Increasing output channels of preamp.

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Hi,

does anyone have a idea how to increase the amount of output channels of a preamp (or mixer)? I'm looking for something to connect on an excisting output, with four (or more) in volume adjustable outputs. I was thinking about placing a buffer after the output of the preamp to prevent too low impedances for that output. After that buffer I would connect the extra pre-amps with the gains adjustable by potmeters.

I could realise this with simple opamps, I know how they work and how to calculate the resistors etc... But I'm not very experienced with opamps practically speaking. Which types are usable for good audio performance?

I could also use transistor stages, it's a little more complicated than previous method, but I can handle this.


If you have any ideas or excisting schematics, please let me know.


Many thanks to this GREAT forum!!

Hugobross.
 
HB

Any competant pre-amp should be able to drive its full rated output into a 600ohm load so it should be possible to use a completely passive arrangement. Four 10kohm pots in parallel should not overload the pre-amp but you would need to keep the connections between the pots and the following equipment reasonably short (< 500 to 750mm) to prevent high frequency roll off.

For longer cable lengths, or to be absolutely on the safe side, a buffer (eg OPA 634, BUF04) or an op-amp wired as a non-inverting unity gain buffer between each pot and output would be required. If there is any doubt about the pre-amp's drive capabilities then a further buffer between the pre-amp and the pots would be desirable. All this is very much as you indicated in your original post.

The buffer ics tend to be relatively expensive so it would probably be more economic to use op-amps. When wired as a unity gain buffer and with a limited (<2Vrms) output voltage swing there is probably very little audible difference between various op-amps once a certain quality standard has been reached. Any differences become far more apparent as the gain and output voltage swing become greater.

In view of the fact that you are going to need up to 10 of each device for your four stereo outputs, I do not think that the cost of something like an OPA627 would be justified unless you have some very high quality equipment in your system. Instead, I would suggest that you consider the NE5534 (or NE5532 dual), which has a very good performance for its price, or the OPA134 (or OPA2134 dual) which is a somewhat more expensive but has a good reputation. Another alternative would be the OPA604 (OPA2604 dual).

If you are interested in some comparison measurements of these devices, take a look at the 'elements of op-amp design' article at Doug Self's website:

http://www.dself.demon.co.uk/ampins.htm

Geoff
 
HB

I have a DIY for guitarists book where they have designed a "spluffer" (splitter / buffer) the idea being to split and buffer a guitar signal into two so that you can drive two amps, or effects pedals etc. I have designed a board for it but doubled it so that I get four outputs for my son's PA system, but have not got around to building it.
I can send the schematic and board if you want.
The board was designed on "Eagle Lite" but if you want I can convert to a GIF or something.

Dieterd
 
thanks,

Geoff, I will check out the types you've mentioned, but right now have to spend some time to study electronics for school - on highschools we've examinations here in Belgium at this time. I will start to design something in two weeks; I will post a reply then to say what I've done to realise that; thanks!

Dieter, can you please post the image in this thread? Then I can compare it in what I've in mind to build.


See you soon (after studying) :rolleyes: ,

HB.
 
Geoff,

Looking to the prices let's start with the cheap ones, the extra amount of outputs are only used in cases we're going to play on events or parties where more than three poweramps are needed.
So I, as the "technician (hmm)" of our group, have to create something in cases we'll need more outputs. So this whole schematic will not be used that frequently, and is only ment to drive heavy power amps playing some loud music. I just wonder if someone can hear in this case the difference in quality between those relative expensive opamps and the cheap ones such as the NE5532?

Do you mean that the ne5532 needs a capacitor (22 pF) between pins 5 and 8. I just looked at some page of rod's:
http://sound.westhost.com/project30a.htm

He uses 22pF between pins 5 and 8; is this what you mean with additional compensation??

What about the TL072 he also uses?

best regards,


HB.
 
HB

Please do not confuse the numbers. You refer in your last post to the NE5532 and then make reference to Rod's diagram (which uses the NE5534) and 22pF capacitors.

The NE5534 is a single op-amp which is stable for gains equal to or greater than three. For gains below this, additional external compensation is required to ensure stability. This takes the form of a capacitor connected between pins 5 and 8 with a typical value of 22pF.

The NE5532 is a dual op-amp that is unity gain stable and does not require external compensation. It is basically two NE5534s in one case, each with additional internal compensation to provide the unity gain stability.

The TL071 (single), TL072 (dual), TL081 (single) or TL082 (dual) could be used in your application and have the benefit of being cheap. However, the NE5532 and NE5534 are better op-amps for audio applications. I would suggest that you use the NE5532 since it is cheaper than two NE5534s and also saves the cost of the compensation capacitors. You will also see if you visit Doug Self's site that it's measured performance, so far as %THD is concerned, is good even when compared with some op-amps costing many times more.

Geoff
 
hugobross said:
I just wonder if someone can hear in this case the difference in quality between those relative expensive opamps and the cheap ones such as the NE5532?
I wonder that too. My guess is, in most cases, probably not. You can always use sockets (not cheap ones!) and try it out.
My understanding is that the NE5532 is a workhorse in the audio and recording industry, so it's a good reference point anyway.
 
AudioFreak

Many manufacturers indicate in the general description or features section of the datasheet whether the op-amp is unity gain stable or not. If it is not, they normally quote the gain below which additional external compensation is required and give suggested capacitor values for different gains somewhere later in the datasheet.

National Semiconductor do not make things so easy in that they do not make reference to unity gain stability. You need to look for statements such as "External components are used to frequency compensate the amplifier." or "The LMxxx is a general purpose operational amplifier with external frequency compensation." or "The amplifier is internally compensated and stable for gains of 10 or greater."

In the absence of any of these statements it is reasonable to assume that the op-amp is unity gain stable. As a cross check, have a look at the pin-out diagram. If there is a need for external compensation under certain conditions of operation you will find a pin labelled 'comp'. The schematics in the 'typical applications' will also show if a capacitor is required, as will the the graphs of 'frequency response for various closed-loop gains' which will normally quote the capacitance value at the various gain levels plotted. In addition, sometimes there is a graph showing the compensation capacitor value at different gains etc.

If you check the Nat Semi datasheet for the LM411 against these criteria, you will find (by default) that it is unity gain stable, despite there being no specific statement to this effect.

Geoff
 
HB
Here is the schematic

Dieterd
 

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Hi, I've finished my exams with success, so I'm back on the road...

I was thinking of something I've publiced here. I'm quite sure it will work, but I've still a doubt: will there be a bigger influence of distortion at the input because of the high impedance of the opamp (is an extra parallel resistor needed here?)

Best regards,

HB.
 

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Congratulations on finishing your exams with success!
A couple of suggestions: You should have an input resistor to ground to keep the first op-amp's input at zero if there's nothing connected. You should probably have an input capacitor unless you can guarantee that your source will always have no DC offset. And don't forget to bypass the opamp's power supply pins.
 
Thanks PaulB!!!

Indeed, you're right, I will place a 10k (or 4k7) input resistor to avoid/reduce distortion. There's no extra capacitor needed because the source (a quality mixer) is DC-protected.
After building this design, I will build a own designed amp that I will publish soon here; I've had bad experiences with other designs on the net so I just decided to do it myself. At school everything runs fine by now, especially on designing amp's (thanks to this forum).

HB.
 
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