JLM AUDIO

A lot of 1540 EQ are Ver 2 unbalanced in and out (NO input and output transformer fitted)
ADM 1540 EQ original schematic & overlay

The input can be modified easily to be electronic balanced in and a output transformer like our JLM111DC added externally wired as 1:2.

1. Cut track to pin 18 on top of PCB
2. Remove R76
3. Check R75 is 3k3 (if not change to 3k3.)
4. Change R94 from 1k2 original to 3k3 with 27pF in parallel.
5. Change R93 to 13k if using a 1:2 output transformer (or to 6k2 if using 1:1 output transformer)
6. Add New resistor from R76 solder pad closest to top of module to edge connector pin 18. 13k if using a 1:2 output transformer (or 6k2 if using 1:1 output transformer)

Depending on the losses of the output transformer the unity level may be slightly high or low in level.
This can be adjusted by making The new resistor and R93 the same amount smaller in value to increase the gain or larger to decrease the gain.

Very nice Joe,
thank you so much for this info

I have ADM mic input and Equalizer modules although unfortunately there's no model number on any of them,
would love to rack them with balanced ins and outs

whoops wrote:Very nice Joe,
thank you so much for this info

I have ADM mic input and Equalizer modules although unfortunately there's no model number on any of them,
would love to rack them with balanced ins and outs

Hope it help anyone racking these

Joe Malone wrote:

whoops wrote:Very nice Joe,
thank you so much for this info

I have ADM mic input and Equalizer modules although unfortunately there's no model number on any of them,
would love to rack them with balanced ins and outs

Hope it help anyone racking these

Thanks so Unfortunately I cant find any info or schematics/pinouts of my modules and I'm completely stuck

here are my modules:

MIC PRE














EQ MODULE

Oooold thread this but im hoping someone is listening in.
I have three of these and have installed the output transformers, Lundahl LL1527 wired 1:1.
I have lots of lundahl line isolation transformers LL2603 that i was hoping to put to use as an input transformer for these. I have tried to put them in circuit but i'm getting A LOT of gain. +4 on the input gets me +16ish on the output.

How would you go about installing an input transformer to these? Are the passive components before the input transformer necessary? Are they an attenuator? I can't read the values properly in the schematic? Padding the input before the input transformer would help with the added gain but would this hurt anything else? Ive tried to change the gain on A1 with a 5k trimmer instead of R76 but at minimum setting i still get the +6dB added by the output amp driving the output transformer.

I added a 47k resistor to ground at the input transformer secondary but not sure if thats the correct value as the transformer im using probably isn't the same turns ratio as the original.
/
Emil

I think i worked it out by myself.
There's a huge pad on the input, 40db. It's extremely hard to read from the schematics.
What i ended up doing is this.
A new pad in front of the input transformer (Lundahl LL2603 1:1) thats attenuating 25dB.The secondary of the 2603 is terminated via 2k plus 1,6nf in series, according to lundahl spec. I rewired the output transformer (Lundahl ll1527) to 1:2. Then i replaced R75 with a 5k trimmer and adjusted to get unity gain. It works on the bench, will need to pass some audio for a while to evaluate.
Any input on this is appreciated. I. don't. really. know. what . I'm. doing. =)

I now have 600r input impedance where the original unit (with transformers) have 10k. I don't see a scenario where this would be an issue though, for how I'm using them.

Edit:
The input pad is 4k7 resistors in series with hi/lo and a 600r shunt across.

studiomollan wrote:I think i worked it out by myself.
There's a huge pad on the input, 40db. It's extremely hard to read from the schematics.
What i ended up doing is this.
A new pad in front of the input transformer (Lundahl LL2603 1:1) thats attenuating 25dB.The secondary of the 2603 is terminated via 2k plus 1,6nf in series, according to lundahl spec. I rewired the output transformer (Lundahl ll1527) to 1:2. Then i replaced R75 with a 5k trimmer and adjusted to get unity gain. It works on the bench, will need to pass some audio for a while to evaluate.
Any input on this is appreciated. I. don't. really. know. what . I'm. doing. =)

I now have 600r input impedance where the original unit (with transformers) have 10k. I don't see a scenario where this would be an issue though, for how I'm using them.

Edit:
The input pad is 4k7 resistors in series with hi/lo and a 600r shunt across.

OK cool sounds like a good solution and you sill have 10k input impedance