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Old 5th May 2017, 11:22 am   #1
indigo.girl
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Default Can I play both channels of stereo amp through one speaker?

Hi everyone,
I've been playing around with the Bardwell stereo amplifier today and I want to use it in something with a single 4ohm speaker.

Naively I thought it would be as simple as connecting the L and R outputs together and connecting to the speaker....but its not! I get a high pitched whine and from reading elsewhere on the forum it says this can damage the IC.

What is the best way to wire it up so I make full use of both channels but only one speaker. Must I also connect the two inputs together? Also, what is going on when I connect the two outputs and why is this so wrong?

Many thanks,
Nicola

PS: Edit - I've just tried connecting the inputs as well as the outputs and this is not good either. Is it just impossible and I should just use one of the two channels?

http://www.bardwells.co.uk/product-category/modules/

Last edited by indigo.girl; 5th May 2017 at 11:28 am.
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Old 5th May 2017, 11:40 am   #2
ms660
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Default Re: Can I play both channels of stereo amp through one speaker?

Use one of the two channels.

Lawrence.
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Old 5th May 2017, 11:43 am   #3
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Default Re: Can I play both channels of stereo amp through one speaker?

Basically, don't do it! Use one channel and leave the other alone, loading it with an 8R resistor to keep the IC happy.

Commercially, it's sometimes done by using a LS with two voicecoils, or by configuring the amp in a "Bridge Tied Load" mode, i.e. while one half is "pulling", the other half "pushes", but I don't think this will be possible with your PCB and IC.

If you need mono, look at the cheap Chinese kits that use the superb TDA2030 IC.
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Old 5th May 2017, 11:53 am   #4
indigo.girl
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Default Re: Can I play both channels of stereo amp through one speaker?

Thanks - will just wire up one channel
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Old 5th May 2017, 12:19 pm   #5
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Default Re: Can I play both channels of stereo amp through one speaker?

Some amps can be bridged to parallel both channels for double the power but they have to be designed to allow that facility. This is a simple IC based amp that will object if subjected to that configuration!
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Old 5th May 2017, 12:32 pm   #6
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Default Re: Can I play both channels of stereo amp through one speaker?

Ah I remember those amps well! Many a childhood Saturday spent there. Those amps came from a late 70's Wye Electronics Hifi I believe.

Great little amp.
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Old 5th May 2017, 12:44 pm   #7
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Default Re: Can I play both channels of stereo amp through one speaker?

More info from David here: http://www.vintage-radio.net/forum/s...d.php?p=739279
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Old 5th May 2017, 2:17 pm   #8
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Lightbulb Re: Can I play both channels of stereo amp through one speaker?

Hi Indigo.Girl,


You could have a transformer with two separate windings (primary's) and feed the left and right channels into these. With a single secondary as an output, you would have added the two channels together as a single output.
The transformer would have to be specially made (I would think), but it would work.


Cheers,
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Old 6th May 2017, 5:20 pm   #9
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Default Re: Can I play both channels of stereo amp through one speaker?

Amplifiers modules are so easy to get these days at a low price I would start again with a mono amp and do it that way. The Bardwell amp uses obsolete chips so if you do blow it up it will be difficult to repair. The squealing noises you are getting appear to be the same as I experienced with mine. When you turn up the volume the amp can suffer from instability, it would seem. Saying that, once I built mine into a proper case it behaved itself so it could have just been because I lashed it up on the bench and it didn't like it. High pitched shrieks and instability will cause the chips to overheat and destroy themselves if you are not careful. It's a bit like driving the amp at full tilt and they don't like it. Hope I haven't put you off experimenting.
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Old 6th May 2017, 6:11 pm   #10
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Default Re: Can I play both channels of stereo amp through one speaker?

If you decide to go for a mono amp this kit from Magenta will produce 20w into 4 ohms in bridge mode.

David
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Old 6th May 2017, 8:59 pm   #11
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Default Re: Can I play both channels of stereo amp through one speaker?

Why not buy a car stereo sub amp, there are plenty of these about secondhand, made to drive one channel from a stereo source. I know most where built to drive a subwoofer, but connected to a hifi speaker they sound quite good. You would obviously just need a 12v power supply to feed the amp.I know most car speakers are 4 ohm but most car amps will run 4/8 ohm happily.

just an idea
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Old 6th May 2017, 10:41 pm   #12
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Default Re: Can I play both channels of stereo amp through one speaker?

Quote:
Originally Posted by indigo.girl View Post
Hi everyone,
Must I also connect the two inputs together? Also, what is going on when I connect the two outputs and why is this so wrong?
You are quite free to connect the two inputs together with no nasty consequences.

Things go wrong if you connect the two outputs together.

The output stages of transistor amplifiers are usually emitter followers and so give quite low output impedance. Transistors produce quite a large amount of distortion, so in order to combat this, they employ large amounts of feedback. As well as reducing the distortion the feedback also scales down the output impedance. So the output impedance of even basic amplifiers is very low indeed.

With two amplifiers driven by the same signal, fairly small differences in gain will create small differences in the output voltages the two amplifiers wish to create. Connect them together and the small voltage differences will combine with the very low output impedances to create troublesome levels of current flowing from one amplifier into the other as the two amplifiers fight to impose their own idea of the required output voltage.

David
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Old 7th May 2017, 5:16 pm   #13
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Default Re: Can I play both channels of stereo amp through one speaker?

I understand the problem with connecting the outputs of a stereo amp together until I came across this stereo cassette radio today.
It is an ITT Super Touring Cassette 350. If I understand it is supposed to have 3 sets of speakers. The normal stereo pair, which in the case of my purchase this morning are both missing, and a 3rd or 5th speaker as it calls it which is a combination of the outputs from both power amp IC's.

Here is a snippet from the schematic showing the output stages.
Click image for larger version

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I suspect that the Cassette Radio, when connected with its two detachable speakers will act as a full stereo unit, with the speaker in the main body of the unit acting as a bass speaker.
When the 2 detachable speakers are disconnected, I think the internal speaker in the main body of the unit will act as a full range MONO speaker. At the moment there appears to be a problem as the unit works fine (and very loud when on mains) with a pair of external speakers connected but there is no sound from the "5th" speaker in the body of the main unit, with the external speakers connected or disconnected.

Would anybody care to comment and elaborate on how this works.
And can this arrangement be used for the question that Nicola asked in the original post.
thanks

Mike

Last edited by crackle; 7th May 2017 at 5:28 pm.
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Old 7th May 2017, 6:20 pm   #14
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Default Re: Can I play both channels of stereo amp through one speaker?

Fig 23 in this data sheet shows two of them driving a motor between the outputs:

http://pdf.datasheetcatalog.com/data...cs/TDA1908.pdf

Lawrence.
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Old 7th May 2017, 6:45 pm   #15
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Default Re: Can I play both channels of stereo amp through one speaker?

As the output impedance of the amps is of the order of 0.1 ohms. you are basically shorting out both outputs. if you did that and turned up the volume you would expect something to pop would you not?
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Old 7th May 2017, 7:25 pm   #16
crackle
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Default Re: Can I play both channels of stereo amp through one speaker?

Hmm, would ITT have designed it like that if it was going to fail prematurely.
Something has gone though as there is nothing coming out of the "centre speaker" in the main body of the radio, but I suspect this to be the speaker itself as the two IC amps are working very well.

Mike
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Old 7th May 2017, 7:32 pm   #17
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Default Re: Can I play both channels of stereo amp through one speaker?

Often, the "centre" speaker is nothing of the sort. It gets wired between the left and right channels positive. That gives sum and difference output, or "poor man's surround".
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Old 7th May 2017, 7:39 pm   #18
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Default Re: Can I play both channels of stereo amp through one speaker?

There aren't many amps you can use to power one speaker using both channels, but in reality the circutry to allow it to happen is as simple as a few resisters and some diodes.

So if a system was supposed to do this and does not, then I would have to imagine one or more diodes went open.
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Old 7th May 2017, 8:12 pm   #19
crackle
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Default Re: Can I play both channels of stereo amp through one speaker?

I don't see any diodes in the OP circuit to the speakers. There are 2 inductors in series, and what looks like a 3.9 ohm resistor which is switched when the external speaker plug is inserted.

Mike
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Old 7th May 2017, 8:59 pm   #20
ShambleS1980
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Default Re: Can I play both channels of stereo amp through one speaker?

Probably functionality is in the IC then, so don't know what to do about that.
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