Jens Rasmussen Leach clone group buy

[tryonziess]

On Jens ten transistor pcb the Vbe is mounted under the main pcb. Where would I check for the currents and voltage in this circuit and what value would represent the proper adjustment.

I have been reading several threads of late and much talk seems to be centered around this particular part of the amplifier design. By using an item that I already have and many here are familiar with I hope to gain some knowledge of the workings of the Vbe.
I am running 4281/4302 output devices from On Semi. I used standard off the shelf diodes recommended by Jens in the BOM. I have set the per rail bias to 750ma.

I have grasped the reason for the Vbe and I now wish to learn optimal values. I am aware that the values change with temperature so I would like to make some comparisons of efficiency in the diode placement and transfer rates from OP through the heatsink to the diode string.

One day I may have absorbed enough information to build a crude amp of my own design. Tad

[AndrewT]

Hi,
leave long legs on the output emitter resistors, i.e. spaced above the PCB.
You can clip on to these legs and measure Vre for all ten resistors.
That will allow you to measure how well you matched your devices and what each is passing as bias current.

[evette]

i would like at least 10 boards, it doesn,t matter how many transistors are used in the final stages. Thank YOU Very Much

[Luke]

Im interested in 2 of the 10 transistor version.I bought 2 of the 6 but I came across a 55 0 55 Torroid and it would be silly to buy another.
Anyone want to trade 10 transistor for my 6 then pm me.

[hags]

I have four untouched/unused boards from this group buy.
Still in original wrap.
I am asking $60 shipped to the contiguous U.S.
International buyers please PM or email for shipping.

[evette]

YHi there, I would like to buy those 4 boards from you. Please tell me how to pay you,Thank You.

Originally Posted by hags

I have four untouched/unused boards from this group buy.
Still in original wrap.
I am asking $60 shipped to the contiguous U.S.
International buyers please PM or email for shipping.

[WJH]

I recently finished up the power supply and boards for the 10 transistor version of this amp. I put together the dual power supply described by Rod Elliot (Power Supply for Power Amplifiers) using an 800VA 2 x 40 VAC toroidal transformer with two 11,000 uF caps per side. I added fuses for the secondaries and a delay relay. The output is +58VDC. As a beginner in electronics, I would like to know how to determine values for the line, secondary, and rail fuses before powering up the boards.

In Sloan’s book, “High Power Audio Amplifier Construction”, I found the following equations:

Line fuse A = transformerVA/120VAC

Secondary fuse A = transformerVA/secondary VAC (number of secondaries)

This works out to 6.7A for the line fuse and 10A for the secondary fuses. When testing the power supply, I used 8A slow blow fuses for both the line and secondary fuses and experienced no problems. Am I good to go?

Is there an empirical way to determine the value for the rail fuses? I ask this because Jens’ parts list shows that the rail fuses are 12A and somewhere in this thread, 4A was mentioned as the value to use. Leach uses 5A fast blow fuses and Sloan recommends this value, since “it is unlikely that speaker load currents will exceed 5A”.

I have found that I can’t be too cautious when working on electronics projects. Thanks in advance!

[AndrewT]

I built the 3pair Leach clone for 8ohm duty.
I used surprisingly small fuses in the supply rail and they survived high power testing into 4r0.
As a result of this and reading other recommendations I have adopted the following.
F fuse rating = Ipk / 2 when driven to full power into the nominal impedance load value. For my 3pair which delivers 170W into 8r0 (40+40Vac transformer) I used F3.1A fuses in the supply rails while testing to full power into half load resistance.
Some others suggest
F fuse rating = Irms / 2 when driving full power into nominal load impedance.

If you expect 400W into 4r0 then the higher fuse rating would be F7A and the lower fuse rating F5A. Try both and see what happens.

In general my rule is to use the lowest value fuse that does not suffer nuisance blowing. If that size of fuse offers little protection (to setting the house on fire while the equipment is unattended) then I look at what needs to be done to allow a lower fuse rating to be used.

BTW,
an 8A fuse in the mains supply could pass 16A for many minutes and maybe even hours before rupturing.
How much damage to surroundings could be done if you pulled 1.9kW from the mains while waiting for the fuse to blow?

[JensRasmussen]

Originally Posted by AndrewT

BTW,
an 8A fuse in the mains supply could pass 16A for many minutes and maybe even hours before rupturing.
How much damage to surroundings could be done if you pulled 1.9kW from the mains while waiting for the fuse to blow?

In Denmark, new houses or houses with updated mains installations have electronic fuses of maximum 13A... that blows quickly. But you are right it is important to adopt a fuse design that takes your mains installation into the equation.

For the fuses on the board, if you use fast fuses and big capacitors on-board for rail decoupling, the fuse has to withstand the energy surge it takes to charge the capacitors... if you have slow start no big deal, but if you just kick the PSU in you will see a huge energy surge through the fuse.

I found that a fast 12A was the best choice when I did the first re-design 6 years ago... you should consider the value with care. Fuses are among the most difficult parts to choose correctly.

\\\Jens

[tryonziess]

On all of the amp projects I assemble I use a 7-10 amp fast sensing circuit breaker where the wall current enters the amp. This will handle the transformer, bridge rectifiers and cap charging/shorting. I then use the smallest fast blow fuse I can get away with for protecting the circuit components and output. The nice thing about Jens 10 transistor board is the location for this fuse. It is after the major power supply items, bridge and transformer. I was able to get away with a 3 amp fuse here but, installed a 4 amp for convenience. With a total of 2 amps bias per board there is still some wiggle room. I have also found that not all fuses are the same. Some from one manufacturer may be a whole lot more sensitive than others. I like the circuit breakers because they sense a short to earth ground very fast and do not just read current draw.

With luck some of us may be soldering components on the new boards next week.

Tad