ARCHIVE: Australian Ham Radio Discussion Forum ( AHRDF )

Hello Gurus,

I have been playing around with interfacing a small mixing console (Behringer Q502USB) with my bench radio (Yaesu FT-857D).

There are a couple of reasons I am thinking of doing this. Firstly the microphones I want to use are all low Z stage mics so I need a "z transformer" to overcome the 20dB difference in output between a low Z microphone and a hi z microphone. 

Next is that after over 40 years competitive shooting, about 30 years teaching diving and 40 odd years in and around the music industry my ears ain't what they used to be and some EQ on the headphones and shack speakers wouldn't go astray.

And of course, the main reason is the console and microphones (and all the bibs and bobs for it) are within reach and sitting idle...

The main out from a console is generally said to be around 22dBu, which is way too high for a microphone input.

My plan is to place a 600Ω:600Ω isolation transformer followed by some sort of resistance between main out and mic in.

In my feeble mind I thought that a 1kΩ log variable resistor across the transformer output might do the job. However, I also have had thoughts about doing a voltage divider with a series resistor before the variable resistor. Only I have no ideas on what would be some starting values for the said resistors.

The transformer and resistor/s will be installed into the box on the desktop that already has the 1/4" microphone jack and PTT switch. (Although I just got a router so a new box may be in the offing).

Has anyone done this?

Any ideas what the divider network values might need to be as a starting point?

Colin

Start with a 680 ohm (or 820 ohm) resistor across the secondary of the 600 ohm transformer, then a series resistor to your 1K pot upper contact.  The 680R resistor is there so that the impedance reflected back to the primary/source is in the vicinity of 600 ohms rather than some completely unknown value, and stays relatively constant regardless of the pot wiper position - and thus loading.

22dBu is about 60 millivolts so might already be close to the level required for the FT857 but that is the job the pot has.   The series resistor plus the 1K value (ie the pot value) form a simple voltage divider so that is easy to work out. For example, a series 1K gives a 2:1 voltage division as a maximum depending on the pot rotation.  And then you can always turn it down !  

The FT857 manual does not quote a mic output level, just a nominal 600 ohm TX input impedance, so it is guesswork anyway.

Doug

Hi Doug,

If I understand your meaning is this what you are suggesting?

If so, what is the series resistor value? (well around what value).

I hope as today is Sunday there might be time for a scrounge in the junk cupboard/box for the bits. I know I have the transformers, some verocard and 680R resistors. The log taper pot, not so sure but we will see.

[attachment=823]

Many Thanks

(edited to actual add the image)

---

Wait. I have to stop doing this stuff before my second coffee.

This is a simple calculation....

(This is why we read all those books before we sit our exams...)

The other option is to input line level audio via the CAT interface connector on the DIG IN pin.

Some menu manipulation is required (Menu 038) - basic info pages 63-66.

(I don't have an FT857, just info I found on a forum on the 'net)

Colin

Exactly.  I would start with the series resistor around 1K, but increasing this simply means turning the pot further upwards.
Start with the pot at minimum, sound source on, TX on, increase the pot position/output level until the RF output power rises to (probably) about 70-75 watts. Done.

Doug

PS  The pot can be linear or log, it just affects how much the level changes with angle of rotation.

Hey Terry.

The data in and out is otherwise plumbed in to a line isolation break out box for digital modes.

The Q502 desk doesn't have enough usable channels to do both.

I wonder if the music place in Gawler has any leads on a second hand 8 channel desk. Although a bigger mixer means I need a bigger radio desk.

Doug,

Thanks. Looks like a quick run past Jaycar may be in my future (well if they actually have any components any more that is...)

The log pot idea was simply because I thought I had one in my luthier (guitar tech) bibs and bobs already, but it doesn't appear so.

Colin

A final comment : make sure you use good RF filtering techniques on all leads (In, Out, Power) to preclude any RF->into->audio issues (also known as audio rectification).  Effective direct RF grounding of any added devices is totally important too.

Good luck.

Doug

Thanks Doug.

As a matter of fact I have a bunch of ferrites on their way, for just that purpose.