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Old 3rd May 2003, 08:47 PM   #1
svokke is offline svokke  Belgium
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Default check my preamp + supply

Before I go out buying components, could you check out my schematic?
The preamp is based on Rod Elliott's designs. I added a parallell opamp output buffer because I have to use long interconnects to my computer/amp.
I haven't decided on witch opamps I'll use yet, so I drew the schematic with the best opamp there is, the ideal one
The filter is a 3-way active filter in the same enclosure, also Rod's design but with the parallell output buffers.
The powersupply is pretty straightforward, not shown are the 10µF electrolytics+100nF caps close to each opamp for decoupling.
The 22nF capacitors will be ceramic, all others are electrolytics or philips mkp.
The transformer is 2*18V 30VA toroidal, the biggest one that fits in a 1unit rackmount enclosure.
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Old 3rd May 2003, 08:48 PM   #2
svokke is offline svokke  Belgium
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And here is the powersupply
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Old 4th May 2003, 05:54 PM   #3
moamps is offline moamps  Croatia
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Hi,

1. ps
ok, but don't use ceramic caps
7912 in and out?

2. preamp
too much opamps for my taste, input caps too large maybe, output caps too small (I prefer 10uF), paralleling opamps is for me bad idea, using buffer amps is better ( BUF634 like)(look PMAs posts)

I hope that my comments will encourage more people to comment.

Regards
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Old 4th May 2003, 07:55 PM   #4
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You have a mixup about the transformer connection.

If you use 2 rectifier bridges you connect each winding SEPARATELY to the bridges, don't connected them and make a center tap.

IF you use a center tap, use only ONE bridge.

Think once more of the maximum output impedance and drive capability! Use one opamp and 220 Ohms is series or a buffer i.e BUF634, if you want more power. It's nothing wrong with paralleling opamps, but do you need it?
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Old 12th May 2003, 09:44 PM   #5
svokke is offline svokke  Belgium
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Thanks for the replies, I'll use seperate windings on the transformers.
Concerning the rest of the circuit: back to the drawing board. I'll make an equivalent schematic of the cable used and the gainclone input and calculate if the parallell opamps are really needed. I think i got carried away with the parallelling opamps idea. Thought it wouldn't hurt, but now I see some of the disadvantages...

Regards, Svokke
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Old 12th May 2003, 11:08 PM   #6
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Thumbs up Why not

I like the idea of the paralleled op amps and I will bet no one can give you a good reason it won't work fine with the 100 ohm output resistors. What is your cable capacitance?

Can someone give me the URL for the IDEAL corporation and which op amp of theirs you would recommend?
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Old 13th May 2003, 05:40 AM   #7
Werner is offline Werner  Europe
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Either ditch the input capacitors, or the output capacitors. Using JFET opamps there is no need for them.

Keep perhaps the input caps, as you don't want any accidental DC on the volume pot. You may want to increase R1 to 100k or more, and use smaller input caps, so that the Mundorf M-Cap Zn or Audyn Zns (same thing) become feasible. Then again, for a first project ...

R2 needn't be 100k. You can use a 20k Alps Blue there, as it is properly buffered by U1.

Is there a real need for U2? Its gain can be absorbed into U1 or the downstream stages, and since the downstream stages are hi-Z anyway, there is no need to protect the volume pot from them.

If you use JFET opamps then balance the impedances around them: Z as seen from + must equal Z as seen from -, where possible. Or use an inverting stage.

Put a lot of though into wiring and grounding, as this can make or break a circuit. It helps if the circuit is simpler.

The voltage regulators can be followed by CRCRC filters, R 1-10 Ohms (LFD even use 75 Ohms after 7800s!)

Drop the Cs over the bridge diodes, but put small resistors between bridge and the first Cs.

Well, these are some of the things I would do anyway ...
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Old 13th May 2003, 06:18 AM   #8
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Talking Re: Why not

Quote:
Originally posted by Fred Dieckmann
I like the idea of the paralleled op amps and I will bet no one can give you a good reason it won't work fine with the 100 ohm output resistors. What is your cable capacitance?

Can someone give me the URL for the IDEAL corporation and which op amp of theirs you would recommend?
Hi fred, maybe this URL is helpfull for svokke
http://www.dwelle.de/rtc/infotheque/...opamps_02.html
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