, make 3 or 4 PCBs before trying this one.
I suggest not building this amplifier now, but it's good to know what you are getting into.
Start by making an LM386 amp.
Build and enjoy it. It is fun.
Later, build , say, some 15 to 30W with a TDA2003, TDA2030 or even TDA2050.
Build and enjoy.
Later, an LM3886 project. (50W)
Loud enough to play anywhere.
Go step by step, enjoy and learn along the way.

I see you live in Peru. Check what amplifier kits you can buy in an Electronics Shop.

Hello again , i wasn't too happy with the ic based power amp sound so i decided to build the power amp above based on Darlington transistors .. and the results were better than with the TDA7293 power amp.
Louder and punchier ..
Here the original schematics:


now my questions are:
if i want to mod it to mixed feedback is the following correct?
and what would be the ideal values ?


i really appreciate your help. Cheers from Peru.

Quote from: phatt on August 22, 2012, 09:36:29 AM
Hi Allan,
If you wish to take on high current DC power Amp design then head over to ESP site where you will find pages of darn good advice that might save some fumes and smoke. winky.

http://sound.westhost.com/projects.htm

Don't expect to read them all in one sitting the site is VAST.

The site is a gold mine for those wishing to explore the hidden details of such things as DC coupled Amp design and audio in general.

For guitar amplifiers, the more basic designs may well sound just as good as the more complex designs. 8|
Phil.

cool thanks for the link .. now i have something to read this weekend..  :tu:

Quote from: Roly on August 09, 2012, 11:16:09 PM
http://www.eetimes.com/design/audio-design/4015876/IC-Audio-Power-Amplifiers-and-Zobel-Networks-One-Size-Does-Not-Fit-All

Very good article thank you..  :dbtu:
This last few days i was reading a lot about mixed mode feedback and lastly zobel network or boucherot cell.. and it`s a lot more than one resistor and one capacitor  :duh
i'm not too good with theory .. but i`m more like an experiment guy hehe..
in practice i found a few things:   
Mixed mode feedback it`s a lot more interesting in Transistor or Darlington transistors based power amps .
Zober network don't have much impact in overall sound (in my opinion).
and following the article you posted sometimes can be dangerous for the IC 
i`ll keep experimenting and after some fumes  :lmao: will find what i`m looking for : A powerfull amp with lots of bass and almost the same response   of a Randall amp   lml.
Any comments are wellcome  :dbtu:

Quote from: J M Fahey on August 09, 2012, 12:59:02 AM
*Leave them there* !!!
They are a Zobel network and absolutely necessary.
Yes, the amp will sound somewhat punchier, nothing revolutionary but it helps anyway.
Y ..... Argentina y Perú un solo corazón !!!! :dbtu:
Hasta nos prestaron aviones Mirage en las Malvinas !!! :dbtu:
Gracias !!!!!!  :dbtu:

Zobel network mmmm i think i'll do a google search to learn about that..

Siempre al  lado  a nuestros hermanos Argentinos.. somos un solo corazón !!!

Yesterday made the changes to the power amp and the sound is a little bit brighter than before .. but with more output. 
I forgot to mention    that there is a resistor (10 ohm) and a condenser (100n) in series between the two extremes of the output..one to the positive side and the other to ground.

my question is: should i leave them there or put them away of the circuit?
Tanks ..  Greetings from Peru.         

Quote from: J M Fahey on August 06, 2012, 12:53:19 AM
Basically yes.
New capacitor 100uFx25V
Resistor on speaker ground= .22 ohms. 7 to 10W
New resistor= 470 ohms.
Do not fit R2 and C2.

Great thanks for your support you're so kind.. 

i`ll test that config and post the results ..  :tu:

i spent a lot of time reading about mixed mode  feedback and it was very interesting 
i need some help with the amp i just made.. based on this schematic found on the TDA 7293 Datasheet..


and would like to modify to mixed mode feedback so i came to this:


So my questions are:
are the modifications i made correct?
What would be the right values of the Resistors and the condenser?

i'm using the amp with 8 Ohm Speaker Cab.
Thanx in advance     :dbtu:

Quote from: J M Fahey on April 26, 2012, 04:02:45 PM
Start by linking to the project you found.
Schematics are easy to find, worst case you use one from a commercial amp (such as the Randall itself).
PCBs are the problem ... that's why I had to learn to roll my own.
Or bite the bullet and buy a chipamp kit, which can be modded.
*or* (heresy), buy/get/beg some dead "known" amp (Peavey/Fender/Laney/Crate/etc) from a garage sale, Craigslist or even ... a dumpster and use it as a half built kit.
Why a "known" one? : to easily get its schematic, and often a layout .
And commercial guitar amps usually already include mixed feedback.

Here the project:
http://www.eleccircuit.com/circuit-power-amp-otl-100w-by-transistor-2n3055-with-pcb/
the Original Schematics of the Randall RG100 ES .. i don't know if its right to post it here.. so i won't ..
give me green light and i'll post it   :dbtu:

Quote from: phatt on April 25, 2012, 09:35:20 AM

Quote from: allan_belt on April 04, 2012, 12:15:14 AM
Hello from Peru and congrats for the Randall Hybrid .. recently finished this project only have two issues:
1. Couldnt find the 2n5484 Jfet so i replaced them with 2n5457 i wonder if both have the same gain???
2. tested the sound out of the preamp section into another guitar amp and the sound is great!!! punchy and Heavy  :dbtu: but when i connected to the power amp section presented here the sound is weak and too trebly for my taste, is it normal??

on my experience with power amps the transistor ones are most pleasing sounding for guitar, and adds a little distortion to the signal when driving hard.. anybody know a 100 watts transistor power amp project with pcb and layout?? please let me know...
thanks for the project and please post some advances or maybe an audio demo..  :tu:
Allan Belt.

Re 2/ above?
What do you mean exactly?
If you plugged into the front (Where you normally insert a guitar ) then yes a big big difference would be heard as you have two preamps in series.

If you like it use it that way and save a power amp build. :tu:
Sorry i meant i plugged on another power amp (Effects return)

Re poweramps.
If 100watts or more is desired then yes plenty of circuits abound if you take time to search.
Rod Elliot site ESP has some good big amp circuits.
yeah i just found one in rod Elliot's site but i did't tried it yet.

But at 50 watts or less just use a chip ,,I have built and use an LM3886 chip which is a serious 40~50 ish watt output.
Phil.
I think i need more Power because of the volume drop on  the Extra Sustain mode.    And maybe Transistor based Power amp adds a little distortion of its own at a certain level  :tu: