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Author Topic: Adapting a preamp with single power supply to dual  (Read 7188 times)

Aleksandar

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Adapting a preamp with single power supply to dual
« on: December 04, 2010, 06:46:01 AM »
Hi! I found this interesting circuit : http://www.redcircuits.com/Page120.htm, which I would like to use as a preamp for a power amp with TDA7294 chip. But the power supply needed for TDA7294 is dual, and for the RED circuit is single. Is there any way to convert the RED circuit into a dual supply, and how?

DJPhil

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Re: Adapting a preamp with single power supply to dual
« Reply #1 on: December 04, 2010, 11:05:18 PM »
It depends on the working voltage of the TDA7294. It can be powered from as little as 20V (+/- 10V) or as much as 80V (+/- 40V).
The preamp is capacitatively coupled (via C8) so it can be powered from any potential that's between 18V and 30V and the output can be ground referenced by the power stage, no worries.
This means you can run it off of any combination of power rails that won't blow it up, and add regulation (voltage regulator or zener regulator) to trim the supply to where you need it.

For example, let's say you're using +/- 35VDC to power the TDA7294.
Vcc sits at +35VDC
gnd sits at 0
Vee sits at -35VDC

Vcc and gnd is about 35VDC, a bit high for you. A linear regulator is going to drop that voltage down for you, and spend the rest as heat. Given that the preamp won't be using much power, this isn't a big concern, so let's say a target of 24VDC to power the preamp. This is a convenient value, as there are three pin regulators at 24V (google 7824) that you could wire up according to the datasheet and essentially be done. Alternately you can use a zener diode and a resistor, as explained here.

The output from C8 can be hooked directly to the input of a standard TDA7294 power stage, and essentially replaces C1 in the reference circuit on the first page. Capacitors will only pass changes in voltage, so the output of the preamp will be referenced to the ground of the power stage by R1 in the reference circuit.

I hope that helps, I'm a bit scrambled today.  :duh

phatt

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Re: Adapting a preamp with single power supply to dual
« Reply #2 on: December 04, 2010, 11:44:26 PM »
Hey DJ Phil,
                Far from scrambled mate,, a darn fine explanation of the situation. :tu:

My concern is the preamp  :-\ It looks a little suspect?
If time permits I might set it up again and try a shootout between my setup.
I've never been able to get the noise down enough when using Fets like this.
Hey that could be just me missing something? :lmao:
Phil.

J M Fahey

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Re: Adapting a preamp with single power supply to dual
« Reply #3 on: December 06, 2010, 08:10:51 PM »
"Redcircuits" are unchecked , many of them unfortunately do not work, or at least are a little weird, the site owner just picks schematics here and there and posts them.
Since you already have a split power supply, I'd use Rod Elliot's guitar preamp, which uses a split supply too, and is professionally designed.
It works and sounds very well.
It will cost the same or less than the "Red" one and is far more stable, doesn't need careful matching of Fets, is easier to mod, etc.
Just my 2 cents.
PS: it even admits the excellent Phatt's tone control instead of the original one.

phatt

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Re: Adapting a preamp with single power supply to dual
« Reply #4 on: December 07, 2010, 08:59:08 AM »

PS: it even admits the excellent Phatt's tone control instead of the original one.

Awwh shucks,, thanks chum but I think you are pulling my leg. ;D

Re the Red Fet Circuit,,, a quick sim but got cut short (wife wanted to go shopping :loco Grr)
BIg Massive bass loss thru that by the looks of it.

I actually built my PhAbbtone both ways, Opamps and Fets.
I really wanted the Fets to work so I bootstraped the input,,, tried mu fets for bigger swing,,, you name it,,,,but alas after much testing,,,, the opamp won.
Phil.

Aleksandar

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Re: Adapting a preamp with single power supply to dual
« Reply #5 on: December 11, 2010, 09:06:34 AM »
Thanks for your replies. Actually, I already have built an opamp pre, but wanted to build one more channel for versatility I guess, and I read this stuff that FET transistors act similar to tubes and what not, but then again, I decided to stick with this one channel that I have already, which is built around two TL072 chips, and I actually like how it sounds.

However, reading this topic http://www.ssguitar.com/index.php?topic=1885.0  on the following schematic I saw those two diodes 1n4148 right in the very input, which are connected to the supply:



Could please someone explain  what is the purpose of those diodes, and how would they affect the sound if I put them in my preamp?

Thanks!

DJPhil

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Re: Adapting a preamp with single power supply to dual
« Reply #6 on: December 11, 2010, 10:39:55 AM »
I believe those are input protection diodes. They'll limit any incoming voltage to about 1.2V peak to peak to prevent damage to the rest of the preamp in the event of someone hooking up something silly (within reason). They'll have no effect on sound unless your guitar is cranked up to super hot and you're picking like a fiend (though this depends a lot on the pickups), and then you'll hear the sound break up during the peaks. If you add those two diodes and find you can't crank things up like you want without clipping you can add another pair in series with them and that will double their effective range from 1.2VPP to 2.4VPP. If you're putting out that hot a signal though you'll find you have less room on the overdrive knob before it gets really dirty.

The short version: they are safe to omit if you're careful about what you plug in, though you might want to experiment with them just to see if you like what they do.

Hope that helps. :)

J M Fahey

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Re: Adapting a preamp with single power supply to dual
« Reply #7 on: December 11, 2010, 11:05:44 AM »
They are protection diodes as correctly stated above, but they are not grounded, please check that the "free" end goes to either +15V or -15V; so the actual threshold is a whopping 15.7 V .
They will not clip a guitar signal, they are there to protect the input op amp from those guys who enjoy plugging a speaker out into another amp's input.
You say those don't exist?
I know a few !!!! :duh
They do not even be too  :loco , most people think that headphone out can be used as line out.
Definitely not, most bean counting designers save a 0.001 cent resistor and headphone outs slam the full speaker voltage (think 16 to 40 V RMS) into high impedance loads (pre inputs).

Aleksandar

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Re: Adapting a preamp with single power supply to dual
« Reply #8 on: December 11, 2010, 12:18:52 PM »
Huh, then I won't need those, definitely  :) I thought that it might have something to do with hum or something...

Thanks guys, you've been a great help!

DJPhil

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Re: Adapting a preamp with single power supply to dual
« Reply #9 on: December 12, 2010, 02:54:59 AM »
They are protection diodes as correctly stated above, but they are not grounded, please check that the "free" end goes to either +15V or -15V; so the actual threshold is a whopping 15.7 V .
They will not clip a guitar signal, they are there to protect the input op amp from those guys who enjoy plugging a speaker out into another amp's input.
You say those don't exist?
I know a few !!!! :duh
They do not even be too  :loco , most people think that headphone out can be used as line out.
Definitely not, most bean counting designers save a 0.001 cent resistor and headphone outs slam the full speaker voltage (think 16 to 40 V RMS) into high impedance loads (pre inputs).
Ah yes. To get the effect I was describing regarding clipping you'd have to connect them to ground, not the power rails. I misread the schematic, I apologize.  :-[

J M Fahey

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Re: Adapting a preamp with single power supply to dual
« Reply #10 on: December 12, 2010, 06:47:41 AM »
Don't need to apologize, the intuitive explanation that pops out at first  is what you "saw".
I catched it, say, after I saw it about 12574 times :'(
Oh well.