There is also a 35watt Jensen Mod (in 10 and 12 inch) that can be had for a bit cheaper than the 50 watt. For a 18watt amp, it should be plenty strong enough to handle all the output power you can muster...

here's a more moderately priced lm3876/3886 amp kit: https://secure.vividcluster.crox.net.au/jaycar2005/productView.asp?ID=KC5150&CATID=25&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=557

Quote from: MonkeyBoi on September 18, 2008, 05:10:32 PM
No load would mean nothing connected to the output of the secondaries? I do have a Fluke that can read AC, DC and Amperage

With no load means with nothing else connected to the output of the secondaries. Of course you have to connect your multimeter, or you ain't measuring squat!  Also, it doesn't matter to what side you connect the negative or positive... either way will work and give you the total volts present. Remember, however, if your tranny is center tapped, and you connect between the CT and one end, then you'll only measure half the voltage of the total secondary winding, which is good for estimating what you'll get in terms of + and - DC rails after rectification (multiply the AC voltage by 1.4 to get approximate DC voltage)...

Hey Bro, if you are definately want to build from a kit, you should veer away from the Velleman ones and more towards something like this: http://www.chipamp.com/lm3886.shtml You'll get a better quality kit with better quality parts, for the same price, and it will actually work as described. I haven't built one of these kits (I've never built a poweramp from a kit, actually), but I have/am building an amp with the same chip (lm3886), and a quick perusal of the archives here will show you that this is really the best way to go... Many of us have had/are having great success building lm3886 amps with ROG preamps. Look over in the preamp forum for many great posts on using ROG circuits as preamps (something I've actually been discussing a lot in there)
good luck and happy building!

Well, the simple answer is: figure out the voltage of the secondaries given the primary voltages, then figure out the amp rating of the transformer, and multiply them to get the VA (VoltAmp) rating of the tranny. This may not be as easy as it sounds. Best would be to find some sort of indication written on the xformer itself. Barring that, you can use any standard multimeter to get the AC voltage of the secondairies (make sure you read the instructions for your multimeter thouroughly first! and just be sure to take all the necessary precautions when dealing with mains (potentially lethal) voltages...) You just hook up the multimeter across the secondaries (no load) and set the multimeter to the proper range of AC volts. Getting the current rating with a multimeter is slightly more problematic. First you must have a multimeter that can read AC current (mine does only DC current), then I believe if you measure the current on the xformer secondaries in a dead short (essentially connecting the two leads of the secondaries through the multimeter), you should get the max current your tranny is rated for (someone correct me if I'm wrong here).
Cheers,
DDC

hmmm... I took a look at the original princton schem (http://www.freeinfosociety.com/electronics/schemview.php?id=802) and the feedback loop does connect to the output of the output tranny. It also connects to the grid (? I think it's the grid, I'm not a tube guy) of the 6y6, which I think is what the ROG guys were copying by having the loop derive from the last FET's drain, although that would go against their logic for the tube to FET switch. Here's what the the ROG guy's say they did:

Quote"We took the 5F2-A schematic and copied it part for part using MPF102 JFETs in place of each tube stage. Each tube Grid was replaced by a JFETs Gate. The tube Plates were analogous to a JFET Drain. Finally, a tube Cathode was replaced with a JFETs Source. We used a 100k trimmer for the plate resistors on the schematic due the fickle nature of JFETs and the much lower power supply involved."

One interesting thing I see when I look at the original princton schem is that the NF loop is connected to both halves of the 12ax7, and the Pof tweed has it connecting just to the second analagous half (ie, the second FET). Does anyone know a practical reason for changing this? Also, it seems that while the (positive) feedback loop of the Prof Tweed connects to the source of FET #2 (second analogous half of the 12ax7), the original schem connects the NF loop to the plates of the 12ax7 (which are supposed to be the analogical drains of the FET's)... Anyone know why that would be done?
  All that being said, why then does the circuit work and even actually sound good? Not ever having heard a real fender princeton, I cannot say myself that the circuit sounds anything like it is "supposed" to sound, but I've read comments on the stompbox forum that indicate that it does actually sound similar. Is this all a happy accident?

   Well, I'm still getting a "500" error everytime I try to post a schem, so I guess I'll have to switch tracks here.
    I looked again at the preamp I started building before I left two months ago, and I realized/remembered that I was actually building a Professor Tweed, not an Eighteen! I remember now that I was going to build the eighteen until I read a couple of posts over at the stompbox forum (http://www.diystompboxes.com/smfforum/index.php?topic=52322.0 and http://www.diystompboxes.com/smfforum/index.php?topic=21396.0) where people were having volume issues with the Prof Tweed, but solved them by changing the value of the capacitor in the feedback loop (the gain is set by an NF loop). The capacitor (C7 in the schem I have) in the original design over at ROG is supposed to be between 10uf and 1uf, but in reality it should be much lower than that to ensure a proper amount of gain. I just finished building the circuit, and I used a value of 0.01uf (I made it socketed, so I can also try other values), and I got PLENTY of volume. In fact I was so impressed with my trial today I made a YouTube vid of me playing through it while it was hooked up on my bench. Here's the link: http://www.youtube.com/watch?v=ErnjbJJwqoM In the vid, I plug the Prof Tweed staight into some crappy computer speakers (which are looking for about a 1v input signal, much the same as a poweramp chip would be), and I get fantastic volume. In fact, I had to dial down both my mic (I'm a harp player) and the volume pot on the computer speakers because I was getting too much feedback!
    So, Armstrong, if you are still working on your amp, I suggest that you switch out the feedback cap before you try any of the other suggestions (the extra op amp stage, etc.)... It should work for you.
   FYI, the only other mods I did to the original Prof Tweed circuit was to increase the decoup,ling caps from 22n to 0.1uf (better for harp), to add an addition decoupling cap tot he input (also 0.1uf), and I changed the value of the caps in the speaker sim section of the circuit from 2n2 to 5n8 following some advice that it would increase the bass response and make it "warmer" which is what I wanted...http://www.diystompboxes.com/smfforum/index.php?topic=63944.0

Okay, I'm having problems posting a schem... I get "Error 500 - Internal Server Error" every time I try to post it!!! Anyway, here is the text the post I wanted to make, and I guess I'll have to try to post up the schematic later on... sorry about that!

Original Post:
"Actually, now that I go back to read that page I see that it was only one of the pages I used when fully understanding how to design my circuit, and is actually not at all the same as the one I designed. I've attached a scan of my pencil drawn schematic, which, now that I've thought about it more, I actually drew more from the first "typical" application schematic in the datasheet than from any design I found on a website. You'll have to forgive me for the mix-up, I designed the circuit several months ago, but haven't gotten around to working on it again until now because I've been working oversea's for the last couple of months! Now I have learned the "hard" way not to post before thinking!"

Hey, I just had another thought. I don't know about your Tripath chip, but the LM3886 has an easily adjustable gain (here's alink to a page describign basically how I'm going to build the LM3886 http://dogbreath.de/Chipamps/ThreeResAmp/ThreeResAmp.html) You just adjust the value of two resistors (Rf and Ri in the schematic). If you are having low volumes with the preamp (prof tweed) you built, can you not just rais the gain of the poweramp chip? I know when I was designing my amp, I made sure that I would be matching the amount of gain of the poweramp stage to the output level of the specific preamp I am using (eighteen). Now of course I have not finished yet, so I'm not sure if this will work out. Does anyone see any issue with this? It would certainly be best if one did not have to build in yet another gain stage between the pre and power (simpler is better, right?)...
Cheers,
DDC

Hey Armstrong... I just wanted to drop a note to say I'm basically at the same stage as you, both in my electronics education and in my amplifier build... Instead of the Prof. Tweed, however, I'm going to try the the Eighteen, as I've read that it has been used as a preamp with better results than the Prof tweed. Also, my poweramp is an LM3886 which I'm powering at +-18v (so it should be in the 40ish watt range)... I'm 90% done with the Eighteen (I need to add the pots and properly bias the transistors), and I have not yet assembled the LM3886, but I have the power supply finished (~80% of the work!), so I should be ready for testing in a week or two (with my busy schedule). When I get it built and tested, I'll let you know my results. Also, if you end up going with an IC voltage amp stage between the pre and power, let us know how it works, and what, exactly you did. I'd be very interested!

Cheers,

DDC

Hi Streel,
  I think that I am the only one on this forum that has actually built this preamp. Before I answer your questions, I must say that while this preamp may not "sound like tubes", it does sound very good. If you check my posts on this preamp in the preamp forum, you will see links to some YouTube videos of me playing harp through it. I cannot vouch for how it will sound with a guitar, however.
  Now to answer your questions. The trimpot at R23 IS the gain pot. I'm not sure why he wanted it to be a trimmer in the original schematic as it is much more functional as a user adjuxtable pot (I used a 100k linear). There are no tone or volume control at all in the second schematic as he suggests each builder would want to add their own prefered circuits here. I agree with that. I added these directly to the output of this circuit with good results (after C4). I used the "stupidly wonderful tone control" and the "better volume control" circuits. You can find them easily by googling those names.
   I did not try to build his first design as it was way to complicated for what this thing does. You should get good results by building the circuit as shown in the second schematic, with the drive mod, using a real pot for the gain, and adding some simple tone and volume to end. I never added a "body" control as a) i have no idea what "body" really means, and b) I never saw where one could add such a control into the second schematic

Cheers
Darwindeathcat

Looking at your schematic and also taking Teemu's wise advice into acount I can tell you the following: The "Zobel Network" is the series cap and resistor to ground at the output of the lm386. This is used to prevent high frequnecy oscillation when driving a speaker load, so you don't need it when coupling to another amplificatin stage (and Teemu confirms this in his post). He also explains that you don't need such high decoupling capacitance (the 220uf one) when coupling to high impedance loads. Your poweramp's input impedance is set by the resistors at it's input. currently you have 4k7 + 10k to ground when the trimmer is set all the way up. You'll have to calculate the output impedance of the lm386, which if i remember correctly put out about 1/2 watt of power. You'll likely need to figure out what the voltage of the output signal is (find the circuit's current draw and use Ohm's law, or just use your multimeter). Your poweramp probably want's to see an input signal of no more than 1v, so you'll need to calculate the resistance you will need to drop the signal to that voltage. Then your 100k pot in the position you have it will work fine as a VC, and you will not be overdriving the poweramp at it's most open position.
As for the new value of the decoupling capacitor, I'm not sure how to figure those calculations, so maybe Teemu or others can help you there....

Also, thanks Teemu for your advice on the FX loop idea. Of course it is a good one, and I have seen sever schematics (including in your e-book) for doing it. My major problem is that I am working with a "vintage" chassis (an old tube radio), so I was trying to limit myself to the number of holes that were already drilled in it in order to preserve the "look of the thing". So I had come up with that idea as an "unorthodox" way of doing the job while sticking to only two jacks... However, after hearing your advice, now I am considering to *gasp* drill another one myself... Hope it don't mess up the looks of the thing!

I'm actually right in the middle of doing the same thing with my LM3886 amp (but using the proffessor tweed circuit from runoffgroove.com for the pre), and was just researching the best way to add a preamp out jack. What I am going to do is to actually just have the leads for the out jack connect directly to the lugs of the master volume pot. This can be done two ways: one way has you connect to the "out" lug so that the connection comes before the VC, and the other way is to connect the jack to the "in" lug, so that the position of the VC pot has no affect on the volume of the preamp out. Here is the interesting thing, however: Provided that you build a simple bypass switch to the internal preamp, you can use the preamp out jack in reverse (that is, you plug an external preamp into it, and feed signal in to the power amp). Why do it this way? Well, 3pdt switches are expensive, and you would need to use one if you wanted a true bypass (one that shorts both signal and ground) so you could then just use the normal "in" jack in bypassed mode. But i am poor! I have some DPDT switches (much less expesive and common), and I have some stereo jacks. It is well known that when you use a stereo jack with a mono plug, you can use it as a switch (connecting the ground with ring two of the jack to make a ground connection, and thus switching on the circuit). Combine this ablilty with the simple DPDT switch and you've got a true bypass when the input jack is unplugged and the DPDT is switched over. But you need to feed signal to the power amp, so you feed it through the jack that would normally be used to send the preamp signal out to the PA... Get it?
Anyway, that's a bit complicated... To address your question, it does not really matter if you keep the 10k trimmer or not. Just know that any resistance in series adds up to the total resistance your signal will have to cross before it enters the amp. The both the VC pot and the 10k trimmer will act as variable voltage dividers sending varying amounts of the signal to ground, depending upon how you adjust them. You don;t have to totally remove the 10ktrimmer, though, you can just turn it all the way open so that it sends all the signal through...
Actually, I took a look through the ruby/little gem schematics again, and I noticed that all of them have that unusually large output cap (220uf) originally, I misread it for 220nf, which is a normal size for an output cap. I also noticed that it is an electrolitic, so it must have to do with a requirement of the lm386 rather than normal DC decoupling. In that case I would keep it there... 220uf is quite high, so I would defintately keep the 0.1uf cap in the input of your poweramp. The ruby is meant to drive speakers directly, so that might also have something to do with it.
I also noticed that the "little gem" has it's VC AFTER the lm386 http://www.runoffgroove.com/littlegem.html You could probably just do your master VC just the way it's done in that schematic. I once built a "noisy cricket" which is a ruby-type amps that has similar tone controls to what you propose to do, so I would go over to beavis audio research and read up on how he does it...
Hope that helps...

Teemu, Yet again you have been able to teach something to me that I did not know before! Thank you... I now fully understand the use of zener diodes in this application. I now wonder if three-pin voltage regualting IC's (like lm7812) work in the same way? I mean are they basically that circuit but in IC form? I suppose I could just use one of those regulators for this purpose (duh, why didn't I think of this earlier!), which would be doing the same job!

Armstrong, looking at the schem you linked to, I note the following. The stock 10k trimmer works with the 4k7 resistor to set the input impedance of the amp, and thus work as a VC. Replacing the trimmer with a 50k log pot will allow you to dynamically change the input impedance over a larger range, but not that much larger. To be honest, you may want to bump up to a 100k pot for even better control.
The power amp schematic also has a 0.1uf input coupling cap, so you actually do not really need the output coupling cap in the ruby at all (the DC voltages will already be isolated by the poweramp's input cap). The couplng cap value matters only in that it effects the lower end of the frequency spectrum let through to the next stage. Larger cap values let more bass through than will lower values. 0.1 uf will let PLENTY of bass through, so you can go smaller (0.047 or 0.022uf) if you want less bass. Also, FYI, the VC pot you are planning to put in will work with any output cap on the ruby to create a highpass filter, so the if you choose to keep an output cap, be aware that the tone will probably change a bit as you adjust the volume.