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[Andrew Hamilton]

I'm looking for a circuit or parts ideas that will enable me to build a pair of (balanced) buffer amps for placing after a pair of balanced mono P&G faders (taken from a Ward Beck console).  I'm ideally looking for +24 dBu headroom and 0 dB gain and both amps need to be 4-to-6 k Ohms Z (and balanced).  The more help the better. ( :


thanks,
    _andrew

[Dave Hecht]

First, out of curiousity, why are you running balanced through  the faders? More common, and simpler to run unbalanced.  

 As for a balanced buffer circuit following the fader, you have several possibilities. You can feed the fader's balanced outs to the differential inputs of an opamp - anything from a 5534, to any of the discrete audio amps available. Not sure what you're referring to by saying the amps need to be balanced. If you're referring to the input of the amp, that's essentially what they're designed for. If you're referring to the output, typical opamps are unbalanced out ( I think Valley People made an amp with balanced outs called a Transamp years ago, can't think of anything current), so you would need an extra component (or components) to achieve this. One possibility would be using a transformer to balance the signal after the opamp. Another option would be to use a balanced line driver chip. Burr Brown makes a good one, the DRV135. That Corp also has a balanced line driver, can't recall the part number, but I believe they call that series "OutSmarts".

 I'm guessing this is for a post fader recovery amp / output stage. The +24dbu should be no problem with easily available components. As for the 0db gain, are you referring to the overall circuit, or the amp stage itself? Either is pretty easy to do, but need to know which you require.

 If you can give me a little more information, shouldn't be too hard to give you some more concrete suggestions.

Dave Hecht

[Jim Williams]

Very high quality differential opamps are now available from TI.
These are the THS6000 series of high speed, low noise/distortion diff opamps with differential inputs AND outputs. These were designed for driving balanced A/D converters and they have better THD specs than most single ended output opamps, including most all the audio grade parts. Some spec at -100 db THD at very high frequencies, like 10 mhz, quite a design feat. Extrapolate these specs down into the audio band and you will find extremely low distortion figures with the benefits of high speed design.

[Dave Hecht]

Jim,

  Thanks for that info. I'm not familiar with those particular amps. I'll have to look into those.

Dave Hecht

[Andrew Hamilton]

The faders I already have are balanced mono.  The channels they would be used to attenuate are balanced, as are the destination inputs, downstream.  In fact, the whole rig, from transfer D/A to rack to A/D to monitor D/A to preamp are all balanced ins and outs.  The monitor amp is balanced ins, as well.  I know that this isn't the only way to go, but, since I am able to stay balanced, I figure, why change at the trimming stage?



I need the amp to provide no gain, really.  I would like the top of fader's "-0" indicator to mean "Unity Gain" from the D/A channel that feeds it.  I think this means that both the circuit and the amp are 0 dB.  However, when the fader is at -3, I'd like the circuit to provide clean "gain = - 3dB."


I'll look at that differential I/O OpAmp.  Any more circuit ideas (how to breadboard them - I'm not an E.E., but I'm licensed to practice magnetic healing in three counties (j/k)), schematics, or DIYs would be super.

Thanks,
    _andrew    

[John Monforte]

I am not sure what you mean by "balanced P&G Faders". They did make stereo ones which can thoretically be used that way.

You might find that the faders will be the problem. The tracking of P&Gs can be pretty good, but even still, once optimized at one position, the degree of balancing will deteriorate at others. This is why nobody works this way. Common practice is to unbalance external signals, perform your processing, then re-balance afterwards. Sounds complicated, but it isn't and works quite well, in fact.

[Andrew Hamilton]

John Monforte wrote on Wed, 17 January 2007 12:30

I am not sure what you mean by "balanced P&G Faders". They did make stereo ones which can thoretically be used that way.

These are model #1550, which, in the incarnation that I possess, is a Ward Beck-destined balanced mono fader with 600 Ohms total Z.    
They have a stereo balanced fader, model 1552, also.  My understanding, which may be mistaken, is that if I were to use one of these faders to attenuate both left and right channels, in single-ended configuration, rather than the + and - leads of one channel only, the impedance would drop to 300 Ohm/channel and that would not work with the capability of the D/A, which can drive 600 Ohms/channel, at a minimum (says designer).


A knowledgeable man also said:
"The amplifier's input should be AC coupled to avoid any noise-generating DC on
the fader."

John Monforte also wrote on Wed, 17 January 2007 12:30

You might find that the faders will be the problem. The tracking of P&Gs can be pretty good, but even still, once optimized at one position, the degree of balancing will deteriorate at others. This is why nobody works this way. Common practice is to unbalance external signals, perform your processing, then re-balance afterwards. Sounds complicated, but it isn't and works quite well, in fact.

Thanks for the heads up.  If it works at all, I'm not planning on riding through a given song at any lower than the -3 position.  Also, I'd be using a DK Audio vector-scope, which I have to use, already, since some of my balancing is with continuously variable pots.  Whereas, I was planning on simplifying most of that fuss by setting and forgetting the pots on the EQ and comp that offer fine-tuning (not stepped), and driving them, as needed, with the post-DAC slidewires (deliberately not linked).  During an album transfer, besides the tedium of moving several pairs of cont. var. pots an exactly equal amount of L-R rotation per unit to address level differences from song to song, many a mix is off center and wants one-sided correction, which can only be achieved through the "subtle, but deliberate deviation from unity tracking".



Best Regards,
    Andrew  

[Dave Hecht]

If you want to stay balanced, the amps Jim recommended, or the That balanced receiver/driver are probably the way to go. You're best bet would be to go to the website and get the data shets and any relevant application notes. That's site usually has pretty detailed notes and schematics. Try contacting them directly if you need more specific details.

Dave Hecht

[Andrew Hamilton]

Many thanks, Dave.  I will check out those O/As and web sites.  

Best wishes to all,
    _andrew
 

[Andrew Hamilton]

As a follow up, I've talked with THAT Corp. and have ordered some 1200P08-U balanced line receivers that are 0 dB.  They also had some neat demo boards that provide housing and XLR I/0.  How's THAT?  

http://www.thatcorp.com/1200desc.html


Thanks, again,
    _andrew