Author Topic: Connecting right and left output on amplifier board  (Read 8885 times)

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Offline Audioguru

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Re: Connecting right and left output on amplifier board
« Reply #25 on: March 22, 2018, 03:31:48 pm »
Since you are using a small box then how will you cool the amplifier??
 

Offline chrsm

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Re: Connecting right and left output on amplifier board
« Reply #26 on: January 02, 2021, 06:37:56 pm »
Hi
I have figured out how to configure the amplifier board to have a mono output with an stereo input.
I have between the bluetooth controller and the TDA7492P amplifier inserted 3 resistors that combine the input from left and right to the right input of the amplifier.
Hope this can be helpful for others.
 

Offline CaptDon

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Re: Connecting right and left output on amplifier board
« Reply #27 on: January 03, 2021, 02:24:23 pm »
Since you now have a true 'mono' sum input it may be possible
to parallel the outputs + to + and - to -  which although will not
increase the output voltage it will increase the output current.
This allows the use of a speaker with half the impedance of a single
channel (use 4 ohm instead of 8 as an example) and that single
speaker will be driven with twice the power for a 3db increase
in volume. For testing if this is feasible you could place a 10 ohm
resistor on both outputs and use a scope to verify that the signal
on both outputs when measured + to + and - to - are 'in phase'
and equal amplitude. In other words there is no differential voltage
from + to + and - to -, if that is true you can parallel the outputs!!!
This is commonly done on very powerful mono-block amplifiers by
paralleling the stereo outputs and inputting a mono source. The only
advantage come when you half the speaker impedance also.
 
Collector and repairer of vintage and not so vintage electronic gadgets and test equipment. What's the difference between a pizza and a musician? A pizza can feed a family of four!! Classically trained guitarist. Sound engineer.
 

Offline tooki

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Re: Connecting right and left output on amplifier board
« Reply #28 on: January 03, 2021, 06:26:44 pm »
Since you now have a true 'mono' sum input it may be possible
to parallel the outputs + to + and - to -  which although will not
increase the output voltage it will increase the output current.
This allows the use of a speaker with half the impedance of a single
channel (use 4 ohm instead of 8 as an example) and that single
speaker will be driven with twice the power for a 3db increase
in volume. For testing if this is feasible you could place a 10 ohm
resistor on both outputs and use a scope to verify that the signal
on both outputs when measured + to + and - to - are 'in phase'
and equal amplitude. In other words there is no differential voltage
from + to + and - to -, if that is true you can parallel the outputs!!!
This is commonly done on very powerful mono-block amplifiers by
paralleling the stereo outputs and inputting a mono source. The only
advantage come when you half the speaker impedance also.
Hi! I’m writing this here because private messages don’t support attachments unfortunately.

FYI, please don’t enter manual line breaks. Not only is it not necessary, as text will wrap automatically, it often produces horrific results on smaller screens. I’ve attached a screenshot of what a recent post of yours looks like on an iPhone, as an example. It’s better to just type away and use line breaks only to separate paragraphs.

Thanks!
 

Offline tooki

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Re: Connecting right and left output on amplifier board
« Reply #29 on: January 03, 2021, 06:30:15 pm »
Hi
I have figured out how to configure the amplifier board to have a mono output with an stereo input.
I have between the bluetooth controller and the TDA7492P amplifier inserted 3 resistors that combine the input from left and right to the right input of the amplifier.
Hope this can be helpful for others.
That’s basic passive summing, the worst way to mix signals (short of bridging them without resistors).

See https://sound-au.com/articles/audio-mixing.htm


Since you now have a true 'mono' sum input it may be possible
to parallel the outputs + to + and - to -  which although will not
increase the output voltage it will increase the output current.
This allows the use of a speaker with half the impedance of a single
channel (use 4 ohm instead of 8 as an example) and that single
speaker will be driven with twice the power for a 3db increase
in volume. For testing if this is feasible you could place a 10 ohm
resistor on both outputs and use a scope to verify that the signal
on both outputs when measured + to + and - to - are 'in phase'
and equal amplitude. In other words there is no differential voltage
from + to + and - to -, if that is true you can parallel the outputs!!!
This is commonly done on very powerful mono-block amplifiers by
paralleling the stereo outputs and inputting a mono source. The only
advantage come when you half the speaker impedance also.
 
What you’re talking about is a BTL (bridge tied load) configuration. But if you look at the amp IC’s datasheet, you’ll see that that amp already uses a BTL configuration internally. So you absolutely cannot bridge its outputs!

Chrsm’s choice of grounding the unused input via a capacitor is pretty much the only option here.
« Last Edit: January 03, 2021, 06:31:58 pm by tooki »
 

Offline CaptDon

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Re: Connecting right and left output on amplifier board
« Reply #30 on: January 03, 2021, 09:53:47 pm »
Guess you haven't messed around with BTL configurations very much. It is true that both the - and the + are active and not ground referenced exactly and the speaker is definitely not connected to ground but 'across' the class D switched mode outputs and inductors. If there is no differential between +L and +R and no differential between -L and -R than you can in fact tie the outputs together and get the same voltage but twice the current for a 3Db power gain. This would be paralleling the outputs and summing two equal amplitude in phase signals. In this case you could drive a 4 ohm speaker safely when each output was designed to drive 8 ohms independently. There is another way that big power amps can be bridged. Take the case of a stereo amplifier designed to be 500 watts at 4 ohms each side. There is a bridging mode where you input a mono signal, flip a rear panel switch and one channel will get an equal but inverted input signal. Then you connect a speaker of double the impedance (no longer 4 ohms on each side but a single 8 ohm) across the outputs from +L to +R and get 1000 watts at 8 ohms. Again, if the original poster sees no differential between +L and +R and no differential between -L and -R he can parallel the outputs and use a lower impedance speaker and take advantage of a 3db power gain. On the switching side of the inductors it makes no difference if the switching frequency is in phase or not, it only matters that the audio frequency after the inductors is equal and in phase and if that is true there is no reason the outputs can't be safely paralleled. The Carver PM2.0-T is bridged parallel for over 1000 watts at 4 ohms although it consists of two independent 'stereo' 500 watt channels at 8 ohms each. It uses a common ground which also happens to be tied to chassis as is the + and - power supply returns. The QSC units are different as both the - and + outputs move in opposite directions as measured to ground and the speaker is across these outputs and in bridge mono one of the channels gets an inverted input signal and the load gets connected +L to +R. The QSC amplifier output stages resemble the original posters amplifier but in his case he will be driving an in-phase audio signal and paralleling the outputs.
Apparently the majority of posters here don't use smartphones and also use hard coded line breaks. you are the first to mention it.

 
Collector and repairer of vintage and not so vintage electronic gadgets and test equipment. What's the difference between a pizza and a musician? A pizza can feed a family of four!! Classically trained guitarist. Sound engineer.
 

Offline CaptDon

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Re: Connecting right and left output on amplifier board
« Reply #31 on: January 04, 2021, 12:31:03 am »
The original poster noted that he is only driving the right side with his mono sum from the resistors. He would need to bridge the mono sum to both inputs which would drive both amplifiers in phase and at the same amplitude making the outputs also appear as identical in phase and equal amplitude. With paralleled outputs there won't be an additional voltage gain but there will be additional current gain which 'could' drive a lower impedance speaker with 3db more power. Is it worth it for 3db? The fact that the outputs are class D switching is noted and that the + output and the - output are both active and the load is across an already bridged style of output. Again, if the left channel output signal is identical to the right side signal due to the fact that the input signals are identical and in phase then you can parallel the outputs regardless of what the spec sheet says. The spec sheet won't let you parallel the outputs if you have true independent stereo inputs that have 'differential' content as that difference on the paralleled outputs would resemble the outputs fighting each other in a manner similar to a short circuit. Believe what I say, I work on multi-kilowatt amplifiers with switched mode SMPS power supplies as well as class 'D' switched mode outputs. I stand by my reputation so much so that local musicians brought their Peavey Deca power amps to me for repair and modifications while they were still in warranty even though there was two authorized Peavey repair centers in the county and I wasn't one of them!! I am also the guy who wrote the 32 page repair manual for ONLY the power supply section of the Carver PM2.0-T a.k.a. the Carver/Clair Bros CBA-1000 in all three of its iterations. The amplifiers were bulletproof, the power supplies....not so much!!!
 
Collector and repairer of vintage and not so vintage electronic gadgets and test equipment. What's the difference between a pizza and a musician? A pizza can feed a family of four!! Classically trained guitarist. Sound engineer.
 

Offline tooki

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Re: Connecting right and left output on amplifier board
« Reply #32 on: January 04, 2021, 01:00:51 am »
Guess you haven't messed around with BTL configurations very much.
No, not really, haven't needed to. But since I spent about half of last year working on the design of an active speaker project at work, I had to immerse myself in the topic. And I think you dismissed my comment without actually having carefully read what I wrote.

It is true that both the - and the + are active and not ground referenced exactly and the speaker is definitely not connected to ground but 'across' the class D switched mode outputs and inductors. If there is no differential between +L and +R and no differential between -L and -R than you can in fact tie the outputs together and get the same voltage but twice the current for a 3Db power gain. This would be paralleling the outputs and summing two equal amplitude in phase signals. In this case you could drive a 4 ohm speaker safely when each output was designed to drive 8 ohms independently.

[snip section below that goes into a different topic before you come back to this one]

Again, if the original poster sees no differential between +L and +R and no differential between -L and -R he can parallel the outputs and use a lower impedance speaker and take advantage of a 3db power gain. On the switching side of the inductors it makes no difference if the switching frequency is in phase or not, it only matters that the audio frequency after the inductors is equal and in phase and if that is true there is no reason the outputs can't be safely paralleled. The Carver PM2.0-T is bridged parallel for over 1000 watts at 4 ohms although it consists of two independent 'stereo' 500 watt channels at 8 ohms each. It uses a common ground which also happens to be tied to chassis as is the + and - power supply returns. The QSC units are different as both the - and + outputs move in opposite directions as measured to ground and the speaker is across these outputs and in bridge mono one of the channels gets an inverted input signal and the load gets connected +L to +R. The QSC amplifier output stages resemble the original posters amplifier but in his case he will be driving an in-phase audio signal and paralleling the outputs.
Right, that's classic paralleling. The thing is, that's all fine and true in theory, but modern class-D amp ICs use all sorts of clever sorcery to eke more power out of less, in particular, weird proprietary modulation schemes. So while you definitely can parallel the smoothed output of two completely independent amplifiers, you definitely cannot make that assumption about two amplifier blocks within the same IC. Some class-D amplifier ICs today have explicit PBTL support which you must enable in order to use them in that configuration (for example, the TI TPA3156D2).


There is another way that big power amps can be bridged. Take the case of a stereo amplifier designed to be 500 watts at 4 ohms each side. There is a bridging mode where you input a mono signal, flip a rear panel switch and one channel will get an equal but inverted input signal. Then you connect a speaker of double the impedance (no longer 4 ohms on each side but a single 8 ohm) across the outputs from +L to +R and get 1000 watts at 8 ohms.
So… bridge tying the load. Which as I explained, in the TDA7492P (like the TPA3156D2) the outputs are already configured as a BTL. As I understand it, you can't bridge the bridged outputs again, because you're already at the limits of the power supply voltage x2 (p-p).



Apparently the majority of posters here don't use smartphones
No idea what the stats are, but I'm definitely far from the only person to access the forum by phone. I, of course, use the forum on all my devices (computer, tablet, phone), just depends on the situation. How the forum gets rendered on a given device will vary; I think I remember others saying it's also really annoying on the mobile forum skin and on tapatalk.

Apparently the majority of posters here […] also use hard coded line breaks.
Absolutely not. There's a tiny handful (like… 2 or 3?) of intransigent die-hards who refuse to stop putting in line breaks even after the problem has been explained to them, but the overwhelming majority of forum members do not use hard line breaks every line. They use line breaks the way they're intended to be used on the web and in word processors: for semantic grouping, i.e. separating paragraphs.

Thank you for being receptive to the feedback, it's much appreciated!!

you are the first to mention it.
I may be the first to mention it to you. But I know the topic has come up a few times elsewhere on the forum (not regarding you specifically, but others who've done the same).
 

Offline JohnnyMalaria

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Re: Connecting right and left output on amplifier board
« Reply #33 on: January 04, 2021, 01:44:07 am »
Be aware that any phase difference between left and right will cause cancellation and reduced in volume.  Any in phase signals left and right will be increased in volume.  Simply adding L and R signals together never sounds as good as you hoped.  It's often better to just listen to left or right.  To achieve that you would connect the Left (or Right) input to both Left and right inputs.

All commercial music is mixed so that it will sound acceptable in mono. If you experience significant cancellation, something else is wrong.
 

Offline CaptDon

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Re: Connecting right and left output on amplifier board
« Reply #34 on: January 04, 2021, 02:06:37 pm »
I love your use of the operative word 'acceptable' and I would say that's true. Any english word which carries a higher value than acceptable would start to stretch the truth a bit. I worked as a broadcast engineer for 13 years back in the 1980's through the 90's and we would sometimes gather in one of the production studios and play around with audio mixing and tweaking and phasing and so forth just for fun. It was nice to have an X/Y oscilloscope to display phase differences in the stereo mixdowns and to see what signals would either 'go away' or become accentuated in a mono mixdown. One of the funny things about commercially released productions is that if you invert the phase of one channel the vocals tend to go away but the reverb of the vocals remains. Also the bass and kick drum tend to disappear. I was at a live gig years ago and if you stood stage center in front of the F.O.H. system the low end completely disappeared. The sound man had his two bass cabinets hooked up to a stereo type amplifier in a normal fashion but he had the amplifier in the bridge mono mode so one channel was inverted and if you stood at the correct sweet spot there was no low end even though I could see the grass moving in front of his 18" loaded E.V. scoops!!!
Collector and repairer of vintage and not so vintage electronic gadgets and test equipment. What's the difference between a pizza and a musician? A pizza can feed a family of four!! Classically trained guitarist. Sound engineer.
 

Offline JohnnyMalaria

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Re: Connecting right and left output on amplifier board
« Reply #35 on: January 04, 2021, 02:14:37 pm »
Indeed.

Cancellation of the vocal by inverting the phase of one channel is how some karaoke machines work. Anything panned dead center in the mix will disappear (which usually includes bass and a lot of drums - depending how the kit was mixed).

I love my old Tektronix 465 'scope because of how glorious things look on XY. Can't say the same about my modern DSO.
 

Offline CaptDon

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Re: Connecting right and left output on amplifier board
« Reply #36 on: January 04, 2021, 03:38:23 pm »
I have found the DSO type scopes to be 100% useless for displaying an A.M. modulated (music) R.F. carrier!! At any reasonable sweep speed the R.F. tends to alias and the display simply looks horrible. Your 465 would be the much better choice!!! My favorite for years was the older 453 'Silver Anniversary' portable scope. The 465 and 465M are also timeless and very affordable these days!!! My DSO is a TDS644B and the little lunchbox TDS2024C. My analogs are the SC502 (15meg) SC504 (80meg) and the little guy SC501 (5meg?). I recently grabbed a fully functional analog storage scope of the 51XX series from the company dumpster. There were several vertical modules and a couple of delayed timebase horizontal modules in the dumpster with it. I think the mainframe is 60mhz?
Collector and repairer of vintage and not so vintage electronic gadgets and test equipment. What's the difference between a pizza and a musician? A pizza can feed a family of four!! Classically trained guitarist. Sound engineer.
 

Offline JohnnyMalaria

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Re: Connecting right and left output on amplifier board
« Reply #37 on: January 04, 2021, 03:41:51 pm »
My bad - it's a 475 (manufactured in 1974).

Here's a clip comparing XY mode with my DSO:



To be fair, seeing both reveals information about the signal that using just one of the 'scopes would miss.
« Last Edit: January 04, 2021, 03:45:11 pm by JohnnyMalaria »
 


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