Anyone know anything about wiring a Mic Input Transformer?

[robanimator]

Hmmm... Something to play with for sure.. I bet it is more for what you were saying about eliminating background noise.. Like maybe it's main purpose is really as a passive crossover.. Could prove to be a cool effect devise.. We'll find out. :D

If you don't mind, I had another question, maybe it has a quick answer so it doesn't take up much of your time (which I really appreciate).. Here goes..

The Bogen tm-200 input transformers
primary: 200 ohms
secondary: 50k ohms

What scenario are these a good fit? Is it dictated by the resistor that is grounded right before the tube? Or does the secondary Ohms value of a mic input transformer not really matter at all?

Thanks in advance,
Rob

[Lotus 7]

Originally Posted by robanimator

...
The Bogen tm-200 input transformers
primary: 200 ohms
secondary: 50k ohms

What scenario are these a good fit? Is it dictated by the resistor that is grounded right before the tube? Or does the secondary Ohms value of a mic input transformer not really matter at all?

Thanks in advance,
Rob

The load impedance, which in a vacuum tube circuit is almost entirely represented by the grid resistor, should be approximately equal to or higher than the rated output impedance of the transformer. Using a High-Z transformer to drive a low-Z load will usually result is reduced bandwidth and sometimes even distortion since the core can "saturate".


With those winding impedances the Bogan is intended to be another tube input transformer. Transformers intended for use in bipolar transistor circuits (before FETs became available) generally have secondary impedances between 2K and 10 K ohms.

It will work very much like your Stancor, but with a 50K ohm instead of the 89k ohm secondary, it will put out a signal at the tube grid which is a little lower (probably an almost insignificant 2 dB or so less. It can be used with a grid input resistor as low as 47K ohms, but will also be fine with a 100K or a 1meg resistor.

Those transformers were optional in the old Bogan "CHS" Series PA amplifiers and have a bandwith of 70 Hz to 12kHz (+/- 2 dB) with a gradual roll-off above and below those frequencies. It's certainly not a "communications" transformer, but it's not "hi-fi" either.

It's not a "high-end" mic transformer like a Lundahl (my very favorite mic transformer company). Lundahl knows better than most how to make transformers and have models with a frequency response of 10 Hz to 100k Hz (+/- 0.3 dB). I have Lundahl transformers in my Jon Ulrigg ribbon mics and they do give a very accurate, extended sound unlike traditional "old-school" ribbon mics.

[robanimator]

Wow! Once again you lift the veil of mystery with impunity!

Thank you so very much,
Rob



Oh and I did look up "communications voice frequency" and found this page:
Voice frequency: Definition from Answers.com

Your a wizard!

[robanimator]

I just read this entire thread top to bottom and it is like Christmas morning!

Thanks sooooooooooooooooooooooooooo much!

Rob

[Lotus 7]

Rob,

No problem. I never mind sharing any knowledge I might happen to have when someone wants to know the "why" or "how" about something, but I usually don't get involved in the much more typical Gearslutz exchanges about "which mic, preamp, or monitor speaker" is going to meet my personal (unexplained) taste as the very best.

Have fun with your new "iron-core" friends.

[robanimator]

"You could then play around trying different combinations of multiple transformers in series. Because transformers can sound "different", depending on how they're driven and terminated (source impedances, load impedance, frequency dependence of those impedances, etc.), connecting two slightly different transformers in series may have a different sound depending on the order they're connected."

Hmmmmmm.............. going to try this, this week..

:D

[Lotus 7]

Originally Posted by robanimator

"You could then play around trying different combinations of multiple transformers in series. Because transformers can sound "different", depending on how they're driven and terminated (source impedances, load impedance, frequency dependence of those impedances, etc.), connecting two slightly different transformers in series may have a different sound depending on the order they're connected."

Hmmmmmm.............. going to try this, this week..

:D

Rob,

If you connect two transformers in series, another easy thing to experiment with is to add a load resistor across the connection between the two transformers, It will change the level of the signal, and can change the sound characteristics, especially if the two transformer inpedances are mismatched. If, for instance you use a 250 ohm to 250 ohm line transformer and then a 600 ohm to 100K tube amp input transformer next to drive a tube amp, placing a resistor with a value between 150 and 600 ohms across the "transformer-transformer" connection will change the loading on the first transformer and may change its sound slightly for the cost of a 20-cent resistor. It's just another option to explore.

See attached schematics.

[robanimator]

Wow, Let me explore the possibilities with this since I picked up some more input transformers, just got a NOS Communications Range input transformer so I'm wondering what that will sound like once I recap my amp and swap out a few bad resistors, CAN'T WAIT! My mail box should be full by the end of the week with new parts..

Thanks for the tip, I sure have enough input transformers to play with..
Rob

I'll keep you posted..

[robanimator]

I love the whole idea of this..
Rob

[robanimator]

Hi Lotus,
I am still in the process of restoring my 2 channel Rauland amp, I have all caps replaced, and resistors that were bad have been replaced, but I have a line-up of microphonic tubes and a weak rectifier and one of the tubes is humming bad so it all boils down to cash for more tubes.. ( and 2 highly mismatched 6L6's when the schematic calls for 6L6G's), so I should have my input transformers installed and experimented will last.. All boils down to cash flow, lol

I thought of something that you may be able to answer in the meantime, I also have a Bogen PH10, I love the tone, the input is an RCA jack. I am wondering if it would be a candidate for an input transformer and a mic?

http://www.rjlmultimedia.com/pdf/Bogen-PH10.pdf

Above is a link to the photofact including description of use and inputs, and schematic. It mentions on page 1 that the input should capable of delivering 1 volt across 500,000 ohms.

Can I hook a mic input transformer to this and hook a mic to it? Maybe even a hot mic that takes 48 volt phantom power? If not, can the 6SL7GT be replaced with a different preamp tube that could do the job like a 6SJ7 or 6SC7?

Does the Bogen PH10 even have a preamp?
Thanks in advance,

Rob

[Richard Crowley]

Wow. Bogen and Rauland were the low-end trash even back in their day. Now they are somehow "classics"? LOL!