Harrison 32C transformerless preamp racking.

Wow, my first own topic, awesome!

I did a search on this console but i could only find stuff about the transformer version.

Well anyway i wanted to see if anyone had any experience with this piece as i bought a preamp card and i'm in the works of putting it in a rack. I got the schematics and i've done my homework and most of it is pretty clear but there is one thing in particular i was wondering.

I saw a picture of the transformerless card and it had 6 LM394 transistors mounted, mine only has 4. The seller said thats how it's supposed to be but considering the picture was in a Harrison broschure i'm not really trusting him. My question is can ut run with only 4 of them? Or will i have to try and source a couple of these out to even get it running? I'm still waiting for a couple of parts so i havnt been able to try it yet hence the reason i'm asking. I'm probably gonna have to get a couple anyways just in case some of them breaks but it would be really nice to have it running next week when i get the last parts.

Second question is not that important but while i'm at it i'll give it a go. The preamp is supposed to run on +/- 18 volts, but i have mounted a +/- 15 volt transformer in there now just because it was laying around. I've read on both the HA4605 opamp and LM394's that it's neither too little or too much power so it should work, but do anybody know if it will? It's just a single preamp so in my world it should, but i'm very new at this so i dont know.

Thanks in advance!
/Chris

It should work as is, but to restore it to spec you'll want to get your missing parts. A 15 volt supply should work just fine, you'll loose a little bit of headroom but not enough to loose sleep over.

Thank you for your fast response! Thats good to hear, then i should have it up and running as soon as i get the last parts. And start scavage the internet for a couple of them to fill it up and for future replacement!

I know for a fact that the preamps were designed with 4 LM 394's & have the schematic to prove it. The mike pre is over designed & could be successful with only one 394. I knew Dave Harrison & was running console checkout at MCI when he spent a month in the lab designung & proving the circuits for his idea of a super console. The I.O. module used around 25 opamps as I recall. Jeep Harned MCI's president refused to manufacture Harrisons design, he went back to Nashville & produced the first Harrison console in just over 90 days; that was the end of his relationship with MCI.

Dave Harrison was the originator of the track assign switching & licensed the concept to MCI as it was implemented in the MCI 416 console, also included the black face plates w white lettering. Thats why the MCI 428 used a lighter color.

If you need some LM394's I have a number of them; they have been out of production since 07 or 08.

Ah cool so the seller was right. I would really like to take a look at those schematics if i may? Would be really interesting to get to know as much as possible of this preamp. The 4 i have now is located in socket 3,4,6 and 7, is that the correct way or doesnt it really matter?

I would deffinatley want a couple of them from you, maybe 4 or so depending on how likely they are to break?

Thanks alot for this information Bill!

Well i got it racked and working, sounds really nice and as soon as i get my silkscreen kit i'll make it look as nice as it sounds. Cant wait to start using it for real

just thought I'd nose in, just in case this hadn't been addressed elsewhere. I just finished racking 8 of these myself

you can install 4 LM394's for use with conventional impedance mics (around 150-250 ohms). The other slots are there for using the pres with 50 ohm microphones like old ribbons.

One issue I noticed and am curious about: this preamp has SUCH low noise until you get to the end of the gain knob travel, at which point it pops and all the noise comes in at once. I'll never need to run the pres this hot (It's got a ton of gain!) so I was thinking of adding some additional series resistance to get it to stop doing that. Not much more than annoying with a dynamic, but with phantom power on it sounds like something's being damaged when it happens!

Hope this helps and let me know if you're having a similar issue please. I used the standard 5k rev log pot as per the service manual for gain. what did you use?

kpearsall said:

just thought I'd nose in, just in case this hadn't been addressed elsewhere. I just finished racking 8 of these myself

you can install 4 LM394's for use with conventional impedance mics (around 150-250 ohms). The other slots are there for using the pres with 50 ohm microphones like old ribbons.

One issue I noticed and am curious about: this preamp has SUCH low noise until you get to the end of the gain knob travel, at which point it pops and all the noise comes in at once. I'll never need to run the pres this hot (It's got a ton of gain!) so I was thinking of adding some additional series resistance to get it to stop doing that. Not much more than annoying with a dynamic, but with phantom power on it sounds like something's being damaged when it happens!

Hope this helps and let me know if you're having a similar issue please. I used the standard 5k rev log pot as per the service manual for gain. what did you use?

Click to expand...

Oh sorry i lost track of this post.

Just opened up my case to add the JLM gobetween kit with a 48v switched system. Isnt working out too well. Got alot of hum and no 48v on the pins, i'm only down to like 1v. Any suggestions?

Yeah i got the same problem with the pot, it as tons of gain so i dont ever go over that last step but it is a bit annoying. 5k Rev log is what i used.

Got it to work now, missed linking the 0v from the Go between to the ground on the XLR, got 48v now with clean and great sound.

Glad to hear it's working for you. I've got too many projects right now to fix something that's just a little annoying, but I can tell you that adding in the series resistance didn't get rid of it. Oddly enough, that same thing happened, although the gain of the circuit decreased. I'm stumped. Did you tie the wiper and the CW lug together? I was thinking of trying it without that.

Yepp they're tied together.