API-312 output attenuator and level

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Spencerleehorton

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Hi,

Busy tracking some stuff with my APIs and into my apollo and luna, getting a pretty good sound but as I had just put all my Neve preamps through the scope and improved them greatly I thought it was time to take a quick peek at my api-312s.
Had to replace an led so did that then found a thread on putting a 1k log pot between the secondary on the output txf.
So before i wired it in I checked ac output voltage, it was only 1.4v, I then wired the output txf so it was a 4:1 rather than a 2:1 and this improved output level to about 2.8v, then I wired in the pot and I then had full control of output level with a bit of reduced level, down to about 2.4v.
I then put a connection from 0v to  the end lug on the pot and the output level shot up to 5.2v, same output now as my Neves.
So quite happy with this unit and when i installed back into patchbay and put the kick mics through it, they sounded so much better!!
This then led me onto my next unit which i then put in the 1k pots but with this unit i cant reduce the output level down to zero as per previous unit it only takes the output level down a bit?
This one has a fab microphone output txf with 4 windings, I've used one winding for primary and wired up the other 3 in series for secondary.
Bit stumped as to why this isn't taking all the level down to zero?

Looking at the schematic I can see I only need two windings for secondary,  so will change and check the correct colours are where they should be, also I havent put in the 0v connection as per my first unit.
 

ruffrecords

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Spencerleehorton said:
Hi,

Busy tracking some stuff with my APIs and into my apollo and luna, getting a pretty good sound but as I had just put all my Neve preamps through the scope and improved them greatly I thought it was time to take a quick peek at my api-312s.
Had to replace an led so did that then found a thread on putting a 1k log pot between the secondary on the output txf.
So before i wired it in I checked ac output voltage, it was only 1.4v,

Under what conditions? Does not mean a lot as it stands. What level were you expecting and why. 1.4V is just over +4dBu. Seems about right to me.

Cheers

Ian
I then wired the output txf so it was a 4:1 rather than a 2:1 and this improved output level to about 2.8v, then I wired in the pot and I then had full control of output level with a bit of reduced level, down to about 2.4v.
I then put a connection from 0v to  the end lug on the pot and the output level shot up to 5.2v, same output now as my Neves.
So quite happy with this unit and when i installed back into patchbay and put the kick mics through it, they sounded so much better!!
This then led me onto my next unit which i then put in the 1k pots but with this unit i cant reduce the output level down to zero as per previous unit it only takes the output level down a bit?
This one has a fab microphone output txf with 4 windings, I've used one winding for primary and wired up the other 3 in series for secondary.
Bit stumped as to why this isn't taking all the level down to zero?

Looking at the schematic I can see I only need two windings for secondary,  so will change and check the correct colours are where they should be, also I havent put in the 0v connection as per my first unit.
[/quote]
 

boji

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I'd experiment with 'wrong' connections to rule out a factory wire color mismatch / polarity issue.


Edit: Should have read more carefully, I see your DUT is a mystery tx.

This one has a fab microphone output txf with 4 windings, I've used one winding for primary and wired up the other 3 in series for secondary.

Fwiw (2503 in 1:3 config):
bv55yTz.jpg


1:6 would tie Violet & Blue together as well Green & Yellow, top of pot would get moved to Orange.
 

iprovlek

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Here is 2503 specification from Capi site Spence...

You can check colors of wires....

 

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Spencerleehorton

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Yes, thanks for that guys, all works now apart from a bit of the bottom of the waveform distorts when I bring up the gain and reduce the output?
If I have lowest gain there is no distortion anxbic I have highest gain there is none, just when it's about half way?

Btw I'm using 990 op amps in this one  but have 2520 in all my others.
 

ruffrecords

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ruffrecords said:
Under what conditions? Does not mean a lot as it stands. What level were you expecting and why. 1.4V is just over +4dBu. Seems about right to me.

Cheers

Ian
I then wired the output txf so it was a 4:1 rather than a 2:1 and this improved output level to about 2.8v, then I wired in the pot and I then had full control of output level with a bit of reduced level, down to about 2.4v.
I then put a connection from 0v to  the end lug on the pot and the output level shot up to 5.2v, same output now as my Neves.

The change fron 1.4 to 2,8V is simply because you double the ratio so you doubled the voltage.

Connecting one side to 0V and suddenly getting 5.2V does not make sense. I suspect your measurement method. If the transformer is producing a balanced output then you should simply measure between hot and cold of whatever output connector you are using. Ov is NOT a signal conductor in a balanced system so connecting one side of the pot to is should make no difference unless you were measuring originally between hot OR cold and 0V which is an invalid measurement.

Cheers

Ian
 

Rob Flinn

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Sussex, UK
You should feed the 1k pot through 2x 300R resistors 1 on each side of transformer output.  That way even when you have the pot turned all the way down the output isn't driving a short.
 

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