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Old 17th February 2009, 04:27 PM   #711
housing is offline housing  Hong Kong
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Default AMB Laboratories' 24: A discrete, cascoded, fully-differential power amplifier

AMB Laboratories' 24: A discrete, cascoded, fully-differential power amplifier


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Old 17th February 2009, 06:13 PM   #712
fotios is offline fotios  Greece
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Default Re: AMB Laboratories' 24: A discrete, cascoded, fully-differential power amplifier

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Originally posted by housing
AMB Laboratories' 24: A discrete, cascoded, fully-differential power amplifier


Freshly brewed.
Hmmm....
Good work in general... But i must to point out, the right choice and orientation of toroidal xformers. You are using instead one monster of, say, 500VA two of 250VA. That means less magnetizing current and consequently less radiation of magnetic flux arround. The second is the vertical mounting of toroids related to orientation of pcbs. This ensures that the magnetic flux radiated from toroids, can't affect the noise sensitive stages of amplifier boards. This amplifier must be very quiet without buzz or crackling noise assuming that the star point of gnd nodes has been done correctly, so there are not gnd loops.
Bravo housing

Fotios
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Old 17th February 2009, 10:06 PM   #713
fotios is offline fotios  Greece
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Default Why a carefully designed project can fail.

In the picture bellow you can see the project implemented. It is a class AB power amplifier of 2 X 180Wrms/8 in dual mono arrangement. Although the first impression can bee: WOW! what a nice construction fotios; there are some unexpected bad results.

Click the image to open in full size.

The main problem is the presense of buzz (or crackling) noise, be it so very small of 1,2mVrms, in output. Under normal listening conditions it is unaudible except if you stick your ear on the cone of woofer under absolute silence... because i am strange man and i have the habbit to test a finished project for a couple of days as much, i am interested mainly for the performance. I haven't time to enjoy with my constructions. Thus my thought it is clearly of mechanical way.
According this, the only acceptable noise in output must be only a little hiss caused from the electronic parts. It is a painfull work to obtain someone a noiseless project which have a Av=43, in conjuction with easiness in repair work. An easy repair work can be obtained only if the amp. pcbs are accessible directly when the bottom cover is removed. The difficult way is when the pcbs are mounted parallel on heatsink thus vertically related to xformer position. In this amplifier on the picture, the amp pcbs are mounted vertically on heatsink and alongside with the xformer. The result is that the sensitive input stages of the two channels are infected from a straight magnetic flux radiated from the 500VA toroidal xformer. Don't be deceived by the supposed immunity of transformer with the bowl, because it is from steel. Steel it is a good conductor of magnetic flux, thus the steel bowl is uselless. The only cure, it is the mounting of pcbs parallel on heatsinks. I hate this thing, for the reasons mentioned above. I can't turn the xformer vertically because its 14,5cm diameter, instead the height of amplifier is 10cm.
Let's go now to the subject called toroid xformer. Before one year, i had not so many knoweledge about this kind of xformers because i used EI type xformers. Toroids are in the round about 25 years. Theoretically, are less noisy from EI xformers. From the other hand, EI xformers have a technological and historical background of enough decades ago. In comparison, toroids are yet IMHO "tabula rasa". In my career, i don't remember to have seen inside a measuring instrument (included medicals) a toroid xformer for power supply. I have seen only EI xformers of different shapes, with double belly bands electricaly earthed and in many cases covered with nickel plate (or mumetal). This make sense. After enough searching, i resulted in this: very few constructors arround the world can build correctly a toroid. Do you now that, the famous Plitron in their LoNo series toroids make use of a belly band from copper tape (earthed) between primary and secondary windings? Ha! Where is gone the guaranteed screening formed only from the copper windings of toroid, as reffered in wikipedia? Why the need for cylidrical covers from special alloys? Aluminium by alone it is not enough! Mumetal it is prohibitivelly expensive. The big secret it is the alloy of the spiral core. The higher the magnetic permeability ( ) of alloy, the less the magnetizing current thus the less magnetic flux radiated. Constructors usually make use of a steel - Fe band to form the core. Fe has a mid to high "". The issue it is that, the high magnetizing current cause mechanical noise except mag. flux radiation in a distance arround 3 to 4cm from the circumference of a toroid.
As for EI xformers, today offered a variety of alloys from which are constructed their E and I laminates. Double belly bands earthed offers a good screening. As for mechanical noise, NO! EI xformers are "open" after their completion for a good impregnation with epoxy varnish into a vacuum can. After 2 - 3 days, when the varnish dried thoroughly, the laminates are sticked between them and the core becomes still like a rock.
I have read somewhere a report of N. Pass about a Plitron toroid: "With a stethoscope, we could hear a little noise from the core of transformer"...
And i am wondering. Are toroids for causing impression? An EI xformer it is not "a must" inside a Hi-End amplifier?
For me, either of the two types is equally good, but only and only if it is constructed correctly!
BTW, in Greece there is a laboratory of EI xformers which can produce top grade xformers, yet for the bigger demand you can imagine. For toroids... a lot of labs operated from "proffesional" amateurs.

Fotios
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Old 18th February 2009, 07:38 AM   #714
housing is offline housing  Hong Kong
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Default Re: Re: AMB Laboratories' 24: A discrete, cascoded, fully-differential power amplifier

Quote:
Originally posted by fotios


Hmmm....
Good work in general... But i must to point out, the right choice and orientation of toroidal xformers. You are using instead one monster of, say, 500VA two of 250VA. That means less magnetizing current and consequently less radiation of magnetic flux arround. The second is the vertical mounting of toroids related to orientation of pcbs. This ensures that the magnetic flux radiated from toroids, can't affect the noise sensitive stages of amplifier boards. This amplifier must be very quiet without buzz or crackling noise assuming that the star point of gnd nodes has been done correctly, so there are not gnd loops.
Bravo housing

Fotios
Fotios, thanks for all the kind words.

Yes, the power amp is dead quiet by design and layout. It's a balanced amp.
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Old 18th February 2009, 12:57 PM   #715
Bonsai is offline Bonsai  Taiwan
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"In the picture bellow you can see the project implemented."

fotios, is this one of your projects? Can we se e some mor e pictures?

Looks very good
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Amplifier Design and Construction for MUSIC! http://hifisonix.com/
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Old 18th February 2009, 06:40 PM   #716
fotios is offline fotios  Greece
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Quote:
Originally posted by Bonsai
"In the picture bellow you can see the project implemented."

fotios, is this one of your projects? Can we se e some mor e pictures?

Looks very good
Thanks to my respectable on-line friend Bonsai, i can post as many pictures you want. Because the batteries of my camera must be recharged, i ask your patience for one day.
Also, if you wish, drop me a link to send you schematics etc.
Yes, it is at whole my own design including an error analog computer for any possible abuse of amplifier, a very simple soft start circuit (arround a Rhopoint SurgeGard ntc), error leds, and two balanced to single ended converters implemented with smd discrettes on the back plate.

Fotios
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Old 22nd February 2009, 12:05 AM   #717
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Here is my latest "add-on". A simple, but effective and high quality A/B speaker switch. I used an 8 pin PIC12F675 to do everything - debounce the momentary switch, drive the LEDs and drive the relay. Simple software.
The relays are 12V coil 250V/16A each, paralleled for extra capacity. I was going to use latching relays, but couldn't find really suitable ones, so in the end used Potter & Brumfield RTD14012F. The boards measure 85x50mm and one board is used for each channel.
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Old 27th February 2009, 08:43 AM   #718
roender is offline roender  Romania
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Look mama, my DAd(C) is singing
RMI-DAC1
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Old 27th February 2009, 08:48 AM   #719
roender is offline roender  Romania
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another one
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Old 27th February 2009, 08:49 AM   #720
roender is offline roender  Romania
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The last one, for folks who love pictures
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