Something stirs in the Undergrowth

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simon
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#2326 Re: Something stirs in the Undergrowth

Post by simon »

Yes, James did a very good job indeed.

I think you're right about the padding resistor. I replaced it with half a Lpad (as a variable resistor) and thought I'd got it nailed. Then sometime down the line I tried increasing from around 2R up to the max 8R expecting the bass to be curtailed, but got the opposite effect. At that point I shrugged my shoulders and thought these crossover things are the devil's work!
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#2327 Re: Something stirs in the Undergrowth

Post by Ant »

The big fane has a qts of 0.7, would that suit?
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IslandPink
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#2328 Re: Something stirs in the Undergrowth

Post by IslandPink »

No, there's no problem with the bass driver itself. & I don't think the Fane would beat the FX120's for mids.
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#2329 Re: Something stirs in the Undergrowth

Post by IslandPink »

simon wrote: Wed Mar 27, 2019 11:55 pm I think you're right about the padding resistor. I replaced it with half a Lpad (as a variable resistor) and thought I'd got it nailed. Then sometime down the line I tried increasing from around 2R up to the max 8R expecting the bass to be curtailed, but got the opposite effect. At that point I shrugged my shoulders and thought these crossover things are the devil's work!
Aha, that makes sense. You probably knocked down the lower mids and upper bass with 8R but the peak at 50Hz would be relatively more obvious. The driver impedance is shown as about 180R at resonance so even 8R in series would hardly attenuate at that frequency.
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#2330 Re: Something stirs in the Undergrowth

Post by IslandPink »

I've just found some resistors and clip leads and got something going here. I've now got 2.7R into the Supravox and an 18R in parallel with the driver, that will be about 3.5dB attenuation. It already sounds a fair bit better - more balanced and bass lines more defined as you'd expect without the big swings of impedance and amplitude ( hence phase errors ) . I got James on the email today and he said he'll run some sims on it at the weekend, to be able to comment some more.

Later... rhythmically tighter on Sweetnighter. Excellent.
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#2331 Re: Something stirs in the Undergrowth

Post by IslandPink »

Damping factor -
I'll leave out the wires and choke for now ( both will reduce these values slightly )
Amp assumed 2R out.
James's original case ( more of less direct drive to Supravox ) : DF = 4:1
My case with simple 3.3R resistor in series : DF = 1.5 :1
Current mod with 2.5R into Speaker+18R parallel : DF = 2.23 :1
This is assuming the parallel resistor is part of the damping, alongside the feed from the amplifier.
So, not ideal, but it seems there is definite improvement.
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#2332 Re: Something stirs in the Undergrowth

Post by IslandPink »

Well the wooden cockroach has come to the end of its time here in my front room. I've learned all I can from it.
My friend Mark from work came round just now and we loaded it into his car.
Mark tells me it's going off to a nice farm, where it will be looked after.
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#2333 Re: Something stirs in the Undergrowth

Post by IslandPink »

Steve ( then ) Bench does make a rather good case for PP in the bass here :
http://diyaudioprojects.com/mirror/memb ... teres.html
I hadn't thought about the varying anode resistance.
It does also indicate why big SE valves have better bass than small SE valves.
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#2334 Re: Something stirs in the Undergrowth

Post by chris661 »

... Or solid state? :D
Simon C visited yesterday, and I think he's convinced me to attend Owston. I'll be bringing along an amplifier that's renowned for it's capabilities in the bass department - exceptional control of the cones. Will be interested to see what you think.

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#2335 Re: Something stirs in the Undergrowth

Post by IslandPink »

Well that's good to hear... but doesn't necessarily address my problem of an amp that can run the bass and mids up to approx. 5kHz with this series crossover.
I suspect Nick's latest amps would do a very good job though.

Meanwhile, more detail work. Second boxed-up crossover finished (phew, glad that's over), and the first 'cowl' for the tweeter fitted on the RHS baffle.
I have been playing in stereo. Not bad, but need to get zobels fitted to both pairs of FX120's as previously trialed.
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#2336 Re: Something stirs in the Undergrowth

Post by IslandPink »

Just been trying to estimate the Le for the Fostex FX120's a little better.
I'm not convinced the estimate of 0.05mH is correct. Some of the Fostex drivers like the 126e are quoted with stupidly low Le vales despite the rising impedance on the graphs.
The impedance vs. frequency graph I found yesterday here, with a proper scale ( unlike the Fostex sheet ) is useful :
https://hificompass.com/en/speakers/mea ... stex-fx120
It's bang-on 20R impedance at 10kHz.
If I assume Z(f) = Re + 2Pif(Le) , then I get 0.2mH.
The only fly in the ointment is that the plot from HiFi Compass there, has an Le calculated at the side of the graph, and it shows 0.67mH. That seems to make no sense.
I'll re-calc the zobel values for the 2x FX120 based on 0.2mH, but if someone like Scott or Chris can shed any further light on the issue, I'd appreciate it !

Edit : OK, that gives somewhat different values - 1.2uF and 18R . Hmm...
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#2337 Re: Something stirs in the Undergrowth

Post by chris661 »

Best bet IMO would be to measure the impedance of the actual drivers. You could then also ensure that your Zobel filter is working correctly to give a flat impedance.

Easy enough to do - a cheap USB soundcard, a 100ohm resistor and a few cables will be all that's needed. You'll be connecting directly to the drivers, so no worries about interfacing with the rest of the system.

IIRC, calculating just on the inductance of the voicecoil doesn't always work for a Zobel. While the coil itself is an inductor, the mass of the cone adds additional impedance at high frequencies - sometimes you get inductance figures at 1kHz and 10kHz, and they can be very different numbers.

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#2338 Re: Something stirs in the Undergrowth

Post by IslandPink »

Well, OK. The HiFiCompass curve impedance is very helpful. I don't think I'd be able to improve on that, to be honest.
But it's a good point about checking the impedance curve afterwards - really I can just use a signal generator and the Avo Meter, as i once did with the horns - put 100R in series and measure the voltage across that. Same principle as what you're suggesting, without buying anything.
I can start with the new values I just calculated, and see how that looks.

Edit : Actually that might fail as the Avo probably won't read correctly on AC voltage over a range from 1kHz to 10kHz. Maybe I could use what you are suggesting. Do you have a link ?
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#2339 Re: Something stirs in the Undergrowth

Post by IslandPink »

D'Oh ! Just realised I have a thing upstairs called an 'oscilloscope' that will do the job.
Just set up so the AC voltage across the resistor at eg. 500Hz is an exact number of divisions, that ramp up the freq to 10 or 12kHz.
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Nick
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#2340 Re: Something stirs in the Undergrowth

Post by Nick »

And while you are at it, measure the voltage on the avo at the same time so you can see how accurate it is at higher frequencies.
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