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-   -   Volume at the output of preamplifier matching transformer (https://www.diyaudio.com/forums/tubes-valves/354933-volume-output-preamplifier-matching-transformer.html)

Glass_painter 23rd May 2020 09:05 AM

Volume at the output of preamplifier matching transformer
 
1 Attachment(s)
Hi,

Please look at the schematic.
This is presented as a clone of REDDI DI box in a different forum.
The pot at the output will short the transformer at the extreme position.
Am I missing anything here?
Thank you
Glass painter
Attachment 846586

JonSnell Electronic 23rd May 2020 09:29 AM

1 Attachment(s)
Not a good idea to mismatch the transformer.
Fit any volume control to the input side of the valve in the recognised way; wiper to the grids or

Schmitz77 23rd May 2020 10:05 AM

A level adjustment pot has to be either on the input side of the pre or between the two tube stages.
This 1K pot is at the wrong place and wrong value in the schemo.

JMFahey 23rd May 2020 11:08 AM

Wrong as drawn
MIGHT work if shorting pot placed across the XLR pins, *after* the 300 ohm resistors R5 and R6 but still the proper way would be to wire it as a non shorting volume control.

Transformer secondary end to end, "ground" and wiper to pins 3 and 2 respectively.

1k pot value is fine there.

analog_sa 23rd May 2020 12:39 PM

It is a drawing error. With the pot on the right side of the 300ohm resistors it will work perfectly.

analog_sa 23rd May 2020 12:52 PM

What kind of input voltage is anticipated? Without a level adjustment at input distortion may be higher.

Schmitz77 23rd May 2020 01:56 PM

Quote:

Originally Posted by analog_sa (https://www.diyaudio.com/forums/tubes-valves/354933-volume-output-preamplifier-matching-transformer-post6215763.html#post6215763)
It is a drawing error. With the pot on the right side of the 300ohm resistors it will work perfectly.


No, its not. Its the complete wrong pot type in this application.

If you wish to adjust a symmetric 600 Ohm line then its necessary to put a symmetric 600 Ohm balanced attenuator into it, which reflects always on in- and output the constant 600 Ohm.

As this method is very oldfashioned and out of style, nowadays we put those kind of pots shown here with different values between the voltage amp tube stages.
Thats good design practise. Or maybe on the input of the circuits. But never on the outputs.

analog_sa 23rd May 2020 03:07 PM

Quote:

Originally Posted by Schmitz77 (https://www.diyaudio.com/forums/tubes-valves/354933-volume-output-preamplifier-matching-transformer-post6215829.html#post6215829)

If you wish to adjust a symmetric 600 Ohm line then its necessary to put a symmetric 600 Ohm balanced attenuator into it, which reflects always on in- and output the constant 600 Ohm.



The circuit provides a perfect balance, an acceptably low output impedance and does not overload the amplification stage.

It does not have a constant input impedance but does it really matter?

Schmitz77 23rd May 2020 03:36 PM

Its not any longer a balanced output

JMFahey 23rd May 2020 03:54 PM

Sorry but you are wrong.
It stays a balanced output for the very good reason that itīs floating, completely unreferenced to ground, it comes from the secondary of a transformer.

If it drives a true balanced input, either active or best, a transformer, it works as intended.


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