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Old 12th December 2010, 08:48 PM   #1
andb is offline andb  Czech Republic
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Default Trying to build a PLLXO

I'm trying to build a PLLXO and could use some double checking. I've been using the reference at
Passive Line-Level Crossover
to try to calulate it and checking with the xls from this diyaudio post:
PLLXO (v AXO v PXO) benefits/ Model?

Ill be using a triode st-70 clone for the highpass amp, which pages found on google says has an input impedance of 470K.
I'm trying to make a second level lowpass at 110hz and a second level highpass at 400hz.

Low pass seems straighforward, my results coincide with the xls.
R1 5.0 k
C1 289.37 nf
C2 28.94 nf
R2 50.0 k

The problem is in the second level high pass.

R1 and C1 are based on the values from the lowpass filter, as I've understood.

Calculation of c2 isn't clear. The issue is the text of the last paragraph:
"If the input impedance gets lower, this has two consequences: the insertion loss gets higher and the xo frequency is shifted upwards. To compensate, calculate C2 in the lowpass as follows:"

I believe this should read ..."calculate C2 in the HIGHpass as follows:"

However, I am not sure if C2 is right in the sheet, it reports .01pF.

The R2 I have calculated is different than the sheet. In fact in the xls, changing amp impedence doesnt affect the R2 value at all, but based on the formula it seems it should. Im getting 56k in my calculation, the sheet gives 50k.

Could I ask someone to help me determine the C2 and R2 component values in a second level highpass PLLXO at 400hz, with an amp impedence of 470k (I hope this figure is right)? With the final results, I can check if there is an error in the xls or if its all correct.

Later, I want to also make a highpass for an audiosector LM3875, but so far haven't found its input impedence documented. Is this documented anywhere or will I need to calculate it?

Thanks!
Andrew
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Old 21st December 2010, 03:38 AM   #2
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Yes, I'm not sure that spread sheet works correctly. I can't decipher the original site, either, so I'm not sure. Both of these "tools" need to be upgraded.

Peace,
Tom E
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Old 21st December 2010, 04:24 AM   #3
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Quote:
Originally Posted by andb View Post
I believe this should read ..."calculate C2 in the HIGHpass as follows:"
I've fixed that.

dave
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Old 21st December 2010, 04:41 AM   #4
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Easy solution: download LTSpice and model the response. Start with the basic equation given to determine values. Use a resistor from output to ground equal to your amp's stated input impedance as a load. You can adjust it to see what effect it has on the response of the filter.

Considerations - if impedances of the PLLXO parts are significantly less than the amp input impedance the latter has little effect. However, if the filter presents too low an impedance to the driving stage you could overload the stage and induce distortion. A n active buffer with a high input impedance lets you choose reasonable values for your filter components, which is why I prefer active. Use a jfet buffer if you son't like op amps.
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Old 21st December 2010, 06:13 AM   #5
Francec is offline Francec  Australia
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Why not try a Rod Elliott P125 - 4 way or P09 - 3 way LR 24 dB active crossover?
Easy to configure and use, and I don't see any value in a PLLXO with its problems.

Frank
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Old 21st December 2010, 07:13 AM   #6
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...I don't see any value in a PLLXO with its problems.
A PLLXO is simplier, much less costly, and avoids all those opamps.

If your system fits the limitions, they are a whole lot better than active anything.

dave
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Old 21st December 2010, 07:36 AM   #7
Francec is offline Francec  Australia
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Quote:
A PLLXO is simplier, much less costly, and avoids all those opamps.
If your system fits the limitions, they are a whole lot better than active anything.
Simplier (sic) does not make better, probably cheaper and avoiding opamps is just a prejudice, without evidence, of dave's. As is the conjecture about being "a whole lot better than active anything".

Frank
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Old 21st December 2010, 07:55 PM   #8
andb is offline andb  Czech Republic
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Default In my case...

In my case, simple and cheap with decent sound for this specific build is the goal.

I just happened to start playing with the MA chr-70's while preparing to build some Austin FR horns. With weak bass in a sealed enclosure I was thinking if I should build better enclosure for them or try a sub. Since the MJK H frame with eminence 15a's was cheap and easy, I thought that could be more fun and also useful in future builds. (I'll probably pull the chr's out of the sealed enclosure and put them on an OB soon)

So with the H frame boxes nearly done and the st-70 kit just about ready to fire up (bad choice of verbs?), I'm looking for something to make just these drivers play music together - this isn't the final step but the first. I've even considered a behringer cx2310 but would prefer to make something that will have nice sound.

Actually, I would have built an active crossover / DAC with the miniDSPs if I wasn't really satisfied with my WM8741 based DAC. This is why Im looking for a really light way to cross over in analog.

Andrew
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Old 22nd December 2010, 07:01 AM   #9
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Quote:
Originally Posted by andb View Post
I'm trying to build a PLLXO and could use some double checking. I've been using the reference at
Passive Line-Level Crossover
to try to calulate it and checking with the xls from this diyaudio post:
PLLXO (v AXO v PXO) benefits/ Model?
I have reworked that calculator (and linked it to the PLLXO page)

http://p10hifi.net/planet10/TLS/down...ulator.xls.zip

dave
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Old 29th December 2010, 10:14 PM   #10
andb is offline andb  Czech Republic
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Default failed attempt at a prototype

I tried wiring up the low pass side of the pllxo as per the worksheet. As a first test, I want to make just the 1st order low level crossover. The schematic is as easy as can be, but somehow I've done something wrong. I'm sure its a really basic mistake, I'm very new to electronics and just barely starting to get oriented. If you see what I'm doing wrong, please let me know.

The worksheet values I wanted to realize are:

Lowpass
70 hz
N.A. amp resistance (ony used in calculating highpass)
10k resistor
227.3642 nf (I used .22uf)

I built a rough prototype. The solder work is not pretty, I don't know what the RCA sockets are made of, but they just don't heat up. All 8 connections are solid though, I've tested this. I also know that some of the wire shielding is damaged, the wire is from a failed attempt to open cat5 cables. The wires don't touch each other so its not a problem for now.

The only thing this circuit does is add a healthy dose of hum.

On the board I have:
Row2: RCA+ in and 10k (leg1)
Row14: .22uf (leg1), RCA- (in and out)
Row16: RCA+ out, 10k (leg2), .22uf (leg2)

I've tried also reversing the RCA positives and grounds just to be careful, with no change.

I'm sure I'm making a real noob mistake. I've attached a picture of the breadboard. Thanks for helping figure out what I've done wrong.

Andrew
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