Crossovers and bridging for sub - pi project

[new_bloke]

Hi all,

I am looking for some help / pointers for a raspberry pi based audio player project I am working on. I'm converting a dead Bush DAC10 like this

into an internet radio, using a raspberry pi, a hifiberry amp zero and volumio. I've done this before with a DAC90a and was able to reuse the single '50s vintage celestion speaker to give it a suitably 'period' (and mono!) sound. This time, the weird speaker in the DAC10a (it has an internal voice coil):

has given up the ghost. Due to the limited amount of space inside the case, I'd like to use 2 main drivers and a single bass driver. I found this video where someone has done pretty much the same thing, but he doesn't really give much detail on how he connected the bass driver to the crossovers, and how he did the bridging.

https://youtu.be/7NESVK89Rho?t=1m5s

Apologies for the very childish diagram, but it's the bit in red that I don't understand:


How should the connections be made? The videos talks about resistors - how do I figure out what they should be? If anyone can give any advice on crossovers, that would be brilliant - I have no experience whatsoever with them. The speakers would all be 4 ohm impedance - the left and right are rated 100 - 20000Hz. I am thinking of a bass driver with response 47 - 7000Hz

Any and all advice gratefully received. If there's any more information that I can add, please let me know.

cheers, NB

Edited by new_bloke on Thursday 2nd March 13:59

[Bullett]

From my electronics course in the dim past I recall that it's an inductor/capacitor combo, one in series and one in parallel with the speaker.
The values and which way round give you high or low pass and the crossover point.

[Bullett]

I had a think as I was bored.

I think the problem is most crossovers are designed for 1 signal in and 1 out not 2 in one out. Which makes sense as you'd install one in each speaker of a pair.

Options
Forget a sub and only go with a pair of full range speakers
Change the amp for one that supports what you need - I found this after a google - https://www.allo.com/sparky-eu/piano-dac-2-1.html
Find a crossover that supports 2 in and one out, couldn't find anything like this.
Find a dac that does 2.1 then add in 3 mono amps.
Only run the crossover on one channel

Splitting an amplified signal requires heftier components.
Splitting the unamplified signal requires more amps

Bi amping is usually seen as better so I'd look at that path.

[new_bloke]

Thanks for this. That piano dac does look interesting. I'm still trying to understand what the guy did in his video (linked above). He seems to combine (bridge?) both low frequency outputs from the crossovers into the sub. He makes mention of using some resistors to do it, but not what they are, or how they are connected...

[Bullett]

The video was confusing and I had difficulty understanding him.

[TonyRPH]

What I think he did was this:

In order to bridge the two channels, he would have either performed some phase inversion somewhere on one channel - or he took the (+) of the channel to one resistor, and the (-) of the other channel to the other resistor.

EDIT: Just watched the video again and this is wrong.

I just watched the video again (about the 5th time!) and he's got some strange wiring going on with those resistors.

I found a Facebook page for the channel and project - I have asked for details on how he wired them up.

Meanwhile here's an image from the FB page.



And a capture from the video:



And my incorrect diagram!

Edited by TonyRPH on Thursday 2nd March 21:13

[Crackie]

http://www.ebay.co.uk/itm/like/122033509892?lpid=1...

Left / right in at line level ; left / right / mono sub out at line level. Easy to use; line level RCA connections in and out.

If the budget allows then I'd recommend a MiniDSP 2x4 running this 'plugin' software https://www.minidsp.com/applications/digital-cross...

You buy the hardware from CPC for just under £100.
http://cpc.farnell.com/mini-dsp/mdsp-24/digital-si...

Seigfreid Linkwitz is a legend and uses a bigger version of the miniDSP to run his reference speaker design. Have a look at the reviews.....best speaker in the world is too subjective but many claim its the best they've heard. http://www.linkwitzlab.com/LX521/Description.htm

Edited by Crackie on Friday 3rd March 08:02

[red997]

agreed - do it a line level, not speaker level.
If you use the circuit diagram above, with the resistors to the sub, your are looking at a whole heap of trouble.
for one thing, your sub will be taking full range - its before the crossover in that circuit !
you will also mix a % of left to right and right to left through it.
the impedance that the output stages of the amps will see will be complex
just basically a not good thing to do.
simplest way is always slept signal frequencies at line level (before power amplification) and then amplify the individual speakers.

[new_bloke]

Thanks to all for the replies. Some really useful (and interesting) stuff here. Looks like I might be barking up the wrong tree with my original plan. I'll be interested to see what the video creator comes back with (if he does), more out of curiosity than anything else...


NB

[heisthegaffer]

Years ago you used to be able to buy basic passive crossovers for car audio i.e a two channel amp would connect to 2 speakers with a 3rd channel outputting to a sub (all at pre-arranged and un-adjustable cross over points). Not designed for really powerful gear but would suit you.

Or you could go down the route of a 4 channel car amp and bridge 2 of the channels for a sub output and would be fairly easy to adjust the crossovers etc to the sound you'd want. I don't imagine powering it would be too hard either (but I'm no expert).

[TonyRPH]

Following up on my post above.

The constructor of the Epic radio project has still not replied to me, either on Facebook or Youtube.

ETA: 07/03 - he has replied to me this morning to say he'll do a diagram later in the week.

Regarding the crossovers - he is using JBL crossovers from this speaker kit (JBL CS760C)

The resistors...

In one of the Youtube comments, someone asked him what they were for and his response was:

"To reduce the current flowing from the AMP to the subwoofer speaker after bridging low frequency outputs.?"

So my guess was correct, he has bridged the amp outputs via the resistors to drive the sub.

However...

Given that those amps operate on the BTL principle, I suspect that what he has done* will ultimately end up blowing the amp.

I wonder if this is why I've had no reply from him?

• assuming my guess was 100% correct.

As others have said above - this really isn't the way to add a sub.

You will always want three channels of amplification, left, right and sub, fed via an electronic crossover.

If you really wanted to cut costs, you could leave the electronic crossover out and simply put a huge inductor in series with the sub.

Additionally, place a 47uF capacitor from the *output* side of the coil to the (-) lead - this will add an additional layer of hf filtering.

Excuse the rough diagram - I'm not at my PC where I have appropriate software installed.

But the rough gist, you could simply drive the sub amp of one channel (nearly all bass is mono anyway) or you could combine the left and right channels with a couple of 10k resistors.

This is a really crappy way of doing it though...



EDIT: If you used another stereo amp to drive the sub, you might be able to bridge the left and right channels anyway.

Edited by TonyRPH on Tuesday 7th March 09:18

[kenny.R400]

I too am experimenting with the pi and at present have the pi + Iqaudio dac fronting a quite decent system, and in fairness making a mockery of some multi thousand streamers/dacs.

Great write up OP by the way, I too am interested in active.

Any views on this? If picore/LMS supported it I'd be ordering one up now along with the KALI clocker.

https://www.allo.com/sparky/piano-dac-2-1.html

Edited by kenny.R400 on Sunday 12th March 11:29

[TonyRPH]

@OP

I see you also posted a request for the schematic on his Youtube channel.

I don't think he's going to be forthcoming somehow!!

[new_bloke]

No - it doesn't look like it, does it! I'll rethink the speaker config, I think. Depending on how much space I've got inside the radio once I've removed the dead workings (which I will keep hold of, but live chassis valve stuff scares me!), I'll either go with two full range drivers, or high and low range drivers for each channel with a crossover.
At the moment, I'm struggling to get the touchscreen working, but that's another story...
If I do hear anything back from him, I will of course post it here. Thanks again to all!