Zobel Network

JaredC79

Banned
2015-09-02 4:32 am
NJ
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.
 

Cortez

Member
2002-12-26 9:45 am
Hungary
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.
 

JaredC79

Banned
2015-09-02 4:32 am
NJ
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?
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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.
 

JaredC79

Banned
2015-09-02 4:32 am
NJ
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
 
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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Cortez

Member
2002-12-26 9:45 am
Hungary
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.
 
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.

You are right. Zobel network has nothing to do with overall stability of the amplifier, nor with good working at low loads, nor EMF etc. The only purpose is to stop any HF oscillations caused by base-emitter parasitic capacitance of emitter follower finals transistors. Serial inductance helps to avoid any effects of capacitances at the output (wiring, crossover filters...) and when well designed and damped, have no effect at all about sound quality, and even can help to reduce distorsions at 5...20 kHz.
 
You are right. Zobel network has nothing to do with overall stability of the amplifier, nor with good working at low loads, nor EMF etc. The only purpose is to stop any HF oscillations caused by base-emitter parasitic capacitance of emitter follower finals transistors. Serial inductance helps to avoid any effects of capacitances at the output (wiring, crossover filters...) and when well designed and damped, have no effect at all about sound quality, and even can help to reduce distorsions at 5...20 kHz.
I can't say I care about original intent. I have an amp designed in 1966 that didn't have one. The vendor added one in a re-design in 1970. The 1966 version is pretty easy to drive crazy. Especially if modern 4 mhz Ft transistors are installed instead of unobtanium 200 khz Ft transistors typical of 1966.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
He's conflated the ideas that inductance = iron = bad for audio. That's a common generalisation about the output coil network run wild. However, the Zobel in the OP here is a simple, series RC combination, so why all the angst about inductors anyway?

HF is high frequency (3- 30 MHz) and VHF is very high frequency (30-300 MHz). These are the regions where audio amplifier instability usually manifests itself - sometimes destructively. Mosfets amps can show instability even up to 100MHz or so. Ignore it and any design measures included to fix it, at the amplifier's peril.

I think like Cortez, that in a forum, jumbling everything you want to say down in one rambling, single paragraph post is going nowhere. For starters, it can't be easily deciphered and your meanings become unclear. There's no harm in relating your practical experience but folks assume you haven't learned much if you also take those good/bad rule-of-thumb approaches to design and commentary.
 

JaredC79

Banned
2015-09-02 4:32 am
NJ
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.

Knowledge capital - not just doing things because "He did it".
I am running a lot of 4 ohm loads
I want to know how Krell and Parasound does it
There's a lot of reasons


Purpose of not having inductors I see a lot of high quality designs without coil inductors. Most are lfets - maybe they are using inductors of another source and I read that it can be achieved with a transformer.

At the end of the day I want to understand as much as possible to create the ideal and most stable amp with minimal part count. Not adding things because it can possibly be a good idea.
 
Ian, I fully agree. A combination of many statements in one starting post, especially if some of them are wrong / misleading / confusing, accompanied with many questions at once - result in a thread going nowhere.

On top of that, there is a therminology mess - yes, Zobel network is just an RC for protecting the amp from the speaker's back EMF, inductor is part of the Thiele network, protecting the amp from capacitive loads.

Now, coming to the particular point. Guys, this 1-2 uH air-core coil does not influence anything within the audio frequency range (please try to convince me if you think it does). However, it isolates the output of the amplifier (equipped with the global feedback loop, which is also important) from capacitive loads at much higher frequencies, where there is a danger of converting the whole thing into a "screaming oscillator".

See how it works >HERE<
Pictures there are showing the wave forms behind the coil (left) and in ftont of the coil (right). The load is 1.1 uF in parallel with 8R.
 

Jay

Banned
2003-02-11 9:02 am
Jakarta
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?

ZOBEL is Resistor-Capacitor network. In speaker design, a zobel is to compensate for impedance rise/irregularity at HF. In amplifier design it is for stability, almost similar, or work together with input filter (C), Cdom and the output inductor...

The difference between zobel for speaker impedance and zobel for amplifier stability is in the frequency they work on. For stability, the zobel works on very high frequency (above 100kHz). The R usually smaller than 10 Ohm, sometimes as small as 1 Ohm. And this must be put very close to the amplifier, not in output terminal.

Speaker impedance zobel, is of course better put in speaker (as part of crossover design) instead of in amp. At worst, when you have commercial speaker, and you want "better" load for your amp, you may want to put in speaker input terminal.

You asked why we need to use inductor, stability zobel, etc? Because it is part of the trade-off that we choose in amplifier design. If we can make the amp super stable, even with very capacitive high end speaker cable, you don't need them. Think of it as a cure... or to increase safety factor...

If you are the amplifier designer, you need to set a standard at how much is not too much. This can be seen as "phase margin" or "gain margin" or from user's eyes, the value of the capacitance (Cdom)...

From my perspective, you don't want any low pass filter that affect phase at audible range. For this, may be you need to set the -3dB high enough so at 20kHz it is still flat, etc. etc.
 
I can't say I care about original intent. I have an amp designed in 1966 that didn't have one. The vendor added one in a re-design in 1970. The 1966 version is pretty easy to drive crazy. Especially if modern 4 mhz Ft transistors are installed instead of unobtanium 200 khz Ft transistors typical of 1966.

200 kKz Ft transistors do not need any Zobel network... Inside resitance of the base lead is very high and damps all sorts of high frequency instabilities. But high frequency response of the amplifier is poor.
 
vzaichenko said:
On top of that, there is a therminology mess - yes, Zobel network is just an RC for protecting the amp from the speaker's back EMF, inductor is part of the Thiele network, protecting the amp from capacitive loads.
No. Whatever it is called, the CR network ensures that the amp sees a lowish and not too capacitive impedance at RF frequencies. It does this by swamping any capacitance with a parallel resistor, but includes the series capacitor so this only happens at higher frequencies. The inductor, if present, assists this by isolating the load at higher frequencies. Nothing whatsoever to do with back EMF.

Including these is not a sign of poor or sloppy design, but wise precautions. There is a barmy school of thought in audio which says that an amplifier which is fussy about what you connect it to has 'fine discrimination' (or something like that); I suspect that some 'high-end' designers propagate this myth to hide their incompetence at electronic design.