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Can one turn a Stereo amp into a Parallel Monoblock? (Not Bridged!)

dlaloum

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So reading random interesting bits of the internet, I came across the concept of the Parallel Amplifier....

At the component level, two amplification circuits, run in parallel, keeping the same V output, but doubling the current - sort of the Yin to a Bridged setup's yang...

The amp would have a lot more current and ability to handle difficult loads (low impedance, capacitive loads) - but it would not get higher power output.

At least I think that is how that would work....

Complexities include needing to match the two channels, ensuring no DC offset... others?

Apparently in the early days of the Quad 405, Quad used to have a Parallel amplification conversion kit.

This piqued my interest, as I may have a spare channel lying around, once I use one of the channels of a stereo amp for my center.

Thought I would throw it out there to the collective brain trust.

(instructions and details of the Quad 405 conversion are here: https://www.dadaelectronics.eu/uplo...Documents/Quad-405-Monoblock-Instructions.pdf )
 

Suffolkhifinut

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Perfect or even near perfect matched outputs would be impossible to get for affordable amplifiers.
 
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dlaloum

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Perfect or even near perfect matched outputs would be impossible to get for affordable amplifiers.
Yes a basic matching circuit is needed - as shown in the Quad 405 example - you may have missed it as your answer was so quick, and I added the link as an edit!
 

Suffolkhifinut

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Yes a basic matching circuit is needed - as shown in the Quad 405 example - you may have missed it as your answer was so quick, and I added the link as an edit!
Thanks had forgotten about the Quad 405. An idea not taken up by anyone else as far as I know, Quad ditched the idea fairly quickly. Component matching issues?
 

egellings

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Sometimes a very small resistance is placed in series with the output of one amplifier, and the other end is connected to the output of the other amplifier. This allows for a small output voltage mismatch, while still letting almost all the power out of the combination of the two amplifiers. Connecting two hard voltage sources directly together is not recommended.
 

Suffolkhifinut

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Sometimes a very small resistance is placed in series with the output of one amplifier, and the other end is connected to the output of the other amplifier. This allows for a small output voltage mismatch, while still letting almost all the power out of the combination of the two amplifiers. Connecting two hard voltage sources directly together is not recommended.
Not sure how successful it is? It will mean the outputs can be paralleled the resistor will absorb the difference in the output powers. However sonically what effect does it have? Resistor power ratings could also be a problem, too low and their heat dissipation will be a problem.
 

MakeMineVinyl

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Paralleling two amplifier channels is frequently done in vacuum tube amplifiers. Honestly I wouldn't muck around with trying this with solid state amplifiers, even with summing resistors on their outputs (which would also reduce damping factor a bit).
 

egellings

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You either tolerate the small effect of a small resistance in series with one of the amplifiers or risk destruction of one of both amplifiers. Take your pick. Paralleled voltage sources do not get along with each other at all. A resistance of maybe 0.1 ohm or so may be all you need for the isolation resistor. Power rating isn't an issue, since it's easy to get as large a resistor as needed, and the small ohmage adds to the ease of finding one large enough. Forget about inductance in a resistance that small. Of course, it is very important to match the levels as closely as possible, (to within 10's of millivolts). Best solution: get a single amplifier with the power you need and be done with it. Or, consider bi-amping the speaker using the 2-channel amp.
 

Suffolkhifinut

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Agree with you egellings. If you’ve got current hungry speakers, why not just buy a more powerful amplifier?
 

MakeMineVinyl

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Of course, it is very important to match the levels as closely as possible, (to within 10's of millivolts). Best solution: get a single amplifier with the power you need and be done with it. Or, consider bi-amping the speaker using the 2-channel amp.
In addition to level matching, it would be a good idea to verify that both amplifiers operate with the same polarity (if they are different amplifiers). :eek:

As mentioned, the easiest choice is to simply get a larger amplifier (they're cheap enough with class D) or bi-amp or something like that.
 

DVDdoug

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I would NOT risk it unless you don't care about the amp. You risk blowing the amp with the likelihood of no benefit.

Agreed. Same polarity is a must!
Unless you want to run it bridged. ;) If you're an electronics hobbyist (or pro) it's pretty easy to build an inverting buffer with an op-amp so you could invert one channel and run it bridged. But there is some danger there too... You're doubling the voltage which also doubles the current for 4-times the power. If the amplifier is not capable of double the current the amp might burn-up. But for example, if the amp is capable of driving 4-Ohm speakers that's also double the current compared to 8-Ohms so it should be OK to run it bridged at 8-Ohms. There is still the possibility of something going wrong...
 

MakeMineVinyl

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This topic is one of those where if somebody has to ask how to do it, it shouldn't be tried. As a matter of fact, trying to do this is something even I wouldn't bother to do unless I didn't care squat about either of the amplifiers, and/or I wanted some not-so-cheap thrills. :facepalm:
 

radix

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I agree with the others, I wouldn't try to do this ad-hoc to existing amps without understanding their circuitry. There's plenty of bridged mono amps out there to choose from if you want the flexibility of adding wattage later.

There is a Wikipedia on these two designs (parallel vs bridged).


Analog Devices also has an article on parallel amplifiers


Parallel amps have been used extensively in RF, e.g. this article by Jacobs



Screen Shot 2021-12-13 at 12.59.24 PM.png
 

Suffolkhifinut

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This topic is one of those where if somebody has to ask how to do it, it shouldn't be tried. As a matter of fact, trying to do this is something even I wouldn't bother to do unless I didn't care squat about either of the amplifiers, and/or I wanted some not-so-cheap thrills. :facepalm:
Couldn’t agree more! If the load needs a high current amplifier make sure it has a stiff power supply. Years ago had an amplifier rated at 160 Watts a channel wimped out when played loud. A very well reviewed piece of kit with a limp power supply.
 

Kal Rubinson

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Apparently in the early days of the Quad 405, Quad used to have a Parallel amplification conversion kit.
My old Dyna Stereo 150 had a similar parallel option built in.
 

sergeauckland

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At the time I had my ill-fated HiFi shop, we did a few parallel conversions to Quad 405s to drive Mission Argonauts which had impedance dips to under 3 ohms. For various reasons to do with appearance, two 405s were deemed preferable to one more capable amplifier. I have to say, the conversions worked very well. The increase in output impedance was something like 0.1ohms, so not of much concern. Setup did need instrumentation, so not something most customers could do for themselves, but it did seem stable and effective.

S
 
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dlaloum

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At the time I had my ill-fated HiFi shop, we did a few parallel conversions to Quad 402s to drive Mission Argonauts which had impedance dips to under 3 ohms. For various reasons to do with appearance, two 405s were deemed preferable to one more capable amplifier. I have to say, the conversions worked very well. The increase in output impedance was something like 0.1ohms, so not of much concern. Setup did need instrumentation, so not something most customers could do for themselves, but it did seem stable and effective.

S
The Quad 405's were a particularly robust and stable design.... would this work with other amplifier types? Could you take a current constrained class-D amp and turn it into a heavy duty monoblock?
 

MakeMineVinyl

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See above warning.....if you have to ask. ;)
 
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dlaloum

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See above warning.....if you have to ask. ;)
I read that.... but I also saw the Peter Walker approved mod for his 405 design.... a highly respected amplifier designer - and if you read the linked PDF in my OP - you will see the circuit and setup/tuning details.

Yes the channels need to be matched.

There may be other "gotcha's" with different designs - eg: if the two channels being linked differ in their response to the environment (thermal or load response) - the more different the channels are the more issues are possible.

But the design assumes individual testing of each amp to match the channels.

Are there additional risks, with differing amp designs?

Is this something that Walker could do with his design, partly due to the relatively indestructible nature of his design? (some of these legendary amps survived extended periods of short circuit, in use at the BBC)

And if you do achieve it successfully - what would be the expected result?
What would be the expected change in specification of the resulting Frankenamp?
 
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