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Help with better understanding of "bridged" stereo amplifier

stbourg

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Hi all,
First of all, I'm a beginner with DIY audio so please forgive me if my question is dumb. :confused:
I've this idea of a "mono streamer" but I'm not not very clever when it comes to electronics so after googling and reading now for a couple of hours and still not feeling any wiser I thought I post my question since there are so many brilliant people here.

I've just one 6 ohm three way in-ceiling speaker which I would like to drive with a "Rpi streamer" setup consisting of a Rpi Zero W equipped with a IQAudio DigiAMP+ Hat (link > : https://datasheets.raspberrypi.com/iqaudio/iqaudio-product-brief.pdf ). The speaker only have one + & - connector.

I'll be using Picoreplayer as SW and as such can convert the stereo to a mono signal digitally with ALSA mixer so that's fine (it means that L & R will be combined digitally and the same combined signal will be sent to both L & R of the DigiAmP hat).

So my question regard the power amp part, how should I connect the speaker? In my confused mind I'm kind of thinking connecting both L & R plus and minus on the DigiAMP to their respective speaker terminal will give me a power boost since the DigiAMP is "2x35W".
But now after googling and reading I do understand that it might not be that simple? So what do 'ya say, is there any point of connecting like that or should I just use one of the channels?
 
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Trdat

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I am sure the others with enough know how will chime in, but in the meantime this link will give you some idea on bridging.

 

pjug

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The DigiAMP+ doesn't look like it is designed for bridging. Maybe there is a way to do it but then you probably need to use a higher current power supply than the normal one.
 

DVDdoug

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The datasheet for the TAS5756M chip indicates it's already bridged.

You'll maximize power output by maximizing power supply voltage.*

But there is a very-unusual "parallel" mode that allows you to use both channels together. See section 8.4.4.2 of the datasheet. Normally, "shorting" amplifier outputs together will fry an amp but apparently this amp is designed for it. That allows more current to the speaker but you won't actually get any more current without lowering the speaker impedance (or putting a pair of speakers in parallel.)

Help with better understanding of "bridged" stereo amplifier
Normally (with a non-bridged connection) the '-' speaker connection is grounded (not really negative).

And the audio signal to the '+' terminal isn't actually positive... It's AC so it's positive half time and negative half the time. When it swings positive the speaker pushes-out, and during the negative half-cycle the speaker sucks-in.

In a bridged connection both terminals are driven in opposite directions. (There is no ground to the speaker.) When the the + terminal is positive the - terminal is negative, and vice-versa. That doubles the voltage across the speaker coil. When you double the voltage you also double the current and you get 4 times the power. (That's assuming the amplifier can supply the current without burning-up.)



* You can calculate power as Voltage squared/impedance. That's the RMS value which is about 70% of the peak voltage. The peak voltage on a non-bridged amplifier can be almost equal to half the power supply voltage with some loss through the amplifier. With a bridge amplifier, double that with peak voltage almost equal to the power supply.
 

NTK

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From what I read from TI's datasheets, it can't be easily done.

The DigiAMP+ board uses the TI TAS5756M chip. The output mode of the chip is software configured. The chip does support the mono PBTL (parallel bridge tied load) configuration, but this has to be configured through the TI HybridFlow software (HF4 mode).
TAS5756M datasheet: https://www.ti.com/lit/ds/symlink/tas5756m.pdf
HybridFlow user's guide: https://www.ti.com/lit/ug/slau577a/slau577a.pdf

TI1.png



TI2.png
 

pjug

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But there is a very-unusual "parallel" mode that allows you to use both channels together.
This parallel connection isn't what I think of as BTL, seems confusing that TI would call it such. On page 13 of the TI datasheet, it does indicate that you can get more power compared to stereo mode into the same load, though. But not the same kinds of gains we are used to seeing from doing BTL connection of two single ended amplifiers.
 

rdenney

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My advice: just use one channel and call it good.

Rick “if 35W isn’t enough (though I bet it is), get a bigger stereo amp” Denney
 

pjug

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My advice: just use one channel and call it good.

Rick “if 35W isn’t enough (though I bet it is), get a bigger stereo amp” Denney
I agree, but the TI datasheet (p12) seems to indicate that power into 6-ohms will be more like 25W.

edit: actually you are correct it does do >30W into 6 ohms on 24V supply. The 6 ohm power knee is not between 4 ohms and 8 ohms. According to Figure 31 in the TI data sheet.
 
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stbourg

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@Trdat, thanks for the link. It make a little bit of sense to me. But is it 100% applicable to my ideas? If I understand your discussion right in the link It's about analogue balanced in/outputs. It makes sense and I think I understand what you're doing there but feel it's not 100% applicable to what I ask about?
And @DVDdoug and @NTK I'm terrible sorry but I think I don't understand since from what I read you two seem to say the opposite? Also since both of you refer to the TI TAS5756M chip official spec I take it as you're knowledgeable of digital audio engineering? Just so you know, as I wrote I basically know nada about that. I've some experience with audio gear from playing in bands for many, many moons ago but when combined with digital as now I get lost. Anyway I just went ahead and did an experiment ...so as i wrote above I set Picoreplayer to transform stereo to two mono signals via ALSA mixer which in turn sends this to the DigiAMP card. E,g the digital input of the DigiAMP have two mono signal instead of L & R to amplify to the speaker outputs. Then i tied the two + cables together and connected them to the speaker + terminal and the same with the - cables connecting them to speaker - terminal. Then I powered it up. And it works, "works" as in I get sound ... all though I'm sure I'm doing something wrong here since it seems like when I crank the volume a bit over 50% it seems like the amp or the Pi gets problem somehow? One could think that it's audio drop-outs but seems more likely that I get the amp to clip? This since as soon I turn the power down a couple of steps it seems to play fine again. I use a 19VDC/4.74A/90W SMPS if it helps anyone to help me. Also if it would be of any help to anyone, I know how to use a multimeter and got a pretty decent Fluke so if it helps anyone I can do measurements on the card if you guide me exactly of where. Sorry I'm not a theorytical type of person and get confused by reading too much info that I'm not 100% familiar with.
 
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stbourg

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I agree, but the TI datasheet (p12) seems to indicate that power into 6-ohms will be more like 25W.
My advice: just use one channel and call it good.

Rick “if 35W isn’t enough (though I bet it is), get a bigger stereo amp” Denney
Sorry guys I missed your inputs while writing what's above. Would also like to add that I've a Lab PS that's capable of 0-30VDC @5A would that do any good for any type of test?
 
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NTK

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I'd recommend following @rdenney's advice and use just one channel. It is not simple to bridge the DigiAMP+, and the benefits are minimal. Parallel bridging (PBTL) only increases the current capacity and without increasing voltage capacity. It is only beneficial if the speaker is low impedance. If your speaker is truly 6 ohm, you can only expect very marginal improvement.

You can see it in the power output curves from the TI datasheet. TI doesn't even list 6 ohm load output for the PBTL configuration (only 2, 3, and 4). Judging using the 3 ohm curve for PBTL, the point where THD starts rapidly increasing is 70 W. For 6 ohm, it will therefore be 35 W (half of 3 ohm). Compare to the 6 ohm curve for BTL mode, the max power for PBTL is not appreciably higher.

PBTL mode is also less efficient than BTL and will result in higher heat dissipation (as will increasing the supply voltage). It can be an issue since the DigiAMP+ is without any heatsink. I'd also strongly recommend against the method you tried by connecting the two channels together. I suspect the dropouts you're hearing were actually the amp went into protection.
 

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stbourg

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I suspect the dropouts you're hearing were actually the amp went into protection.
Yes that's what I was thinking as well since as soon as lowering the volume the dropout seemed to disappear. I must say I'm so grateful for your time spent answering my dumb questions. Looking at the graphs you attached and reading your text again and again looking back and forth on the graphs. Learning a lot :).

I'd also strongly recommend against the method you tried by connecting the two channels together.
The thing I did with connecting the two channels; exactly what did I do? I mean what is it technically speaking that makes it a bad idea? Or maybe what sound disadvantages do I create by such a connection. In my novice mind I think I'm thinking that having a L & R signal (e,g different signal) on the L & R outputs I kind of could think it's not very clever, I've no good technical explanation why not all though. So then I guess was thinking if it's the same signal on both outputs what' the difference when combining the two channels? (now when I write this it strikes me that I keep on forgetting that speakers are AC, probably because having the red/black connectors to relate to all the time. Ok it makes it more understandable. But I guess. Really haven't got a clue to be honest ...)

And @pjug who wrote "edit: actually you are correct it does do >30W into 6 ohms on 24V supply. The 6 ohm power knee is not between 4 ohms and 8 ohms. According to Figure 31 in the TI data sheet." > What would one say is the optimal power supply spec for this card? Asking since I could get almost any type an spec for almost nothing. The SMPS I'm using was just something I had laying around and as such haven't decided what to use yet.
 

pjug

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Yes that's what I was thinking as well since as soon as lowering the volume the dropout seemed to disappear. I must say I'm so grateful for your time spent answering my dumb questions. Looking at the graphs you attached and reading your text again and again looking back and forth on the graphs. Learning a lot :).


The thing I did with connecting the two channels; exactly what did I do? I mean what is it technically speaking that makes it a bad idea? Or maybe what sound disadvantages do I create by such a connection. In my novice mind I think I'm thinking that having a L & R signal (e,g different signal) on the L & R outputs I kind of could think it's not very clever, I've no good technical explanation why not all though. So then I guess was thinking if it's the same signal on both outputs what' the difference when combining the two channels? (now when I write this it strikes me that I keep on forgetting that speakers are AC, probably because having the red/black connectors to relate to all the time. Ok it makes it more understandable. But I guess. Really haven't got a clue to be honest ...)

And @pjug who wrote "edit: actually you are correct it does do >30W into 6 ohms on 24V supply. The 6 ohm power knee is not between 4 ohms and 8 ohms. According to Figure 31 in the TI data sheet." > What would one say is the optimal power supply spec for this card? Asking since I could get almost any type an spec for almost nothing. The SMPS I'm using was just something I had laying around and as such haven't decided what to use yet.
The TI datasheet says 30V absolute max power supply voltage, 26.4V is max recommended. So I wouldn't use higher than the recommended, and you probably should have something like 4-5 amps. What is the speaker? It would be nice to know the actual impedance if someone has measured this.
 
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stbourg

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The TI datasheet says 30V absolute max power supply voltage, 26.4V is max recommended. So I wouldn't use higher than the recommended, and you probably should have something like 4-5 amps. What is the speaker? It would be nice to know the actual impedance if someone has measured this.
It's this one > https://www.monitoraudio.com/en/product-ranges/controlled-performance/cp-ct380idc/
I measured 4.7 ohm. One thing though is the filter switches, the HF & MF doesn't do much with the measurement but with what MA call "Boundary compensation switch" active the resistance changes drastically. There's likely a capacitor somehow connected 'cause it seems like I'm charging it with the Fluke. I'll not most likely use it though since I'm planning using EQ in the SW of the RPi if I feel I need to tweak it when I finally have it monuted in the room.
 

rdenney

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Your Fluke is only measuring the DC resistance, which is only part of the impedance. The impedance will be higher, nominally, but will vary significantly across the frequency spectrum.

But why use a chip amp? I must have missed something in the original post. Any amp of reasonable quality will work fine, including inexpensive used stuff. If one channel of the amp you have isn’t enough, flog it on eBay and get one with more power.

It’s voltage (the amplitude of the waveform) that makes the signal, and current that keeps the speaker impedance from absorbing all of it before the wave peaks. The lower the impedance, the more current is required for a given voltage. Increasing current capability (parallel) only works if the voltage gain is already enough (and if it is, you can use one channel and probably be fine). Bridging (which runs the channels in series rather than parallel) increases the voltage without increasing the current, and thus doubles the minimum acceptable speaker impedance to prevent starving the waveform (= clipping).

Rick “power is cheap these days” Denney
 

pjug

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It's this one > https://www.monitoraudio.com/en/product-ranges/controlled-performance/cp-ct380idc/
I measured 4.7 ohm. One thing though is the filter switches, the HF & MF doesn't do much with the measurement but with what MA call "Boundary compensation switch" active the resistance changes drastically. There's likely a capacitor somehow connected 'cause it seems like I'm charging it with the Fluke. I'll not most likely use it though since I'm planning using EQ in the SW of the RPi if I feel I need to tweak it when I finally have it monuted in the room.
I emailed Monitor Audio asking for curves and their USA rep Kevro kindly sent these (the one labeled Imp Plot is the impedance curve). Interesting that they are Klippel, which might speak well for Monitor Audio. Anyway the impedance dips to just above 4 ohms.
 

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stbourg

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I emailed Monitor Audio asking for curves and their USA rep Kevro kindly sent these (the one labeled Imp Plot is the impedance curve). Interesting that they are Klippel, which might speak well for Monitor Audio. Anyway the impedance dips to just above 4 ohms.
Sorry for my ignorance. but is it good or bad?

Also sorry for asking this again but I really would like to understand why it's inadvisable connecting the two channels together by just twinning the two cables together on each speaker terminal? As I said I sort of understand it's problematic with a L/R source but I kind of have hard to understand why it's a bad idea when I've exactly the same mono signal on both channels? What can happen technically speaking?
 

pjug

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Sorry for my ignorance. but is it good or bad?

Also sorry for asking this again but I really would like to understand why it's inadvisable connecting the two channels together by just twinning the two cables together on each speaker terminal? As I said I sort of understand it's problematic with a L/R source but I kind of have hard to understand why it's a bad idea when I've exactly the same mono signal on both channels? What can happen technically speaking?
The speaker impedance is fine, just you might want more current from your power supply compared to if impedance were higher. But really this might be overthinking. I would just use the 19V supply or try your lab PS at ~26V. If you are happy then great. If not then maybe consider another amp as Rick suggested instead of trying to squeeze every drop of power out of the DigiAMP+.
 
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stbourg

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Thousand thanks for kind help for my ignorance guys. Things are much clearer now.
 
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