Balance Input with +/- 5db Control
« on: January 10, 2021, 09:26:05 PM »
Trying to sort out a circuit for an balanced line input section with stepped gain and attenuation +/- 5db.
Is it as simple as a differential input buffer then an amp with 10dB of gain then attenuating?
Looking for something akin to the Maselec MTC-1 input section but I can't find any information on its architecture.


john12ax7

Re: Balance Input with +/- 5db Control
« Reply #1 on: January 11, 2021, 03:01:14 AM »
You should put the attenuator between the input receiver and amp.

Tekay

Re: Balance Input with +/- 5db Control
« Reply #2 on: January 11, 2021, 07:09:01 AM »
As there are more than one input, the variable gainstage is probably after the input selector.
Thomas "TK" Kristiansson
----------------------------------
"The Sound Is In The Iron"
www.vintagedesign.se  www.tkaudio.se

Majestic12

Re: Balance Input with +/- 5db Control
« Reply #3 on: January 11, 2021, 07:56:28 AM »
I have made a Mastering console frontend with input selector, filter and +-6dB of gain with an simple inverting opamp stage and a stepped feedback resistor switch. Works without any problems and you also have the "gain boost" option as in the Maselec unit by reducing the resistor value that goes into the inverting input of the opamp.

Re: Balance Input with +/- 5db Control
« Reply #4 on: January 12, 2021, 09:27:28 AM »
I have made a Mastering console frontend with input selector, filter and +-6dB of gain with an simple inverting opamp stage and a stepped feedback resistor switch. Works without any problems and you also have the "gain boost" option as in the Maselec unit by reducing the resistor value that goes into the inverting input of the opamp.

Thanks! how are you creating the -6dB attenuation? Would love to see a drawing if you wouldn't mind, even if its just gain blocks. or maybe an equivalent schematic. (I really feel like I'm overthinking this.)

dbelousov

Re: Balance Input with +/- 5db Control
« Reply #5 on: January 12, 2021, 11:44:28 AM »
Look at this http://www.douglas-self.com/ampins/balanced/balanced.htm

There are some schematics for "high/low" and variable gain.
Dmitry

Majestic12

Re: Balance Input with +/- 5db Control
« Reply #6 on: January 14, 2021, 01:29:54 AM »
Thanks! how are you creating the -6dB attenuation? Would love to see a drawing if you wouldn't mind, even if its just gain blocks. or maybe an equivalent schematic. (I really feel like I'm overthinking this.)


Just an inverting opamp gain stage:

https://circuitdigest.com/sites/default/files/inlineimages/inverting-%20opamp.png

Set Rin to a fixed value  and put a variable pot or stepped resistor in place of Rf

Lets say Rin = 10k and Rf is a stepped resistor going from 5k to 20k

Rf = 5k : 5/10 = 0,5 (equals -6dB)
Rf = 10k : 10/10 = 1 (unity gain)
Rf = 20k : 20/10 = 2 (equals +6dB)

A "global" +6dB boost can be archived by putting another 10K resistor in parallel to Rin, making it 5k in value.

Rf = 5k : 5/5 = 1 (unity gain)
Rf = 10k : 10/5 = 2 (+6dB)
Rf = 20k : 20/5 = 4 (+12dB)

Just remember that your signal has inverted polarity after this stage, which you will have to compensate by putting another simple inverting unity gain stage after it. In my case the stage is followed by a THAT1646 balanced driver and I just swap the + and - outputs.
« Last Edit: January 14, 2021, 03:49:42 AM by Majestic12 »

Tekay

Thomas "TK" Kristiansson
----------------------------------
"The Sound Is In The Iron"
www.vintagedesign.se  www.tkaudio.se


 

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