1200 ohms??

[Not_So_New]

Hi all.

Just finished my A12's LOVE them, very nice. So over at Gearslutz thethrillfactor said this (below) and I wanted to see what people here think of this and if there is any reason to not try it?? Any advice? Thanks in advance.

For the longest i could not understand why the modules in the console sounded so much better than when in the lunchbox.

The reason is the Api modules are used to seeing a different load when in the console.

Being loaded down differently affects the freq response.

In the console they are creamier and wider sounding.

In the lunch box they are thinner,brighter and a little mid foward.

This is the sonic characteristic i've seen a lot of people attach to API modules.

In talking with a number of people(including API) i've come to realize that the modules are used to seeing a different load on the output.

API recommends a 600ohm resistor.

I tried it and it wasn't as close to the sound of the console.

Again in doing research i ran into a conversation with John Klett and he said he normally sets them up with a 1200ohm resistor on the output.

Tried it and voila!!!

The closest sound to the console out of the lunchbox.

Its not exactly there but pretty close.

I suggest you try it and see for yourself.

[tele_player]

Uh... why not try it?
You can do it without modifying the A12. Leave J5 off, and modify a cable with a 1200 ohm resistor.

[Not_So_New]

Uh... why not try it?

Uh.. because I don't want to waste my time if it is not worth it and I don't want to hurt the pre amp. I am not sure about hurting the pre (I doubt it would) but I want to check first. I have never built anything this complicated before and in my world it is better to be safe than sorry, I don't have time or money to smoke an A12.

So anyway I thought it would be a good idea to see if anyone else has tried it here but thanks for the positive and oh so helpful response.

[tele_player]

It's perfectly safe.
Think of how J5 works. When it's installed, the 600 ohm resistor is in parallel with your load. When J5 is open, an infinitely high resistance is parallel with your load. It's reasonable to extrapolate that something between the two is safe.

[Not_So_New]

It's perfectly safe.
Think of how J5 works. When it's installed, the 600 ohm resistor is in parallel with your load. When J5 is open, an infinitely high resistance is parallel with your load. It's reasonable to extrapolate that something between the two is safe.

I did not know that with J5 open you have an infinitely high resistance in parallel so I guess that would have been kind of hard for for me to extrapolate huh???



So just keep the 600 ohm on for J5 then slap two 1200 ohm resistors on pins 2 and 3 of an external cable and it should be good to go for a test?? That is what I was planning on but wanted to check.

Back to point #1 has anyone here tried this?? Is it cool or am I wasting my time? (I know, that is up to me just wanted to hear it from someone else Iguess)

Thanks Tele...

[Category 5]

Hmmm....Very interesting.

I am thinking that if you have the 604 ohm resistor jumpered in (which is recommended) you could just make a special female to male cable adapter with about a 1.8k resistor across pins 2 and 3. I think this would cause the preamp to see a 1200ohm load and you wouldn't have to modify anything from Tim's spec to try it.

Or you could just do as you say, and leave the 600ohm jumper open in the preamp, and tie a 1200ohm resistor across pins 2 and 3.

I am curious to hear what your results are though. The A12s sound awfully fantastic when built according to Tim's design. The first words spoken by anyone who hears them (especially in phones) is always "wow!"

Shane

[tele_player]

No, you can't do it without removing the jumper. There's nothing you can parallel with 600 ohms resistance to increase the impedance. Whatever you add will decrease the impedance.

[Not_So_New]

Yes the A12's are very very good sounding for sure, I am very happy with them.

Cat5 said

I am thinking that if you have the 604 ohm resistor jumpered in (which is recommended) you could just make a special female to male cable adapter with about a 1.8k resistor across pins 1 and 2.

Forgive me for my newbie'ness (I am really really new to DIY but I want to learn more). I assume Cat5 was talking about pins 2+3 not 1+2, I think it was a typo but I wanted to check. Also why a 1.8 resister? My A12's are built already and in my rack, they are working just fine so I would rather not have to rip everything apart to pull the jumpers so what cables would I need to build to try this?

To answer that I guess I need to know are the A12's built off of the same design as the 312/512's modules that would go in a lunchbox?

What I mean is it sounds like Thrill is saying that the modules don't get the same load in the lunchbx as they would in the console so they don't have the same output. Are Tim's A12's loaded like a 312/512 module in a lunchbox or did Tim's design take the console loading into account already??

I found a few more people ver at GS talking about this....

Max said

This has come up in a couple of threads, but I thought I'd start a new one since several people have asked about it.

Someone suggested soldering a 600ohm resistor across the output of a 512c when it's in a lunchbox to give it a load similar to what it would see if it were in a console (perhaps this info came from API?) Someone else suggested that a 1200ohm resistor would be more appropriate (I think it may have been Thrill Factor.)

I scrounged, and all I had laying around was a 1000ohm so I thought I'd try it. The result is that I'll never go back to using the preamp without the resistor. It's much more 'cushy' and 'luxurious' with the resistor. Rounder and more colorful.

I'm going to try some different resistor values when I can, but for now, I suggest doing this.

Thrill said in the same thread

Hi Max,

I am glad you are liking it.

Yeah i suggested 1200ohms.

It sounds closer to what the modules sound like in the console.

I wish i could take credit for it but the suggestion came from John Klett.

[Not_So_New]

No, you can't do it without removing the jumper. There's nothing you can parallel with 600 ohms resistance to increase the impedance. Whatever you add will decrease the impedance.

Thanks Tele, just saw your post after I submitted mine above.

So you are saying that no matter what I need to pull apart my rack to even try this then? What a pain in the a$$. There has to be an easier way to try this right? Please say there is......

[tele_player]

If the 600 ohm jumper is installed, there's no way to try a higher impedance without removing it.

Anyway, it's definitely worth trying different load resistors. Since most users are probably sending the signal to a device with a high input impedance, the load seen by the A12 output tranny is mostly determined by the internal load resistor. A smaller resistor represents a greater load, requiring more current through the secondary (for a given voltage swing) than a higher load impedance. It might be interesting to see freq. response charts into various loads. I'm sure Tim can provide those in his copious spare time.

[Not_So_New]

If the 600 ohm jumper is installed, there's no way to try a higher impedance without removing it.

*sigh*

So take the rack apart and take off the 600 jumper (on J5 is it?) then make a cable with 1200 ohm resisters on pins 2 and 3?

That is the only way to do it then?? And the above is correct?

[Category 5]

Thanks Tele for setting my info straight. I guess parallel resistances won't work in this situation as I was thinking.

Yes, I meant 2 and 3 though...that was a mistake on my part.

I have a strong urge to try this as well, but no desire to rip the rack apart right now. It would require removing the modules entirely.

Tele, why wouldn't the paralell resistor work? I was assuming the tranny would see the paralell network of 600 + 1.8k as 1/2 of 2400=1200.

I believe you, I just wondered why it wouldn't work like that.

Shane

[tele_player]

Cat5 - the reason is that you don't have the formula for parallel resistances correct. The formula is simply stated as R = product / sum, for any two resistances in parallel. The simple special case where the two resistances are equal is R / 2 (where R is the value of either resistor)

There is probably an algebraic proof that the result of ((R1 * R2) / (R1 + R2)) can never be larger than the smaller of the two, but it's easier to just use a commonsense analogy.

Imagine you have a fat water pipe, and you run a skinny hose in parallel with it. Regardless of the size of either pipe, together they can flow more water. Since resistance is, by definition, opposition to the flow of current, increased flow equals decreased resistance.

[Not_So_New]

So take the rack apart and take off the 600 jumper (on J5 is it?) then make a cable with 1200 ohm resisters on pins 2 and 3?

That is the only way to do it then?? And the above is correct?

[tele_player]

Or make a jumper that has a 600 ohm resistor in it. That would be in series with the 604 on the board, yielding 1200 ohms. I haven't looked at the board lately, but if there's room, this could be easily done using two jumpers.

Somebody else can verify - but I went to school for this stuff, I don't make most of it up.