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Messages - Katoda

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16
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
f=1/(2*pi*sqrt(L*C))
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.

17
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.

18
Yes, just buy those plastic jacks, they are way more reliable anyway, IMHO.
Check out musikding, they have low shipping rates for small orders.

19
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.

20
The vulcan circuit is a preamp. Basically every effect is a preamp, considering it amplifies and shapes the signal. There is no difference strictly speaking. That being said, most people are used to the standard gain stage + TMB tone stack, since this FMV configuration cuts mids usually at 400 or 800 Hz, and that sounds pleasing to the ears.


21
If you are just after more distortion, maybe stick some antiparallel diodes in the first op amp feedback loop, or put in a fet boost as the input.

22
Preamps and Effects / Re: Fet version of the JCM800
« on: January 09, 2018, 01:33:18 PM »
Well, I don't think soldering smd components is that difficult, I soldered 5150 circuit with the same topology and components on a protoboard. That being said, I think you can substitute ES1J for any ultra fast high voltage rectifier diode, two paralleled bav70 for a single 1N4148 and the two bat54s for 4 1N4148, but wiser members than me should confirm or deny that.

23
The Newcomer's Forum / Re: First real build. LM386.
« on: January 02, 2018, 08:37:37 AM »
Oh right, sorry, I wasn't looking at the schematic, didn't see that the gain control was actually feedback control. For that configuration you might need an anti-log pot. Also, datasheet specifies you should use a capacitor in series with the pot, but that's not really the problem.
For the pots - I usually use Alpha brand, they're cheap and of all-right quality.

Sorry I couldn't be of more help, good luck.

24
The Newcomer's Forum / Re: First real build. LM386.
« on: December 31, 2017, 11:26:24 AM »
Doesn't really matter what brand you use. But before you order a new pot, you might want to try soldering a 1.5k resistor from wiper to ground, which will somewhat simulate the log curve.

25
Amplifier Discussion / Re: Peavey Studio pro 40 has squeel
« on: December 28, 2017, 03:00:54 PM »
DC voltage on speaker is definitely not good. I think something is causing the power transistor on the negative rail to conduct more current than the one on the positive rail.

26
Tubes and Hybrids / Re: Peavey VTX Classic 212 65w Need Help???
« on: December 28, 2017, 04:04:31 AM »
Ok, so for checking the DC voltages:
If you look at the schematic, you will see that each op amp has +15 and -15V connections, which connect to the pins specified on the schematic (like U1 has them on pins 4 and 8). Keep in mind that U1A and U1B are the same chip, just different sections. When you locate the op-amp power supply pins, you connect the black probe of your voltmeter to ground (or chassis) and check those pins with the other probe. The voltmeter must be set to DCV, with the range set accordingly.
You check the power supply board and tubes the same way. Basically, if it says there should be DC voltage on the schematic, you should locate that point on the layout and measure it. If it differs from whats written on the schematic a lot, you might have found the problem.

Wow, seems like a cool job you have there. Are you trying to make us jealous?   :)


27
Amplifier Discussion / Re: Bass amp input tweaking
« on: December 28, 2017, 03:20:14 AM »
Well, you could, but if you've already replaced the caps, there wouldn't really be any point and/or audible difference as far as bass is concerned.

If you are just looking to do some tweaks, I'd try using some LEDs as clipping diodes at the IC2A output, but maybe put the diodes in series with a capacitor, which would limit the clipping to upper frequencies.

28
Amplifier Discussion / Re: Peavey Studio pro 40 has squeel
« on: December 26, 2017, 04:38:49 PM »
12/26 update. I reordered the exact reverb tank I needed. It's a little better.  This may be a clue: The channel 2 input is fine. The channel 1 input (the louder one) is where the problems comes in. If I had to describe it, I'd say its overloads and shuts the signal off. The symptom is only when the "saturation" and the "reverb" controls are past 3/4. Maybe its ok????  Any opinions.

Hmm, perhaps there is something wrong with transistor bias, perhaps there is DC voltage on one of the pots which would mess with that. A leaky cap maybe?

29
Amplifier Discussion / Re: Bass amp input tweaking
« on: December 25, 2017, 03:28:29 AM »
Yes, it is indeed a high pass filter, C1 and C2 in series along with R19 give the rolloff frequency of 67 Hz, which isn't that low. The op amp configuration even flattens that a bit. The circuit probably adds the lowest of frequencies later, especially if it has some distortion or clipping afterwards. If you want more bass in the preamp, you can replace C1 and C2 with 22nF caps, which would yield a cutoff frequency of approximately 33Hz.

EDIT: C1 is also a coupling capacitor, it prevents any DC from going through your bass guitar, but since the circuit has dual power supply, it is redundant. So instead of replacing both caps, you might want to try just shorting C1, since it would have the same effect.

Hope his helps

Cheers

30
Tubes and Hybrids / Re: LND150 tube emulator attempt.
« on: December 23, 2017, 06:52:43 AM »
You are right about twice the current of course, but there is not a wholotta current flowing there.
I did find out that when you put two diodes in parallel, the voltage drop across the pair is slightly smaller than across a single diode. Maybe that is good for something.
But actually, I think my problem is that I don't really understand the "grid" limiter thing of wich the diodes are part...

The diode conducts when there is a voltage greater than cca 0.7V across the conducting direction.  So when the signal swings in the positive direction, voltage on the source rises. If the source voltage is then greater than 0.7V  to base, the diode clips the signal. The 1k resistor limits the current through the diode, making the knee of the clipping softer.

It simulates the asymmetry of valve clipping.

I hope I'm not completely wrong, somebody please correct me if I am.

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