Vivek said:
Vivtek,
If you were to BiAmp to use an active crossover after the pre-amp for the subwoofer and the infinity woofers are just drivers with no plate amplifier then you could go active on the amplifiers.
I am not sure what your reference to "cheaper" implies. Systems with active crossovers are not cheaper as rule than the passive based crossover systems in overall costs.
That said active based crossover systems are much better. Even the difference of going to a BiAmp crossobver where the woofer has own amplifier and the midrainge/tweeter are still a passive based system is a major improvement in audio quality. This is because IT and TIM are reduced as each amplifier is handling a narower frequency rance to reproduce, you eliminate the first and major challenge to the amplifier in terms of speaker load by elimiating the large inductors and capacitors associated with the woofer section of an active crossover and you increase damping of of a speaker when it is directly connected with no passive crossover inbetween with the amplifier. The last point is most important for woofers.
Your savings are more in the reduce power required from the amplifiers you need to build to drive the speakers, woofers in your case. Your woofer has two ratings, a 90dB and a 300W. The first rating you need to look at is the sensitivity rating which is the 90dB. That means a at 1W your woofer will reproduce 90dB at 1 meter in an anechoic chamber. Woofers in most normal listening rooms will sound louder than 90dB as the room is much smaller than the 1/4 wavelength of the woofer frequencies. For the sake of this discussion I will stick with the 90dB. At 2 meters the sensitivity would be 84dB based on the inversely proportional behaviour of sound and light. 6dB is the dB equal to twice as loud or have as loud depending which way you are going. Your woofer would need 10W (+9.97dB) to reach 100dB, and 101.6W to reach 110dB for an anechoic chamber when driving the speaker directly with no passive crossover as is done for speaker sensitivity tests and listed ratings.
As a room is not an anechoic chamber there will be gains and losses for room volume/boundaries and absorbing. It is far too complicated to model, let alone calculate these effects in any detail. So some simple rules are applied based on speaker position to wall(s) and floor. For each boundary the woofer is close to in terms of 1/4 wavelength one adds 3dB. For room volume I cannot guide you on the gain there. For room dimensions and boundaries there will be peaks and dips that restul from the room dimensions. These room effects are not impacted by a active or passive crossover design speaker system.
The point here is you can generally have 6 - 12 dB gain due to room volume and boundaries for your woofer and decrease as one goes up in frequency. This means for 10W (+9.97dB) 106-112dB, 100W (+12.58dB ) 119-125dB and 200W (+22.93dB) 119-125dB with room effects added in. These are ear damaging levels for anything from short term to instant listening.
Because your speaker has a 300W rating you do not need to go to 300W. 300W (+24.7dB) is about 115dB in an anechoic chamber and about 121-127dB in a room.
The cheaper is a result of needing less power to drive the speaker which means less current demands on the amplifier, hence smaller transformer and not as many or less powerful output devices, smaller heatsink with a smaller PSU filtering capacitance and filter capacitor voltage. The savings is mostly in the PSU and output device heatsinking. Each amp module for a 4 ohm is 10W/6.33V/1.6A/10VA, 100W/20V/5A/100VA, 200W/28.3/7.1A/200VA and 300W/34.7V/8.67/300VA before bridge, filter capacitor. PSU sagging and MOSFET losses which can be 12-17V P-P. Using the speaker impediance/2 rule of thumb to estimate the reactive load on the amplifier with a speaker per amp module is 10W/4.5/2.24/10A/45VA, 100W/14.15V/7.1/100VA, 200W/20V/10A/200VA, and 300W/24.5V/12.25A/300VA ignoring transformer sagging, PSU losses and MOSFET losses of about 14-20V P-P.
Regards,
John L. Males
Willowdale, Ontario
Canada
04 August 2007 (07:30 -) 10:19
04 August 2007 (10:20 -) 10:30 Added in 200W calculations. jlm
Official Quasi thread Researcher
Vivek , if I'm not mistaken the paper used by our transformer winders is called leatheroid. It comes in different thicknesses and colours.It's available from any retail enamel wire dealer . They cater to the small transformer winders.
Removing the core is not very easy. The whole core is dipped in insulating varnish . Small manufacturers use air drying varnish and bigger guys dry them in an oven and just a handful will vacuum varnish them.
You will have to hold the transformer in a vise and keeping the laminations that you plan to knock off , free of the jaws of the vise.
Then you remove one " I " lamination and using a very thin screwdrive or steel sheet ( not another lamination ) you should slowly knock out the E layer. This could be quite tough and you can damage the transformer coil if you are not very careful.
Try it on an old junked transformer and practice till you are confident of doing it without damage.
Once one E-I set comes out it gets easier removing the others.
It is a time consuming job removing them all !
Have fun.
Cheers.
Removing the core is not very easy. The whole core is dipped in insulating varnish . Small manufacturers use air drying varnish and bigger guys dry them in an oven and just a handful will vacuum varnish them.
You will have to hold the transformer in a vise and keeping the laminations that you plan to knock off , free of the jaws of the vise.
Then you remove one " I " lamination and using a very thin screwdrive or steel sheet ( not another lamination ) you should slowly knock out the E layer. This could be quite tough and you can damage the transformer coil if you are not very careful.
Try it on an old junked transformer and practice till you are confident of doing it without damage.
Once one E-I set comes out it gets easier removing the others.
It is a time consuming job removing them all !
Have fun.
Cheers.
John, Thanks for that reply. But I am sorry I did not mention that the subs I am building are for HT. The fronts are a pair of Vifa MTMs.
The reason I was asking whether 200W would suffice is because there might be a tendency to pump up the volume on the sub amps. I was worried about me driving the 200W amp into clipping. Also, I was told by my friends in the JBL store that this is a power-hungry woofer. As I understand it, more the power, cleaner the sound, right?
What I meant by cheaper is that, if a 200W module is good enough for each driver, I would be saving a lot as compared to building two 300W ones.
Ashok, this tansformer I have is so thick in varnish. I do not think it will be easy unless I forcibly rip out the insulation. In that case, I think it will be very easy to damage the winding.
Cheers,
Vivek
The reason I was asking whether 200W would suffice is because there might be a tendency to pump up the volume on the sub amps. I was worried about me driving the 200W amp into clipping. Also, I was told by my friends in the JBL store that this is a power-hungry woofer. As I understand it, more the power, cleaner the sound, right?
What I meant by cheaper is that, if a 200W module is good enough for each driver, I would be saving a lot as compared to building two 300W ones.
Ashok, this tansformer I have is so thick in varnish. I do not think it will be easy unless I forcibly rip out the insulation. In that case, I think it will be very easy to damage the winding.
Cheers,
Vivek
Vivek said:
Vivek,
I was replying in the context of HT, be it you have a single subwoofer from the HT amplifier to the subwoofer or if you were subwoofering each channel of the HT. (I intend to subwoofer each channel for my HT.)
The points I made about using an active crossover remain the same. In the HT case the .1 is the subwoofer centre channel. If your HT amp already has a LP filter for the subwoofer out of you HT amp then all you need to do is build one quasi amp which will be your subwoofer amp. In terms what size of quasi to build I would be certain 200W RMS would be more than plenty of power for the subwoofer.
The reference to the subwoofers you wish to build being power hungry is a myth. If your completed subwoofer system has a very low sensitivity then that is what some might call power hungry. As an example if you could buy two different completed subwoofer with no plate amp and one had a sensitivity of 81dB/1W and the other was 87dB one then the 81dB speaker would need an amplifier 4 times the power to product the same sound volume. So if you used a 50W amplifier with the 90dB/1W speaker you would need a 200W amplifier for the 81dB amplifier. That said if your MTM speakers were 90dB sensitivity and were connected to a 50W amplifier, then the 87dB/1W subwoofer would need an amplifier of 100W to match the MTM speakers and the 81dB/1W would need a 400W amplifier to match the power and dynamic levels of the MTM amplifiers all else being equal.
As you can see a speaker, subwoofer or any other speaker, with a lower sensitivity needs more amplifier power to acheive the same volume as a speaker with a higher sensitivity. It is like comparing a small car to a B747-500. The small car uses less fuel to reach and maintain the same speed compared to a B747-500. An extreme example, but it helps make the point about efficiency and what is power hungry. It is all relative as then again the A380 will have different fuel demans than a B747-500.
The one certain thing is a dedicated subwoofer will need a smaller aplifier than if the subwoofer is connected to the MTM via a passive crossover in the subwoofer. There are very few passive crossovers in a subwoofer built today commerically and most DIY would not use a pasive crossover for the subwoofer.
Where you have to be careful is on the total power handing of the subwoofer. For example if the subwoofer has a sensitivity of 81dB and a rated power handling of 300W (+24.7dB) then at best you can only reach 105.7dB excluding room factors, whereas if the MTMs were 90dB sensitivity they would only need 32W to reach 105dB. So if you need the subwoofer to go run at or go above 105dB you will damage your amplifier due to amplifier clipping which will not only damage the subwoofer amplifier, but will fry the subwoofer voice coil in process likely starting at 102dB. Going back to the small care vs B747-500 example, the small car will never acheive air speeds the B747-500 can on the road and if you try you will burn out the engine of the small car.
The difference between subwoofers and the rest of the speaker is the room tends to add volume to the subwoofer and remove volume from the MTM mid/high speakers due to the difference in 1/4 wave distance, room nodes and more upper frequency absorption by small soft surfaces.
So you first need to look at the sensitivity of all of your speakers you wish to use for the HT system and determine the lowest and highest sensitivity, then for each based on their power handling rating determine the most they can handle in dB terms and then decide if you have enough power and what power you need. I have a 8 ohms rated speaker syste, using a passive crossover and my typical loudish levels measure about 2-4Vrms which is about 1-2W and these speakers are handling the low frequencies as well. My current HT amp allows me to not use a subwoofer and send the subwoofer signal to my L and R speakers.
The only reason I can think of a speaker being refeered to as "power hungry" is because of a low sensitivity. No speaker you raise the volume on requires more "power" than another unless the speaker is so poor design to have all sorts of reactive elements to it's impediance. It that were case the speaker would sound like garbage in all likelyhood.
I forgot to mention in my last post that although you need to calculate power of impediance/2, you should also calculate impedance*10 so the amplifier has enough voltage to drive the subwoofer through its resonant frequency. At resonant frequency the impedance peaks very high. so while the current demand will be lower due to the impedance peak, the voltage required to drive the subwoofer will be a much higher need. This can confuse people as one may need a rails of >=60V to accomodate the impedance peaks people will assume that the speaker and amplifier can run at the full power of the rails for extended time. Not the case. One overbuilds to accomodate the impedance peaks that are with any speaker, woofer, mid, tweeter, etc, the current demands for normal and dynamic headroom plus reserve. Bear in mind if one does not provide a PSU to compliment the designed abilities of the amp module then the amp module will be limited in providing the power the amp needs leading to a lower power amp than perhaps on thought might be what they had designed for.
In many HT systems the subwoofer one buys has a plate amp built into the subwoofer and usually has two types of inputs - Line and Speaker. Most of these plate amps will have some controls for selecting the crossover frequency for the subwoofer and matching the level with the rest of the speakers in the HT system. The active crossover that is part of the plate amp is built as part of the front end of the plate amp. The plate amp active crossover is redundant if the HT amp already has a LP filter on the .1 subwoofer out. A HT with a subwoofer out is line level no matter if it has a LP filter for the subwoofer or not.
In terms of costs, I answered that in my previous reply and where the costs are saved.
Regards,
John L. Males
Willowdale, Ontario
Canada
04 August 2007 (13:50 -) 15:10
04 August 2007 15:15 Added a one line answer to one question and made a typo correction. jlm
Official Quasi Thread Researcher
Hi Vivek,
Just one comment here.
Avoid very inefficient speakers. Most of the power you deliver to them is turned into heat. The hotter a voice coil gets, the less efficient it will be and the more the specs will change on it. You need to keep the voice coil as cool running as you can.
For sub woofers, you want to build the most efficient system you can so that you have large headroom for big sounds. I did build 8" systems that had a -3dB point below 30 Hz. 😀 People kept looking for the non-existant subs. I've also built subs in large boxes (more efficient) that had their -3dB points below 20 Hz, the boxes were 8,000 cu in. - two in the system. A 50 wpc amplifier was all it took to shake everything in the house. Satisfied owner - you bet.
There is no fooling physics. If you want efficient sub woofers, the boxes will be large but the power requirement to drive them will be low. This is obeying those laws. A tiny box will require a large amp and do not sound nearly as good as a proper B4 type box.
Look for a real subwoofer, I think JBL forgot how to build speakers. The less power you need, the cleaner the sound will be. I think they forgot how that relationship works as well.
-Chris
Just one comment here.
Avoid very inefficient speakers. Most of the power you deliver to them is turned into heat. The hotter a voice coil gets, the less efficient it will be and the more the specs will change on it. You need to keep the voice coil as cool running as you can.
For sub woofers, you want to build the most efficient system you can so that you have large headroom for big sounds. I did build 8" systems that had a -3dB point below 30 Hz. 😀 People kept looking for the non-existant subs. I've also built subs in large boxes (more efficient) that had their -3dB points below 20 Hz, the boxes were 8,000 cu in. - two in the system. A 50 wpc amplifier was all it took to shake everything in the house. Satisfied owner - you bet.
There is no fooling physics. If you want efficient sub woofers, the boxes will be large but the power requirement to drive them will be low. This is obeying those laws. A tiny box will require a large amp and do not sound nearly as good as a proper B4 type box.
Look for a real subwoofer, I think JBL forgot how to build speakers. The less power you need, the cleaner the sound will be. I think they forgot how that relationship works as well.
-Chris
Good point Chris,
Lets hope the kids in them "doof doof" cars never realise that a good PA driver (fs 40Hz, Eff 100dB+) can destroy the competition with only a single 100 watt amp.
Q
Lets hope the kids in them "doof doof" cars never realise that a good PA driver (fs 40Hz, Eff 100dB+) can destroy the competition with only a single 100 watt amp.
Q
Vivek,
Chris makes excellent points regarding box size and driver size. Another advantage of smaller drivers is they are faster as well which is still important for low frequencies as well as tweeters.
Also bear in mind sealed enclosures have a slower rolloff from the -3dB point so they will actually have more useable frequency below their -3dB point. Vented systems have a lower -3dB point, but cut off at a steeper slope so vented (not Transmission Lines) enclosures actually have less low frequency an octave or two below the -3dB point depending on the speaker Q's involved. One does need a speaker with higher power handling and better Xmax for sealed enclosures than vented for the same -3dB point which is one reason vented systems are more popular. The other reason vented systems are more popular is generally they require a smaller box volume compared to a sealed system. for the same -3dB point with the same speaker system Q.
So if you can take Chris' comments and effect them you can reduce the costs of the amplifier you drive the subwoofers with.
Regards,
John L. Males
Willowdale, Ontario
Canada
04 August 2007 (19:35 -) 20:06
Official Quasi Thread Researcher
Chris makes excellent points regarding box size and driver size. Another advantage of smaller drivers is they are faster as well which is still important for low frequencies as well as tweeters.
Also bear in mind sealed enclosures have a slower rolloff from the -3dB point so they will actually have more useable frequency below their -3dB point. Vented systems have a lower -3dB point, but cut off at a steeper slope so vented (not Transmission Lines) enclosures actually have less low frequency an octave or two below the -3dB point depending on the speaker Q's involved. One does need a speaker with higher power handling and better Xmax for sealed enclosures than vented for the same -3dB point which is one reason vented systems are more popular. The other reason vented systems are more popular is generally they require a smaller box volume compared to a sealed system. for the same -3dB point with the same speaker system Q.
So if you can take Chris' comments and effect them you can reduce the costs of the amplifier you drive the subwoofers with.
Regards,
John L. Males
Willowdale, Ontario
Canada
04 August 2007 (19:35 -) 20:06
Official Quasi Thread Researcher
John, Chris, Quasi, you guys are just great. It's all becoming clear now.
My Denon receiver does have a LPF in the .1 output.
The boxes I have built are 60L each (wife is complaining about too many boxes in the house) and will be sealed. So in that way, going by the point Chris made, I think I am on the right path.
About the efficiency of the driver, I suppose a 90dB driver is not all that bad. In the MTMs, the woofers are 86dB and the tweeters are 92dB. That can be fixed I suppose.
Ultimately, I am convinced that 2x200wpc would be kind of sufficient.
My Denon receiver does have a LPF in the .1 output.
The boxes I have built are 60L each (wife is complaining about too many boxes in the house) and will be sealed. So in that way, going by the point Chris made, I think I am on the right path.
About the efficiency of the driver, I suppose a 90dB driver is not all that bad. In the MTMs, the woofers are 86dB and the tweeters are 92dB. That can be fixed I suppose.
Ultimately, I am convinced that 2x200wpc would be kind of sufficient.
Hi John,
Actually, a vented B4 box is approximately twice as big as a sealed box. The efficiency is the same in midband. The port serves to damp the woofer near resonance and extend the usable range down about an octave. If your -3dB point is low enough, there is no worries about the woofer becoming unloaded (which happens below the box tuning, or port tuning frequency).
In other words, a 6.5" woofer is a poor candidate for a ported box as a sealed box provides damping all the way down in frequency. A ported box would unload the woofer too high in frequency. Most commercial enclosures are too small, that is where some misconceptions are.
I built a double tuned chamber described by David Weems. It loads the woofer over two octaves and so provides lower distortion. This design allowed me to get down below 30 Hz in a tower design (one box atop the other) with an 8" woofer. Normally a 10 " would be used. Proper ported boxes for 12" and 15" woofers get mighty large very quickly.
In short, if your design will cut off low in frequency, use a real B4 ported alignment. Higher frequency cut off points would be better served with other designs, such as a sealed box.
-Chris
Actually, a vented B4 box is approximately twice as big as a sealed box. The efficiency is the same in midband. The port serves to damp the woofer near resonance and extend the usable range down about an octave. If your -3dB point is low enough, there is no worries about the woofer becoming unloaded (which happens below the box tuning, or port tuning frequency).
In other words, a 6.5" woofer is a poor candidate for a ported box as a sealed box provides damping all the way down in frequency. A ported box would unload the woofer too high in frequency. Most commercial enclosures are too small, that is where some misconceptions are.
I built a double tuned chamber described by David Weems. It loads the woofer over two octaves and so provides lower distortion. This design allowed me to get down below 30 Hz in a tower design (one box atop the other) with an 8" woofer. Normally a 10 " would be used. Proper ported boxes for 12" and 15" woofers get mighty large very quickly.
In short, if your design will cut off low in frequency, use a real B4 ported alignment. Higher frequency cut off points would be better served with other designs, such as a sealed box.
-Chris
Which other FETs are recommended for the NMOS200 apart from IRF840? I think one of the guys had used 540. I have some 510s at home. However, I think the 510s are of a far lower voltage.
Vivek said:
The IRF510 is not suitable as it has a Pd of only 43 watts. The The IRF540 comes from a range of manufacturers. It is available in a range of Pd up to 140 watts. Note both are 100v devices this means rails must never exceed 50 volts.
Cheers
Q
Vivek,
Be aware if you design for 200W for 4 ohms the rail voltage will be 50V (192W) per quasi's "Power selection guide" and 107W at 8 ohms.
You can use the IRF740, the IRF640N (better than the IRF640, but the IRF640 is ok), IRFP240, IRFP250, IRF260 (but not best choice as Ciss is very high), IRFP460 (but not best choice as Ciss is very high) are some IRF type devices you could use and are all good for rails greater than 50V. I know some APT devices that are good as well. Hari found some good devices he was making inquiries about not that many post ago for 160W+ PCB he is designing that seem to be available at reasonable prices. I would suggested reading back before the referenced posting and a bit after to gain a sense of what you you will be working with in terms of rail voltages, losses, et al for a the 180W versions you wish to build for your 4 Ohm subwoofers.
There will be some difference of opinion on how many devices and the PSU design to meet the 200W/20V/10A/200VA for a 4 ohm/2 load. Some will say it is ok to use the 10ms SOAR limits on the device datasheet, others will say the 100ms SOAR and a some will say the DC SOAR. You will be safest with the DC SOAR. You will find some 180W rated devices (before Tc derating) will have a DC SOAR of 1-2A in the 50V range and about 5A in the 100ms range. You will find many manufactures will not list a DC SOAR for more than 10ms, though it is not that hard to extrapolate the 100ms and DC SOAR from the graphs. As this is a 4 ohm subwoofer I would think you will need at least 2 pairs for each module for the typical 180W device before it is derated for Tc. Given it is a subwoofer I would be inclined to suggest 3 pairs of the 180W type devices to handle the 10A current loads.
Regards,
John L. Males
Willowdale, Ontario
Canada
06 August 2007 (12:30 -) 13:19
Official Quasi Thread Researcher
Be aware if you design for 200W for 4 ohms the rail voltage will be 50V (192W) per quasi's "Power selection guide" and 107W at 8 ohms.
You can use the IRF740, the IRF640N (better than the IRF640, but the IRF640 is ok), IRFP240, IRFP250, IRF260 (but not best choice as Ciss is very high), IRFP460 (but not best choice as Ciss is very high) are some IRF type devices you could use and are all good for rails greater than 50V. I know some APT devices that are good as well. Hari found some good devices he was making inquiries about not that many post ago for 160W+ PCB he is designing that seem to be available at reasonable prices. I would suggested reading back before the referenced posting and a bit after to gain a sense of what you you will be working with in terms of rail voltages, losses, et al for a the 180W versions you wish to build for your 4 Ohm subwoofers.
There will be some difference of opinion on how many devices and the PSU design to meet the 200W/20V/10A/200VA for a 4 ohm/2 load. Some will say it is ok to use the 10ms SOAR limits on the device datasheet, others will say the 100ms SOAR and a some will say the DC SOAR. You will be safest with the DC SOAR. You will find some 180W rated devices (before Tc derating) will have a DC SOAR of 1-2A in the 50V range and about 5A in the 100ms range. You will find many manufactures will not list a DC SOAR for more than 10ms, though it is not that hard to extrapolate the 100ms and DC SOAR from the graphs. As this is a 4 ohm subwoofer I would think you will need at least 2 pairs for each module for the typical 180W device before it is derated for Tc. Given it is a subwoofer I would be inclined to suggest 3 pairs of the 180W type devices to handle the 10A current loads.
Regards,
John L. Males
Willowdale, Ontario
Canada
06 August 2007 (12:30 -) 13:19
Official Quasi Thread Researcher
The IRF840s were available in the market. I bought 12 of them. I also bought BD139, BD140, BC546 and BC556 transistors.
I am going to buy a new toroidal transformer. So I should get something like a 40-0-40 12A transformer I suppose.
I have been following Hari's PCB design too. I will do some more reading on this thread too.
And is there a parts list anywhere I can refer to?
Cheers,
Vivek
I am going to buy a new toroidal transformer. So I should get something like a 40-0-40 12A transformer I suppose.
I have been following Hari's PCB design too. I will do some more reading on this thread too.
And is there a parts list anywhere I can refer to?
Cheers,
Vivek
Vivek said:
Vivtek,
I believe the BD139/140 will be under dimensioned for a PSU with rails around the 56V range the 40-0-40 torid will give you. You may need to use the MJE340/350 or KSE340/350. The "experts" will be better able to comment on your choice to use BD139/140 for a 40-0-40 transformer.
Are buying two transformers, one for each amp module?. The transformer needs to be rated at least 10A a side of the CT winding which means the full secondary winding needs to be 20A.
You can refer to a parts list quasi posted some time ago that may need to be refined a bit with updates and corrections since the parts list initial posting 05 January 2005.
Regards,
John L. Males
Willowdale, Ontario
Canada
06 August 2007 (14:30 -) 14:56
Official Quasi Thread Researcher
Hi,
40-0-40 @ 12A is 960VA.
That size of transformer is good for 300W+300W into 4ohms. Voltage and current are just about perfect.
Now find some 160mF 63Vdc smoothing caps.
40-0-40 @ 12A is 960VA.
That size of transformer is good for 300W+300W into 4ohms. Voltage and current are just about perfect.
Now find some 160mF 63Vdc smoothing caps.
I could get a 35-0-35 (12A) if a 40-0-40 is not really required. Also, I have one 25-0-25 (12A) if that can be considered. But I guess that would give me a lot less power.
John, I know I would perhaps be better off with two transformers but that would mean rising costs again.
John, I know I would perhaps be better off with two transformers but that would mean rising costs again.
Vivek said:
Vivek,
As Andrew pointed out you would be fine with a 40-0-40 12A transformer. That said for sure a 35-0-35 12A would do you just fine as well. In fact with a 35-0-35 12A you have more current headroom. A 35-0-35 would allow you to use 63V PSU filter capacitors which are much easier to find and are also cheaper. It would also mean the PSU filter capacitors would have a longer life as they would be used at a lower voltage than with the 40-0-40 all else being equal.
Your peak power will be about 360W and 180RMS. That is not going to make a difference compared to 200W RMS/400W P-P in dB terms. What this translates to is about 9.5A P-P at 4 ohms resistive and the impedance/2 rule 19A P-P. 12A RMS P-P is about 17A. This is where you see why I suggest two transformers may be a consideration. That said, you are not likely to push the amp to the limit as I am sure your HT amp is not 180W RMS a channel, nor will it likely have a transformer as near as powerful as the 35-0-35 12A one you like to buy. With a 3dB dymanic headroom for odd peaks in the music your current demans fall to 9.5A P-P and with 6dB headroom those current demans fall to 4.75 P-P. This would suggest a 35-0-35 would do just fine for your subwoofer application.
Where you need to be careful and do some simple testing if you do not have the HT amp schematic is to match the quasi amp modules gain to the HT gain. Forst to ensure levels are matched and second to ensure you do not drive the quasi modules for the subwoofer much higher and perhaps at the limits of the PSU and modules. You do not need to have the volume high in the HT amp to do the test adn I woudl keep it low for the test. Apply a known signal and measure both the RMS of the signal at the input of the HT amp and one channel of the HT speaker outpur and then you can divide the output by input to determine the HT gain. Once the HT gain is known you build your quasi modules with the same gain.
Regards,
John L. Males
Willowdale, Ontario
Canada
07 August 2007 (08:06 -) 08:34
Official Quasi Thread Researcher
So I think we are settled on the 35-0-35 12A transformer.
Today, I went out to buy the resistors and capacitors. I found that the stores in the electronics market would not give me 1/2W metal film resistors in small quantities. So I had to buy 5% carbon composition resistors. I would have liked MFRs but it couldn't be helped. With the 35V AC transformer, I think I will be able to use the BD139/140.
The ceramic disc capacitors available were only 100V ones. I could not find the 39pF caps. Non-polar 47uF caps were also not available. This is the position as of now.
Now, I will get the PCBs made and get the transformer too. Once the two modules are built, I can test them and my HT receiver.
Today, I went out to buy the resistors and capacitors. I found that the stores in the electronics market would not give me 1/2W metal film resistors in small quantities. So I had to buy 5% carbon composition resistors. I would have liked MFRs but it couldn't be helped. With the 35V AC transformer, I think I will be able to use the BD139/140.
The ceramic disc capacitors available were only 100V ones. I could not find the 39pF caps. Non-polar 47uF caps were also not available. This is the position as of now.
Now, I will get the PCBs made and get the transformer too. Once the two modules are built, I can test them and my HT receiver.