Bogen J330 Mic Input Problem

[Sylvaniac]

Ok, so you have no transformer, just another plug-in jumper wire. When that can is plugged into #3 socket, all 3 inputs should work the same, as Hi-Z inputs, XLR pin2 hot.

OK, I see. And they do. But I have all low Z unbalanced mics I'd like to use. So, would a transformer replace that plug in jumper? Just to be clear, with that plug-in jumper out, none of the mic inputs will work? Thank you.

[Sylvaniac]

OK, I think I got it. My options are 1) load down or 2) buy a transformer? Would there be any sonic advantages with a transformer compared to loading down? They asking the pretty penny for those damned transformers.

[g1]

I think when Leo said "load down" he meant to decrease sensitivity of the input, but it would still only take a Hi-Z source.
There are various cheaper transformer solutions.
Your " inline transformer to hi Z phone plug" would work if you changed the 1/4" to xlr wired pin2 hot, pin1 gnd.
(unless the transformer is built right in to the 1/4 inch plug itself, then you would need a 1/4" to xlr adapter)

[Leo_Gnardo]

OK, I see. And they do. But I have all low Z unbalanced mics I'd like to use. So, would a transformer replace that plug in jumper? Just to be clear, with that plug-in jumper out, none of the mic inputs will work? Thank you.

With low Z UNbalanced mics I'd try going straight in no transformer. Of course the pin 8-9 jumper would have to be in place. Wire an XLR connector for the mixer end of your cable pin 1 ground/shield, pin 2 signal, pin 3 not connected.

If you get a signal through with this setup, and find it sounds very bright, here's where the load R's come into play. You can tack one across the Bogen's mic connectors pin 1 to 2. And here's where your ear also comes into play. Try different values say 10K ohm down to whatever R matches the mic's claimed impedance, usually 600 or 150 or 50 ohms. At some point you'll hear excess hi end sizzle tame down and then you can fine tune it to taste.

First let's get a mic signal through those inputs. Correct, without that 8-9 jumper no signal can get to the preamps.

[Sylvaniac]

I think when Leo said "load down" he meant to decrease sensitivity of the input, but it would still only take a Hi-Z source.
There are various cheaper transformer solutions.
Your " inline transformer to hi Z phone plug" would work if you changed the 1/4" to xlr wired pin2 hot, pin1 gnd.
(unless the transformer is built right in to the 1/4 inch plug itself, then you would need a 1/4" to xlr adapter)

Oh, OK, got it, thanks!

[Sylvaniac]

With low Z UNbalanced mics I'd try going straight in no transformer. Of course the pin 8-9 jumper would have to be in place. Wire an XLR connector for the mixer end of your cable pin 1 ground/shield, pin 2 signal, pin 3 not connected.

If you get a signal through with this setup, and find it sounds very bright, here's where the load R's come into play. You can tack one across the Bogen's mic connectors pin 1 to 2. And here's where your ear also comes into play. Try different values say 10K ohm down to whatever R matches the mic's claimed impedance, usually 600 or 150 or 50 ohms. At some point you'll hear excess hi end sizzle tame down and then you can fine tune it to taste.

First let's get a mic signal through those inputs. Correct, without that 8-9 jumper no signal can get to the preamps.

What a neat workaround! I know my old mics' sounds and trust my ears. But I've located and purchased a Bogen T-155 nine-pin transformer that matches the configuration shown in the schematic and that has the correct ohms for my mics (150 - 250 ohms). I may try the resistors meanwhile, just to learn.

This thread been a great and fun learning experience for me! Thank you all, and I'll get back to you with the results.

[Enzo]

One of these is a simple solution:

Hosa XLR Female LO-Z to 1/4in TS Male HI-Z Microphone Input Impedance Transformer | Musician's Friend transformer&index=4

[Sylvaniac]

One of these is a simple solution:

Hosa XLR Female LO-Z to 1/4in TS Male HI-Z Microphone Input Impedance Transformer | Musician's Friend transformer&index=4

Thanks, Enzo, yes, I have a couple of those somewhere. Here's what I'm using till the transformer arrives:



My J330 Bogen has XLR male input connectors rather than the usual female:

[Leo_Gnardo]

What a neat workaround! I know my old mics' sounds and trust my ears. But I've located and purchased a Bogen T-155 nine-pin transformer that matches the configuration shown in the schematic and that has the correct ohms for my mics (150 - 250 ohms). I may try the resistors meanwhile, just to learn.

Good to have the expected transformer on hand, my worry is that with the xfmr in circuit, you might wind up with a lot more signal than you expect at the input tube grid. As you said, try with and without, your ears will know what's best.

Low-impedance unbalanced just lets you run a little longer mic cable before you start picking up hum, compared to hi impedance. Pro mics are balanced, you can run hundreds of feet of cable, no hum pickup, but you definitely need a transformer at the preamp end to maintain the balance, otherwise the hum-bucking feature doesn't work.

Bogen apparently was having gender-confusion back in the 50's - my MXMA mic mixer has "backwards" input connectors too.

[Sylvaniac]

Good to have the expected transformer on hand, my worry is that with the xfmr in circuit, you might wind up with a lot more signal than you expect at the input tube grid. As you said, try with and without, your ears will know what's best.

Low-impedance unbalanced just lets you run a little longer mic cable before you start picking up hum, compared to hi impedance. Pro mics are balanced, you can run hundreds of feet of cable, no hum pickup, but you definitely need a transformer at the preamp end to maintain the balance, otherwise the hum-bucking feature doesn't work.

Bogen apparently was having gender-confusion back in the 50's - my MXMA mic mixer has "backwards" input connectors too.

I wonder if the owner's manual back then explained why they did that. Anyway, you got me thinking... Since the amp, stock, is set up for Hi Z, it makes sense to use it as intended by utilizing the inline adapter / transformer. I do understand about the advantages of Low Z with long cable runs. And from experimenting, I've never heard any difference in signal strength and audio quality between Hi and Lo Z (so long as properly matched, of course). I'll still go ahead and hear the mic using the Bogen T-155 transformer, just to hear, yes. But I must ask, since the transformer is made by Bogen specifically for this application, would a lot more signal at the grid be harmful and in what way? To the tube?

Also, this is interesting and I'd like everyone's take on it: My Bogen VP17X record player 10 watt amplifier mic input has ONLY 1/4" phone plug Hi Z. When I use a Low Z mic, with inline transformer to Low Z phone plug, I get a much stronger signal than I do on my 30 watt J330. I know this may be due to old, weak output tubes, but I think I need to correctly wire a cable to optimize my mic signal. Right? OK, so I would use a Low Z mic, with inline transformer to Hi Z phone plug, but then I'd have to make a cable that has a phone jack to XLR, but wire pins 1 and 2 only, correct? I just want to optimize the signal and be sure I'm getting full signal.

[g1]

OK, so I would use a Low Z mic, with inline transformer to Hi Z phone plug, but then I'd have to make a cable that has a phone jack to XLR, but wire pins 1 and 2 only, correct? I just want to optimize the signal and be sure I'm getting full signal.

I'm not sure what a couple items are in your photo. I think at the base of the mic stand is just a coupler? Then the old round cylinder is a transformer? Out of that goes to your 1/4 inch plug? If that is correct then you should be able to just replace that 1/4" plug with a female xlr, pin 2 hot, 1 ground.

Don't worry about putting too much signal into the tube, you can't damage it, it will just distort.
As far as the gain differences between the 2 amps, I'm not convinced the VP17X has not been modified. The other issues you have with it in the other thread make me think it may not be stock.

[Enzo]

Bogen did it because back then, many if not most mics were Hi-Z, and transformers were not cheap, so the typical school PA system with a Hi-Z paging mic sitting next to it didn't need to spend the extra money.

[Sylvaniac]

Bogen did it because back then, many if not most mics were Hi-Z, and transformers were not cheap, so the typical school PA system with a Hi-Z paging mic sitting next to it didn't need to spend the extra money.

Well, I mean, why did they have the male XLR instead of the common female XLR for the input? You know what? It could've been something as simple as an attention getter to make their amp stand out and be noticed in the stores! ???

[Sylvaniac]

I'm not sure what a couple items are in your photo. I think at the base of the mic stand is just a coupler? Then the old round cylinder is a transformer? Out of that goes to your 1/4 inch plug? If that is correct then you should be able to just replace that 1/4" plug with a female xlr, pin 2 hot, 1 ground.

Don't worry about putting too much signal into the tube, you can't damage it, it will just distort.
As far as the gain differences between the 2 amps, I'm not convinced the VP17X has not been modified. The other issues you have with it in the other thread make me think it may not be stock.

Yeah, good point. I wonder...

So, that tube distortion--that's what the guitarists love, huh? And that they prize certain tubes not only for their tone but for overdriven qualities. Ah, OK.

Yes, the can thing is a transformer and the base--the mic cable just runs through it out the bottom rear. This is all part of a nice eBay score of original base, mic (EV 655...like Art Linkletter used, if you're of age to remember his TV show, even before Kids Are People Too) and Shure transformer. Yes, would be nice to just replace the phone plug with female XLR wired 2 hot 1 ground. But I kinda don't want to alter the originality of it all. Also, another thing I've yet to look into is to open the transformer. It is selectable in there. Maybe I can play around with other impedances? Hear how it sounds; see if I get more output.

Also, my current setup terminates with another in-line transformer that takes the phone plug Hi Z and makes it Low Z into the Bogen. Yet it works! AND all three pins of that in-line XFMR are wired. So, I'm betting, if I replace that in-line low Z transformer with a phone jack to female XLR, wired like you say, it will now match perfectly and give better output. From what I've learned here, it's got to work. And this is what I meant by why the J330 sounds weaker than the VP17X, at least in theory (and also yes taking into account the VP could be modded).

[Enzo]

Bogen amps and their competitors back then were not sold in stores. They would have been bought from catalogs by contractors. The wrong gender XLR was just something they did. Standards were not as well developed back then. I wouldn't read anything else into it. If you asked someone in the mid 11950s what a mic connector was, they describe that threaded screw-on thing with the center contact. Switchcraft : Vintage Microphone Connector

I would swap out that male XLR for a proper female in a heartbeat. This is not a museum piece, this is something someone will want to use.