Viking Valiant line level input

[WB5IRI]

OK, audio gurus, I need some advice. I can do RF OK, but I am a newbie when it comes to all things audio. I want to feed my Valiant from my ART Tube MP preamp/CRL 300A modulation limiter setup, and the logical place to do that is through the phone patch input on the back of the rig. But in my reading about how to do this the right way, I am getting conflicting info. The line from the phone patch input goes directly to the cathode of the second audio amp, 1/2 of a 12AX7. I fed my Astatic Model 77 into the Tube MP preamp and then through a .1 uF cap (per the Valiant manual) into the phone patch input, and I must say it sounds really good through my monitor headphones. Looks good on my REA modulation monitor, too.  Some say just feed it right in like that, others (TimTron among them, whose knowledge and experience I respect mightily) say feed it to the top of the audio control pot at the grid of the same tube. So, which is better, or does it make any difference? Before I wrestle the beast out of its case, flip it on its side, and start moving things around, I want to know, does it make any REAL difference feeding the audio to the cathode or the grid? Lower distortion? Better control? More drive? I am not a fan of making changes unless there is a real benefit to be had.

Another question: the input to the phone patch is unbalanced, nothing wrong with that, but all my audio fanatic friends who do really top end audio stuff say I should use a balanced input (XLR as opposed to RCA phono plug), but they can't tell me why. My setup provides both balanced and unbalanced, but I wonder if I need to bother. Again, anyone have any ideas about this? I'm thinking balanced input and output is good for instruments and music, but does it provide any benefit for stuff like this?

Please excuse the newbie, probably naive questions. I am in a learning mode here. Someone on one of the threads here (K1JJ?) said this audio stuff can become its own hobby. I am just starting on that journey.

Doug, WB5IRI

[DMOD]

Since most professional preamps and limiters have low impedance output circuits (10-300 ohms), I would come out of the line level XLR jack and feed the Valiant directly into the phone patch as apparently you are doing now.

An advantage of feeding the second stage's cathode directly is you avoid the noise and non-linearities of the first stage.

I would increase the coupling capacitor's value to 33 uf (35V) to reduce the possibility of loosing some low frequency content.

Phil - AC0OB

[nq5t]

Feeding at the top of the gain control will allow you to use the Valiant's gain control to set the audio level, probably easier than fiddling with the output level on the CRL limiter.

I feed my Globe Champ 300 at the gain control (which is where the PP input is on this radio) through a Jensen JT-11P-1 transformer.  I run it balanced from my limiter (CRL SMP-950 strapped for mono) up to the transformer, and then unbalanced for just the 12" cable run to the PP input jack.  Did the same thing with a Globe King when I had one.  Works fine, which isn't to say that something else wouldn't. 

Grant

[W9ZSL]

In addition to being a Ham, I've also operated various recording studios since 1965 building much of the equipment.  The major difference between balanced and unbalanced is low-impedance (50-600 ohms) balanced lines such as mic cords can be run far longer distances with minimum hum pickup.  The two wires provide the signal path while the shield, the third wire, is the ground.  In addition, the mic preamp is further isolated from ground by a transformer or differential circuit in place of the transformer.  

With unbalanced, the shield actually completes the signal path as well as acting as the ground and is usually used with high impedances over shorter distances.  Unbalanced cables have to follow strict guide lines in order to avoid ground loops which are unwanted currents that can develop in the connection between two pieces of equipment which should be at the same potential, usually ground, but for some reason are not, resulting in hum, buzzing and other noise.

My current studio is all unbalanced with cable runs no longer than 7 or 8 feet.  Because most of the equipment has the 3-wire grounding plug, all the gear is at the same potential and the rest have the polarized plug.  One of my mixers actually has a ground lug which is wired to a nearby water pipe. I have absolutely no problems.

Mike - W9ZSL / Passage Recording Studio