Loading the secondary on output transformer with 600 ohms

[Category 5]

I was just looking through the user guide for the Great River ME-NV2 (a fine preamp in its' own right)

The ME-NV has a loading switch for connecting or disconnecting the 600 ohm load resistor accross the secondary of the output transformer. Dan K. suggests that with the output UNLOADED there is a noticable increase in high frequency response (ala. and air band eq) that could be found desirable, but that the standard configuration is for this resistor to be switched IN for flattest and most natural response.

On the A12, you give the option of connecting or disconnecting this resistor as well, but I got the impression that it was supposed to be disconnected for normal usage (and that connecting it is more experimental). The Great River guide made me think the opposite...that the resistor should normally be connected, and disconnected for experimenting...

I am interfacing with pretty much the normal average studio gear, primarily my RME balanced AD inputs (which I am sure is the most common usage for your products)

My Questions....

1 - Which is the standard position? In other words, which will give me the flattest and most natural response? I assume now that a real 312 has the 600ohm load connected in normal cirumstances.

2 - If there is enough difference in sound between the two options (600ohm in and out) is there anything that would prevent me from adding a switch (simply wired to the jumper) to enable switching the resistor in and out at random (and would this cause trouble if I switched the load in and out with power running through the preamp, and connected)?

3 - If the answer to #2 is YES, install a switch, would I have to worry about the switch quality? Since the switch is not actually passing audio my belief is that I could use any small & cheap old push-button switch based on how easy it would be to modify my faceplate for mounting.

Thanks in advance for your answers, and thanks again for putting out these kits. I am learning SO much by building them!

Shane

P.S. I replaces the metal barrel front panel switches from my first A12 with the same ones you sent me in the second kit (the process sealed variety). Getting the old ones off was impossible so I had to cut the leads and pull the pins out one at a time. ....just sharing.

[tpryan]

My answers:

1 - Which is the standard position? In other words, which will give me the flattest and most natural response? I assume now that a real 312 has the 600ohm load connected in normal cirumstances.

I'd leave the jumper in place unless you're actually driving a device with a 600 ohm input impedance.

2 - If there is enough difference in sound between the two options (600ohm in and out) is there anything that would prevent me from adding a switch (simply wired to the jumper) to enable switching the resistor in and out at random (and would this cause trouble if I switched the load in and out with power running through the preamp, and connected)?

Sure, you can add a switch. It wouldn't cause any problems, but the switching will almost certainly be audible.

3 - If the answer to #2 is YES, install a switch, would I have to worry about the switch quality? Since the switch is not actually passing audio my belief is that I could use any small & cheap old push-button switch based on how easy it would be to modify my faceplate for mounting.

Well, the switch would be passing audio, and full-range audio at that. It would just be passing it to ground, and not to the external load. I'd use a switch on a par with the toggles that come with the kit. No need to go crazy, but really cheap parts are usually a false economy.

[Category 5]

Just to clarify, the input impedance of my AD converter is >5k ohmks. So you are recommending the jumper be ON, thus connecting the load?

Also, couldn't the extra output from the transformer be made to drive a VU or led meter? Just a thought. Maybe an additional unbalanced output?

Shane

[tpryan]

Just to clarify, the input impedance of my AD converter is >5k ohmks. So you are recommending the jumper be ON, thus connecting the load?

Yep.

Also, couldn't the extra output from the transformer be made to drive a VU or led meter? Just a thought. Maybe an additional unbalanced output?

Sure.

[bradb]

I just got an AM-864/U federal limiter/compressor... On my SCA pres, I'll need to remove this 600Ohm jumper to drive the am864's input impedance of 600Ohms, correct?

I'm assuming the jumper puts the 600 Ohm resistor in parallel with the input of the next stage, thereby dropping it to just under 600 Ohms. if i drive the am864 with the jumper in place, its like the SCA pre's will be seeing a 300 Ohm load. (right?)

If i am correct.... is this a no-no?

thanks,
brad