Jensen JT-10KB-DPC for microphone stepup?

[gnd]

Hi.
I have two Jensen JT-10KB-DPC 4:1 stepdown line input transformers , and am thinking if they could be used backwards connected for microphone to get 1:4 stepup?
http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-10kb-dpc.pdf

Thing is I have this tube mic (external power, no phantom power needed) which needs a bit more gain, so my preamp gets a bit noisy at 50dB gain. So I was thinking if I put those 4:1 TX in between, backwards connected, I'd get 1:4 stepup transformer for mic, which would get me 12 dB of gain for free.

Any value in such thinking? Would it work? What problems can I expect?

 

[joaquins]

  It would work just fine but is not optimized for the task, a dedicated mic input would give better performance.

  It's DCR resistance is a bit high for the job and would make just a bit higher input noise, not a big problem but to consider in a real design. I don't know all the specs but there might be some other trap.

JS

 

[abbey road d enfer]

Thing is I have this tube mic (external power, no phantom power needed) which needs a bit more gain, so my preamp gets a bit noisy at 50dB gain. So I was thinking if I put those 4:1 TX in between, backwards connected, I'd get 1:4 stepup transformer for mic, which would get me 12 dB of gain for free.

Any value in such thinking? Would it work? What problems can I expect?

Remember that a step-up xfmr also steps up the impedance; you 150-200r mic would look like 1800-2400r to your preamp. That would likely introduce a loss of about 6dB when meeting the input Z of your mic pre, so instead of 12dB you would end up with only 6 dB additional gain. That really depends on the characteristics of you preamp.

 

[gyraf]

..and core material in line transformers is NOT optimized for the very-low signal signals in mics. You want something with very low hysteresis losses, like nickel-alloys or amorf.

So yes, it would work - but it probably wouldn't sound too good. And that before we start discussing shielding.

Jakob E.

 

[CJ]

what kind of mic

what kind of preamp

 

[gnd]

what kind of mic

what kind of preamp

CJ, mic is SE Gemini, cca 10 years old, tubes are new. Preamp is Joemeek VC6Q going into Neve clone (just line preamp/DAW frontend) then going into converters and into DAW. Joemeek gets noisy at higher gain (with soft vocalist), so idea was to increase mic level before going into preamp.

 

[abbey road d enfer]

CJ, mic is SE Gemini, cca 10 years old, tubes are new. Preamp is Joemeek VC6Q going into Neve clone (just line preamp/DAW frontend) then going into converters and into DAW. Joemeek gets noisy at higher gain (with soft vocalist), so idea was to increase mic level before going into preamp.

So the preamp is a SS device; showing it a 2k Z instead of 200r will increase its noise factor by about 10 dB.

 

[gnd]

So the preamp is a SS device; showing it a 2k Z instead of 200r will increase its noise factor by about 10 dB.

So, I've done nothing then, noise going down 12dB due to Tx, then 10 dB up due to impedance mismatch.

Maybe I'd be better off by building another mix preamp with lower noise, and use Joemeek at line level as eq/compressor.

 

[abbey road d enfer]

So, I've done nothing then, noise going down 12dB due to Tx, then 10 dB up due to impedance mismatch.

Yeah, that's about it.

Maybe I'd be better off by building another mix preamp with lower noise, and use Joemeek at line level as eq/compressor.

I guess the Joe Meek uses a bipolar input stage, that may not be optimized for VLN operation. Several integrated solutions offer superior noise pertormance (THAT1510/12, INA103, SSM2019).

 

[ruffrecords]

I am surprised the Joe Meek is noisy at 50dB gain. Even with an EIN of only -120dBu, the output noise at that gain should only be -70dBu which, at normal monitoring levels,  should be inaudible. Has it always been like this?

Cheers

Ian

 

[ruffrecords]

I just checked the JO Meek specs and its EIN is -125.5dBu at an unspecified gain so I would not expect it to be audibly noisy at 50dB gain. I notice you connect this to the DAW via a Neve clone. Why??

Cheers

Ian

 

[gnd]

I guess the Joe Meek uses a bipolar input stage, that may not be optimized for VLN operation. Several integrated solutions offer superior noise pertormance (THAT1510/12, INA103, SSM2019).

Joemeek uses SSM2017 afaik.

 

[gnd]

I am surprised the Joe Meek is noisy at 50dB gain. Even with an EIN of only -120dBu, the output noise at that gain should only be -70dBu which, at normal monitoring levels,  should be inaudible. Has it always been like this?

I just checked the JO Meek specs and its EIN is -125.5dBu at an unspecified gain so I would not expect it to be audibly noisy at 50dB gain. I notice you connect this to the DAW via a Neve clone. Why??

Cheers

Ian

Joemeek noise at 50dB gain is on a border of being audible, you can hear it at normal level on monitors or with headphones. If Joemeek compressor is turned on, it gets even a bit noisier. It is fullrange white noise, no buzzing or such. After vocal is compressed in DAW it comes out even more.

I really cannot say if it was always like that, I have it since cca 20 yrs, cannot remember. But with loud singer there is no problem, no need to drive it 50dB then. Only when singer is really soft then noise comes up.

I thought a quick hack with step-up transformer would help a bit, but I guess it is not a good solution. Maybe Joemeek VC6Q needs servicing. I have 4 units (3x VC6 - with exciter and 1x VC6Q - with eq), so I can measure them all and compare noise levels. Maybe just PSU caps need replacement.

And I go through Neve clone to make me feel like I get that Neve sound... I don't hear a damn difference, but you know how they say that Neve iron always sounds better no matter what. That Neve was a costly DIY, all original parts including original Marinair Tx, and to not use it I would feel even more stupid than how I feel now using it even though I hear no difference. Yea, studio fun...

 

[ruffrecords]

You may be hearing the noise of the Joe Meek PLUS the Neve. Have you tried it without the Neve in series?

Cheers

Ian

 

[abbey road d enfer]

If Joemeek compressor is turned on, it gets even a bit noisier.

That is predictable; usually compression involves increasing idle gain. I'm afriad you're just reaching the limits of physics.

 

[Monte McGuire]

I am surprised the Joe Meek is noisy at 50dB gain. Even with an EIN of only -120dBu, the output noise at that gain should only be -70dBu which, at normal monitoring levels,  should be inaudible. Has it always been like this?

Cheers

Ian

Perhaps for a finished, processed recording, but a preamp used for production needs extra headroom / lower noise that will get used up along the way to the final mix. 70dB is barely passable. Also, EIN is usually rated optimistically, at maximum gain. Most circuits have a fixed noise component along with a component that varies with the gain, so at lower gains, most mike amp circuits have even worse EIN, chewing away at that already paltry 70dB.

 

[joaquins]

Perhaps for a finished, processed recording, but a preamp used for production needs extra headroom / lower noise that will get used up along the way to the final mix. 70dB is barely passable.

  There is a limit, for a 150Ω mic, 25ºC room you won't get under -130dBu EIN, that's -80dBu noise floor at 50dB gain, better than -70dBu but still there. If you want lower noise than that you have few options, record at -150ºC will give you an extra 4dB. Other option is to use a 50Ω mic and a mic preamp optimized for such low impedance. Is not hard to find a 50Ω condenser, the mic preamp is another story...

Also, EIN is usually rated optimistically, at maximum gain. Most circuits have a fixed noise component along with a component that varies with the gain, so at lower gains, most mike amp circuits have even worse EIN, chewing away at that already paltry 70dB.

  You are talking of EON, the noise referred to the output, that is not affected by the gain setting. EIN is one thing and easier to measure at max gain, EON is another and easier to measure at min gain. With both figures you can estimate the noise at any gain.

JS

 

[Whoops]

CJ, mic is SE Gemini, cca 10 years old, tubes are new. Preamp is Joemeek VC6Q going into Neve clone (just line preamp/DAW frontend) then going into converters and into DAW. Joemeek gets noisy at higher gain (with soft vocalist), so idea was to increase mic level before going into preamp.

Your setup doesn't make sense, and I think that might be your problem and where the noise is coming from like Ian pointed out.

You don need a Line amplifier between a Mic Preamp and the Line input of the converter.
Mic preamp job is to amplify to line level, once at line level it goes straight into the converter input.


If you want to feel confident and have the Neve sound, why not just built a Neve 1290 type mic preamp, and then skip the joemeek and the neve line amplifier?
I guarantee you will not have problems with noise

Otherwise connect the Joemeek directly to the converter input