[Paul Hynes]

Should you wire your line array drive units in parallel or series/parallel? Here are some useful observations that may be of interest to array builders.

Moving coil drive units have complex reactive impedance characteristics. If all the drive units are wired in parallel the reactive impedance is relatively benign as the amplifier has direct control over all the drive units.

Wiring drive units in series/parallel causes considerable additional electrical reaction and interaction between the drive units. This is because the amplifier can no longer exert control as it is attempting to drive some drive units via the reactive impedance of the other drive units.

My eight-way Visaton B200 open baffle line arrays are wired in such a way that I can effectively program parallel or series/parallel operation using 4mm plugs and sockets.

If I choose parallel operation I have a convincing illusion of real musicians. The music is effortless, focussed and full of inner beauty and I don’t suffer fatigue no matter how long I listen to music.

If I choose series/parallel operation the musicians get lost and confused in the mists of time and space. The confusion with this setting is tiring and fatigue sets in rapidly.

The nominal impedance of eight B200 drive units in parallel is 0.75 ohms. This looks like an amplifier punishing load. However most solid-state amplifiers will muster a few watts into such a load without self-destruction as long as you are careful with the volume control. Some valve amplifiers may also work well if they have low impedance taps on the output transformer. I wouldn’t recommend connecting such a load to the 4 or 8 ohm taps of a valve amplifier as it may move the valve operating characteristics out of their linear and/or safe area. The efficiency of eight B200 drive units in parallel is around 105 dB per watt at 1 metre over much of the operating frequency range and a few watts go a long way with this level of efficiency. Last week I drove the B200 line arrays with a hybrid circlotron amplifier rated at 100 watts into 8 ohms. The amplifier coped admirably and achieved realistic sound pressure levels with the lines.

The drive units in most line arrays are wired in series/parallel to ensure a relatively easy load for generally available amplifiers so they don’t suffer overload. The sonic difference between parallel and series/parallel operation is not subtle, yet most line array builders are unaware of the benefits of parallel operation because they automatically go for series/parallel operation to be kind to their amplifiers.

“Tight lines” as they say in the angling world.

Warning! Rick Craig has rightly pointed out that I had omitted to include a warning to the inexperienced Audiocircle readers about using low impedance loads on standard amplifiers. If you don't understand the issues involved here do not follow my suggestions. For those of you who have a sound knowledge of the issues, and wish to do a short trial to verify the benefits of parallel drive operation, go easy on the volume control and if the amp sounds stressed in any way stop the trial. An alternative is to look for an amplifier that is designed specifically to drive low impedance loads. If there is enough interest in such an amplifier I would be pleased to offer a DIY design to the circle.

Regards
Paul

[Rick Craig]

Should you wire your line array drive units in parallel or series/parallel? Here are some useful observations that may be of interest to array builders.

Moving coil drive units have complex reactive impedance characteristics. If all the drive units are wired in parallel the reactive impedance is relatively benign as the amplifier has direct control over all the drive units.

Wiring drive units in series/parallel causes considerable additional electrical reaction and interaction between the drive units. This is because the amplifier can no longer exert control as it is attempting to drive some drive units via the reactive impedance of the other drive units.

My eight-way Visaton B200 open baffle line arrays are wired in such a way that I can effectively program parallel or series/parallel operation using 4mm plugs and sockets.

If I choose parallel operation I have a convincing illusion of real musicians. The music is effortless, focussed and full of inner beauty and I don’t suffer fatigue no matter how long I listen to music.

If I choose series/parallel operation the musicians get lost and confused in the mists of time and space. The confusion with this setting is tiring and fatigue sets in rapidly.

The nominal impedance of eight B200 drive units in parallel is 0.75 ohms. This looks like an amplifier punishing load. However most solid-state amplifiers will muster a few watts into such a load without self-destruction as long as you are careful with the volume control. Some valve amplifiers may also work well if they have low impedance taps on the output transformer. I wouldn’t recommend connecting such a load to the 4 or 8 ohm taps of a valve amplifier as it may move the valve operating characteristics out of their linear and/or safe area. The efficiency of eight B200 drive units in parallel is around 105 dB per watt at 1 metre over much of the operating frequency range and a few watts go a long way with this level of efficiency. Last week I drove the B200 line arrays with a hybrid circlotron amplifier rated at 100 watts into 8 ohms. The amplifier coped admirably and achieved realistic sound pressure levels with the lines.

The drive units in most line arrays are wired in series/parallel to ensure a relatively easy load for generally available amplifiers so they don’t suffer overload. The sonic difference between parallel and series/parallel operation is not subtle, yet most line array builders are unaware of the benefits of parallel operation because they automatically go for series/parallel operation to be kind to their amplifiers.

“Tight lines” as they say in the angling world.

Regards
Paul

I'm sorry Paul but this is really bad advice. With that low of an impedance you risk damaging your amplifier and any tube amp with an average or high output impedance will also react to the low impedance load and skew the response. I've built several arrays with series/parallel wiring and they work fine.

[JohninCR]

I'm lucky, my Decware tube amps prefer low impedance and can handle anything down to a dead short.  I too have always preferred the sound of parallel connections compared to any series wiring, not just with arrays and not just with these amps.  I ran across what seemed to be a valid engineering explanation once over at DIYaudio, but unfortunately I can't remember what it is.

[Paul Hynes]

Hello Rick,

I understand your concerns regarding driving low impedance loads, which is why I suggested care with the volume control. A few watts into 0.75 ohm requires around 2.5 amps and most solid state amps would not see this as a problem for a short trial to verify the benefits of parallel operation. If you were over enthusiastic with the volume control then there could be problems. I had meant to include this caveat in the post and will return and edit it so that the inexperienced understand the dangers of overdriving into such a load. Thank you for bringing this omission to my attention. The last thing I wish to do is encourage people to destroy their amplifiers.

I did in fact say that I didn’t recommend doing this with valve amplifiers with 4 or 8 ohm taps, only those with specific low impedance taps, for instance JohninCR’s Decware amplifier.

Regards
Paul

[Occam]

..... An alternative is to look for an amplifier that is designed specifically to drive low impedance loads. If there is enough interest in such an amplifier I would be pleased to offer a DIY design to the circle.

Yes please, as its rather difficult to find a ML-25. With the requisite rails, at least you minimize the issues of secondary (voltage dependant) output device (BJT) breakdown.....

[Paul Hynes]

Hello Occam,

I see you have noticed that there aren’t too many amps around for this job.

Thanks for starting the ball rolling.

Perhaps you and any other interested members could begin a wish list of parameters that are relevant to your particular line arrays. This would be very helpful with regard to sizing the proposed amplifier. Details like nominal line impedance, overall efficiency, SPL Target. I suspect that there will be quite a variation in requirements.

The two designs I have in mind are simple and adaptable. One is single-ended pure class A (no requests for a one kilowatt version of this please), the other is based on a balanced bridge which can be biased in class A for low powers or high bias class AB for higher powers. Lets see what the wish list turns up.

Regards
Paul

[JohninCR]

Maybe Occam can shed some light on the series/parallel issue.  I've noted obvious sonic differences.  Is it the amplifier acting differently or does sending the current through one driver then the next in a series connection cause degradation in some form (phase shift, back emf, just the increased inductance, etc.)?

[DaveC113]

Would an autoformer (like the Speltz zeroformer) be able to multiply the impedence of a line array so most amps would be able to drive them with the drivers wired in parallel?

Dave

[FredT300B]

I'm seeing two issues here. One is about the effect of series/parallel wiring on an amplifier's ability to control the drivers. The other is whether an amplifier can safely drive a very low impedence load.

I don't know the answer to either question, but I am aware that it costs more to build an amplifier that has the high currrent capacity to safely drive a very low impedance load. Many mfg don't build that capability into all their products because it would require a price increase to satisfy the small number of audiophiles who need that capability. For an example, note the differences between the Van Alstine 240EX, the 260EX, and the 260EXH in the link below:
http://www.avahifi.com/root/equipment/amplifier/comparison_grid.htm

[Occam]

Paul,

That 'yes please' was based upon my (minimal and misdirected) intellectual curiosity on amp topology and implementation, and the fact that I'm the facilitator of the Lab Circle, and like diy projects.  I'm not an array/line source sort of a guy, so I can't offer specific requirements.

I also tend towards a 'optimal' (with 15-25mv across the emitter resistors) AB BJT EF output stages which are obviously not your intended direction. (horses for specific courses).
I am curious as to your potential choice of a balanced bridge topology. Given the impedances you're dealing with, I'd imagine the rails would be sufficiently below the secondary breakdown voltages, at least for a BJT output stage, that bridging wouldn't be that appealing, at least insofar as you'd have to (approximately) halve the rails, and each half would be 'seeing'  a nominal impedance of 1/2 that 0R75, .375Ohms.[obligatory - All Things Being Equal] Such an amp would be a Herculean accomplishment.
I'm really looking forward to an amp design that could elegantly and economically deliver to such a punishing load. I'm having a difficult time conceptualizing the net result of a series and/or parallel reactive network, Paralleled the capacitance adds , the inductance [1/(1/I1 + 1/I2+...)]'s, but I believe each drive sees its prorata portion, assuming a perfect source, but the series(/parallel) connections are far more difficult for me to grasp.

Alternatively, a purpose build output autotransformer, similar to Paul Speltz's -
http://www.zeroimpedance.com/
might do the trick.....
(I see Dave already suggested this)

Regardless of the chosen topology, if you're going to use a BJT output stage, and if paralleling of the output devices is required, the newest OnSemi audio power devices are amazingly well matched both within type and for their complements. They're reasonably priced and available, at least in the States, and if needed, a group buy could be arranged. But then again, what you've described has a serious MOSFET vibe about it.

Regards,
Paul

[Paul Hynes]

Hi all,

Before the flames are lit, let me point out that I am not a loudspeaker designer by trade. I am an enthusiast who has built many different types of loudspeaker over the years and I am looking for my Holy Grail just like many of you. My knowledge of drive unit behaviour is consequently limited to my practical experience. In this field I tend to rely on logic to provide me with answers and I use my ears as the final arbiter as I am the one who has to listen to my system.

Hello JohninCR,

I think the interactions are quite complex but I suspect that back emf plays a large part in what we have both experienced with series drive unit connection.

Parallel connection would reduce the overall inductance of the array, and each drive unit would see the damping effect of the amplifier output impedance.

Series connection places a boingy electrical spring in series with the drive units reducing the electrical damping. This has to alter the vector of the music waveform envelope in an audible way.

Hello DaveC113,

A transformer designed for this function would be one possible solution. I haven’t tried this so I can’t comment further on the sonic issues.

Hello FredT300B,

I think the series/parallel debate is well worth pursuing, as the benefits of parallel operation are audible.

I wouldn’t advocate using an amplifier designed for normal operation above 4 ohms permanently on lower load impedances. Sooner or later an accidental abuse will occur and many amplifiers will not like it. For permanent use the amplifier must be designed to work safely into low impedance loads. I’ve tried to illuminate the issues related to my initial post as clearly as possible. If it will help remove any further consternation I will go back and edit out all reference to conducting a careful trial using a standard amplifier into a low impedance parallel line from my posts. Please advise me of your opinion here, Fred.

Hello Paul (Occam),

I am sorry to say that time has run out and I must go to work. I will answer your post later when I get free.

Regards
Paul

[FredT300B]

If it will help remove any further consternation I will go back and edit out all reference to conducting a careful trial using a standard amplifier into a low impedance parallel line from my posts. Please advise me of your opinion here, Fred.

My opinion is that you should leave it just as it is.  This is a good thread that had prompted some useful information. Nobody should hesitate to start a thread or to "sanitize" it just because it might spark some disagreement. Sacred cows aren't sacred in this circle.

I have noticed that people (like me) who are into special interest components (line arrays, OB speakers, single driver speakers, SET amps, etc) tend to be more passionate about our interests than other audiophiles who's systems are comprised of mainstream components. So what's wrong with a little passion as long as we don't get too upset with anybody who disagrees with us. 

[Paul Hynes]

Hi Paul (Occam),

Hi Fred (FredT300B)

Sorry about the tardy reply, Guys, I’ve been snowed under with work trying to get all current orders out before we relocate to the Isle of South Uist during the next month.

Paul, I take your point about not being a line enthusiast. However, there are other applications for a low impedance amplifier, for instance driving ribbons, so others may find the project of interest.

Fred, I wasn’t worried about sparking disagreement, just concerned about anyone getting into trouble with my original trial suggestion. Thanks for the support. Let the passion begin.

I have given the parallel drive unit array project further thought, in the odd free moment since my last post. The sonic benefit of parallel operation over series/parallel operation is not subtle as far as I am concerned. There appears to be little interest in parallel operation at present but the result is well worth the effort. In order to encourage constructors to experiment I will prepare an amplifier project for DIY. This may take a few months to prepare, due to the logistics of our home re-location and the need to get business activities back on line as soon as possible. In the meantime, if anyone wishes to discuss the matter further, feel free to post. I will try to answer any questions directed at me as quickly as possible, but please understand that over the next month I may be unavailable for periods of up to five days at a time while moving the office and home goods to Uist.

From an engineering aspect, I think that a modular approach to amplification would be the best path forward. This is because some constructors may decide to build a floor to ceiling array using a large quantity of small drivers, which makes a one size fit all approach more complicated due to the very low impedance such a line would present to the amplifier. Sizing the output stage/power supply to drive up to 8 parallel drive units seems a sensible approach. Line arrays with more drive units can be divided into sections with an equal number of drive units and an output stage for each section. Due to the efficiency gains of the line array approach, the amplifier does not have to be particularly large for decent sound pressure levels, provided drive units of reasonable efficiency are chosen. Comments, ideas and suggestions are welcome.

On the issue of driving low impedance loads the mosfet output stage I am using at present on my Visaton B200 parallel line array has been happily driving a nominal impedance of 0.75 ohm for 9 months continuously.

Regards
Paul

[darkmoebius]

I have given the parallel drive unit array project further thought, in the odd free moment since my last post. The sonic benefit of parallel operation over series/parallel operation is not subtle as far as I am concerned. There appears to be little interest in parallel operation at present but the result is well worth the effort. In order to encourage constructors to experiment I will prepare an amplifier project for DIY.

I've just started doing research on arrays, but the ability to design a dedicated amp for each array in 3-4 way arrays seems like a great way to achieve optimal performance. Especially one for the bass and mid-bass arrays.

How have you progressed with the concept in the last 7 months?

[FredT300B]

A "distribution" amplifier might be a good match for a line array with parallel-wired drivers. Here's one with 12 channels stable into four ohms:

http://www.ati-amp.com/at6012.html

[Andrikos]

Interesting subject.

I cannot attest to Paul's claim that paralleled array drivers sound better (more time-phase focused) than series-// drivers but I will take his word for it.

My line arrays are series-// connected to ~6ohm load for each of the midwoof (8 x Vifa PL19W09) and 6 x BG Neo8PDRs.

I'm willing to experiment on this and convince myself.
I do agree that theoretically the concept has its merits especially when we're talking about non-matched production variable drivers.

I can see how the amp can "see" different loads (RLC) from the series-// combos.
I can also visualize how an all parallel configuration can make the amp "see" the individual driver coils and be able to drive them with more cohesion.

My recommendation would use high Re drivers put them in parallel and keep the combination to 2ohms minimum.
For more drivers, add more amps.
Naturally, I'm talking about active line arrays.
I wonder if the results will justify the cost penalty.

[darkmoebius]

A "distribution" amplifier might be a good match for a line array with parallel-wired drivers. Here's one with 12 channels stable into four ohms:http://www.ati-amp.com/at6012.html

I remember looking at AT's amps a few years back, that company delivers great performance at surprisingly low prices. But....

I'm not sure they are good to the 0.75 ohm array load that P. Hynes is talking about. That's dangerously close to a short circuit.

A good, low-cost, alternative would be something like a 20 year old 255wpc Hafler DH-500 amp. One of the original commercial "Monster Amps" famous for handling ever the toughest of loads. The manual, which I have, claims:

The over-sized power transformer, the conservative operating levels of the  MOSFETs,  the computer-grade  electrolytics  totalling 40,000 microfarads, the enclosed relay, the use of film capacitors in signal circuits-all are evidence of the design efforts to achieve exceptional reliability simultaneously with state of the art  sonics  and specifications. So rugged is the DH-500 that it can deliver over 20 amperes into a short circuit!

Power Rating:  Less than  0.025% total harmonic distortion at any power level up to 255
watts continuous average power per channel into 8 ohms at any frequency between
20 Hz and 20  kHz with both channels driven.
IM Distortion (SMPTE): Less than  0.007% from 1 watt to 255 watts into 8 ohms
Typical THD at 255 watts into 8 ohms:  1 kHz-0.002%;  10 kHz-0.007%
Frequency Response into 8 ohms: -3 dB, 0.5 Hz to 120  kHz at 1 watt
kO.5 dB, 5 Hz to 40  kHz at 255 watts
Typical Channel Separation at 1 kHz:  60 dB
Signal to Noise Ratio:  Exceeds 100  dB  referred to 255 watts into 8 ohms, unweighted
Exceeds 90  dB referred to 1 watt into 8 ohms, A weighting
Input Impedance: 47,000  ohms
Input Sensitivity: 2.35  volts for 255 watts into 8 ohms; 0.145 volts for 1 watt
Damping Factor: 200  to 1 kHz into 8 ohms; 60 to 10  kHz into 8 ohms
Rise Time: 10 kHz, 80 volts p/p square wave, 10% to 90%: 2.5 us.
Slew Rate: 1 kHz, 120 volts p/p square wave: 45 V/us.
Semiconductor Complement: 27  transistors, 12 power Mosfets, 33 diodes, 8 zener diodes,
one integrated circuit, one diode bridge.
Power Consumption: 240 VA quiescent; 1200 VA @ rated power into 8 ohms, 2 channels

There are some mod's that can be done to these relics that would supposedly improve the overall sonics. But, I actually wonder how they would sound stock on a ~100-105dB parallel array where the amps would be using less than 0.5% of their full output potential most of the time. Even under the worst conditions, I can't see demand going over 50 wpc in most demanding dynamic bursts.

The home version of the DH-500's usually sell for $250-350 in near mint condition these days. One  stereo amp for each matching two arrays. So, three for a 3-way at total cost of $750.

[FredT300B]

Regarding the 12 channel amp, I was thinking you would parallel channels so each channel was connected to only two eight ohm parallel drivers, an easy four ohm load. This takes care of the low impedance issue.

[darkmoebius]

Regarding the 12 channel amp, I was thinking you would parallel channels so each channel was connected to only two eight ohm parallel drivers, an easy four ohm load. This takes care of the low impedance issue.

  sorry, I should have made that connection. Could be a great solution. I wonder how they sound and if their presentation on regular 3-ways is any indication of their sound on arrays?

[Paul Hynes]

Hello darkmoebius,

I have been too busy dealing with the issues of moving into an uncompleted house renovation and setting up the new workshop/office to get on with the parallel driver low impedance amplifier project. Also, as little interest has been shown on the thread, I didn’t give it any priority as I already have the amplifiers in my system, so I wasn’t missing anything.

I did order all the materials to prepare the project and I verified that the mosfet, heatsink and transformer suppliers would be happy to supply individuals on a worldwide basis. I could re-introduce the project to my work schedule if enough interest is shown. So, if anyone is interested in such an amplifier, make yourself known on this thread.

You are right about the power requirements. If you chose high efficiency drive units with the efficiency gain of the line source you will need very little power for realistic sound pressure levels. I’m still using the original 2 watt into 0.75 ohm amp in my system. The system can go very loud. Only rarely do I feel the need for more power. This could change if I ever find time to build a dedicated listening room.

Hello FredT300B,

If the distribution amp sounds musical then this would be a good solution.

Hello Andrikos,

I did look into high impedance drivers but couldn’t find a likely “off the shelf” candidate at the time. I also approached a drive unit manufacturer about having specials done but he didn’t seem too enthusiastic about making a relatively small batch for me. So I thought, what the hell, I will design a low impedance amplifier and then the drive units will no longer be a problem. At the time I was assessing the Visaton B200 in open baffles and liked what they were doing enough to want to rig up a B200 line source. Fortunately they are very efficient drive units requiring little power to move a lot of air when configured in a parallel driver line source. I am still very happy with the results.

Regards
Paul