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Old 25th September 2013, 01:48 PM   #1
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Default Using the heater supply to get a DC source

Hi,

I want to put some solid state circuitry into a tube amp (namely an opamp to get a cheap and simple low impedance line-out from between the tube stages).

The question is, could I use rectified heater AC (6.3 VAC rectified to around 8.8 VDC) and use that to power the opamp? The heaters would still be powered by AC, of course.

Is this a good idea, or should I forget it and throw in a 9V battery or use a wall wart instead? Would the heater AC somehow be disturbed by having a diode bridge across it?
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Old 25th September 2013, 02:06 PM   #2
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What opamp are you going to use? Don't most of them require a split supply. And your 6.3v ac will only give you about 6.8v dc oh! I know 6.3 X 1.41 gives you 8.8v but you lose about 2 volts in the rectifier.
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Old 25th September 2013, 02:15 PM   #3
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As your initial post indicates that you wont subscribe to snake oil, then the effect of any electronics on the heater supply probably wont affect you.

As Woody says, the raw 6.3VAC wont be enough, but voltage quadruple it to approximately 30V will give you a useful supply, use a virtual ground for the 0V and you have +/- 15V. The op-amp will only require 100mA or so, so the draw by the quadrupler would be about 400mA. Just check that the heater supply can manage the extra half amp.

By the time you've gone to all that bother, you might be just as well off with a 5VA transformer, they cost pennies.
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Old 25th September 2013, 02:22 PM   #4
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Here's the basic concept. You would need to scale the caps to suit your circuit of course.
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Old 25th September 2013, 02:46 PM   #5
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Originally Posted by KatieandDad View Post
As your initial post indicates that you wont subscribe to snake oil, then the effect of any electronics on the heater supply probably wont affect you.

As Woody says, the raw 6.3VAC wont be enough, but voltage quadruple it to approximately 30V will give you a useful supply, use a virtual ground for the 0V and you have +/- 15V. The op-amp will only require 100mA or so, so the draw by the quadrupler would be about 400mA. Just check that the heater supply can manage the extra half amp.

By the time you've gone to all that bother, you might be just as well off with a 5VA transformer, they cost pennies.
Ah, I completely forgot about voltage drop, as it is usually a non issue when rectifying higher voltages.

As for the voltage demands of opamps, I've successfully run an OPA2134 on 5V (USB-power, virtual ground at 2.5V) in the past so I think it would work as long as I don't need much gain. But the quadrupler looks simple enough (thanks for that!) and it would give ample headroom.

A separate small transformer (indeed very cheap, I notice) so that's also a good option I hadn't thought about.

Thanks for your help!
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Old 25th September 2013, 03:06 PM   #6
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just be aware that with the doubler or quadrupler you will be referencing the heater supply to the ground of the amplifier circuit. That shouldn't be a problem, but in case you already have your heaters referenced to the ground or other voltage in the amplifier, you may be creating a short and/or loop.
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Old 26th September 2013, 05:22 AM   #7
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I have removed all discussion of primary side connections due to the danger and violation of forum rules. As the OP had no intention of doing this the thread has now been re-opened.
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Old 26th September 2013, 08:11 AM   #8
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I have removed all discussion of primary side connections due to the danger and violation of forum rules. As the OP had no intention of doing this the thread has now been re-opened.
Thanks!
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Old 26th September 2013, 10:35 AM   #9
M Gregg is offline M Gregg  United Kingdom
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The problem is,

Sometimes you lift the heater supply to remove hum via a potential divider on the B+ supply.

If the heater supply is centre grounded then you have a supply that is already ground referenced, if you then try to ground another part of the voltage doubled supply you could cause a short.

the other thing is to make sure you fuse the supply to the voltage doubler if a cap or diode goes short your transformer will be damaged.

Make sure you only use a heater supply used for signal not rectifier duty. For some rectifiers the heater supply is part of the B+ circuit.

I have used doubler circuits in the past to supply control for an amp, however the supply would have to be regulated and well smoothed for OP amp supplies.
As mentioned the diodes cause a voltage drop. It depends how much current you intend to draw and making sure the supply is rated for it.

Regards
M. Gregg
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Last edited by M Gregg; 26th September 2013 at 10:40 AM.
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Old 26th September 2013, 11:52 AM   #10
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Thanks! My heater supply is referenced to ground via 100K resistors, but this question is now theoretical as I already ordered a small transformer for the opamp. It went for a steep 4 euros.

I'll see how this turns out, and will save this good knowledge about using the heater supply for later.

Cheers,
/Fredde
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