syn

balanced bridged T position of insertion?
« on: November 28, 2009, 05:06:19 AM »
Well, this one looks simple, but I got into the brain loop I can't get out on my own.
Is it better to have it before the primary, or after the secondary and before the grids?
Now, if I have it before the primary do I  match impedance of the attenuator to the
source impedance or to the reflected impedance of the  secondary to the primary?
The levels fed can be handled by the transformer primary i need to drop them for the grids sake...
Thanks


abbey road d enfer

Re: balanced bridged T position of insertion?
« Reply #1 on: November 28, 2009, 12:28:30 PM »
Well, this one looks simple, but I got into the brain loop I can't get out on my own.
Is it better to have it before the primary, or after the secondary and before the grids?
Now, if I have it before the primary do I  match impedance of the attenuator to the
source impedance or to the reflected impedance of the  secondary to the primary?
The levels fed can be handled by the transformer primary i need to drop them for the grids sake...
Thanks
The briged-T attenuator is intended for applications where BOTH the input and the output (read source and load) must be matched. If this condition is not needed, you'd better use a simple potentiometer.
Putting the BTA after the xformer would not make much sense because the grid is HiZ.
Putting it before the xformer is the "vintage" approved way.
But, as you say your xfmer is capable of handling the max input level, the best solution IMO is a potentiometer to the grid.
In the absence of a schematic and the operating environment, that's all I can say.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

syn

Re: balanced bridged T position of insertion?
« Reply #2 on: November 28, 2009, 01:06:43 PM »
abbey road d enfer thank you very much. what i meant is if my source impedance is 600ohm and my input impedance is 10K do i need than 10k or 600ohm  BTA or it does not matter? I'm messin' with Altec 436c like circuit and need to pad the level to the grid, but need some more positions to the pad than shown in this schemo. BTA could show 20k loading to the transformer and 20k loading to the grid? No? Sometimes I like to hit a transformer a bit harder to get some sound out of it, so if I pad before it I loose that opportunity. If I use pot before grids won't I mess up transformers loading? I'm not sure about any of this.
Thank you

abbey road d enfer

Re: balanced bridged T position of insertion?
« Reply #3 on: November 28, 2009, 01:21:43 PM »
abbey road d enfer thank you very much. what i meant is if my source impedance is 600ohm and my input impedance is 10K do i need than 10k or 600ohm  BTA or it does not matter?
It depends if your source NEEDS to see 600R load. It depends if your input xfmr NEEDS to see a 600R source.
Quote
I'm messin' with Altec 436c like circuit and need to pad the level to the grid, but need some more positions to the pad than shown in this schemo.
It's much clearer now with the schemo. Clearly what you need is a DUAL pot with good matching between tracks (I recommend you use a 2x 50k Lin pot with a 10k resistor between wiper and ground).
Quote
BTA could show 20k loading to the transformer and 20k loading to the grid?
You would need a very costly dual BTA, for no reason. 
Quote
Sometimes I like to hit a transformer a bit harder to get some sound out of it, so if I pad before it I loose that opportunity. If I use pot before grids won't I mess up transformers loading? I'm not sure about any of this.
The input transformer on the original schemo is clearly designed to handle large variations in its source loading. This is not always the case. Some xfmr's need to tame their HF peaking with a Zobel network. The only way to know is inject a square wave and look at the secondary waveform with an o'scope.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

syn

Re: balanced bridged T position of insertion?
« Reply #4 on: November 28, 2009, 01:32:39 PM »
O.K. all clear. Dual pot will be. Thank you very much.


 

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