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Old 18th February 2004, 10:26 PM   #1
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Default anyone have any experience with these amps.

http://www.ampslab.com/products.htm

These amps look pretty cheap and seem easy to build. I am curious as to what people think of there sound quality.

I am looking at building two of the Bi70's http://www.ampslab.com/bi70.htm
i think this should be plenty fo power to power some halway decent monitors

If any of these setups on this site look better than the one im thinking of building plz tell me, also any other kits that would cost about the same, plz tell em also.

thnx
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Old 19th February 2004, 02:15 AM   #2
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Haven't used these amps BUT, the PC boards look well made & thought out, the fact that they are using MJ15024/25 outputs would imply to me that they want the amp to live even when in rough use, I would say yes to buying them.

Side note:
Funny thing is that I actualy only saw their site today also, when searching for replacement MJ15024/25 transistors, for another amp that did see the end of it's output life, because of a crewl owner..
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Old 19th February 2004, 11:03 PM   #3
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haha nice

anyoen have any suggestions on whether to go mosfet , power ic's or bipolar. I hear mosfet isnt that great sounding but produces more power for input power.
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Old 20th February 2004, 12:30 AM   #4
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You'll find proponents of each type.

I've had all over time. Haven't built a decent gainclone yet, but have had some horrific chips in store bought stuff, some OK ones, too. Comparing a Hafler DH-500 (MOSFET) to a Yamaha M-2 (Bipolar) by swapping their positions (sub/main) I preferred the Hafler for the mains.. I bought a Yamaha DSP-A1 (Bipolar I think) and like it better than the Hafler, now back on sub duty.

I built a Leach amp (Bipolar) and like it a lot, better than the DSP-A1. I just finished an A-75 (MOSFET) and like it better still.

It may be a case of liking the newest stuff I have but I don't think so.

The kits you mentioned should be fairly easy to assemble, but I have no personal experience with them. If you want to try something else, I heartily recommend the Leach amps. Well documented, and you can get boards from Professor Leach. I managed to get mine together without a hitch, despite not owning an oscilloscope or Variac at the time. Rod Elliot's P-3A has a strong following, although I have not built one.

I had a little more trouble with the A-75, due to rushing at the end. story at Another A-75 comes to life

Good luck, take your time building and double check everything before applying power. If you don't have a variac, put a 100 ohm 1/4 watt resistor in series with the rail leads. If you power it up without smoking the resistors, you're probably OK.
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Old 20th February 2004, 02:38 AM   #5
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ive been thinkng bout doing one of these, but im curious as to wrre is houdl get the psu, anyone know of any good psu kits that would suit the set up im thinkign of

thnx
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Old 20th February 2004, 11:14 AM   #6
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PSU KITS? We don't need no stinkin' PSU Kits!

Class AB Amp PSU= Transformer + Bridge +capacitors + Switch, and sometimes inrush limiting. Bolt the pieces to your case and wire it point to point.

Apex Jr. may have a suitable transformer (call Steve and ask if you don't see what you want on the website), otherwise Plitron, Toroid of MD, Avel, Parts Express (sells Avel), Viictoria Magnetivs are sources. Secondary voltages should be in the neighborhood of (rail volts)/1.3 - you'll get more or less depending on the transformer and your amp bias - class A types tend to be less. It is generally not critical - most amps can handle a range of rail volltages.

ApexJr is one of the likely suspects for caps, you can also buy them from digikey, mouser, etc. More is better - to a point, although some gaincloners think otherwise. Get caps rated at least several volts higher than your expected rails. Too high just wastes money though.

Bridge rectifiers are standard parts, or you can go exotic - there are quite a few threads discussing the merits of exotic rectification. Choose >3 times your transformer secondary voltage and 25 amps or more. Charging pulses on the caps are short, resulting in high peak currents.

The switch can be anything that suits your fancy, as long as it can handle 2-3 times your expected load. You can use a triac and a smaller switch like the A75 or you can use a small, pretty switch to activate a relay. You can even find circuits in this forum taht allow you to use a momentary pushbutton to toggle your creation on and off.

There are several methods of limiting the initial surge when you power up you amp. The simplest is to use an inrush limiter, such as CL-30, etc. (Moouser/digikey, etc) This works for me - with a 1KVA toroid and 180,000 uf total. Rod Elliot and others have plans for relay operated limiters if you are worried about losing a little power due to the drop across the CL-X0.

I am sure that others would prefer more scientific methods of choosing components, but in the DIY world, we tend towards overkill anyway.

Have fun!
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