Tweakers' Asylum

• RE: Class A SS or chip amp tda2040 - rcollege 01:24:25 11/30/10 (0)
  In Reply to: RE: Class A SS or chip amp tda2040 posted by rcollege on October 22, 2010 at 01:02:17

  Thanks everyone again for your responses.
  I admit the lm1875 chip sounded very good...when I wired it all up. Funny, though...the single supply version sounds more musical than the split supply.
  The TDA 2040 sounds accurate and quite clear if not too bright.
  
  So an update...I've been going full speed...I have tried different mosfets with the class A design. I'm to the point of creating a class a using a single BJT or Darlington. In my experimentations I did a BJT with very little amplification...so it needs more biasing. What I did find "cool" was how little heat was produced with it. So if I'm correct, I'll have a decent class a amp with manageable heat.
  I've been using a spice program...so we'll see.
  
  Are there any circuits for the lm 1875, tda 2030 that are different than the data sheet schematic?
  One thing I have to admit is that I kinda feel the rediculous notion that making a chip amp is kind of cheating...especially when I see the block diagram of what is in the lm1875...good lord its a lot. I almost feel that I'll stick to my vintage Marantz...I want simpler sound, without the complexity of several transistors. A couple more stabs at the Class A then if that don't work...I'll go chip.
  Normality frightens me- its what they use to lure you into the cookie cutter.
  Roger
  
  

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• RE: Class A SS or chip amp tda2040 - Johnny2Bad 16:25:04 11/18/10 (0)
  In Reply to: RE: Class A SS or chip amp tda2040 posted by rcollege on October 22, 2010 at 01:02:17

  " ... Has anyone here heard a Class A single ended solid state amp?
  Or a class A push /pull transistor or even darlignton amp?
  What about a tda2040 chip amp?
  
  Is there a large difference in sound? ..."
  
  In my experience, assuming you can hear differences in amplifiers (some claim they can't) there is a fairly distinct "family" sound between Class A single ended, Class A push-pull and a Class AB amp such as the 2040.
  
  Personally I would rate them just as you listed them for SQ (i.e with the Class A single ended the better of the lot).
  
  Having said that they can all sound good, and the differences in the real world are more likely to show up based on the attention to detail employed by each design and the builder.
  
  If money is an object, you may well find the order is reversed; it's easier to build a good sounding amp with limited funds with something like a TDA2040 than a Class A design. The push-pull configurations (A or AB) are easier to get higher power out of, if that's important.
  

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• I'd go chip - Owen 08:50:56 11/15/10 (0)
  In Reply to: RE: Class A SS or chip amp tda2040 posted by rcollege on October 22, 2010 at 01:02:17

  Quick, easy, inexpensive, upgradable yada yada yada. Much like a class A homebrew with one exception. Chip amps you can re-use old CPU heatsinks with, so they are definitely in the inexpensive camp. And small heatsink = small project, and small project = easily hid from S.O. (or housed in a pretty box)
  
  The sound from them can be very suprising.
  
  
  Owen
  
  
  

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• Maybe a little of both? - Jon Risch 19:39:38 10/25/10 (0)
  In Reply to: RE: Class A SS or chip amp tda2040 posted by rcollege on October 22, 2010 at 01:02:17

  Regarding gain clone type amps, see this thread:
  http://db.audioasylum.com/cgi/m.mpl?forum=tweaks&n=148236&highlight=1875
  
  Having since heard what was supposed to be a "hot" little LM3886 based amp, I still think the LM1875 sounds pretty decent, more "tubelike" if you will.
  
  However, I never did hear the LM3886 with the power supply rail pull-down trick, where you load the output to the negative rail through either a current source or a simple load resistor, and "force" a modicum of Class A operation instead of full-blown push-pull flip-flopping. This might help quite a bit, and make the difference. If so, it would be a formidable sounding amp then.
  
  
  Jon Risch
  
  

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• I was successful with Class A - rcollege 23:10:15 10/24/10 (0)
  In Reply to: RE: Class A SS or chip amp tda2040 posted by rcollege on October 22, 2010 at 01:02:17

  Hello and thank you for your responses.
  I realized that theTDA2040's needed a split power supply after I just bought a small 25v .7a power supply.

  So I did end up wiring up this Class A single Ended amp- one channel. http://diyaudioprojects.com/Solid/ZCA/ZCA.htm

  I ended up using an inexpensive K1378 Mosfet. I used the switching power supply and the amp had ZERO hum without any filtering other than what was listed on the amp schematic. I didn't need the power schematic.
  
  I didn't know if the speakers were on until I turned on the music.
  Heat really was minimal...but the power wasn't a ton. At full volume it would drive my Boston A100's 10" 2 ways enough to fill a medium living room, but nothing much more. Still I was turning it down.

  It sounded quite nice clear and sweet- but very base heavy and subdued highs. My wife was impressed...but I like sound that has the breathier voice, chimes and high hats balanced in...which was lacking with this amp. It sounded very similar to the other tube amp I heard at an audiophile store- heavy looser bass, high hats and symbols faded to the background. I just don't think its the sound I prefer. It's not going to replace my vintage Marantz.

  I don't have a picture of the wires and components spread all over the floor....I put it all away ready to take apart for a chip project down the road...unless I can figure out how to get less bass and more highs.
  Probably a different mosfet.

  If I get the highs and less bass...it just might replace the Marantz...but the chips need to take a stab at it.

  I am dying to use that 15- 20lb aluminum heatsink I have.

  
  Normality frightens me- its what they use to lure you into the cookie cutter.
  Roger
  
  

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  Edits: 10/24/10
• Class A single-ended requires a transformer or coupling capacitor. - Al Sekela 15:15:28 10/22/10 (2)
  In Reply to: RE: Class A SS or chip amp tda2040 posted by rcollege on October 22, 2010 at 01:02:17

  "Class A" means the output device never shuts off. There is substantial DC current flowing through the output device all the time. In a single-ended amp, there is no way to balance the bias current, and the speaker cannot tolerate this. Single-ended amps made of tubes require specialized output transformers that can tolerate the DC, and these are wound to match the high tube plate impedance to the low speaker impedance.
  
  I don't know if anyone makes a specialized output transformer that matches low transistor impedance to the speaker, while tolerating the bias current.
  
  An alternative would be to couple the speaker with a huge capacitor, but this would thoroughly degrade the sound.
  

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  • Really? - Ric Schultz 16:35:07 10/22/10 (1)
    In Reply to: RE: Class A single-ended requires a transformer or coupling capacitor. posted by Al Sekela on October 22, 2010 at 15:15:28

    Single ended transistor circuits can be designed using plus and minus supplies. The Pass Labs Aleph series is like this. You have a differential front end and a single ended output that has a current source for the other half. You bias it as high as you like and with a cap in the feedback to ground you get around 40mv offset. Amps like this can sound fantastic. The Aleph series has a sliding bias output stage to give it more current as the load demands. While this makes for more power into lower impedances it messes up the sound. A pure fixed bias output stage sounds best. I have done it both ways.
    
    The trouble with single ended class A circuits is that they are so inefficient. I was biasing my amp at 3 amps bias and so I was using 150 watts a channel to get 25 watts.....ridiculous. Tons of heat and tons of heat-sinks and tons of capacitance in the power supply to get rid of the hum.
    
    I would go with the Tripath. I heard a Vinnie amp that ran off batteries and with a few mods was guite good. I am done with class A single ended (at least for now).
    

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    • Thanks. I did not know this had been done. - Al Sekela 15:04:55 10/23/10 (0)
      In Reply to: RE: Really? posted by Ric Schultz on October 22, 2010 at 16:35:07

      Agree about the heat issue. There is no point in listening to class A if the air conditioning has to run to keep the room temperature under control.
      

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