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Old 28th January 2010, 09:13 PM   #1
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Default Krill – The Evolution

Here is a next logical step for the 50W amp. This is what PH104 (Phil) is building at the moment. I think he changed some resistor values in a couple of places. He can inform us what those changes were. No, I do not have a problem with his doing that. He knows what he is doing, and is welcome to make the changes.

As I said, this is a 50W at 8 Ohm amp. With adequate heat sinking and a large enough transformer, this amp will deliver 100W at 4 ohms and 200W at two. Adding two more pair of output transistors will allow 400W into one ohm.

The original 50 and 100W amps are documented on my web site, thanks to Rob_S, PH104 and c2cthomas. I would like to concentrate now on units that drive more difficult loads and/or put out more power into those loads. Of course, if someone decides to build anything I have posted so far, I will be glad to assist in any way I can.
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Old 28th January 2010, 09:20 PM   #2
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Nice piece of circuitry! But I can´t see why you would want the input lowpass filter really, allt it does is ruin the slewrate.
A question, are there any special reason to use such heavy transistors for Q13 and Q15? I bet there are smaller and faster ones...

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Old 28th January 2010, 09:45 PM   #3
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Quote:
Originally Posted by Rikard Nilsson View Post
Nice piece of circuitry! But I can´t see why you would want the input lowpass filter really, allt it does is ruin the slewrate.
A question, are there any special reason to use such heavy transistors for Q13 and Q15? I bet there are smaller and faster ones...

I disagree about the slew rate. The low pass is there to help keep HF noise out of the circuit. As for my choice of transistors for the drivers, if more outputs are added for difficult loads, as I mentioned, the current demands on the drivers can be fairly high with reactive loads. These transistors are fast enough for audio, and using the same transistors for drivers and outputs avoids having to source two more part numbers in order to save perhaps $1 per channel.
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Old 28th January 2010, 11:35 PM   #4
wahab is offline wahab  Algeria
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if the source , let say a preamp has a caps at his output,
the input differential will see unbalanced bias resistors....
since the output stage is not included in the feedback loop,
there will be some distorsions...
equally, it is assumed that the front end is stable without
added compensations....
can you please shed some light(s) about these matters?
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Old 29th January 2010, 07:06 AM   #5
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Quote:
Originally Posted by Steve Dunlap View Post
I disagree about the slew rate. The low pass is there to help keep HF noise out of the circuit. As for my choice of transistors for the drivers, if more outputs are added for difficult loads, as I mentioned, the current demands on the drivers can be fairly high with reactive loads. These transistors are fast enough for audio, and using the same transistors for drivers and outputs avoids having to source two more part numbers in order to save perhaps $1 per channel.
Well, you are correct about the HF noise, but isn´t the circuit quiet enough without the filter? If not, I would try to make it quiet. Slowing down the slewrate isn´t going to help with performance, especially if it turns out to be unnecessary.

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Old 29th January 2010, 09:50 AM   #6
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Quote:
input lowpass filter really, allt it does is ruin the slewrate.
Rikard, the input filter stops HF coming in from outside. That has really nothing to do with slew rate or the noise of the circuit. Feel free to leave it out, but be aware that your amp could theoretically suddenly play radio with the bipolars demodulating incoming RF.

Also using faster drivers will not give you a faster amp. Speed is usually dictated by the VAS-transistors, or more correctly by their Cob times gain (=CMiller).

Have fun, Hannes
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Old 29th January 2010, 10:19 AM   #7
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Yes, I understand the radio problem, but at the same time I think this would only happen if you have really poor cables and ground loops or similar. The input cap can surely short such interference, and also some of the internal noise, but it will still ruin the slewrate. There is no chance you could ever obtain the best slewrate with a cap across the input.

And I wouldn´t quite agree about the drivers either, slow drivers = slow amp just as any other transistor in the signal path will affect speed.

Last edited by Rikard Nilsson; 29th January 2010 at 10:24 AM.
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Old 29th January 2010, 10:28 AM   #8
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Rikard, RF does not need cables, or is your portable radio wired?

Standard RCA plugs are not RF-proof, neither are openings in your case.

You make me curious, who or what makes you believe a cap on the input affects the slew rate of an amp? It solely affects how fast an input signal slews, but has nothing to do with the amp itself.

Have fun, Hannes

PS:
Quote:
slow drivers = slow amp
Well, if the voltage gain stage is 10 times slower than the drivers, are drivers with half the speed really a big deal?

Last edited by h_a; 29th January 2010 at 10:31 AM.
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Old 29th January 2010, 11:09 AM   #9
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RF needs antennas, and poor wiring is what could be expected to work as an antenna. And if the slewrate is reduced by the input cap or the amp itself doesn´t matter to me, it´s still ruined.
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Old 29th January 2010, 11:15 AM   #10
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Quote:
Originally Posted by Steve Dunlap View Post
Here is a next logical step for the 50W amp...
Hi Steve!
Good that Krill is back

Dear Guys,
please read previous threads and posts about Krill
and then send yours.

The project is fully debugged and is working!

High frequency limitations of the transistors for this amp are not important, it works without GNFB!

cheers!
__________________
regards, Pawel
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