DIY symmetrical 400W amplifier almost finished.

Discussion in 'Archived Threads 2001-2004' started by Michael R Price, Jan 27, 2003.

  1. Michael R Price

    Michael R Price Screenwriter

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    This is my latest project, and it has taken a much longer time than it should have. It is the Symmetrical MOSFET amplifier designed by Anthony Holton of www.aussieamplifiers.com. A three-stage, Class AB amplifier, it can deliver somewhere above 200W into 8 ohms and 400W into 4 ohms.

    [​IMG]

    The problems I have had getting this project up and running are almost too long to list, and most have been because of my own stupidity. In short, one channel of my amplifier is almost completely dead because of problems with the output stage (and it's awaiting replacement parts). The other channel is working, but I had a close call with it... I connected the speaker negative lead to a spot on the PCB that looked like the right place, but it had a 10 ohm 0.6W resistor between it and ground. That resistor went up in smoke, quite literally. Luckily, replacing the resistor and connecting the output directly to ground solved the problem.

    This channel has a bit of uneven bias: readings across the source resistors in the output stage are between 0.014-0.020 volts, indicating between 64 and 91 mA of bias per Mosfet. But that doesn't get in the way of operation. The power supply varies between +/-73-77V based on line conditions, it consists of a 750VA 50V Plitron toroid, 400V 35A bridge and a pair of 23,400uf capacitors (per channel). DC offset is between 6-8 millivolts and there is no turn on thump. With this amount of bias the output into 4 ohms should be Class A for the first 3 watts or so... I may try to increase the bias later since I have the power and heatsink to handle it.

    This project cost around $400 for a pair of monoblocks built with quality parts: IRF Mosfets, polypropylene and polyester capacitors, etc... and overkill power supplies and heatsinks. With any luck it should have sound quality up there with very good quality high power Class AB amplifiers. As soon as I have two channels [​IMG], I'll post my impressions of the sound and compare it to my old amplifier as well as the Class A Zen amp.

    I just tested this thing hooked up to a single Kit281 (87db/2v) a bit, slowly increasing the volume until my voltmeter (which has significant lack of high frequency response) was reading between 8-12V AC. This means somewhere around 10-30 watts continuous and 90-95db at my seat. The sound was not harsh or "loud", still detailed and has a much more dynamic, stronger sound than I've had before...at least as far as I can tell at this point. And yes, it will go much louder. A simple calculation tells me that 75V rails should be good for peaks of around 550W, although its ability to hold that power continuously is questionable. At this point it doesn't really matter; I was just looking to squeeze whatever dynamic capability I could out of these speakers, which had better not require 500 watts.

    I didn't try to turn it up more because I am concerned with the thermal coupling between Q10 (the Vbe multiplier) and output stage Mosfets... at the moment it is superglued on top of one of the IRFP240s, with a dab of silver thermal paste in the middle. You don't want to know what happened to the amplifier where the Vbe multiplier was on a separate heatsink...

    I strongly suggest those looking for a high power solid state amp consider this design. It has serious power, is turning out to sound really good so far, and the designer has been very helpful in responding to my silly questions. Holton also offers a PCB for a similar design with 800 watts into 4 ohms, which would be a very nice subwoofer amplifier.

    Thanks for listening.
     
  2. ShaneH

    ShaneH Stunt Coordinator

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    Good post. Keep us up to date on your progress.
     
  3. Eduardo

    Eduardo Agent

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    Great job. [​IMG]

    I'm very interested in the Kit281 speaker, did you build them? I plan on building a pair of front speakers and the Kit281s is #1 on my list. Any perspective would be helpful.
     
  4. Michael R Price

    Michael R Price Screenwriter

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    Eduardo,

    Yes, I built them (with the help of my friend Elliott and his limited selection of power tools) and it was my first DIY audio project. You can probably tell the construction quality is shoddy and they are not finished at all, however the sound is very good. In short they are a large inefficient tower speaker with a really clear sound, large and precise soundstage and very good bass and lower midrange smoothness. Their disadvantages from what others say would be upper midrange and treble quality although I haven't had the chance to listen to a speaker with truly good mid/treble sound; and with the Zen Class A amplifier these create very real sounding voices and other sounds that are very smooth and "float" in midair, so I would not consider this a weakness. Adire should have a 3-way version of this speaker available soon using a 5.25" midrange between 350 and 3000Hz (IIRC) to increase midrange quality, off-axis response, and power handling. I would recommend this speaker if you are looking for an easy to build, mid-priced large speaker kit with good bass and dynamics (when given an appropriate amplifier) and a clear soundstage.

    Efficiency is a consideration here given the Kit281's rating of 92db/2.83v which is realistic for an in-room measurement at one meter. The real anechoic sensitivity is 86.7db/2v (~1 watt). I would expect these speakers to be capable of clean reference level movie production when set to small, using a 100 watt amplifier in a medium size room. These speakers have a very clean sound and it is sometimes difficult to tell when it is loud - you may find yourself turning it up because it sounds so good, and then running into amplifier-induced distortion. I sure did with my 100 watt amp at levels above 90-95db continuous, and that's why I have this one. The HE10.1/12.1 may be a better choice if you'd like to use a Class A or tube amp, although they have lesser bass/lower midrange output capability and are unshielded; their tone is similar to the Kit281 and I haven't heard about the midrange and treble quality much.

    It's funny how the speaker makes the amplifier look small, although it's not such a big speaker, because I took the picture standing up. The heatsink is 24 by 7 by 3 inches (most audio components are 17 inches wide), each capacitor is larger than a soda can and the transformer (under the bed) weighs 12 pounds... this is the amplifier for one channel.

    Because of all the mistakes I've made I must say: Please do not attempt this amplifier for your first project unless you really know what you're doing. The high complexity and finicky setup of this design, along with the high power components make this quite unsafe and prone to damaging components or zapping yourself. As my second amplifier project (after a very well thought out and simple Pass Zen V4), this proved challenging. Errr... it's still proving challenging.
     
  5. Troy_j

    Troy_j Agent

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    IMHO, the coffee cup is at the root of all of your trials, not your experience...[​IMG]
     
  6. Eduardo

    Eduardo Agent

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    Michael

    Thanks for the response. I only have a Sony reciever that will powering the speaker. It will support 4ohm speakers.

    When did you hear about the new speaker from Adire. I'm going to have to send an e-mail to Dan.

    Eduardo
     
  7. Michael R Price

    Michael R Price Screenwriter

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    Troy,

    What, that piece of *&(*^! Radio Shack soldering iron I've been using? (Does anyone else use a coffee mug to hold theirs?) [​IMG]

    Eduardo,

    If you have some extra cash an upgrade to your front 2 channels of amplification may be very worthwhile in the future. Don't worry about it for now, the Sony will work fine, but as you can see I really appreciate the difference good amplifiers are making with these speakers.

    Since the introduction of the LCC center kit which uses the midrange, we've all assumed there would be a similar kit to replace the Kit281. Dan Wiggins hinted at this too, but I don't think they said anything specific. If you e-mail them they may give you an idea of when it will be available.
     
  8. Gary Joe

    Gary Joe Stunt Coordinator

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    I've em'd Adire a month ago about the 3 way tower to match the LCC. They said around March for the release.
     
  9. Scott Simonian

    Scott Simonian Screenwriter

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    Yeah I talked to Dan Wiggins and Dan Hyre when at CES 2k3 and I asked them about an LCC conversion of the kit281 and he said that they may be available around the spring time.

    Just in time.


    Hey Michael, Ive seen this project before! Looking for great amp kits for some time now. I was expecting something new. Comeon now [​IMG]

    I still just want to make a decent amp that has a sturdy 3db of headroom capability. I want dynamics and power. *Ahem* "...for cheap." [​IMG]



    EDIT: Just checking out the website now. Did you see that the schematics for this amp had been improved? I see the S/NR is @ -122db! Wow! Damping factor of 200. How can you increase this, to maybe 1000?
     
  10. Dave Poehlman

    Dave Poehlman Producer

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  11. Michael R Price

    Michael R Price Screenwriter

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    Scott, this design has been around for awhile and has been updated a lot (mainly to use the IRF Mosfets I think) but I've never seen evidence that anyone else has built it. Maybe there's a reason. [​IMG]

    If your power supply is stiff and your output stage can take the load there's not supposed to be any headroom! My old amp had headroom but that's just because the maximum power the actual circuit could provide could be supplied to it only momentarily because the power supply was wimpy.

    To increase damping factor the simplest thing I could think of would just be to increase the feedback (lower R11 I think). You could contact Mr. Holton and he'd probably suggest a better modification but he is sometimes difficult to reach via e-mail. Besides, 200 isn't enough damping?

    Oh I like that alien too. It was a party favor, oddly enough. I haven't been beamed up yet or anything... so I keep it. On second thought I really never did come up with an explanation with why the input stages on my other board blew...
     
  12. Scott Simonian

    Scott Simonian Screenwriter

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  13. Peter Johnson

    Peter Johnson Stunt Coordinator

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    No, DH is not what you want.

    Take a look at the specs for a lot of cost-no-object SS amps. Most have almost zero headroom. Stiff power supply.

    There is some info on this in the AA archives. Search for posts from Trevor Wilson.
     
  14. Scott Simonian

    Scott Simonian Screenwriter

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    Okay then, how do you recreate these moments of pure dynamics in movies without the extra oommmpphhh from the amp?
     
  15. Michael R Price

    Michael R Price Screenwriter

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    Scott,

    The ooomph comes from the fact that the amplifier itself is capable of delivering that huge power all day if it wants to. [​IMG]

    The amplifier circuit can provide a certain peak power based on its voltage rails and the load impedance. Its ability to supply this amount of power continuously is limited by the power supply (a wimpy power supply will drop voltage and cause clipping when stressed) and the robustness of the output stage and its heatsinks. "Headroom," then, means the peak power the circuit is capable of delivering is greater than the continuous power that can be supplied to it; this is generally considered a bad thing.
     
  16. Scott Simonian

    Scott Simonian Screenwriter

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    Then what is the point of peak power at all? Why even give it in some situations. I thought Cinepro made some of the best amps. I always looked at them to reference. Reviews would talk about how the extra power reserves made it possible to experience some of these dynamic moments. Like in their ads they would have some chart with ID4 blowing up the white house and such. Saying how with dialog you only need about 10-15watts and how these peak moments need upwards of about 800watts!

    I hope I am wrong. I really do.

    EDIT: So how are some amps dynamic and others not? ANd I mean between top-of-the-line amps. Is it just as simple as being able to keep power levels stable like you guys are telling me? Just design an amp that doesnt have power that droops? *sigh*

    If I want that kind of performance, what do I do? Except buy an amp, no, I want to build one. Just like the speakers and subs. I love building things now. It feels so good.

    [​IMG] [​IMG]
     
  17. Michael R Price

    Michael R Price Screenwriter

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    Scott,

    I really don't understand enough the technical reasons why amplifiers have traits such as dynamics, soundstaging, etc... I imagine power supply stability and filtering are only parts of the equation.

    Whatever it is that makes an amp dynamic, I have a feeling many reviewers don't understand it either. Basically, saying an amp's dynamic capability is due to its ability to only provide its maximum power for short peaks... is a bit unfounded.

    A watt may be more than you think. Although many times the average power may be required for peaks, this average power is usually quite small. Generally "reference level" implies an average level around 85db during a movie, which could mean anywhere up to a couple watts. 90db efficient speakers should be good for reference level peaks (105db/channel) with a 100-200 watt amp in most situations.

    I'm at this point sort of ignoring the huge compression and distortion coming into play with most speakers at the levels involved. I'm guessing this is why normal speakers can play loud enough to "drive you out of the room," yet really dynamic speakers (horns, etc) could play even louder without sounding unpleasant. So it's kind of pointless to pursue these extents of amplifier power where the speakers will waste it away as ours do. However, speaker distortion is a less noticeable thing... the sound sort of becomes less clear and distinct as the speakers start to take the heat, and in peaks we may not notice this compression; I think we're used to it in fact. On the other hand if your amplifier is asked to produce 6db more than it has the power for... your ears will know it.

    If you want the kind of performance that blows you away, you don't want 800 watts. You want more efficient speakers so that you'll only need 50 watts. I do too, but the Kit281s are here to stay; hence this big amplifier (compromise).
     
  18. Scott Simonian

    Scott Simonian Screenwriter

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    Another good reason for me to keep on saving for a line array kit. [​IMG]

    I do understand all the factors that come into play when power starts becoming more than the speaker is willing to use. Ive seen it before. I dont like it at all.

    Ever since I really jumped into HT about 6 or 7 years ago, Ive been in search for the dynamic feeling you get during a pictures first showing in the cities best cinemas. Back then I was just in middleschool, my ears were not trained like they are today. Unfortunatly, like more people than me have too, have been increasingly hard to please. And with the theaters in my town getting worse and worse sounding, (I think that has something to do with the management though) I havent heard a truely terrifing experience in sound. The last movie I heard that sounded incredible and I mean INCREDIBLE was when I saw Episode 1 for my 3rd time. The first time was in an average theater. The 2nd was in Monteray and in a theater converted to a cineplex. Wow was it horrible. Then I saw it again in one of the better cinemas(back at home) and it was great. Before that was seeing SPR rereleased in the same theater.

    Sorry for the ramble.

    I guess by the time I move out and get a place of my own Ill have some mulah to make me some arrays and HUGE amps. Cant wait.

    Keep us updated on progress with the amp, I really want to hear about it.

    [​IMG] [​IMG]
     
  19. Michael R Price

    Michael R Price Screenwriter

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    I don't know Scott, I haven't really been able to listen to this big new amplifier yet (parts for the blown channel should be here any day now). I think it should capture a bit of that dynamic feeling - after all, a pair of AV8 woofers per channel isn't *that* bad. So I'll post my impressions once it all works.

    When it's not distorting (not very loud), the Zen 15 watt amplifier actually has a hint of that lively "big" sound to it as well, more so than my old 100 watt amp. It is not only a matter of maximum power, although it seems that maximum output is the best predictor of a speaker/amplifier's dynamic capability.

    BTW, do you know how the dynamic capability of a line array compares to that of horn loaded systems? This is a curiosity to me... can you match horns' dynamics by stacking 6-8 8" woofers and using a 6' ribbon mid-tweeter for example?
     
  20. Scott Simonian

    Scott Simonian Screenwriter

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    I do not know for a fact. Just gettin that out of the way.

    When you horn load a woofer or tweeter you pretty much just increasing the sensitivity. The driver now can go louder without having to work so hard.

    In a linearray it is similar. I dont know if the overall sensitivity raises or what. I do know that since the array of drivers are sharing the load on the amplifier the signal is split amongst them. Now they only need to work at a fraction of what they previously did when only using 1 (or 2, etc.) driver.

    I really like the sound of line arrays. Effortless is the word I like to use. That is something I like to hear, I am so used to cringing during some passages in movies with my speakers. Not the sub though, it is great. Most of the time when the room is just being punished (or supposed to be) my system will compress into a flat soundstage of nothing. Not very impressive. The volume then goes down to compensate. Then it sounds a little better. Just cause it isnt working too hard. But then it is quieter and not so in-your-face.

    Something that would be quite interesting would be if someone made a horn-loaded line array. Geez! I know the cabinets would have to be huge. Yeesh.
     

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