ALL RIGHT now we'talking NEW La2A just born (twins)

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mata_haze

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Joined
Dec 20, 2005
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810
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Back in London!
here they are in all their beautifulness..
identical and amazing sounding La2A.
thank you Drip Electronic for PCB (very well done!!!)
thank you maeve61 for the original UREI T4B (they sound awesome)
and THANK YOU Brian Sowter for designing the INPUT and OUTPUT transformers for us. they sound as we expected: brilliant!

thank you everybody at Prodigy as this would not have seen the light without you all!!!
DIY FOREVER!









ROCK AND ROLL!!!!
 

drpat

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Joined
Mar 30, 2005
Messages
1,095
Location
NorCal
Who looks at the meters on an LA2A anyways??? I keep mine covered to avoid ass chewings by meter maids. Let's face it, these things aren't precision instruments.

Just put your left hand on the GAIN knob, right hand on the REDUCTION knob, close your eyes and ride it like a Harley!!! Vroooom Vrooooom!!!
 

Purusha

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Jan 14, 2006
Messages
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Somewhere on the Planet
[quote author="SSLtech"]Did you calibrate it?

Define 'way off'. Do you mean in GR mode or in signal mode?

Keith[/quote]

It's in GR mode. I need yet to solve this with my Sifam LA29 meters and LA-2A.
It looks like it needs some additional circuit to respond to the scale.
Mine shows -20db on GR meter scale when it's actually doing only 7-8db of GR.
 

peter purpose

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Jun 3, 2004
Messages
2,082
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London
[quote author="drpat"]Just put your left hand on the GAIN knob, right hand on the REDUCTION knob, close your eyes and ride it like a Harley!!! Vroooom Vrooooom!!![/quote]

Maaaaahvellous stuff.. :thumb:
 

SSLtech

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Jun 3, 2004
Messages
5,447
Location
Florida (Previously UK)
You adjust R25 until it matches. Without calibrating R25, they don't track accuratley on ANY LA-2a.

Replace R25 with a trimpot of about 1.5 or 2 times the stated value of R25 then start it halfway and NEVER take it all the way to either extreme...

Somewhere in there will be a setting which makes the meter agree when switching between GR and +4, after setting a tone to 0VU output in +4 mode with no GR present. Increase the GR and set it so that it matches at around -5dB (5dB gain reduction). It will be maybe a little off at -3dB, maybe a little off at -8dB... a little off all over the scale, but I find that matching the meter at-5dB is usually an excellent compromise.

The only way to get them to track correctly is to MATCH the optos in your T4b. Then you re-adjust R25 and try again.

Read the comments by David Kulka and myself in the T4B threads, and you should be able to find out a lot. T4B matching and behaviour research is something that I've done a LOT of...

If you buy a NOS T4B from David and plug that in, you should find that there is a better match between optos, since they DID select and match the optos. Setting R25 will then give you about the closest match that you can get.

However, if you've not adjusted and set the value of R25, you'll NEVER getting them to track would be like winning the lottery, and if you put a different T4 in there, it wouldn't track again, until you adjusted R25 again.

Keith
 

Purusha

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Thanks Keith, this is just the info I was looking for :thumb:

If you were in my neighborhood I would buy you a :guinness:
...ups can't do, alcohol is sinful so I guess it would have to be a
strawberry juice with a weep cream on top :sam:

:green: :green: :green:
 

SSLtech

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Jun 3, 2004
Messages
5,447
Location
Florida (Previously UK)
I actually just did this on a re-T4'd UA LA-2a this morning. I used a 50kΩ variable resistor. The value in the end turned out to be 24.2kΩ, so I put a 22k resistor and a 2.2k resistor in series for R25. Tracks within about .025dB, but that's with a genuine UA T4b... random optos in a T4 will probably be within a dB or so.

Keith
 

CJ

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Joined
Jun 3, 2004
Messages
14,594
Location
California
Crooked T4, nah nah nah..nah nah.....nah..!
:grin:
Just kiddin.
Seriously though, all you crooked T4 people out there, simply unscrew the cover and rotate the base. There is a tensioner in the form of a warped snap ring, but it can be persuaded.
We do have our techniques, 007.
 

Purusha

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Joined
Jan 14, 2006
Messages
1,868
Location
Somewhere on the Planet
[quote author="SSLtech"]You adjust R25 until it matches. Without calibrating R25, they don't track accuratley on ANY LA-2a.

Replace R25 with a trimpot of about 1.5 or 2 times the stated value of R25 then start it halfway and NEVER take it all the way to either extreme...

Somewhere in there will be a setting which makes the meter agree when switching between GR and +4, after setting a tone to 0VU output in +4 mode with no GR present. Increase the GR and set it so that it matches at around -5dB (5dB gain reduction). It will be maybe a little off at -3dB, maybe a little off at -8dB... a little off all over the scale, but I find that matching the meter at-5dB is usually an excellent compromise.

The only way to get them to track correctly is to MATCH the optos in your T4b. Then you re-adjust R25 and try again.

Read the comments by David Kulka and myself in the T4B threads, and you should be able to find out a lot. T4B matching and behaviour research is something that I've done a LOT of...

If you buy a NOS T4B from David and plug that in, you should find that there is a better match between optos, since they DID select and match the optos. Setting R25 will then give you about the closest match that you can get.

However, if you've not adjusted and set the value of R25, you'll NEVER getting them to track would be like winning the lottery, and if you put a different T4 in there, it wouldn't track again, until you adjusted R25 again.

Keith[/quote]

Thanks Keith. It works like a charm :thumb:

BTW, is there any special reason why to change the trimpot with the fixed resistor after the calibration?
 

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