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Old 27th February 2016, 04:47 PM   #1
JaredC79 is offline JaredC79  United States
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Default Zobel Network

I know a solid nobel network and overbuilt amp is the key to driving low impedance loads however i am not keen on using an inductor in my amp for various reasons. I know ICs can act as a zobel without the inductance or emi/rfi potential issues as well as some transistors. What can be done in place of a zobel at the amp level and since my amps are built to pair specifically with my speakers, can i use a zobel on the speaker ins prior to the XO for same results? Most designs are using active crossing with bi/tri amping depending on the speaker array. for the rack mount version, what can be done to eliminate the need for an inductor? I would like to be stable at 2 ohms with possibility of 1 ohm stability.
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Old 27th February 2016, 08:36 PM   #2
Cortez is offline Cortez  Hungary
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Zobel at the output of an amp ensures a "default load" on high frequencies.
That's necessary because the output stage can oscillate without any load
due to his tendeny to self resonance.
The output inductor is there for a similar reason: with a specific capacitive
load the outoput stage can go to self-oscilaltion again.
And being so they should be put directly to the output of the amplifier.
And as far as I know (with a proper front-end and compensation of course)
they are not related with low impedance stability at all.
Using a zobel or an inductor may depend on the specific layout but using
them is a good general rule against output stage self-oscillation.
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Old 27th February 2016, 10:37 PM   #3
JaredC79 is offline JaredC79  United States
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I was under the impression from another post a solid zobel was needed to insure stability at heavy loads. I am over building everything and it's almost identical to the 100 watt ultimate amp listed above with some changes and components. Wired point to point. I need to make sure it's stabile at 2ohms. Any assistence?
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Old 27th February 2016, 11:01 PM   #4
Cortez is offline Cortez  Hungary
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With a proper design stability is not a problem with low impedance load.
It's rather just a question of max output current/power capability.
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Old 28th February 2016, 12:12 AM   #5
Ian Finch is offline Ian Finch  Australia
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Zobel Network
According to Doug Self, the simple RC Zobel network is a HF load on the output stage and it's damping effect on oscillation is greatest at VHF. It's a one-size-fits-all device but has little or nothing to do with stability problems specific to an amplifier driving very low impedance loads. There, you need to apply other measures like base stopper resistors, and small bypass caps to squish oscillation.

Low impedance designs generally require more output devices, bigger power supplies, more copper and duplication that requires longer runs of circuitry. This can result in less stability due to more stray reactances, regardless of the load impedance and higher currents.

I think the term "2 ohm stable" is easily misunderstood by people throwing it around as a classification of amplifier types, without being aware of the issues and how they are commonly dealt with at the design level. The term "2 ohm capable" might be more appropropriate if you just want the security of knowing an amplifier can drive such low impedance loads to high power levels if ever needed.
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Old 28th February 2016, 05:40 PM   #6
JaredC79 is offline JaredC79  United States
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Sorry 2 ohm capable. I need it unconditionally stable at 4 ohms. I know Krell is power slobs and can get to 1ohm - ideally I would like to be there. My last kit was a Holton and i changed a bit from his design - not to say he isn't a genius, I wanted to minimize 3 areas of biasing to 2 and use DC servo or 2 bias areas or transistors with thermistor or biasin - think that was the alternative I saw the other day... Holton is unconditionally stable at 500w 4ohms using 4 pairs of Exicon lfet. 2 ohm stable. 55/55 80va toroid kicked up to 78v rails. I get toroids for practically nothing. 70 for a 1kva w 3 additional windings from a local I have a wholesale/OEM account with. So psu isn't an issues. Since Holton is able to max the chips with 4 output pairs - what other stages need to be beefed up? Possibly current to make it faster? More stable "driver" I was going to driver w a mje350 only but may use a pair funneling into a single 350 to drive if it will add stability. Beefing up VAs would only increase rail voltage correct? Output devices are bootstrapped with caps and I have some 1uf .1uf and think a few pf caps but will get more. When you say HF load, you mean high frequency? I rather not have any inductors in the design and know other ways around it. I went thru a ton of effort to pick parts that weren't magnetic or inductive. I am not familiar with all the lingo - what is a base stopper resistor? U mean high values at the base of a transistor? I am building 150w 8 ohm/75w 4 for my 5.25s and tweeters and 500w/4ohms. I may be able to use a larger Arnp for the 2 5.25s. I don't mind adding extra outputs but based on holtons designs I don't believe they are nessesary for the 500 or even 800w models but lower powers seems like it's important. What other value caps do you recommend having on hand for bypassing
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Old 28th February 2016, 09:48 PM   #7
Cortez is offline Cortez  Hungary
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Jared, what exactly is your goal, it's still not clear to me.
What do you want to achieve? Why is this 2 ohm thing important to you?
And please if possible just use concise sentences or a bullet point list.
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Old 29th February 2016, 10:40 AM   #8
indianajo is offline indianajo  United States
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The no inductors idea is definitely daft in 2016. Everybody is carrying a portable radio in their pocket these days. They are called cell phones. Zobel keeps RF from backfeeding into the high gain circuits from the speaker terminals and causing oscillations. Your ears can't hear 1 mhz oscillations but they will definitely melt your transistors if you don't stomp that tendency out. 12 turns around a AA battery form is not a big high fidelity killer. Most amps have got them parallel a 10 ohm resistor to the speaker, and the ones that don't were tested in a lab, not a living room.
I had a serious problem in a mixer picking up a CB transmitter driving by emitting offensive music in dog barks continuously. Inducttors in DC power supply coming in , disk capacitors to ground on the input, and inductors on the output were required to get rid of that interference. If you live in the country 2000' from the nearest road you might be able to get away without RF interference parts. Even out at my sommer camp by a lake, airplanes fly overhead emitting RF. I'm only 60' from the road here in town, and that is pretty far for the suburbs. And I'm the last guy in the world who takes the battery out of his cell phone when I'm not traveling. So, RF is everywhere these days.
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Last edited by indianajo; 29th February 2016 at 10:51 AM.
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Old 29th February 2016, 10:56 AM   #9
Cortez is offline Cortez  Hungary
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I don't think that the zobel and the inductor is basically for preventing HF getting into the amplifier.
Zobel is just a "dummy load" to the output stage @ HF and the inductor is against self resonance
formed by the inductive output impedance of the output stage and a capacitive load.
But not against uF-s (though the inductor has a big effect here as well of course), but typically in the
range of 1n-10nF. The emitter followers are prune to oscillation with capacitive loads in this range.
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Old 29th February 2016, 03:44 PM   #10
DF96 is offline DF96  England
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I'm still not clear what the OP has against inductors.
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