LA3A 500 BUILD
- Thread starter peter purpose
- Start date
Help Support GroupDIY Audio Forum:
dmp
Well-known member
I haven't seen a calibration procedure, but I'll summarize what worked for me.
From looking at Universal Audio LA-3A schems/docs:
RV31 - stereo adj - fully cw nominally (looking from front). Matching two units is accomplished by turning the trimmer ccw
RV28 - HF control - fully ccw for flat: "CW adj of R29 increases the gain reduction circuit for freq > 1kHz"
RV7 - output amp bias - adjust so out pin of amp pcb is 6-7v
RV3 - compress / limit option, 1.3k nominally, adjust to taste I suppose
For the meter:
R24 NI
R13 10k
R22 47k
RV36 - 3.9k
RV23 - adjust to zero meter
From looking at Universal Audio LA-3A schems/docs:
RV31 - stereo adj - fully cw nominally (looking from front). Matching two units is accomplished by turning the trimmer ccw
RV28 - HF control - fully ccw for flat: "CW adj of R29 increases the gain reduction circuit for freq > 1kHz"
RV7 - output amp bias - adjust so out pin of amp pcb is 6-7v
RV3 - compress / limit option, 1.3k nominally, adjust to taste I suppose
For the meter:
R24 NI
R13 10k
R22 47k
RV36 - 3.9k
RV23 - adjust to zero meter
imo
Well-known member
curious to hear peoples "T4B" solutions. I used some door liner which works pretty well, but still lets a bit of light in along the top. Might just put some gorilla tape over that. What about getting the light panels directly on the LDR's? On a real T4, they are snug against the LDR, but there is still a bit of play with mine, and i think it is affecting the drive voltage
IAn
IAn
Winetree
Well-known member
I used some wide weather stripping and cut the center out to fit around the two photo sensers.
( like a one piece oblong doughnut) I've got the pins on the panel board cut and the panel's touching the top of the mill max sockets and yes there is still a gap between the panel and the sensers. The only way to lower it futher is to shave off the top of the millmax sockets. I'll wait to see how it reacts. Just got my meter Zero'd, gonna check the bias voltage, and run some audio through them.
( like a one piece oblong doughnut) I've got the pins on the panel board cut and the panel's touching the top of the mill max sockets and yes there is still a gap between the panel and the sensers. The only way to lower it futher is to shave off the top of the millmax sockets. I'll wait to see how it reacts. Just got my meter Zero'd, gonna check the bias voltage, and run some audio through them.
kato
Well-known member
I put two plastic washers under each opto cell to raise them up off the board before soldering. The cells are just a hair away from the panel that way.
peter purpose
Well-known member
I don't think this is super critical. Once in the rack, very little light penetrates.
imo
Well-known member
I have my LA3A fully calibrated and used it this afternoon in a session. Wow! It is much smoother than my other unit and i can push it much harder without it breaking up. Good work Peter!!
I was thinking that it would be a good idea to come up with a calibration procedure.
I found that it was best to calibrate the output section(RV7) first to make sure the unit is operating linearly.
The stereo and hf control can be done at any time
RV36 and i think Rv3 have to be done with no DC power to get the measurements right. I thought i had a faulty trimpot until i tried this.
Rv23 can be done after the unit is racked up
Thanks again!
Ian
I was thinking that it would be a good idea to come up with a calibration procedure.
I found that it was best to calibrate the output section(RV7) first to make sure the unit is operating linearly.
The stereo and hf control can be done at any time
RV36 and i think Rv3 have to be done with no DC power to get the measurements right. I thought i had a faulty trimpot until i tried this.
Rv23 can be done after the unit is racked up
Thanks again!
Ian
Winetree
Well-known member
What test points are used to calibrate the output (RV7) of the amp card?
imo
Well-known member
i found it a little confusing to follow the schematic with the daughter boards and the additional, non schematic additions to the circuit, but Peter posted the layouts on page 2 of the thread. you can infer general areas of the schematic if you reference the boards from the layout.
If you look at the schematic is says to ' adjust for bias to between 6-7 volts at 'tp1'. you can see that is between R18 and R19. This is a DC reference, so probe from ground to 'tp1' and adjust until you have the correct voltage. It is paramount to have all boards connected when you do this so you are properly adjusting the bias voltage
Hope this helps
Ian
If you look at the schematic is says to ' adjust for bias to between 6-7 volts at 'tp1'. you can see that is between R18 and R19. This is a DC reference, so probe from ground to 'tp1' and adjust until you have the correct voltage. It is paramount to have all boards connected when you do this so you are properly adjusting the bias voltage
Hope this helps
Ian
nielsk
Well-known member
2 little questions:
bi-color LED orientation?...(one + lead is longer)
An idiots guide to matching the photo cells enough to work?
bi-color LED orientation?...(one + lead is longer)
An idiots guide to matching the photo cells enough to work?
I am just getting started on building the main board. Realized that rv23 and rv31 need to be placed on the bottom of the board. Are there any other parts that need to be placed on the underside as well?
Also, any pics of finished boards would be great.
Thanks
Ian
Also, any pics of finished boards would be great.
Thanks
Ian
peter purpose
Well-known member
Ian,
The only things that absolutely have to go on the underside are those trimmers.
Depending on the length of the pins you use for the amps, some stuff below them might need to go on the underside.
peter
EDIT.... forgot that the leds go underneath too.
The only things that absolutely have to go on the underside are those trimmers.
Depending on the length of the pins you use for the amps, some stuff below them might need to go on the underside.
peter
EDIT.... forgot that the leds go underneath too.
peter purpose
Well-known member
bi-color LED orientation?
Depends what colours you have and which way round you want them.
An idiots guide to matching the photo cells enough to work?
I wouldn't bother, unless you're making a stereo pair.
Do a search on google for the complexity factor.
Depends what colours you have and which way round you want them.
An idiots guide to matching the photo cells enough to work?
I wouldn't bother, unless you're making a stereo pair.
Do a search on google for the complexity factor.
kato
Well-known member
peter purpose said:
Blokes who bought kits from me: the bicolor LEDs alight RED and GREEN.
- Longest leg (middle) is common cathode.
- voltage to medium-length leg = red light
- voltage to shortest leg = green light
In other words - shortest leg towards the VU in both cases if you want pushbutton-in to be green. Makes no odds for comp/limit switch unless red/green has a meaning for you in that dept. http://cl.ly/2G1D0S0a3p3a1t02223z
nielsk
Well-known member
So it's add a dash of color to taste?
I am finding a good bit of difference in the photo cells at an all dark state, guess I'll just match based on that.
I am finding a good bit of difference in the photo cells at an all dark state, guess I'll just match based on that.
kato
Well-known member
nielsk said:
I wired mine red with switch disengaged, green with switch engaged.
The only other bicolor LEDs I had easy access to switch between red and blue - which might look cool against the blue face panel - for those who like to pimp their rides. But blue doesn't necessarily indicate on in my mind.
im almost done with mine! have a few questions about mounting the t4b el panel. When i took apart my t4b, the panel was touching the "little round things" ha, dont know what they are called... i installed the "little round things" on the board according to what numbers they where hooked up to on the t4b. i also hooked up the el panel to the micro board with the corresponding numbers. but, my pins are a bit to tall so the panel wont be touching the "little round things". do i need to trim the pins so that they have direct contact like when they where in the T4B canister? also, my el panel has the wires along the shorter side of the panel, ie, not like in duantro's or peters pic.... as long as they are hooked up to the right numbers, is everything ok?
also, there was a resistor or capacitor attached to one wire of the el panel, do i need to keep this in the circuit? doesnt look like it from duantro's pics, but just thought id ask....
i installed the "little round things" on the board according to what numbers they where hooked up to on the t4b. i also hooked up the el panel to the micro board with the corresponding numbers. but, my pins are a bit to tall so the panel wont be touching the "little round things". do i need to trim the pins so that they have direct contact like when they where in the T4B canister? also, my el panel has the wires along the shorter side of the panel, ie, not like in duantro's or peters pic.... as long as they are hooked up to the right numbers, is everything ok? also, there was a resistor or capacitor attached to one wire of the el panel, do i need to keep this in the circuit? doesnt look like it from duantro's pics, but just thought id ask....
peter purpose
Well-known member
The little round things are LDRs... light dependent resistors and yes... trim the pins so that they touch the EL panel.
The EL runs on AC, so wiring is unimportant.
Forget the resistor.
The EL runs on AC, so wiring is unimportant.
Forget the resistor.
kato
Well-known member
I raised mine with plastic washers. I should've mentioned that for long-pinners. Clipping them shorter will work too. Probably easier if you've already soldered the opto cells.
