Transformer for guitar pre balanced output


2016-04-14 10:17 pm
Hello, I'm new to the site. So sorry if I don't make this very streamlined.

I have a Peavey Rockmaster guitar pre that I have modded to my liking. When I record, I take the signal from the Rockmaster -> API 312c -> ADC etc... Then I use cab impulses in Cubase. The results are great.

Here's the kink. The Rockmaster balanced output is a dud compared to the unbalanced outputs. It just doesn't sound very good. The 312c lives in a studio that I have access to, but is not mine. I'm planning on getting DIYRE CP5's for my home, which do not have a hi-z input. So I either need to get a nice DI, or find a nice transformer to put in the Rockmaster. The latter sounds way more fun to me, and potentially cheaper (I'm a broke student).

I don't know much about transformers. So if you want to weigh in on this and help me find a good replacement, I'd appreciate it. I can't find specs on the transformer. Im pretty sure it's from TMI (Electrical Windings), single primary, single secondary, 1:1 (maybe).

Measured resistance of the primary (accounting for the extra resistors) 121Ohms
Measured resistance of the secondary 165Ohms
Part# stamped into the case 70500094 682


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Odd that you would think a high Z to low Z transformer would have a 1:1 ratio, especially when the winding resistance is different.

You can put a different transformer in it, but be aware that good quality transformers like that are not cheap.

That 705 number is the Peavey part number, and if you contacted customer service at Peavey, they may be able to give you the specs for it.

Dud isn;t very descriptive, how would you characterize the sound problems through this transformer?


2016-04-14 10:17 pm
Hey Enzo. Thanks for the reply. I don't know much about transformers. I just figured I'd tap into the wealth of knowledge here before spending the hours of studying. I'm a student, so my study time is packed full.

The balanced output sounds like it is attenuating certain high frequencies, and maybe some high mids. It doesn't sound quite like a high shelf cut however.

Subjectively, it sounds dull or boring. Like playing a nice head through a cheap cab. You can try to eq it, but it doesn't have the same effect.

I suppose the ideal goal is to find a replacement transformer that adds a nice color. I've been using an API pre, so it's safe to say I like that "sound", but I'm not married to it. I'd just like some suggestions.

Just incase this is helpful, the two unbalanced outputs are labeled
1V RMS 0dBV and 3v RMS +10dBV

The balanced output is labeled -10dBV

I'll email Peavey. Thanks for the tip.
Well, when you do that (or call on the phone - recommend right after they open at 8AM central time), you might ask about a couple other transformers. I have two types they used in some mixers.

One is the 70500139, which was used to convert the mixer Hi-Z outs to balanced in MC series mixers, and the other is the BA143, which probably also had the number 70500143. It was used in the MC series mixers at the channel inputs, converting low-Z mic inputs up to the high Z inputs.

One of those might work for you and could be had from me CHEAP.
The BA143s are small shiny hermetic boxes. 22x27mm and about 25mm tall on the board. Six terminals for pc mount, the low Z side has two taps, one was unused in this application, but appears to be still usable. The hi-Z side has three pins, but I suspect only two of them are connected. The center one is the hot, and they grounded both end pins, so I can't imagine both are on the winding. I have 12, still on the board with 36 1/4" Switchcraft jacks.

The 138s are plain old EI types, though small, and have four wire leads. Only have three.