Pavel Macura buffer as a preamp.... - diyAudio
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Old 10th November 2003, 02:14 PM   #1
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Default Pavel Macura buffer as a preamp....

HI!

I've just finished a buffer by Pavel Macura using it as a preamp instead. I think his design may have been discussed before - couldn't find any threads though.

It utilizes OPA627 with a BUF634T in it's feedback loop. BUF643T can handle current of uptil 250ma, so it should have what it takes to drive a power amp.

It sound nice, stereo perspective is wide and noise is all most nonexisting. Putting my ear to the speaker I can only hear plenty of "nothing"....... BUF634T runs in class-A with an Iq of 15mA. I gets a little varm and seems to sound better after running an hour or so.

I have done a few mods that I would like to have some feedback on:

I have increased power to +/- 18 volt to have more dynamics.

Is this a good idea?

I would like to have more gain. I have changed R4 to 22K. Can I just raise it until I get the gain I want?

Can I remove C1 (1uf) and the 470p cap sitting over R2?

Can I omit C10 (18p) going from pin 2 to 6 on OPA637T?
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Old 10th November 2003, 02:16 PM   #2
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Old 10th November 2003, 02:34 PM   #3
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Hi jazzpeter,

you can use supply voltage of +/-18V. In any case I would recommend to mount small heatsink onto BUF634T when operated with Iq=15mA.

Voltage gain can be reasonably increased up to +20, in case that you need it.

Input cap (1uF) prevents the buffer from possible DC shift due to preceding circuits, you can omitt the cap, but probably it is safer to keep it. Foil cap would do no sound harm, IMHO.

The 470 pF cap creates a HF filter with R1 and it is better to keep it, it also decreases possible intermodulations with HF.

18pF cap is not needed for high gain as you have, it is connected for buffer application (G=+2 unloaded).

OPA637 can be used for gain higher than 5, as is your case. Not for buffer application (G=+2).

Pavel Macura
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Old 10th November 2003, 02:58 PM   #4
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Default More gain....

HI Pavel,

How do I increase gain? Raise the value of R4? By how much? Now it's 22K which doesn't seem enogh to give the "swing" I need to feed my poweramp.

Is there a formel to calculate exact gain?

PS: I forgot to mention: This circuit was built in two hours a sounds great . . Highly recommended for a compact preamp. Noise is incredible low using OPA627AP.
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Old 10th November 2003, 03:12 PM   #5
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Hi Peter,

the gain can be calculated as G = 1 + (R4/R3).

Now (for R4=22k) you have voltage gain of 7,67 (17.7 dB). For voltage gain of 20 (i.e. 26dB) I would recommend to change R3 to 1k2 and R4 to 22k (as you have it now). The gain will be
G = 1 + 22/1.2 = 19.33, i.e. 25.7 dB.

P.S. The calculations hereabove are valid without terminating resistor R12 50 Ohm (that should be placed at the end of output cable). R12 creates a divider with R10 and reduces calculated gain to one halve compared to calculations. With R12 the gain is:

G = 0.5 x (1 + (R4/R3))
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Old 10th November 2003, 03:50 PM   #6
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Per-Anders just sent me a link to his page. He says that Your design is very similar to his.

Look at: http://home.swipnet.se/~w-50719/hifi/qrv04
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Old 10th November 2003, 06:23 PM   #7
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I have been utilizing the circuit circuit similar to Audio Buffer since 1997 for measuring purposes. In fact it is a slightly modified datasheet circuit by Burr-Brown. In spring 2002 I tried it for audio - and it succeeded. I know about Per's pages.
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Old 10th November 2003, 06:27 PM   #8
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Quote:
Originally posted by jazzpeter65
Per-Anders just sent me a link to his page. He says that Your design is very similar to his.
Meta42, qrv04 and Pavel's design is more or less the same and also what the datasheet of BUF634 says.

Peter the reason that I emailed you was to avoid attention from somebody.... and the reason was just technical talk, no spam.

Pavel, I think we have discussed it before but I don't understand your cable interface. Why not just add so much resistance so you will get the output impedance equal to the cable? If the cable has 50 ohms (just an example) you chose 51 ohms as series resistor and also as load in the other end. Isn't this the normal procedure?
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Old 10th November 2003, 06:30 PM   #9
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Per, that's exactly how it is. Please visit my www (just click the button below), and see Audio Buffer. There is a column about placement of the resistors, R10 is in series with the cable and R12 is at the end as a terminating resitor.

From the engineering point of view, R12 seems to be useless for audio frequency band. In fact this resistor changes the sound impression in a very positive way. It is worth trying
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Old 10th November 2003, 06:31 PM   #10
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Jazzy Peter,

I would also lower R2. With the gain of one you have it is OK, but raising the gain to 22 means you will also amplify any thermal noise of the 100k R2 22 times. 10K would still be high enough to cause no trouble to your sources. But you would want to increase the input coupling cap to keep the same lf cut-off point.

Jan Didden
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