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Old 20th March 2003, 05:46 PM   #1
pundip is offline pundip  Australia
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Question PreAmp Regulated Supply

I am currently in the process of building a power amp and the preamp will be sharing the same case. After advice from this forum I settled on using http://sound.westhost.com/project97.htm
this project as my preamp and tone control. I have come accross a fair bit of information but need of advice and clarification on the following points.

firstly I am looking for -+15v to feed 3 tl072 (this will be swapped with opa and lm opa amps to paly around)

I am thinking of using 78x regs 1A or 2A a
seprate ei core transforer will be used.


1)Are 78xx regulators sutiable for this task and if not is there a comparativly simple solution that will offer greater benefits.

2) I am thinking of putting a set of 1000uf caps after the rectifier/before regulator and a set after the regulators. Is there a configuration that offers any significant benefits without too much complication.

3) The power op amp is being feed with supply with two rectifiers (one for each rail) and the terminals are being bypassed with
100nf ceramic caps. Its this two rectifier setup suitanble for a pre
amp supplies as well. Soft recovery diodes? are they suitable in this situtaion.

4) Am i correct in assuming that using this supply to feed anything else but the preamp (eg clip indicator, peakmeter) will comprimise the quality of the supply. Is the simplest solution with the least impact to quality of sound is to take power off the main supply and run it through the 78reg to step down. if this is the case what is the best point to tap main supply. I have +-35v being run by 300va torid.

I have a limited understanding of electronics and am really crap at doing pcb layouts so simplicity will be greatly appreciated.
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Old 20th March 2003, 06:30 PM   #2
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well, they are somewhat better than adequate, but remember that 78LXX and 79Lxx regulators regulate and <b>add</b> some tens of millivolts of noise.

there are several regulated power supply threads at diyaudio. From my personal experience, the best <b>simple </b>regulated supply I have built for a phono + preamp stage used a pair of LM317/LM337's in the positive and negative supply rails. I.E. the first LM317 regulated to 18 volts, the second LM337 regulated to 12 volts, and vice versa with a pair of LM337's on the negative rail.

Oh, and by the way, make sure that all the power supply pins of the opamps are decoupled with 100N bypass capacitors.
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Old 20th March 2003, 06:51 PM   #3
mlloyd1 is offline mlloyd1  United States
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Look at the bottom of this page for via circuit using the 78xx, 79xx parts .

Nice high-end touches.

mlloyd1
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Old 20th March 2003, 07:11 PM   #4
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Default Hagerman just got a nice review in AX

AudioXpress just had a review of phonopreamps -- Hagerman (whom you cite in the link) came out on top -- uses the OPA2134 I believe.
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Old 20th March 2003, 11:02 PM   #5
mlloyd1 is offline mlloyd1  United States
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Default hagtech riaa preamp wins shootout

yep. should do a lot for mr. hagerman's business.

i bought a board myself out of curiousity

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Old 21st March 2003, 12:09 AM   #6
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Default REGS.

Hi,

Generally speaking: the mua78/79xx series is pretty much outdated...unless you use them for a particular purpose.

To me, they're just fine for heater reg. in tube circuits.

Mind you, you can put their limited performance to your advantage.For example, you can take advatage of their poor bandwidth for filtering out digital nastities.

Other than that the LM series is quite superior in performance.

Cheers,

/Horses for courses.
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Old 21st March 2003, 12:10 PM   #7
Morph is offline Morph  United Kingdom
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There is good design for a pre amp powere supply on rods site. Why not use that as you are build the pre amp using his design as well.
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Old 21st March 2003, 01:03 PM   #8
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Default improved 78xxx/79xxx

How about a 78xxx ( or 79XXX) followed by a capacitance multiplier ? It should filter out the noise ?
Cheers.
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File Type: jpg 78xxx.jpg (6.8 KB, 1985 views)
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Old 21st March 2003, 02:43 PM   #9
pundip is offline pundip  Australia
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Jackinnj,
I did consider using the LM regulator. Do you happed to have a schematic for -/+supply that you used.

Does anyone else have an opinion one way or the order regarding using one or two rectifiers and the placement of ripple filter caps.
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Old 24th March 2003, 11:53 AM   #10
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Default Double regulation.

You can use double regulation, with the 78xx and 79xx .
That is, use 7818 and 7918 first and then 7815 and 7915.
Before the first regulators I would use 3300 uf, 1000 uf sounds weak to me.
After the regulators, something like 100 uf is fine.
Always bypass with 100 nf ceramic caps.
It's a simpler design than the LM317/LM337, but the LMs are better.
And double regulation with LMs?
Oh man, this never stops...

Cheers
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