GreeneKing Posted February 20, 2009 Share Posted February 20, 2009 I've been meaning to get around to producing a short post that illustrates how a filter based pre-amplifier works. It really isn't rocket science, it's just a different approach to the usual 2/3 EQ and you can't really relate to one via the other imo. In addition to a volume/blend knob basically you have: 1. A Low Pass Filter The ACG01 fitted to Alan's basses has one per pickup. The ACG02 retrofitted to a (cough) few of my basses has just the one. Imagine it's a bath tap. When the tap is shut it's leaking just low frequency sound, about 20Hz if I remember correctly. As you open the tap it allows progressively higher frequencies to be added to the flow. When the tap is wide open it's allowing everything through from the 20Hz right up to 3kHz. There's another 'little tap' on top that let's through a little extra just at the cut off frequency i.e. when the big one is shut that'd be 20Hz and when it's wide open 3kHz and wherever it's set in between. This allows you alter the shape of the frequency spectrum. This knob I will call the resonant peak boost, cause that's what it does. So here's some sound samples, they're just chromatic scales to illustrate the operation of the filters. First up just the neck pickup, with no boost, the filter is being incrementally opened from 20Hz to 3kHz. [url="http://www.zshare.net/audio/559253145e7fc64e/"]Neck pup no resonant peak boost[/url] Now with half boost [url="http://www.zshare.net/audio/55925786add6ed8f/"]Neck pup half resonant peak boost[/url] Finally with max boost [url="http://www.zshare.net/audio/5592607118b32063/"]Neck pup max boost[/url] Next is just the one sound sample going through the whole above process with the bridge pickup low pass filter: [url="http://www.zshare.net/audio/5592702673b51f1f/"]Bridge pup opened with a range of boost positions[/url] The low pass filters are always letting sound through. This tone is what I'd call the 'meat'. It's always there. The High Pass Filter can be shut right down like a 'non leaky' tap. I'd describe this as the 'gravy' in that you can add some if you need to but it's not essential. If the tone through the low pass filters up to 3kHz is good for you you can ignore the high pass filter. I'll continue the thread in a min looking at the High Pass Filter. Quote Link to comment Share on other sites More sharing options...
steve Posted February 20, 2009 Share Posted February 20, 2009 Nice thread Peter, I'm sure I'll be referring to it in a several months time Quote Link to comment Share on other sites More sharing options...
jmanfunk Posted February 20, 2009 Share Posted February 20, 2009 Great stuff, just what I was looking for! Thanks for the clips. I've been playing around with my Schack pre, setting different frequency points, using the dip switches and loving exploring the different timbres I can get. Downside is the awkwardness of adjusting it. This pre seems like the next logical progression. Shame the ACG01 is only available as a proprietary design but I suppose that's the point. I'll add a Skelf to the GAS list Quote Link to comment Share on other sites More sharing options...
GreeneKing Posted February 20, 2009 Author Share Posted February 20, 2009 (edited) Thanks Steve Now the: 2. High Pass Filter It's not so easy to demonstrate what it does because it's not possible to listen to it's output in isolation like it is with the low pass 'jobbies'. It works in the same way but the other way around. When this (hot) bath tap is shut it just leaks the very highest frequencies (sizzling) and as it's opened progressively lower frequencies are added in right down to about 1kHz. So you can see that it overlaps the low pass filter when wide open with it's lowest mids lower that the low pass filters highest mids. The top knob on this filter isn't a resonant peak boost (this is important) but a straight boost with minimum boost being zero. So it can effectively be turned off. In order to illustrate it's operation I've started with the High Pass filter wide open (all frequencies from sizzle to mids) and it's boost set to max. I've then incrementally shut it off. The bridge and neck pups are balanced and set to a fairly 'dubby' tone that's overlaid at all times by the 'gravy' of the high pass filter. [url="http://www.zshare.net/audio/55928716d3690819/"]High pass filter operation[/url] Finally a couple of points. How do I set my bass up and why bother with all this filter malarkey? Here's how I go about creating my available range of tones: 1. Isolate high pass filter by shutting down it's boost and removing it from the tonal equation. 2. Blend to the neck pup and using low pass filter get the tone I want from it (I think more 'P' tone here). 3. Use the resonant peak boost to shape the tone further. 4. Blend to the bridge pup and repeat 2. and 3. above to get tone required (I like some jazzy honk). 5. With the 'meat' sorted middle the blend and introduce higher frequencies (gravy) where you want them and at the level (amount of boost) you want them. 6. The above is all done as preparation. For tone when playing just use your blend to go from neck to bridge pups and access a whole seamless range of tone. So why bother? A normal 2 or 3 EQ pre allows you to tweak the colour that that individual pre-amp creates. The ACG filter based pre-amp allows you to create the colour itself. That's how I see it. Just to add, setting up the ACG02 with just the 1 low pass filter I do this way: 1. Zero the boost on the high pass filter to remove it from the tonal equation. 2. Middle the pup blend, if you have one, and zero the resonant peak boost on the low pass filter. 3. Adjust the 'meat', the bottom ring of the low pass filter to get the broad tone you want, shape it with the resonant peak boost. 4. Add the 'gravy' where and how much you want it with the high pass filter and boost. 5. Use the blend to achieve subtle tonal change and the 'meat' to get a more radical shift in tone. 6. Tweak any knob a little as you go. Peter Edited February 22, 2009 by GreeneKing Quote Link to comment Share on other sites More sharing options...
skelf Posted February 20, 2009 Share Posted February 20, 2009 Many thanks for taking the time to do that Peter. From the very start it has been difficult to explain how the pre-amp works in writing it makes it seem a lot more complicated than it actually is. Using it is easier than writing/talking about it. Just to add a few bits myself. The low pass filter now goes up to 6.3 Khz and the high pass filter now goes down lower into the frequency range of the lowpass filter so there is now a larger overlap between the two controls. Adjusting the area in the overlap with either control gives a different sound to the bass. Alan Quote Link to comment Share on other sites More sharing options...
niceguyhomer Posted February 20, 2009 Share Posted February 20, 2009 I'm getting well used to mine now - it makes sense to me and I'm thick. Quote Link to comment Share on other sites More sharing options...
Owen Posted February 21, 2009 Share Posted February 21, 2009 I am VERY interested in this, but the links do not actually load to sound. Is it me? Is it something I said? Quote Link to comment Share on other sites More sharing options...
GreeneKing Posted February 21, 2009 Author Share Posted February 21, 2009 (edited) They are working okay, just checked. They may take a while. Has anyone else had problems. Any recommendations for a better sound file host? Peter Edited February 21, 2009 by GreeneKing Quote Link to comment Share on other sites More sharing options...
Owen Posted February 21, 2009 Share Posted February 21, 2009 [quote name='GreeneKing' post='416178' date='Feb 21 2009, 03:04 PM']They are working okay, just checked. They may take a while. Has anyone else had problems. Any recommendations for a better sound file host? Peter[/quote] It's been connecting for 30 mins for me now It is probabaly just me. Ho hum. Quote Link to comment Share on other sites More sharing options...
GreeneKing Posted February 21, 2009 Author Share Posted February 21, 2009 I'm going to try with a different host - watch this space Peter Quote Link to comment Share on other sites More sharing options...
7string Posted February 22, 2009 Share Posted February 22, 2009 That explanation is really helpful. Makes this less hit-and-miss. I've been enjoying playing my ACG so much that I haven't really played with the pre too much. I've just found a good sound and stuck with it. On my ACG, I also have the series/parallel switches (which I think GreeneKing has as well) and the piezo bridge. Changing these controls gives even more tonal variation. Good fun indeed Quote Link to comment Share on other sites More sharing options...
GreeneKing Posted February 22, 2009 Author Share Posted February 22, 2009 (edited) I've hosted the samples on a faster site and split the bridge pickup sample into 3. It was painfully slow downloading and the samples were the lowest quality I can record The low pass samples are from the neck and bridge pickups with the filter being incrementally opened with 3 settings on the resonant peak boost, min, mid and max. [url="http://www.plunder.com/-download-7e45661f.htm"]Neck low pass min[/url] [url="http://www.plunder.com/-download-6bd5ec60.htm"]Neck low pass mid[/url] [url="http://www.plunder.com/-download-1a6ffbb2.htm"]Neck low pass max[/url] [url="http://www.plunder.com/-download-ca166d0c.htm"]Bridge low pass min[/url] [url="http://www.plunder.com/-download-4db791c8.htm"]Bridge low pass mid[/url] [url="http://www.plunder.com/-download-9eac5039.htm"]Bridge low pass max[/url] and finally the high pass with boost set to maximum, both pickups set very 'dubby' incrementally wound from high plus all mids to just high. [url="http://www.plunder.com/-download-16931b9d.htm"]High pass demo[/url] That's it Peter Edited February 22, 2009 by GreeneKing Quote Link to comment Share on other sites More sharing options...
chris_b Posted February 22, 2009 Share Posted February 22, 2009 [quote name='GreeneKing' post='416495' date='Feb 22 2009, 09:38 AM'][url="http://www.plunder.com/-download-6bd5ec60.htm"]Neck pickup low pass filter boost mid pos'n[/url] If anyone out there can have a go at downloading this and tell me if it works okay? Peter[/quote] Hi Pete, works much better. Quote Link to comment Share on other sites More sharing options...
GreeneKing Posted February 22, 2009 Author Share Posted February 22, 2009 Thanks Chris, I've rehosted the lot in another post. Quote Link to comment Share on other sites More sharing options...
Owen Posted February 22, 2009 Share Posted February 22, 2009 Yep that works, thanks for your work. Unfortunately it has confirmed that I need one as well. Quote Link to comment Share on other sites More sharing options...
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