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Old 16th February 2004, 08:47 PM   #1
mikelm is offline mikelm  England
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Default First Watt balanced gainclone idea

whereas: I just noticed that my CD o/p sounded much better with LM6182 biased into class A with resistors ( CCS trial coming soon )

and whereas Nelson ( and others I'm sure ) believe that the sound quality of the first watt is the most important

and whereas I like designing balanced working amps

therefore I present the idea of a balanced wkg gainclone. Powered from 3 x 12.7 V batteries per channel. Perhaps using 2050's cos they sound good with batteries and can supply 5 amps. With the first watt ( or two ) biased into class A with CCS's and the o/p lifted a few volts to maintain max o/p swing.

I won't be building this one for a while, too busy on other stuff, but based on my previous experience I think this could be quite nice.

All comments welcome

mike

p.s. I drew this diagram on a train, so please make allowances
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Old 18th February 2004, 10:41 PM   #2
Steven is offline Steven  Netherlands
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Hi Mike,

I don't think it is required to lift the outputs a few volt for maximum swing. In most amplifiers the lower side voltage dropout to the power supply is often close to 3 V, which is enough for a CCS to work. So the CSS will not limit the swing.

Steven
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Old 19th February 2004, 06:53 AM   #3
mikelm is offline mikelm  England
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OK that's interesting - the only thing is, in a reply to Fred Dieckmann about biasing op amps into class A scott wurcer said the following

....The question remains as to N-ness or P-ness in the direction of the CCS, you will have to experiment and determine for yourself. I did this experiment years ago on a classic Bipolar process (bad pnp) and the pnp (pull up) was better, go figure....

I will have to hope that with the chip I select the pull down sounds the best otherwise I will loose this advantage.

perhaps we will resurrect these threads if and when one of us builds one of these cct's

regards

mike
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Old 19th February 2004, 09:10 PM   #4
Steven is offline Steven  Netherlands
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Hi Mike,

Well, it is just a matter of trying. Your circuit will work as well with a current source as with a current sink.
My circuit (in http://www.diyaudio.com/forums/showt...476#post325476) already uses both current sinking and sourcing. I pressume that with LM3886 amplifiers both halves work quite well as class A output. As long as they stay in class A there will be no crossover anyway.

Steven
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Old 6th November 2004, 11:25 AM   #5
kafka is offline kafka  Poland
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>mikelm

I'm going to build such amp, but I'm wondering how much bias should I apply? 200mA per chip? Not enough? 500mA?

I want to have 5, 10W in A class.
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Old 6th November 2004, 05:09 PM   #6
mikelm is offline mikelm  England
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Quote:
Originally posted by kafka
>mikelm

I'm going to build such amp, but I'm wondering how much bias should I apply? 200mA per chip? Not enough? 500mA?

I want to have 5, 10W in A class.
looking at my diagram it looks like I was thinking to have about 500mA but I think starting at 200mA is a good idea.

perhaps make it possible to change the current source resistor easily so that you can try some different values.

Make sure that you use big enough heatsinks !

good luck

mike
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Old 6th November 2004, 06:26 PM   #7
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Mike, this is such a simple test that I was thinking in making for some time that now I have the opportunity and this is it.
I'm makin' a small amp for my bench/computer.
Inverting T-network LM1875, with OPA2132 input buffer and a small unregulated (tweaked ) +/- 14V PSU.
This is a ~10W amp.
It has no case yet, it's still a bunch of aligators as connectors and it's playin' for some days now, with Visaton FR10-8 fullranges that will have an enclosure today, along with small tweeters.
Anyway, I've just took two 68ohms/3watt resistros from my parts box and put them there (this gives ~205ma biasing).
DC goes a little higher, but nothing special: 40mv.
So, music it is.
There seams to be something more goin' on around here, but it's difficult to say with these speakers.
As I have some enclosures to make today (and tomorrow), I will have no time to test this in my main system.
But I put here a challenge: who dares?
Seriously guys, try it, it's so simple...
Say what you think.

Note: the 3w resistors get very hot.
But the heatsink is cool, due to the low voltage of the PSU. This amp doesn't get hot at all.
If I use this on the final amp, I would rather take the voltage from the buffers' regulators, and not from the unregulated PSU.
IDEA: maby I will use a switch to activate or not the class-A biasing!
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Old 6th November 2004, 06:40 PM   #8
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Carlos: I have no doubt that the resistors get hot!

If you do your IxIxR = watts you get 27.2 watts, pretty good if the 3 watters can stand it.

When they are hot check the voltage drop ot monitor current and see what the real resistance is when hot.
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Old 6th November 2004, 07:04 PM   #9
mikelm is offline mikelm  England
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Carlos,

I am delighted that someone has tried this idea. I look forward to hearing what you think when you can listen more clearly.

Do you think the resistor is going to be superior to a CCS ?

I currently have on my workbench 4 half finnished choke and gyrator regulated chip amps based on TDA2050.

Do you know of any reason that these should not work OK using a T network ?

If they work OK I will then try this biasing mod as well so I get to hear it myself

and then that only leaves an input buffer.....Mmm tube or 627 ?

mike
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Old 6th November 2004, 09:57 PM   #10
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Quote:
Originally posted by SheldonD
Carlos: I have no doubt that the resistors get hot!

If you do your IxIxR = watts you get 27.2 watts, pretty good if the 3 watters can stand it.

When they are hot check the voltage drop ot monitor current and see what the real resistance is when hot.

Yes, but this is what I had for the value I wanted to test.
I listened for around 5 minutes LOUD AND CLEAR, then removed the resistors, listened again, then put them back, listened again.
The 3w carbon resistors got very hot, but no trace dark colour or (worse) red incandescent colour or (even worse) bang!
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