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Ian Mackenzie
06-18-2004, 04:29 PM
Okay,

We have had some fun posting our grungy experiences....mostly amplifiers.

So I figured its time to de grung all you wonderful formites with a new project.

Every now and then a really great idea or invention comes along, they are often very simple.

This invention will have a bigger single impact on the sound of your JBL's than anything you have done before!

And you can make it your self and enjoy the satisfaction of hearing it go!

I will be presenting simple but unique amplifer project over the next month or so based on the super class A amplifier I have built and referred to from time to time.

The project can be scaled in size to suit particular needs and level of experience. From a small toaster to a furnace.

It's fairly easy to make and the subjective performance will astound you.

I will be supporting the project with pcbs on request, matched parts and detailed technical assistance.

To be continued.

scott fitlin
06-18-2004, 04:59 PM
About how big and how much would a 100wpc@4ohms be? And can this amp be done with an input sensitivity of 3/4 volt?

Ian Mackenzie
06-18-2004, 07:17 PM
Hi Scott,

Thanks for your interest.

I have not done any costings recently so I will let you know. Most of the cost is in the heatsinks and power supply.

The size of such a device would be like a 3 unit rack size amplifier but much heavier (mono blocks).

Suggest you hire the movie "The Forbidden Planet". The immense power of such an amplifier is breathtaking.

Sensitivity can be set to suit any level. The input should preferrably be balanced and I propose to include a unbalanced to balanced class A preamplifer line stage.

Ian

Ian Mackenzie
06-19-2004, 06:27 PM
Scott,

Based on your power requirement the approx cost;

Semiconductors $150
Heatsinks $150*
Transformers & $300*
Capacitors

This is an indication only and you would need to check pricing from local vendors. The final cost will also depend on your individual approach to construction and your application.

Such a project would take some time to source parts, construct and commission and you would acquire many new skills and an appreciation of this kind of amplifier. There is also the self satisfaction and the results that would compare favourably to any amplifier at any price.

Amplifers on this scale tend to be conservatively designed and parts rated accordingly.

Here is a gallery showcasing some recent diy efforts.

http://www.passdiy.com/gallery/alephx-p3.htm
http://www.passdiy.com/gallery/alephx-p2.htm

Of course not everyone needs a monster from The Forbidden Planet, and all you may require is a 30 watt amplifier to bi amp your 4430/4435 or a 75 watt (8ohms) version to bi amp your 4343 etc and use a conventional amplifier for the bass.

So I intend to provide templated options for the above power levels, but if you have specific needs I am happy to consult.


Ian

*There is a tendancy to use less volume with such a very dynamic & high resolution amplifiers because you can hear significantly more information even at low level.

boputnam
06-25-2004, 08:41 AM
Originally posted by Ian Mackenzie
This invention will have a bigger single impact on the sound of your JBL's than anything you have done before! Lovely Karen...!!

Oh, sorry - wrong Thread... :slink:

Ian Mackenzie
06-25-2004, 01:19 PM
I doubt if she or her husband would appreciate that!

Karen for those who are unaware, is the DIY Diva at Passlabs.

No love letters please, Karen is married with 3 daughters.


Ian

Ian Mackenzie
06-25-2004, 10:16 PM
As to the program for rolling out this project, some description and story behind the design is in order and then a step by step plan of how to build it is in order.

Firstly, some background.

This amplifier is a diy version of the most recent technology from the Nelson Pass company Passlabs. Diyaudio forum guru Grey Rollins designed the circuit based on the Pass patents and presented the amplifier on the Diyaudio.com Passlabs forum some 24 months ago.

The design is protected by several patents but this version can be constructed for diy purposes on a not for profit basis. In fact over a 1000 diy pcb boards have been made world wide.

Sufficient numbers of this diy amplifier have now been successfully built and tested that with some careful attention to detail, the average constructor can attempt and commission the project with only a minimum of test equipment. No less used in tuning a bass reflect enclosure!, an audio ocsillator, multimeter and low voltage DC bench power supply.

I conceptual terms, the amplifier is solid state (FET) pure class A but its topology is termed balanced single ended. The earlier Aleph patented current source and the more recently patented Super Symmetry (su) have been mixed to create an amplifer with the velvet smoothness of SE Class A, but with the brilliant dynamic contrast of that Su X delivers.

For the technically minded, the amplifier uses only a small amount of feedback to make the balanced function of the amplifier perfectly symmetrical, thus enabling 100% cancellation of all distortions without resorting to the more commonly used negative feedback.

To assist in providing detailed background information I shall use hyper links where possible. There have been dozens of threads and thousands of posts on the diyaudio.com forums regards this particular amplifier, so please do not stray from the marked trail or you will get lost in hyper space.

Go to this link for official background information on the X Aleph project. http://www.diyaudio.com/wiki/index.php?page=Aleph-X

Ian Mackenzie
06-25-2004, 10:46 PM
Okay,

At this point if you are interested in the project, you are probably saying what power level do I need and how do I scale my project .

I anticipate everyone has their own idea of how much power they think they need, but practicality, cost and degree of competence must be considered.

So to rationalise this issue I plan to present 2 power levels which should suit most members needs and slot in nicely for some typical JBL bi amplifed monitors. The powers levels chosen will also enable construction without being overly expensive while also being practical.

The power levels will be 30 watts (8 ohms) for the 4430/4435 biamplification of the HF horn and other 2 way HF applications, and a 75 watt 8 ohm version for bi amplification of the 4343/4345 and 4350 monitors and the smaller JBL monitors.
(Both versions can be adjusted to operate in a 4 ohm load to product the same or proportionally higher power output).

Some of you may dissmayed at the modest power levels here compared to your industrial pro amps but I assure you the dynamic qualities and high resolution of these amplifiers are sufficient for the highest level of musical enjoyment.

I will provide an Excel spread sheet calculation for scaling both versions two interested members.

I do not propose to publish the actual circuit design here for reasons of potential patent infringment but I can email to those seriously interested or it can be located in the above hyperlink.

Next week I provide an overview for the basics of contruction for the project.

Here is a glimpse of a finished X Aleph project!http://www.passdiy.com/images/gallery/ax-p5-f3.jpg



Ian

Ian Mackenzie
07-01-2004, 02:47 AM
Okay,

The design calculations for both versions have been completed using an excel spreedsheet.

I will start presenting the kit this weekend by building up the higher power version myself as this will be shipped to the USA when completed for one lucky member.

The concepts and design are common to any scale in the design so you can follow the basic steps.

I will start be looking at the PCB, this can be either made by your self following some simple instructions and a few basic tools, or I can provide it perhaps or I can refer you to a supplier..

Making you own pcb is the fun part so keep your eye on this thread.(its easy)

I will then provide a parts list, followed by instructions on stuffing the pcb(s), assembly and mounting of power fets on the heatsinks, assembly of the power supply, guidance to fabrication of a chassis, and testing.

To be continued.

Ian


:cool:

Guido
07-01-2004, 04:30 AM
Thanks Ian!

I'll follow for sure...

Ian Mackenzie
07-01-2004, 05:06 AM
Well you will be busy for a while, but cosy when you are done.

They do burn off a bit of heat....keep your pussy nice and warm in winter.

Ian:rotfl:

Ian Mackenzie
07-04-2004, 04:09 AM
The beginning.........................


Here I will discuss some pre design aspects of the project and an introduction to Class A.

Quoted from Erno Borbely........World authority of Class A amplifiers, former director of Engineering for Dynaco and the David Hafler Company.

"The quest for class A starts here. Many listening tests have proven that amplifiers working in "pure"class A sound better than those in class AB. Users report an improvement in low level details and ambience as the bias current increases. They normally attribute this to the fact that the active devices (bi polar or mosfets) are operating in their linear region, without abrupt nonlinearities or turn-on/ turn off delays.

Equal weight is put on the current capability of class A amplifiers. By definition they are high current amplifiers and they are capable of driving difficult speaker loads better than class AB amps.

The definition of "pure class A" can be very different from one manufacturer to another. Often this is nothing more than marketing hype, the question of current is probably enough up to 1 watt, but nothing more.

The definition of Class A is simply that the device always conducts current and never goes into cut off. This is especially important to keep in mind when you consider the output stage, where large currents flow. Looking the output stage of most amplifiers, the push pull, complementary emitter-follower or source follower stage. In such an amplifier, the peak output current in class A is equal to twice the idle or bias current."

In preparation for the project it is necessary to specify the DC bias current conditons for these Class A amplifiers.

In order to do this we need to look at the load conditons represented by the intended speakers.

This afternoon I measured the impedence of the JBL 4345 system and then the midrange high frequency section as used in the bi amplified mode.

Interestingly, in the normal passive mode the 4345 system presents a load of around 4.5 ohms from the bass to midrange frequences and then rises through the HF region.

I then measured the system in biamp mode, the midrange and HF both measured around 8.0- 9.5 ohms and then increases in the HF region.

The low impedence of this system and most likely the 4343 and 4315 monitors suggests why some amplifers would not give good or best performance in passive mode while bi amp mode being a much easier load for the amplifier also gives better performance.

To determine the bias current we use several formulae to arrive at the peak voltage and peak current.

For example if we want 75 watts into 8 ohms:

The RMSvoltage = V(P x R ) where P =75 and R =8

Equals 24.49 V RMS.

This is the equivalent of 34.64 volts peak. So the supply voltage must be a minimum of 34.64+-.

I Bias = 1/2 VC/R load where VC =34.64 and R=8
Equals = 2.165 A

The output stage will deliver a peak current of 2 x I Bias =4.33 amps before leaving class A, which is the equivalent of 3.06 amps RMS.

The class A output power into 8 ohms is

Po =Vrms x Irms = 24.49 x 3.06 = 75W.

In the next post I will apply these bias requirements to the actual design using an excel speedsheet

Ian

Ian Mackenzie
07-05-2004, 04:24 AM
Here we have some specific design requirements for the amplifier.

The attachment file is a screen capture of an Excel 5.00 spreadsheet (If you would like a file copy pm me).

The overall idea is to balance a variety of criteria based on a few facts and some certain known assumptions. Then apply some practical experience to get a workable design. I have built 5 similar amplifiers and they all work very nicely.

In the above thread we looked at a known speaker load and came up with some minimum output requirements for the amplifer to operate in pure class A.

The spread sheet below is a neat and convenient way to flesh out the amplifier key operating parameters. Output power into 8 ohms or higher impedance in this instance, DC bias current, supply rail voltages, number of output devices, power dissipation etc.

I worked several simulations framed by the desired output power, the known power dissipation and thermal ratings for a given size heatsink equivalent to 2 x 0.25c/watt.

To round out the design we have to allow for practical losses. To cover losses 20% more bias current was added to the above formulae ensuring complete class operation right up to full power and beyond giving 5.20 amps DC bias current.

As this particular class A design uses an (ccs) active current source that shares 50% of the AC Current Gain (Patented by Neslon Pass) the amplifier DC bias current conditons are a bit more realistic for practical design amplifier of this size.

As the current source is actively sharing the ac current gain the load effectively seen by the amplifier is a negative impedance twice that of the speaker. ie 16 ohms.

The target of 75 watts output is a conservative operating conditon and the power dissipation of 200 watts for one channel conveniently fits the heatsink K/w ratings I have on hand of 2 x 0.25C/watt.

Heatsinking is a major design factor for class A operation and cannot be overlooked. I will refer to this in more detail later in the thread. By the way, we normally use sufficiently large heatsinks that fan cooling is not required. That way the ambient noise from even the quietest fans does not mask the resolving power of the amplifier.

Ian

Ian Mackenzie
07-05-2004, 04:35 AM
In this instance, we are not concerned about driving an impedance lower than 8 ohms, however this graph illustrates the amplifier has sufficient reserve for more power into lower impedances.

In the event an individual constructor wishes to operate a lower or perhaps higher impedance load, the design can be revised for that application.

Ian

Ian Mackenzie
07-05-2004, 04:54 AM
This aspect of the spreedsheet deals with more the business end of the amplifier. Heat as mentioned above and effective ways to get rid of it must be managed.

The design uses the rule of thumb that 25c above ambient , or 50C is hot to touch but not that hot as to cause pain or user damage. This is a known threshold in that 55 c is almost too hot to touch and 60Ci s even more so. None the less the amplifier will survive under these conditions day in and day out until the Sun cools and becomes a Red planet with an afternoon glow!

Just think, in the middle of your North American winter you will be able to come home after work, crack a can of beer and enjoy a nice warm listening room after a short warm up.

The output devices are an IRFP240, rated at 150C junction temperature. In this design we use 12 per channel based on the above mentioned heatsinks. The design is conservative and can be revised for even lower junction temperatures by using 16 devices per channel if required! (These devices are particularly tough, unlike your garden variety plastic Bi polar transisters in many consumer amplifers)

Ian Mackenzie
07-05-2004, 05:27 AM
So tonight we have covered most of the important theory.

I will also post the design details of the smaller version shortly.

We will then consider the practical steps to construction of the amplifer.

Here is a simplified schematic of the amplifier which some will find interesting.

It has but only 2 gain stages, a simple differential input pair and a follower output stage cleverly biased with an active current source.

The design however is not so intuitive, with the way the current source works and the super symmertry (Su) that is imposed within the balanced operation. The practical operation of such as design mystified the inventor for years (not shown). Once unlocked you have an amplifier that can deliver the best qualities of balanced symmetrical operation and pure class A.

To be continued......

Ian

Ian Mackenzie
07-05-2004, 08:44 AM
Perhaps a few practical construction insights are in order following the above math.

Here we have a typical heatsink that will be used for the above design.

This particular hestink has dual flanges that enable mounting the power Fets to the flange via M3 fixing bolts and nuts as opposed to tapping M3 holes in the surface of a flat machined heatsink like that below.

Either way if you have a drill press it makes the job a lot easier to get nice plum holes. A reasonable amount of precision is required to ensure the pins of the Fets line up correctly with the Pcb that holds the source resistors.

Tapping the auminium bolt holes is not that difficult but it cannot be rushed. A broken tap means the end of the job and you have to start over.

Ian

Ian Mackenzie
07-05-2004, 08:52 AM
This is a pair of two 0.21 C/watt heatsinks used on an earlier 100 watt version.

Note the silicon grease used to improve the thermal bond between the Fet and Mica washer. No grease hi performance washers are also available.

The red sticky labels are used to identify and label the Fets which are matched for Vgs. I will refer to this latter on when to look at the hardware proper.

Ian

Ian Mackenzie
07-05-2004, 09:09 AM
Here is the completed pcb holding the bulk of the circuit less the power fets and associated source resistors.

I used screw clamp connectors to add in assemble of this prototype.

All these components are practical aspects of the amplifier project that can bring a great deal of pleasure from doing it yourself!

Ian
:)

Ian Mackenzie
07-05-2004, 09:26 AM
The other major component is the power supply.

The power supply shown here was used in an earlier Aleph 2 mono block.

The power supply for a class A amplifier must be carefully designed for continuous high current operation. We will look at the power supply in great detail later in the thread.

Ian

Ian Mackenzie
07-14-2004, 02:42 AM
I will post details of the PCB shortly, and as I am making pcbs else where I make some for this project.

Ian

Ian Mackenzie
07-14-2004, 08:11 AM
Here are the parameters of the smaller version suitable for driving horns. Again this is a cerservative design and can be optimised for different impediances.

Ian

Ian Mackenzie
07-14-2004, 08:19 AM
The other business end of this design.

Ian Mackenzie
07-14-2004, 08:44 AM
The pcb layout.

For those specifically interested I can zip the pdf and component overlay.

As I have a transformer and suitable heatsinks on hand I will build up the smaller version over the weekend.

Ian

Ian Mackenzie
07-14-2004, 09:15 AM
A quick simulation of the power supply.

Here we use RC filteriing and liberal filter capacitance to remove ac ripple from the supply rails.

The residual is less than 28mv! More effective filtering can be obtain with LC filtering but the chokes would be large and expensive, so we burn off a few watts instead.

The total filter capacitance for one channel here would be 180,000 uf!

Ian

Ian Mackenzie
07-15-2004, 05:47 AM
This is the power supply simulation for the 74 watt X Aleph.

The ripple has been well filtered being 15 milli volts with more capacitance as the rms current is higher. The transformer in this case should be rated at 500 VA with dual 16 volt secondaries, while the small amp has a 300 VA transformer with dual 12 volt secondaries.

The bridge rectifier should be a high current version with an adequate heatsink. We will look at a special slow turn on circuit to prevent the otherwise high inrush current shortly along with other details of the mains wiring.

Ian Mackenzie

Ian Mackenzie
07-15-2004, 06:02 AM
Time permitting I will post either a working version of the smaller or large amp over the weekend.

We will then detail the actual construction of the Pcbs, assemble of the power fets on the heatsinks and the power supply over the next few weeks.

Yes, I will draw up a parts list when time permits.

Below is a typical layout of such an amplifier, simple but neat wiring with adequate space for cooling. This Box took some 6 weeks to fabricate. Weight= 25 kilos.

Ian

Ian Mackenzie
08-08-2004, 04:24 AM
This project is on hold till early September.

Ian

boputnam
08-08-2004, 11:03 PM
Originally posted by Ian Mackenzie
This project is on hold till early September. Try and sober up, dood - people are relying on you!! :scold:

:rotfl:

Ian Mackenzie
08-09-2004, 05:19 AM
Unfortunately I have other urgent priorities at this particular point in time.

However, I do plan to use some of my annual leave to finalise this project in September.

Those genuinely keen to tackle for project can note their intent.

I think Bo you can pass on that, I'm making yours for you.


Ian

boputnam
08-09-2004, 06:52 PM
Originally posted by Ian Mackenzie
I think Bo you can pass on that, I'm making yours for you. :dancin:

Ian Mackenzie
08-10-2004, 07:55 AM
Your damn right.

You'll be jumping like a red beatle with this thing designed to run at 122 F (50c) all day every day.

Try doing that with your PA amps, fans or no fans its game over Man, game over.

Ian Mackenzie
08-21-2004, 06:08 AM
In preparation for construction of the amplifer I will be testing the smaller version on Monday to verify its DC operating parameters.

I will then detail the construction the following week commencing 30 August.

The aim with this project is to provide an overview of the design (see earlier posts) and a step by step guide to the contruction, testing and commissioning of the amplifier.

I will also provide insights into key design and construction appoaches as we progress through the project.

This should provide interested members a complete "how to guide" while also giving you the opportunity to develop new skills.

For those who take up the challenge you will have the satisfaction of having built the amp yourself and the musical enjoyment it will bring for years to come.



Ian

Ian Mackenzie
08-22-2004, 06:06 AM
I have built up and tested the smaller version and it tests correclty and plays very nicely.

I will post a parts list tomorrow.

Ian

Ian Mackenzie
08-31-2004, 09:04 PM
Update:

I am reviewing some of the parameters of the smaller version at the moment.

Specifically, the input Gate capacitance of a given power Fet is non linear with voltage and at low Drain (Vds) to Source voltages the capacitance increases significantly such that it may effect high frequency performance depending of the source impedance.

An alternative is to cascode the Fet or use a higher supply rail and devices with lower Gate capacitance. It is anticipated the extra power dissipation will be in the order of 50 watts with the latter.

Nelson Pass says and he prefers to run Fets with a some voltage across them and ample bias current so that's what we are going to do.

My simulation of the smaller version with the updated supply voltage of +- 25 volts shows total power dissipation of 150 watts (1 channel) and power output of 36 watts into 8 ohms and much higher into 12 and 16 ohms.

This approach may bring benefits to those with 16 ohms compression drivers...running lots of clean watts for those devices is hard to come by.

The choice of the number of output devices is largely a matter of power dissipated and heatsink real estate. I plan to use a minimum of 12 IFRP 240/244 per channel (they are only a $2-3 dollars each.). I will discuss this more later on.

From a project perspective I will standardise on +-25 volt supply rails and run the larger version on this voltage as well to mitigate any potential technical issues there. That way if you buy the appropriately rated transformer you can use it for either version or if you decide to up date to the larger version later. (500-750VA toroidal). In terms of numbers the larger version will dissipate about 260 watts per channel and have a power output of 108 watts into 8 ohm.

I have just completed developing the main PCB. Here is an image of the PCB showing the green photo resist.

I used a 40 watt Fluoro tube 5 cm from the PCB with 10 minutes expose time.

Ian Mackenzie
09-09-2004, 05:48 AM
I was going to make the smaller version as a project, however my current resource of parts suggests the larger is more appropriate.

So here we have the key elements. Once you have sorted out the power level and purchased the major components its time to idealise how its all going to come together.

We do this prior to detailed construction so that we can overview the wider aspects of the project. The chassis design and layout is a major aspect of it.

With class A we have energy and heat to deal with and also a means of holding everything together in a space in which the parts will occupy.

After trying several ideas I came up this this layout. The two transformers rated at 300va each will be mounted on the floor of the chassis base plate. This is done to provide the coolest part of the enclosure for the transformers and also keep them a reasonable distance from the main driver PCB.

Next we have the reservoir capacitors mounted above the transformers. They need to be in close proximity to the rectifiers ,transformers and the output stage to keep high current wiring as short as possible.

The main PCB is located near the top of the amp chassis. Here it is central for shortest wiring and also accessible for final assembly and adjustment.

At the sides are the heatsinks which will transfer thermal energy to the outside.

I then worked on how to fasten the whole thing together and will show some aspects of that phase over the weekend, time permitting.

This phase do not preclude the actual assembly of the PCBs, which will discuss in great detail shortly. We now have all the parts and I will start off with testing and match the mosfets in the next post.

Ian

Ian Mackenzie
09-09-2004, 11:42 PM
Major Parts: What you need and where to buy :

Okay,

I have not posted a complete parts list yet and will do so in detail shortly..Oops!

Incidentally, main difference in parts between the two power levels will be the source resisters and some other values for biasing the current source of the input pair and active current source used in the output stage.

The main parts of the amplifier are the HexFets, transformer, capacitors for the power supply and the heatsinks.

The Power fets are IRFP 240/ or IRFP 244, you will need to order a factory lot coded tube of 50 and match them for sets of 16 with VGS within +- 0.10 volt.

Also P Channel 9610 2 matched pairs and 2 single devices for the front end current source.(More on this later so don't stress over it now)

Note these devices are sensitive to static discharge so take precaution and handle with care.

The best place to buy is IRF.COM for factory lot coded tubes.

The transformer should be a toroidal type and should be rated at 2-3 times the power dissipation of the amp, about 500-750 Va depending on how the manufacturer rates the transformer, I will post specific details once I have done some more simulations.

600 va 5% regulation
Primary voltage as per your mains supply
2 x 22 volt secondary (full load)

Check you Yellow pages for transformer suppliers.

The capacitors should be large can types or computor grade types rated for high current/long life and rated at 2-3times the DC supply voltage. The ripple current rating for the capacitor should be 3-4 time the current drawn. So in this case we are looking at 6 times 25,000uf capacitors rated each at 15 amps ripple current rating and 50-75 volt. (This amplifier is relatively immune to capacitor quality they need to be very heavy duty.)

6 x 25,000 uf 75volt Computor grade electrolytic
Try Mouser, Digikey Allied Electronics, RS Components, Farnell

Alternatively you can use multiple smaller capacitors like I have in the picture to arrive at this specification. Some may wish to consider so called audiophile parts. I recommend Jamicon, Panasonic or Cornell Dup (CE) large cans. They will be about 2-3 inches in diameter and about 4-6 inches tall.

The heatsinks are most important for the reliability and survival of your project. We aim for about a 55-60 C temperature on the heatsinks after running for 60 minutes. Thats hot but not too hot to touch. (your pussy will love it!) Ideally we calculate this on the basis of a 25-30 C rise above ambient based on the C per watt rating of the total combined heatsinks.

So here we have about 250 watts and the total C/watt rating needs to be about 0.1 C - 0.12 per watt. Its unlikely you will find one single heatsink to do that so consider two or even 4 if necessary.

My solution was to use two 0.25 C / watt heatsinks which I already had on hand. My temperature rise above ambient will be 31 C so I will have 56 C temperature thereabouts.

2 x 0.25 C per watt 8 Hexfets per heatsink or
4 x 0.5 C per watt 4 Hexfets per heatsink

Try your Yellow Pages for Heatsinks or Mouser, Digikey Allied Electronics, RS Components, Farnell (Conrad Engineering in Aust)

Ian

Ian Mackenzie
09-10-2004, 03:49 AM
The parts list excluding the above items.

Ian

Ian Mackenzie
09-10-2004, 04:33 AM
Now for some technical enlightenment.

At this stage it is assumed you have ordered your parts and are ready to commence construction.

With this type of amplifier we must match certain components within a reasonable close tolerance just like they do in US$10,000 High End Amplifers.....The ones on 5th Avenue NY your old lady won't let you look at!

Don't let the matching put you off, it just takes a little time and quite straight forward.

Read these links by mosfet class A legend Nelson Pass:

http://www.passdiy.com/pdf/mos.pdf
http://www.passdiy.com/howto/matching.htm

Your bias per Fet is 0.65 amps for the power fets
(R1 = 17 ohms for the N channel IRF240..match to 0.1 volts)

Note the active side or negative side of the output needs the fets to be matched closely on both sides of the balanced output.

So you will need 8 IRFP240 with a VGS within 0.10volts

Your bias per fet is 10 millamps for the differential 9610 fets
(R1 = 1100 ohms for the P channel IRF9610..match within +- 1.0 millivolt)

You may find it useful to buy some small round sticky coloured labels to identify your matched sets. An Excel spread sheet can also be handy to catalogue your matched Fets.

*A small regulated bench top power supply rated at 1.00 - 1.5amp 15VDC is required. Check out Radio Shack or you local Electronics Hobby store. It should be less than $60 bucks and will be quite useful.

*A Digital Voltmeter will be require. No an expensive model but one capable of 3/1/2 or 4 digits and a low DC volt range ($50)

This bit of equipment will be your buddy right through to the project.

When you are done here we are ready to start baking that Mud Cake with Big Fat Moma.

Ian

Ian Mackenzie
09-10-2004, 05:36 AM
Heatsink Assembly.

At this stage you will have matched and batch coded your power Fets and will need to start considering how you are going to mount the Fet packages on the heatsinkand associated circuitry.

Firstly, you will need to arrange some mounting hardware.

Qty 32 TO3P/TO247 Mica washers or high quality non grease washers and silicon grease. (normally in a white in a small tube).

Refer to Mouser or Digikey online catalogue.

Depending on whether you mount your Fets on a flanged Heatsink or directly onto the back plate you will need some M3 bolts, nuts and lock washers and a drill tap and wrench.

If your heatsinks are flanged like mine you can drill through and mount the Fets on the flange with a nut and bolt, or if mounting direct you will need to drill and tap your M3 holes.

The taping is not difficult but it is recommended you have a drill press handy. Taps and drills can be purchased from your local industrial hardware supplier.

(Take care with the taping, take your time, a broken tap is impossible to remove!)

The Power Fets and associated Source/Gate stopper resisters need to be mounted on a PCB or wired point to point if your are very handy with wiring. I recommend the PCB and will post some artwork when I can locate it.

These are small PCB strip boards with precisely located tracks and holes for the positioning of the Fets on the heatsinks so everything is aligned.

When you have drill/taped your holes and removed all burrs with a fine file you are nearly ready to mount the Fets.

The legs of the Fets will need to to bent at 90 degrees if mounting direct on a backplate I recommend making a jig.

Using a 4mm aliminium plate with 3 countersunk holes to poke the FET legs through and bend gently with the a bar.

(Do Not Use Your Fingers)

Alternatively, with a fine pair of pliers carefully bend the legs to the required length with a gentle radius.

Next apply liberal quantities of silicon grease on both sides of the Mica washers and the underside of the Fet.

Then position the washer on the heatsink and Fet carefully using the drill hole as a guide.

Place a washer on the Fet package then insert you M3 Bolt and tighen gently but firmly. Do Not Over Tighten. Repeat this for all the FETS.

Locate the PCB board and align the holes for the Fet legs and gently insert the legs. The PCB is supported by a standoff and this should be secured with an M3 bolt at this stage.

Once you are satisfied the Fet legs are all located and aligned in the PCB holes the legs can be soldered. I recommend you ground yourself and use a small crock clip to short the Gate to drain of the Fet when soldering as they are sensitive to static charges.

The Source and Gate stopper resisters can then to mounted on the PCB's and soldered in position.

That about does for now, next we will look at the main PCB.

Ian

Ian Mackenzie
09-10-2004, 05:42 AM
Here is a close up of the IRFP240 heatsink assembly.

Ian

Time for an ale :o :cheers:

Ian Mackenzie
09-11-2004, 08:39 PM
With the main PCB, mount the smaller 1/4 watt resisters first, then the larger 3/6 watt resisters. The resisters for the output current share sense will need to be carefully soldered side my side then piggy backed as there are 8 x 0.77 ohm.

Next mount the 220 uf electros, the 10pf silver mica, the diodes and finally the 9610 Fets. Prior to this stage however you will need to arrange a small heatsink for the 9610 Fets and small solid bar of aluminium for the input pair to share. Its important that they be thermally bonded.

The same rules apply here regards static discharge and insulation using mica washers. When all the parts are soldered in place check carefully at for correct orientation and also the any cold solder joints.

When you are satisfied everything is correct clean the board with some spray on some PCB cleaner and apply spray on board lacquor.

I will post an image of the board later.

When the above stage is completed we will start assembly of the main chassis and construction of the power supply.

Ian Mackenzie
03-20-2005, 12:50 AM
I'm re starting this project and it will be continued in a private area of the forums for IP reasons.

If you would like access, pm me.

Ian