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SteveT 3rd May 2005 11:36 AM

Leach amp power supply?
I am going to build a Leach amp as a subwoofer amp. What transformer rating would I need for the single amp? I would guess 3-4Amps. Any suggestions?
Also, the power supply diagram on the Leach web site shows 4 output fuses- 2 on each rail-Why?

Thanks for any help

audiofan 3rd May 2005 02:21 PM

4 fuses for a stereo amp one fuse for each chanel.

About transfo rating if the amp is for a subwoofer remember that low frequency signal at high level requires much current from supply use a big transfo and also heavy heat sink

AndrewT 3rd May 2005 04:19 PM

and a lot of PSU capacitance and onboard decoupling.

AndrewT 3rd May 2005 04:28 PM

what impedance are you driving.
Standard Leach is 120w into 8r, assuming 6 ohm minimum impedance, this gives Ipk about 42/6 = 7A from each rail alternately. So the average is about 3.5A, right in your suggested range giving a transformer of about 300va.
But if you drive 4r then all ratings need to be doubled. i.e. 14A/7A/600va.

SteveT 3rd May 2005 06:40 PM

Driving 8ohm, but things are always changing.
Thanks guys.

BrianDonegan 3rd May 2005 07:55 PM

Check out Victoria Magnetics. They make some torroids specifically for the Leach.

SteveT 4th May 2005 06:50 PM

Thanks for the link , but I hate to think what the shipping will cost to the U.K.

amplifierguru 5th May 2005 06:30 AM

You could probably do it economically with 200VA as Ihalf =Io/pi
but the smaller the transformer usually the poorer the regulation and the greater the bounce at power envelope infrasonic frequencies and I doubt this ampwould cut it with it's RC rail filters even with the cascode. This is serious modulation. It throws the onus onto the power supply, giving worthwhile (audible) improvement with overbuilding, to compensate. When people say it benefits from supply overbuilding you can rest assured it's weak in the PSRR dept and/or conditions are onerous. Sub amp conditions are.

A chip front end with reg supplies would eat it for b'fast IMHO.

jaycee 5th May 2005 01:39 PM

Theres no harm in oversizing the PSU. for a sub i'd go for a 500VA or 625VA unit, with plenty of capacitance, 10,000uF or greater per rail.

amplifierguru 6th May 2005 02:26 AM

Sadly, Jaycee, we all live on a planet being poisoned by excess - much of it through poor design of products, or ones that celebrate inefficiency. Look at Class A.

In this case overbuilding can provide some compensation for weak design ( or one unintended for the purpose) but there's every harm in it. If everyone had this mindset.....

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