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Old 5th February 2009, 12:04 AM   #1
njepitt is offline njepitt  United Kingdom
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Default Planning an F5 build - some beginner questions

While I have read a great deal on this forum over the last year or so, this is the first time I am posting (anywhere on the internet, in fact), so let me start by thanking everybody for the help I've found on this forum, and on diyaudio in general.

To give a little background, I am an expatriate Brit living in Brasilia. I started messing around with diy audio stuff about a year ago, after I decided that (here in Brazil at least) the options for improving the sound coming from my computer were (a) expensive and (b) pretty rubbish, so having dim recollections of using a soldering iron in adolescence I decided to "have a go"... Since then I have made several projects, the most successful of which are versions of the B1 preamp, two Hiraga Le Monstres and one 20W Le classe A, being fed by a USB DAC. (I'll spare you the *less* successful stories...)

Now a recurring problem here is that when I try to build any project there are parts that are simply not available here, and importing is slow and expensive. (We appear to have 70% import taxes on top of the value of the product *and* the postage.) I usually end up having to use what I can find locally (sometimes in the mail from Sao Paulo) instead of what would be "ideal" and often have to improvise, particularly with anything heavy such as transformers or heatsinks. (A good source of almost free heatsinks is to ask at car audio repair places for burned-out power modules. Some of these have huge finned aluminium cases, and you can get reasonable heatsinks for the price of the scrap value, although they are usually ugly indeed...)

To sum up, so far my projects have been cheap, mostly ugly, loads of fun and sound pretty good, but now I want to see if I can go a step further and make something that sounds better still and that my wife will actually let me put in the living room. So I have started an F5 build, and have some questions which have turned up in the planning/design.

1. For the first time I have paid real money for two big heatsinks (which should arrive from SP any day) for the mosfets. These are each a little more than 6" wide and 10" high, with a bunch of 2" fins, anodized black. The website says a four inch section (I presume unanodized) is rated at 0.74 degrees C/W, so according to the long F5 thread if I cut them in half and put one mosfet on each this should be fine - less than 0.7 C/W each. (For some bizarre reason buying four 5" heatsinks or one 20" heatsink were both 30% more expensive than two 10" heatsinks...) In any event, I am wondering if I could use one for each channel without cutting them, making a taller, tower-like amp. I realise this would be thermally less efficient than cutting them and separating them horizontally, but has anyone done anything similar? Am I likely to lose too much in efficiency? (If it helps I could post photos of them when they arrive.)

2. I plan to put the power supply in a separate case from the amp itself, as I did with the Hiragas. This has the advantage that I could reuse the power supply if I decide to make an F4 or something (F6? F7?) at a later date, but also lets me use a bigger box, tucked out of sight. (And over-engineer things...) Is there any disadvantage to this that I have missed?

3. I cannot find toroidal transformers here at any price, so it'll have to be EI. It is easy to find transformers with a centre tap, but transformers with two competely separate secondaries as shown on the F4 manual have to be wound specially, which is expensive and of unpredictable quality. Can I use two ordinary 18VAC transformers instead of one with two secondaries? Any down side?

4. If the answer to 3 is "no", I might try to use the capacitance multiplier PSU circuit from the ESP website

http://sound.westhost.com/project15.htm,

which I used for the Hiragas with some success. Is this likely to be comparable to the F4/F5 design? If not, can anyone give another suggestion?

Well, I guess that's it for now, although doubtless I'll have other questions later. Special thanks to Nelson Pass for making his designs available to us in this way. I am very happy with the B1, and hope my F5 will be as successful.
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Old 5th February 2009, 06:42 PM   #2
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Welcome to the club.

There is no issue with using two transformers, just give each its
own rectifier bridge to cover any issues.

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Old 5th February 2009, 07:53 PM   #3
njepitt is offline njepitt  United Kingdom
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Many thanks for the reply, Nelson. Two transformers it is. (Or should it be: "Two transformers they are"?)

I am pretty happy right now, since the heatsinks have just arrived and they look terrific. (Prompt service and nice finish from a company called Franab, if anyone in Brazil is interested). I'll try to see if I can post a couple of photos, and I'd be most grateful for opinions on question 1 above.
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Old 5th February 2009, 07:53 PM   #4
njepitt is offline njepitt  United Kingdom
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I have attached (I hope) a photo of one of the heatsinks. (Pen included for scale...)
Attached Images
File Type: jpg img_2814.small.jpg (72.3 KB, 1612 views)
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Old 5th February 2009, 07:53 PM   #5
njepitt is offline njepitt  United Kingdom
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Here is another...
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File Type: jpg img_2815.small.jpg (80.1 KB, 1470 views)
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Old 7th February 2009, 12:39 AM   #6
njepitt is offline njepitt  United Kingdom
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Here's another question. I spent a couple of hours digging around in the junk room at the Elec. Eng. dept. at the university today, to see if I could find some transformers. (I have spent most of my budget for the month on the heatsinks...) If my calculations are right, I need two 18VAC transformers rated at 5A or so in the secondary (they don't quote power ratings in VA here, for some reason, instead they speak of a 2A or 5A transformer). Now there was nothing exactly like that (too much to ask, maybe...) but I *did* find a huge collection of old transformers which were used to heat filaments in valve experiments way back when. They are pretty solid and appear to be in good condition, and choosing output wires appropriately give an output of about 19.5VAC, with no load, which should be about right. They are not big enough individually, however; best guesses from the people around are of about 2.5A, so I helped myself to four (from about 100 or so...), planning to wire them as two pairs in parallel. (Same voltage, twice the current, right?) My question is whether I should parallel them before the two rectifier bridges that Nelson recommended, or use four bridges and parallel the output from these..... Anyone have an opinion?
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Old 7th February 2009, 01:32 AM   #7
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I built an amp using 4 transformers -- 2 in parallel for each channel. I used one rectifier for each channel. All 4 of the transformers were identical -- not sure if yours fall into this category.

At any rate, it worked perfectly -- no hum or blown fuses at all.

When wiring up the transformers, I hooked up the secondaries to an oscilliscope, and switched around the wires until I got identical outputs on the scope.

JJ
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Old 7th February 2009, 08:38 AM   #8
njepitt is offline njepitt  United Kingdom
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Yes, all four are identical - at least, they are the same brand and model so they are the same to within commerical tolerances anyway. I don't have an oscilloscope, so I'll have to try it and see if it works. (I suppose I can always put two more bridges in later if necessary.) Thanks for the help.
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Old 7th February 2009, 10:05 AM   #9
h_a is offline h_a  Europe
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Quote:
I should parallel them before the two rectifier bridges that Nelson recommended, or use four bridges and parallel the output from these..... Anyone have an opinion?
Naah, Nelson recommends individual bridges for individual transformers. You are not forced to do that, but it nicely avoids humm in the case that windings are not perfectly equal and cause DC.

So individual bridges and connect the outputs.

Have fun, Hannes
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Old 7th February 2009, 10:22 AM   #10
njepitt is offline njepitt  United Kingdom
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Thanks for the reply, Hannes. Since I actually have four bridges in my junkbox I'll go with that, I think. (I thought a little more about jupiterjune's reply, and I am not sure his topology is the same as mine - I am using one PSU for both channels, for a start.)

I am also going to have to test the transformers for current rating, since there isn't anything written on them to help. A little later I'll go out and try to get hold of some resistors I can use for dummy loads. The secondaries appear to be one winding of 19.5V or so, tapped to give 6.3V and 12V. The current rating should be the same for all, I think (??) so I am going to test the 6.3V so that the resistors stay within a cheap-ish power rating. (Don't want to test to failure, though, of course...)

Am I missing any obvious indications of power rating? How about weight?

Thanks for all the help.

Nigel
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