TVC Driving a 600 Ohm Input Transformer

discussion of magnetic volume control design for both line and speaker levels.

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westernelectric300b
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Joined: Fri Apr 30, 2021 1:29 am

TVC Driving a 600 Ohm Input Transformer

Post by westernelectric300b »

I have a pair of TVCs (not AVCs), and I would like to now use them in a different application. Note that their input and output will be used in a true balanced configuration.

They will be driven by various sources, each having a very low source impedance (600 ohms or less, and all sources provide a balanced output), so that should not be an issue here.

However, my new application entails using the secondary of the TVCs in the balanced configuration to drive a high-quality line-to-grid input transformer having a primary impedance of 600 ohms. The secondary of this input transformer will be unterminated; it will be looking into a very high load impedance, that being the grid of a vacuum tube.

I understand these TVC units are designed to work best when the secondary is seeing a high load impedance, ideally a purely resistive load, typically on the order of 10K ohms or higher. How will the performance of the TVC possibly degrade when the secondary of the TVC is terminated in the 600 ohm load I had described above? Will there be any impairment to the high-frequency response, etc., of the TVC?

I can see where this could possibly be an issue when the TVC is being used at minimum attenuation; the output impedance of the TVC in this case could conceivably be higher than the 600 ohm load impedance, and that could create an issue. But that is not a typical operating condition; the TVC would usually be operating in the range of perhaps 10 to 20 dB of attenuation in my system, in which case the output impedance of the TVC is probably substantially lower than the 600 ohm load impedance.

Am I also to assume that the 600 ohm reflected impedance presented by the input transformer will not pose an issue for source equipment having a source impedance on the order of 600 ohms, maximum? As mentioned above, the source equipment presents a balanced input to the TVC, which of course will be also configured for balanced operation. I have no idea as to what the impedance ratio is for the TVC, once you increase the attenuation beyond minimum attenuation, so I cannot calculate the reflected impedance as seen by the source.

I was also giving my particular application some additional thought yesterday. Recall that the input transformer I will be driving with the TVC has an input impedance of 600 ohms when resistively terminated in its secondary impedance, but in my application, the transformer secondary will be unterminated; the secondary of the transformer is looking into the grid of a tube, so the load impedance is very high. This means that in reality, the input transformer primary impedance as seen by the secondary of the TVC is not 600 ohms, but a higher impedance. The unloaded primary impedance at the very lowest audio frequencies is limited only by the inductance presented by the input transformer, but as the frequency increases, the unterminated secondary reflected impedance of the input transformer as seen at the primary, may in fact be substantially above 600 ohms. This being the case, I suspect the load impedance as seen by the TVC may be substantially higher than if the TVC were to be operated into a purely resistive 600 ohm load, or if the secondary of my input transformer was resistively terminated in its secondary impedance.

There is another consideration here as well; having the secondary of the TVC in parallel with the input transformer results in a substantial decrease in the total inductance of the two devices, as inductances in parallel do not of course add; they are subtractive!

And to complicate the issue further, as the TVC attenuation varies, so does the total inductance of the TVC and the input transformer, and this could have a negative and variable impact to the low frequency response.

I think Thomas Mayer in Germany uses the same design approach as I have described, that being a TVC (or an AVC) driving an input transformer within his 2-stage power amplifiers, and his products are very highly rated as being extremely musical, so who knows?

Hopefully, this all makes sense to you.

In summary, does anyone here have any guidance as to the expected behavior of the TVC when operated into the input transformer and circuit I have described, with regard to the low and high frequency response performance?

Thanks for your guidance!
dave slagle
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Re: TVC Driving a 600 Ohm Input Transformer

Post by dave slagle »

Hey,

you answered all of your questions and the load the TVC will see is not 600Ω since the secondary is left open. The primary inductance will appear in parallel with the reflected load but at anything below unity that value will get very large since it will be multiplied by the turn ratio squared. 10hy of primary inductance and 12dB of attenuation puts 160hy in parallel with the primary inductance of the TVC.

dave
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westernelectric300b
Posts: 28
Joined: Fri Apr 30, 2021 1:29 am

Re: TVC Driving a 600 Ohm Input Transformer

Post by westernelectric300b »

Hi Dave,

Sorry for the late reply here. I should check this site more often for responses!

At first glance, that 10hy of primary inductance and 12dB of TVC attenuation puts 160hy in parallel with the primary inductance of the TVC, looks to be a good thing, yes?

I understand these TVC units are designed to work best when the secondary is seeing a high load impedance, ideally a purely resistive load, typically on the order of 10K ohms or higher. How will the frequency response of the TVC possibly degrade when the secondary of the TVC is terminated in the 600 ohm load I had described above, which of course will be using an unterminated secondary winding? Obviously and in this scenario, the primary impedance will be higher than 600 ohms, due to the unterminated secondary. That said, will there be any impairment to the high-frequency or low frequency response, etc., of the TVC?

This thing of the TVC driving a recommended load impedance of 10K or higher keeps staying in my mind.....

Thanks!
dave slagle
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Re: TVC Driving a 600 Ohm Input Transformer

Post by dave slagle »

I do not think it is fair to comment on inductive volume controls as a whole since there are many design considerations and your best approach is to measure and listen to the specific case to determine suitability. In speaking about my autoformers, they do really well driving difficult loads but ultimately the lighter the load the better I find them sounding. The key point here is if part of the requirement of the device is to drive a difficult load then the autoformer is a good choice but adding additional loading just because "you can" results in a subjective step backwards in sound.

dave
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