Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Please login or register.

Login with username, password and session length
Advanced search  

News:

SSGuitar has moved to PHP 7.1, enjoy the speed!

collapse
Pages: [1]   Go Down

Author Topic: Is it enough power, cap'n?  (Read 468 times)

HamSandwich

  • SSGuitar Apprentice
  • **
  • Chip Points: 0
  • Posts: 5
    • View Profile
Is it enough power, cap'n?
« on: April 15, 2018, 08:42:01 PM »

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
Logged

Loudthud

  • Elite SSGuitarist
  • *****
  • Chip Points: 29
  • Posts: 185
    • View Profile
Re: Is it enough power, cap'n?
« Reply #1 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.

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.
Logged

HamSandwich

  • SSGuitar Apprentice
  • **
  • Chip Points: 0
  • Posts: 5
    • View Profile
Re: Is it enough power, cap'n?
« Reply #2 on: April 16, 2018, 06:40:06 PM »

Thanks for the reply!

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.

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?

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?
Logged

phatt

  • Legendary
  • ******
  • Chip Points: 217
  • Posts: 1857
    • View Profile
Re: Is it enough power, cap'n?
« Reply #3 on: April 17, 2018, 07:43:52 AM »

I Quote from Data sheets;
"The LM1875 delivers 20 watts into a 4Ω or 8Ω load on ±25Vsupplies.
Using an 8Ω load and ±30V supplies, over 30 watts of power maybe delivered."

You may find it's a lot easier to just stack two 15VAC wall warts and you can run split rails.

15VAC x 1.4 will give you around 21-0-21VDC rails.
As long as they are 2Amp units then that will give you at least 10Watts into 8 Ohm, maybe more if the current ability of the wall warts is higher.

There is a catch with voltage doublers. xP
There is more current loss than a full wave bridge, not sure of the numbers but your 2.5Amps is the rating of the secondary voltage which is AC volts. BUT you have to De-rate that Current ability depending which type of Rectification you wish to use.

I think the current limit on LM1875 cuts in at about 4Amps so if your using wall warts of at least 2 Amps it will be ok. Of course more current will give a bit more power but on such a small power chip, I doubt you will notice.
FWIW,
I used a pair of old HP printer wall warts to run a lot of power amp ideas, they were 37 Volts DC and quite large in size and filtered,, no need for extra rectifier parts. I just wired them up to a 3 terminal block giving me instant *And SAFE* split rail supply to test many ideas on my bread board. So the idea is valid. :tu:
Phil.
Logged

Loudthud

  • Elite SSGuitarist
  • *****
  • Chip Points: 29
  • Posts: 185
    • View Profile
Re: Is it enough power, cap'n?
« Reply #4 on: April 18, 2018, 12:15:48 AM »

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.
Logged

HamSandwich

  • SSGuitar Apprentice
  • **
  • Chip Points: 0
  • Posts: 5
    • View Profile
Re: Is it enough power, cap'n?
« Reply #5 on: April 18, 2018, 04:02:43 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.
Logged

Loudthud

  • Elite SSGuitarist
  • *****
  • Chip Points: 29
  • Posts: 185
    • View Profile
Re: Is it enough power, cap'n?
« Reply #6 on: April 18, 2018, 11:53:05 PM »

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

Yes.

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.
« Last Edit: April 19, 2018, 12:05:33 AM by Loudthud »
Logged

phatt

  • Legendary
  • ******
  • Chip Points: 217
  • Posts: 1857
    • View Profile
Re: Is it enough power, cap'n?
« Reply #7 on: April 20, 2018, 05:10:38 AM »

I'm no expert but I had no issues with isolated power (i.e. plug pak/wall wart)

These are the 2 transformers I mentioned which powered many power amp circuit ideas. I can't see any issues *As long as the audio circuit common is connected to a metal enclosure*
AFAIK, IF? the Metal frame of a transformer is bolted to the case and there is no earth then you might have hum issues but that is likely unsafe anyway. xP

As can be seem in pic, these transformers have no earth pins and the case is plastic (Double insulated)
By simply connecting the secondary output neg of TR1 to the pos of TR2 you have +30/0/-30VDC. You now have a split rail supply :tu:
Zero volts becomes circuit common which you ground to the case.


As for Smode supply,, it's a hit and miss game.
Some might work but a lot will induce a high frequency whine on the audio signal and filtering that out is no easy task.
Unless you can find a Sw-mode that is dedicated for audio use I'd be wary. 8|

Those LM1875 chips are found in a lot of small budget amps and the transformers in those are often under rated yet they still punch out a good 10 watts.

A power amp can only output the power that is available from the transformer so the transformer current is the limiting factor.

Let's say a given amp circuit needs 4 Amps to deliver 50 Watts, now if the supply can only deliver 2 amps then the wattage will be more like 25 watts. As the current limit is reached the supply rails droop.
In the case of my two transformers shown that 37VDC drops down to 28 Volts because they can only deliver 400mA each.
But still some of my tests where loud enough to keep up with a drummer.
The transformers only ever ran warm.
I had no hum issues except when I forgot to ground something in the audio path.
Phil.
Logged
Pages: [1]   Go Up
 

* User Controls
 
 
Welcome, Guest. Please login or register.
Did you miss your activation email?
* Recent Posts
Yamaha G100-112 - Journey by Enzo
[Today at 02:27:37 PM]


parts by dlbraly
[Today at 07:56:04 AM]


How to build an amp to run on solar power by dlbraly
[July 18, 2018, 09:22:25 PM]


can anyone find my bike by dlbraly
[July 17, 2018, 10:31:41 PM]


Yamaha G100 112 - Transformer? by Jazz P Bass
[July 16, 2018, 11:03:13 AM]

* Sponsors