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DIY Stompboxes => Building your own stompbox => Topic started by: bluelagoon on November 19, 2020, 07:46:48 AM

Title: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: bluelagoon on November 19, 2020, 07:46:48 AM
Hello Fellow Board Members,
                                             I am in a bit of a quandry, trying to properly figure just how to correctly convert the Boss FA-1 circuit from single 9 Volt supply to a Dual rail +/- 9 volyt supply with a centre 0v ground. I realize I shall need an appropriate charge pump to get the dual supply, but really need help to identify just which of the original grounds become -9volt and which stay as 0 volt ground, I understand that the op amp half supply voltage goes to 0 volt just mainly curious on the section circled in orange trying to get my head around, particularly where the 3M3 resistor is supposed to end up either 0V ground or -9volt? also the 1M pull down resistor does it go to --9volt or ground? I see that also the voltage dividing network in there can probably be scrapped just need some sound advice on the matter. the rest of the circuit seems a bit more straight forward, but please chime in with every detail needs correcting for dual +/-v supply.

Please see attached FA-1 Schematic.

Thanks for any help you can offer.

Best Regards,

TTay
(https://i.postimg.cc/gxrwRr8P/FA-1-Circuit.jpg) (https://postimg.cc/gxrwRr8P)
Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: iainpunk on November 19, 2020, 08:25:01 AM
all the Vr points go to the new ground, that includes the 3M3 resistor.
RPD 1M still goes to ground,
the whole Vr circuit encircled in orange can be omitted,
the rest can stay the same.

cheers, Iain
Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: antonis on November 19, 2020, 08:39:46 AM
all the Vr points go to the new ground, that includes the 3M3 resistor.

Dramatically altering Q1 Gate bias.. :icon_wink:
(as it is, Gate is biased at 3V above GND..)
Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: iainpunk on November 19, 2020, 09:19:26 AM
all the Vr points go to the new ground, that includes the 3M3 resistor.

Dramatically altering Q1 Gate bias.. :icon_wink:
(as it is, Gate is biased at 3V above GND..)
bruh...
you have 18V of headroom for the input signal, a few volts of mis-bias won't make a damn difference here.

cheers, Iain
Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: antonis on November 19, 2020, 09:24:15 AM
you have 18V of headroom for the input signal..

No, unless you connect R3 to -9V (instead of GND)..
(I didn't see any relevant proposal..)  :icon_wink:
Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: tonyharker on November 19, 2020, 10:24:41 AM
Have you seen this  https://www.ebay.co.uk/itm/15V-Dual-Output-DC-DC-Converter-Step-Up-Boost-Voltage-3-3V-5V-12V-to-15V-Module/252996434711?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649
Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: iainpunk on November 19, 2020, 11:24:19 AM
you have 18V of headroom for the input signal..

No, unless you connect R3 to -9V (instead of GND)..
(I didn't see any relevant proposal..)  :icon_wink:
shouldn't you bias the base to Vpower/2, which is 0V in the case of a bipolar supply?!
the -9V can be quite noisy with a charge pump, biasing to this point can introduce said noise to the signal.
and we probably only need 3V pkpk headroom anyways since its probably used for instrument level.

cheers, Iain
Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: antonis on November 19, 2020, 02:16:24 PM
shouldn't you bias the gate to Vpower/2, which is 0V in the case of a bipolar supply?!

Iain, plz..  :icon_smile:

There shouldn't be any bipolar supply for Q1  with its Source resistor left connected to GND..
As it is, VGS is -1.5V, considering Source at 4.5V..
With R3 to GND & for same Drain current, Source has to lie at 1.5V, leaving only 1.5V for negative undistorted signal headroom..
(actually, much less than 1.5V due to R3 and and succeeding impedance voltage dividing effect..)

Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: iainpunk on November 19, 2020, 02:59:35 PM
shouldn't you bias the gate to Vpower/2, which is 0V in the case of a bipolar supply?!

Iain, plz..  :icon_smile:

There shouldn't be any bipolar supply for Q1  with its Source resistor left connected to GND..
As it is, VGS is -1.5V, considering Source at 4.5V..
With R3 to GND & for same Drain current, Source has to lie at 1.5V, leaving only 1.5V for negative undistorted signal headroom..
(actually, much less than 1.5V due to R3 and and succeeding impedance voltage dividing effect..)
so i was thinking that it was a BJT, not a JFET, im an idiot for not noticing that earlier. i noticed it when i read the words ''Source resistor''

sorry and cheers, Iain
Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: antonis on November 19, 2020, 03:04:48 PM
 :icon_lol: :icon_lol: :icon_lol:

Cheers but forgiven..!!
(at least, not until I finish your on the house beer..) :icon_wink:
Title: Re: How to Convert single supply voltage circuit to Dual +/- voltage supply??
Post by: antonis on November 20, 2020, 06:22:19 AM
In case of OP is confused due to the above argument ..

(https://i.imgur.com/5551NmB.jpg)
edited, thanks to Stephen.. :icon_wink: