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Port tuning and velocity


funkydoug
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Thanks to all who have given their advice on this. I added to 100mm port today. Also lined it and changed the speaker for a very good 16ohm SICA that I picked up on ebay for £40. It is ideal. 

The idea is to run this as a 16ohm small combo that I can hook up to an 8ohm 210 with two of the same drivers and with the same tuning.  Very happy with the result. Next job is to upgrade the tweeter. It is certainly a fun  project :)

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Quite a bit more bottom end in a pleasing way. Happy with the result.

WinISD suggests that is tuned to about 57 hz with the current port length but is very close to Agedhorse and Stevie's suggested dimensions. If they'd be happy then I'm happy:)

Probably  going to swap the cheap tweeter for an APT80 or a BGH25-8 and will no doubt spend the next month agonising over that 🤦‍♂️

 

 

Edited by funkydoug
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Yep I will do. It currently has a simple 1st order filter on the tweeter, with nothing on the woofer side. Will start to look at filters and crossovers now,  starting by revisiting the BC cab threads. 

Suggestions from you learned lot will be much appreciated. 

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I specify the BGH25-8 in my Simplexx bass cabs. In part that's because it doesn't extend higher in response than what electric bass needs, reducing hiss, in part because with a 2nd order low pass/4th order high pass crossover it can run to 2kHz. That eliminates the typical upper mids hole in cabs that cross over an octave or more higher, and provides dispersion above 2kHz that no woofer can match. 

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  • 3 weeks later...

Another update on this project, and another question.

I added an APT80 to it which was a drop-in replacement for the cheap piezo that Ashdown had used. I also used a custom 2nd order crossover (thanks to Blue Aran's great email support) which crosses over at 3.5khz but was built for my 16ohm woofer and the APT80 at 8ohms. 

Controlling the level on the tweeter was important as it's both very sensitive and is an 8ohm unit. My aim was to be able to run the tweeter 'loud' in the event that this combo sits on top of a tweeterless cab but dial it down in normal use. I used an eminence LPad to do it. Seems to work very well.

I had to remove the wadding I'd added to do all that, but will re-do it once I'm sure Im happy.

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The question is about using distortion with a tweeter. It all sounds good at any realistic setting but if I fully max what is a very mild dirt dial on the ABM, max the bass and max the preamp gain even at very low volume (like bedroom volume) I get a horrible, unmusical distortion sound from the tweeter that is in the 'malfunction' ballpark - not musical in anyway. It did this with the piezo and with the new tweeter.  Is it just that running that much bass overpowers the HF protection offered by the crossover?? Or maybe it is the sound of the input stage or valve being overpowered?  Anything to worry about?? I wouldn't run it like that in any real situation - only did it as part of testing everything - but I am slightly concerned it is a warning sign that something isn't right.

Edited by funkydoug
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Thanks Bill. In that case I'm especially glad I added the Lpad, and that in all real world situations  my dirt pedals all have some kind of LPF / cab sim.

I have to say I'm surprised at how bad it the dirt sounds up in the tweeter territory.  I'd read alot about distortion sounding nasty with a tweeter without LPF but this was far worse than I expected. 

Im happy to report that in all but those extreme tests the combo is now doing great. 

 

Edited by funkydoug
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One of the issues with most tweetered cabs is that they don't take account of the difference in output between the woofer and the tweeter. It ia not just about putting an L-Pad attenuator in circuit. you also need a properly designed crossover to make sure there are no peaks or troughs in the response.

While bass guitar and upright bass do not need to go flat to 20Khz, any shortcut in the design will show up in the sound and most quality HF transducers will go up towrads 20KHz. A good trick is to listen so some audio from CD. Choose a track you know well but one with say female vocals, Sade's first album is a good one to start with. @stevie also uses Dire Straights, don't use a track with distortion initially as it can mask what is going on.

Edited by Chienmortbb
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While it's true that clipping your power amp will dump lots of high frequencies into your tweeter, this doesn't seem to be what's happening here, as the problem exists at low volumes.

There are too many variables to troubleshoot reliably, but the cause might be:

1. The system is reproducing what it's being fed and the Ashdown distortion isn't very nice or has been voiced for a non-tweetered cab (no HF rolloff)

2. A 2nd order filter at 3500Hz isn't steep enough for the APT 80 and you're hearing distortion from the tweeter below the crossover frequency. 

As @Chienmortbb rightly says, you'll probably find it easier to troubleshoot using a good recording of voice.

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Thanks Stevie, those 2 possibilities  are what I also arrived at.

I've got this right, feeding it a clean but hot and very bass heavy sound will test whether the problem is too much bass for the crossover (in which case the problem will reappear) or an unfiltered distortion (in which case it will not reappear).

I'd hope that it is scenario 1 (of stevies 2) because 3.5khz 2nd order crossover is what eminence  recommend for the APT80. 

In the event that I need more HF protection it isn't game over, I'll just need to go back to the drawing board for HF protection. 

 

Edited by funkydoug
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It's not a matter of HF protection. When you clip the amp or use distortion the high frequency content can be 10dB or more higher than normal compared to the low frequencies. It might be improved somewhat with a higher order filter (I haven't used a second order high pass for 50 years) but even 4th order filtering won't stop the highs that are so irritating. The only full on cure is to somehow attenuate the high frequencies when using distortion. Or do what I do, which is no distortion. I leave the guitar toys to guitar players.  😊

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Well it has passed the female vocal test with flying colours.

Played Sade, Stevie Nicks, London Grammar and Bjork. All sounds brilliant and far better than my Sony hifi :)

 

Also pinned the issue down to preamp distortion. Turning the Lpad right down is one way to dirt-proof the box, another is to use the onboard preamp as it was intended ('grind', not wild distortion) and leave any real dirt to pedals with tone dials / cab sims which seems to be a pretty standard approach.

I am amazed this doesn't happen more often though, especially on cabs with tweeters crossed over at 3.5khz and lower ... or do all dirt pedals really roll off the highs sufficiently to prevent this?  Curious because as we know, Barefaced cabs crossover low, as do Bill's, and both are well regarded and road tested by countless players.

Anyway, this is now just a matter of curiosity. The combo works very well for my intended use. It has lovely low end, crisp top end and as a small amp it is very satisfying. The modelling suggests that there are no problems in terms of driver excursion or port chuffing up to 100w which is as much power as it will see with the 16ohm driver and play testing supports that, at least to my ears. It will marry up with an 8ohm cab (2x10) without asking too much of the onboard driver and port, and the tweeter can be turned up in that situation to balance the 3x10s rather than 1x10. And it looks smart! Very happy.

Thanks for all your input guys.

 

 

Edited by funkydoug
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  • 2 months later...

Bit of an update on this little combo…

I’m happy to report that it still sounds very good to my ears. I’m now thinking about the extension cab and have realised that finding matching drivers for the 210 to live underneath it is near impossible, they’re discontinued.

 

This leads to 2 questions:

1) what are the advantages / risks of having the exact same driver vs a matching pair of different but good 10s? My decision is, do I buy the best 210 I can find for the job or do I also need to replace my existing 10 to make a set of 3 matching drivers. 
 

2) does a no-tweeter extension need to be low passed to make way for the tweeter in the combo? Concern here is whether the highs coming from the bottom woofers would interfere with the tweeter. 

Lastly, remembering that this is a learning project more than anything else…  if you had a driver with lots of highs like a whizzer type design, would interference with the tweeter be an issue??


 

Thanks again, learned scholars of BC.

 

Edited by funkydoug
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You don't need to use identical drivers, but they should be spec matched as close as possible, as should the enclosure. There's no need for a low pass filter in the extension, the woofers aren't doing much above 3.5kHz anyway. That's one reason why a 2kHz capable tweeter gives a better result.

If you're going to run into problems it would be using an 8 ohm 210 along with the 8 ohm 110. That puts half the power into the 110, half into the 210. Doing that you might as well have the extension be a 110 as well.

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Brilliant, thanks for your answer Bill.

Feeling pleased with myself here… I chose a 16ohm driver for the combo for the reason you point out. Also because the port in such a small box couldn’t handle more than about 100watts. My 210 will be another two 16ohm 10s, so all should be good on that front.

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