Can I use SMPS to power bass guitar amp instead of HUGE transformer?

[guitylerham]

Hey everyone, I hope this is in the right forum (solid state).

I am a DIY'er bass player and want to build a tube preamp and mate it with a solid state power amp. I have acquired a non-functioning 250W (@8ohm) pre/power amp head that is meant for bass guitar amplification. The preamp section is toast (is on its own separate pcb) and I'd like to remove it and incorporate a new tube pre inside the enclosure. I've been trying to get into the lightweight realm with class D power section but at the end of the day, I wouldn't save much money building my own from modules compared to buying a commercial pre/power amp unit so I'm trying to innovate with what I have on hand. I'd plan on using a separate but smaller transformer for the tube pre section but I'm curious about using an SMPS module to replace the large and heavy power amp transformer that is in the unit right now. It'd sure save a lot of weight and I'd be able to reuse the chassis for the most part.

Is this possible? I'm trying to educate myself on the ins and outs of switch mode. From what I've gathered, as long as it's powerful enough, an SMPS can run a mosfet poweramp, correct?

Is it as simple as buying a sufficiently powerful module (for a 250W amp, maybe a 500W power supply?) with the correct output voltage and hooking it up correctly? Would I run it through the onboard filter capacitors that were used to clean up the old power supply?

I know switching noise can be an issue. I've been googling for hours but would love some people's discussion on this idea. It's not unheard of for me to try and tackle ideas that just don't make sense and I'm wondering if this is one of them!

What do you all think? I'm in between jobs at the moment so my time value isn't too high

 

[lsheng]

Hooking-up-corrctly has many levels, connectting right according to schematic is one level, treat any wire with parasitic resistance and connet them in a way that have minimum inpact on the sound is one level up, and much difficult. If you see wire and component with their electromagnetic field, and treat them accordingly, that is another level up.

To the last level, SMPS will make good sound. Sorry the advice is not specific.

 

[JonSnell Electronic]

Answer, Yes. Absolutely no issues at all.

 

[mcd99]

Answer, Yes. Absolutely no issues at all.

Only as long as you consider possible galvanic isolation issues.

 

[brig001]

Not all SMPSs like a capacitive load, so you may have to add a small choke between the SMPS and the amplifier board (depending on what capacitance there is on the rails). I have used a small ferrite cored choke for this in the past, but can't tell you the value as it was just one from the junk box.

HTH,
Brian

 

[JonSnell Electronic]

Obviously the loading situation must be taken into consideration but a good switch mode power supply will put up with anything that you throw at it. They are inherently very stable and don't care what sort of load is on the end as long as the power supply can produce about 1.3 times what is expected of it. Capacitive, inductive or resistive loads will make little difference as they can detect very quickly changes in voltage rails and therefore loads. Pick a good known power supply design if you fancy making one or just browse the well known auction site for a goodly choice.

 

[JonSnell Electronic]

Only as long as you consider possible galvanic isolation issues.

What galvanic issues? Do you mean galvanometric issues? There is an isolation switch mode transformer and an opto isolator producing stabilization.
Sorry but I don't understand your remark.

 

[brig001]

Capacitive, inductive or resistive loads will make little difference as they can detect very quickly changes in voltage rails and therefore loads.

I'm sorry, but this isn't the case.
Read:
http://www.murata-ps.com/data/apnotes/dcan-58.pdf
FAQ B4 on this page Switching Power Supply - Mean Well Switching Power Supply Manufacturer

There's more, but you get the idea.
To be fair, they also say that inductance is bad, but I found a little bit to be beneficial with a highly capacitive load.

 

[nigelwright7557]

I'm sorry, but this isn't the case.

There's more, but you get the idea.
To be fair, they also say that inductance is bad, but I found a little bit to be beneficial with a highly capacitive load.

I found problems with SMSP with large capacitive loads.

Large capacitors on power up act like a short circuit. An SMPS will allow a short time during power up for a short but not so long as it takes to charge say 10,000uF. In reality you don't need the big capacitors as the SMPS will react quickly enough to keep the volts stable.

 

[JonSnell Electronic]

There is no high capacitive load in a guitar or any amplifier. The power supply doesn't need large capacitors as it stabilizes itself at a rate fast enough to cope will all requirements from a power amplifier. How do you think the Crown Power range and all the other commercially sold power amplifiers work successfully? Most if not all LCD TV screens and Plazma screens have smps, they don't have issues.

 

[brig001]

Sorry, I should have been more specific. I'm talking about the existng power supply smoothing capacitors. As you rightly say:

The power supply doesn't need large capacitors as it stabilizes itself at a rate fast enough to cope will all requirements from a power amplifier.

So, the existing capacitors could/might have to be replaced with a much lower value.

Brian

 

[JonSnell Electronic]

The switch mode power supply will have its own tank capacitors and all that will be required is a couple of 100uF decoupling on the rails at the amplifier end.
If one uses large tank capacitors, the power supply may overshoot and give odd voltages.
Most guitar amplifiers use 4k7uF smoothing or 6k8 at most. Remove them and fit the lower value caps in their place. Don't forget they mainly use a +ve and -ve supply. So use a suitable twin output power supply!

 

[Ian Finch]

Have protagonists for DIY switchmode supplies checked forum rules, note 2?
This isn't the first time the issue has been raised and of course, it does not address current commercial design or practice which most us are well aware of, driven as it is by cost as much as weight.
Even if there is true isolation in the supply design, the distinction between commercial, sealed SMPS products and DIY builds or modifications requiring isolation transformers needs to be made - prefererably in the power supply forum, I should think.

 

[nigelwright7557]

Have protagonists for DIY switchmode supplies checked forum rules, note 2?
This isn't the first time the issue has been raised and of course, it does not address current commercial design or practice which most us are well aware of, driven as it is by cost as much as weight.
Even if there is true isolation in the supply design, the distinction between commercial, sealed SMPS products and DIY builds or modifications requiring isolation transformers needs to be made - prefererably in the power supply forum, I should think.

I don't think the OP was considering making his own SMPS.

He should be ok with a professionally made company SMPS as it will be checked for safety and emissions.

 

[brig001]

Is it as simple as buying a sufficiently powerful module

The OP was definitely asking about COTS, so shouldn't conflict with forum rules.

 

[Ian Finch]

You may be correct there, Nigel. The problem, as a friend found last year with with his Ebay purchase of a couple of 24V Chinese supplies, was that were not fully isolated. An unmistakable shock from accidentally touching chassis earth to the 24V output says so.

Some supplies are not enclosed (e.g. Hypex) and rely largely on user measures for shielding and safety in their fitting, adjustment and use. See the VSSA thread for what I refer to. There is a big difference in the precautions taken by professionaly experienced manufacturers, engineers, technicians etc. to guesswork by DIY hobbyists and those just seeking to make matters easier for their hobby.

DIYs often assume commercial products to be safe but maybe not understanding the documentation or the nature of the products, can they really understand the risks among the many types available?

 

[mcd99]

Ebay purchase of a couple of 24V Chinese supplies, was that were not fully isolated.

This comes as no surprise at all. Some manufacturers just don't care.

Approval marks anyone?

 

[JonSnell Electronic]

Have protagonists for DIY switchmode supplies checked forum rules, note 2?
This isn't the first time the issue has been raised and of course, it does not address current commercial design or practice which most us are well aware of, driven as it is by cost as much as weight.
Even if there is true isolation in the supply design, the distinction between commercial, sealed SMPS products and DIY builds or modifications requiring isolation transformers needs to be made - prefererably in the power supply forum, I should think.

I should not imagine anyone trying to make a smps without an isolation transformer unless copying a design from the Thorn/RBM days of the early Colour TV's. Sensible to point it out as one never knows.
They used a live chassis with an auto-transformer as a choke with a recovery diode and chopper transistor. Dead basic and the isolation was in the aerial socket but it worked!

 

[mcd99]

They used a live chassis with an auto-transformer as a choke with a recovery diode and chopper transistor. Dead basic and the isolation was in the aerial socket but it worked!

Do you mean the socket in the wall or within the TV?

 

[JonSnell Electronic]

Within the TV. The aerial socket used to be an isolator when the chassis all were live. ITT kb Hand wired colour tv's were nice to work on as were the Thorn EMI 2000 range.
Workshops didn't have isolating transformers either so the chassis went to Neutral. The newer Philips Monochrome tv's used a bridge rectifier at the front end and the chassis was always half live! That was the start of smps units in tv. Circa 1975