Marantz MA-500 Bridged for B&W 805S, is it safe?

[longhornguy]

Hello,

I am currently driving my B&W 805S with a Marantz SR8001 Receiver (~100W/Ch; 5 Channels Driven). It is in a Bi-Wire configuration (according to the manual), where I am using the 2nd zone receiver channels to power the high's of the B&W's. Basically, there are 4 receiver channels powering my front L and R's now.

I've recently upgraded my Center channel to use a Marantz MA 700 Monoblock (200W/ch) and I loved it. The voices are now clearer and the available headroom is very noticable.

Now I am thinking about upgrading the amplification for my B&W's with 4 Marantz MA 500 Monoblocks to really make them sing. As I've heard the 805S' really crave a lot more power than their 120W handling spec.

My plan is to use two MA-500's for each speaker in a bridged configuration, which should push 360W to each L/R speaker.

My question is has anyone attempted this set-up? I am unsure if the bridged configuration will do damage to my speakers. The manual states that the amp bridged will see an 8 ohm load and the B&W's are 8 ohm speakers, so it sounds like I would be ok. However, I've read on Audioholics that bridging an amp actually makes it see a load to half of what it usually produces. I do not want to damage my prized possession by bridging these amps to it!

This forum has really helped me a lot with my HT and appreciate any input anybody has in advance.

 

[TLS Guy]

Hello,

I am currently driving my B&W 805S with a Marantz SR8001 Receiver (~100W/Ch; 5 Channels Driven). It is in a Bi-Wire configuration (according to the manual), where I am using the 2nd zone receiver channels to power the high's of the B&W's. Basically, there are 4 receiver channels powering my front L and R's now.

I've recently upgraded my Center channel to use a Marantz MA 700 Monoblock (200W/ch) and I loved it. The voices are now clearer and the available headroom is very noticable.

Now I am thinking about upgrading the amplification for my B&W's with 4 Marantz MA 500 Monoblocks to really make them sing. As I've heard the 805S' really crave a lot more power than their 120W handling spec.

My plan is to use two MA-500's for each speaker in a bridged configuration, which should push 360W to each L/R speaker.

My question is has anyone attempted this set-up? I am unsure if the bridged configuration will do damage to my speakers. The manual states that the amp bridged will see an 8 ohm load and the B&W's are 8 ohm speakers, so it sounds like I would be ok. However, I've read on Audioholics that bridging an amp actually makes it see a load to half of what it usually produces. I do not want to damage my prized possession by bridging these amps to it!

This forum has really helped me a lot with my HT and appreciate any input anybody has in advance.

No it will not be safe. In bridged mode that MA 500 will need to see 8 ohms, or it will blow up. Those speakers drop impedance to four ohms.

What you are doing now buys you nothing. You are passive biamping, through the speakers passive crossovers. The tweeter takes only a watt or two at most, so you are gaining no power advantage, just complication.

Rather than invest in amps I would move up to the 803S or if you can spring it the 802D

120 Watts should be plenty for those speakers. You risk thermal compression effects pushing them harder, and possible damage of your ears can take it. If you want more power you need to add speaker voice coils first. Those are relatively small speakers you have, so you need to use common sense.

 

[longhornguy]

Thanks for your input, I knew A'holics would be able to give some good guidance.

One thing I want to make sure I understand clearly though:

You mentioned that the speakers will drop impedance to 4 ohms, is this because of the bridged power from the MA 500's?

The B&W specs have the 805S at 8 ohms, so I don't understand what it means when you mention they will "drop" to 4 ohms.

 

[WmAx]

Manufacture impedance specs mean nothing in my experience. Often, they are seemingly made up from thin air..... Most likely, with the driver configuration your speaker has, it dips to at least 4 Ohms such TLS Guy told you.

I would use at least 300 watts at 4 Ohms rated amplifier for those speakers to get great dynamics. If you can afford it, 500 watts would be optimal. I recommend forgetting 'consumer' amps if money is tight. The extra power, really, is just to take care of momentary transients. Pro amps offer more power, better build quality, and amplification as transparent as a good 'audiophile' amp. Yamaha makes a line of amps with no fan noise (P2500s, P3500S, P5000S and P7000S). Now, these have only balanced inputs and expect a rather higher input voltage. Use a device like the Art Cleanbox or Samson S-convert; these are quality line driver devices that up the consumer voltage from RCA jacks to a suitable ideal voltage to feed pro device XLR or 1/4" TRS balanced inputs.

As TLS Guy said, upgrade the speakers if possible - as this will always be your greatest sound quality improvement (along with the room acoustics).

-Chris

 

[TLS Guy]

Thanks for your input, I knew A'holics would be able to give some good guidance.

One thing I want to make sure I understand clearly though:

You mentioned that the speakers will drop impedance to 4 ohms, is this because of the bridged power from the MA 500's?

The B&W specs have the 805S at 8 ohms, so I don't understand what it means when you mention they will "drop" to 4 ohms.

Actually they drop to 3.7 ohms.

This issue has to be driven home again and again in these forums.

A speaker load is not like a resistor, and is a reactive load. So impedance varies with frequency. There is a tendency for impedance to rise with frequency.

However the big hooker is diffraction, or step loss compensation. This occurs from the frequency loss due to the narrow baffle, which for your speakers will be a 6 db per octave roll off starting about 600 Hz. Now this is compensated for in the crossover. However all compensation schemes involve dropping the impedance, to allow the amp to deliver the increased power to correct the 6db/ octave first order loss. So for every 3db boost double the amp power is required.

Now the acoustic spl power divide is 400 Hz. But the power delivered to a diffraction compensated speaker involves at a minimum 2/3 of the power delivered below 400 Hz.

So most of the power will be delivered into a four ohm load or less. There will be two tuning peaks of impedance related to the box tuning, but otherwise impedance will fall in the lower octaves.

Now speaker manufacturers average the impedance curve with frequency, but not the power curve. This creates the useless nominal impedance spec, to make you buy the speaker, because far too many amps don't perform well into 4 ohm loads or less, and in actuality, in most receivers, the power delivered to most speakers is half what is rated.

The bottom line, is that assume any decent modern speaker is actually four ohms or less and you won't go far wrong.

The situation is actually worse than I stated, because in inductive loads, voltage and current are out of phase. So the current required is much higher than ohms law would suggest based on phase angles, which few manufacturers publish.

I'm not a fan of bridged amps. I have seen too many disasters. The other problem is that the two channels have to be tweaked to be absolutely identical. This is because one amp is providing the +ve part of the wave form and the other the-ve. So this is a classic push pull arrangement. Any small difference between the amps result in heinous crossover distortion.

So except for some sub applications I steer people away from bridging.

 

[longhornguy]

Thanks for such detailed answers to my questions. Once again, this forum has helped me make the right decisions in building my HT and also educating me on the inner workings of Hi-Fi electronics.

Appreciate the explanations and suggestions. I will definitely stay away from the bridged MA-500's and probably have my eye on an Emotiva XPA-2 or Rotel 1080.

I naively was following the manufacturer's specs on impedance literally and it makes a lot of sense that they would fudge these numbers to makes sales easier.

 

[TLS Guy]

Thanks for such detailed answers to my questions. Once again, this forum has helped me make the right decisions in building my HT and also educating me on the inner workings of Hi-Fi electronics.

Appreciate the explanations and suggestions. I will definitely stay away from the bridged MA-500's and probably have my eye on an Emotiva XPA-2 or Rotel 1080.

I naively was following the manufacturer's specs on impedance literally and it makes a lot of sense that they would fudge these numbers to makes sales easier.

It is good to know you can make use of the information.

 

[PENG]

Before saying "No it will not be safe. In bridged mode that MA 500 will need to see 8 ohms, or it will blow up", let's consider some facts:

- If you look at most impedance/freq curves you will see the dips typically occur in narrow bands and not always in the power demanding low/mid low frequencies. Manufacturer's quoted 8 ohms nominal would actually be quite useful.

- Most mid range amps and receivers can handle such dips without problem provided they are of short duration (split seconds).

- Large phase angles that result in poor p.f. (hope you know what it is) don't necessarily coincide with low impedance.

- Most mid range amps/receivers deliver more power into 4 ohms, not halved, it could be closer to halved if the impedance is near 4 ohms all the time, but not if those dips are only at certain frequencies and are therefore of short time nature in real world music signals.

I am in agreement that more power is better but as Chris said it would be mostly be helpful for peaks. It is not hard to prove that you just don't get much more than 10 to 20 dB peaks very often in most recordings. Also, in not too large rooms listening at 75 to 85 dB SPL the average power draw by a speaker is only a few watts peaking to may be 50 to 60W, so you get the picture.

The OP will likely be fine and won't blow anything unless he cranks those Marantz monoblocks right up. So in a way it is a waste of time because he won't gain much, but it is not right to say it will blow up (unqualified).

Most any amp will blow something including itself if you turn the volume right up so what the heck is the big deal in this case. As long as those amps see around 6 to 8 ohms most of the time they will be fine and will have no trouble seeing 3.7 ohms or 1.85 ohms (amps bridged) effectively occasionally. It is no different than using an amp that is rated for 4 ohm nominal speakers when we know full well such speakers could dip well below 4 ohms.