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February 16, 2025, 08:22:23 AM

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#1
Amplifier Discussion / Re: Ruby amp from LM386M-1 mod...
Last post by blackcorvo - February 15, 2025, 12:55:55 PM
I fixed it.


The main issue was the MT3608 DC-DC boost converter I was using. I think it was inducing high frequency oscillation into the circuit. Subbing it for an XL6009 solved it.
I also isolated the input jack from the panel because I think it was causing a ground loop with the output jack.
#2
Schematics and Layouts / Re: FAL Super 100 4 channel PA
Last post by tonyharker - February 15, 2025, 11:24:13 AM
That appears to be derived from an old (now) RCA circuit. Note the transistor types 40xxx all RCA type numbers.
#3
Schematics and Layouts / Re: FAL Super 100 4 channel PA
Last post by J M Fahey - February 15, 2025, 09:46:22 AM
Quote from: Bandito on February 14, 2025, 04:41:04 PMExcellent! I am East Devon, UK. Done the cleaning. The Amp works but sounds weak and different on channels 1/2 compared to 3/4.
It should.
Channels 3-4 are Instrument (Guitar/Bass) level so  more sensitive, 1-2 are high impedance Line level, think a Grammophone with a crystal/ceramic pickup, a radio tuner or tape recorder output, etc.
Any modern pedalboard would work fine there.

If you want to convert 1-2 to 3-4 specs, replace current 1M pots and 1M mixing resistors with 50k pots and 47k rtesistors, look at inputs in the schematic kindly provided by G1

QuoteHas 3 sockets for speakers.  1 and 2 (straight parallel connection) work, but the slave (3) does not.
1 and 2 are regular speaker outs, 8 ohm impedance *total* so 16 ohm each if using both.



The 3rd one, clearly marked Slave, is NOT a speaker out but a Line out, meant to drive an extra power amp with its own speakers.

Old school design, not surprising in an early 70s amp.

QuoteIs this a "good" amp, worthy of investment?

You should ask your Financial Manager for that.

In my book, for a Musician, Amps (and Instruments and any Musical stuff) are either useful, good sounding, etc. or not.

I reserve the word "investment" for Tesla stock, gold, Bitcoin and similar stuff.

FWIW that amp is quite old style but very well made, working fine (except for some "cleaning" issues, of course) for 50 years now.
#4
Schematics and Layouts / Re: FAL Super 100 4 channel PA
Last post by Bandito - February 14, 2025, 04:41:04 PM
Excellent! I am East Devon, UK. Done the cleaning. The Amp works but sounds weak and different on channels 1/2 compared to 3/4.  Has 3 sockets for speakers.  1 and 2 (straight parallel connection) work, but the slave (3) does not. Is this a "good" amp, worthy of investment?

#5
Amplifier Discussion / Re: Ruby amp from LM386M-1 mod...
Last post by blackcorvo - February 14, 2025, 09:34:09 AM
I'll try playing it through another speaker, I don't really want to mess with dismantling the build because I used the box from the old multimeter for the finished thing.

Just in case, I'll try swapping the J-Fet for another one today (I also ordered more, cus I only have like 2 of them left).
#6
Schematics and Layouts / Re: FAL Super 100 4 channel PA
Last post by J M Fahey - February 14, 2025, 06:25:49 AM
Thanks G1

Looks VERY much like an unauthorized Traynor clone, which also made a similar 4-passive-input mixer PA, YVM something.  YVM4?

Doubt anybody will "repair it for fun" though.

But maybe all it needs is some love and care: clean all pots and jacks with Deoxit or similar good quality product, (NOT WD40), check all wires are intact, etc.
Schematic apparently hints at a plug-in board.
If so, pull it, clean socket with compressed air, squirt Deoxit on contacts and wipe them with clean paper towels, etc.

It "should" work.

Do NOT start pulling parts at random "justt check them", you will needlessly damage the PCB.

A gut picture could help
#7
Amplifier Discussion / Re: Ruby amp from LM386M-1 mod...
Last post by J M Fahey - February 14, 2025, 06:11:00 AM
That FET takes 25V with no problem, LM386 does not.

Your first video had better sound, it used a different speaker.

To my ears your amp now is unstable and jumps into oscillation, giving an ugly buzzy sound.

IF you have a scope, check that.

If not, pull board from new one and mount it in the old multimeter one and check.
#8
Amplifier Discussion / Re: Ruby amp from LM386M-1 mod...
Last post by blackcorvo - February 11, 2025, 10:00:53 PM
And here it is, finished!


Although I'm suspecting I should replace the MPF102 because there's some strange underlying distortion I didn't have before. I accidentally had the DC-DC converter board set to 25v and fried the first LM386 amp board, but I'm not sure if the J-Fet didn't also get nuked. I'll do it tomorrow cus it's midnight right now.
#9
Schematics and Layouts / Re: FAL Super 100 4 channel PA
Last post by g1 - February 11, 2025, 04:02:59 PM
Maybe this one (corrected version)
#10
Amplifier Discussion / Re: How to setup headphones an...
Last post by J M Fahey - February 11, 2025, 01:12:36 PM
You trust our divination skills too much  8|

WHICH Amp? Brand and model?

Post a picture of headphone out.

Is it a Guitar or a Hi Fi amplifier?

Show the speaker output jacks/terminals too.

Any speaker selector switch on the front panel?

LOTS of questions.