So many projects on the fire....
I've been plugging away on this preamp for a while. Time for an update.
So far here is what has been done:
- swapped out the stock (China) transformers for Jensen JT-11K8-APC. Let Jensen know that these are for a Presonus and get a discount ($30 each)
There is a good writeup on how to do this here:
http://www.imjohn.com/PresonusJensenXfrmrs/TransformerSwap.htm
- Installed BB OPA627PA IC in place of the 5534
- Installed 3 new insert jacks to replace those damaged by a PO using screws that were too long on the cover.
- Installed a new set of original knobs.
Changing the transformers is a big job that should be done carefully and without haste. Most can do it if careful.
The top cover uses 1/4" long screws (I need to measure so don't quote me). Some were lost by a PO and longer screws were used. There are a few places where the longer screws went down into the insert jacks and messed them up. This is a good time to talk about Chad Kelly and Presonus support.
In looking for replacement jacks I contacted Presonus. The degree of support hat I got from Presonus and from Chad Kelly was outstanding. I can recommend Presonus fully based on the support I got. It turns out that Chad had done the final testing on this unit when it was new. His signature was in it. Support above and beyond! Plus a lot of information on the unit.
TODO: Right now the biggest thing is that the gain pots are noisy. I have a set of replacement pots coming from Presonous that I will install soon.
I might replace the output coupling caps. They are Nichicon and not bad but I might want to try some Muse there. Chad says that there is minimal (if any) audable gain.
Testing:
I have done a few tests just singing (if you cn call it that) into the preamp. I'll post the clips sooner or later for your review. The tests are:
CH xformer, stock 5534 IC
CH xformer, OPA627 IC
Jensen xformer, stock 5534 IC
Jensen xformer, OPA627 IC
THe signal chain is an Oktave MK-012 SDC (with modded electronics), Presonus, Tascam BR-20 and later to CD on a Tascam CDRW-5000 and then to an mp3.
Those 4 tests were done at the same time and in the same configuration (distance to mic and levels).
I hope to add some like tests later of other preamps (DMP3 before and after mods, GR MP2, DIY MPC1, and a few others)
FUTURE:
Well I have 8 Chinese transformers to play with. I'm thinking that a FET to OPA627 to 49860 preamp might be nice to DIY....
Regards, Ethan
PS: I really want to say thank you to Presonus and Chad Kelly for their outstanding support.