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Chip Amps Amplifiers based on integrated circuits

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Old 17th May 2009, 06:20 PM   #1
SM7UYJ is offline SM7UYJ  Sweden
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Default CG to drive DIY ESL

I have just built a couple of hybride ESL-speakers and impedance measurement show they are a quite nasty load for my amps!

Click the image to open in full size.

There is an old thread on the subject, but it does not realy answer my question: -Is it worth the trouble off parallelling chips for higher current capabilitys?

I'm using an old LM3875 amp to drive them right now and when crank the volume up, shure they get hot but the thermal limiting has not set in so far.

But as the load is well under 1 Ohm for a great portion off the audio band, I'm thinking of building a more suited amplifier.
I have 4 LM4780 and 4 LM3886 just laying around waiting to play music.

Any sugestions?

/R
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Old 18th May 2009, 10:47 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
go and read the Borbely variations and note that the Electrostatic compatible version has enormous output capability.
Forget chipamps driving electrostatics.
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Old 18th May 2009, 11:45 AM   #3
SM7UYJ is offline SM7UYJ  Sweden
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Why should I forget chip amps?

I understand that a descrete design made specialy for an ESL is the best choice, but that is not the question here.

The LM3875 is doing a fine job on it own, but would parallelling chips be worth the effort?

Will the increased drive capability come with instability problems - pros vs cons ?

/R
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Old 18th May 2009, 12:13 PM   #4
AndrewT is offline AndrewT  Scotland
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Default Re: CG to drive DIY ESL

Quote:
Originally posted by SM7UYJ
..................the load is well under 1 Ohm for a great portion off the audio band, I'm thinking of building a more suited amplifier..........................
Any suggestions?
what was the question?
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Old 18th May 2009, 01:01 PM   #5
SM7UYJ is offline SM7UYJ  Sweden
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AndrewT: I now see the problem!

But it's posted under Chip Amps, so that was the amp type in mind.

My daughter is sick today, so I put my LM3875 on the bench for a test drive into a 1 Ohm load - did not look pretty!

4V P-P and then the thing starts to oscilate!

With my Class-A amp I get 6V P-P before clipping, but no oscilation.

But it would be nice with a small GC that could handle the load!

/R
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Old 18th May 2009, 01:46 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
was that a 1r0 resistive test load?
or
a 1ohm reactive speaker load?

I'll wager that Spike came in before oscillation started. It was probably Spike switching the outputs off and on repeatedly that you could see/hear.

A resistive load is relatively easy to drive, but not with a chipamp which cannot deliver the necessary current.

A reactive speaker load is much more difficult to drive. Electrodynamic drivers can demand around three times the current that a nominal resistance would predict.
eg. 1r0 could expect to demand 2Apk when driven with 2Vpk.
A 1ohm speaker could demand 6Apk when driven with 2Vpk.
An electrostatic speaker is even worse and the stress on the output devices is much worse.

I'll say again, forget chipamps driving electrostatic speakers.
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Old 19th May 2009, 07:23 AM   #7
SM7UYJ is offline SM7UYJ  Sweden
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Hi,

I just tested with a resistor as load!
They were all (4pcs) 1 Ohm wirewound so not a pure resistive load but pretty close.

Tested with two LM1875 in parallell (+/- 17V) as I had an amp on my workbench with these chips, and It drives the 1 Ohm dummyload much, much better but it needs a bufferstage.

Tested it with my speakers os load and the switched back to my LM3875 amp: -I'm intrigued by how well the LM3875 handles the load of the speakers?

/R
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