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 Post subject: 6888 Transmitter stereo to mono input
PostPosted: Jan Fri 10, 2020 1:00 am 
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Norm's original transmitter uses a "step-up" transformer to boost the input signal.

I have a Heathkit AA100 amp with one bad OPT.

The good OPT is 6600Ω primary with 4-8-16Ω outputs. Can this be used with stereo inputs to the 8 and 16 taps? Seems this would be perfect. I thought I would ask before I tare it apart.


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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Fri 10, 2020 1:06 am 
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I think I just answered my own question after drawing it out. No center common ground for the 8Ω but I might try the 4Ω taps. Opinions?


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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Fri 10, 2020 8:44 am 
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Mike Toon wrote:
I might try the 4Ω taps

I'm confused. There's more than one 4Ω tap?

If you're connecting your inputs on opposite sides of the common then the Left and Right will be out of phase, and audio that's common to both Left and Right (i.e., center stage) will cancel out. Most noticeable effect will be lack of bass.


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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Fri 10, 2020 1:53 pm 
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I would try combining L and R channels through each side of two 10 ohm resistors tied together. Then connect the
center connection point of the two resistors to one side of the transformer input. The other side of the transformer would get connected to the common side of your source input.
This would get you around any phase issues like discussed above.

Assuming you are using a smart phone or MP3 player with low impedance headphone output, this should give you a pretty good impedance match to the transformer. You could experiment with the different taps to see what sounds best.

Attachment:
Revised Ipod Adaptor1.jpg
Revised Ipod Adaptor1.jpg [ 31.34 KiB | Viewed 1650 times ]

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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Fri 10, 2020 6:40 pm 
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BobWeaver wrote:
Mike Toon wrote:
I might try the 4Ω taps

I'm confused. There's more than one 4Ω tap?

If you're connecting your inputs on opposite sides of the common then the Left and Right will be out of phase, and audio that's common to both Left and Right (i.e., center stage) will cancel out. Most noticeable effect will be lack of bass.


This won't work?


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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Fri 10, 2020 6:48 pm 
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Suppose this won't work either?


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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Fri 10, 2020 6:51 pm 
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Mike Toon wrote:
BobWeaver wrote:
Mike Toon wrote:
I might try the 4Ω taps

I'm confused. There's more than one 4Ω tap?

If you're connecting your inputs on opposite sides of the common then the Left and Right will be out of phase, and audio that's common to both Left and Right (i.e., center stage) will cancel out. Most noticeable effect will be lack of bass.


This won't work?


You could try it and it wouldn't damage anything, but the phase cancellation might be noticeable as an overall loss of volume and bass.

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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Sun 12, 2020 12:03 am 
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If the bad OPT has only an open on half of the primary and its secondary is intact, it is useable for this project instead of the good one.

The taps of interest are common, 4 ohm and 16 ohm. The primary to secondary voltage ratio is proportional to the square of impedance ratio. Therefore the number of turns between the 4 ohm and common taps is the same as the one between the 16 ohm and 4 ohm taps. When two 4-ohm windings are connected in series, you have 16 ohm (4 x 4 ohm).

Open one of the bells and see if you can separate the two wires at the 4 ohm tap. If you do, you will end up with two 4 ohm windings: 16-4a and 4b-common. Connect the left input to the 16 ohm tap and the right to the 4b tap. Tie the 4a and common taps together and this will serve as the common input. The voltage step up is about 40 (32 db).

Edit:
Attachment:
OPT_Mod_for_Stereo_Input.jpg
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In general, when a Z1 winding is in series with a Z2 winding, the resulting winding is for an impedance of:

Z1*(1 + sqrt(Z2/Z1))^2 or
Z1 + Z2 + 2*sqrt(Z1*Z2)

In the special case where Z1 = Z2 = Z, the resulting impedance is 4*Z. For example, two 150-ohm windings in series become 600 ohm.

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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Sun 12, 2020 2:04 pm 
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bb.odin's suggestion to separate the winding at the 4 ohm tap will fix the center channel cancellation problem, but there is still a problem with isolation. Any winding on a transformer can and will act as both a primary and a secondary. So, even though the left and right channels are connected to different windings, their signals are not isolated from each other (except for the DC component). Left will load down right, and right will load down left.

Processhead's suggestion to use resistors is the only single transformer method I can see that would isolate left and right.


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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Sun 12, 2020 6:56 pm 
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I removed the good transformer and as Bob predicted bass is compromised.
I'll remove the bad one and do some more testing.

I have been using 10Ω resistors since I built Norm's transmitter in 2006 even though the suggested resistors then were 100K Ω.

To further complicate things, the amp/mp3 player is PWM and the channels are isolated, no common ground so I had to connect the grounds together through 10Ω resistors also. I am testing with a regular mp3 player.


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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Sun 12, 2020 7:06 pm 
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BobWeaver wrote:
So, even though the left and right channels are connected to different windings, their signals are not isolated from each other (except for the DC component). Left will load down right, and right will load down left.

Agree with that.

For the purpose of simulation, this OPT can be modeled as three coupled inductors. Let L1 and L2 represent the 4-ohm windings with an inductance of 20mH each. L3 with an inductance of 8H represents 1/2 of the primary. The voltage step-up ratio is N = 20. Perfect coupling is assumed. R1 and R2 of 1 ohm each are used to measure the current flow in L1 and L2 respectively.
Attachment:
Coupled_Inductors.jpg
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The transformer performs an average of the left and right inputs. If both inputs are perfectly in-phase with each other and have the same amplitude, the output has the same amplitude. For example, injecting a 20Hz 1V sine wave to both inputs results in a 20V sine wave at the output as shown below. In the figure, Red = Ouput, Green = R1 Voltage, Blue = R2 Voltage.
Attachment:
Waveform_1.jpg
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When the right input amplitude is reduced to zero, the output amplitude is halved (10V). The right input winding also sees half of the left input amplitude (0.5V). The danger here the current flow in the right widing, which could be large, is limited by the resistances of the winding and the signal source. To limit this current, a resistor of suitable value needs to be placed in series between signal source and the transformer input.
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Waveform_2b.jpg
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The worst case happens when both inputs are out-of-phase with each other. The output is close to zero. However the magnetizing inductances L1 and L2 of the two input windings cancel each other. The series resistance in the circuit is the only one that limits the current flow.
Attachment:
Waveform_3.jpg
Waveform_3.jpg [ 254.88 KiB | Viewed 1504 times ]

If you still want to use a transformer to couple a stereo signal to a mono input, it's better to use one designed for the task such as the Edcor WSM6400 or PC6400. The high resistances of the windings will limit the current in the worst scenario.

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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Sun 12, 2020 11:04 pm 
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Mike Toon wrote:
I have been using 10Ω resistors since I built Norm's transmitter in 2006 even though the suggested resistors then were 100K Ω.

To further complicate things, the amp/mp3 player is PWM and the channels are isolated, no common ground so I had to connect the grounds together through 10Ω resistors also. I am testing with a regular mp3 player.

The low impedance transformer input requires small value resistors, 100K, even 10K would appear as a open circuit. I use 10K with the tube preamp(line) input.

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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Mon 13, 2020 12:32 am 
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Tom, what is the advantage in using any resistor in series with an 8Ω winding on a transformer? I figured most mp3 players were designed to operate using earbuds of about 16Ω per channel. My reasoning in adding a 10Ω resistor would bring the "load" up to about 18Ω.


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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Mon 13, 2020 2:12 am 
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Mike, I have run both channels directly into an 8 ohm transformer primary without the resistors. I was using an old Ipod touch or a Samsung Galaxy phone.

It works and sounds fine, but I added the 10 ohm resistors as discussed to provide some isolation between left and right outputs of the MP3 source. My reasoning was that I just didn't think it was ideal for the amplifier outputs of the Ipod. YMMV.

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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Mon 13, 2020 3:32 am 
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processhead wrote:
Mike, I have run both channels directly into an 8 ohm transformer primary without the resistors. I was using an old Ipod touch or a Samsung Galaxy phone.

It works and sounds fine, but I added the 10 ohm resistors as discussed to provide some isolation between left and right outputs of the MP3 source. My reasoning was that I just didn't think it was ideal for the amplifier outputs of the Ipod. YMMV.

Agreed...

I'm currently running a 6GY6, 6AV6, 6SN7 transmitter that only has one input, R & L are fed through a Y cable. I really can tell no difference from one with a pair of resistors.

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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Mon 13, 2020 9:08 pm 
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I would suggest trying resistors that are about 10% of the DC resistance of your transformer, give or take. That puts the majority of the signal into the transformer, instead of loosing it in the resistors. The resistors are there just to avoid shorting the outputs together. Think of it as a voltage divider, at a given instant L is outputting 2V, R is outputting 0V. You have 2 resistors in series with 2V at one end, 0V at the other, so you will see 1V at the junction.

If your amp has the channels isolated, can you put left and right in series instead of parallel, eliminating the stereo to mono issue and basically doubling the output at the same time?


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 Post subject: Re: 6888 Transmitter stereo to mono input
PostPosted: Jan Tue 14, 2020 2:55 am 
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classicelectronicsguy wrote:
If your amp has the channels isolated, can you put left and right in series instead of parallel, eliminating the stereo to mono issue and basically doubling the output at the same time?


This seems to work very well. What is the downside?


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