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Messages - Katoda

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The Newcomer's Forum / Re: Mosfet JCM800 by KMG
« on: October 02, 2018, 02:16:41 AM »
If you want to use the pcb layout from the project site, there is a way to transform that pdf into an image and then turn that image into a gerber.

The Newcomer's Forum / Re: Guitar plus iPod inputs
« on: February 08, 2018, 01:29:38 PM »
You mean to plug both of them in the same input? It would probably work, but not very good. I'm guessing that part of the iPod output signal would prefer to travel through the lower impedance that your guitar pickups provide, which might cause problems. You could build a buffer for each of them and mix them after it. That would provide some isolation.
Still, don't expect the results to be very good, guitar and line audio have quite different characteristics in terms of the signal processing needs.

Amplifier Discussion / Re: Opamp slew rates
« on: February 06, 2018, 07:56:11 AM »

Slew rate is how fast can the op amp change from low to high, so 4 V/us means it can get from 0 to 4 V in one us. It is important in high speed switching, but not so much in audio. If you have rail to rail signal swing, say 30V (not in real life), it would take 7.5uS to swing it (7.5us of rise time). That is far beyond audio range, since frequency = 1/Tr.
You don't need that speed in audio, much less in guitar audio. For the sallen-key filter , you can just throw in any op amp you have, it won't make much of a difference.

EDIT: Terribly sorry for spreading misinformation, after a thorough reading I realized I was wrong. Apparently for the sallen-key filter, the gain bandwith product should be greater than 100*fc. Did not know that. So uA741 is out, but the others are good replacements.

The Newcomer's Forum / Re: First real build. LM386.
« on: January 31, 2018, 11:23:15 AM »
It is just a plastic jack, no need for a name. You get these mostly everywhere they sell components, from Ali to Farnell and everything in between.

The Newcomer's Forum / Re: First real build. LM386.
« on: January 31, 2018, 09:26:57 AM »

A jack like this one, but a mono version. Or stereo, doesn't matter. Plastic 6.3 mm jack

:lmao: I wasn't expecting that at all  :)
If everything is securely in place and it won't move, then I guess it's OK. If the brown empty pcb is intented for the preamp, then you might have some mains hum when it gets amplified through it. Keep the input signal as far away as possible from that psu.
+1 for creativity  :)

Yes, that's where the preamp will go. I'll move the input jack farther away from the PSU and the PCB input will sit right underneath it, so the wire will be a few millimeter long, like 12 mm or so. Hope that works just fine. I guess I should have mounted the PSU with the mains facing the opposite side. Will fix that. Man, I appreciate your input, really do, Thank you!

No problem at all. Good luck with your build then  :dbtu:

 :lmao: I wasn't expecting that at all  :)
If everything is securely in place and it won't move, then I guess it's OK. If the brown empty pcb is intented for the preamp, then you might have some mains hum when it gets amplified through it. Keep the input signal as far away as possible from that psu.
+1 for creativity  :)

Cheap practice amps are just that - cheap practice amps. The design goal is the same in all of them - lowest cost possible. That's why there are op amps with clipping diodes, fets are more expensive.
At least that's what I think...

Crap, i was hoping there would be a cable... I wouldn't recommend mounting it inside the chassis, the coils would probably radiate too much for such a small enclosure. If the smps circuit has a transformer (as in - not just an inductor), you could fit in in another enclosure, one that would provide a connection to earth, but that's quite an advanced challenge, because you're dealing with potentially lethal voltages there.
If it's just a buck converter, there's nothing you can do but to buy another PSU. No mains isolation is a no-no.

Yes, the earth should be connected to the negative terminal, the ground.

Awesome, now that you know the source of the hum, you can take measures to eliminate it. It might be as simple as opening the hard drive PSU and connecting the chassis to the output ground, or as annoying as having to buy/build another PSU, if the current doesn't even have a 3-pronged mains cable.

If you are feeling adventurous, you could open the hard drive PSU, desolder the current 2-prong cable and replace it with a 3-pronged one, connecting the mains earth to the output ground (be careful if you do that, small mistake could mean a large fire and/or electrocution!!!)

No problemat all, we're all here to learn :)

If the hum is mainly 100Hz-ish, the smps switching noise might not be the problem after all, since they operate at higher frequencies. If you get the power from a switcher, the ground is probably not connected to the "earth" connector of the mains. That would mean that you could be picking up mains noise from everywhere around you. Does the amount of noise reduce if you touch the ground with your finger? If it does, there's your problem. If not, the input might still be picking something from the mains, if it is not completely encased in grounded metal.

I didn't mean that it is better (or more usable for that matter) to have 4 conductor shielded cable, I just wanted to say what to expect. You can, however, use two or more of the conductors for the pots in the same signal path, like on a volume/gain pot - one pin is ground, and the other two can be soldered to different wires inside the cable. It just saves some space and results in a cleaner looking build, but the downside is that there is a lot of tension on the individual cores (if the plastic insulation is too thick), which might result in breaking wires.

Your cable wiring is correct, the shield is ground and the centre is the signal. You must watch for ground loops here, make sure that there aren't multiple connections to different grounds from any single point.

A choke is an inductor, a coil of wire wrapped around a magnetic material (ferrite, in this case). If the switching noise is really the problem, then try googling LC low pass filter. Because you dont know the coils inductance, it might be a hit or miss, but taking that toroidal psu coil and soldering some leads together, so it has all the wraps around the core in series and in the same direction, and then placing it in front of your amps power supply (so that it is followed by paralleled 1000uF and 100nF caps) might help a bit.

Keep the input of the power supply short and away from the preamp, as the switching from the external unit might be creating some quite strong EMI.

EDIT: The -3dB cutoff frequency of an LC filter is given as
So if the switching frequency is the same as the LC cutoff frequency, the power supply ripple will be 3dB smaller. Now, electrolytics have higher ESR and ESR, so for effective high frequency decoupling you want ceramics. A lot of them in parallel. Measure the coils inductance if you can, and then try to make the cutoff frequency a couple of times lower than the switching frequency.

Yes, definitely use shielded wire. If you're a cheap scavenger like me you can reuse some old usb cables, they have good shielding and 4 cores.

Since you are using a hard drive (probably switching) power supply, you need to filter the hell out of it, using big electrolytics paralleled to small ceramics, maybe even throw a choke in ( you have some of these if you raided the computer PSU ). Otherwise there will be noise in the circuit. Perhaps you could even add in a small LDO voltage regulator for the preamp circuit to minimize the noise.

Yes, just buy those plastic jacks, they are way more reliable anyway, IMHO.
Check out musikding, they have low shipping rates for small orders.

The Newcomer's Forum / Re: Charging questions
« on: January 24, 2018, 04:31:11 AM »
AFAIK most chargers supply pulses at a certain frequency, varying the amount of current and voltage that goes into the battery in relation to the charge. Connecting a battery straight to DC might damage it. If you do decide to put a charger inside an amp, be wary of possible EMI issues, as the pulsating high current will probably radiate a lot of interference.

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