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.

BTW: I soldered the resistor into a mic cable, so no messing about inside the preamp (way above my head!)

I suppose this could have been posted in 'Geekslutz' but I remember seeing the original thread here (I think.)

Hope this info helps!

Thanks Max, it does help....although....

WHERE DID YOU BUY THE 'SOLDERING RESISTORS INTO CABLES FOR DUMMIES' manual? heh heh
In other words...howdjado-dat??
Does the same apply for the the 512,550's, 560's and 525?

Has anybody tried soldering it inside the lunchbox? Cause my lunchbox is sitting next to the patchbay, and I guess it'll be tough to fit the resistor into a TT cable...But then again, I'd need that dummies manual to be 100% sure...

Gone Fishin'

Redsandblu

Just listening to the A/B tracks; the sound without the resistor isn't bad at all. It's just different. I can imagine using the preamp both ways.

What I'm really loving these days is tracking vocals/acoustic guitars with the API525 compressor in 'OFF' mode (as opposed to the hardwire bypass mode.) The 'OFF' mode runs the audio through the circuitry but none of the controls are functional. Very 'PFATT' sounding! I've used it this way during mixing to 'phatten' vocal tracks that have come in tracked elsewhere.

Redsandblu:

I'm good at making cables and doing slight repairs, but some of this goes way over my head. But, if I were gonna do it, I'd cut the TT cable and solder the resistor across the two conductors, then carefully make sure there's no short and seal the whole deal in heat shrink tubing.

(Any of you technically minded folk wanna check me on that?)

For my personal use, anything to not hafta work on the high dollar stuff. For one thing, it's pretty easy to 'F' up a board if you're...uh...me.

Yeah, I'd love to try this also, but I'm kinda on the slow side when it comes to electronics. Are there any premade solutions, or dummies guide or something I've used a soldering iron before, but it was years ago. Still I might be able to manage if I knew what I was looking for when I go "resistor shopping". Where do you get resistors, and do I need to tell the guy at the counter anything more than "I need a 1200ohm resistor"? Thanks!

-Mike

Good to hear about your use of the 525; On a side note I just noticed you can latch the ratio buttons just like another black face comp/limit. Although I haven't listen to the audible effect, if there is one !

As a owner of a 10 space api rack and a 2 space horizontial, I'd be curious if the ApiSez crew could chime in here. What's the deal??? Why wouldn't they implement the resistor in the first place?

Hold on; I'll try it right now...


Holy Smokes!!! It smacked it into oblivion!!! Excellent! Super brickwall.

I just sang some 'shoo-be-doo's' into PTLE, and I could hear the 'nukeage' right away. I don't know why I wouldn't have ever tried that, but there ya go.

Oh, as far as the 1000 ohm resistor (again, not an officially suggested value, just what I had on hand; I'm practicing medicine without a license here.) If the signal goes up to 'zero' on the VU meter on the 512c, which is where I usually try to hit is, the saturation with the resistor is a bit much. I found a nice 'sweet spot' at '-6'. Maybe 600 ohms would be more sensible?

damn, I need to go to my shop now!!! AWESOME, I LOVE THE 525!!!

max,

please tell me more about the resistor in the wire thing. i feel like watching a series and right on the most intense spot, there is that advertisement break...

you mentioned the output - i guess, you mean soldering a 600 across the xlr pins ?
please correct, or shed some light on it.
thank you very much !!!

The theory behind attaching a resistor across the output is that it will provide termination for the transformer.

An unterminated transformer may have an overshoot or ringing which can affect the upper frequency response, making it peaky. Not all transformers will have this problem, however. Neve transformers, for example, do require termination and in a Neve console the termination was provided within the console itself. When modules are removed from a Neve console, they need to be terminated externally.

Are you sure that the API requires external termination?

If you add more termination that is required, you may cause an 'overdamped' response and what's worse you may possibly "overload" the output of the circuit.

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.

Hey, I know what you mean. I'm ok with some of this stuff, but only in the 'cookbook' sense. If I have to use my imagination, I'm lost.

Anhow, all you guys who are thinking of doing this, I'm going to the electronics supply guy later today, and I figure I'm gonna try some different values. Some of the mics I have are sounding pretty saturated at 1000 ohms when the track is solo'd, but when I drop it into a mix, it's perfect.

One problem is that 512c's are all I really ever used with a couple of exceptions. So I don't have any relativity as far as how saturated and 'round' a mic preamp can/should be. I trust my ears, but it's nice to have some kind of benchmark. I feel like the real deal is gonna be to make some little pigtails with different values, then I could change them in a couple of seconds.

zmix: as to your question, I have no idea what the word from 'above' is on the external termination. Like I said, I'm practicing medicine without a license.

The impetus for doing this came from this thread:

https://www.gearslutz.com/board/showthrea...ight=api+klett

where the idea for a 600 ohm resistor is attributed to API and the idea for a 1200 ohm resistor is attributed to John Klett. While the character of the preamp is (to me) definitely API with and without the 'modification' the sound makes more sense to me when I hear the preamp with the resistor in place.

Try Resistors-R -Us. Putting it in XLR would be easier, also a 1200 ohms is not an absolute. 2200 to 1K would still work. Consider this; an 1176 black face is a true 600 ohm input so it by its self would load the output transformer even more. In fact then the 1200 ohm and the 600 ohm would be in parallel making it more like 400 ohms. This is one of those reasons why a discrete op amp is far better than a IC because it can handle the lower impedance with little or no effect on performance. Another real life config, 2 1176's driven by the same output would look like 300 ohms...I hope none of this helps...Just kidding oh great audio lovers.Sorry for the rambling, had about 300 cups of coffee and IM wide open with the knob broke off...

thank max !!!
there was the little info that i needed.

before i start slamming and burning my nails while soldering, a short question:

i solder the resistor (as soon as i got it, any specific) across the female xlr pins carring the signals and leave the ground untouched ? right ?
anything more to bear in mind ?
something that i have forgotten to consider that will smoke any other following devices ?

please give your oks and opinions - i guess i found my evening job because there is just boring stuff on telly

Say I'm going... mic->512c->550b.
Where would I put the resistor?
After 512, or after 550?

Andrew

You won't need one between the pre and the EQ, so this depends on what the 550 is feeding. Be aware that loading the output with a resistor can cause reduced headroom and reduced high frequency response. Same as pluging a guitar into a low impedance input.

dose anyone have a digital camera?
is this using the unit as a stand alone or in the lunchbox?
i was thinking in the lunchbox but maybe i am lost.
could a box me made like a old substitution box and used when need?

Specific instuctions please! Anyone?

As Zmix mentioned, most output transformers must be loaded into the proper load that they were designed for. 600 Ohms is one of the old standards that is still in common use.

To terminate the output, you would connect the load resistor between pins 2 and 3. Now what the optimum value would be is another story since it depends upon the transformer's design.

The way I determine the value is to apply a 1 kHz square wave to the input of the amplifier and experiment with resistor values across pins 2/3 to find the optimal square response as seen on the oscilloscope.

Ideally, the resistor should be OPTIONAL with the gear, either via a "trick" cable or a switch. Ampex recorders had the "termination" switch on the rear of the audio electronics for that purpose...it either connected or disconnected the 620 Ohm load R. You do NOT want to have the termination if the gear is driving a piece of gear with a 600 Ohm input impedance, like a LA2A, etc.

Bri

So I am assuming this same concept would apply when trying to insert a vintage LA-2A on an SSL 6000? This particular LA-2A sounds great when inserted on a Neve console but on the SSL I get "overload" lights and the signal is super low. There was a 600?*resistor in the box that we got this LA-2A in and we didn't seem to need it to make it work with the neve. So if I am getting this right, I should put the resistor across the + and - terminals of the output on the LA-2A and everything should be OK? Would it be safe to assume that I would HAVE to remove the resistor to make it work with the Neve again or would it work OK?

Thanks guys
John

Touche

Yes, most gear with an output transformer needs to have a termination resistor between the + and - output line IF it's driving a higher impedance (aka bridging) input.

I'm unsure what the deal is with inserting the LA2 into the SSL. Something else sounds wrong there.

Bri

I spoke with Avedis at BAE today about the resistor philosophy with the API's and he said it was interesting but totally unnecessary and that it would end up adding saturation, if that's what your looking for then there you go. He said that it is needed to terminate the neve style pres though. I dunno I'm no tech head. I just use the stuff.

thumbsup

Indeed, some transformers are much more affected by output loading compared to others. The only way to tell is run some tests, especially with a square wave signal while monitoring the output on a scope.

Bri

Something you could do is make a series of termination points on you patchbay. I.e have a couple of patch points with 600 Ohms strapped across them and a couple of 1200 Ohm patch points etc. This way you can check very quickly the effect it has on your gear & use it if & when it's called for.

Justin.

Ampex, et al included termination switches on their gear that was built in the "transition times" when both bridging and terminating inputs co-existed in the studios. The AG-440 series, sold from the late 60's into the late 70's, was probably the most commonly supplied unit with a "term" switch.

I emulated that design when racking up some Audix modules with an extra balanced line driver built around a pair of Neve LO-2567 output trannies. The "term" switch dropped 820 Ohm terminators, which seemed to be a nice compromise.

Bri

This is one way you could easily check the with/with out term.resistor. If you have an 1176 or Purple Mc-76 for example: first create a mult, API to Mult, one to the input of the 1176, 2nd to input of monitor or line input on console ect.. Then with a source, snare, guitar amp ect listen as you patch and unpatch the mult going to the 1176 and see what difference you can hear. Don't listen to the output of the 1176. THE 1176 has a true 600 ohm input which will load the API pretty well.

Interesting idea. But if you are essientially Y cable-ing out the load would be 300 ohm since the other mult has no load, right?

Me likey that sweet saturation. heh

No, if you were multing 2 1176's yes, I was refering to 1 1176 and 1 input in the order of 10 to 20K such as a line in of some consoles, for monitoring.