[BUILD] 1108 DIY project and PCB layout

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Swapped all transistors, hiss stayed the same. Even tried multiple J309s (socket)

Tried with the Cinemag disconnected and hiss stayed the same once again (infact noise went up a bit). Can we discard the input tranfo & input cabling?

Checked all resistors and R14 is 830K instead of the original 820K, this shouldn't be a problem right? Apart from that all other values are correct.

Any ideas?
 
Just noticed that when I move the on-board switch to "Low" a loud oscillation appears at 25Hz + harmonics.

What can be causing this?

Related to my hiss problem perhaps?

Here is the noise viewed with a spectrograph. The 50Hz and armonics are from the power transformer (I will be putting the PSU in a separate box later, want to fix the hiss first)

Thanks
 

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Its posible to add some input buffer for Di?

It will be a very tight fit. Did you see the picture I posted? I've built 4 of these in 500 series format.
To fit a DI (and especially a buffer), you'd have to leave something else off.
 
That transformer should have at least dual 18V secondaries on it. (probably higher) This preamp only needs a single 24V supply, so it may work just fine. Wire the secondaries in series and measure the AC output. You'll definitely have enough for the preamp, but I don't know if you'll have enough for phantom power (if you even need it).
 
Regarding the input transformer.  Do you think 1:5 ratio 200ohm to 5k would be adequate or is that just too low of a step-up?

 
Hello all! First off thanks to groupdiy for such a wonderful and informative site, thanks to Mnats for the beautiful 1108 boards and power supply pcb.

I'm building two channels of the 1108 and have a few questions, I'm new to building things of this scale so bare with me. (I'm used to building pedals).

My build consists of: i eliminated the Vactrols and have wired up 1m pots. Input transformer is Cinemag CMMI-7C and output transformer is Hairball Audio's 5002. (Thanks for the great site Hairball and transformers!)

now, my questions are:

1. According to the schematic on Mnats page there are two connections near R20, labeled "E" and "5". I've seen other members have thrown a resistor in the spot. What's the purpose of the connections, or what size resistor should I use? (I couldn't  make out the color bands from any photos.)

2. When wiring the output transformer, the yellow and orange wire should be cut and taped off as they are not needed? That's what I gather from the photos and schematic (again, I'm new to this so my "gathering" is probably not correct!)

3. When wiring the input tranny, is it advisable to skip the s3 (impedance) if I know I'm not going to be running ribbons? I can just cut secure the cable and wrap it with tape if I change my mind. And should I go hi or low in regards to the "set it and forgot" impedance?

Sorry for so many questions, i will keep searching and reading in an attempt to figure it out on my own, I just wanted to double check before I wire it up fully and test it, or break something.

Thanks again for any and all help.

Dkm
 
Im finishing dual chanel right now
I had some problems with it but all now is on a good way!

1Meg pot is a good choice:)

1."E" and "5" you will not find in the pdf. This is really important part of the circuit. I missed this point and there i had problems.
You need put here 8,2K resistor and all should works fine. It's a feedback resistor.

2. Yellow and orange wires should be soldered with eachother and secured with tape or heatshrink tubing

3. Ofcourse you can wrap it with tape but without shorting both wires. I think better will be make for a start a switch of impedance. Hi/Low switch is usable too.  If you don't want to have switchable h/l section go to hi position. For impedance switch you will need dpdt type switch and for h/l spdt or dpdt.

This circuit is very sensitive in case of grounding. It need to be well grounded. If you will have still some problems or questions fell free to ask.

Cheers!
 
Thanks a ton ln76D!

I should have this thing wired up by the weekend. (I need a few more switches, I was being lazy about the impedance switch. Seems silly not to just add it.)

I will post some final pics (when that day comes) and I'm sure i will have at least one more question!


Thanks again!
Dkm
 
Hey everyone,

I'm looking for the dimensions and hole spacing of the mnats variable + phantom PCB.

We just purchased 8 of these for use in a preamp project for our studio, but they're still in the mail and I'm designing CAD files for enclosures. I'm trying to get the hole spacing right...

Judging from the two prototype units we've already put together, it seems it might be something like 52mm on the short dimension and 86mm on the longer dimension, but I can't be sure because our holes weren't drilled with much precision.

Incidentally, these are phantom supplies for REDD.47 tube preamps. They've worked well on our prototypes.
 
Here it is:

3mm holes x 4
center to center: 52.1mm x 86.4mm

This is all measured though.  Not sure what the actual intended measurements were in the design.

Hope that helps.

Mike
 
Been messing with this pre circuit used a Sowter 9045 1:7 mic transformer & used a diy La4a to drive a green LED & put LED in a big blob of blu tack facing a LDR connected to 1108 no audio is used from the LA4A the 1108 works beautifully for recording vocals cant make  it sound bad
 
recently finished mine! This was my first mic pre build and I am very happy with the results. I am attaching photos of my work. It's kind of ugly, but it does the job! Sounds great Mako!
 

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I'm sorry to intrude, and also sorry for not having read all topic pages yet.

But I made a search on the site using the word Vactrol, as I am exploring implementing a limiter with a Vactrol or Silonex optocoupler, and I wonder if use any of them as such.

Did you?
 
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