Line Level Audio Transformers

[n3lrx]

Hey Guys,

Not quite a toob reference but oh well. I'm looking for a 600 ohm to 1k ohm audio transformer to go from balanced line level on my audio chain to unbalanced line level on the transmitter inpoot. I'm not having much luck. I have a 1:1 at 600 ohms. Will that kill the audio or is it OK to use?

My equipment says it will do balanced and unbalanced but I prefer to use balanced as much as I can and keep the impedance matched.

My next question is am I gaining/losing anything by going direct or is it better to use an audio balun?

I guess what I'm trying to say is, does the impedance mismatch really matter? If it does, does anybody know where I can find the matching transformer? I don't need a big honking transformer just a tiny one for a few millivolts. Through hole if possible.

Thanks.

[N2DTS]

I have a bunch, but they are all 600 ohms to 20,000 ohms, or 10,000 ohms.
600 is close enough to 1000 ohms i would think...

[KA0HCP]

It is ideal to have an exact transformer for each transition.  However 600 ohm to 1000 ohm is very close that a transformer is probably not necessary.  Going from low to higher impedance will give maximum power transfer.

I wrangled with this on one military project with various field radios being matched to custom and OTS commercial gear.  In the end the best engineering advice was "Try it and see how the audio sounds and make adjustments from there".  bill 

p.s. If the audio level was simply low, it can be compensated by raising the input level.   A confounding factor can be ground loops developing if a transformer isn't used.   Radio Shack has a $3  8 ohm to 1000 ohm transformer the size of a sugar cube. I've used them by the handful for various projects over the years.

[KA2DZT]

600 to 1000 ohms.  Close enough, no need for any matching xfmrs.

Fred

[n3lrx]

I'm just wondering if there is any advantage to running balanced line.

All my gear in is the same rack so my runs are probably short enough not to worry about balanced. I guess I'll just run it in unbalanced mode.

I did find an 2:1 (or near enough 1.2k x 600) audio transformer on Allied, but it's $21 shipped. All that for a 1/2" cube.

In any event I'll just scrap the matching transformer and run it direct.

Thanks for the input.

[w4bfs]

I'm just wondering if there is any advantage to running balanced line.

All my gear in is the same rack so my runs are probably short enough not to worry about balanced. I guess I'll just run it in unbalanced mode.

I did find an 2:1 (or near enough 1.2k x 600) audio transformer on Allied, but it's $21 shipped. All that for a 1/2" cube.

In any event I'll just scrap the matching transformer and run it direct.

Thanks for the input.

there are advantages to balanced audio with an input transformer .... common mode hum rejection generaly caused by ground loops can be easily better than 60 db making that a non-problem .... don't forget noise rejection and rf rejection .... the old application in toob gear was line in to control grid, single ended or pushpull ... this is high impedance and allows a step up tranny .... say 600 ohm to 60kohm is a 1:100 z raise which is a turns ratio of 1:10 and will yield a 10x voltage gain .... the value of this was generally that one voltage gain stage could be left out of the audio chain .... some gear allows you to directly drive your phase invertor for the pushpull driver/modulator

[W2VW]

600 to 600 will work just fine into a 1K load. There isn't any significant power being transferred. A low source impedance can usually play into a higher load impedance. This is akin to plugging a nite lite into a 20 ampere house outlet.

Audio squeels and howling can be a real problem which gets much worse with hokey antennas and lots of audio compression.

A balanced connection right to the transmitter should help with that problem. The transformer will make more problems go away if it is right at or even inside the tx.

A few tricks.

Use an attenuator within the tx and run the whole chain hotter if the audio components can take it. Most balanced audio gear makes enough soup to blow riceboxes away so it's easy. 0 to +4 dBm right up to the radio and set the attenuator for 100% mudulation with a 1 khz tone.

The static shield of the balanced feed can sometimes be left disconnected right at the audio xfmr balanced in connection instead of tying it to the radio chassis. W3GL's radio is set up that way along with a half dozen other active AMers I know of.

[n3lrx]

 This is what I'm using as the balun http://www.rane.com/note110.html Figure 2 Very simple except I'll be using a 1/4 TRS for the balanced line. I don't have any chassis XLR's to use.

[W3RSW]

Good document:

Ground loops notwithstanding (and agree for that, if nothing else, you might want a transformer for isolation), 600 ohms or 1000 ohms are frequency, complex impedance dependent.

Both signal and source have complex impedances so the difference at audio frequencies between 600 and 1000 ohms is probably not significant unless you have very specific requirements in your equipment manuals or are matching power loads, also mentioned.

Look at your circuits.  If either source or load has differential amplifiers calling for a balanced connection, I'd seriously consider unbalanced to balanced transformation regardless of how achieved, copper or solid state.  I think the studio world must come across this problem frequently with suitable transformers available, perhaps surplus from someone's shop.

Or use option three,

The Last Best Right Way To Do It

and as the guys say, "hook it directly up or just try it."  No hum, little attenuation, no sparks.. you're there.  

[W1RKW]

I use one of these: http://www.mcmelectronics.com/product/MCM-CUSTOM-AUDIO-555-8485-/555-8485

Works pretty good. I have it installed inside the 813 rig.  The unbalanced line-in from it is only 12" long to the audio amp.

[n3lrx]

Well, all my gear is in the same rack so the interconnects are as short as I could find 1.5'. I'll see what happens direct and with a transformer and see what I like best. The audio chain has unbal. capabilities just plug in a TS 1/4 phone jack and fly.

I've already got the transformer, the bud box to put it in, etc. I'm waiting for my new soldering iron to put it together. So buying another assembled one would be a waste of money. Plus it gives me something to do.

[n3lrx]

My simple audio matching transformer. I haven't done any soldering on it yet. I just wired it up. I'm waiting on a new soldering station that I'll order tomorrow. I don't want to do a bastard job on it with my used up slobering iron. I've also decided to put it in a smaller more appropriate box. So I'll have to find one close to the right size instead of wasting my larger box on a tiny board.

[W1AEX]

I've had surprisingly good results with the STANCOR TTPC-2 transformers that are sold by Allied Electronics for around 6 bucks:

http://www.alliedelec.com/search/productdetail.aspx?SKU=70213325

They are 1:1 600 ohm audio transformers that are listed as 300 cps - 3500 cps but they go below 100 cps and extend out to at least 6 kc easily. Looking at the scope while operating AM I can see some roll-off in audio energy below 75 cps when I plug one of these into my audio chain, but to be honest, I roll that stuff off with a pre-emphasis network anyway so I don't really miss it.

I use one of these with the balanced output of my audio rack into the unbalanced input of a TS-590S with good results. I also use one to couple the balanced line output of my audio rack to the balanced line input of my Flex 5000 and another one at the balanced output of my sometimes RFI prone RANE VP-12 which drives the audio system of my Viking rig. These transformers clean up any RF sensitivity very nicely.

While chatting with Mike WZ5Q one morning he mentioned the EDCOR transformer company. They've got some really interesting custom made audio transformers at very reasonable prices. Unlike a of lot stuff that's out there, they're listed as being able to pass 20~20K Hz quite nicely. Next time I stock up I'm going to give them a try.

http://www.edcorusa.com/p/143/wsm600-600

Rob W1AEX

[AB2EZ]

For applications where I am trying to eliminate hum, and where the signal source has a low output impedance (a few hundred ohms) and the signal sink had a high input impedance (e.g. 10000 ohms), I use the 600 ohm:600 ohm (i.e. 1:1) Xicon transformers that are sold by Mouser (around $2.00 each). If they are fed by a low impedance signal source, and if they are feeding a high impedance signal sink, they have a frequency response that is flat from below 50Hz to well beyond 7.5 kHz.

In this type of application, the amount of power flowing through the transformer is much lower than the transformer's power rating (e.g. 2 volts peak output across 10,000 ohms = 0.2 milliwatts). Since the signal sink has a high impedance, the transformer's leakage inductance doesn't roll off the high frequencies as quickly as it would with a lower impedance signal sink. Since the signal source has a low impedance, the transformer's magnetizing inductance does not roll off the low frequencies as quickly as it would with a higher impedance signal source.

Stu