L'Amp: A simple SIT Amp

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#341
luvdunhill said:
...so I can pop one of the Semisouth devices in to play with as well. Any idea which one would be recommended...
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You could try the PASS-SIT-1 (Sorry. Naughty.)

R085 perhaps, if you build the adjustable negative bias.

Check with Stephano B., I think he did some work with that one.

It's not a SIT, of course, so any load line cancellation will not be as dramatic.
 
#342
Thanks! I found some posts. I think with a CCS that my 25 VAC transformer will work. I can drop lots of volts with a pair of MOSFETs (one in the PSU and one in a CCS) or less volts with BJTs. Back of the envelope calculations say I should be able to hit my target voltages this way. Next, to draw up a little schematic.
 
#343
The hum is the same....cynch in or out....

I got to manage the exact values 13,9V ds and 2,55A. I did not yet look at the distortion...but I heard again two hours music......

Very very very very fine sound, so clear and distinct.

It is less sharp than two days ago, maybe it is because I took out two 10W resistors out of the chain and could replace them by 30W monsters or because I managed now better to meet the sweet spot.

Today or tomorrow I will start to rebuild the PSU to get rid of the hum.....

Gerd
 
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#346
Oh thank you very much tea-bag!

I use the step up from Jack Elliot electra print


PVA-2n Constant load input transformer for OpAmps to standard Amplifier Input. 75 to 600 ohm to 10k terminated. Nickel core 1 to 8 voltage gain. Other step-up ratios are available. $60.50

http://www.electra-print.com/specialuse.php

I ordered the 1:5 version, but you must wait until it is produced....

And I put the F4 input stage in front of the xformer and use only one output pair in F4 that is my crippled F4....
 
#348
yes I supposed so, but was not quite sure, thanks!

Moreover the FW PSU normally have a very extended region for earth that is in my use no more earth but middle of the voltage, maybe in this big area also hum can be induced ..

I just build a PSU like you suggested....
 
#354
I used 50 Watt Dale power resistors attached to a small heat sink (piece of aluminum angle). 20 Watts or larger is recommended. I like to comfortably oversize these parts. I got a bunch cheap on Ebay.

You can also series/parallel a bunch of those 3W Panasonics to get what you want. I take the expected dissipation and multiply by three or four (or more 🙂)

ra7's air core inductor recommendation also does a great job. That's what I'm using in my spiffed-up deluxe version.
 
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#356
hi Franz,

very nice......!

I always take the triangle of the idiot U in the top and I and R in the basement put your hand on the value you want and you see the solution.

I am so old now but still easy to disturb in maths....like in my youth.
 
#357
Franz Gysi said:
Any experience/opinion about this cheap china 100W resistors, available at eBay?

100W 6 Ohm 5% Screw Tabs Aluminum Wire Wound Resistor (280792618317)

An externally hosted image should be here but it was not working when we last tested it.


Franz
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Scalling by the hand in the picture, it looks like a typical Dale or Mills 50W. They are actually a few turns of robost wire like material (depending on value) incapsulated in plastic and stuffed into the gold anadized housing. Without bolting them down to a heatsink, they are no better than about 10W. When you blow them up the plastic will break in half and one side will pop out of the aluminum housing and unravle the resistor material. A real 100W version of that design is about 35-40mm x 110-130mm with screw terminals. The actual size difference between the picture and the 100W'ers I've seen is at least a 4:1 difference. Most of the ones I've seen do have a very good temp coefficieint of 50ppm or better and I beleive Dale even offers a low noise version. 😀
 
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