xaxxon

LA2A r7 and r25
« on: December 05, 2017, 12:20:24 AM »
Depending on what schematic I'm seeing, either r7 is fixed at 2.7k, or varies between 1k and 2.7k.

r25 can vary between 27k and 100k, with both resistors seeming to relate to the T4B. 

I'm not quite grasping the variance, since the factory would be using a uniform part (I think?).

Could somebody give me guidance as to what values I should use, based on which aftermarket T4B I would use (if that's significant)?



Rob Flinn

Re: LA2A r7 and r25
« Reply #1 on: December 06, 2017, 07:25:35 PM »
Not quite sure how to get the right value of R7.

R25 scales the meter so it shows the correct gain reduction.   You can determine the correct value by subbing in 100k pot.   Make sure the meter is on zero when switched to GR.  Start with the GR pot down. Switch the meter to +4 inject a signal & turn the input up until you see 0VU on the meter.  Turn the GR pot up until the level on the meter drops to -10dB.  Switch the meter to GR & adjust the subbed 100k pot until you meter reads -10db.   Either leave subbed pot in or take out measure resistance & put a fixed resistor in that is the same value.
regards Rob

Capital letters.  The difference between helping your uncle Jack off his horse, & helping your uncle jack off his horse ........

xaxxon

Re: LA2A r7 and r25
« Reply #2 on: December 06, 2017, 09:02:27 PM »
Rob -

I'm assuming that the 100K pot should be linear and not an audio taper pot? And would injecting a 1KHz 1V p-p signal suffice?

Thanks,

Bob

Re: LA2A r7 and r25
« Reply #3 on: December 07, 2017, 11:37:05 AM »
Not quite sure how to get the right value of R7.

R25 scales the meter so it shows the correct gain reduction.   You can determine the correct value by subbing in 100k pot.   Make sure the meter is on zero when switched to GR.  Start with the GR pot down. Switch the meter to +4 inject a signal & turn the input up until you see 0VU on the meter.  Turn the GR pot up until the level on the meter drops to -10dB.  Switch the meter to GR & adjust the subbed 100k pot until you meter reads -10db.   Either leave subbed pot in or take out measure resistance & put a fixed resistor in that is the same value.


0 dBm is 0.775 volt across 600 ohms.  +4 (dBv) above 0.775 volt is 1.23 volts.  I suggest feeding 1.23 volts into the input (at 1 kHz), then turn the input up to 0 VU on the meter.   Should use a known signal reference as input to calibrate the gain reduction.   Important to calibrate to a reference input level.  My manual (from 21 Dec 1965) from my LA2 units does not specify a certain input level but does specify "For ease of control and to prevent overload of the input transformer, sufficient fixed pad should be placed ahead of the LA-2A to allow normal output at approximately 50% setting of the gain control."  There are several pages of set up procedure in the manuaI.

I prefer to calibrate my gear to a standard input.. helps if something goes wrong or a unit needs to be changed out quickly... The LA-2A input transformer (UTC HA-100X) is rated at +16 dBm max,  so calibrating the unit with  +4 input level at 400 Hz or 1 kHz should work fine....

Rob Flinn

Re: LA2A r7 and r25
« Reply #4 on: December 07, 2017, 04:44:12 PM »
Rob -

I'm assuming that the 100K pot should be linear and not an audio taper pot? And would injecting a 1KHz 1V p-p signal suffice?

Thanks,

Bob

Linear is easiest but either would work.
regards Rob

Capital letters.  The difference between helping your uncle Jack off his horse, & helping your uncle jack off his horse ........

Rob Flinn

Re: LA2A r7 and r25
« Reply #5 on: December 07, 2017, 04:46:50 PM »


0 dBm is 0.775 volt across 600 ohms.  +4 (dBv) above 0.775 volt is 1.23 volts.  I suggest feeding 1.23 volts into the input (at 1 kHz), then turn the input up to 0 VU on the meter.   Should use a known signal reference as input to calibrate the gain reduction.   Important to calibrate to a reference input level.  My manual (from 21 Dec 1965) from my LA2 units does not specify a certain input level but does specify "For ease of control and to prevent overload of the input transformer, sufficient fixed pad should be placed ahead of the LA-2A to allow normal output at approximately 50% setting of the gain control."  There are several pages of set up procedure in the manuaI.

I prefer to calibrate my gear to a standard input.. helps if something goes wrong or a unit needs to be changed out quickly... The LA-2A input transformer (UTC HA-100X) is rated at +16 dBm max,  so calibrating the unit with  +4 input level at 400 Hz or 1 kHz should work fine....

So long as you're not overloading the circuit I don't think it will make much difference.
regards Rob

Capital letters.  The difference between helping your uncle Jack off his horse, & helping your uncle jack off his horse ........

xaxxon

Re: LA2A r7 and r25
« Reply #6 on: December 11, 2017, 12:28:22 AM »
I appreciate the replies.

I don't have a stand alone signal injector. I'm trying to use a tablet, phone, laptop , etc.

These apps warn that they are not happy with an input as low as 600 ohms. I strongly suspect that this is a hardware issue.

Could using a 50K ohm resistor in series with the signal correct this problem?

One app suggests not grounding the injection device to the LA2A, but I think this would remove the accuracy of the p-p volts signal's amplitude, which could affect overall accuracy.

Any thoughts?

core13

Re: LA2A r7 and r25
« Reply #7 on: December 12, 2017, 08:39:52 PM »
Use signal generator of your daw

If you use kenetek classic curve, R25 is 31K for 7db gain reduction setting
I already did the pot trick

xaxxon

Re: LA2A r7 and r25
« Reply #8 on: December 13, 2017, 10:42:27 PM »
core13 - Merci!

It looks like I was just being  paranoid about injecting a high impedance signal into a low impedance input.

And to Rob - kind of goofy of to me not to realize that the linear or audio pots would lead to the same conclusion.

Bob