and now for something completely different part 3

[Satyrnine]

10 minutes ago, dip16amp said:

ran my CFA2LT for 70 minutes at 140ma and it got up to 49c
used a fluke 87v with thermocouple probe on the middle 2sa1860 mounting screw

Excellent! Thank you! What voltage are your grlvs’s running at? 
 

Also, I know “max temp” is 150c, but whats a realistic/safe case temp maximum? How far can it reliably/safely be pushed?

Edited July 9, 2023 by Satyrnine

[dip16amp]

3 hours ago, Satyrnine said:

Excellent! Thank you! What voltage are your grlvs’s running at? 
 

Also, I know “max temp” is 150c, but whats a realistic/safe case temp maximum? How far can it reliably/safely be pushed?

that was with my +/- 16.7v grlv
i ran it again for an hour on my +/- 28.8v grlv at 150ma
min temp
0  32
5  49
10 56
15 59
20 61
25 60
30 60
35 60
40 62
45 63
50 63
55 63
60 62

it could probably run to 100c but should not get higher than 70c

 

[Satyrnine]

37 minutes ago, dip16amp said:

that was with my +/- 16.7v grlv
i ran it again for an hour on my +/- 28.8v grlv at 150ma

it could probably run to 100c but should not get higher than 70c

 

This is super useful data! Thank You!

[audiostar]

13 hours ago, Satyrnine said:

Also, I know “max temp” is 150c, but whats a realistic/safe case temp maximum? How far can it reliably/safely be pushed?

To quote Nelson Pass: 

As a rule of thumb, the output devices should not be operated at more than about half their maximum rating, and generally the case temperature needs to be under 100 deg C. For most amplifiers this means a heat sink temperature of about 50 to 55 deg C., which is the temperature that you can put your hand on for about 10 seconds.

[justin]

First 2 chassis just arrived. Getting it tested!

 

 

[Voltron]

Is that made by RebelAmp?

[n_maher]

#toosoon

[justin]

as Al would probably say, better sooner than later

 

[bdinnev]

Hi All,

 

As with most of us, "life" caused some delays with my CFA3 build and I am just getting back around to trying to finish it off. I have 2 x GRLV's and 2 x CFA3 boards ready to go, just need to get initial setup done and then will case it all up.

 

I have got the general idea of initial setup steps but is anyone able to assist with a step by step guide for a CFA3 - I.E. what do you do when powering up the first time? I understand I need to pull the op-amps out, power a board up and adjust trim-pots for bias, but can anyone help with where I should be measuring and what I should be aiming for, what steps should be taken and what safety measurements should be taken to ensure everything is functioning correctly?

 

Thanks!

 

Ben

[Pars]

The first thing to do is power each of the GRLVs up (not connected to anything) and verify that the output voltages are correct. Once that is done you can start connecting one of the CFP3 boards at a time to the GRLV, and begin testing. You will want to remove the servo opamp, and measure:

1. DC offset voltage from output + and - to gnd
2. Measure the bias across the 1 ohm resistors and calculate the bias current from that

These measurements should be done with the inputs shorted to ground. If you have a volume pot connected, turned down all the way works fine. I believe the 5K trimpots (RV1 and RV2 on the schematic) are for the bias, and the 10K trimpot (RV1) is for the offset. All of this is from memory though, so I could be wrong.

I would strongly suggest using Kevin's protector board with this amp as it is capable of easily toasting a set of phones.

Edited August 1, 2023 by Pars

[bdinnev]

37 minutes ago, Pars said:

The first thing to do is power each of the GRLVs up (not connected to anything) and verify that the output voltages are correct. Once that is done you can start connecting one of the CFP3 boards at a time to the GRLV, and begin testing. You will want to remove the servo opamp, and measure:

1. DC offset voltage from output + and - to gnd
2. Measure the bias across the 1 ohm resistors and calculate the bias current from that

These measurements should be done with the inputs shorted to ground. If you have a volume pot connected, turned down all the way works fine. I believe the 5K trimpots (RV1 and RV2 on the schematic) are for the bias, and the 10K trimpot (RV1) is for the offset. All of this is from memory though, so I could be wrong.

I would strongly suggest using Kevin's protector board with this amp as it is capable of easily toasting a set of phones.

Thanks Pars - I dialed in the GRLV's many moons ago, they are both running at 19.995v +- .002 on all outputs. 

When you say "across the 1ohm resistors" - is that across 1 of them, or across the pair on each circuit?

 

I have the protector board ready to wire in, that will happen as part of the casing process, just want to get the CFA3 boards sorted ATM, one step at a time

 

Thanks,

 

Ben

[Pars]

Isn't 20V kind of low for a CFA3? I thought most were running at 30V?

For bias, measure across each of the 1 ohm resistors. Hopefully they are about the same. I don't recall what the target output current is for a CFA3.

EDIT: it appears that most have gone for 120-150mA, but some run at 200mA. Unclear whether the ones going higher were using lower voltage or not. The only CFP3 I have listened to was at +/-30V.

Edited August 1, 2023 by Pars

[bdinnev]

When I was getting all the parts some time ago, someone who has built one recommended I get 21v transformers, I actually ended up with 24v transformers. My understanding is that the output of the GRLV's will / should be lower than the input voltage so 20v seemed about right? 

 

Circa 150mA bias is what I have seen throughout this thread as well so seems to be on the money, my only question is is that when measuring across 1 of the 2 1ohm series resistors, or across both (mine read 2.3 ohm when measured across both on both the +ve and the -ve circuits).

My measurements at present show a DC voltage of approx 150mv on both +ve and -ve after a quick tweak on the 5ohm pots, however the voltages are not stable, they are consistently rising (they are not oscillating, just increasing), the rising does slow down after a few minutes, but they do continue to go up - caused by temp increase across the circuit? 

 

If the measurement is across 1 or the 2 series resistors, then 150mv will be circa 150mA, but if I need to measure across both resistors in series, then I need to bump up the bias by double - a considerable increase. 

[Pars]

Measure across 1 resistor only. Then across the other one. Yes the voltage will change as the amp warms up. Ideally, you would do this with the amp cased and having warmed up an hour or so. Some might go as far as having the top on (or partially on) when doing this as well.

When I build one, it will run at 30V. I believe Dukei's do as well (at least the one I worked on was running 30V supplies).

[bdinnev]

OK, will go across 1 resistor, makes the math easy

 

Hopefully last question - what dc offset voltage should I be aiming for? My research leans towards "as close to 0+- as possible" as any voltage whilst "idle" simple gets turned into heat / noise on the transducers?? 

[kevin gilmore]

+/-24v is recommended mininum for lowest distortion

[audiostar]

Running 30v GRLVs as well.

Probably the cheapest option for a transformer with a 30vac primary would be the Antec AS-1230. 

[Satyrnine]

44 minutes ago, bdinnev said:

OK, will go across 1 resistor, makes the math easy

 

Hopefully last question - what dc offset voltage should I be aiming for? My research leans towards "as close to 0+- as possible" as any voltage whilst "idle" simple gets turned into heat / noise on the transducers?? 

without servo opamp, around 40mv is "normal". With servo opamp installed, it should start on turn on around 40mv-ish, and slowly go down to very near zero or zero over 30sec or less.

Can also verify the Antek as-1230 is a great choice. Using 2x for my dual mono cfa3.

Edited August 1, 2023 by Satyrnine

[bdinnev]

I am reading -500mv on one board with the 10k pot trimmed to the max (or min depending on how you look at it) , the other board is reading around -2.5v! Any ideas where to look?

[Satyrnine]

13 minutes ago, bdinnev said:

I am reading -500mv on one board with the 10k pot trimmed to the max (or min depending on how you look at it) , the other board is reading around -2.5v! Any ideas where to look?

I had the same issue, turned out to be a single transistor backwards on its pad. I repeatedly missed it after rechecking things numerous times. Also make sure all resistor values are correct and your non/pnp’s are in the right spots.

-2.5v is extreme. Something likely very off/failed on that one. 
 

Note that shorting the wrong pins on any output device will make the 1R’s go nuclear and possibly take other bits with them.

 

Edited August 1, 2023 by Satyrnine

[bdinnev]

I have visually checked all the components on the boards and tested what I could with a meter and all seems OK - all resistors are in the correct location and of the correct values (as per the boards), all the caps seem to be in the correct orientation, all the PNP's and all the NPN's in the correct location, diodes etc, and the same across both boards. All the transistors are in the correct locations (15030 v 15031) and there does not smell or appear to have been any magic smoke come from anything. 

 

Is there anywhere on the board I can probe for a voltage / resistance to try and narrow down the issue?

[Satyrnine]

2 hours ago, bdinnev said:

I have visually checked all the components on the boards and tested what I could with a meter and all seems OK - all resistors are in the correct location and of the correct values (as per the boards), all the caps seem to be in the correct orientation, all the PNP's and all the NPN's in the correct location, diodes etc, and the same across both boards. All the transistors are in the correct locations (15030 v 15031) and there does not smell or appear to have been any magic smoke come from anything. 

 

Is there anywhere on the board I can probe for a voltage / resistance to try and narrow down the issue?

When I was in this same boat, it was recommended to me to get two boards powered up, one "good" and one with problems. Print out two schems and measure voltages at each leg of each component and note on schem. Comparing the two should shed some light. Did you use parts from a known/trusted official supplier? Did you match transistor pairs? All this, and much much more is within this thread. I know it's a long read, but reading it all truly does help. It did for me.

I know it sounds absurd, but I too "thought" I'd checked every component three times, and still found that a transistor was in backwards. It was the right transistor part number (which i verified thrice), but backwards.

Because this circuit is fully dc-coupled diagnosing is a bit trickier than in ac coupled one. Any small dc offset (from an error or mismatched/failed part) early in the circuit gets amplified as it goes along.

Comparing to a known-good board may be the best method to get to the bottom of it, short of shotgunning things, assuming you have spare transistors. You could swap out pairs and test between each replacement until you (hopefully) find the culprit. While they're out, I'd suggest measuring their hfe so you can check/resolve any significant mismatches as well. 

Edit: Since this is a CFA3, maybe pull whatever socketed input transitors (THAT, jfets, etc, quite a few options) you went with and see if offset is affected. Not sure that guarantees its those specific transitors, but I know the CFA3 can be picky with some inputs iirc. I built a 4x CFA2 + ubal2baltube to make a cfa3, so no direct experience with ss cfa3 boards.

Edited August 2, 2023 by Satyrnine

[MLA]

8 hours ago, bdinnev said:

I am reading -500mv on one board with the 10k pot trimmed to the max (or min depending on how you look at it) , the other board is reading around -2.5v! Any ideas where to look?

For what it's worth, my CFA3 bords fell 2,5V short in terms of offset when I built them. In my case, swapping all the red LEDs to a version with much less forward current than the ones I installed first (I'm now using LTL-4221, which has 2mA I believe) put all numbers back in the right places. 

Edited August 2, 2023 by MLA

[bdinnev]

8 hours ago, MLA said:

For what it's worth, my CFA3 bords fell 2,5V short in terms of offset when I built them. In my case, swapping all the red LEDs to a version with much less forward current than the ones I installed first (I'm now using LTL-4221, which has 2mA I believe) put all numbers back in the right places. 

I have noticed that the LED's on the -ve side of the board don't glow as bright as those on the +ve side of the board - is that normal or has anyone else noticed this in correlation with an issue? I will try changing the LED's and see what happens. 

[Pars]

I do recall on the one CFP3 I worked on, half the LEDs were different, so I changed them all out for the correct ones. Later, the builder said he ran out of the normal ones and used what he had. Not the right move.