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Old 26th July 2002, 02:22 PM   #1
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Question preamp input and output levels

hello all,
im building an mosfet power amp and offcourse i also need a preamp thingy. I saw a nice and not to complicated scheme @ http://sound.westhost.com/project02.htm using NE5532 opamps.

i have a couple of questions about it

1) my preamp has 1.1 V RMS input sensitivity. i want the preamp to delever exactly 1.1 V RMS when volume is @ max. I also want to be able to use this preamp eventually with other amps, which may have a different input sens, so this should be adjustable. How can i do that ? I have not a lot of experience but i see two possibilities:
- making the gain of the final stage adjustable with a trimmer (pcb pot)
- setting the gain of the final stage to a rather high value and use an adjustable output resistor devider to adjust it to 1.1 V RMS

with both possibilities it is easy to adjust the output to a given value (eg 0.5 v rms, 1 v rms, 1.1 v rms), enabling the use with other preamps.
i think the first solution is preffered. Are there other ways to do this ? The pot wil be carbon and not a regular metalfilm resistor. Is this bad ?

2) the input levels.
I want to use the preamp with eg cd player or a tuner or a soundplaster (not phono). These have different output levels (ranging from 500 mv rmd for a tuner (?) to 2 v rms (?) for a cd player). When i attach a cd player instead of a tuner, i dont want a volume boost. So, all the entries to the preamp should eventually be at the same level. How can i accomplish this. I see one possibility: take down all te voltages to a predetermined value (say 250 mv RMS) with adjustable resustors and then set the gain of the preamp so it produces say 1 V rms (adjustable according to 1) output).

what are the usual solutions to these 'problems' ?
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Old 26th July 2002, 02:29 PM   #2
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another thing:

when i make a pcb for the circuit:
- is the use of a ground plane better or worse than tracks for the ground ?

- is it prefferend to mount the components around the treble and bass pot (the 3 resistors and 2 caps) on the pots themselves and then connect the circuit to the pcb, or mount the resistors and the caps on the pcb, and then wire the pots to the pcb ?

- same question for the vol / balance pots
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Old 27th July 2002, 10:28 AM   #3
trwh is offline trwh  United Kingdom
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Hi

I suggest you build Rod Elliottís revised preamp design instead, which is here http://sound.westhost.com/project88.htm. This is an improved design, and uses the OPA2134 opamp, which offers a worthwhile improvement over the NE5532 you mention. Also, PCBs are available for the revised design - I strongly suggest you use them. The PCBs even include spaces for jumper switches to adjust the preamp gain, which sounds just like what youíre after. On top of this Rodís prices are very reasonable, and successful operation is almost guaranteed first time.

I think you are a little confused in your calculations. You quote figures for the maximum output level of your sources - these are quite valid. However, not all CDs are recorded so as to cause the player to constantly hit 0dB (around 2V RMS output as you say). I have some classical and blues CDs that peak at only 10-20% of this figure for most of the time. Only heavily compressed mass-market music continuously peaks at 0dB - it seems that if a CD sounds loud, Joe Public loves it even more!

So, you need to have enough gain to allow quieter recordings to drive your amp to full output, with the volume control at, say a 1-2 oí clock position. FYI, my power amp has an input sensitivity of 1V, and I use a preamp with a gain of 5 successfully. If youíre bothered about source level matching, you can simply include a preset pot at the input of the louder sources, as you say. However, the level jump really doesnít bother me - I rarely move sources during a listening session.

Hope this helps,
TRWH.
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Old 27th July 2002, 04:44 PM   #4
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Sound advice, TRWH

I am building a pre-amp using Rod Elliot's PCB. He makes the units very flexible and easy to construct. I strongly suggest that you use his design. He delivered his product within a week of payment (not bad for around the world) and kept good correspondence with me.

If you would like to keep your levels resonably close between sources, you'll have to use some resistors (a voltage divider will work) to bring down the loud sources. Then set the gain of your pre-amp accordingly. Rod's pre-amp utilizes DIP switches to set the pre-amp gain. But, a number of solutions are available to you.

Good luck,
-Dan
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Old 31st July 2002, 08:33 PM   #5
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Normally inputs must be around 150 mV to 250 mV and outputs at 770mV to 1500mV. Thats an amplification of 5,5 -6,5 times as in most preamplifiers.
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