ATC Mid question

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TwangBar

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Joined 2003
#1
I am converting my passive 3-way (ATC woofer, ATC mid and Seas Excel tweeter) to an active system (at 400hz) between my woofer and mid/tweeter (keeping the passive xover between the mid and tweeter).

My question is, if I do this, should I protect my ATC mid from amp turn-on thump using a cap? And if so, what frequency should it be protected from?

Thanks for any help you can provide.
 
#2
Yeh, an interesting one. You've got several options. (A) get an amp' that doesn't thump. This is the v.expensive, "smart alec" solution and probably the last thing you wanted to hear. (B) it is possible to get an add on 'anti-thump' circuit. Down our way (Australia) there was a magazine DIY kit that did two things. It had a relay connection between the amp' and speaker and there was a timing circuit that delayed connection of the speaker until the amp' stabilised (say 3-5 seconds). This solved the thump issue. It also had a DC detector that disengaged the speaker in the event of a "major incident" in the power-amp output stage. This is a anxiety preventing option that will appeal to anyone who owns an expensive midrange but on the other hand it does have a relay in the signal path and many people won't want that. (C) Go with your cap' option and use it creatively. I've been thinking about this issue for a while and one possibilty is this. MAKE THE CAP PART OF THE CROSSOVER. For example. If you were thinking of a 18db/octave active design then change that to a 12 db active circuit and make the high level cap the appropriate value to give you the 3rd leg of you slope. That way you don't actualy increase the number of reactive components in the signal path and you have peace of mind by protecting the speaker and the slope you want. To get a Butterworth shape you can have a 2nd order active arrangement with a bit of a peak at the c-over frequency etc. all covered with normal filter theory. In answer to your question; in this instance ther value would be around the size needed for a 6db slope in a high level passive cap. which is something around the 30uF (check tables and real world speaker impedence) size for a 8ohm speaker and while good polyprops are expensive you can use a motor start cap (almost all are polyprop' and they're not to expensive) But I guess if you've got ATC's then you've probably set you mind on the path of poverty anyway!!!!! (ATC's are desgned by an Australian living in the UK I believe)
 
#4
If your amp has turn on thump, then it's rubbish or it needs fixing. What is it?

If you can afford ATC drivers, then you can afford a decent amp to drive them. If buying new is too expensive, then look at the LM4780 gainclone in the chip amp forum, pretty cheap to build, and should match up well with the ATCs. You might even be able to build it in your existing amp's chassis.
 
#6
Thanks for all your replies.

The amp I am using is a self-built Aleph 2. Maybe thump is not the best term - I just notice when I put it on the bass driver really moves. I do not actually hear the thump. Should I not be concerned with this?

As for rumours of me being wealthy... the truth of the matter is that I got mids by bying a pair of 1990s Proac Studio towers. ATC mid, 10" ATC woofer (LC version), scanspeak tweeter... all for $800US!!

The design of the Studo Tower was that of an EBS system - and I do not like how the woofer sounds - flat and cold. I want to use my Marchand XM9 24dB xover to release my mid and woofer from the shackles... and boost the bass by about 3dB.....

~Scott
 
#7
Hi Scott, (1) Yes you should be worried by that movement. It won't hurt the woofer but the mid will not be amused atall. It may well experience death with a single blow. And while you may have got the originals as a bargin a replacement mid will be unlikely to jump out at you at the same bargin price. (2) As to the thump problem you might run the issue past the Pass Lab guys on their own part of the Forum. They will have thought about it/ cured it etc. Now I'm not an authority on Pass stuff but if the DC thump is the output cap charging up on 'switch on' because it is a single rail design then the obvious thing to do is replace it with a smaller one. Again I'm not looking at a diagram but most output caps are usually over 2,000uF so as to achieve good bass extension (some are much bigger, Sudgeon can be 10,000uF). But if you're only wanting the amp as a mid-upper amp you don't need it to go down to 20c/s. Stick with your original plan and put a smaller cap in there instead of the existing output cap rather than an additional one. If its around 40-60uF then it won't have the charging issues of a big cap. It can be a plastic flim type and therefore better quality than the exisiting electrolytic. You are then in a win/win situation which doesn't appen often in this game....enjoy it while it lasts.....(3) I think you may have misunderstood us. We don't think you're wealthy atall. We think you're on the path to poverty!!!!! Cheers and good luck.
 
#10
Sorry for delay. Re themistor. Not sure where it is in the design. Some of the early JLH Class A amps had themistors on the mains side of the transformer to reduce current during switch on. Other designs use RC components in the small signal parts of the circuits to slow the rise of the DC voltage. From what you've said I'd still look at your original proposal. A cap in the mid/high speaker lead has a lot going for it. It is a 100% reliable protection device for DC, won't effect the sound much and will reduce any hum left on the output, save the speaker from too much low frequency rubbish i.e. dropped tone arms etc. From what I've read you'll get such a noticable improvement from the bi-ampping that you won't worry about anything else.
 
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