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OFFER #2

LAKLAND LH-3 BASS PREAMP

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These are the last photos I did.

I still have this pcb as its one of the best sounding eq/preamps for bass. Active OR passive pickups.

I also have the wiring as a pdf. Very clear and well laid out.

All we need is a schematic, so thats up for grabs. If I had better skills, I would have done this.

Everything except the values of the caps are pretty clear. Pots are 50KB.

40Hz bass
6Khz Treble
Selectable Mids 200-1100 hz with the DIP switch all positions select a different freq.

The trim pot...this provide TONS of clean gain. This makes this preamp so much louder than Bartolini and Aguilar and others. It has serious volume output.


I WILL BE ADDING THE OLD KEN SMITH ELECTRONICS SOON. NOT THE MODERN STUFF, BUT THE OLD EQ FROM THE 80'S THAT HAS THE BUILT IN MUTRON AND OUTBOARD POWER SUPPLY.

I've been cleaning up a few things. Here's my trace. It's been on my "to do" list for a while.

Keep in mind the cap values are not known. I've added some values as examples but do not assume they are in anyway the real values.

It is possible to measure some cap values with any disassembly. C4, C4A, C4B, C4C could be measured by setting the dip switched to all off and just measuring the caps in circuit. (Measure on the PCB not the part and don't push too hard!).

Capacitor C11 (Treble Wiper) could be measured by desoldering the treble wiper wire then measure the part in situ. (Don't pull it from the PCB it is too easy to damage the PCB)

C5 Bass and C10 Mids can only be measured by removing them from the ckt - unless you have access to an LCR bridge which can measure in circuit. It is possible to back engineer C10 from the C4 etc. values and the frequencies. Another way to work out C5 is to use an oscillator and measure the gain (=input/output) at a few frequencies with mid & treble centered and the bass at full, centered, and minimum.

The circuit has a stack of inductors and small caps. Keep you mojo in your duds, they don't serve any audio function. They will be small values like <1uH and < 33pF. They are there to stop pips and stuff from mobile phones getting into the audio. If you want build one you can remove these. Maybe replace L1 with a 1k resistor.

V1.0 to V1.1 update.

I might be able to add a bit more info.

In ChicagoMike's post on he states,

40Hz bass
6Khz Treble

however, he doesn't give a +/- dB spec.

I don't know where those specs originate from, but, if we assume ,
1) Bass/treble spec +/-15dB (a reasonable assumption)
2) Set the mid frequency to the default 600Hz
3) Use the C10=33n, C4A=27n version of my mid cap options for 600Hz

when we choose,
- Bass cap C2 = 68n, and
- Treble cap C11 = 8n2

Then surprisingly we get ChicagoMike's specs!

Bass: +15dB at 43Hz
Treble : +15dB at 6kHz

With C11 = 8n2 the mid-caps need a little tweak
to get the correct frequency centres,

275Hz: C4 = 120n (19.8dB @ 276Hz)
600Hz: C4A = 27n (12.9dB @ 583Hz)
900Hz: C4B = 12n (8.7dB @ 893Hz)
1100Hz: C4C = 8n2 (6.9dB @ 1.1kHz)


Maybe solved?

I forgot this thread!

the eq points come from Laklands adverts. But I never asked if they're truly accurate or how much cut / boost.

I need to get another one of these preamps and really get some good pics to double triple check everything.

interested!

the eq points come from Laklands adverts. But I never asked if they're truly accurate or how much cut / boost.
I need to get another one of these preamps and really get some good pics to double triple check everything.

Good to know the specs come from an ad. I'm highly skeptical matching specs because when I check specs against circuits I know are correct they rarely match. They can only be used as a ball-park.

BTW, I spent ages doing that one - lots of hidden tracks and stuff.

I didn't desolder anything on it since that particular unit I was still using.

I plan on getting a couple more since I do like them and I can take even more detailed shots if need be then.


Happy to post whatever I can that might not be around these parts!

I didn't desolder anything on it since that particular unit I was still using.

I wouldn't go desoldering anything.

One very useful test would to switch-out the mid caps then measure the values in-circuit. That would help verify the values or at least get better estimates for the caps.

So the idea is to open-up all the switches then measure C4, C4A, C4B, C4C.
Don't push the probes in the caps themselves. Try to push on the probes on the PCB.
Once side of the caps are all connected together.

okay, no problem.

I might be able to do that this weekend!