525 build thread

I'll try to get things together later today or early this week and try both the 10K and 100K version. I'll see if I can post something once I get them adjusted and up and running finally. I had to set these aside for other commitments and another housing move.

So a ruff estimate would be attack of 15us to 4ms with the 10K and 15us to 40ms with the 100k, comparison wise thats very useful but it's something that your ears must judge.

Thanks again for the information and graphs. I had looked at other schematics and was trying to get the same thing but I overlooked the simple fact that with the attack were trying to slow down that C3 cap time charging up. Thats really what this is about learning more and we get some cool tools out of it as well.

Cheers,

Kevin

Good observations on the metering.  Have to look into mod possibilities there; haven't yet. 

I built up a seperate meter circuit on a breadboard but didn't get much improvement by adding components to change the voltage range for the chip. Did add a little bit more range in the first few LEDs as suspected, but not a big difference at all in practice. 

In your earlier comment about the first 5 LED's, are you seeing useful 0.4ish dB step resolution, or are they really all coming on at the same time, or something in between?  I'm not against super-res at the bottom, but all 5 at once would be crazy. 

It is small dB steps for the first few LEDs in my experience. Only for certain settings (high compression) it appears they all go on at once. As I've been using these 525s over the last week for guitar and vocals the first LEDS are quite useful. High compression has a very unmusical distortion so they are limited to subtle use, ~5dB. In my experience so far for my builds.
As a rough measurement I recorded at what gain reduction (dB) the LEDs would go on (there is some measurement error for these):

LED          as original pcb            mod on breadboard
1              0.05                          0.58
2              0.76                          1.72
3              1.85                          2.15
4              3.14                          3.26
5              4.14                          4.51
6              6.42                          6.71
7              7.79                          8.33
8              9.47                          10.23
9              15                            14.63
10            20                            20

I got one of my two dead 525 working this evening. there were a few combinations of things that were causing the problems, I wasn't getting enough V- to the opamps, turned out to be the V- at A3 where I had drilled out the hole to fit sockets and the top side wasn't connecting to the socket, same thing happened with the Out of A1. Now it's working fine, just one last thing, when the 525 is powered on and in bypass it's dropping my signal by 6 db, when I power it off, it returns to unity, so I take it I have a loading problem correct? How should I fix it?

Thanks guys. I may look at this attack thing too now that I've got a working unit.

I haven't checked if mine shows the same 6dB drop, but the input opamp has 10k resistors attached on the input that are not switched out when the bypass is engaged. A 10k / 10k voltage divider gives a -6 dB drop, so it could be part of the design. 20*log10(10k/20k).
The in/out switch is 4 pole, so with a little mod, the input opamp could be switched out on bypass.

Oops, the voltage divider idea I just threw out doesn't make sense. It would depend on the source impedance.

benlindell said:

I got one of my two dead 525 working this evening. there were a few combinations of things that were causing the problems, I wasn't getting enough V- to the opamps, turned out to be the V- at A3 where I had drilled out the hole to fit sockets and the top side wasn't connecting to the socket, same thing happened with the Out of A1. Now it's working fine, just one last thing, when the 525 is powered on and in bypass it's dropping my signal by 6 db, when I power it off, it returns to unity, so I take it I have a loading problem correct? How should I fix it?

Thanks guys. I may look at this attack thing too now that I've got a working unit.

Click to expand...

Yeah, I drilled out the 2520 pads to accomodate sockets and had a little bit of the same fuss with getting the drilled pad to take the solder.  Once I found where the ground pin of one of the opamps(i can't remember exactly which one) wasn't seeing ground and got it reflowed, things started behaving.  One of them still sounds like it has a gentle low pass on it at around 2k, compression or no, which I'll trace out next time through.  Pretty satisfying to be listening to music through these after tweaking with them for so long, though.   

I notice a little hum on both of my 525s... has anyone else noticed this?
With no input signal, the hum is most noticable with the ceiling at ccw positions. With the ceiling fully cw, at 20, it is gone. The output pot has no effect.

I think I found the proverbial smoking gun with these 525s.
The ones I built both had hum and a non-musical distortion (same as discussed on  pg 14 by Mich and Peter). The distortion was not caused by the FET - it was caused by the meter circuit. After turning R55 all the way off (so the meter would not light at all), the distortion disappears. There's a ground problem when the meter circuit shares the same ground as the in/out pot grounds. I think the current from the meter circuit was causing a ground loop between in/out pots and A3. Easy fix - I cut the trace leading from the neg side of CM1 to the GND on the meter board, and ran a wire from Neg CM1 to audio ground at the hole by gold finger 5. With this change, both of my 525s are quiet & distortion free. Even at heavy compression they sound smooth - a huge improvement. And the hum is gone
A second mod I'd recommend - after tightening down the mounting nuts a few times (or a zillion in my case), the silkscreen over the ground plane developed shorts, which connects audio ground to the chassis, not desirable. Easy mod was to cut the traces by gold fingers 5 & 13 to disconnect the pcb ground plane from audio ground. Then the short to the chassis through the mounting bolts makes the pcb ground plane a chassis ground, which is better for noise rejection.
With these mods the 525s are fantastic. I just played through them with guitar and singing and they are smooth and clean.

first unit is finally (almost) working! after using a melcor opamp in A4 the unit is working when more/less switch is on pos 12 to 20.
wasnt able to build a working peter-opamp. but ill try again!

at positions 2 to 10, the meter isnt responding, and i cant hear any compression... maybe an error on the switch?

(btw: its not properly calibratet yet)

connecting both att points together brought just distortion...

Brizco said:

first unit is finally (almost) working! after using a melcor opamp in A4 the unit is working when more/less switch is on pos 12 to 20.
wasnt able to build a working peter-opamp. but ill try again!

Click to expand...

still no love for the a4... still using melcor 1731...

at positions 2 to 10, the meter isnt responding, and i cant hear any compression... maybe an error on the switch?

Click to expand...

fixed: bad wiring caused this problem

now a new problem with this unit: meter doesnt work anymore, changed ic, nothing happened, there are +15.2V on the cap, but 0V at the LED's... have to check that when i have time.


good news: second unit worked from first start! (with melcor 1731 on A4)

Finally, after 2 years i finished the Module today. Unfortunately i have to wait until next week to get the backpanel from the 51X PSU to finish the PSU and wiring.

Thanks for all support.

Briz,
You must have ignored the A4 mod.... there are so few components on there that it must be the case... or you're using the wrong nails.

peter purpose said:

Not being a cap snob and lacking the lug holes to tell the difference, I shove whatever I have in there.

peter

Click to expand...

Yes, I know it's an old thread but I agree, like Keef, with what is said. I'm sick of tossers who know
more about adjectives & fashion than they will EVER know about electronics.


Frank

dmp said:

Easy fix - I cut the trace leading from the neg side of CM1 to the GND on the meter board, and ran a wire from Neg CM1 to audio ground at the hole by gold finger 5.

Click to expand...

Being a Noob or too blurry eyed from the solder, I'm not sure where the "gold finger 5" is that you're referring to in this mod. Could someone enlighten me? 

Pin 5 on the edge connector son.

Thanks Peter, I should have figured that out!

dmp said:

Anyone know of a supplier for Omeg pots in the US? I think this would be good - ECO1MLIN

Click to expand...

Has anyone found a supplier for these Omeg 1meg pots in the states? Or anyone across the pond care to mail me a few? ;D

Pm me with you qty, I'm ordering by Friday.