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Started by Neosho, September 09, 2009, 10:11:40 PM
Quote from: Neosho on September 13, 2009, 09:30:01 PMI want to try to design an SS amp that has some reasonable limiting on the preamp stages, but it is very hard to "softly" clip due to the crest factor problem. The amplitude of the guitar signal's peaks are very large compared to the average signal. So I haven't figured out any good approach that seems worth building. I have tried the easy circuits (like diode limiters) that you see in the old schematics but they always sound very bad compared to a good tube amp or the digital processed modelers. Anyway, I keep looking. Thanks.
Quote from: Neosho on September 17, 2009, 10:59:54 PMHi phatt:You asked, "why not use an output transformer?" Good question...I am not sure if it would help or hurt. It seems like it would be very good at preventing DC offsets from getting to the speaker and damaging it. Instead, the amp's transformer would become damaged. At least you wouldn't be frying speaker coils due to a transistor getting shorted to the power rail, so that's some progress.I know that old amps like Acoustic used transformers, but maybe it was because they were limited on the availability of transistors (only NPN? was available for awhile).The hi-fi amp books I have read argue against output transformers. I have Doug Self's book and Slone's book. But their interest is sound reproduction. The cons are: transformers add cost, aren't really needed, cause phase shift, make the amp more narrow band, add weight and mechanical complexity. Certainly you never see output transformers in newer SS amps built by Fender and Tech21, etc.I have seen the theory that transformers have some ability to saturate non-linearly, and that is part of vacuum tube tone. I think there is even some patent where the author introduces DC offset on purpose to try to imitate tube tone. I don't know if these things are provable or just wishful thinking. I know the some of the tube people think that it's helpful to use a high class transformer to get the best sound, and there are differences in the construction that are important. Mercury Magnetics sells an upgrade transformer kit for the Valve Junior for asking price of $299... seems like a lot of money for an amp that can be seen on craigslist for $100.Do you have a theory about output transformers?
Quote from: armstrom on September 23, 2009, 03:16:25 PMWay off topic here, but just to respond to R.G.'s last post:This is why I plan to build a nice, low wattage tube amp that has exactly the sound I want (Maybe a Fender Champ design, AX84 P1, or even a 1W firefly) then get the best dummy load you possibly can and feed it into something like a LM3886 power amp.The trick is finding a good dummy load and voicing the power amp/speaker combination correctly. My end goal is to have an amp that I can switch between having the tube amp drive the speaker directly or have it drive a dummy load and let the SS power amp drive the speaker. The only noticeable difference should be the maximum volume available (at least that's the goal!). We'll see if I ever really get there It all boils down to how do you want to achieve the "tube tone" in a louder, lighter and more reliable amplifier package. I see two possibilities, you can either get the tone in the same way people have been doing it for decades, then just make it louder with a high quality SS power amp. Or do you invest lots of time and effort trying to "emulate" the sound of a tube amp without using tubes? Depends on what you're after really. It's almost a guarantee that the cost of building a hybrid amp will be significantly higher than an emulating/modeling amp just because of the "iron" required by the tube section.It seems to me that the "tube sound" has almost been relegated to only a few genres of music (dirty blues, for example) and a set of purists who must have it for what ever reason. If you look a areas that used to be dominated by tube amps (hard rock, heavy metal, etc...) you have to admit the trend seems to be moving toward using extensive effects systems (pedals, rack-mount modelers, whatever) feeding into a super clean solid state amp like a JC-120 or something similar. The wall-o-marshalls just doesn't seem to popular any more It's just my $0.02 though. I've likely completely missed the mark on this.