A high-fidelity interface for coverting line level audio (600ohm) to high-imp

[W2XR]

As mentioned here, some modulators can be VERY linear..tube or solid state, and thus are able to impose the exact waveform on the RF carrier that is fed into it. Steve's two modulators...PDM and linear are flat within the audio range of near DC to 7kc or higher...able to pass a square wave pretty accurately. 

[/quote]

Hi Dan,

Can you clarify one thing for me with regard to your post?

Concerning the statement, "Those tube modulators with just a modulation transformer (no grid transformer) can get close to this. " When you refer to (no grid transformer, are you referring to the lack of a driver xfmr to the push-pull modulator grids, or the audio input xfmr that is generally utilized to split the phase to the audio driver?

Perhaps you are referring to the later Gates rigs with cathode follower drivers to the push-pull class B modulator grids; this circuit eliminated the need for the audio driver xfmr, but I believe still required an audio input xfmr to split the phase at the input to the driver/modulator audio deck.

How would the elimination of one or both xfmrs improve the asymetrical modulation characteristics of the xmtr? In terms of square pulse response, I can clearly see how this would improve in terms of transient responce and a reduction in overshoot and ringing, but not necessarily the ability of the rig to faithfully pass an asymetrical waveform.

Thanks & 73,

Bruce

[W2XR]

Reposted due an error at my end in the "quote" function! Let me ask the question once again:

"As mentioned here, some modulators can be VERY linear..tube or solid state, and thus are able to impose the exact waveform on the RF carrier that is fed into it. Steve's two modulators...PDM and linear are flat within the audio range of near DC to 7kc or higher...able to pass a square wave pretty accurately. Those tube modulators with just a modulation transformer (no grid transformer) can get close to this. "
[/quote]

Hi Dan,

Can you clarify one thing for me with regard to your post?

Concerning the statement, "Those tube modulators with just a modulation transformer (no grid transformer) can get close to this. " When you refer to no grid transformer, are you referring to the lack of a driver xfmr to the push-pull modulator grids, or the audio input xfmr that is generally utilized to split the phase to the audio driver?

Perhaps you are referring to the later Gates rigs with cathode follower drivers to the push-pull class B modulator grids; this circuit eliminated the need for the audio driver xfmr, but I believe still required an audio input xfmr to split the phase at the input to the driver/modulator audio deck.

How would the elimination of one or both xfmrs improve the asymetrical modulation characteristics of the xmtr? In terms of square pulse response, I can clearly see how this would improve in terms of transient response and a reduction in overshoot and ringing, but not necessarily the ability of the rig to faithfully pass an asymetrical waveform.

Thanks & 73,

Bruce

[W1DAN]

Hi Bruce:

I was thinking in terms of phase shift over the frequency range of interest (and beyond) changing the phase of an asymetrical (or symmetrical) signal at the frequency extremes.

With more than  one transformer, it is easy to get up to 180 degrees of phase shift at the extremes of the audio passband...resulting in a conversion of normal negative feedback to positive feedback at the high and low end.

If you feed audio that has been diode clipped at the negative excursions into a modulator that has phase shift, the areas that are supposed to be flat will get tilted by the phase shift.

Normal audio's phase will also be changed at these extremes, but it is a dice toss as to whether it will be good or bad (i.e. make negative going spikes where you do not want them).

The least amount to phase shift can be had with a modulator (like a Gates BC1T) that has a cathode follower driver and a mod transformer. I'd do an active phase splitter at the modulator input as well. Reactive elements are still reactive and in my mind should be minimized.

This can be demonstrated by running signals through a Viking II or Valiant, and then changing the driver circuit to an active splitter...much wider frequency response can be had. The Modulation transformer still is the limiting item (next to the power supply).

Asymetrical modulation normally requires a modulator that can provide 125% or greater positive swing (and thus power). This obviously means a beefier power supply and modulator tubes.

In the end, the closer you can pass a square wave with enough amplitude, the more fidelity you will pass through the modulator. This is usually very hard to do with transformers.

Hope this helps...

73,
Dan
W1DAN

[W2XR]

Hi Bruce:

I was thinking in terms of phase shift over the frequency range of interest (and beyond) changing the phase of an asymetrical (or symmetrical) signal at the frequency extremes.

With more than  one transformer, it is easy to get up to 180 degrees of phase shift at the extremes of the audio passband...resulting in a conversion of normal negative feedback to positive feedback at the high and low end.

If you feed audio that has been diode clipped at the negative excursions into a modulator that has phase shift, the areas that are supposed to be flat will get tilted by the phase shift.

Normal audio's phase will also be changed at these extremes, but it is a dice toss as to whether it will be good or bad (i.e. make negative going spikes where you do not want them).

The least amount to phase shift can be had with a modulator (like a Gates BC1T) that has a cathode follower driver and a mod transformer. I'd do an active phase splitter at the modulator input as well. Reactive elements are still reactive and in my mind should be minimized.

Asymetrical modulation normally requires a modulator that can provide 125% or greater positive swing (and thus power). This obviously means a beefier power supply and modulator tubes.

In the end, the closer you can pass a square wave with enough amplitude, the more fidelity you will pass through the modulator. This is usually very hard to do with transformers.

Hope this helps...

73,
Dan
W1DAN

Hi Dan,

Many thanks for the usual detailed and thoughtful reply.

You are correct; I neglected to consider the issue of phase shift thru the magnetics within the audio path. Changing or shifting the phase relationship thru the audio path will plainly cause a shift in symetry. I can see this also when I make a significant change in the equalization of my audio; if certain frequency bands are boosted, the symetry (or asymetry) of the modulation characteristic will change.

Thanks again!

73,

Bruce

[W1DAN]

Bruce:

yes the same physics apply. I was gonna suggest your sending a square wave or voice through an EQ and look at the results when you hi pass and low pass, change frequencies, etc.

Let us know what you end up doing on your modulator!

73,
Dan
W1DAN

[Opcom]

this conversation's really as broad as it is deep. Behringer since it is always mentioned, makes something called a Behringer US600. When properly applied to the microphone signal, it seems to fill out the speech bandwidth and increase clarity intelligibility a little, using the slightest amount of frequency shifing and mixing it with the straight signal. It seems to add more waves to the voice wafeform filling up some empty amplitude space within the half cycle. It was designed for guitars, but has some interesting effects on the voice and since it is DSP there's no having to hit the gate hard to make it work. It's sort of like having two voices at once, exactly in time, but the benefit is lost or ruined if too much shift or effect is used. Needs 10 turn pot. This thing is cheap too <$30. I just got it to play with, Not sure it would do anything good on the air.