Best simple power supply

[john & Jake the dog]

Hi,
Putting together a simple power supply for my Ferrograph boards.
Could anyone please say if there is any advantage in using FIG 2 over FIG 1.
FIG 1 is the original design in the Ferrograph.
The voltages are about the same.




If FIG 2 is better would two separate bridges be an improvement?

[John Caswell]

Hi John,
The less you couple the two channels together the better so if you just have one secondary winding on the power tx that is the final arbiter. Two bridges, and separate smoothing will certainly be better, Figure 2 will be the best and as I say two rectifiers.

John

[john & Jake the dog]

Hi John, thanks, I'll sort out two rectifiers as I do only have one output on the transformer.

Fitted new power transistors and some better caps.

Wired it together this afternoon on the table with a cheap pair of bookshelves

No nasty noises and it played music first time.
Initial impressions are very favourable.

I have to wire it up into an old Exposure case now.
I'll concentrate on getting the most stable power supply I can, may be very beneficial.

[John Caswell]

Hi John,
Yes that is the best way to go. Getting a stable power supply does not necessarily mean a stabilised power supply. Very often just a rectifier and large cap give the best results as there is no limiting of current in the way.

John

[john & Jake the dog]

Thanks John.
In the pic above, next to the power transistors there is a potentiometer.
It is connected to both sides of a resistor.
I believe it is for setting the quiescent current 25 to 30 ma and not above 45.

Is it possible for me to measure that using only a multimeter and where would I stick the probes

[colin.hepburn]

Hi John
WARNING you can blow the transistors doing this
Don’t try this until you know exactly where you put the red probe the black probe is croc clipped to ground it is best to post up the amp schematic first and let john explain
I know how to do it on my pioneer SA500A as there is a test point for this but without seeing you amp it is difficult to direct you but please make sure you disconnect the speakers and all inputs and you may have to connect a dummy load the speaker post I am sure john will take you thought it if you post the schematic

[john & Jake the dog]

Thanks colin,
here's part of the scematic, I think the relevant resistor is R506 as R518 is the potentiometer.
It is attached to the two ends of R506.
Could you also please tell me the purpose of R715 as it is the last component before the speaker and is in the signal path.
It is only 0.35 ohms so can it be removed.
[IMG][/IMG]

[John Caswell]

Hi John,
Yes it looks as though Ferrograph have placed a variable R across R508 as it says "set Iq".
So place an DMM set to 200mA in place of the fuse and adjust the variable R for what you require, but about 30mA should suffice, then replace the fuse. You can ignore the current that the pre-amp stages take as it will be negligible WRT the main amp.
No do not remove R715 (0R35) as it provides some protection against low L/S loads

John

[john & Jake the dog]

Hi John, thanks, did you mean remove r506 and put the multimeter in it's place, adjust and then replace the resistor.
Would anything nastyhappen if the probe came off whilst the amp was running?

Not using the pre-amp section, running the cd directly to the two power boards via a blue alps.

[john & Jake the dog]

Hi John,just cottoned on I think.
By the fuse you mean the DC fuse

I haven't got them in circuit as it is all laid out on the coffee table at the moment.

So my guess is I place the meter across r506 and twiddle the potentiometer until I get 30 with the spaeakers dis-connected.