question about preamp direct to tape

[SteveM]

I have two preamps I'm running directly out of to the back of a Tascam 38. A UA 610 solo and Trident 4t. They both sound great like this except one problem, the UA seems to track a little hissy I wind up with quite a bit of air on top, though the fatness and richness of going direct is obvious to the ear. I was wondering if anyone could tell me how to solve this. I'm using homemade cables, with resistors designed to go from +4 to the -10 input. It's not the cable because the Trident doesn't have this problem. I'm thinking it's something I'm doing wrong with the UA. Here are the specs for the UA http://www.uaudio.com/products/analog/SOLO610/specs.html if someones got a second could you tell me what the problem might be? maybe it needs a different cable? Thanks

 

[evm1024]

interesting problem

You might hook a set of headphones to the UA output and listen for hiss. Something that does not take a lot of power. Its a simple test that divides the preamp from the cables/deck.

Are you using the same mic type into both preamps? Does the UA mac need more gain?

Loosely speaking you do not have to level shift from +4 to -10 unless you want the preamp and the dec to both read 0 VU at the same time. You might think of it as 14 dB (I know apples and Oranges) more headroom. Of course I don't think that your resistor network is the cause of the noise.

Just some rambelings off the top of my head.

--Ethan

 

[SteveM]

You might hook a set of headphones to the UA output and listen for hiss. Something that does not take a lot of power. Its a simple test that divides the preamp from the cables/deck.

Are you using the same mic type into both preamps? Does the UA mac need more gain?

Loosely speaking you do not have to level shift from +4 to -10 unless you want the preamp and the dec to both read 0 VU at the same time. You might think of it as 14 dB (I know apples and Oranges) more headroom. Of course I don't think that your resistor network is the cause of the noise.

Just some rambelings off the top of my head.

--Ethan

Hey EVM, thanks for the tips. Yeah, I don't think it's the resistors, I think it's the power of the tubes the UA, it's just a little noisier. The Trident is solid state. And it DOES seem I need a to crank up the volume with the resitor cable. By not "level shifting" are you suggesting trying it with a regular cable and lower volume?

 

[evm1024]

Tube noise could be less

Hey EVM, thanks for the tips. Yeah, I don't think it's the resistors, I think it's the power of the tubes the UA, it's just a little noisier. The Trident is solid state. And it DOES seem I need a to crank up the volume with the resitor cable. By not "level shifting" are you suggesting trying it with a regular cable and lower volume?

Yup. Direct cable.

What tubes are you using?

 

[FALKEN]

this is how I recorded, but without the resistors. What value are you using and which end of the cable are they wired into? And how are they wired? I think you would only need one resistor...I'm confoosed.

 

[Beck]

Could be a noisy tube. They use the JJ Tesla tubes, which are nice, but you can get a hissy one now and then of any brand.

Other than that it could be a gain staging problem. If you don’t have the drive level hot enough (drive LED should be green or orange all of the time) you have to open up the output level too much which can result in more hiss.

I would also try it with a regular XLR-to-RCA cable without the resistors. The output level knob is just an attenuator… so if you adjust the input to optimum level you can use the output level control as a pad in this case.

It’s very important that the resistor arraignment from a balanced to unbalanced circuit is like in this pic, which I posted in another thread earlier. But since the 610 has a tube driving the output, the resistor pad may be throwing something off.

As a rule a pad like this actually increases headroom and gives the output control a “normal” range of adjustment, but there are exceptions. Usually without the pad the output control will jump from low to high levels with just a nudge, which makes it a bit useless for fine adjustment. But try it without the pad and see.

Almost forgot... what kind of mic are you using? You're hi-z/lo-z switch on the front may be set wrong and the mic/line switch on the back should be set to line.

 

[FALKEN]

ok...that makes more sense.

 

[SteveM]

I (think) I'm good now. Trying it without the resistored cable it seems to be quieter and work fine. I think I wind up having to turn the gain up too high with the resistors in, though I'm still not going higher than 7. Tim, yes that's how I set it up except, and to answer Falkens question, I put the resistors on the rca side. I don't think it's the cable though, because it works fine with the Trident so I think it's a combination of the condensor mike with the tubes and the resistor. Or something like that. But if it's ok to run it without the resistors then I think I'll be alright.

As a rule a pad like this actually increases headroom and gives the output control a “normal” range of adjustment, but there are exceptions. Usually without the pad the output control will jump from low to high levels with just a nudge, which makes it a bit useless for fine adjustment. But try it without the pad and see

Tim, the logic here makes complete sense to me. That's what I'd think too, but in the case of the UA it seems to adjust smoothly without the pad. Also, I always run the mic input into the green.

Almost forgot... what kind of mic are you using? You're hi-z/lo-z switch on the front may be set wrong and the mic/line switch on the back should be set to line.

Just caught this question. The mic is a condensor, set on lo-z and I set the output switch to "line" because the mic setting doesn't give a strong enough signal, in either case, with or without the resistor cable.

 

[FALKEN]

perhaps he needs to float the - side ? or connect it to shield with a 4.7k resistor to balance it? is either of those a possibility?

 

[SteveM]

perhaps he needs to float the - side ? or connect it to shield with a 4.7k resistor to balance it? is either of those a possibility?

I was told from someone else to not connect the sheild at all and to tape it back from making any contact. Unless I was confused as to what he was talking about by shield. Being the foil that wraps around the cable. The ground and the - side are both wired through pin 1 and 3.

I afraid this is another one of my questions that could have a million different reasons. Mainly though I'm concerned with the connection between the amp and the deck and what should equal a good signal.

 

[FALKEN]

I am thinking not to ground the - side of the output from the amp. Since it is a transformer-balanced output, it should be fine like this.

 

[Beck]

I was told from someone else to not connect the sheild at all and to tape it back from making any contact.

Don't do that! You would only leave the shield disconnected in a balanced-to-balanced cable, but you only leave the shield disconnected on one side. It's basically a ground-lift if needed for hum problems.

Don’t worry, the configuration in the photo can be found in any standard sound/recording handbook, such as those by John Woram, Bruce Bartlett, etc. It does two things… converts balanced to unbalanced and pads the signal down by 12dB (the difference between –10 dBV and +4 dBm).

If you hear distortion or hum you can disconnect pin-3. This might occur with electronically balanced outputs… it depends on how well it’s designed. However, if your output is floating-transformer coupled, pin-3 should be as pictured.

 

[SteveM]

I'm confused.


Tim, I was just referring to the foil that surrounds the three wires not the ground wire. But should I keep the ground and negative seperate to their respective pins, 1&3 rather than have them both on both pins?

No hum or buzzing though. Basically sounds good but I want to make sure it's right.

 

[Beck]

The third wire on a 3-wire cable is the ground, AKA shield and it connects to pin-1.

Shield and ground are two terms for the same thing. The shield/ground is the third wire, but it surrounds the two center wires that carry the + and - signals in a balanced line. It’s braided around the other two wires.

Some cables also have an additional foil shield for better rejection of interference. The foil is in contact with the ground wire for the length of the cable, so you don’t attach the foil to anything.

If you’re going to use the resistors it’s easiest to put them inside the XLR connector housing. You would just use buss wire or a piece of wire cut from a resistor to connect pins 1 & 3, and then add the resistors as shown in the first pic.

If you’re not using the resistors you can leave the - wire (pin-3) unconnected, or connect it at the RCA side together with the ground... whatever sounds best. See pic below.

Whatever you do you must have the ground/shield wire attached when going from balanced to unbalanced.

 

[SteveM]

Tim, thanks!