I'd buy a kit/pcb, but id also be interested to see and learn from the schematic!

Quote from: Loudthud on April 18, 2018, 12:15:48 AM

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.

Thanks for the tip, I'll check out the site.

I'll look into using two wall transformers, but would that have the same issue with the him?

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.

Thanks for the reply!

Quote from: Loudthud on April 16, 2018, 02:11:48 AM
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.

Got it! I'll up the cap farad's. Shouldn't be too much of an issue with size or price as they won't need too high a voltage rating.

Quote from: Loudthud on April 16, 2018, 02:11:48 AM
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.

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?

Quote from: Loudthud on April 16, 2018, 02:11:48 AM
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.

Thanks for this! I'll run it as you say. I took a look at the Lead 12 schematic and it makes sense.

I'm open to other ideas (15V or 24V mentioned supplies, etc) as well as different power amp topographies, if transistor set up is better suited to wall mount transformers?

I'd like to build a solid state guitar amp. I am confident in the preamp design, and the power amp schematic (as the power amp was swiped from the AC30VR), but it's the power supply for the power amp that concerns me. As I've read, the power supply is the most important factor in a successful power amp.

I'm happy with lower voltages, and I've built a few tube amps, but I'm not comfortable with designing my own layout that deals with mains wiring, so I'm trying to stay away from that. Thus, this AC wall wart idea was born (/stolen from the internet). To circumvent the issue of having mains inside the box, I thought I'd try to power an LM1875 using a 15VAC 2.5A wall wart using the following schematic:

https://imgur.com/a/bISQ7

The other option is a 24V 2.7A adaptor, but I guess each rail is really only going to have 1A available from the 78xxx and 79xx regulators.

So I guess, at +/- 24V or +/-15, is 1A per rail going to be enough? And what might be a predicted wattage of this set up / would the 3875 run on either of those power configurations with any better results?

Thank you very much

Hello Enzo, thanks for the reply. I have a handful of schematics I'll link to tomorrow for what I was looking at that use the mix mode feedback rather than a straight copy of the datasheet/gainclone.

I'll have a look at what the power supply is doing as well! Cheers!

Hola - first post here!

I'm looking at chipamp poweramps and came across some posts about changing them from voltage amps to mixed voltage and current amps in order to make it a little more... forgiving, ala Project 56, Figure 2, at Rod Elliot's Sound Product site.

I won't pretend to understand a lot of what's going on here, but I've been looking at poweramps used in production models, such as the Marshall 8040, and trying to figure out why values were chosen.

I'm somewhat of a novice with LTSpice, so when I try to simulate figure 2 from ESP, I get the 18.8V output without the 8ohm load, but when I make the connection to an 8ohm resistor, I get a square wave at 200mV on the output of the amplifier. So before I can use the equations to figure out output impedance, I need to get that voltage under load somehow.

In any case, does anyone have any tips on how to figure out that voltage before I continue on the path of misunderstanding?

Also, I did what I would consider a pretty thorough search on chipamps set up like this for guitar amps, but I can't seem to find many comments any projects posted here, or elsewhere. Of course there are a ton of people using the datasheet examples of chipamps for power amps, but nothing similar to this. I thought that was a bit odd as prevalent as these chips are in current commercial amps. So if there is something similar going around, I'd love to know about it! Thank you.