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What is opamp design?

Discussion in 'Amps and Cabs [BG]' started by Arranger, Apr 18, 2004.


  1. Arranger

    Arranger

    Mar 9, 2003
    Pennsylvania
    ThunderFunk describes its design as pure opamp and transistor solid-state. What does opamp mean? Thanks-
     
  2. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Op amp means "operational amplifier". It's basically a bunch of transistors on a chip (an "integrated circuit"), wired to create a "theoretically perfect" amplifier. Electronic designers like op amps 'cause they're easy to work with and design around. Just as with any other electronic component, there are good op amps and bad ones. The 741-type op amps are probably the most common, but the 5532-type op amps are more expensive and sound a lot better.
     
  3. Arranger

    Arranger

    Mar 9, 2003
    Pennsylvania
    Thank you. Great description. So, how would you know what type of op amp is embedded in the ThunderFunk TFB420?
     
  4. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Well, normally that would be shown on the schematic, which Thunderfunk may send you if you ask. Or, you could open up the cover and look inside to see whether the components on the circuit board are visible. Sometimes they embed the boards in epoxy so you can't see (and reverse engineer) the circuit, but the pic on the web site shows what looks like an ordinary open PC board, so if that's the case you might be able to read the numbers on the chips. If you can, and you can post them here, I can probably tell you what they are. Is it my imagination or does that amp look a lot like an early SWR?
     
  5. Arranger

    Arranger

    Mar 9, 2003
    Pennsylvania
  6. Thunderfunk

    Thunderfunk

    Mar 27, 2004
    McHenry, IL
    TL072's are used except the first stage is an NE5532. TL072's are about 25 years old, yet they are the basis of the new Fender Bassman (SWR) amps. Opamps are actually analog computers. They are called operational amplifiers because they perform math operations, like addition (mixing of signals in a mixer), subtraction, multiplication, division, integration, and differentiation. There's about 26 transistors in each one. They're easy to design with because by changing a resistor value you can adjust the gain, and are basically gain blocks with two inputs and one output. They can be set to invert a signal, or not invert the signal. They have four basic specs to be concerned with; gain bandwidth product (GBP), noise, distortion, and slew rate. The 741 was one of the first designs and suffered from a low GBP meaning you couldn't generate a high gain at a high frequency (GxB=P). Today it's considered to be a junk chip.

    http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm

    http://www.thunderfunk.com/TFB420_Preamp.html

    Service information is available. There's nothing secret in my amp. It's a matter of using good quality parts, and not skimping on the power supply. The original design (early SWR) also used a single sided board to be cheap, and some of the jumper wires carried the signal. That made it prone to noise. Stick your finger near one or move it, and you'd see it in the output signal. nonsqtr is perceptive in saying it looks like an early SWR. It also looks alot like the Bassman 400, the Eden WT350, and probably the Walkabout. The difference is in not counting pennies in the design, and also in not over engineering it. I looked at the Bassman 400 and found it has too much (IMHO) low frequency distortion (I can tell that from the schematic) which would give it a lack of low end definition. They also "fixed" an engineering "mistake" which would make it sound hi-fi like an EBS amp. If you like that, then it's the amp for you. It has only three parametric stages (like the Eden) but has a nice mute, and clipping LED circuit that I don't. The fan turns on when the heatsink gets hot (no user control over it) which keeps it fool proof, but I use an aluminum chassis to help dissipate the heat without the fan (turn it off in the studio). If you're playing loud enough to need the fan, you won't hear it anyway. The Bassman uses six 200-watt bipolar output transistors, whereas I use four. I suspect this is why they allow a 2-ohm load which I don't. I don't know why they use a 10A fuse though, except if you put a two ohm load on my amp it exceeds the 5A fuse I use, which seems more appropriate to the Bassman. As I've said before, the 2-ohm load thing is not a good idea, especially with bipolar transistors (beta droop). You don't magically get more power. The limit is still what the transformer will put out. You might generate more "average" power, but I'd expect at the expense of less dynamics (less peak power). The Bassman has a nice standard current limit protection design in the power amp that will protect it from abuse, but might also affect the sound (not sure).

    Another interesting thing about the Bassman is they use the 3080 compressor chip. I'm pretty sure this is the same chip as in the Dynacomp compressor pedal. I use a limiter design which works very well. I'd like to hear comparison comments when someone gets both amps in the same room. So all in all you get a lot for your money with the Fender. But does it sound as good as a Thunderfunk? :)
     
  7. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Wow, there it is, straight from the designer. I love this forum!

    Thanks Dave, that's great info. About the limiter, is that a FET attached to the feedback loop, like in the old SWR's? I like that design, it sounds good and works great. Reminiscent of the 1176's, right?
     
  8. Thunderfunk

    Thunderfunk

    Mar 27, 2004
    McHenry, IL
    Yes, it's a FET after a full-wave bridge so you get the pos and neg wave triggering it. A timing cap to keep it from riding the low freqs and you're there.
     
  9. Arranger

    Arranger

    Mar 9, 2003
    Pennsylvania
    :smug: Well, this is all way over my head now. Nice to have you covering the bases (basses?), Dave. It sounds like the new TFB420 has been given the utmost consideration in its development.

    Now I can go have a beer in peace knowing that all is well in the higher scheme of things! Thanks again.
    Bob G,