JLM99*MB Kit documentation

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baadc0de
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Re: JLM99*MB Kit documentation

Post by baadc0de »

Ah, I see, it's "up to 36V" for a single (71) and up to 18V for a double (72). Thanks for all the help.
Last edited by baadc0de on Fri Sep 16, 2011 4:37 pm, edited 1 time in total.

baadc0de
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Re: JLM99*MB Kit documentation

Post by baadc0de »

I will be using this preamp to interface either to modern A/D converters or vintage gear like a pultec or la2a. Is it better to install an output transformer (I thought of going with Jensen) or leave the circuit as - is without the optional transformer? I understand the signal post R26 and R27 are already inverted one to the other?

I was thinking of going JT-123-ALCF

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Re: JLM99*MB Kit documentation

Post by Joe Malone »

baadc0de wrote:I will be using this preamp to interface either to modern A/D converters or vintage gear like a pultec or la2a. Is it better to install an output transformer (I thought of going with Jensen) or leave the circuit as - is without the optional transformer? I understand the signal post R26 and R27 are already inverted one to the other?

I was thinking of going JT-123-ALCF
This PCB can be configured for the second opamp to be a gain stage (non - inverting) which uses a dual gain pot and output pot or inverting output which gives 6dB extra gain and only has a single gain pot and no output pot option available. Either way will work with modern A/Ds
Joe :-)
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baadc0de
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Re: JLM99*MB Kit documentation

Post by baadc0de »

Hi,

finally putting this together... took a long time to get 990c from JH, but now am itching to get this done and hear it in action :)

I have your 10k dual input potentiometer, lundahl 1538 equivalent from haufe and two 990c opamps from JH installed. I've followed the schematic, but also compared with this BOM:
http://www.jlmaudio.com/Stuffing%20Shee ... %20Pre.txt

and there are certain discrepancies. Since i'm a newbie when it comes to understanding how these values affect the preamp, I thought to ask..

On the BOM:
R3 is not mentioned, but 10k in schem
R6 is 100, but 120 in schem
R8 is 33 but 68 in schem
R11 is 0 (link) but 10 in schem
R12,14 are omitted but 1M in schem
R13 is omitted but ? in schem (I assume link goes here?)
R15 is 1k8..1k but 2k in schem (BOM talks about gain, should I put 1k in there if I want high gain for ribbon mics?)
R17 is omitted, but 22k in schem if inverting jumper is installed?
R19,20 are omitted but 2.2M in schem
R13 is omitted but ? in schem (I assume link goes here?)
R22 same as R15

C19 is bipolar 100uF, but here is no C19 in schem .. I assume that's a typo since there are two C9 in schem.

Could you explain the differences or tell me what I should pick as values for a clean sounding ribbon preamp with the options mentioned above? I don't know what setting (invert or not invert) is preferable for this application, so I would like your advice on that as well if possible.

Thank you very much in advance,
B.

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Re: JLM99*MB Kit documentation

Post by Joe Malone »

The BOM is correct for Dual99v mic pre with no dual DC servo and will work for the LL1538 and 990c with no DC servo as long as you pick the correct Rload Cload RZobel CZobel parts for the transformer you are using. C9 and C19 are both marked C9 on the PCB but are the same value part anyway.

All parts not listed on the BOM for Dual99v mic pre are not fitted for NO dual DC servo. If you want to try Dual servo you just need to the download the circuit from Jensen Transformers web site and cross match the the part numbers to add the extra parts.
Joe :-)
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baadc0de
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Re: JLM99*MB Kit documentation

Post by baadc0de »

Oh yeah that clears up things indeed :) should have known it would be that way because that one didn't include the third opamp.

Yeah I'd like to try DC servo very much.. I assume this is the schematic I should reference with.

I'll look into it and come up with an alternate BOM that we can discuss a bit if you would be so kind.. I've very rarely customized other people's work so I need my hand held a bit :)

Cheers,
B.

baadc0de
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Re: JLM99*MB Kit documentation

Post by baadc0de »

Ok I'll just jot things down as I see them and maybe you can correct me if there are things I didn't catch.. or thought wrong.

* R10 is the loading resistor, I assume both jensen and lundahl being of the similar quality, impedance and turns ratio should be about equivalent or at least the loading resistor specified is a good starting point. I'll add a socket here and put something between 6k and 7k5 in there, then experiment from there on the scope.

* zobel networks are usually omitted with lundahl transformers, so I'll just omit them too

* R13 seems to be 15k in the jensen schemo. Let's go with that.

* since in the jensen schemo the output of the opamp is only connected to the feedback loop and the input of the second opamp, I'll just omit R24, C23 and R26 so output A is unused. Actually I could make R26 a wire link so that output A is at ground reference for the output trafo

* the local filters for V+ / V- of both opamps can stay there unmolested, they're probably there for a reason

* the feedback loops seem to be pretty different.. but it's built to use a dual 10k rev log pot in your version that I have here, so I'll just build your version and see if it works. I guess it kind of interacts with the input to the second opamp with R16, so I hope removing R24 and load that would be present there will still work for me.

* R21 should be 300k-ish

* In the output of the second opamp, I could go with R25 = link, C24 = link. Now I also need to find an output transformer.. I understand this could use a 600:600 transformer since the 24V opamps should be able to move quite a bit of juice and will work well with that kind of load..

Thanks for looking this through :)
B.

For starters I'm going to use this unbalanced (and measure DC offset first) but later I would give it a JT-11SSP-8MA output transformer from jensen :)

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Re: JLM99*MB Kit documentation

Post by Joe Malone »

From a quick read that all seems correct.
Joe :-)
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mac
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Re: JLM99*MB Kit documentation

Post by mac »

Hi Guys,
I know this thread is a bit old....but it's relevant to my current build.

I have the JLM99*MB kit and am putting it together with a pair of Fred Forssell fet992 DOA's... Looking to use an OEP on the board, so Joes schem for the dual 99v with the OEP provides a great starting point.

also found a schem from Fred which shows the servo option, which I am implementing ......

http://www.forsselltech.com/media/attac ... ematic.pdf


So now some questions.

I want to implement a High/ Low switch with a DPDT toggle, which throws the ratio jumper on the OEP. CL will remain fixed at 220pf for both ratios, but the table on the JLM website suggests 27k for the low ratio, and 100k for RL in the high ratio. Any comment on how critical RL would be here, or should I switch via a relay that throws the jumpers and the RL values?

Fred's schemo uses 33pf for c10 and c20, and 10k at r16 and r23, so I shall use these values.

R13 and R21..... The Jensen schematic discussed earlier in this thread applies 15k and 301k in these positions, but Freds design is using 49.9k for both positions.... (I don't think I understand this at all...)

R24 and R25 are slightly less at 49.9 ohms (which is probably insignificant), however the Forssell design omits the output caps at C23 and C24, which I presume is because there is no output transformer. I am planning to do listening test without transformers and then with a few different trafos. Should I omit these?

Mac

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Re: JLM99*MB Kit documentation

Post by Joe Malone »

The circuit you have linked to is a fixed gain line amp which is very different to a variable gain mic pre circuit on the JLM99*MB.

The normal values in the parts list for the JLM99*MB will work fine with the 992 opamps with DC servo as well. No changes needed.

You can switch the input impedances and leave 27k load if you want 150/600 input impedance switching.

If you test the outputs and there is no DC offset which means the DC servo is working correctly then the output caps can be left out.
Joe :-)
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slb1959
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Re: JLM99*MB Kit documentation

Post by slb1959 »

Hi Joe.

This circuit JLM99 works in a +/-16v?

I sent you a email about Go Between Plus.

Thanks, Luís

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Re: JLM99*MB Kit documentation

Post by Joe Malone »

slb1959 wrote:
Mon Feb 19, 2024 6:09 am
Hi Joe.

This circuit JLM99 works in a +/-16v?

I sent you a email about Go Between Plus.

Thanks, Luís
The JLM99*MB pcb runs the JLM99v on whatever power rails you send in which used to be +/-24vdc and then makes +/-15v rails for the dc servo ic. You could just run in +/-16v power in and link over the dc servo regs so it all runs on +/-16v just fine.
Joe :-)
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