Parallel-mono operation: why aren't many amps able to do it? What are the downsides?

Kreshna

Kreshna

...but I have to know.
Bridging seems to adversely affect an amplifier's capability to drive low impedance load. An amplifier rated to drive a 2 ohm stereo load (2 ohms on each channel) is only going to be able to drive a 4 ohm (or higher) bridge-mono load. A 2 ohm bridge-mono load will more than likely destroy the said amplifier.

An amplifier running in bridge-mono operation produces twice as much voltage across the load.

However, there is also parallel-mono operation, which produces twice the current instead of twice the voltage. An example of amplifier which can run in parallel-mono is Crown Com-Tech series, like Crown CT 210, Crown CT 410, and Crown CT 810.

On page 35, Crown Com-Tech series manual, there is table showing minimum guaranteed power for Crown Com-Tech 210. And indeed, the differences between dual channel, bridge-mono, and parallel-mono operation are apparent.
  1. Dual channel has power figures for 4 ohms load and 8 ohms load.
  2. Bridge-mono has power figures for 8 ohms load and 16 ohms load.
  3. Parallel-mono has power figures for 2 ohms load and 4 ohms load.

Crown_CT_210_parallel_mode.jpg

Minimum guaranteed power for Crown CT 210. Notice the load differences between dual channel, bridge-mono, and parallel-mono.

I can see parallel-mono being useful for hard-to-drive loudspeakers. For example, Bowers & Wilkins DM302 has impedance drop as low as 3 ohms between 200 Hz and 300 Hz.

Mtz1rBc.jpg

B&W DM302 impedance curve.

So, my questions are: why aren't many amplifiers able to do parallel-mono operations? The only models I knew capable to perform parallel-mono operations are Crown Com-Tech series, while many other amps does not support such mode. For example, Crown XLS 1002 can only be bridged, but it cannot be paralleled.

What are the major downsides of parallel-mono, that keep many amplifiers from supporting such operation? Perhaps more distortions? Or perhaps such operation requires more difficult design and/or more expensive circuitry?
 
The answer, though not simple, mostly lies in Ohm's law. In bridge operation you drive one channel inverted. That means when one voltage goes up, the other goes down. Where one channel see a grounded load, now it can see the same load with the opposite supply on the other side. Twice the voltage, so twice the current, but four times the power. Power supply and output stage have to be designed to handle it, and "it" is way more stressful than non-bridge operation.

Parallel connections have their own problems. You have double the output stage, so current and dissipation hopefully won't be a problem. What is a problem is getting the two channels to share the load equally and be stable. Imagine if the gain is slightly different (and it always is). If the output impedance is low (good damping factor) almost infinite current will be drawn, trying to get the voltages equal. The channels will fight each other to the death. I suspect either some extra resistance has to be included to isolate the channels a bit, which is wasteful and wrecks damping factor, or some games have to be played with the feedback network, essentially turning two amplifiers into one. The design also has to be stable and not oscillate or misbehave under a wider range of conditions.

Being a bit vague and maybe clear as muc, hopefully others can explain this better.
 
IMO the biggest problem with parallel operation is that the amps have to be exactly matched. Most of the problems with parallel chip amps are due to slight differences in gain, causing one of the amps to fight with the other. The other factor is price. It is a lot cheaper to build an amp with more output transistors (i.e. for a lower impedance load) than to build two exactly matched amps.
 
McIntosh is the only HiFi brand that offers this feature that I'm aware - and only on amps with autoformers. In the pro world, it's a staple and all of those units are direct coupled.

Any stereo amplifier that offers mono-bridge and mono-parallel modes is quite flexible indeed.
 
ConradH said:
The answer, though not simple, mostly lies in Ohm's law. In bridge operation you drive one channel inverted. That means when one voltage goes up, the other goes down. Where one channel see a grounded load, now it can see the same load with the opposite supply on the other side. Twice the voltage, so twice the current, but four times the power. Power supply and output stage have to be designed to handle it, and "it" is way more stressful than non-bridge operation.

Parallel connections have their own problems. You have double the output stage, so current and dissipation hopefully won't be a problem. What is a problem is getting the two channels to share the load equally and be stable. Imagine if the gain is slightly different (and it always is). If the output impedance is low (good damping factor) almost infinite current will be drawn, trying to get the voltages equal. The channels will fight each other to the death. I suspect either some extra resistance has to be included to isolate the channels a bit, which is wasteful and wrecks damping factor, or some games have to be played with the feedback network, essentially turning two amplifiers into one. The design also has to be stable and not oscillate or misbehave under a wider range of conditions.

Being a bit vague and maybe clear as muc, hopefully others can explain this better.
Click to expand...
damacman said:
McIntosh is the only HiFi brand that offers this feature that I'm aware - and only on amps with autoformers. In the pro world, it's a staple and all of those units are direct coupled.

Any stereo amplifier that offers mono-bridge and mono-parallel modes is quite flexible indeed.
Click to expand...
I see. Thanks for the explanations, folks. I browsed quite a lot last night, and it seems parallel-mono operation is also called "amplifier strapping". Is that correct?

Also, it seems parallel-mono is simpler than bridge-mono, cabling-wise. You need to invert one input channel for the bridge-mono, while you only need to duplicate an input channel for parallel mono.


sregor said:
IMO the biggest problem with parallel operation is that the amps have to be exactly matched. Most of the problems with parallel chip amps are due to slight differences in gain, causing one of the amps to fight with the other. The other factor is price. It is a lot cheaper to build an amp with more output transistors (i.e. for a lower impedance load) than to build two exactly matched amps.
Click to expand...
Now, if an amplifier is not specifically mentioned to be able to do parallel-mono, but I connect the L/R channel for parallel operations anyway, what the worst could happen? For example, Crown XLS 1002 can be bridged, but nowhere the manual says it can be paralleled. If I wire the L/R channel for parallel mono operation, can the XLS 1002 handle it?

Also, what amplifier models are specifically mentioned to be able to handle parallel-mono? The only model I knew is Crown Com-Tech (CT 200, CT 210, etcetera); at least the manual mentions it. How about other Crowns?
 
Kreshna said:
I see. Thanks for the explanations, folks. I browsed quite a lot last night, and it seems parallel-mono operation is also called "amplifier strapping". Is that correct?

Also, it seems parallel-mono is simpler than bridge-mono, cabling-wise. You need to invert one input channel for the bridge-mono, while you only need to duplicate an input channel for parallel mono.



Now, if an amplifier is not specifically mentioned to be able to do parallel-mono, but I connect the L/R channel for parallel operations anyway, what the worst could happen? For example, Crown XLS 1002 can be bridged, but nowhere the manual says it can be paralleled. If I wire the L/R channel for parallel mono operation, can the XLS 1002 handle it?

Also, what amplifier models are specifically mentioned to be able to handle parallel-mono? The only model I knew is Crown Com-Tech (CT 200, CT 210, etcetera); at least the manual mentions it. How about other Crowns?
Click to expand...
You may enjoy this read - http://audiokarma.org/forums/index....s-strapping-the-differences-explained.688545/
 
ConradH said:
IMO, if it doesn't mention it, don't do it. The worst that could happen is you'll blow the output stages and/or the power supply. The typical amplifier is not going to be able to do it because of the mismatch problem. https://en.wikipedia.org/wiki/Bridged_and_paralleled_amplifiers
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I see. Thanks for the heads up. However, I'm still curious about a thing (please see below).

damacman said:
You may enjoy this read - http://audiokarma.org/forums/index....s-strapping-the-differences-explained.688545/
Click to expand...
This is what the OP said about parallel mono:

When it comes to HiFi power amplifiers, McIntosh is the only company I'm aware of that permits this and this applies only to McIntosh amplifiers with autoformers. In the pro industry they all do it - Crown being I believe the earliest adopter - and they're all direct coupled designs. Many McIntosh and Crown power amplifiers allow either Bridged-Mono or Parallel-Mono wiring schemes. This makes a power amplifier incredibly versatile.​

"In the pro industri they all do it." Does it mean all, or at least most, pro amp is designed to be strappable, even though it's not explicitly mentioned in the manual?
 
"All", "none", "always" and "never" are words that should raise alarm bells. If it isn't in the manual, or you didn't check with the factory and have them OK it, don't do it.

Now, I use the motto, "I fixed it once, I can fix it again." If I had some great burning desire to parallel channels on an amp with no data, I'd first put it on the test bench and adjust or alter components until the gains were exactly equal. Then I'd make the offsets as close to zero as possible, a few mV at most. Then I'd put about 0.25 ohms in series with each output before connecting them. Then I'd try it and see if there was any overheating or other misbehavior.
 
Kreshna said:
I see. Thanks for the heads up. However, I'm still curious about a thing (please see below).


This is what the OP said about parallel mono:

When it comes to HiFi power amplifiers, McIntosh is the only company I'm aware of that permits this and this applies only to McIntosh amplifiers with autoformers. In the pro industry they all do it - Crown being I believe the earliest adopter - and they're all direct coupled designs. Many McIntosh and Crown power amplifiers allow either Bridged-Mono or Parallel-Mono wiring schemes. This makes a power amplifier incredibly versatile.​

"In the pro industri they all do it." Does it mean all, or at least most, pro amp is designed to be strappable, even though it's not explicitly mentioned in the manual?
Click to expand...
They all do it, but not on all models. You'll note that a rear panel switch for this mode is provided for models that have this feature. If the amp lacks it, I sure wouldn't go about doing it anyhow - well, not unless it's your objective to destroy the amp on power up.
 
It all depends on the components that make the amplifier. Take a Mac MC 7200 for instance. rated at 200 watts and capable of oh so much more at 8 ohms. And what it will do at 1.66 ohms is scary as does not use autoformers. But it does need whisper fans when used at 2 ohms over a large spectrum pushing out 625 watts and close 725 watt peaks at 2 ohms and not 10 milli sec peaks like crown but 40 millisecond peaks. Macs current amps have a 3 db peak rating. So if you by a 611 thats over 1200 watts guys. The older 601 would put out close to 1000 watt peaks and close to 900 watts continuous way below rate specs of .005% distortion with 124 + db signal to noise. The new Stereo amps because of their Quad balanced configuration are just Stereo amps. But then the new 462 can put put 900 watt peaks and over 650 watts continuous. THe the little 152 rated at 150 watts a channel puts out close to 240 watts continuous at rated specs. But if you are looking for a great deal on A mac amp keep your eyes open for a 352 or a MC 500. They aren't Mac favorites but they can be had for steal some times and they are great performers. The 500 has over a 100 DF rating with autoformers and much higher below 7000 hz and thats amazing. Even though the new amps DF rating is only rated above 40, below the HF they are almost 10 times that high. So Mac continues to push the envelope. Its unfortunate the Church sold Crown to Harmon. Yes Harmon pushed Crown forward, but the service after the sale was never the same and they never strove for better sound. Sure they got more power out of a small chassis and they perfected remote controlled operation and monitoring, but that does us no good in our homes. If you really want outstanding Crown amps look for their Reference series. They only made two models, but they were real performers and leaders at the time of their production.
 
damacman said:
Oh, I see what you’re asking. I’ve only ever seen that feature on one amplifier - the Crown MacroTech 10,000 - of which you may never actually see one in the wild. But it’s discussed at length in the OM - https://www.crownaudio.com/en/products/macro-tech-series

Oh, and you'll need four of them . . .
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Yes, exactly. I just happen to have 4 Boothroyd Stuart Meridian 105s I've been meaning to do just that with. However, it should be mentioned that increasing output doesn't necessarily ensure sq will stay at par. It would serve as an interesting experiment, though. :)
 
Superampman said:
Yes, exactly. I just happen to have 4 Boothroyd Stuart Meridian 105s I've been meaning to do just that with. However, it should be mentioned that increasing output doesn't necessarily ensure sq will stay at par. It would serve as an interesting experiment, though. :)
Click to expand...
I had to google those - is that part of their feature set?
 
damacman said:
I had to google those - is that part of their feature set?
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The 105s were their entry into recognition in 1978 to compete with Naim. They were serious contenders and imo better. Extremely fast yet very musical and transparent. What they've produced since is no comparison.
 
damacman said:
They all do it, but not on all models. You'll note that a rear panel switch for this mode is provided for models that have this feature. If the amp lacks it, I sure wouldn't go about doing it anyhow - well, not unless it's your objective to destroy the amp on power up.
Click to expand...
Aw, I see. What amplifier models can be strapped? The only models I knew is Com-Tech series, like CT 200 and CT 400.
 
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