Ping PJ, PB,DOC

[Thermioniclife]

Hello,
I am awaiting a BeePre recently ordered and I have been reviewing the schematics.
I would like to add a level control for the Balanced output, here are my thoughts.
First the info that I can give you, I plan to use the unbalanced outputs to drive a pair of 300B amps that have a 100k ohm input impedance.
I would also like to drive a JBL powered subwoofer using the balanced outputs (LSR2310SP) that I do not know the input impedance of. the only spec they give is a input sensitivity of -10dbv. I spoke to Harman group tech support and they could not confirm the input  imp. of the amp. so this is where I make assumptions, input imp. is prob 10k or more.
having said that please look at the highlighted area of my attachment and advise if this circuit would seem plausible.
or if you see any unreasonable loading of the output. I can use an unbalanced input on the sub also.
Any thoughts will be greatly appreciated.
btw I did not include the various component values of your design as I do not know how you would feel about that.
I'm sure you know what part of the circuit I am speaking of.

 

[Paul Birkeland]

I would like to add a level control for the Balanced output

Why?

having said that please look at the highlighted area of my attachment and advise if this circuit would seem plausible.

How about just using the stock BeePre circuit and using the level control on the sub?

or if you see any unreasonable loading of the output. I can use an unbalanced input on the sub also.

The loading of the BeePre won't be a problem, but your volume control will give -6dB of attenuation when it's all the way down and -12dB when it's all the way up on the +Ve pin, which isn't much range.  It will also put some of the +Ve signal onto the -Ve pin, but at a lower magnitude.  This would be enough of a reason for me to suggest not doing this, especially when there are other options available that are built in already.

 

[Paul Joppa]

I think it will work (depending on whether PB or I has more correctly interpreted your sketch!). I certainly agree with PB that you should use the level control on the sub if it has one for the balanced input

If you do this, I strongly recommend that you place the components inside the red square at the sub end of the balanced cable, so that the balanced line maintains its low impedance and high noise rejection. If you do that, then you can build the preamp stock, which we always recommend for the initial build in any case.

Strictly speaking, the unlabelled resistor that corresponds to Rg (475K) should be paralleled with a 100K resistor to maintain the balance which is disturbed by the 100K load on the unbalanced output. But that will make only a very small difference, unlikely to be worth the trouble.

 

[Paul Birkeland]

(depending on whether PB or I has more correctly interpreted your sketch!).

Yes, I ignored the resistors without values, which perhaps wasn't the wisest decision!

You can post the values we use, the explanation for this arrangement is published in a reasonably popular paper by Jensen Transformers, and it's certainly not a huge secret.

 

[Thermioniclife]

thanks for the reply gentleman,
as to the question of why I would like to do this is manifold, besides being old, fat and lazy I would like the ability to control the output signal going to the sub at the preamp. I will build the project as designed at first but will eventually modify it to be operated by infra red remote control. I have been working on a "passive input selector project for the past year that incorporates remote control operation of 4 inputs, 3 line level and 1 Bluetooth, main volume, sub volume and mute. it also has individual adjustable input attenuators for each input. 0db -3db -6db -9db and -12db. in addition of those features it has 2 separate 12vdc triggers that output to 2 solid state relay controlled 120vac outlets, one to turn on my 300B monoblock amps and one to turn on my powered sub.
the sub indeed has an input control on the back of the cabinet but I would like to control the input to the sub via remote control.
I would use the BeePre volume control to adjust the overall volume and control the balanced output separately for the sub level.
the sub also has unbalanced inputs that could be used if necessary.
Attached are some lousy photos of my remote input/ volume control. Please be kind its a prototype housed in a cardboard box at the moment.

 

[Thermioniclife]

as I have said,i  am old and do not understand the ways of the internet, I tried to upload? a number of pics in my last post so I will try it again.

 

[Thermioniclife]

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[Thermioniclife]

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[Thermioniclife]

oh well,
I will figure this out someday.

 

[Paul Birkeland]

You could feed the balanced output of the BeePre to a balanced remote volume control. 

 

[Thermioniclife]

True enough, although I would like to have ir control of it.
the circuit I showed in my original post is what those remote vc's are using.
the thing that I am unsure about is whether it will work with a capacitor based output as opposed to a transformer output.
Regardless I will build the preamp as stock and experiment after. I've been known to put the cart before the horse on more than one occasion. Thanks.