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Old 17th July 2013, 10:33 PM   #11
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Quote:
Originally Posted by gadut View Post
nice project, you always come with a fresh design. how does it sound compare to a standard LM3875?

Regards
Better !!!!

To me, it sounds like a completely different amplifier.

Cheers, Joe
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Old 17th July 2013, 10:40 PM   #12
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Quote:
Originally Posted by Loudthud View Post
Is the DC Trim pot shown correctly on the schematic?
It must be, since it actually works!

Quote:
Originally Posted by Loudthud View Post
Is there any reason that other power amp ICs won't work or not be suitable for this design?
Would I use this exact circuit? No, as the bandpass filter on the input and the DC trimming may be completely different - so while it is possible to use others, I cannot foresee what needs to be sorted.

Cheers, Joe
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Old 28th July 2013, 04:26 PM   #13
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This seems a very interesting project to try. Thanks Joe for sharing. I already have on hand two 400VA transformers with 2 x 24V output. Except for giving away a couple of watts at the output can I use them to give a try to your design with my Elsinore speakers?

Last edited by petefrontiers; 28th July 2013 at 04:28 PM. Reason: text modification for better understanding
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Old 28th July 2013, 11:19 PM   #14
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Quote:
Originally Posted by petefrontiers View Post
I already have on hand two 400VA transformers with 2 x 24V output... can I use them to give a try to your design with my Elsinore speakers?
Sounds like a good idea to me.

Just a quick one, are you able to do a basic impedance plot on your Elsinores? It should not be required, but a good idea to do before current driving them - it is not a hard thing to do.

Cheers, Joe

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Old 29th July 2013, 01:23 AM   #15
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Quote:
Originally Posted by Joe Rasmussen View Post
Sounds like a good idea to me.

Just a quick one, are you able to do a basic impedance plot on your Elsinores? It should not be required, but a good idea to do before current driving them - it is not a hard thing to do.

Cheers, Joe

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What is a basic impedance plot, Joe?
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Old 29th July 2013, 01:26 AM   #16
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I remember an Analog Devices EE who had a transconductance amp design in one of their Data application books, back 20 or so years ago. Rated for 50w, you were allowed to build two amplifiers without consent for personal use, more contact AD. It was a very high end design where the designer didn't spare quality. Even when a typical Zener diode would have been used was made discretely (they are quite noisy). Outputs were IGBT's. Specs were very impressive. Listed parts included the PCB layout et al.

Joe, wondered why you left off the output filter that prevents ringing with capacitive loads. Unnecessary unless using this type of load I would expect, contact you for mod if using this type of setup? Or just slap on a 0.7uH in parallel with a 10Ω in series with the output as in the data sheet pdf?
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Old 29th July 2013, 02:56 AM   #17
bimo is offline bimo  Indonesia
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Voltage gain = Speaker impedance / 0.33
Because speaker impedance vary with frequency then voltage gain vary with frequency too.
I saw such as design from Elektor magazine many years ago. But this design never success commercially.
Maybe people don't like hear from speaker that drive by current.
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Old 29th July 2013, 03:01 AM   #18
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Ok, A little research told me what is an impedance plot. Sorry for my not perfect understanding of english language. But I would like to know how do you measure it.
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Old 30th July 2013, 06:52 PM   #19
Sjef is offline Sjef  Netherlands
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Interesting. How stable is this circuit, since the LM3875 is pretty prone to oscillation. The floating input could be a bit tricky. Would like to try such amp for my open baffle mids (Phy-Hp H21LB15) but the floating input might be a little concern in a three-way active system where proper earthing is a bit of a hassle. Have had an with floating inputs before and that simply didn't work, lot's of disturbance. The amp on it's own was o.k. but in a complex active system it was the cause of earthing problems.

Could I use an input transformer to tie the primary to PSU ground and leave the secondary floating ?

I have been thinking of building a Pass F2 before but the enormous power use of these monsters held me back. My system is almost always on, from 4 to 16 hours a day, every second I'm home. So this could be a nice alternative to a Pass amp.
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Old 30th July 2013, 11:39 PM   #20
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Quote:
Originally Posted by Sjef View Post
Interesting. How stable is this circuit, since the LM3875 is pretty prone to oscillation. The floating input could be a bit tricky.
I would say, go for it.

You will note that I supplied a wiring diagram and not just a schematic. Build them as mono-blocks solves a number of issues, and actually hum is as big as any of them. But fixing the hum this way you will also make it stable from oscillation.

But not sure why you are describing this as floating earth? It isn't, follow the wring to that Green circle where everything is referenced to, including the ripple current in the power supply. In fact, it is the speaker return that must not be grounded as all the current must go through the current sense resistor - but the other end IS grounded to the same as input and power supply. It the speaker is in way floating - and that should not prove to be a problem if you know about it.

If you are referring to 22R (not a critical value), that can be eliminated (shorted) altogether, but the resistor will suppress ground-loop type hum, that's all.

Cheers, Joe
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