[BUILD] SSL 9K2

[tomas1808]

Great!

Its good to give back a little after getting so much from this forum.

 

[gemini86]

I'm excited to correct this on the preamps I've built. They work fine for me, but I let my friend borrow one and he popped it into an API console at the studio he was tracking in, he said it would ring at really high gain. I couldn't duplicate the problem in my studio, so I never got to figure out what was happening.

Glad to finally have some closure. I've thought about going back to this project and revamping the layout, simplifying and shortening some of the nets, cleaning it up a bit and adding more features or maybe small addon boards like signal and clip leds, filters, etc.

 

[tomas1808]

Signal and clipping LEDs would be great.

 

[3nity]

I dont think signal and meter is really a necessity youre better of with the 9k5
The best thing to do will be maybe move the pushbuttons towards the front edge and 48V led.

Im so excited to have the rest working as they should!

Thanks.

 

[okgb]

So this last mod is clipping one trace and NO jumper ?
I still have a couple of these boards I haven't gotten to,
A bi, or tri l.e.d. for present / clip would be handy but I don't want to buy
another board when I have these still !

 

[tomas1808]

You need a jumper between C93/C94 to pin 2 of the TL072

 

[tomas1808]

Can anyone please measure your noise of your 9K2? I would like to compare.

If my measurements are correct I am getting an EIN of around -107dBu which is not great.

For some reason all 4 preamps I built have significant HISS and mediocre EIN levels (Green, 1108 and this) I am starting to suspect I am doing something wrong  :'(

-------

Here is how I made the measurements just in case:

I fitted a 150ohm resistor between pins 2 and 3. At full gain I read -50dBFS on my DAW (44.1KHz, 24bits).

Considering the nominal level of my soundcard is -14dBFS (+4dBu) then -50dBFS equals -32dBu.

If I subtract the 75dB of gain I get the -107dB EIN.


Is this the correct way to do it?

 

[gemini86]

If you can actually hear more noise vs your other preamps, there's a problem. Should be very quiet. It is worth nothing, though, that measuring EIN at max gain is not a great indicator of real world performance. Nobody uses a mic pre at max gain, at least nobody that understands good gain staging. Set up your preamp for a gain of, say 55 or 60 then measure again.

Using the google search function here brings up some good discussion on the subject of EIN measurement.

 

[tomas1808]

The thing is that all my preamps are DIY, so I have no "golden" reference to test by.

I only have two pres at the moment, 9k2 and 1108. Both have a similar EIN (112dBu on the 1108 and 107dBu on the 9K2).

Rane website says that EIN measurements should be made with max gain, but I see what you mean. I dont have the cable I need to measure gain on the preamps yet.

However I dont think testing at other gain levels will change much. Hiss seems to increase evenly with gain. In practice when using a SM57, the HISS is significant.

It is usable, but I would like a REALLY quiet pre. Thats why I built these in the first place.

I am kind of going crazy with the hiss issue on my preamps 

 

[gemini86]

I gotcha.

Since this is essentially the same circuit as all the other 9k preamps, it may help to look through those threads, since those builds are much older and people have been messing with them for years.

I seem to remember that resistor matching is important for very low noise operation, but I don't know if that is for self noise or for noise rejection.

 

[tomas1808]

Yeah I should check there.

BTW are your 5534 getting kind of warm? (40-50ºC / 104-122ºF)?

 

[gemini86]

I'll have to check. All of the ICs on all of your preamps are this warm?

 

[tomas1808]

Only the 5534

 

[gemini86]

But which one? All of them?

I'm not going to find the time to open up mine on the bench and run tests (after doing the last trace edit) anytime soon. Anyone else have hot ICs? (although, that type of heat isn't a killer, and I'm not entirely sure it isn't normal.)

 

[3nity]

you mean all of them?? 5534s?

 

[tomas1808]

Oh, sorry. Yes all 5534.

I mean they are cool enough to touch without pain. But its almost on the threshold.

Thanks

 

[tomas1808]

Just built my second 9K2.

Exact same amount of hiss. Its not that bad, but I would like it to be less prominent.

The 20dB PAD doesnt change hiss levels.

BTW, I think I exaggerated about the heat on the 5534s. They are okay.

Any ideas?  Could somebody please measure their 9K2?

Thanks

 

[3nity]

do you remember if hiss was present before ic31 trace fix??
did can you trace where hiss comes from??
the only hiss i hear is when 48v is on and i turn it off like cap discharging.

 

[tomas1808]

Before the IC31 trace fix the output was all distorted. It was impossible/useless to measure the hiss back then. I dont think IC31 is the problem because now the circuit is exactly like Keith's original 9K. I dont understand how you guys got this sounding ok (albeit intermittently) without the IC31 fix.

I'll try to trace the hiss, not sure I know how to, but I'll try.

Have you tried your 9K2 with a dynamic mic while using headphones? I can hear the hiss that way.

Can somebody measure the dBFS levels on your DAW when placing a 150ohm resistor between pins 2&3 on the input and turning the gain to the max? (Specify your soundcard)

 

[gemini86]

I'm interested to see what the hiss/noise level VS max signal is. Do you have RMAA?