[BUILD] CAPI FC526~500 Series~FET Limiter Kit~Official Support Thread

[kante1603]

In which position will they have the most coloration ? Any tips ?

ABI?


8)

 

[kosi]

I meant the OP Amps , in which position do they give the most color,
Signal Input ?
Receiver ?
or Output ?

Didn't have so much time to play around, cause I use them always...

 

[jsteiger]

I think Dan Deurloo has messed around a lot with opamp placement. Maybe he will chime in. I have really only used then with all reds or all gar1731's. Most of my Nashville pals like both for different uses.

Another thing to do is intentionally increased the THD with the null trimmer. I would find the low spot and then turn the trimmer CW up to maybe 0.5% or 0.75%. Typically, the highest THD during normal use will be when the unit is knocking back 6-7dB of gain reduction.

 

[steveau]

Jeff,
I'm about ready to start calibration, but I notice my DCV's are slightly off,
at -10.03 at TP9 and 2.53 at TP11,
my voltage at the CAPI  PSU is 16.6 V,
my voltages at the DOA are  +15.84/-15.88,
Is that close enough to start biasing the VVR?
(I did use the 750ohm for R41),
Thanks,
-Steven G

 

[jsteiger]

Yeah, you look good to go Steven 

 

[emrr]

The rack is now right, the voltages look right everywhere, and I'm still a fail.  I can get to the proper green LED starting point now with 2.487V. 

At 20:1, once I trim RV8 to 2.245V it always lands at -5dB GR.  I eventually maxed out RV8 doing the RV1/RV8 trimmer dance, never seeing any difference in GR reading.  Maybe I'm totally misreading the directions somehow, the voltage difference at each comparison just got wider and wider apart with no significant change in meter result.  After each retweak of RV1 the GR meter always read high.  On the last go at RV8 it dropped to -6 somewhat later than previous attempts. 

If I switch to 4/8/12:1, it reads -7dB.  2:1 reads something like -1dB.  Maybe this indicates a problem in the ratio switch?

No time to try unit 2 tonight....

 

[jsteiger]

At 20:1, once I trim RV8 to 2.245V it always lands at -5dB GR.

Doug, I'm not sure I follow you here. Do you mean that with 2.245V at TP10, the LED's are only lit up to the -5 yellow one, the -7 doesn't come on?

 

[emrr]

At 20:1, once I trim RV8 to 2.245V it always lands at -5dB GR.

Do you mean that with 2.245V at TP10, the LED's are only lit up to the -5 yellow one, the -7 doesn't come on?

Correct for 20:1.  When finished with the RV1 set, it's up at -10, then trimming RV8 to the correct voltage it drops to -5.  If I switch to the other ratios from 4 to 12, it reads -7 as it should, with 2:1 reading -1 or so. 

 

[jsteiger]

Well, what its interesting here is that the comparator string doesn't know or care what the ratio is. Its just looking at the incoming DCV from the meter circuit. The meter will show something different if the only control changed is the ratio BUT, that is only because the amount of GR will be different. If you set the ratio to say 4:1, and then adjust input and output for exactly 7dB of gain reduction, the meter will work exactly as before. 2.245V will trip the -7 LED.

So the ratio switch will change the amount of gain reduction but it does not alter how the meter works. The meter will show the proper amount of gain reduction regardless of the ratio switch position.

 

[emrr]

Meter readings for actual -7GR at 20:1, consistent at each attempt to align, up until RV8 hit an end stop. 

2:1 -1
4:1 -7
8:1 -7
12:1 -7
20:1 -5

 

[jsteiger]

Meter readings for actual -7GR at 20:1, consistent at each attempt to align, up until RV8 hit an end stop. 

2:1 -1
4:1 -7
8:1 -7
12:1 -7
20:1 -5

What is the DCV at TP10 for each one of those?

 

[jsteiger]

BTW Doug, have you seen the "Classic-FC526-Test-Points-Guide-Rev_2.pdf" that I posted last week at the beginning of this thread? The last part of it gives a little insight into the meter.

 

[emrr]

+2.245V at TP10 at 20:1, I didn't make note of it at other ratios. 

I did note that once RV1 is set for +2.487 (1 green LED lit) that it stays the same for the lower ratios as well, though it does jump and has a resettling time when the ratio switch is changed. 

I have to find time to go through the next unit, see if there's any comparison.    I do have the new docs.

 

[jsteiger]

.... I do have the new docs.

OK. All of the ratio switch and other things aside, the meter is just a simple set of comparators much like any other LM339 style meter. The only difference is that I am using them "backwards". So, once the incoming test voltage drops below the respective comparators reference voltage, that LED will light. If the meter reference string is correct (see the meter test points), no setting on the comp will alter this.

For example, if you run bipolar 16V to the meter PCB along with a 0V ground reference, you can use the FC526 meter to indicate any incoming DCV from 0 to 2.5V. This can be easily tested without being connected to the comp's main PCB.

The fact that 2.245V does not trip the -7 LED tells me that your reference string as shown in the test points doc is not right. Meter point #3 cannot be 2.253V.

 

[emrr]

The fact that 2.245V does not trip the -7 LED tells me that your reference string as shown in the test points doc is not right. Meter point #3 cannot be 2.253V.

Ah, but we tested the reference string a few weeks back, and it was all dead on once the rack offset was discovered.  I posted measurements previously.  This weeks results are with the basic +2.5V and -10V confirmed again before starting. 

What would make the 20:1 setting read LESS GR than the lower ratios, with -7 dB actual measured at 20:1? 

I'm gonna run RV1 and RV8 back mid position and start the whole thing over again with Q bias, although this has not appeared to shift once I've run it once.  I might tackle unit 2 before doing this, I might not. 

 

[jsteiger]

What would make the 20:1 setting read LESS GR than the lower ratios, with -7 dB actual measured at 20:1? 

Only something funky with the meter board. What's important to note is what the DCV is at TP10 when this unexpected result happens. Just remember, all each individual comparator channel is doing is looking at the reference voltage and the incoming test voltage. Once the test voltage drops just below the reference, that LED will light. That part is pretty straight forward and will never change.

 

[emrr]

I should phrase it another way.

Regardless of meter alignment, if you change ratios and have less actual GR, why would a higher ratio show less (correct or incorrect) GR than the lower ratios? 

I have to wonder if that points to a problem external to the meter circuit. 
Other thought is around the previous attempts to align while the rack offset voltage existed, can you get RV1/8 to points which are too far off base to use as starting points, but which still look like they are working?

 

[jsteiger]

Regardless of meter alignment, if you change ratios and have less actual GR, why would a higher ratio show less (correct or incorrect) GR than the lower ratios?

Well, setting the unit up  20:1, attack/release both full CW/fast, input/output to a resulting 7dB of actual gain reduction, then only changing the ratio switch will alter the amount of GR. This is simply a matter of how the 1176 circuit works. This will not at all change the meter but will change the voltage at TP10, which will change what the meter shows.

Meter LED's aside, with the above typical 7dB of GR setup, when monitoring the output level so that 0dB is showing (+7dB at the input).....

switching to:
12:1 will yield -1.45dB
8:1 will yield -1.94dB
4:1 -1.74dB
2:1 +1.92

 

[emrr]

It feels like we are talking at cross-purposes, maybe not. 

Yes, change to a lower ratio will alter the amount of GR to a lesser amount than found at a higher ratio. 

I'm wondering if there's something about this particular 20:1 not being right, which would feed the meter incorrect information, or if it's an idiosyncrasy of the 1176 circuit that looks more obviously strange with peak metering versus VU metering. 

Meter LED's aside, with the above typical 7dB of GR setup, when monitoring the output level so that 0dB is showing (+7dB at the input).....

switching to:
12:1 will yield -1.45dB
8:1 will yield -1.94dB
4:1 -1.74dB
2:1 +1.92

Those ratio change dB numbers are relative output change or metered GR?  I think I'm reading it as relative output change. 

More measurements to do, theorizing isn't revealing anything. 

 

[jsteiger]

Yes, change to a lower ratio will alter the amount of GR to a lesser amount than found at a higher ratio.

No, not necessarily. There are many other things at play like onset of compression and knee. This will change depending on how much GR is taking place. Depending on how much GR is taking place, the opposite will happen.

I'm wondering if there's something about this particular 20:1 not being right, which would feed the meter incorrect information...

This is possible but the if the test voltage going to the meter is 2.245V, then the -7 LED will fire. If not, its a problem on the meter board. No matter what the ratio setting is, if the comp is producing 7dB of gain reduction, TP10 will be 2.245V if all is cal'ed correctly. I have verified this with every ratio setting.

Meter LED's aside, with the above typical 7dB of GR setup, when monitoring the output level so that 0dB is showing (+7dB at the input).....

switching to:
12:1 will yield -1.45dB (resulting in 8.45dB of GR)
8:1 will yield -1.94dB (resulting in 8.94dB of GR)
4:1 -1.74dB (resulting in 8.74dB of GR)
2:1 +1.92 (resulting in 5.08dB of GR)

Those ratio change dB numbers are relative output change or metered GR?  I think I'm reading it as relative output change. 

More measurements to do, theorizing isn't revealing anything.

Yes, relative output change. I have added some info to them to clarify.