Gemini PA-7000 Preamp mic jack

mariov

New member
I have a older Gemini PA-7000 preamp. I tried to use the mic jack and nothing but a pop, when the mic is plugged in, and as I start to turn the mic level up, then nothing. I am trying a Behringer xm8500 dynamic mic, a xlrf to 1/4" trs cable. Then I added a 1/4" trs to ts adaptor and the same thing. I have attached the schematic. I don't know if this is the correct mic setup or if the preamp mic circuit is bad. Everything else on the preamp works fine.
Thanks
Mario
 

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If you have an instrument/patch cable (1/4" TS both ends) plug it in and touch the tip of the end of the plug of the cable while increasing the gain on the amps mic channel. You should hear some hum.
If you do hear hum I'm inclined to think the TRS to TS adapter may be a problem and I don't believe the XLR to TRS cable will work. If you hear hum we can work on finding a cable that may work.
The schematic of the mic input jack on the amp I 'think' is wired with a shorting contact so that when no plug is inserted the output of the last mic gain stage is shorted so as to kill any noise of that preamp channel may produce with nothing plugged in. It may be in part the reason for the pop when you plug into it when it opens the short.
 
The amps mic input has a 3kohm input impedance. My first thought was that transformer as well and I have several around from my early years with old 1/4" input recorders and amps that had hi-z inputs. Might work, but I was thinking more more of an XLR to 1/4" TS cable that unbalanced the 1/4" connection (XLR pin 3 to shield/Gnd) to the amp which I've used occasionally with some gear I have.
Foremost the OP needs to be sure the input is actually working, which I tend to believe it would be.
 
I have tried a guitar cable and yes I do hear a hum when I touch the tip. The mic level does not change the hum but the volume output of the preamp does change the hum. Also I looked inside the unit the mic jack is in a plastic modular case with a clear top and I could absolutely see a set of contacts making and breaking. When I plug the microphone with the XLR to TRS quarter inch cable into my guitar amp and I turn it up pretty loud I could hear my voice just a little louder than normal speech. But when I add the TRS to TS adapter I don’t hear anything
 
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Somehow the hum is bypassing the mic preamp so the mic level is not affecting it.
- Are you pretty sure the 1/4" plug was all the way in?
- Do you have another way to check the mic is working (another mixer or amp)?
- Did the mic input ever work that you know of?

Using the XLR to TRS cable you have and no mic connected to it, plug the TRS end into the mic jack. With a paperclip, nail, or similar piece of metal you could use as a test probe, hold it in your fingers and poke it into the pin 2 and pin 3 (small numbers may be near where the pins go) of the XLR end of the cable one at a time and try the mic level control again listening for hum. I'm wondering if the jack is not wired to do what I think it might as it's drawn in a way that doesn't seem correct or clear on its function.
 
The amps mic input has a 3kohm input impedance. My first thought was that transformer as well and I have several around from my early years with old 1/4" input recorders and amps that had hi-z inputs. Might work, but I was thinking more more of an XLR to 1/4" TS cable that unbalanced the 1/4" connection (XLR pin 3 to shield/Gnd) to the amp which I've used occasionally with some gear I have.
Foremost the OP needs to be sure the input is actually working, which I tend to believe it would be.

Yes the input Z is about 3k but the transformers have a high secondary resistance and will 'current drive' that low Z input.
This means that the amount of gain lift and the loading on the mic is up with the fairies and cannot be known. Will work ok but expect a pretty weird frequency response!

Dave.
 
Yes the input Z is about 3k but the transformers have a high secondary resistance and will 'current drive' that low Z input.
This means that the amount of gain lift and the loading on the mic is up with the fairies and cannot be known. Will work ok but expect a pretty weird frequency response!

Dave.
I've tried one of those transformers with a dynamic mic into a consumer type camcorder mic input (no clue as to the input Z). It did have a 'weird frequency response' you mention, but also somehow seemed to have a sort underlying ringing sound in the audio which I never figured out. The mic straight into the camcorder with a cable to unbalance the connection sounded more like the mic was supposed to and had adequate gain.
These transformers I have were originally acquired many years ago to use with an old Tascam 246 Portastudio that had 1/4" unbalanced ins that were specced at 100kohm impedance. They worked well with that. They're gathering dust in a drawer now pretty much.
 
I am attempting to acquire a second Gemini PA 7000. That way I can test for similarities and differences. Plus I really like this preamp and I’ll have spare parts and I’ll be able to experiment with upgrading the Op amps. The thing I really don’t know is if this input is balanced or unbalanced ( trs or ts) maybe next time I open it up I’ll get on the circuitboard and do some testing. By looking inside the unit I really can’t tell because the mic jack is in a housing and I don’t want disassemble it. I really don’t mind buying small items and testing but first I need to find out if this input works. I was trained in electronics but I’m an electrician I don’t have any test equipment like oscilloscopes and I don’t have a service manual the manufacture said one was never made for this unit. So hopefully I get the second unit and I can start there. Thanks all.
Mario V.
 
I am attempting to acquire a second Gemini PA 7000. That way I can test for similarities and differences. Plus I really like this preamp and I’ll have spare parts and I’ll be able to experiment with upgrading the Op amps. The thing I really don’t know is if this input is balanced or unbalanced ( trs or ts) maybe next time I open it up I’ll get on the circuitboard and do some testing. By looking inside the unit I really can’t tell because the mic jack is in a housing and I don’t want disassemble it. I really don’t mind buying small items and testing but first I need to find out if this input works. I was trained in electronics but I’m an electrician I don’t have any test equipment like oscilloscopes and I don’t have a service manual the manufacture said one was never made for this unit. So hopefully I get the second unit and I can start there. Thanks all.
Mario V.

From the schematic posted the mic input is not balanced and converting it to a transformerless balanced circuit would be quite a task and could be prey to RFI. You could incorporate a transformer which would need minimal extra circuitry, mainly just a Zobel network.

Op amp changes? The LM4562 is about the quietest of the common bipolar amps (about 3-4dB quieter than the NE5532 and that is no slouch!) but if they are not standard DILs and in sockets? Tricky. If they are Surface Mount you are rather stuffed.

If you are going the full nine and fitting an XLR for the mic you should consider phantom power. You could get it from a voltage multiplier from the transformer or a DC-DC converter block. If you are going to dabble with transistor audio I strongly recommend you get a copy of Small Signal Audio Design. D.Self.

Dave.
 
Thank you Dave. I scored the Pa 7000 on eBay. So I will have the second one to experiment with. I will look into the book.
 
Have to ask, is the mic actually working as i would have thought that connecting it to a guitar amplifier even if using an TRS plug there would have been fairly good volume --- even if not sounding all that great !!!

Before anything else, I would get an XLR to TS lead and try this in the guitar amplifier. A friend of mine (during rehearsals) does this with his Fender amp and has enough volume to easily get above a 20 piece swing band.

Once you know that the mic is actually working then you can start searching for other problems.

The schematic for the Gemini unit definitely shows an unbalanced mic input, so you will require the XLR to TS lead to connect to the Gemini.

Just some thoughts

David
 
Hi David.
an XLR to TS will not give a differential input to the mic and it could be hummy also you cannot guarantee such a cable will be wired correctly (if OP have the means I am sure he would have rolled his own?)

The transformer do three very valuable things...
1) give the mic a differential input.
2) Boost the signal by at least 10dB.
3) Stops RFI which plagues such simple mic stages.

They also cost little more than another cable. Back in the day we used Reslo RB ribbons in the 'group' but these were high impedance types and went straight into the 50W Carlsbro amp (along with two guitars!)

From another,

Dave.
 
I have rung out my xlr to trs cable, looking at the female xlr end 2 is left, 1 is right and 3 is below. And I have 2 to the tip, 3 to the ring and 1 to the sleeve.
The trs to ts adaptor seems incorrect. It has 2 and 3 at the tip and 1 on the sleeve.
I have a xlrf to ts cable coming in the mail, I believe the tip should be 2 and the sleeve 1 and 3, correct me if i am wrong.
The transformer? is this the inline type. Are they available?
 
I have rung out my xlr to trs cable, looking at the female xlr end 2 is left, 1 is right and 3 is below. And I have 2 to the tip, 3 to the ring and 1 to the sleeve.
The trs to ts adaptor seems incorrect. It has 2 and 3 at the tip and 1 on the sleeve.
I have a xlrf to ts cable coming in the mail, I believe the tip should be 2 and the sleeve 1 and 3, correct me if i am wrong.
The transformer? is this the inline type. Are they available?
The connections highlighted in bold in your quote will correctly unbalance the connection into the preamp and typical what I use (home built cables).

Transformer is inline and generally plugged into the jack on the amp or other device you're wanting to match up to. I wouldn't normally use it on the preamp below, but just an example how it would plug in. Hold off getting one until you know the preamp input jack is working, either on the one you have or the one you're getting.
 

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So new (used) preamp showed up and my xlr to ts cable showed up and now I have vocals. Wow, the old preamp, every thing worked but the mic circuit. Weird, but I am not the original owner.. I sure did learn a lot here, as for the op amps they are soldered in so they will stay. I may want to have the first unit repaired, If i cannot figure out what is wrong with it. but for now back burner. I wanna rock! Actually I am an novice guitarist. But now I have more options. Thanks.
 
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