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Old 10th August 2006, 08:19 PM   #1
JaRoD is offline JaRoD  Finland
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Default Diy preamp/active crossover

I've started thinking about making a simple preamp with active crossovers after I found that using the kx drivers for a soundblaster soundcard didn't work that well. Anyone have a design to share or a link to some simple preamp?

The crossover part is easy but I know almost nothing about how to design a preamp. Have been looking around a little and people seem to either build "complex" discrete component preamps or passive ones. Haven't really found any inbetween, why is that? Wouldn't it' be enough with just a few opamps since the amplification doesn't seem to need to be that high in preamps?

Have also realised that I might be able to use this as a final project in school too. But just a preamp/crossover would probably be too simple. So since I've use microcontrollers a little I could add a display and a remote. After reading around here I found a thread where they spoke about using LDRs/LEDs for a volume control. Implementing a remote and using that wouldn't be that hard when using a microcontroller.

In the crossoverpart I'll most likely use 4th order LR crossovers for the tweeter/mid/sub. So what do you people think? And please do tell me what to think of when designing the preamp part.

And thanks beforehand if someone has a good tip/idea and forget to thank them!
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Old 10th August 2006, 09:11 PM   #2
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I'm far from an expert, and I've never built a phono stage, but these two are the ones I've found attractive as I've looked for easy & nice phono stages to build in the future:

VSPS - the Very Simple Phono Stage
http://www.geocities.com/rjm003.geo/.../diy_pho5.html

NSSPS - the Not So Simple Phono Stage, a fashionably (and IMO sensibly) tweaked version of the VSPS:
http://www.geocities.com/rjm003.geo/.../diy_pho5.html


Edit: huh? Why did I get the feeling you were asking for a phono stage, when you weren't? Christ. Oh well. The links get to stay.
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Old 10th August 2006, 09:17 PM   #3
preiter is offline preiter  United States
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A pre-amp can be as simple or as complex as you want. Possible features are volume control, source switching, tone controls and balance control, RIAA equalization for a phono input.

Figure out what features you want to include, then go looking for the circuits to implement them.

I built a pre-amp / active EQ. It's just an input stage with gain -> pot for volume control -> high pass and low pass filters (plus a unity gain buffer for a full range output)
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Old 10th August 2006, 09:46 PM   #4
JaRoD is offline JaRoD  Finland
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lol, thanks anyway kristleifur

Quote:
Originally posted by preiter
It's just an input stage with gain -> pot for volume control -> high pass and low pass filters (plus a unity gain buffer for a full range output)
What's a unity gain buffer? And something similar to what you've built sounds right for me.

Will have to install some dipole compensation too since my main speakers are dipoles. And then there's the problem if I change the elements/build a new speaker. But I guess it's not thar much trouble to remove and put in new resistors/caps in the crossover part.

Oh, and what opamps are great for audio? The crossover I have now uses TL074 and according to what I've read they are good but not great. But atleast they are really cheap. I've heard that burr brown has good opamps but it seems my local distributor doesn't have them.
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Old 10th August 2006, 10:12 PM   #5
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OK bleh, the NSSPS link was even wrong. For reasons disclosed below, I'm posting the correct one:
http://www.ecp.cc/NSSPS.html

Quote:
Originally posted by JaRoD
What's a unity gain buffer? And something similar to what you've built sounds right for me.

Oh, and what opamps are great for audio? The crossover I have now uses TL074 and according to what I've read they are good but not great. But atleast they are really cheap. I've heard that burr brown has good opamps but it seems my local distributor doesn't have them.
A unity gain buffer is sort of like an opamp with the following characteristics:
* High imput impedance
* Good output capabilities, usually higher current output than ordinary opamps
* A gain of 1 ("no gain", that is -- or unity gain). It means that the signal's power is multiplied by a factor of one.

The opamp has an easy time driving the buffer because of the buffer's high imput impedance (because the opamp doesn't have to push as much current into the buffer as it would when driving lower-impedance loads). The buffer then drives whatever is connected, for example longer cables than would work with just an opamp. This configuration is also used for headphone amplifiers, for example.

There's quite a lot of good literature on headphone amplifiers. Headphone amps aren't that dissimilar from a preamp.

Here's a nice page about different opamps by the respected Tangent:
http://tangentsoft.net/audio/opamps.html
This should give you a good start.

If you look at the phono-stage links I posted, you'll notice that they use the AD825,NJR5532 OPA627 or OPA134.

Tangent's writeup on opamps is focused on opamps driving headphones directly. The requirements for an opamp in a preamp are a bit different, but in general, the preamp could be considered an easier job for an opamp. For example, the TL07 is considered rather bad for driving headphones, but is considered workable in a preamp, though not great AFAIK.

Actually, the phono stages I posted are pretty close to preiter's description of a preamp. The gain is different, probably quite a bit higher than you'd want in a normal line-level preamp. There's also RIAA EQ-ing for vinyl audio in the phono preamps, which in essence is the same as high-pass or low-pass filters you'd find in a normal preamp, only with different filter values. You could therefore get a pretty good idea about how preamps work by looking at the phono stages I so rabidly posted, if you don't find any good literature that answers your specific questions about preamps. The VSPS is a very simple design, which means that it should be good for looking at and learning from.

The M^3 headphone amp can also serve as a preamp:
http://www.amb.org/audio/mmm/

It's considered quite a good design. In this case, each opamp drives a set of MOSFET transistors. In the M^3, these MOSFETs serve as the "unity gain buffer". It's a bit expensive for a preamp, and very likely overkill for your purposes, but it's likely that some education could be had by studying it. In the end though, I'm sure that you'll get good links to "purer" preamps from other members here. I mostly know about headphone amps, and not that much, even

Good luck, and keep us posted.
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Old 10th August 2006, 10:19 PM   #6
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Krister - I thought the same thing since the question was in the analogue forum.

JaRoD, have you looked at www.sound.westhost.com? IIRC Rod has some preamp projects that may be appropriate.

As for opamps, I'd stick with moderate speed devices like the OPA2134 or OPA2132 unless you are experienced laying out circuits. The high speed favorites like OPA627, LM6172, AD826 are fussy about power supply bypass and circuit layout - they'll oscillate relatively easily. I like the NE5532, although many rail against its age and bipolar inputs. It handles low impedance loads with ease.

For a basic preamp/XO (assuming that you have or don't want a phono preamp you could go as simple as a pot feeding your XO section (with source selectors if required). I do that with my office system driven by a computer sound card. A simple buffer with around 6 dB of gain driving the pot would probably suffice in most low level source situations.

A buffer can be an op amp. For unity gain, you connect the output and negative input. Drive the positive input - be sure to have some resistance to ground from the positive input. For 6 dB of gain you'll connect a 10K resistor from output to negative input, and another from the negative input to ground.

You can make buffers and gain stages from single transistors or discrete opamps with many transistors. I'd recommend sticking with opamps, if this is your first attempt. The OPA2134 is fairly forgiving of circuit layout and sounds decent to boot.

As for the crossover parts, I'm running a group buy of crossover boards that is open until August 18th. They offer a few more functions than Rod's boards (several EQs, Linkwitz transform and Phase correction) that may or may not be useful to you. More information
Here or click on my www button for a description, component calculation spreadsheet, and ordering information. I am also buying power supply boards, and producing PSU kits with all of the required parts.

Hope this helps.

PS - if you aren't happy with the sound of your sound card based XO, chances are you'll be dissatisfied with LDR volume control, too. Take a look at a digital pot based solution - also microcontroller controllable, or my favorite would be to use a microprocessor controlled relay array. You could even put attenuators AFTER each crossover section to attenuate the noise generated by the previous stages as well as the signal. Put the attenuators between the filter outputs and the buffer stage and you've got a killer project.
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Old 11th August 2006, 05:25 AM   #7
JaRoD is offline JaRoD  Finland
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Out of the opamps you all have mentioned it seems my local store only has the NE5532. But I just realised that they can special order stuff, it's just that it can take weeks to get it but it doesn't cost extra.

And thanks for the answers so far, been very helpful And concerning the soundcard crossover, I have no idea why it sounded bad but it degraded the sound compared to the active filters I normally use.
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Old 11th August 2006, 11:15 AM   #8
owen is offline owen  United Kingdom
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Dont rule out the LF353 - it's available for peanuts everywhere, and to my ears sounds very good as a unity gain buffer, but then it does share a topolgy similar to the high powered opamps used by chipamp fans everywhere...

I set up the original buffer as inverting, as there are benefits to doing so - this however creates a phase inversion problem, so unless your amp is inverting too (as it was in my example) then set it up as non-inverting...

Owen
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