This is my take on the Randall RG-100ES amp minus the original poweramp and reverb section - not to make it as complicated as hell for some beginners. For simplicity, poweramp is a TDA7293 based from Rod Elliot's application. Reverb section based from BELTON reverb brick.
Few notes:
1. In the reverb buffer section(starting from the 82n capacitor), I just directly input values from the Rebote 2.5 delay as this was originaly my plan to put a delay instead of a reverb. Feel free to experiment on the values of each resistors and capacitors or better yet use the values recommended according to the datasheet of Belton Reverb Brick.
2. Transformer is of course, as seen on the attached file, 25-0-25 / 3A to 4A.
3. 50K trimmers(0.5W) are used instead of fix 33K resistor(can be also used either) to adjust the correct Drain voltages of each FET according to the schematic or for your own tuning.
4. I didn't provide pads for the channel switching( just refer to schematic, it's very simple).
Comments, suggestions, and (violent) reactions are welcome. :)
Enjoy!. ;)
Thank you so much! Do you have it up and running? How does it sound?
Quote from: tra on October 12, 2011, 12:50:10 PM
Thank you so much! Do you have it up and running? How does it sound?
Sounds like a Mesa..:)..just kidding. You'll never now if you won't try, buddy. Haven't build it yet but I'm pretty sure that thing will work.
Looks good :tu:
Thanks for sharing :D
"100n C" = 100n Multilayer Ceramic Capacitor
(50K trimmers are used instead of fixed 33K resistors to adjust the correct Drain voltages of each FET or for your own tuning.)
What affect does this due to the circuit? and how does it change the sound of the pre-amp? more gain?
I have an old rg100es and looking to do a few simple mods with it.I am wondering if changing the fixed resistors to trim pots will help me get different tones/gain.
thank you!
Your RG100 is "the real thing", properly biased and powered with, say, 24V or higher.
The trimmers are a kludge to make poorly designed preamps work, even if fed absolutely inapproppriate voltages such as 9V.
You turn the little wheel or screw until you get 4 or 5V at their Drains, no matter how poorly biased they are.
Leave your excellent Randall alone, they have practically extracted the maximum from what they used.
Add some clean boost pedal between your guitar and it to slam it into heavier distortion, that´s all it takes.
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.
Quote from: allan_belt on April 04, 2012, 12:15:14 AM
1. Couldnt find the 2n5484 Jfet so i replaced them with 2n5457 i wonder if both have the same gain???
Those FETs are pretty close. The transconductance of the 2N5457 is spec'd at 1000 to 5000 uMho. The 2N5484 is spec'd at 3000 to 6000 uMho so on average, it's a little higher but it depends on the exact part you are using.
Quote from: allan_belt on April 04, 2012, 12:15:14 AM
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??
I would look for a bad component or wiring error. Try feeding another preamp into the power amp. Or just feed your guitar straight into the power amp.
Quote from: Loudthud on April 04, 2012, 08:27:01 PM
Quote from: allan_belt on April 04, 2012, 12:15:14 AM
1. Couldnt find the 2n5484 Jfet so i replaced them with 2n5457 i wonder if both have the same gain???
Those FETs are pretty close. The transconductance of the 2N5457 is spec'd at 1000 to 5000 uMho. The 2N5484 is spec'd at 3000 to 6000 uMho so on average, it's a little higher but it depends on the exact part you are using.
Thanx a lot for your support. i recently found the 2n5484 Jfets and the sound is improved now very tight and gainier than 2n5457.
Just one more question...the power suply needs 25-0-25 volts 3 or 4 A ... my question is :is necesary to use 1/2w watt resistors at least? or is ti ok with 1/4 watt? i mean in the preamp section.. Thanks in advance..
Quote from: Loudthud on April 04, 2012, 08:27:01 PM
Quote from: allan_belt on April 04, 2012, 12:15:14 AM
Quote from: allan_belt on April 04, 2012, 12:15:14 AM
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??
I would look for a bad component or wiring error. Try feeding another preamp into the power amp. Or just feed your guitar straight into the power amp.
Yes you are right i had a mismatched component :-[ now the power amp sounds great and powerful ... thanks for your help.. i `ll post the amp when i put it in a Dynacord Chasis i have empty.. and maybe some demos too... ;)
While this TDA7293-based power amp may sound very good, you won't get the original amp's response with it. The RG100 uses mixed-mode feedback to raise its output impedance.
Hello again .. 8|
Does anybody know any project to make a power amp close to the Randall RG100 with PCB and Layout?
i mean that uses Transistors intead of IC... ;)
Not that I know of.
And I have searched, that and many other things.
I laugh when people say "you can find *anything* on the Net"
Yea, sure !!! Keep searching !!!
You'll find (maybe) *some* of what anybody posted ... and what never was, never was.
You'll find lots of stuff on classic 40 years old tube amps, little on modern tube ones, almost nothing on SS ones.
Closest solution would be getting some classic discrete 100W into 4 ohms power amp kit and mod it slightly to add the mixed mode feedback.
You'd save 90 or 95% of the work.
Good luck.
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:
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.
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.
Quote from: J M Fahey on April 24, 2012, 11:34:26 PM
Not that I know of.
And I have searched, that and many other things.
I laugh when people say "you can find *anything* on the Net"
Yea, sure !!! Keep searching !!!
You'll find (maybe) *some* of what anybody posted ... and what never was, never was.
You'll find lots of stuff on classic 40 years old tube amps, little on modern tube ones, almost nothing on SS ones.
Closest solution would be getting some classic discrete 100W into 4 ohms power amp kit and mod it slightly to add the mixed mode feedback.
You'd save 90 or 95% of the work.
Good luck.
Yes i spent a lot of time searching for something close .. and i found on eproject with PCB and Layout of a 100W RMS power amp.. but .. i don't know if it will work :o
maybe i'll sepend more time learning how to add the mixed feedback..
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:
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. ;)
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/ (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:
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.
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..
(http://i50.tinypic.com/350j7g1.jpg)
and would like to modify to mixed mode feedback so i came to this:
(http://i49.tinypic.com/2lma2ia.jpg)
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:
Basically yes.
New capacitor 100uFx25V
Resistor on speaker ground= .22 ohms. 7 to 10W
New resistor= 470 ohms.
Do not fit R2 and C2.
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:
Yesterday made the changes to the power amp and the sound is a little bit brighter than before .. but with more output. 8)
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. ;)
*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:
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 !!!
http://www.eetimes.com/design/audio-design/4015876/IC-Audio-Power-Amplifiers-and-Zobel-Networks-One-Size-Does-Not-Fit-All
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 :o
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:
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.
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: 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:
(http://i50.tinypic.com/289ju2p.jpg)
now my questions are:
if i want to mod it to mixed feedback is the following correct?
and what would be the ideal values ?
(http://i50.tinypic.com/2gw6w61.jpg)
i really appreciate your help. Cheers from Peru.
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.
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:
(http://i46.tinypic.com/1zg3qes.jpg)
thank you ... :dbtu:
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:
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.
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:
(http://i47.tinypic.com/2daewew.jpg)
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.. 8)
Randall's power amp is not what you would call "mixed mode" feedback, it's current only feedback with DC correction. C34 blocks any AC voltage feedback, only allowing DC and very low frequencys to pass. The current feedback signal from R63 gets divided down by R52 and R44. C36 and C33 block any DC feedback through that path that would otherwise kill the DC coming back through R45.
This same type arrangement could be used with a Chip or other power amp if desired, although stability might become an issue. C32 is a fairly high value limiting the high frequency open loop gain of the Randall amp.
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 .. ???
Let's take a couple of steps back to the circuit in post #32.
Negative feedback, NFB, is applied to the input differential pair via R4. If this was all that happened then the overall gain would be unity because the input and output voltages have to match in the diff pair.
Now we introduce R3 and create a voltage divider, and if we reduce negative feedback we increase gain. The gain is now approximately R4/R3, 22000/470 = 46.81 times.
If this was also the DC gain of the amp it would tend to be thermally unstable, so we introduce C1 which effectively makes R3 open circuit for DC and the gain therefore falls to 1 for DC.
In round numbers, with +/-40V rails the peak output voltage will be about 40 volts, and if the AC gain is about x40 then the input for full output will be about a volt peak (a bit less actually), so the signal voltage at the base of Q2 will be the same.
Now in mixed-mode feedback you typically have about 10% current feedback and about 90% voltage feedback (i.e. via R4), so if the voltage feedback is a bit less than a volt peak you want about 100mV peak of current feedback at the base of Q2.
Your C5 does much the same job as C1 so I'd start off with a similar value.
Now you need to apply Ohms Law to your values for R9 and R12 so that you develop 100mV across R3/470r when you have full output. This could get tricky without having some gain in this path because you want to generate at least ten times as much voltage so that your added R9 is large compared to R4/22k so it doesn't upset the AC voltage feedback conditions, but you also want to keep R12 as small as possible so you don't waste your output power heating it up.
Me, I'd be inclined to go with a value of R12 that gives you about 100mV for full output, then use an op-amp to bump this right up to say 10 volts (i.e. x100), then you can have a large value for your R9 to bring it right back down again without upsetting the relationship between R3 and R4. This will also let you adjust the amount of current feedback by changing the op-amp gain and not need a power pot at R12.
The amp in post #35 doesn't seem to have AC voltage feedback at all due to the bypassing/filtering action of R46/C34. This helps to explain why the Miller capacitance C32 is so large; the loop may well be unstable at high frequencies without overall AC NFB because the loop gain will be close to the open loop figure, which will be very large; so they apply heavy local NFB to that transistor.
R44 and C33 serve the same function as in the circuit in post #32, except it is only dealing with the feedback voltage arising from the speaker current, i.e. current feedback.
HTH.