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Chip Amps Amplifiers based on integrated circuits

LM3886 opinions sought
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Old 3rd February 2018, 12:52 AM   #31
tvi is offline tvi  Australia
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IC Audio Power Amplifiers and Zobel Networks: One Size Does Not Fit All Walter S. Gontowski Jr., STMicroelectronics
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An audio loudspeaker has a complex impedance, while an audio amplifier is more comfortable driving a purely resistive load. To compensate for this, a Zobel network is placed in parallel with the speaker.

Some designers use a standard "rule of thumb" method, where the value of the resistor is between 2.7 O and 10 O, depending on the speaker DC resistance, and the capacitor is invariably 100nF.

This works for most discrete amplifiers, but selecting component values without understanding the parameters of the speaker can lead to serious consequences for IC power amplifiers.
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Old 3rd February 2018, 08:09 AM   #32
AndrewT is offline AndrewT  Scotland
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Tvi,
this paper is about the Zobel fitted inside the speaker crossover.

It is quite different from the amplifier Output Zobel that is fitted in the output stage of the amplifier.

When I first saw a link to this paper a couple of years ago I made this same comment. And a couple of Members joined in to agree.

This paper has nothing to do with amplifier design, ignore it when designing amplifiers.
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Old 9th February 2018, 07:37 PM   #33
elaar is offline elaar  United Kingdom
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Hi all,

Apologies, I don't mean to hijack the thread, but I'm in a similar predicament so I didn't think it worth creating a new one for it.

I too have two 25v 225va encapsulated transformers, that are pretty decent quality wise, and would like to use them if possible.
I have 2 sets of speakers that are rated for 6ohms (minimum 4ohms (one 4.3ohms)), and a component box with LM3886, LM4780 ICs and various pcbs, where I had planned to make a gainclone back in 2006 (according to order emails), and yet nothing materialised.

Due the lowish impedance of the speakers, and the current limitations of the chipamps, I wondered how best to use this combination of parts (much like the original poster).

According to the LM3886 datasheet, although it can be used with 4-6ohm speakers, the THD is higher than that of 8ohm speakers, and it is marginally cripped current wise. However, when viewing AN-1992, I can see that a PA100 design THD with 4ohms matches that of an LM3886 8 ohm, with each section of the chip sharing the load.
I can also see that the recommended maximum voltage for a PA100 with 6ohm speakers is +-37V.

The 25v transformers I have (94% efficiency), specify a no-load voltage of 26.6v. Therefore, with MUR860 diodes, and minimal load, I estimate around 36.5v, and a nominal voltage around 34v. The graphs for 6ohm speakers show a +-35v supply, so I assume this is roughly the recommended voltage.

Can someone please confirm if this theory sounds okay, and whether they have any recommendations. The only other option I can think of, whilst being able to use these transformers, is to go regulated (to drop the voltage somewhat), or to use another method (as some have mentioned earlier), to drop it.

Many thanks,
Andy.
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Old 10th February 2018, 01:35 AM   #34
john blackburn is offline john blackburn  United Kingdom
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Hi Andy. Feel free to join in, all input is welcome.

I have got the amps done, the inductors are wound and the regulators and a bit of metalwork are ongoing, so it is well underway. There should be some listening impressions of my first regulated amp in the not too distant future all being well. It's ages since I've heard something new, I'm looking forward to it.

John
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Old 10th February 2018, 10:38 AM   #35
elaar is offline elaar  United Kingdom
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Thanks John,

It looks like I'm in the exact same predicament as you, as it looks like at some point I was going to make the CarlosFM design (I seem to have all the parts for it).
Good luck with yours, and please do let me know how it all sounds, and I'll get cracking on mine.

The reason for my previous questions was that I also seemed to have bought some "Unisonus" LM4780 pcbs from this forum about 10 years ago, so I'm just not sure which one to build, the CarlosFM or a PA100 when I'm going to use 6ohm (4min) speakers.
Normally I'd build both and try them both out, but due to the case design, the whole pcb has to be attached to the heatsink that makes up the back panel of the monoblock cases, so I don't have the option of doing both right now.

Andy
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Old 10th February 2018, 01:37 PM   #36
john blackburn is offline john blackburn  United Kingdom
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Hi Andy.

I haven't heard the LM4780, you could have bother getting them now if you don't have the chips.

I am trying the snubberised Carlosfm psu design at 28 volts but going with more of the amp stability parts that were left off my previous builds.

My first was a chipamp.com LM3875 kit which had nothing in the way of DC blocking on the input, no feedback cap, zobel or suchlike, I was lucky not to kill any speakers. It is still happily playing today with a Pass B1 in front of it though, and has outlived any amp I've had before (and one I've had since but that was my fault).

I've had to scrap the idea of monoblocks for now so it will be dual mono unless I turn something up cheap in the meantime.
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Old 10th February 2018, 07:22 PM   #37
indianajo is offline indianajo  United States
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Okay, good luck with the 25 vac transformers. Series pass regulators are simple, but fail in the blow up IC mode unfortunately, all of us who used linear supplies back in the70's remember. Buying the "overvoltage crowbar" accessory was highly recommended in those days, one that blew the fuse when the series pass transistor shorted across.
About the zobel network. In my opinion the position should be as close to the speaker jack as possible . I just added a network to two Allen S-100 organ amps that picked up football scores from the AM radio station 6 miiles away during the first service I powered them up. 20 db down, really annoying. Wrong speaker wire (shielded 16 ga tw) plus no zobel network gets RF interferance. Allen did include a ferrite bead on the speaker hot lead, not good enough. Allen got away with the design by running the speaker output through inductive relay attenuators after the amp, which are quite unreliable due to contact oxidation at this age (37 years). Deleting the relays (and rear antiphonal speakers) got me the above.
Winding 12 turns 22 ga wire around a 10 ohm 3 watt resistor in parallel, and glueing the assembly to a wooden stick screwed right next to the speaker outlet, in series with output, solved the problem. There are 3" of wire from speaker jack to driver board, then another 8" to the output transistors up on the heat sink (on top lf chassis inside a mesh enclosure). This acts as an RF transmit antenna IMHO until the radio is stopped at the outside wall by inductance.
For further experience, the dynakit ST120 I have was subject to something bad until the "TIP mod" of about 197? that put a 1000 ohm resistor series a .1 ceramic cap on the back of the speaker banana jack. See greg Dunn's website about this mod. The inductive speaker coil in these is 11 turns wound around the output capacitor, and not shorting resistor, again about 6" from the output transistors but inside the mesh enclosure. No AM radio from the speaker wires is my experience. When I I started driving that amp with 12' of RCA cable due to a furniture addition, RF interferance happened. Upping the input cap from 33 pf to 68 from center to ring solved that problem.
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