Neve 1290 build completed!!

[mitsos]

Hey Martin,
Just got one channel of these together. I'm using Altran in/out, 1+1:4 on in (used 1:2), and their C4000 gapped trafo on output. Sounds great.... except:

I get some weird oscillations at high gain, usually only at max but sometimes in the setting just before. I measured DC and transistor voltages and everything is normal until the oscillation happens. At that point, DC is normal, and TR 4, 5 and 6 all have normal voltages. TR 1, 2, 3 all read strange voltages:

These are C - B - E

TR1  4.0 - 0.6 - 0.4
TR2  22.7 - 4.0 - 7.0
TR3  20.6 - 6.7 - 6.2

So something is going on in the output stage? I noticed that once the oscillation starts, it continues even after I turn the gain down. At first the only way to get the pre working again would be to turn the power off and on again. But after a couple of these cycles, I noticed that if, after turning the gain down, if I waited a bit and/or tapped the mic a bit hard (SM58 in this case) the signal would come back.  Should I be looking at the gain resistors or in the output stage itself?
thanks!
 

 

[wolfgang]

Hello!

I have build 2 channels some time ago and they are working perfect!
Last week i build another 2 channels and they have the same problem like mitsos.
But this weird oscillations will happen at any Gain position when i unplug the cable from the JLM-DI.

I got the same thing like mitsos: when a tap the microfone i a littlebit harder signal is coming back.
I also wired a 5dbTrim/fullFader-Pot. When i put it in Fader mode and turn the fader back to about 9oclock the signal is also coming back.

First build was with Carnhill iron, JLM Go Between, JLM Fet DI, and 48V SMPSU with the JLM TRex.
Second build has LL1538 InputTX and the JLM OutputTX, JLM GoBetween, JLM Fet DI, and the JLM 3RailPSU.
All other parts are from the same order from digikey.

I changed the DI for a sure working one but but nothing changed. I also did away the trim Pot and used the onboard resistor and jumper- nothing changed.

All voltages on transistors are fine. Checked all Parts for values and direction and also for bad soldering joints.

Where should i take a closer look? I think it could have to do with the output since when i put the fader down the oscillation is away!??!

I am a newbie and i am very helpfull for every help!!!

best regards,
Wolfgang

 

[madriaanse]

Hello,

Both your problems sound like any/all of the following:
1. Not using shielded cable
2. running mic/in next to line/out wires
3. input transformer too close to output transformer
4. Wrong transformers. Stay with Carnhill or original iron; use other iron and anything, and I mean ANYTHING, is possible - seriously not a good idea.

Also, try connecting without the JLM DI and see how it goes.

Best,

m.

 

[wolfgang]

Hello Madriaanse!

Thanks for reply!

for your points:

1. I am using all shieldede cables

2. mic/in and line/out wires are crossing one time at 90degree angle - but i will try to move them a bit!

3. transformers have much space between them!

4. I will try to swap the transformers to Carnhill. But i think ll1538 should be ok and also the jlm since this is a gapped and Joe says it can substitute the LO1166

I will test some things tomorrow and hope i can post some positive results!

Thanks so far for chiming in!

regards,
Wolfgang

 

[3nity]

i had the same problem with mines!
was using JLM di and go between...

I got rid of all the JLM stuff and hardwired phantom its clean and clear now!!

But xtra care has to be allowed to the transformer wiring!

Jorge.

 

[mitsos]

Just to update that I "solved" my problem. I now have an intermittent connection between my input trafo and the PCB to take care of, but the oscillation is gone. I can't say that I know what it was, but what I did was touch up a few solder points on the switch, the 180pF cap (I used 220pF actually), TR1,2,3 and one of the 470uF caps. I also swapped the somewhat thin wire I was using for 0V with a 1mm solid copper wire. 

Anyway, I will do more tests going forward (especially after I get this in a case), but I am happy to say that all gain steps are working!

thanks!

 

[wolfgang]

Hi Mitsos!

Great news!
What Transformers do you use? Carnhill, Original or something else?

I found the problem also! It is the JLM111DC Output TX i wired it 1:2 and this is why i have this wired thing going on! I switched to Carnhill and the problem was gone!

So the preamp pcb is working

Now I tried JLM wired 1:1 and also no problems any more!!

But i am wondering why this happens??? The LO1166 is also step up . Or am i missing something?

regards,
Wolfgang

 

[mitsos]

I wonder if improper biasing can cause oscillation? I've biased this a few times but I still get visibly uneven waveforms in PT when things get loud so I wonder if my biasing technique is correct.  I am setting a sinewave to -20 or -30dB and raising the gain until I see it clipping.  At this point the trimmer works and I can even out the clipping.  But I still get uneven clipping in PT.  Now, if I send a lower level sinewave and the preamp is max gain or close, the clipping is so uneven that I turn the trimmer all the way through its rotation and it doesn't even come close to evening out.  Also, I should be measuring before the output trafo, right? 

Wolfgang, I am using Altran trafos. The C4000 is an LO1166 "descendant". The input is the C3982, which is the same ratio as the Neve input, but smaller and in the Cinemag pcb-mount footprint. Anyway, the pre sounds really good, it is a bit noisy at higher gains, but I think that's just a matter of boxing it up properly (I hope). 



thanks!

 

[3nity]

Mitsos does Altran sells transformers in small number? i want to buy 2 and they dont respond my e-mails.
Thanks.

 

[mitsos]

3nity, I haven't had issues with emailing them. Takes a couple days usually, but Dave almost always gets back.  Try calling, if he's there they'll hunt him down for you. As far as small numbers, yes, they sell their "stock trafos" in quantities as low as 1 and discounts start at 9 or 10 IIRC. These trafos, the API ones, are stock.  For other models I don't know but Dave always offered to sample different ones. Might be a bit pricier than their "stock trafos" but I can't see it as being expensive judging from their other prices.  Anyway, it's definitely one of the best deals around for such nice quality stuff. And I saw a post somewhere that Ed Anderson is working with them on re-doing the C4000 to be closer to the original, so that will be one kick ass trafo! (to me, it only makes sense that they'll re-do the input but I don't want to start rumors, so you didn't hear it from me)  8)

 

[madriaanse]

Hi Mitsos, I always connect the oscilloscope to the secondary of the LO1166.

you wrote:

I am setting a sinewave to -20 or -30dB and raising the gain until I see it clipping.  At this point the trimmer works and I can even out the clipping.  But I still get uneven clipping in PT.

That doesn't quite sound right. For one, ProTools (cal'd to +4dBu=-15dBfs) should be clipping extremely hard before the Neve even gets close to breaking up. I suspect the transformers, or DC offset on your A/D - do your other preamps make PT clip in the same way?

EDIT: also, did touching up the cold solder joints fix up your transistor voltages? If not, check/double check the resistors on the output stage.

HTH,

M.

 

[3nity]

Mitsos Redoing a Carnhill/St-ives/Marinair style sure would cost near the same as a carnhill!
But how you find the sound of that Neve you built?

Thanks.

 

[mitsos]

Hey guys, gonna do some mroe testing and I'll let you know more details.  But speaking after only a few quick tests, the pre is HUGE!  I recorded nylon guitar with an old AKG C3000 and it was amazing out of the box. At times I felt it was lacking in highs, but this would be in comparison to an API or something which is a bit brighter to me. All in all, I like it. Picked up some carnie outputs in the BM and will compare when they arrive, and hopefully I can find an input trafo stateside or something to try as well. My plan is to do 8 if the budget allows!

About the redesigns:  I don't know any more than what I saw in the post somewhere here. Whoever posted said it would take 3 months, so until then, no one will know.

Martin: Sorry that was very badly explained. PT was indeed clipping like mad during biasing.  I was talking about a quick recording I did after biasing.  I could see the waveform clipping on one side while the other had a large peak but did not clip.  I guess maybe I could draw this out to give a visual. I was mostly wondering if I was doing it right.  So I should take the reading on the secondaries, set the sine wave to -20dB or something and raise the gain until I see clipping in the scope?  Or do I have to be at max gain like it says in your assembly manual?

I hear ya about DC..  Touching up the solder joints mostly fixed things, but I suspect it may also have to do with the 180pF cap.  I put in 220, but I will double it to see what happens cuz I did get some oscillation after my last post, but I couldn't repeat it. 

Hope it's not the trafos, but I am getting some UK ones to test in any case... Anyone have an extra carnie input trafo?? 

 

[mitsos]

Actually I'm at a loss. I can now repeat the weird hissing pretty consistently. Here is the thing: It happens with dynamic mics yet not with condensers.  Condensers do work throughout the gain but have a bit more hiss which I assume is from the phantom since it happens when phantom is on, regardless of the type of mic (condenser or dynamic).  So I checked my grounding, but if there was a problem the condenser wouldn't work, since it needs a proper ground reference, right?

The other issue which makes me lean toward grounding is that there is one way I found to avoid the hissing when going to the highest gain:  I must be holding the mic and grounding myself with the other hand. If I do this I get clean gain all the way up to max (with a dynamic... condenser is not so clean as stated above).

3nity, I'm curious how you wired your input XLR ->trafo-> board.  Right now I am coming into the trafo with a shielded cable and I've connected the shield to the low side of the secondary which I also tied to the trafo's Faraday and shielding can.  I normally tie all these trafo connections to ground, but I notice the Carnhill only has one earth pin, which looks like an internal faraday shield so I wonder if maybe I should lift the can pin?  The way it will be connected in the end the can will be attached to the rack case probably with some kind of clamp type thing.

Any ideas welcome. Next week I will redo my house ground wiring cuz I think that my be contributing in general to noise..

 

[wolfgang]

Hello Mitsos!

I was testing my 1290 today and i tried to terminate the secondary on the ouputTx with 620 Ohm and now my problemis gone. Also when i wire it 1:2!!
I dont why it is like this. I send Joe a email maybe he has a answer.

regards,
Wolfgang

 

[beyondimpact]

Do you have any more EZ1290 boards? I tried using my own design but it isn't working out right

 

[madriaanse]

Do you have any more EZ1290 boards? I tried using my own design but it isn't working out right

no problem; email me at martin_adriaanse -at- yahoo -dot- com

Hi Mitsos, let me know what happens when you get the carnhills. Also, besides the 220pF, did you use the parts from my BOM?

I hope we'll get you sorted out soon!!

M.

 

[kambo]

could someone, measure the Volt on R7 47ohm 1/2w on a calibrated unit please...
(one side is connected to 0V)
i know every unit will have different V value, just need to compare some values...
mine workshop  is in a mess, right now ( everywhere is 2254c ).... cant measure it today

thank you
kam

 

[mitsos]

Hi guys...

Martin, I'm afraid you are right about this input trafo. It needs heavy damping to work... I threw together a zobel jig with a pot and an IC socket to swap in caps. The pot didn't make much difference but what worked was a 30nF cap across the secondary with no load R (I know RL is not normally used, but I thought I'd try. Used a 6K2, but it made the output trafo ring audibly at high gain. I could hear this thing ringing from a few meters away!)

So, with just the 30nF in place of the 180pF it works, and does not oscillate.

Have to test phantom later. Last thing to check then next week I'll case it up the way it is and try using it for a bit before I get a Carnhill or the cinemag 3nity used.

I am going to do a couple more tests, but since I won't have a signal generator for another month or so, trafo tests are mostly done for now. It works ok, but not sure how such a large cap will affect frequency response. What do I need to know to figure this out? It's an LC filter right? How do I know the inductance of the trafo?

thanks guys!

 

[danjpiscina]

Hi guys! I've got another 3 pairs of 1290's (almost) completed! These things sound so great and musical it's almost unbelievable. I tracked drums the other day and it was beautiful. Anyway, the reason I'm writing is because I'm having some (strange?) problems. On the last 3 pairs I've built I get proper, clean gain up until the 70dB step (2nd to last). It dies on the last step. I still get signal, just not 5dB louder, more like 20dB quieter and I hear artifacts and odd noises. These units have not yet been properly calibrated so I think it could be to do with that. However, here's the funny part: when I touch (but not switch) the PAD on the Go Between, I get the last gain stage and it sounds fine. Is this a grounding issue of some sort? Any ideas? Thanks for listening!

Dan P.