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Old 4th June 2013, 01:21 AM   #1
m00dawg is offline m00dawg  United States
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Question Transformer Based Balanced Output & Impedance

I'm trying to make a basic balanced output for my Pro-Sounded GameBoy using a transformer and am running into trouble. The full circuit takes a stereo pair, inverts one channel using a smaller transformer, sums it and then goes to another transformer to balance the output.

The noise rejection part works well (it's actually another design I'm adopting). Music just goes over, say, the left channel, and nothing on the right. Inverting the right and summing it basically helps remove noise inherent in both channels. For the GameBoy, it's good enough. But one characteristic of that tranny for the current design I'm using is lack of bass. Pretty sure it's a Xicon 42TM or one like it, and the spec sheet mentions 300Hz for the low side.That's no problem for the noise rejection (most of the noise is in the KHz).

But, if I wanted to use a similar tranny for a balanced output, now I might have an issue. I read that part of the lack of bass could be an impedance issue. The Pro-Sound mod should be line-level (ish?) but I don't know what the actual impedance is. I was thinking of trying the trannys from HammondMFG. They at least go down to 200Hz, which is still not very bassey but better.

I read of some tricks to try to get better response from lower end tranny's but, as I recall, it involved driving them harder (say with an op-amp stage). I also thought about going full active. The potential ground issues aside, I could do a balanced circuit with an op-amp and a line-driver (ie DRV134) or more op-amps. I could even get rid of the noise rejection transformer (I think) but using a dual OPA and inverting one of the inputs and then summing those.

The problem with an active circuit (other than lack of isolation) is power. The box I was looking at doesn't have room for a power transformer without eating up most of the space. And I wanted it to be a stage box so having a complicated power supply brick to go with it isn't ideal. I thought about a virtual ground, but the problem is that there are also some digital circuits in there (an "ArduinoBoy" to sync the GameBoy via MIDI) and I'm guessing even with buffers, there will be a balancing issue if I don't go with a tranny.

So, all that said, am I barking up the wrong tree? If I just drop the whole balanced idea, this becomes alot simpler. But it'd be a nice feature to have so there would be no need for an extra DI box when needed (I think)?

Thoughts / Suggestions / Flames?
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Old 4th June 2013, 02:29 PM   #2
DF96 is offline DF96  England
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If you take a stereo pair, invert one channel and then add to the other channel you will get partial cancellation of bass. No bass cancellation means they were not a stereo pair but merely two separate audio channels.

You will not get any cancellation of noise, unless the noise is correlated. If the noise is correlated then that means there is some design flaw in the source.
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Old 4th June 2013, 04:31 PM   #3
m00dawg is offline m00dawg  United States
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"Stereo" is a loaded term here. It's actually dual mono is a better way to think of it. All the music is only played on one channel. The other is a noise reference. It's the one that gets inverted and then summed back in. It's not perfect, but does a fantastic job at reducing the high pitched whine, especially with the GameBoy Color. In this case, bass isn't an issue because the noise rejection tranny isn't very good at reproducing it, but also there isn't really any bass noise.

The circuit is not my own, it came from this:

PISSbox | kitsch-bent, l.l.c.

I just want to simplify it by removing things I don't want, looking at using a slightly better tranny to see if I get improved results, and stuffing it in this all in one box.

The real conundrum is the balanced output. I think it's just better all around if I not worry about that for this device. The times I need it I can use an Ebtech or even a Radio Shack balun (which apparently work well). I really would like to do balanced, but I think physics and cost are getting in my way
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Old 11th June 2013, 02:59 PM   #4
Pano is offline Pano  United States
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Interesting trick. Maybe you could use one of the EDCOR stereo to mono mixing transformers to do this and get better bass. You'd have to wire it flip polarity on one channel.
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Old 11th June 2013, 03:28 PM   #5
m00dawg is offline m00dawg  United States
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It is indeed a pretty neat trick! I was surprised how much it cuts out. I didn't realize stereo to mono transformers even existed until today I assume you mean one of these? So, if it's doing both summing AND balancing, I can end up simply using one transformer and be done with it?

Not sure how I might flip polarity in this case though. Looking at the diagram, instead of L, GND, R, it would be GND, L, R or something?

I've heard good things about the EDCOR stuff but nothing overly specific, but, wow for less than $8 for a tranny that lists a frequency response form 20Hz-20kHz, that's impressive! Surely there's a catch?
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Old 11th June 2013, 03:40 PM   #6
Pano is offline Pano  United States
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Long wait times can be the catch with Edcor, if not in stock. The value for money with Edcor can not be beat.

Yes, the inverting part is tricky. Because the Edcor transfos have that common pin 3, you can't tie it to hot on left, ground on right for inversion - that would just be a short for the left channel. You really need two separate windings for the primary. That said, I have done surgery on Edcor transfos where I cut the wire on the center pin into two halves. Not the easiest solution.

I'm not sure GND, L, R would work the way you want it. Gotta think about that.....
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Old 11th June 2013, 04:26 PM   #7
m00dawg is offline m00dawg  United States
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Well, reducing the transformer count by one would be nice, but isn't a necessity. The tranny the inverts the input doesn't need to have a wide frequency range. Or, space permitting, I could use two EDCOR's I suppose there.

The output tranny is the one that's more important to have a wider response. Looks like, either way, the EDCOR's are totally worth the wait if those spec sheets are accurate! Thanks for the info!
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Old 11th June 2013, 04:38 PM   #8
Pano is offline Pano  United States
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Sure, glad to help!

I was doing something similar a few years back - extracting the difference signal from L, R and did it with a transformer. You should contact Edcor to see if they have something like that already, that is not in the catalog. Just let them know that you want the difference signal (not sum) from 2 inputs. They can probably supply what you want for not much more than the price of the mixing transformers. Let them know you want line level to line level (1:1) if possible.

It would be nice if they had this as a regular catalog item.
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Old 11th June 2013, 04:41 PM   #9
m00dawg is offline m00dawg  United States
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Ah good idea, I'll do that!

In regards to line to line, seems the safest option. I think line level can be assumed to be the level from the GameBoy, but I'm not 100% sure. I wonder of a 10k to 600 ohm might be a good (the output will be going into a mixer)? Better to have a low impedance output than a higher one?
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Old 11th June 2013, 04:49 PM   #10
Pano is offline Pano  United States
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Low impedance output is good, but to get it with a transformer you have to lose voltage. That's OK if you take a line level signal to a mic input, but not OK if you want to go in line level. Just ask them if they can keep the ratio close to 1:1. 2:1 or 4:1 would be OK.

I'm thinking that a simple center tapped transfo with the center tied to ground and L on pin 1, R on opposite pin could do the trick. Is that right? Or do I need more coffee?
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