JLM mic pre

[Flatpicker]

[quote author="tk@halmi"]There is no place for the output transformer on the JLM board simple as that.[/quote]Oh, I see, it's just stating that the transformer is not on the pbc. When I glanced at it last night I thought it was saying not to put it on the board (sheesh, I must've been sleepy!)
[quote author="tk@halmi"]The 99V has a low open loop gain compared to the 2520 or the JH990 and you run out of feedback at higher gain settings. Thus the recommendation to double them up in most cases.[/quote]But on the schematic it looks they are running in some kind of balanced mode, not adding gain stages... :?

 

[Flatpicker]

But wait... they are adding gains. This circuit gets more interesting the more I look at it. Pretty cool, actually.

 

[jrmintz]

I emailed Joe asking about the 99V in an API 312. I don't think he'd mind me sharing his reply:

The 99v will work in a 312 but the 99v can only do 36dB of the gain
on its own. So R2 minimum value is 270ohm instead of the 200ohm if 20k is
fitted from the output to the negative input. Also a output cap is needed to
remove the +2v DC at the 99v output. A 470uF 10v or higher would work fine.
It will run of the +/16v but I would recommend +/-28v to +/-34v rails. So
with full gain and the 2622 input transformer primaries in series / output
2503 as 1:3 you would get about 58dB of gain and with the 2622 primaries in
parallel / output 1:3 would get 64dB of total gain.

:thumb: :sam:

 

[SonsOfThunder]

PP,

I like your "mic cable". Very hi-end!

Seriously, I want to build a couple of these BAD!

Peace!

 

[adrianh]

PP

I second that!
A real classey SM57 connector.

Thanks for the kick.
I need to send some money to Brisbane AU to Joe.
Anyone want to share the shipping costs?
I am game in the next 30 days.

 

[peter purpose]

How dare you..!!
That is my new (patent pending) twisted pair, unshielded, C37 clamped (it really opens up the soundfield), monster grounded (can I say monster anymore?) ÜBER CABLE ?.

 

[peter purpose]

SEND JOE YOUR MONEY

 

[Kev]

[quote author="tk@halmi"]I love the JLM boards ....
He even makes a very affordable 1:4 mic input transformer. The man is a DYNAMO![/quote]

correct
:thumb:

 

[Bauman]

The 99v will work in a 312 but the 99v can only do 36dB of the gain
on its own.

The 2.5.2.0 is set to 40dB on 312 so it's pretty much the same thing and I like the idea of
1:3 at output...
Now I'm very curious to try one of these 99V.... :razz:

 

[soundguy]

[quote author="peter purpose"]
IT'S LARGE, round, thick and juicy.

Only shoved my voice on a 57 in front of it so far, but it sounds great.
[/quote]


Told ya...

dave

 

[JLM Audio]

Thank you for all your kind words :grin:

How to fit 99v to 312

The JLM99*MB can be configured as an electronically balanced output using the second 99* as a unity gain inverter to drive pin 3 of the XLR. This will give you a 6dB gain increase and 6dB more headroom (+34dBM out).

But you really only ever need an unbalanced output and balanced input for noise cancelling to work as long as you wire pin 3 to pin 1 ground direct or by a 51ohm resistor at the output XLR. So I prefer to use the second opamp and share the gain so both opamps are not working so hard.

This also allows for Passive EQ and compressors or both to be built in between the two 99* opamps. We will soon have a simple one knob opto compressor kit that will plug straight into the small 3 pin jumper plug in the middle of the JLM99*MB PCB. This will only put one resistor extra in between the 2 99* opamps and one LDR to ground so the audio path stays pure. The small PCB mounted to the gain reduction pot will be a simple side chain with the optos for audio and gain reduction meter. You will just need to wire power to it and a meter.

:sam: :sam:

Joe

www.jlmaudio.com

 

[PRR]

> But you really only ever need an unbalanced output and balanced input for noise cancelling to work

Nope, if it ain't BALANCED it can't possibly sound good. Or so a LOT of people seem to think.

I mostly work unbalanced (without even faking a balanced impedance), got paid, clients mostly happy, and when they weren't it wasn't about the balancing. But that ain't hip no more.

I do like differential inputs though I don't use them as much as I should.

The few times that single-ended out to differential input wasn't enough, the real answer was a transformer. I have a couple long lines crossing power grounds in a big hall. One of them gets away with a good transformerless balanced input, no matter what is on the far end (even consumer hi-fi). The other had to have a transformer to be always clean. But these are not small-studio problems.

> We will soon have a simple one knob opto compressor kit

Want it!

 

[PRR]

> output cap is needed to remove the +2v DC at the 99v output.

Joe-

As long as you have DC anyway, and need caps, have you ever pondered this differential amplifier?

Actually it works quite well without caps (or maybe the blocking cap on the gain-set so the DC error isn't multiplied). But with caps as shown, it models well with opamps powered from 0V/+48V and with 0.6V DC offset voltage in the "opamps". The nodes wind up at several voltages but not enough off to crimp headroom. And (if EIN is low) it is good differential input, good CMRR, without input iron. Some of these values depend on the amp you use: U4 sees 660Ω||Load, which could be too heavy for some amps (I sketched an AB output to handle it). The 3K input resistors are for fat BJTs; FETs could use much higher-Z input coupling.

Compared to the classic 3-amp diff-amp, it saves an amp. Without the flaws of resistor-input 1-amp or 2-amp diff-inputs. A drawback is that gain can not can not be set less than 1:2 or +6dB, so input swing has to be less than half the rail voltage, making it pointless for Line Input. But as a mike input, with low-EIN amps, it seems interesting.

 

[JLM Audio]

HI PRR
Yes when it comes to very long runs things can get tricky. I meant more for normal short run studio use. Having worked for our Telecom and balancing hybrid transformers to convert 2 wire phone lines to 4 wire send and receive systems that ran several miles it used to always amaze me what would and wouldn't work each time in what seemed like the same situation.

We use this circuit you have shown for the input of our JLM BAM1 monitors but with no caps and no Rg. The bias point is just ground in the monitors as a 12dB HPF filter follows this and its caps isolate any DC offset. It works very well.

We have some of the BAM1s running on long feeds in a studio here. The cabling there gets lots of dimmer noise due to electronic switching dimmers controlling 240v to 12v drop transformers (which are in the control room) for the 12v halogen lamps but you cannot hear any of it with your ear to the tweeter. Unless you unbalance the input to the speaker which brings the switching noise up to a very loud audible level. Checking one leg of the cable with a Cro is scary.

Joe

 

[peter purpose]

Joe,
Before I order the parts, is there any reason why I shouldn't run seperate pots for each amp?
This would deal with the age old output level problem.

peter

 

[JLM Audio]

Hi Peter
Not sure why you want two seperate pots as minimum gain and maximum gain will still be the same if they are the same value pots as the dual one. But there is no reason not to do it otherwise.

But on the other hand using a switch will let you reduce the minimum gain down to about 18dB less as each opamp can be turned down to unity gain then by open circuiting both or one gain pots.

If you want less gain you can also add a switch to switch between Output A (Dual 99v) and Output B (single 99v) on the JLM99*MB.

Joe

 

[peter purpose]

Hi Joe,
I was thinking more of driving the first amp hard and reducing output from the second, or vice versa.

peter

 

[danielle]

Onya Joe :thumb:
Thanks Peter for reawakening the forum to another gem from an audio legend in Oz.
Looks like it was worth the wait Joe!
What are the tops like Peter, Joe, Soundguy anyone? Do they sizzle, vanilla, cut, slice & dice like a suschi chef, straight silky razor___um!
Shutup Danielle I think everyone gets the idea; don't you know by now that how something sounds is 'effin subjective Goddamit girl! Sheesh!

Regards,
Danielle

 

[JLM Audio]

Hi Peter
It would be interesting to try to see what happens but the 99v has so much headroom that there should not be much colour change until one opamp gets near clipping and/or is at full gain in theory. That's why I think the switch between output A & output B would do what you want. But definitely try it and let us know what you find

Danielle thanks for the legend status :shock: :grin:

The 99v is not lacking in top end at all but it just sounds silky. The main thing is the thickness of the mids which are smooth and lush without any muddiness which makes the whole sound gel together. Well that's my way of trying to explain it in a few words anyway.

Joe

 

[peter purpose]

Joe,
I'll give it a go and report back.

Danielle,
I've only put a 57 in front of it so far and we know that's not the brightest mic in the locker, but there is some serious voodoo going on in the mids.
The best description I can muster is a 'high energy atomic aura'.

:thumb: