What kind of solder are you using? No-Lead solder usually has a high melting point. The lowest melting point is 63% tin, 37% lead if you can get it. Soldering irons can vary in temperature. I use a Variac to vary the temperature on mine, but you can use a low cost light dimmer.

Using chokes is no guarantee that your amp will be hum free. A bad grounding scheme can introduce hum that you can hear when there is no signal. In general you need to know how much current will pass through each choke. Once you get  up to the several Amp territory, you are looking at a 5 pound (2 kilogram) chunk of iron and for a dual rail amp, you will need two. That is the case when you pass all the current for the power supply through the chokes.

Link to data sheet: https://www.mouser.com/datasheet/2/389/tda7293-957143.pdf

The TDA7293 is a 15 pin chipamp so it may be possible to only pass the current for the driver stage through the chokes. (This allows the use of much smaller chokes.) This is uncharted territory, you'll need to experiment and you may blow up some chips before you get it right. A simple RC decoupling network might work just as well.

When a solid state power amp clips to the rails, any ripple on the rail is transferred to the speaker. Tube amps do the same thing. It's hard to hear this hum because the amp is as loud as it will go at that point. If you want to eliminate this hum, you need to lower the Voltage to the driver stage so the it "runs out of gas" before the output transistors clip to the rails. This lowers the power output by perhaps 10%.

Another way to eliminate this "slamming the rails" hum is to not let the power amp clip to the rails. You can do this with a Master Volume control, but this only works if the power amp is acting as a Voltage amplifier with no mixed mode feedback. Mixed mode feedback means current flowing in the speaker determines if the power amp needs to "Go to the rail" to make current in the speaker follow the input. (Flame On Non-believers)

I'm not trying to dampen your enthusiasm, but the chances of that circuit working without modification are not very good. If you want simplicity, try a chipamp circuit. Slightly more complicated are the circuits in this thread:

https://www.ssguitar.com/index.php?topic=3747.0

The circuit I posted in reply #7 can be adjusted to accommodate just about any power supply.

Quote from: Aleksandar on May 05, 2018, 05:50:52 PM
What do you think of this design?

http://redcircuits.com/Page123.htm

That circuit has some problems.
1) It's easy to smoke the FETs if you adjust the trimmers wrong. Slight circuit revision would fix it.
2) The reason for the regulated power supply was probably that the circuit Hummed without it.
3) The LM317T in the power supply is a 40V part in a 50V circuit. It could fail and cause the circuit to hum.

Quote from: Aleksandar on May 05, 2018, 05:50:52 PM
It's really simple, I hope its not too simple. But it's a single supply and  that would suit most the preamps in the previous posts.

Single rail amps get a bad rap here. I think they sound more like tube guitar amps.

Quote from: Aleksandar on May 05, 2018, 05:50:52 PM
Also what exactly is the purpose of the bipolar transistor there, and can it be replaced with a fet or mosfet transistor?

Yes it can be replaced by a MOSFET. Some adjustments to the circuit will be required. See the circuit I posted in this thread:
https://www.ssguitar.com/index.php?topic=3747.0 

The first preamp was discussed in these threads:

https://www.ssguitar.com/index.php?topic=304.0

https://www.ssguitar.com/index.php?topic=3373.0

It will run on higher Voltage if you get the right JFETs. You want an Idss of 10mA or less. I highly suggest that you install sockets for your JFETs so you can swap them around until you find some that work. The same generally applies to the second circuit but you may need to swap out some of the trim pots to get everything working.

Quote from: HamSandwich on April 18, 2018, 04:02:43 PMI'll look into using two wall transformers, but would that have the same issue with the him?

Yes.

Quote from: HamSandwich on April 18, 2018, 04:02:43 PM
Would I be better off using something like a 24V 4A DC switching power supply be better? I guess I'd suffer on the wattage of the amp into a 16ohm load pretty badly.

Most switching supplies have a 3 wire line cord (IEC type) and the ground of the DC output is tied internally to the safety ground. This restricts what you can do for a guitar amp.

You may be able to find some kind of little power supply brick that will convert the DC to +/- 15V to run an opamp based preamp. Running opamps off of a single rail power supply is kind of a pain if the preamp is very complicated.

You can run a Bridge type power amp to overcome the 24V restriction. One problem with switchers is that they act badly when overloaded. They shut off and take a couple of seconds to recover. Bridged into 8 Ohms you can probably get something like 25W and not overload the supply.

The one advantage a switching supply can give you is that it will probably work anywhere on Earth.

Quote from: HamSandwich on April 16, 2018, 06:40:06 PM
Hmm I don't quite get this one. The transformer is in the AC wall mount adapter, which is a 2 prong device. It's a class II power supply, so rather than have an earth connection for safety, it has double or reinforced insulation between the mains and the secondary. This is going to be an issue?

Without some kind of ground, there will be a leakage current on the secondary. Inside the transformer there is capacitance between primary and secondary. You can't shield it because there is nothing to connect the shield to. The capacitive coupling will most often be towards one end of the primary. If you can reverse the  phase of the primary, one phase will give lower leakage current. A safety ground just gives something to short the leakage current to. Without the safety ground, you will have hum and may feel a little tingle when you touch your guitar and something that is grounded like a microphone.

I don't know what country you are in. For USA or Canada you can get wall transformers up to 50VA with a ground from MG Electronics (Google them). I've also seen switchers up to 48V with a ground and universal input 100VAC to 250VAC without a switch.

The power supply you linked does not have enough filter capacitance to deliver 1 Amp. Those 100uF caps just past the diodes need to be increased to 1000uF or 2200uF. The 15VAC 2.5A wall-wart transformer will be enough if you limit the load to 8 Ohms or higher. These type transformers may run a little warm and they usually have some kind of internal fuse or circuit breaker. When they die, you have to buy another one.

Special Note: The transformer you use should have a safety ground on the Mains side or the amp will have a Hum that you won't be able to get rid of. Connect the safety ground to your circuit ground at the power supply.

Technically, the power supply is a Voltage Doubler. While some people here might poo-poo the Voltage Doubler, they actually work pretty well. What won't work is running the LM1875 from the +/- 15V regulated outputs. The power amp chip needs to run right from the filter caps. Call those points the +/- 20V outputs. The regulator chips won't be able to maintain 15V on the outputs. It's better to just run the preamp from unregulated power. Just use a decoupling network of something like 470 Ohms 1W and 470uF cap on each rail and that should be enough. Checkout some of the Marshall Lead 12 schematics, that's how they do it.

Search a site like Mouser for DC-DC converters. You can choose isolated or non-isolated, you want isolated. The next most important selection is the input Voltage. 5V and 48V are common but other Voltages are available. You probably want +/- 15V for output but you need to know how much current is needed. Most DC-DC converters come in some kind of brick form but there are also small PC board and DIP packages. The data sheet for the supply will specify a maximum isolation Voltage.

DC-DC converters can be quite expensive. They get used in military and industrial products. They can be found at surplus stores, but you will need to dig up the proper data sheet to get all the data.

No amount of shielding will fix a noisy power supply. Try running the amp from a 12 Volt battery. Next, measure the Voltage to ground from each IC pin and post the results here.

Did you ever notice that the gain equation for the non-inverting opamp has that silly +1 in it? Because of the massive gain of an opamp, the input waveform on the non-inverting input is basically copied over to the inverting input. So what you say... It means that no matter how you distort the waveform in the RF part of the feedback network, a copy of the input signal (the plus one part) gets added to the output so you get a mixture of distorted and non distorted signal. Yes, the famous Tube Screamer does it that way. If you want to eliminate this, you need to use only inverting opamp gain stages.

OK. I didn't think of the implied 2S prefix because there are parts in the USA market that are "Jxxx".

Many USA JFETs have the Gate on what would be pin 3 of the Toshiba part because they use a transistor lead frame where the Collector goes to pin 3. Collector on a transistor die is the back side contact. Don't worry about getting Drain and Source right. They can be interchanged.

That looks good. Where did you find a J103? I don't have any data on that part.

The load in the 12AX7 X-Y photo is just the 10X scope probe DC coupled. Any load like a tone stack would cause a loop in the X-Y trace at just about any frequency. And speaking of frequency, the gain (Vout/Vin) will change with frequency and signal level so you really can't put a single number on the gain. My objective for this project was to show how, by simple means, the essence of a tube preamp can be duplicated. It certainly doesn't duplicate everything a tube preamp does to the nth degree like some preamps that use 50 or 100 opamps does. I hope people that build it will notice and train their ears to hear the difference between the typical soild state preamp and the preamp with tube-like low order distortion.

Quote from: tarahall on March 11, 2017, 09:28:38 PM
Would there be any difference/advantage in inserting the FETs between the resistor(s) R4 and/or R9 and the caps(s) C1 and C3 instead of between the caps and ground ?

When you overdrive one of these stages, there could be some DC accumulation on C1 or C3 because of the asymmetrical clipping. This DC would interact with the DC bias on the Gate of the JFET. It's' hard to say if it would be audible or if it would sound good or bad. Try it.

FYI: The polarity of C3 might be an issue. The offset of the opamp could go either way. Short out C3. If pin 1 of the opamp goes positive, C3 is OK. If the polarity is negative, reverse C3. If you can find a 4.7uF film cap or a Non-Polarized electrolytic, you can use those without a worry.