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27th April 2004, 05:58 PM  #1 
diyAudio Member

Sub plate amp design
What do you all think of this? I’ve had an impossible time trying to find an easy adjustable crossover circuit for my sub amp project.
Pot1a/b is a 50K stereo pot. The model shows this pot in steps of 10K, The aprox 120hz crossover area (green line) is the pot at zero resistance. The 50K position is a crossover region around 9hz (gray line). I’m using a 50K pot because it is what I have on hand, I may adjust this to a 5 or 10K pot to yield a low crossover point of 50hz or 35hz respectively. The shown circuit will be followed by an inverted TL072 with no gain and a switch to bypass that (phase invert switch) and a pot on the output of that for general volume control. All that followed by 2 paralleled lm3875 or 3886 at 31V DC into a parallel coil 3.5 ohm Shiva (it’s a prototype shorted voice coil Shiva, so it may have a bit lower resistance then the typical) 
27th April 2004, 06:00 PM  #2 
diyAudio Member

This is the output of the circuit.

28th April 2004, 05:52 PM  #3 
diyAudio Member

Updated calculations
No one has offered any help or suggestions yet, and I’m not surprised. This is either a subject no one really cares about, or something that most have decided isn’t worth the trouble.
As a service to others who search for this in the future I will try and document my progress here. So anyone who can offer any help, please do, and with that I’ll get started. I found that my general guessing using the calculator found in WinISD and then modeling things in LTspice (free download) left quite a bit lacking, namely a proper 2nd order curve, and my –3db (corner frequency)calculations. After finding that a SallenKey type filter was the easiest to implement both in number of parts and simplicity. And after doing some searching to find the equasions for calculating for a SallenKey crossover, I found This page at tlinespeakers.org. It has the calculations for picking a capacitor and resistor values based on wanted crossover frequencies. For this project I am only concerned with a unity gain adjustable low pass crossover. For this type of circuit you need to calculate for a second smaller value capacitor, and then for a series resistor to be used in conjunction with a potentiometer of a certain calculated value. The calculations that can more or less be pasted into a spreadsheet program are. Calculation for second cap ="wanted value of first cap" /(4*(SQRT(2)/2)^2) Calculation for resistance at LOW freq =1/(2*PI()*"required low freq"*(SQRT(("value of first cap in uf"*0.000001)*("value of second cap in uF"*0.000001)))) Calculation for resistance at HIGH freq =1/(2*PI()*"required high freq"*(SQRT(("value of first cap in uf"*0.000001)*("value of second cap in uF"*0.000001)))) You use the value of the lower resistance resistor (the resistor for the higher freq) in series with the pot, and the difference between the resistor values will be the value of the pot. An example of how mine worked out, I used an initial value for a capacitor C1 of 0.05uF, and this gave a value of 0.025uF (25nf, or 25000pF) for C2. My low frequency is 50hz, and this gives me a required resistance of 90K, and my high frequency is 120hz, this gives me a resistance of 37.5K. So R1 and R2 will be 37.5K and 90K37.5K=52.5K or about a 50K pot. (The use of a 50K pot will change the low cutoff frequency from 50hz, to 51.4hz, but that’s close enough for the women I go with). Also note that I could not find any 0.025uf capacitors, you could use 2 0.025uf in series, or a 0.022uf or 0.027uf should be close enough too. What your left with should look something much like what I’ve posted below (if you use all my same numbers). Values can be adjusted for capacitors or pots on hand. Generally speaking a larger cap will require a smaller value pot. Next up is summing a left and right channel, as well as accepting speaker level inputs and adding some gain so we have something to attenuate later. Then a phase switch, since this is a second order crossover we’ll defiantly need one of those. 
28th April 2004, 05:53 PM  #4 
diyAudio Member

Notice the much nicer and tighter frequency curves here.

29th April 2004, 08:08 PM  #5 
diyAudio Member

OK, so today we’ll go over summing a left and right channel for those that don’t have a sub out on whatever they are hooking this up to. But first there are a few small problems and oddities with yesterdays design.
1) The capacitor values of C2 are hard to find, and they can be adjusted a bit in either direction, although this does change the crossover slope it is very minor. I’ll just be using two 0.05uf capacitor’s in a row to get my 0.025uf. 2) The resistors R1 and R2 are also of a not very common value, since this really will only change the Freq High crossover point, it is also a minor issue. R1&2 values between 37K and 40K should be attainable and will leave the Freq High point somewhere between 122hz, and 112hz (respectively). I’ll be using 39.2K from here out, giving me a high freq point of 114115hz. 3) As for connection of the pot, only use the center leg and one outer leg to adjust resistance do not connect the leftover leg to ground or do any other stuff like that. Now on to new things! (well new to me anyways) 
29th April 2004, 09:02 PM  #6 
diyAudio Member

ver 0.3 Left&Right sum, and Invert
R4 and R5 are the resistors used to set input impedance and to isolate the channels from each other. The value 39.2K was used as it’s the same as we are using elsewhere, and the less different parts we have floating around that happier I am. R3 sets the gain of this op amp at 6db (2x). Larger R3 values could be used to get more gain (if R3 was 392K you would have a gain of 10 (26db)). I don’t know if we need it or not. And I probably won’t know unless someone chips in here. Chances are I will leave this section of my circuit with some space, so that I can serial a few more of these 39.2K resistors to adjust the gain).
This is also an inverted Opamp and it will give us a nice inverted phase at the output as you can see in the lower output graph. 
29th April 2004, 09:05 PM  #7 
diyAudio Member

Most everything is the same here, Level is shifted 6db up, and phase is reversed.

29th April 2004, 09:54 PM  #8 
diyAudio Member

Ver 0.4 phase switch
Next step is easy as pie too, add an inverting opamp, with no gain and have a switch Leading to the output. After the switch, you can add a pot for overall gain control I’d say you would be ok with pretty much any value from 5K50K and then from that right into the power Amp section of the plate amp.
I’ve set R6 and R7 at 39.2K just because it seemed easy, and since if your using metal film resistors from digikey you will have to get 10 of em anyway. R8 and R9 are 50omh resistors and they may not be needed. It just seemed like a good idea to include them when I was putting this together and I’ve just left em. When I build I think I’ll try leaving them out at first. 
29th April 2004, 09:55 PM  #9 
diyAudio Member

Here are the 2 different phases.

30th April 2004, 04:27 PM  #10 
diyAudio Member
Join Date: Dec 2003
Location: Orange County, CA

No replys
I think you have no replys because this is all circuitry that was worked out by 1975 and holds no mysteries to anyone with over 5 years experience in circuit design. You have here, basically, the crossover circuitry for almost every computer speaker system that has a sub woofer.
Yes, you have to use capacitors in a 2:1 ratio if you want a decently sharp curve. Do build it. It will work fine.
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Dan Fraser 
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