Increasing output channels of preamp.

hugobross · 15th January 2002, 12:05 PM

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.

paulb · 15th January 2002, 01:01 PM

Good opamps.

Geoff · 15th January 2002, 02:03 PM

Hugobross

Do you wish to be able to increase the voltage level of individual outputs relative to the voltage output of the pre-amp or merely to reduce it? The optimum solution will depend on this.

Geoff

hugobross · 15th January 2002, 07:05 PM

The meaning is to be able to reduce the amplitude per channel.
So we will need per channel a potmeter to reduce amplitude.

regards,

HB.

Geoff · 15th January 2002, 08:16 PM

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

DieterD · 16th January 2002, 10:19 AM

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

hugobross · 16th January 2002, 01:49 PM

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)

,

HB.

Geoff · 16th January 2002, 01:58 PM

HB

Best of luck with your exams. When you get back to your project, please remember that the NE5534 will require additional compensation when used as a unity gain buffer. The other op-amps I mentioned are all unity gain stable and do not normally require a compensation capacitor.

Geoff

AudioFreak · 16th January 2002, 02:02 PM

Geoff, is the LF411ACN unity gain stable? how can i tell from the data sheets if an opamp is or is not unity gain stable? Thanks again.

hugobross · 16th January 2002, 05:26 PM

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.

15th January 2002, 12:05 PM	#1
hugobross diyAudio Member Join Date: Jun 2001 Location: Flanders, Belgium	Increasing output channels of preamp. 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.

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15th January 2002, 01:01 PM	#2
paulb diyAudio Member Join Date: Jun 2001 Location: Calgary	Good opamps.

15th January 2002, 02:03 PM	#3
Geoff diyAudio Member Join Date: Apr 2001 Location: UK	Hugobross Do you wish to be able to increase the voltage level of individual outputs relative to the voltage output of the pre-amp or merely to reduce it? The optimum solution will depend on this. Geoff

15th January 2002, 07:05 PM	#4
hugobross diyAudio Member Join Date: Jun 2001 Location: Flanders, Belgium	The meaning is to be able to reduce the amplitude per channel. So we will need per channel a potmeter to reduce amplitude. regards, HB.

15th January 2002, 08:16 PM	#5
Geoff diyAudio Member Join Date: Apr 2001 Location: UK	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

16th January 2002, 10:19 AM	#6
DieterD diyAudio Member Join Date: Dec 2001 Location: Howick South Africa	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

16th January 2002, 01:49 PM	#7
hugobross diyAudio Member Join Date: Jun 2001 Location: Flanders, Belgium	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) , HB.

16th January 2002, 01:58 PM	#8
Geoff diyAudio Member Join Date: Apr 2001 Location: UK	HB Best of luck with your exams. When you get back to your project, please remember that the NE5534 will require additional compensation when used as a unity gain buffer. The other op-amps I mentioned are all unity gain stable and do not normally require a compensation capacitor. Geoff

16th January 2002, 02:02 PM	#9
AudioFreak Warp Engineer On Holiday Join Date: Oct 2001 Location: Queensland, Australia	Geoff, is the LF411ACN unity gain stable? how can i tell from the data sheets if an opamp is or is not unity gain stable? Thanks again.

16th January 2002, 05:26 PM	#10
hugobross diyAudio Member Join Date: Jun 2001 Location: Flanders, Belgium	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.

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