Conjunctive filter values.

[colossal]

Ontariomaximus wrote: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.

Thanks Randy. I did get what you mean about the 'cut' being just dampening, not an actual cut. I have been referencing this Zobel calculator (http://www.carstereo.com/help/Articles.cfm?id=36 ) which I think was posted in another thread.

Ontariomaximus wrote: I have a ton of different cabs, who knows what my customers have, so my 10R/2.2uf is a one size fits all.

That's what I was wondering: if a general assumption of speaker inductances and Re would be sufficient for approximating a Zobel for our purposes here. For fun, I looked at all the Eminence speaker characteristics (thanks for the tip KRX -- I wish Celestion would publish their data like Eminence does) for all of the Red Coat (British flavored) speakers to see if there were some obvious relationships between the resonance peak, Re, Le, and magnet weight. There are some weak correlations but nothing that would let me predict Le based on other parameters. It might be nice to be able to model different R and C values if we had data for each speaker. I know many here use Celestions or close relatives, myself included, and so that's what I would be basing my assumptions on (GH1230 Heritage to be exact) for calculating R and C for a Zobel. Anyway, since I am planning a couple of 18W builds, I am looking into possible mods that might be nice to have on a switch.

As ZP has said, most have not "required" a Zobel filter in their 18W amps, but some of it may just be individual taste as to the qualities of overdriven EL84s (the fizz component) and others may be on the tails of the distribution having actually experienced the dreaded Ruby Fizz or where one or more components (such as their OT) is not really up to par and degrading the tone. It seems that a Zobel would be a useful mod to be able to switch in. I do agree however that it would be important to discern where an amp might actually be sub par and using a Zobel as a patch versus the enhanced tonal properties a Zobel filter could add to an amp that is working correctly.

[colossal]

krx wrote: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.

For fun, I looked at the resonance, Re, Le, and magnet weight data for all of the speakers in the Red Coat series on the Eminence site. For all of their 16R speakers, the mean inductance was 0.71mH +/- 0.04 (5.6%) with a mean resonance of 99Hz for n=6 speakers. I wish Celestion would publish data sheets like Eminence. I'll email them and see if they will help out.

[zaphod_phil]

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.

As long as the filter remains permanently connected to the appropriate tap, I think it shouldn't matter which tap the speaker is actually connected to. The OT secondary will do the necessary impedance transformation, as if it was an autotransformer.

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.

With guitar speakers there tends to be a rolloff starting a little above 5kHz. There is also an upward swing in the impedance curve above 5k. The problem is some speakers have additional peaks or lobes in their response up at around 10 to 15kHz, where the impedance is also quite high. So you may then hit the EL84 fizz problem.

[colossal]

ontariomaximus wrote: 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.

zaphod phil wrote: As long as the filter remains permanently connected to the appropriate tap, I think it shouldn't matter which tap the speaker is actually connected to. The OT secondary will do the necessary impedance transformation, as if it was an autotransformer.

Ok, very cool, so Ontariomaximus' 10R/2.2uF would be fine even if using the 16R tap since damping is occurring regardless of the speaker impedance. This is exactly what I was wondering.

zaphod phil wrote:With guitar speakers there tends to be a rolloff starting a little above 5kHz. There is also an upward swing in the impedance curve above 5k. The problem is some speakers have additional peaks or lobes in their response up at around 10 to 15kHz, where the impedance is also quite high. So you may then hit the EL84 fizz problem.

I did contact Celestion and they said they'd send the Le and Re values to me for Celestions we see most commonly used with 18W amps. I'm not sure however if any of it will actually matter as krx and Ontario pointed out, a one-size-fits-all approach will probably be just fine and the cap values could be tweaked to taste. For now, I checked out this site which shows the damping effect of a Zobel. Maybe not perfect for guitar amps but it demonstrates the rise in impedance at high frequencies. The filter clearly damps both the resonant peak but also the high end.

http://www.carstereo.com/help/Articles.cfm?id=36

The resources page for Eminence drivers includes the impedance vs. frequency curves. I wish Celestion would do this
http://eminence.com/resources/params.asp

[oh7hhi]

Bumping old thread..

Yesterday I played with zobel filter, because my amp (Richie's TMB) has always had too much high-end sizzle for my taste.
I'm using G12H30 speaker and I put 18 ohm + 1 uF at 16 ohm tap.

It's better now, but I think it smoothed it bit too much. Is there a way to calculate roll-off point or should I just experiment?
I'm going to try 0.47uF today...

OT is Hammond 125E. Does anyone have experience with Ampmakers OT and how does it compare with Hammond?