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Old 26th July 2012, 01:17 AM   #1
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Location: New Zealand
Default Gainclone with transformer input question

Honorable members, I've collected the parts for building the inverted gainclone with the LM1875; there was some cheap offering on eBay PCB and all parts. After the kit arrived I checked the layout and was not happy about one channel - some compromises in layout had been made and the output connection was running too close to the input connection for my liking. I decided to use only half of the board (with the proper layout) and get another board and use only that half of the board that has the proper layout part. This has the result that a) the layout is optimum and b) each has their own rectifier and DC smoothing resulting in less crossover issues.

This amplifier is going to be driven by a computer and I have had problems with another amplifier in the past with a groundloop which I never totally got rid off. I managed to score on eBay at an acceptable price (to me) some Beyerdynamic input transformers and want to use those to minimize hum and buzz.

I'll be using a low impedance output from the computer (many soundcards seem to use the headphone output as the line output but I am not to concerned about that - the iPod and the Cowan D2+ are doing the same) and the transformer has a 1:5 ratio, input 200 ohm, secondary 5K.

Questions:
1) Since the inverted gainclone has a feedback loop of 10K/220K does this mean the input impedance of the gainclone is 10K?
2) Do I need to put some loading resistor accross the secondary (e.g. 10K) to ensure the load is the same as the 5K output impedance?
3) input voltage from e.g. a CD player will be too much and the input impedance will be very low: can I just put a resistor in series with the primary to use that as an attenuator or do I need to put a resistor accross the primary in addition to the series resistor when I do this?
4) In light of question 3: Can I just swap the transformer around and use the scondary (5K) as input and the primary as output (200 Ohm), does it matter?

Many thanks in advance

Last edited by AmadeusMozart; 26th July 2012 at 01:22 AM.
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Old 26th July 2012, 04:54 PM   #2
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Join Date: Feb 2011
use a good opamp as buffer, then most probably whatever input impedance will suit You.
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