transformer choice for amp

[Richidoo]

I am building ESP Project 101 , a MOSFET amp. It is my first project.

I have speakers that have low impedence, dipping below 2 ohms in the bass, with some phase angle in there too. They are 95dB sensitive, and rated for 500W/4ohms nominal. Legacy Focus 20/20 impedence curve

The amp design has dual pair of lateral mosfets per channel and is rated at 180W/8ohms, 250W/4ohms with +/- 70V rails. I am worried that I will exceed SOAR if I use 70V rails into such a difficult load.

If I lower the rails to 56V, I can get more current from the amp with lower max power. Since the speakers are 95dB sensitive, I think that may be the way to go instead of going for max power and having problems below 4ohms. Am I on the right track?
Any additional advice/comments are welcome!
Thanks
Rich

[XEAGLEKEEPER]

Are you going to purchase the boards from Rod Elliot? Shoot and e-mail to Rod and get his opinion. I'm running two sets of Yamaha NS-1000's with two 2-channel ESP P101s. Yamaha's are 8ohm with and efficiency of 94db they (P101s)sound better than the 2 Yamaha M-45 amps I was previously using. A good source for toroid is ebaystore Antek inc. I picked up one of his 0-50,0-50 800VA for 59.00 USD but If you need a lower voltage he has all kinds to choose from.

[XEAGLEKEEPER]

Those are some pricey speakers bet they sound fabulous.

[AndrewT]

Hi,
the average of 4ohms hardly does justice to the load imposed on an amplifier.

That dip to <2r and phase angle of <70degrees is a severe load at the extreme bass.

Two pair of output devices would be severely stressed if you are tempted to turn up the wick.
I see two choices.
1. as you have already said reduce the PSU voltage significantly. This eases the burden on output devices by reducing the current that peak voltage can push into the load and also reduces the Vce on each device when it is trying to drive that peak current.

That peak current could be a long sustained note in the bass and that is precisely when the outputs could cry enough. You are rightly concerned.

2. increase the number of parallel output devices to 3pair or even 4pair. Does the PCB allow for this? One of ESP's designs does exactly this, doubling up a 2pair to 4pair for extreme loading.

[Richidoo]

Thanks guys
I heard back from Rod last night, he advised using 56V rails instead of the 70V max to allow for higher current. He said there are a couple installed in rec studios which have not burned up, meaning the amps are tough. Applying EQ to bass guitar and synth notes requires unbelievable power for long periods.

He said the devices can handle 8A each so not to worry, they can handle just about any speaker, but good idea to lower the voltage.

P101 has two versions: low power with one output pair per channel, and high power with 2 pair. They use the lowest power exicon mosfets so perhaps higher power handling devices could be used, but the design would have to change and I don't know how to do that. Changing to 4 pairs per channel will not fit on the P101 board, it is TINY. He has some other amps for subwoofers, etc, but I don't need high power (Watts into 8R) I only need 100W/8 but with high current ability.

I saw Antek on ebay last night while trying to figure out how to make these high end amps cost mid fi price. Nice selection, glad to know someone who actually bought from him and had a good experience. I think I may get a pair of the beautiful toroids Kevin Haskin (diycable.com) sells for the UcD400. Then in the future I can try the UcDs. His toroids are 1kVA, so will not collapse under current draw. Price ($125 ea) is great for the features, and they are exactly 40V output which I want, plus 13-0-13 and 16-0, EMI and Mag sheild, and potted with thruhole! That would cost $300+ each from Avel.

The Focus speakers are a great pleasure to listen to. I auditioned them first, then I tried to find a similar sound at a lower price, but I couldn't believe how much better they sounded on real music than all the big name high end brands. I ended up getting the focus' from my local dealer for a big demo discount, and I couldn't be happier, even after visiting RMAF last week! But my Cary integrated tube amp can't do the bass justice, but sounds heavenly above 200 Hz so I will try bi-amping them.
Thanks a lot for your advice.
Rich

[AndrewT]

Hi,
a solid state push pull amplifier will try to drive double the power in to half the load impedance. It's their constant voltage drive that tries to do this and can't be avoided.

Your +-56Vdc proposal will produce about 130W into 8r and on one pair will only manage about an extra 50% into 4r, but two pair will probably manage 200W into 4r and nearly 300W into 2r but starting to struggle at that loading.

3pair or 4pair will manage the low impedance much better but it will still be a 130W into 8r amplifier and with careful selection of components could achieve 400W into 2r and not blow up when tested into 1r0.

If you think 400W is too much then lower the supply rail voltage even more to maybe +-50Vdc or even +-45Vdc. Then the stress on the output stage is more manageable and it allows beefier components to be used that have less voltage tolerance complemented by greater current ability (the 2sk/j devices cannot take advantage of this).

[djk]

Even on ±56V rails, the amp will try and do ±40V peak at 2 ohms. This will need a minimum of 20A drain current. Trust us, you will need a minimum of three pair of those 8A FETs.

Both Magnatec and Exicon make some double die lateral FETs in the TO-264 package that are rated at 16A, go for those.

http://www.profusionplc.com/pro/gex/...artno=ECW20P20

http://www.magnatec-uk.com/pdf/magnatec/BUZ900DP.pdf

The Magnatec seems to be a bit less at GBP 9.20 vs GBP 11.98 for the Exicon.

Hafler used Exicon in the last production runs for the DH-500s.

[AndrewT]

Hi,

double die lateral FETs in the TO-264 package

that's a good way to stay with the same PCB space.
Check the To247 size against the To264. They are about 3mm or so taller.

[sreten]

Hi,

I'll state one rule of thumb: low impedance loads and mosfet amplifiers
without higher voltage driver rails (than the output rails) do not mix.

You can try, but your wasting your time, build a bipolar current monster.

/sreten.

[Richidoo]

Thanks for the advice.

What is the difference between driver rails and output rails, for example on this design? ESP P101 Schematic

Can I just substitute the higher current rated output FETs for the 8A units? Or will there be other adjustments to the circuit needed to accomodate different output devices?

Where can I find plans for a Bipolar current monster? Sounds scaaaaaary! hehe

This is my first audio project, I am hoping to learn a lot. By using a proven kit, I can be an idiot and if I follow the directions (hopefully) it will not blow up. I am reading Self books and the forums too, but it takes time to soak it all up, fun though!

The exicon EXC10N16 can handle 8A each, or 125W each. With 2 pairs each channel, isn't that 32A capability? My power supply should be up to it, that seems easier to specify.1000VA@+/-40VAC, dual bridges, 20,000uF 4-poles per rail, soft-start, separate supplies per channel. Rod rates the amp at 275W/4ohms with 56W.

Don't forget my speakers are pretty efficient at 95dB/W/m. When I listen to dance music now, I hear gross distortion in tha bass because my tubes are wimpy. I am not looking for a DJ speaker system, just to clean up that bass.

I just took a look at Rod's Subwoofer amp, as he calls it. It has 4 pair. I can build that next if these smaller ones have a problem. I will need the practice anyway. Thanks for your advice guys!
Rich