Making custom inline H and T pads, got some Qs

[Sound Drifter]

This is what I'm looking to do:

I want to make some inline pads for some of my gear. What I don't know is what values I should consider when doing my calculations regarding impedances of the gear it is to interface. These pads are for in between gear, so I can drive some things a little harder for various effects:

Dynamics Toolbox -> inline Pad -> Convertors
or
Dynamics Toolbox -> Line input of another preamp that doesn't have an output gain knob -> inline Pad -> Convertors

This calculator I planned on using to get resistor values:

http://www.nu9n.com/tpad-calculator.html


The main reason is, I'm getting some sowter transformers for my Dynamics tool box and the transformer site behind the output level control. I want to be able to hit the trannys hard to get some more saturation out of them.


So the main questions would be:

1: If my preamp output impedance is say 600 ohms should the input impedance of the inline pad be 10x more than that for maximum voltage transfer or should I match it?

Preamp output 600ohms -> inline pad input impedance ????


2: If the preamp input coming from the inline pad has an impedance of say 40K ohms should the input impedance be 10x LESS than that or should I match it?

inline pad output impedance ????-> Preamp balanced input 600 ohms



3: Is it possible to alter the sound with the pad, like an impedance selector switch on a mic pre? Could I do that by making inline pads of various input impedances of say 600 ohm and 2K ohm?


I've read this: Uneeda Audio - Build your own attenuator pads
And it says this:


The point here is that you can minimize the change in coloration caused by inserting a pad by paying attention to this detail and designing the pad to mimic the conditions present before its insertion.


So this would me mirroring the impedances of the interconnected devices so it would be like this:
Preamp output ="x" ohms -> inline pad input impedance ="Y" ohms/ inline pad output impedance ="x" ohms -> Preamp balanced input ="Y" ohms

My brain hurts.

Thanks for anyhelp.

[Michael Tibes]

I've been trying to wrap my head around this recently...
Just to make things even more complicated: My halfway educated guess is that it might as well (if not mainly) depend on the transformer you use. The turns ratio transfers the impedance as well and the loading of the transformer influences the ringing. I'd just grab a bag of resistors, give it a try and see what happens. OK, maybe first listen to what happens, but checking with a squarewave generator and an oszillloscope might help to get it right. Then again, it might sound better if it is not technically right... :eusa_think:

My brain hurts as well...

Luckily resistors are cheap :icon_lol: Btw, what 'dynamics toolbox' do you use?

Some people with real tech brains hang out here: http://www.prodigy-pro.com/diy/index.php I remember that there was a thread about transformer damping quite a while ago and pads were discussed as well several times.

Michael

[Sound Drifter]

Hi Michael,

Thanks. I didn't even think of asking or looking over at prodigy.

The dynamics toolbox is Safe Sound's Dynamics Toolbox.

http://www.safesoundaudio.co.uk/

They actually published the review I wrote. Should be on their homepage or under news. Pretty cool. :icon_cool: I have some sowter output trannies coming this week to plop in it and I want to see how much character I can drive out of them. Need the pads so I don't clip the next device in the chain or the dacs. The pads would be useful for some of my other gear as well so I can have the ability to push things into saturation.


Thanks for the link. I'll check it out. :icon_biggrin:

[Michael Tibes]

I just tried the calculator and it seems that for an H-pad the range of possible values is rather limited anyway. I guess about 10 to 15 dB are necessary to get you going, you want a not too low in- and as low as possible out-impedance. All this depends on the transformer and the receiving device (probably the DA). I'd just start with 15 dB and the best possible shot. If I got it right, that would be 1971R for R1/2, 532R for R2 and 0R for R3/2. Turns the thing into a U pad...

Michael

[Sound Drifter]

Thanks for the reply Michael.

Is there a rule of thumb though with designing the pads as far as impedances go? Like high in, low out for instance? I can't find any answer in concrete on this and looking at different manufacturer's h-pad products there are all sorts of values :eusa_think:

Oh, I also found another really good calculator that brings in real world values of 5% and 1% resistors. :icon_cool:

http://www.k7mem.150m.com/Electronic_Notebook/attenuators/h_pad.html?Value_Z1=1500&Value_Z2=600&Value_A=10

[Michael Tibes]

I'm really no expert on electronics theory, so anyone with better knowledge is invited to correct me :pop_corn:

Afaik the load on the transformer mustn't be too low, and the interaction with the following input is smaller with a lower source impedance. This leads to a very rough rule of thumb to have a high in and low out impedance. But this is very much depending on the sending transformer, it might be the best to test the real thing with a squarewave and find the perfect load resistor and capacitor. I vaguely remember a thread on prodigy pro about how to do that, but this was quite a while ago. Maybe you can do a search and get lucky. The transformer manufacturer might have the values for the correct zobel network for you model, that might also point into the right direction.

On the other hand I'd not be too crazy about theory. If all that's missing is the pad, I'd really just give it a try and see what happens. Maybe you come to the conclusion that the whole thing is not what you hope for, or you accidentaly find the perfect mismatch that ads up nicely combined with a daw. If perfection in sound was just a low thd number, than Behringer would sound way better than all old stuff. I guess the reason to get out of the box is to get back some of the (hopefully) nice distortions we used to have in analogue. :icon_cool:

Keep us posted how things work out!

Michael

[John Lance]

A bit off to the side here, but the various levels of the loading of the preamp impedance wise could result in different sound qualities from the preamp, for better or worse, or all good but different, or hardly any difference at all, and that capability should be checked out first before you design the pad.

You might end up with a potentiometer and limiting resistor to facilitate a variable load on the output of the preamp if that results in different but good flavors, before feeding your higher z pad network..