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Old 28th May 2015, 04:51 PM   #1
fjc is offline fjc  Taiwan
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Default determine element value for the pre-amp package I am working on

I am building this kit board base on the schematic diagram above. Obviously I like to alternate some of the design value of resistors and capacitors and hope for a better sonic performance based on my DIY experience.
I like to change R1 to be 100 ohm a buffer resistor, and also add an output buffer resistor with the same 100 ohm. As for the first stage amplification, the negative feedback capacitor value C3 of 100 p will be switched to 22p, 100P seems too large for smoothing of the OP ringing effect of square wave, may be the 100p was deliberated added to have a smooth warm effect to the ear. On the second stage amp, the feed back capacitor will be changed also to be 10p-15p. Besides the Impedance resistor R2 and R5 will also be changed from 10k to normal figure of 47k-100k ohm, 10k will be low for an impendence value setup. I will reduce the amplification rate from 6x2=12 to 8.5 by alter the amplification on the second stage, alter R6 and R7 from 2k2, 2k2 to be 1k, 4k or set R6 and R7 to be 0.5k, 2k. Can you generously commit on new values I choose on R1, R2/R5, R6, R7 and C3 (1, 2)? I was never tried to check the sonic difference on resistor value (say, (R6+R7)/R7 on a same amplify rate. This schematic diagram is a simplified version based on the famous MBL 6010D pre-amp.
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Last edited by fjc; 28th May 2015 at 05:34 PM.
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Old 28th May 2015, 05:14 PM   #2
Mooly is online now Mooly  United Kingdom
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Moved to Analogue line level.

What do mean by the 'the element value' ? Do you mean the two presets ? If so then the left one is adjusted for 0.00 volts at N1 output and only after adjuting that is the other adjusted to give 0.00 at N2

Its not an ideal circuit arrangement though.
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Old 28th May 2015, 05:50 PM   #3
fjc is offline fjc  Taiwan
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W1 and W2 are designed to adjusting zero DC output and has the benefit not using out put capacitor or DC servos, thanks Mooly for suggest LT spice. I do not use much of the LT spice and afraid not to get the maximum benefit from it. I will try to learn. I have a question on select the pair resistors value of R6/R7 and on sonic effect. The rest of the change on base on my common experience on DIY pre-amp. I do know MBL has good reasons on the design. They are famous to use power supply shunt capacitors that not showing in the diagram.
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Old 28th May 2015, 06:27 PM   #4
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R6/R7 set the gain of the opamp stage so you need to arrange them to suit your requirements. For such low gain surely one opamp stage would be better than two.

This is the proper way to zero the offset.
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Old 29th May 2015, 06:27 AM   #5
fjc is offline fjc  Taiwan
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Thanks Mooly UK, I went to engineering graduate school of Sheffield Univ. at 1987.
One question about Impedance resistor R2 and R5, I like to change from 10k to normal figure of 47k-100k ohm, will the impedance resistor R2 has any connection with the max. resistor value of the volume control ? I am using 24th step volume control of 20k ohm. I know R2 along with R1, and C1 relate to the range of incoming freq. cut off. So large value should have a wider freq. response, at least 100khz is needed for my requirement even at low noise purpose. If not, I like to switch to at least 47k ohm on both R2 and R5. Do not know why MBL apply 10k ohm on R2, R5, Is it because of recommended 10k ohm variable volume control? And I will try the offset method on my diagram, it is hard to alter the already existed PCB. As time goes by, my pre-amp circuitry goes from end capacitor 4.7uF to DC servo control, now I like to try the trim method to eliminate dc. At modern design and making of the OP, the offset is already very low, unless an unbalanced power failure occurs. Please advise on R2/R5, thank you.
I still wonder about the sonic difference to choose R6:R7 for same power ratio of 1.5.,(ex, 1k, 2k or 0.5k, 1k both gives same amp ratio of 1.5 times)

Last edited by fjc; 29th May 2015 at 06:28 AM. Reason: correction
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Old 29th May 2015, 06:39 AM   #6
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R2 will modify the law of the volume control because it appears as a parallel resistor from wiper to ground. You can actually make a linear pot appear to behave like a log pot by correct choice of value. Your R2 isn't low enough to do that but it will still alter the pots original law slightly.

R5 has virtually no effect and is not needed.

For minimum DC offset you should really aim to keep the bias currents in the opamp input equal... but that can't be done if you DC couple the volume control.
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Old 29th May 2015, 11:19 PM   #7
fjc is offline fjc  Taiwan
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TKS. Since I am using a 20k ohm self-made step ladder, I will fix R2 to be 25 k or 47k ohm. feedback C3 will goes to 22pF high Q grade, and R5 is no need indid nor C2 as a noise reducer by using low noise component and OP, C1 is sufficient to do the job, besides for high impedance purpose, a extra buffer PCB is added in front before the volume that has a 100k ohm R1 configuration. Thanks again for Mooly's effort in discussion. I always thought of asking help when I have doubt about DIY my gears. Last time I was asking components of drives and output power transistors of rehault my 125w power amp. in DIYaudio and got excellent help, Amazing.
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Old 30th May 2015, 06:06 AM   #8
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You're very welcome

Hope it all works OK..... it should do.
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Old 30th May 2015, 08:14 AM   #9
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Last thing, Mr. Mooly, while using a 20k ohm step ladder, what would be the difference if I set up R2 to be 20k, 50k or 100k ohm, I just like to learn a bit more. Thanks.

According to web clone DIY information, to get good sonic performance, matching of quality components, power supply quality and transformer quality are all vital factors besides selective OP.
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Old 30th May 2015, 10:54 AM   #10
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You would need to plot it out or simulate it for each value. Have a read at these two links,

ESP - A Better Volume Control

Potentiometers (Beginners' Guide to Pots)

Good decoupling is always very important with fast opamps.
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