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Help needed: Linkwitz-Riley frequency response curves
Help needed: Linkwitz-Riley frequency response curves
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Old 19th August 2019, 04:20 PM   #21
aboos is offline aboos  Germany
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On the right corner there are diodes for rectification, the smoothing caps and as it looks two regulator chips with the heat sinks. The green connector at the upper right side has three connections so you need 2x15V center tapped AC from a simple mains transformer
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Old 19th August 2019, 04:21 PM   #22
Michael Bean is online now Michael Bean  United States
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Originally Posted by yannikab View Post
Bugs me too. Every time I poke the running man I have to set proper axis parameters. But it's not that big a deal. My initial perception of its graphics capabilities was pretty horrendous and I was actually contemplating going down the complex math route to get my curves or use something like Matlab, Mathematica, etc.

It would be very interesting to find a strong open source version of this tool that included a rich library of components right after installation and that hopefully had a cleaner interface. I'm certain it's out there somewhere.
I'm kind of fond of TI Tina (Here: http://www.ti.com/tool/TINA-TI)...much like LT Spice it's dedicated to TI parts but can create your own spice models if you want. I didn't care for LT Spice because of the steep learning curve, TI Tina is much easier and more intuitive.

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Old 19th August 2019, 04:23 PM   #23
yannikab is offline yannikab  Greece
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Originally Posted by aboos View Post
On the right corner there are diodes for rectification, the smoothing caps and as it looks two regulator chips with the heat sinks. The green connector at the upper right side has three connections so you need 2x15V center tapped AC from a simple mains transformer
Dear aboos, THANK YOU. You saved me from purchasing an additional board for the AC to DC conversion. It did provide both +-15AC and +-15DC but a simple transformer is more straightforward. I had to power that board with 24V DC, so I would have two transformers in actuality.

Last edited by yannikab; 19th August 2019 at 04:31 PM.
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Old 19th August 2019, 04:54 PM   #24
yannikab is offline yannikab  Greece
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So, now I have to get a simple AC transformer.

This unit is based on ESP's design:

Linkwitz-Riley Electronic Crossover

ESP also builds a nice power supply that matches his crossover well:

Power Supply for Preamps

From that page:

"Alternatively, the supply can be run from a conventional split voltage transformer (e.g. 15-0-15V AC). If a split AC supply is used, then the transformer centre tap connects to GND, and the two 15V winding ends connect to AC1 and AC2. Although virtually any transformer of 0.5A or more will work (provided the voltage is correct), there is very little to be gained by using anything more than 30VA (and even that is likely to be overkill)."

I am absolutely convinced that the good people in the far east have integrated part of this power supply on their board, so the text in quotes above applies to their design. I'm trying to figure out the power rating of the transformer to buy.

If I am reading his text right, even a 15VA +-15V AC transformer is enough, supplying the minimum 0.5A current needed. Am I right? Would it make sense to go a bit higher, at 20VA or 25VA?

Lastly, after a little reading about AC transformers (I am a complete electronics newbie), I discovered that toroidal transformers have the property of producing much less EMF interference, and the transformer will sit next to an audio processing circuit. Would you recommend spending the extra cost for a toroidal or will it be something like a 0.5% more noise with a regular transformer?
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Old 19th August 2019, 05:06 PM   #25
PRR is online now PRR  United States
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Help needed: Linkwitz-Riley frequency response curves
> power rating of the transformer

It looks like five dual opamps, unknown type. Taking 5mA/opamp, 10mA per chip, that's 50mA DC. Double that for AC Amps, 0.1 Amps AC. 15VAC+15VAC (30VAC CT) seems ample. 30VAC*0.1A AC is 3VA, the minimum transformer.

> less EMF interference

The really teeny 3VA transformers can throw a lot of hum/buzz for their size. I would be looking at 5VA-10VA types. (30VAC CT @ 0.25A)

I do have a doubt. Those regulator heatsinks are WAY bigger than the opamp chips they power. Typically a regulator dissipates 0.6X-1X the heat in the load (opamps). Did they just get a good deal on large heatsinks? Did they spend 2 cents extra to get sinks that look WOW! in the picture? Also the diodes look like 3A jobs when the 1A 1N400x type should be over-ample. I suspect you are right: the designer and the merchant are totally separate parties, good-looking trumps taut engineering, and you won't get technical info from the seller. But hey, it's cheap!! Inspect for bad solder. Wire it up. See if it smokes. With voltmeter black on ground, you expect +/-22V DC on pins of big caps, +/-15V DC on opamp power pins.
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Old 19th August 2019, 05:31 PM   #26
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by yannikab View Post
I'm trying to figure out the power rating of the transformer to buy.
Most op-amps only draw a few milliamps at most. As an example, the Texas Instruments NE5532 datasheet shows a typical 8 mA draw per dual op-amp, and a maximum of 16 mA.

Your board has five op-amps, so 40 mA typical, and 80 mA max draw. Times 30 volts (+/- 15V) that's still only 2.4 watts, worst case. A 5VA transformer will have a nice big safety margin.

Edit: I see PRR beat me to it.
Quote:
Originally Posted by yannikab View Post
...the transformer will sit next to an audio processing circuit. Would you recommend spending the extra cost for a toroidal or will it be something like a 0.5% more noise with a regular transformer?
There have been millions of audio circuits built with conventional audio transformers, and negligible noise (i.e. far below audibility).

That said, this requires good layout and grounding, and distance is your friend - physical distance between the noisy transformer, and the sensitive front-end electronics.

If the transformer is right up against the circuitry, a toroidal transformer will certainly make it easier to achieve good hum and noise performance. If you can find one that doesn't carry a big price penalty, I would say it's worth getting.

I'm a bit concerned about the three transformer-to-board wires, which carry AC with a nasty spiky waveform (because of the way diode rectifiers work), and run right up to the PCB. Wires carrying AC will radiate some noise no matter what type of transformer you use, and wires carrying AC with a spiky waveform will typically radiate more noise and buzz than wires carrying pure sine-wave AC (some of the noise isn't radiated, but coupled to the circuitry through stray capacitance.)

We can only hope that whoever designed the PCB paid attention to detail and made sure this AC-to-board layout still produces acceptable noise.

If I were building this filter from scratch, I would convert the AC to regulated DC on a separate PCB at a safe distance from the sensitive audio circuitry, and then run three wires carrying only DC (which doesn't radiate electromagnetic noise.)

I suggest trying the board as designed. If there proves to be too much hum and buzz, you can always convert the AC to DC off-board, and feed the board with DC. The on-board diodes won't have much work to do, but the board should work just fine.


-Gnobuddy

Last edited by Gnobuddy; 19th August 2019 at 05:32 PM. Reason: PRR answered before me
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Old 19th August 2019, 05:34 PM   #27
yannikab is offline yannikab  Greece
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Quote:
Originally Posted by PRR View Post
> power rating of the transformer

It looks like five dual opamps, unknown type. Taking 5mA/opamp, 10mA per chip, that's 50mA DC. Double that for AC Amps, 0.1 Amps AC. 15VAC+15VAC (30VAC CT) seems ample. 30VAC*0.1A AC is 3VA, the minimum transformer.

> less EMF interference

The really teeny 3VA transformers can throw a lot of hum/buzz for their size. I would be looking at 5VA-10VA types. (30VAC CT @ 0.25A)

I do have a doubt. Those regulator heatsinks are WAY bigger than the opamp chips they power. Typically a regulator dissipates 0.6X-1X the heat in the load (opamps). Did they just get a good deal on large heatsinks? Did they spend 2 cents extra to get sinks that look WOW! in the picture? Also the diodes look like 3A jobs when the 1A 1N400x type should be over-ample. I suspect you are right: the designer and the merchant are totally separate parties, good-looking trumps taut engineering, and you won't get technical info from the seller. But hey, it's cheap!! Inspect for bad solder. Wire it up. See if it smokes. With voltmeter black on ground, you expect +/-22V DC on pins of big caps, +/-15V DC on opamp power pins.
THANK YOU for the sagely wisdom dispensed. Opamps are the usual suspect, dual NE5532s. Not sure if this changes anything significantly, but in any case you mentioned noise, so a higher power transformer will cover anything surplus. I'm surprised that required power is an order of magnitude less than what ESP mentions, even if it's an absolute minimum calculation.

The way they "work" is probably having the boards almost ready and just soldering 8 resistors to set the frequency. I expect scary things visually and even a short or two. Really pressing fast work conditions, therefore my quotes earlier. I will most certainly make the measurements you mention, and thanks again. I'm grateful for any tiny bit of wisdom/knowledge dispensed.
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Old 19th August 2019, 05:43 PM   #28
yannikab is offline yannikab  Greece
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Quote:
Originally Posted by Gnobuddy View Post
If I were building this filter from scratch, I would convert the AC to regulated DC on a separate PCB at a safe distance from the sensitive audio circuitry, and then run three wires carrying only DC (which doesn't radiate electromagnetic noise.)

I suggest trying the board as designed. If there proves to be too much hum and buzz, you can always convert the AC to DC off-board, and feed the board with DC. The on-board diodes won't have much work to do, but the board should work just fine.
Very interesting. This is in fact what ESP does, separate power supply that feeds the LR board with +-15VDC.

Could you clarify on what is acceptable as input on the three pins of the board since by design it wants +-15VAC (and by +- I mean phase of course). What other configurations could make it work the same way? The way you describe it is sounds like I can just substitute +-15VAC with +-15VDC and eliminate AC noise, if it's there.

Last edited by yannikab; 19th August 2019 at 05:48 PM.
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Old 19th August 2019, 07:34 PM   #29
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by yannikab View Post
The way you describe it is sounds like I can just substitute +-15VAC with +-15VDC and eliminate AC noise, if it's there.
Without actually seeing the board (or better, the schematic for the board), I'm making an educated guess. (It might help if you provide a link to the board you bought.)

I can't actually see that end of the board in the photo you posted, but it looks as though it has four diodes onboard. Almost certainly they are wired as a full wave bridge. The two ends of your 30V AC transformer winding feed the bridge, and the centre-tap (the "0" in the 15-0-15) provides a voltage halfway between the other two.

As designed, the four diodes rectify the incoming AC, steering positive half-cycles to one supply rail, and negative half-cycles to the other.

Now, if you replace your transformer with a +15V DC, 0V, -15V DC power supply, everything still works normally. Only two of the four diodes in the bridge will be in operation (because the incoming voltage never reverses polarity). Those two diodes won't be doing anything useful, because there is no AC to convert to DC. But they'll still conduct, and allow the incoming DC to flow through them.

So yes, all you have to do is connect +DC to the point on the pad where you normally connect one end of the 30V AC winding. 0V DC goes to the same place the centre-tap of that 30V AC winding would otherwise go. And you connect your -DC where the other end of the 30V AC winding would have gone.

15 V AC RMS (one half of the 30V winding) rectifies to about 21 V DC, less two diode losses of about 0.5 - 0.7 volts each. Each voltage regulator chip on your board is therefore being fed something close to 20 V DC, either + or - as appropriate, which is then regulated down to +15V and -15V.

If you choose to power your board with DC rather than AC, you'll want to provide a similar voltage - roughly +/- 20 volts DC.

You can get this from a modern switching supply or two, or from an old-fashioned 60 Hz transformer, toroidal or otherwise. If you use a transformer, you'll want a 15-0-15 VAC transformer...because after you rectify it to DC (off-board), it will turn into the +/- 20V DC you need.


-Gnobuddy
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Old 19th August 2019, 08:37 PM   #30
yannikab is offline yannikab  Greece
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Originally Posted by Gnobuddy View Post
So yes, all you have to do is connect +DC to the point on the pad where you normally connect one end of the 30V AC winding. 0V DC goes to the same place the centre-tap of that 30V AC winding would otherwise go. And you connect your -DC where the other end of the 30V AC winding would have gone.

15 V AC RMS (one half of the 30V winding) rectifies to about 21 V DC, less two diode losses of about 0.5 - 0.7 volts each. Each voltage regulator chip on your board is therefore being fed something close to 20 V DC, either + or - as appropriate, which is then regulated down to +15V and -15V.

If you choose to power your board with DC rather than AC, you'll want to provide a similar voltage - roughly +/- 20 volts DC.

You can get this from a modern switching supply or two, or from an old-fashioned 60 Hz transformer, toroidal or otherwise. If you use a transformer, you'll want a 15-0-15 VAC transformer...because after you rectify it to DC (off-board), it will turn into the +/- 20V DC you need.
Given things on the board are as you suspect, it's just as I imagined. Substitute AC with DC, just with a different amplitude.

I found a completed LR product there in the far east, and zoomed into it's toroidal transformer:

Help needed: Linkwitz-Riley frequency response curves-bingzi1-jpg

And further research shows it's this, it's a PCB mounted version:

Help needed: Linkwitz-Riley frequency response curves-bingzi2-jpg

So the people in the far east are using a 15VA transformer. Perhaps they are aware of noise problems at lower power ratings. Perhaps it costs the same to make a 10VA transformer. I trust you guys however, in that this board's demands are nowhere near 15VA.

This green thing costs something like 20$. I almost got it until I found a different toroidal, 30VA mind you, for something like 13$ so I just snatched it. If it works, cheap is good in this case:

Help needed: Linkwitz-Riley frequency response curves-toroidal-jpg
It has a dual primary winding so I imagine I have to very carefully connect them in series so that it properly works in my country (Greece, 230V-240V) and nothing blows up.

So I am super excitedly waiting for these two components to play with.

A final note: After your earlier calculations about the board's current draw, I had my reservations because of the text from ESP's power supply description: "If a split AC supply is used (such as 15-0-15V AC), then the transformer centre tap connects to GND, and the two 15V winding ends connect to AC1 and AC2. Although virtually any transformer of 0.5A or more will work (provided the voltage is correct), there is very little to be gained by using anything more than 30VA (and even that is likely to be overkill)."

I think my mistake is that this is a description for a generic power supply for preamplifiers. Other boards may have much higher demands, hence the high-ish 30VA rating he recommends (ten times what I need in this case). Regardless, I ended up getting a 30VA after all, just because of the price.

As a second side note, and because LR is pretty much finished in this thread for now, I am installing eSim and KiCad and attempting to recreate/simulate the same circuit I made in LTspice. See if and how things work out.
Attached Images
File Type: jpg bingzi1.jpg (38.6 KB, 211 views)
File Type: jpg bingzi2.jpg (25.0 KB, 208 views)
File Type: jpg toroidal.jpg (12.6 KB, 160 views)
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