Hey Guys! This is my second post here (my first was about 30 seconds ago). I own a Peavey Rage 158 and I absolutely love it. It sounds perfect for the kind of music I play, but it's just not loud enough! So I figured that I would try to build a Rage 158 but with a more powerful power amp. Then I said to myself, "If I'm going to build in more power, why not build in an entire second channel?" So now I'm thinking, two complete Rage 158 preamps (each with a clean volume control, distorted pre-gain, distorted post gain, a switch to choose between clean and distorted on each channel, each with it's own TMB tonestack, and a switch to choose between a vintage or modern countour) running into a much more powerful power amp. The power amp must retain the original sound, just louder! So, should I just copy the output stage of the Rage 158 and run two or three or four of those in parallel? Also, what power supply mods will i have to make?
Here's a link to the schematic (without modern/vintage switch):
http://www.freewebs.com/andrewrovner/Peavey%20Rage%20158.pdf
Thanks in Advance!
Rock'n'Roll,
--Andy
Well, my first suggestion for getting more power would be using a higher power chip from the TDA series since the amp already seems to have one. This definitely also includes changing the power transformer to a proper one; it has to have a higher supply voltage and a higher VA rating. You probably have to redo the whole power supply. Extra bad is that the circuit is single supply, it will undoubtedly cause problems. You shouldn't forget heat sinking either - more power means more iron. Also, if you change the speaker to match up with more power the amp will not have the same tone - even ignoring the speakers frequency response: The power stage runs in mixed-mode (note that feedback path after the speaker) and is very much dominated by the load impedance. Put a new speaker in and the carefully designed circuit becomes... well, not so carefully designed. You have a 50-50 chance of getting a good tone from it.
Taking this path will quite surely mean a complete rebuild and redesign of some parts. The result will not be the same low power amplifier, thus the tone will not be the same either. However, this should be easier than it first seems - at least if you find a high power TDA chip that shares the same pinout and works on single supply. It also could happen that PCB traces are too thin for higher currents and there is insufficient room for a bigger heatsink. Anyway, i'm quite sure that properly done the end result would be far more better than the original circuit though.
Another option is to replicate the power supply and power amp and run the amp in stereo. You have to find a matching speaker though - or at least one that suits you. You could built as many of these as you want, say for example four: That would equal power of 60 watts - which you could get by building only one new power amp and power supply circuit. Probably the same amount of work. I guess parallel configuration wouldwork as well but once again you need a power transformer which is capable for feeding enough power to the amp. Probably you need a higher power speaker too, thus changing the tone. Frankly, i see no point of using two 15 W amps in parallel or in stereo - if this is for loudness reasons just mic the amp or find a more powerful speaker system.
By the way, thanks for the schematic. It's the first "TransTube" amplifier in my collection. :)
I think he meant building a new one, not rebuild his current peavey. Anyway, the easiest way to get more power out of it would be to build a power amp and just line out from the peavey.
As far as power supply goes, you'll need to redesign that quite a bit to accomodate more voltage/amps as well as grab a nice large transformer. Again, it would be easiest to just build a power amp and use the peavey as a pre.
Yeah - I wan't to try and build a completely new amp. So what would be an easy way to get around 100watts of power that wou'ld still sound similar to this original Peavey? I'm assuming that using a chipamp or two would work.
Rock'n'Roll,
--Andy
100watts is easily obtainable with a chipamp. I would recommend using a LM4780 for the poweramp section, this would give you around 100w RMS of power and they are relatively easy to work with. As far as tone goes, 99% comes from the guitar, preamp and speaker choice. A good poweramp will not color the sound.
"A good poweramp will not color the sound."
I have to disagree on this since it's a statement mainly reserved for High Fidelity reproduction of sound, not production that is going on in guitar amps. A good guitar power amp may, or may not, color the sound - depending on how it reacts to the varying load impedance.
I second using the Peavey pre and building a new power amplifier circuit and a matching supply. But there is a but: You want to retain the similar sound of your amplifier, in this case you have to figure out what IS the sound that you like and what makes it: Do you have the "sound" already in the signal that you get from the preamp (I guess you should get a signal out from the CD in jack) or is the speaker and the power amplifier an important factor to it. If you play your guitar "dry" to power amp (from the CD in jack) do you get the "sound" this way instead. (Note that there will most definitely be a terrible impedance mismatch and you should try playing a guitar signal from a tape or computer instead). Is the preamp tone a lesser factor than power amp tone or do they equal. What aspects of the sound you appreciate most.
It might help to point out that all the difference you hear in headphone out vs. speaker is due to either speaker's frequency response or power amplifier reacting with the load impedance. All the difference you hear from the CD in is due to power amp - speaker interaction too. Does the headphone/preamp signal seem to lack bass and highs?
Building a new amplifier will not sound identical, it may sound "similar" though. If you want an easy way to build a high power (100W) amp i suggest you avoid discrete designs. Two parallel chip amps and a rugged supply should be OK but if a huge part of the "sound" lies within the interaction of poweramp and the speaker then you have to match the new and more powerful speaker system to the old. This means at least matching up both resonant frequency and HF roll-off of speakers. Then you probably have to design a mixed-mode power stage that drives your speaker setup properly too, otherwise you will have less effect of speaker's impedance.
I wrote something about this earlier. It might explain what i'm talking about better:
http://www.ssguitar.com/index.php?topic=75.msg785#msg785
Yeah, I guess I should have said that for the most part a poweramp won't color the sound, at least unfavorably. I've tried lm3886 and lm1875s for poweramps and neither one seemed to shape the sound, some very very minor differences were noticed though and I like the lm1875 a little better.
Does anyone have a schematic for a LM3886 or LM1875 based power amp? About what is the maximum output of each chipamp? Also, how easy is it to make these chip amps distort/clip?
By the way - I finally got a schematic for the version that I have:
http://www.freewebs.com/andrewrovner/Rage%20III%20schematic.pdf
Rock'n'Roll,
--Andy
The schematics are readily available.
LM3886
http://www.national.com/images/pf/LM3886/01183301.pdf
LM1875
http://www.national.com/images/pf/LM1875/00503002.pdf
The LM3886 will give you roughly 68watts and the LM1875 only 30W, of course, this is RMS power. Roughly double that for peak.
You can actually get kits from chipamp.com for both of those, and if you wait a little while, you can get one from me for free, just pay shipping. Brian, the guy that runs chipamp.com, is setting me up with some LM3886 and LM1875 kits to distribute here.
**edit**
I listened to your free mp3s on your site, not bad. Where can I get the Big Band EP?
joecool85: You can get The Big Bang EP by sending me an e-mail so I can give you my address and then you can send me $6.00 (plus shipping) and I'll send one to you.
So if I ran two LM3886 in parallel I could be pushing 142watts without clipping in the power amp, right?
Rock'n'Roll,
--Andy
Note that the introduction of mixed-mode output to these designs is no rocket science. Just compare the schematics. :)
Be aware: This circuit is conceptual and i haven't been able to test it in real-life. Simulation shows it works but the real circuit might experience some instability problems - also i suspect the gain might be too high. On the other hand, the introduction of protection diodes and Zobel network might even make it more stabile than the example application circuit.
Noobie question - What exactly is meant by "mixed mode"?
Thanks for those schematics!
Rock'n'Roll,
--Andy
Rod Elliott explains it quite well in his article about variable amplifier impedances (http://sound.westhost.com/project56.htm). The following quote is an excerpt from it:
"Voltage drive maintains a constant voltage across the load, while a current drive circuit will produce exactly the same current into the load - in both cases regardless of impedance."
If you think of the quote in a context of gain it makes more sense. If an impedance at certain frequency is R and P=UI we get: P=U^2/R or P=I^2 * R. You see the R is a dominant factor. In the first formula, large R means smaller power and in the second formula large R means higher power - and opposites. Now think what happens in both cases (voltage and current drive) when the speaker impedance might swing anywhere between 1 and 150 ohms - according to the model of course. Here's a link to a datasheet of one Eminence model speaker. Look at the impedance graph, it's a quite good example of any guitar speaker:
http://www.eminence.com/pdf/legend-1028k.pdf
If you read the thread i posted a link to (http://www.ssguitar.com/index.php?topic=75.msg785#msg785), you already seen the effects of both ways to drive a load: The frequency response of the power amplifier will change from linear a great deal into enhancing bass and treble. Some people regard this as "warm" and "punchy" tone. For high-fidelity systems any colorization is of course a bad thing, but for guitar... well, it's a matter of taste. Mixed-mode drive is just a mix between the two - current and voltage - a compromise between two unideal approaches. If you look at Rod Elliott's article you can see a simple schematic how it is achieved by combining two feedback paths: One for current, one for voltage.
Ok - I understand "mixed mode" now. I'm probably just going to go with a voltage drive circuit. I drew up a schematic for my concept last night (just the signal path, no power supply, yet). Is this the propper way to 142 watts out of the power amp?
Click Here For Schematic (http://s19.photobucket.com/albums/b189/gotchaguys2/?action=view¤t=EntireAmp.jpg)
http://s19.photobucket.com/albums/b189/gotchaguys2/?action-view¤t=EntireAmp.jpg
Be warned - the picture is about 1500x573 pixels! It took alot of room to draw in two completely identical channels.
Comments, corrections, and other observations are greatly appreciated!
Rock'n'Roll,
--Andy
So far it looks good to me, but I just took a quick look. I'm at my parent's house right now.
The volume potentiometer(s) trouble me. If it's not ganged then you have a serious imbalance between the two power amps. On the other hand, if it's ganged then why not use just one?
Yes, my intent was to use a dual ganged pot.
Rock'n'Roll,
--Andy
I looked the schematic more closely and it seems to have at least three identical gain circuits that have no controls. If you only switch between preamp A and B (and do not, for example, blend them) you don't have to build gain stages separately for each preamp. (Ie. Sw1 pole also to sweeper of both 1M gain set potentiometers and to "input" of both 10k gain set potentiometers. The other gain stage looks like it could use a very similar arrangement). If you figure out the switching and signal routing more thoroughly you simplify the circuit a lot. Trust me. You probably have to implement switches with more poles to succeed. Also, i already mentioned about the "master volume" potentiometer.
Edit: I'd use a mono potentiometer and put sw1 before the series 10k resistor and 33n capacitor at the sweepers of 50k potentiometers: You now now need only one 10k resistor and 33n capacitor instead of two.
A thought occured to my mind... If the switching arrangement gets too complex it might be a good idea to replace it with CMOS switches and design a control arrangement for them. This has some additional benefits: The signal path will be shortened and the switch's "life" will be hugely extended. Also, it allows implementing a good foot pedal control. This could be done with discrete components too but i consider using CMOS chips as much simpler solution.
hi,
very late and completely unrelated to the last postings- but: would it work to "tap" the output of the small peavey by using a small transformer and just feed that signal into any large
ss-poweramp?
basically the interaction of rage-amp and speaker should be preserved like this and you can simply plug it into any cheap PA and drive a marshall cab (good for rock'n roll).
Not a bad idea el mo. I was actually thinking about something along those lines where I could just rehouse the peavey in a smaller case with no speaker and feed it into a reactive load and then EQ it so it sounds like it's coming out of a real speaker and then feed it into the PA and then a cabinet. Also, if I could house the reactive dummy load, speaker sim EQ, and power amp all together with little to no knobs, then I could use it with low powered tube amps (am I allowed to say that on this forum?)!
Rock'n'Roll
--Andy
This will be problematic only if you want to switch the Peavey´s speaker off while plugging to PA. But why use a transformer: Just put a high impedance voltage divider to the output (in parallel with the speaker) and buffer it's output. A high impedance (>1Meg) will be virtually "invisible" in parallel with the speaker load and the voltage divider is used to attenuate amplifier´s too high output voltage to 1-2V range - which should be suitable for most line level inputs. Then just feed the output signal from the buffer to the PA. This is the most basic form of the circuit you'll need. If you want to isolate the source from PA (to reduce hum, have balanced input etc.) you can put a 1:1 transformer to the buffer output.