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Article - Sony VFETs part 1
Article - Sony VFETs part 1
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Old 22nd October 2017, 06:37 PM   #1361
luvdunhill is offline luvdunhill  United States
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Quote:
Originally Posted by Nelson Pass View Post
If you

can drive 500 ohms without significant distortion, you are doing fine.
Excellent, here are my results at 500 ohms

buffer_500.JPG

They are cute little guys

IMG_3101.JPG



Quote:
Originally Posted by Nelson Pass View Post
For maximum bandwidth you want buffers that have output impedances

lower than the DC values of the primaries. For the Jensen and Cinemag

examples, 25 ohm is desirable, although things don't fall apart until quite

a bit higher.

I need to think of a good way to test this on my bench.
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Old 23rd October 2017, 10:04 PM   #1362
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I used to do quick and dirty method that would probably get you there. When you load the output with a value equal to the source Z, the output level will be cut in half(-6db). At 10k load, a 25 ohm output Z is relatively meaningless. But with 25 ohm load, 1/2 your voltage is dropped in the output Z and 1/2 on the load
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Old 10th November 2017, 08:34 PM   #1363
luvdunhill is offline luvdunhill  United States
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Quote:
Originally Posted by Nelson Pass View Post
There is a nominal impedance for the transformer primaries, but that's for when

the secondaries are driving their rated impedance, which in these instances

they usually aren't.



With the 150:150:150:150 ohm line transformers we've been using as

autoformers, I have measured mid-band impedances of 5K or so. If you

can drive 500 ohms without significant distortion, you are doing fine.



For maximum bandwidth you want buffers that have output impedances

lower than the DC values of the primaries. For the Jensen and Cinemag

examples, 25 ohm is desirable, although things don't fall apart until quite

a bit higher.


So, plugged them in and generated some smoke

In thinking about it, I have a few questions. Here is the schematic that elicited the smoke, I was using a balanced to SE XLR Adapter

IMG_3160.JPG

1.) I am switching the buffer in and out of the circuit via a relay. I am just moving the input of the buffer, the output stays connected. Will this work? Is it better to switch the output, or must I switch both the input and out of the buffer?

2.) My buffer saw as load the primaries in parallel, which is pretty low and caused a resistor to smoke. This is much lower than the 500R above. This is because the negative input was connected to ground via the adapter. I am doing something wrong here, obviously. How can I fix it?

Thanks!
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Old 10th November 2017, 09:25 PM   #1364
Nelson Pass is offline Nelson Pass  United States
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Article - Sony VFETs part 1
Yes, you are doing something wrong.

The first step on the path to enlightenment.

I don't think you want the output of the buffer and the input signal
connected to the transformer at the same time. Also, always see to it
that the buffer input has something to look at, even if it's only a large
value resistor to ground.
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Old 10th November 2017, 09:46 PM   #1365
luvdunhill is offline luvdunhill  United States
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Quote:
Originally Posted by Nelson Pass View Post
Yes, you are doing something wrong

The smoke was the first indication of that

Ok, so switch the output of the buffer. Makes sense!

It seems that I should also switch the input. Take this example:

IMG_3161.JPG

I seem to loose the benefit of a balanced input, as I have the input resistor to ground? So, I guess if I wanted to have the buffer flexibility, I need switch it completely in/out, meaning both input and output?

In the case of the first diagram, where the switch toggles the buffer to be in-line, doesn’t the buffer still see 25R in parallel with 25R as load?
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Old 11th November 2017, 12:00 AM   #1366
Nelson Pass is offline Nelson Pass  United States
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Article - Sony VFETs part 1
You want the buffer to always have some DC connection to ground,
even if it's 1 Meg (or more likely 100K). Otherwise it might be led astray...
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Old 11th November 2017, 02:03 AM   #1367
luvdunhill is offline luvdunhill  United States
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Quote:
Originally Posted by Nelson Pass View Post
You want the buffer to always have some DC connection to ground,

even if it's 1 Meg (or more likely 100K). Otherwise it might be led astray...


Thank you!

... and the second part of my question I believe is answered by “yes, the buffer has to drive a DC load of 25 in parallel with 25, as the primaries are in parallel.” My little buffer can’t do this.

I think the original question I meant to ask, was “when I am testing my buffer for low distortion, how can I approximate the load and/or use the transformer itself as an load approximation”. I misunderstood your answer of “500” to mean, stick a 500 ohm resistor on the output.

So, given my confusion, what would be the best way to bench test the buffer at a good approximately of the transformer load? Just use the transformer itself with the secondaries open?
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Old 11th November 2017, 11:00 AM   #1368
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Article - Sony VFETs part 1
you're always taking in account secondary load transferred to primary
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Old 11th November 2017, 06:21 PM   #1369
Nelson Pass is offline Nelson Pass  United States
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Article - Sony VFETs part 1
Obviously the best way is to test it into the actual intended load.

That said, I find that a buffer capable of driving the "rated" impedance as a
resistance will do well enough into the transformer.

For example, the Jfet buffers I use won't drive 150 ohms, but are ok
into a 150 ohm tap on a transformer as long as the secondary system is
not driving its rated impedance.

I have a couple of amplifiers with 150:150:150:150 coils as autoformers
giving a gain of 12 dB into 10K ohm. They work fine driven by a pair of
2SK170/2SJ74.
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Old 12th November 2017, 12:58 PM   #1370
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Hi Dear Mr. Pass, Since I built a sort of CSX1 but with balanced couple of Vfets and I am very comfortable with the dynamic of this topology with my speakers, I am interested in those transformers, where is possible to get them? Thanks in advance.
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