Circuit Review: Audio Routing

Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
A Bluetooth transmitter will not have an audio input impedance of 8 ohms or 16 ohms which is a speaker impedance. Instead its input impedance will be 10k ohms or more.
An audio coupling capacitor value is not calculated to pass frequencies higher than you can hear. Instead it is calculated to block DC and very low frequencies that you cannot hear. You want the coupling capacitor to pass audio frequencies.
I assume your audio is speech which goes as low as about 50Hz. Then a coupling capacitor value to pass audio frequencies above 50Hz into 10k ohms is 1 divided by (2 x pi x 10k ohms x 50Hz)= 0.32uF. Use a 330nF film capacitor.
Thank you! That’s very helpful. The only concern is that the audio is not just speech. At least one channel will be music.

Your calculation examples and the typical impedance of a BT transmitter are critical data points.

Thanks again.
 

Wolframore

Joined Jan 21, 2019
2,609
You need 3uF to get optimal response at 50Hz using the assumptions above. Unless you don’t mind the roll off on the bass. It’s difficult to reproduce

E2269053-1C28-4F13-95AC-46450194A80F.gif
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
Hey dj it’s a high pass you don’t want to cut off everything below 20k

That frequency is the -3dB point

Again it depends on where you need the cutoff.

edit- I agree with audioguru 16 ohms would be too low impedance, which could potentially pass too much current... do you have the datasheet?
No datasheet. It’s a commercial product and I couldn’t find the input impedance.
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
You also wrongly calculated coupling capacitor values in your pan pot driver circuit:
Thanks for the data! I know it’s painfully obvious that I’m at a disadvantage when it comes to analog circuits. My introduction to electronics was early TTL ICs. Then, the progression to μP was simple due to my professional programming skills.
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
Hey dj it’s a high pass you don’t want to cut off everything below 20k

That frequency is the -3dB point

Again it depends on where you need the cutoff.

edit- I agree with audioguru 16 ohms would be too low impedance, which could potentially pass too much current... do you have the datasheet?
So you’re telling me that I shouldn’t have used 20kHz in my calculations? That the frequency used in your equation should be the lowest frequency allowed? Instead of the highest frequency expected?

Give it to me straight.. define the frequency used to calculate the capacitor value.

Thanks @Wolframore
 

crutschow

Joined Mar 14, 2008
34,283
define the frequency used to calculate the capacitor value.
Since a series coupling capacitor acts as a high-pass filter, you need to use the lowest frequency you want to pass to determine the capacitor value.
For audio, 20Hz is typically used as the corner frequency for that calculation.
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
Since a series coupling capacitor acts as a high-pass filter, you need to use the lowest frequency you want to pass to determine the capacitor value.
For audio, 20Hz is typically used as the corner frequency for that calculation.
Thanks for the clear explanation. I had it backasswards. I mistakenly believed I was supposed to use the highest frequency I wanted to pass. So many previous responses and multiple threads suddenly make sense!
 

Audioguru

Joined Dec 20, 2007
11,248
You said the audio must be directional. Low audio frequencies are never directional, they go all around.
Will your speakers produce music frequencies below 50Hz? Many speakers cannot do it.

You had the calculation for the coupling capacitor of a highpass filter that passes frequencies from 20kHz to radio frequencies, reducing and not passing audio frequencies.
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
You said the audio must be directional. Low audio frequencies are never directional, they go all around.
Will your speakers produce music frequencies below 50Hz? Many speakers cannot do it.

You had the calculation for the coupling capacitor of a highpass filter that passes frequencies from 20kHz to radio frequencies, reducing and not passing audio frequencies.
I didn’t say the audio MUST be directional. I said the speaker system I am using IS directional.

These aren’t your father’s speakers. They are a specialty speaker system from Holosonics, that uses ultrasound which shifts frequency in air to produce an approximately 5-6 ft. cone of sound, that can’t be heard beyond a strict range.

Frequencies below 50 Hz in this system are NOT omnidirectional. Check out the link above.

As mentioned, I understand my misunderstanding of the calculations for the filter. I’ve updated my circuit pursuant to receiving that info.
 
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Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
@Audioguru

Just a follow up... I have s 5’x10’ are where the guests/audience is located. Using two of the Holosonic speaker systems, there is s left area (5’x5’) and a right area. The audio sent to the left area CANNOT be heard outside it’s 5’x5’ area. Likewise, the audio sent to the right ares CANNOT be heard outside of its 5’x5’ area. The location hints must be reproduced independently in each area.

This is the reasoning for independent mixers sharing eight sources of different single line out signals which is split from a single audio line out channel (one channel; no stereo separation). A source located physically in the far left side of the 5’x10’ area must appear loudest to the far left of the left speaker area and at s low level in the left of the right speaker area... And so on and so forth.

Again, I strongly recommend that you review the specs of the Holosonic sound system.

Best regards.
dj
 

Wolframore

Joined Jan 21, 2019
2,609
Ah I remember these speakers they actually send an inaudible signal and the audio is created using difference tones where they intersect. They had systems for concerts and there were some benefits such as size. There were issues as well.

Dj can you adjust the area by moving the inducers further apart and different angles?
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
Ah I remember these speakers they actually send an inaudible signal and the audio is created using difference tones where they intersect. They had systems for concerts and there were some benefits such as size. There were issues as well.
Dj, can you adjust the area by moving the inducers further apart and different angles?
Apparently not. I was in detailed discussion with their engineers. The conclusion is that to cover a larger area, I need to duplicate the hardware. Hence, two 24i systems have been recommended for my application. Each covers s 5’x5’ area... Two covers my 5’x10’ area. And by using a mixing/pan circuit, I can place each individual source somewhere in that area.

I think with @Bertus’ recommendation, I can meet my goals, with a single source in the circuit.
 

Audioguru

Joined Dec 20, 2007
11,248
The ultrasonic Audio Spotlights have their ultrasonic beam modulated with a sound signal but there are no details about the input impedance, input level or frequency response of the signal source.
They do not say it now but years ago it was said that the sound seems to come from the object that the ultrasonic beam is aimed at. I would guess that the size of the object determines the low frequency response.
 

Wolframore

Joined Jan 21, 2019
2,609
Dj why wouldn’t you just get a decent mixer? If you need a decent but inexpensive option I would recommend the behringer 4 channel mixer. Very reasonable with pretty much all the features you would want. I have a 12 channel that I use for various things like setting up some guitars and mics to pumping music through it. It’s not bad for the price.
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
The ultrasonic Audio Spotlights have their ultrasonic beam modulated with a sound signal but there are no details about the input impedance, input level or frequency response of the signal source.
They do not say it now but years ago it was said that the sound seems to come from the object that the ultrasonic beam is aimed at. I would guess that the size of the object determines the low frequency response.
The Holosonic system is aimed at the listener; not an object. Perhaps you are thinking of another product?
 

Thread Starter

djsfantasi

Joined Apr 11, 2010
9,156
Dj why wouldn’t you just get a decent mixer? If you need a decent but inexpensive option I would recommend the behringer 4 channel mixer. Very reasonable with pretty much all the features you would want. I have a 12 channel that I use for various things like setting up some guitars and mics to pumping music through it. It’s not bad for the price.
I’ve considered a commercial mixer. As I recently discovered, I’m going to need 16 channel per mixer. And 10-20 mixers in total. So, the total cost becomes a budgeting issue. For one thing, by building my own, I can custom populate them with only as many channels that I need and add more only when necessary.

However, I’m open to other options! So I thank you for suggestion.
 

Wolframore

Joined Jan 21, 2019
2,609
10-20 mixers? Maybe you need to build a simplified version. Might be good to reverse engineering a simpler mixer. Could you explain what exactly the mixers do?

This for $300 is a deal and they do a very good job. Everything has balanced connections and I would get another behringer board for general use.

Maybe if you look around you can find a lower priced one. This one has all the features I would be interested in. I wouldn’t even be able to get the pots and faders at this price. Let alone all the other features.

https://www.sweetwater.com/store/detail/X2222USB--behringer-xenyx-x2222usb-mixer-with-usb-and-effects?mrkgcl=28&mrkgadid=3301332098&rkg_id=0&product_id=X2222USB&campaigntype=shopping&campaign=aaShopping - SKU - Live Sound & Lighting&adgroup=Live Sound Mixers - Behringer - x2222usb&placement=google&adpos=1o1&creative=280136034232&device=t&matchtype=&network=g&gclid=Cj0KCQjw-b7qBRDPARIsADVbUbVD020JE2sWUasxmJhqapbDOP_JoFVM9xQag68GzfMEjyTx64usG8MaAmw0EALw_wcB

57C365FA-B47C-4DC6-B703-20ACE0485F28.jpeg



This one is $163 out of China but I’m not famialar with it or the company.
https://www.aliexpress.com/item/330...chweb0_0,searchweb201602_8,searchweb201603_53
 
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