Conjunctive filter values.

wiseowl · Post by **wiseowl** » Mon 07/05/10 3:52 pm

I have a harshness in the amp which I've been trying to tame with tweaking components and experimenting with the nfb. Nothing has really cured it though. I'm considering trying a conjunctive filter across the ot, looking around I've seen values of 2uF and 10r, these seem somewhat extreme to me, can anyone confirm if they are a good starting point?

Also, does the filter need to be close to the transformer, or for experimental purposes at any rate, can it be fitted across the speaker?

Thanks

Martin

krx · Post by **krx** » Mon 07/05/10 4:13 pm

Are you connecting it across the secondary? I've only seen it done across the primary before, which is probably the better place for it.

For an 18W try 0.0022uF/600V (or 1000V if you want a safety margin) and 10k/10W. Good thread on it here:

http://archive.ampage.org/threads/4/gag ... ter-1.html

MadDitch · Post by **MadDitch** » Mon 07/05/10 4:35 pm

Place it accross the primaries. I currently have a .001uf 1200V Orange Drop in series with a 10K 5W resistor on an amp to cure some spikiness in the OT. It's really not noticeable, just gets rid of the hash. You could also do a Zobel filer across the secondaries, 10R 5W and a 1uf or 2.2uf bipolar cap.

wiseowl · Post by **wiseowl** » Mon 07/05/10 4:37 pm

Yeah, of course it should be the primary, had a blond moment there.

Those values sound far more sensible, I'll start with those.

Thanks

Martin

krx · Post by **krx** » Mon 07/05/10 4:38 pm

Any functional difference between a Zobel and conjunctive? Also, if you're going conjunctive, you can go up to a 0.022 or 0.047uF cap if you want to really smooth things out. Definitely put that one on a switch though, not permanently wired in.

MadDitch · Post by **MadDitch** » Mon 07/05/10 4:47 pm

Any functional difference between a Zobel and conjunctive? Also, if you're going conjunctive, you can go up to a 0.022 or 0.047uF cap if you want to really smooth things out. Definitely put that one on a switch though, not permanently wired in.

Zobel is going to depend on you speakers impedance. So you'll have to adjust the resistor value for 8ohm, 16ohm ect. I think the CF does a better job with lower cap values, but your millage may vary.

wiseowl · Post by **wiseowl** » Mon 07/05/10 4:47 pm

MadDitch wrote:Place it accross the primaries. I currently have a .001uf 1200V Orange Drop in series with a 10K 5W resistor on an amp to cure some spikiness in the OT. It's really not noticeable, just gets rid of the hash. You could also do a Zobel filer across the secondaries, 10R 5W and a 1uf or 2.2uf bipolar cap.

Perhaps this is where I got confused with the values.(not difficult).

Thanks
Martin

StarGeezers · Post by **StarGeezers** » Mon 07/05/10 5:41 pm

Isn't that just a Band-Aid approach that disguises the real problem ???

krx · Post by **krx** » Mon 07/05/10 6:20 pm

Isn't that just a Band-Aid approach that disguises the real problem ???

Heck of a lot cheaper than a new OT...

Post by **CurtissRobin** » Mon 07/05/10 8:23 pm

StarGeezers wrote:Isn't that just a Band-Aid approach that disguises the real problem ???

It can work very well if you're being practical or thrifty. If it's a recording amp you'd be justified in doing other things.

KennyO

wiseowl · Post by **wiseowl** » Tue 07/06/10 11:17 am

StarGeezers wrote:Isn't that just a Band-Aid approach that disguises the real problem ???

By the real problem do you mean just the OT? or are there other suspects. I thought I'd invested in a decent OT.

FWIW The speaker I'm driving is a Celestion G12H30, which is beautiful and smooth with my 18 watter.

Regards

Martin

krx · Post by **krx** » Tue 07/06/10 11:57 am

Yes, a conjunctive filter is generally thought of as a fix for a less-than-great OT.

One thing you might want to try is using pure nickel guitar strings like DR Pure Blues. With a simple amp like an 18, you get out what you put in. Tubes can make a big difference, too, of course.

I generally prefer to tweak those sorts of things for the basic tone rather than adding extra things to the circuit like feedback, filters, etc.

Post by **zaphod_phil** » Tue 07/06/10 1:03 pm

Yes, a conjunctive filter is generally thought of as a fix for a less-than-great OT.

Here are a few posts on the subject from Randy Fay aka Ontariomaximus, who builds some pretty cool amps of his own design. His contention is that you need zobel or conjunctive filters in EL84 amps to counteract the inductive reactance of the speaker(s).

ontariomaximus wrote:Every EL84 amp I do has a Zobel, which is a conjunctive filter but on the secondary. 10 ohm 5 W R in series with a 2u2 cap across the 8 ohm connection. Contrary to what some believe, it ain't a bandaid. What it does is alter the load presented to the amp at high frequencies. You wouldn't use a output trans with a 30K primary would you? Well that could be load presented to the amp at high frequencies due to the the speakers voice coil inductance. The Zobel counteracts the rise in impedance due to that inductance. I'm not saying that is the cause of the fizz 100%, but I never hear it. And perhaps that's why some people have it and some not - it depends on the impedance of the speaker.
You have to remember EL84's with out feedback have a very high output z, so a high z load will likely sound like fizz.
JMHO

ontariomaximus wrote:Its not the resonant frequency of the speaker, it's the voice coil inductance you are neutralizing. If you are playing up in the 17th fret, and your guitar 8 ohm guitar speaker has an impedance of 20 ohms up at 4khz, your carefully chosen 5k opt transformer (or whatever) is no longer 5k, it's 12.5k. THAT will sound different. And if your output Z is high, your frequency response will track the speaker impedance. Chances are, that ain't good. Subjectively, the zobels have absolutely worked for me on the other side of the ot and I see no reason why it wouldn't work on the primary.

ontariomaximus wrote:The 10ohm + 2u2 would be average values to counteract the speakers VC inductance, but it can be tweaked to taste.

ontariomaximus wrote:If you want to convert a zobel to a conjunctive filter or vice versa just multiply or divide by 1000. eg. a Zobel with a 10 ohm R and 2.2uf C on the 8 ohm secondary is equivalent to a 10k R and .0022 cap on the primary (which is 8k) Shouldn't make any difference.

PS: I sure hope we get this quotation bug fixed soon.....

StarGeezers · Post by **StarGeezers** » Wed 07/07/10 5:51 am

Hmmmm....??? Interesting !!!! Thanks ZP !!!

krx · Post by **krx** » Wed 07/07/10 7:08 am

Every guitar speaker's impedance rises with frequency. To say you "need" some sort of compensation for that is a bit silly, since it's part of what makes a guitar amp a guitar amp. We're not building hi-fis here

.

Post by **zaphod_phil** » Wed 07/07/10 8:46 am

Well maybe yes and no. With the poor damping factor of EL84s vs beam tetrodes like 6V6s, it appears you can sometimes get the dreaded fizziness problem. Or as Randy says, "You have to remember EL84's without feedback have a very high output z, so a high z load will likely sound like fizz."

In this particular thread, we also see that after trying every possible fix, Jdj was only able to fix his amp's fizz problem through using a zobel filter - http://www.18watt.com/modules.php?name= ... c&p=222567 And we mustn't forget that the original version of the 18W, the WEM Dominator had a conjunctive filter.

So personally, while I've never found a need to use a Zobel filter or conjunctive filter in any of my own amp builds, I wouldn't dismiss their use in some cases.

ontariomaximus · Post by **ontariomaximus** » Wed 07/07/10 10:01 am

I think this "impedance compensation" via conjunctive filter or zobel idea is simple enough that if it piques your interest, try it. If you like it, keep it.
I tend to gravitate toward a high gain sound, which has plenty of higher order harmonics, and impedance compensation is a benefit.

Additionally, if someone wanted to add a true CUT control, the secondary might be the best place for it. Say a switchable 10ohm + 3.3uf to 6.8uf across the 8 ohm winding.

Thanks for all the quoting, ZP!

Post by **colossal** » Wed 07/07/10 11:16 am

Randy,

I PM'd you over at AmpGarage regarding this, but most of your comments on the Zobel filter you use is centered around an 8R tap. If you were using the 16R tap, the values would need to be changed to suit, correct? So, 20R and 550nF assuming the same speaker inductance? Do you have a reference for different speaker inductances when calculating the values for a Zobel? Celestion.com is of no help.

Thanks!

ontariomaximus · Post by **ontariomaximus** » Wed 07/07/10 1:31 pm

I don't spend a ton of time over at ampgarage.

I typically use 10R + 2.2uf on the 8 ohm tap and is permanently wired, but 20ohm + 1uf would be a starting point for the 16 ohm tap. It's really a taste thing though. You may prefer 10ohm and .47 uf on the 16 ohm tap which will cut less in the audio band, but more above 15khz.

The 'cut' not really being a cut, but a lessening of the amp's frequency response tracking the speaker rising impedance.

additionally, I don't have a reference for difference speakers, though one could be done. I have a ton of different cabs, who knows what my customers have, so my 10R/2.2uf is a one size fits all.

krx · Post by **krx** » Wed 07/07/10 1:45 pm

will cut less in the audio band, but more above 15khz.

additionally, I don't have a reference for difference speakers, though one could be done. I have a ton of different cabs, who knows what my customers have, so my 10R/2.2uf is a one size fits all.

Do you think your speakers are really producing anything above 15 kHz anyways? That's tweeter territory. Heck, I don't think a lot of us can hear much above 15 kHz anymore anyways

. I don't question the validity of using Zobel filters in general, but I'm wondering if it really makes a difference for guitar use.

As for different guitar speakers impedance curves, I'd say they're all close enough (see the charts on the Eminence site, for instance) that a one-size-fits-all approach is reasonable.

I'm also thinking a Zobel filter would be a good place to use a non-inductive resistor like a Mills.

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Conjunctive filter values.

Conjunctive filter values.