Small Transformers for stompbox power amp

[TheWinterSnow]

I am looking for transformers so that I can build a small high powered as possible Hi-Fi Solid State power amp for recording sine wave sweeps through a speaker cabinet, hopefully something I could fit in a stomp box like the EH 44 magnum or in my case, I really like the 1590X.  Problem is I can't seem to find transformers that have the current capabilities relative to the secondary voltage .

A 20 Watt amp would be fine, I would like to see more of a 50VA transformer with the secondary around 48V.  With a 16 ohm load a 24-0-24 center tapped transformer supply will be pulling no more than 1.06A at 17.6W.  So I need a 50VA transformer with a 115 primary, 24-0-24 CT secondary with a maximum secondary current of at least 1.2A.

Or am I getting everything wrong here, I just need a transformer that can power a 16ohm load to a little less than 20Wrms.

[PRR]

Go to the rummage store and study hi-fi amplifiers.

Even allowing half of a 2-channel amp, I can't imagine a 20 Watt linear amp in a 4x5x2" box. Not just the power transformer, but also the heatsinking. And filter-caps!

I won't say "impossible". The old Crown D-75 was two 37 Watt channels in 17" by 8" by 1.75". It used a super-custom transformer, did some heatsinking via the rack-ears, and was very tightly packed.

There are now switching-amps about that size. As you say, E-H's "44 Magnum" is one. Often offered at $150. Does use an external power lump.

[PRR]

BTW: constant-drive sweep-tests are often done with one Watt, which sure would be smaller.

BTBTW: speakers on some (tube and transistor) systems are designed for amplifiers with low Damping Factor. Specifically: a large open-back cab with high resonance speakers will slump (due to openback) with high DF, but will bump (due to underdamped resonance) with low DF. If you want "the true tone of the speaker", you may need to know some about the amp it sounds good with.

[Processaurus]

Not to deprive you of a project, if that's your aim, but if you just need a little 20 watt amp, I've gotten a couple of these Class D hi fi switching amplifiers that use a switching power supply, and they work very well, based on the Tripath chip:


http://www.amazon.com/LP-2020A-Lepai-Tripath-Class-T-Amplifier/dp/B0049P6OTI/ref=sr_1_1?ie=UTF8&qid=1350381919&sr=8-1&keywords=lepai

[R.G.]

I did a study on a pedalboard power amplifier. The simplest thing I could come up with was a $12 switching power supply that made ?36V? (it's been a while) and fed it to a TDA2030 in a Hammond box. It got the box too hot and I had to stick a heat sink on top, but that way I got 20Wrms in a stompbox size.

More importantly, it included no high voltage AC wiring at all. I strongly discourage beginners from doing any AC power line wiring until they already know for sure they can do it safely. Stuffing AC line power into a stompbox sized package is difficult to do safely, even for someone who has experience with this. And it's not really good for the noise and hum on a pedalboard to run AC power out to the board. Far simpler to encase the safety issues in a tested-and-certified lump.

And on top of that, your original comments are right - it's hard to find transformers of the right voltage and current for something like this.

[TheWinterSnow]

BTW: constant-drive sweep-tests are often done with one Watt, which sure would be smaller.

BTBTW: speakers on some (tube and transistor) systems are designed for amplifiers with low Damping Factor. Specifically: a large open-back cab with high resonance speakers will slump (due to openback) with high DF, but will bump (due to underdamped resonance) with low DF. If you want "the true tone of the speaker", you may need to know some about the amp it sounds good with.

Would one watt be loud enough?  I have a hard time believing that a one watt solid state amp could drive a guitar amp cabinet to volume loud enough to me mic'd and still have a decent S/N ratio.

As for dampening, I can't really say, I know that I will be trying to get as much of a transformer coupled tube power amp, so it will need a lower dampening factor.  Not to sure how I would simulate that with a SS amp/chip without increasing the output impedance of the amp which would be bad

More importantly, it included no high voltage AC wiring at all. I strongly discourage beginners from doing any AC power line wiring until they already know for sure they can do it safely. Stuffing AC line power into a stompbox sized package is difficult to do safely, even for someone who has experience with this. And it's not really good for the noise and hum on a pedalboard to run AC power out to the board. Far simpler to encase the safety issues in a tested-and-certified lump.

And on top of that, your original comments are right - it's hard to find transformers of the right voltage and current for something like this.

.

I have a degree in electrical engineering, and have been modding tube amps for a few years so I know my way around AC.  Indeed though I would like to stay away from AC if possible, but for more high powered stuff the chances of you getting away from it become more slim.

As for finding transformers I haven't been able to to find anything for audio amps, I guess companies making audio amps are making their own in house or are getting custom transformers made.  I am soon to make a mic pre from an old behringer chassis that died on me, fortunately it has a 115 to a 24-0-24 toroidal for phantom power so that is taken care of.  As for finding generic generic 100-120 Watt power transformers for tube amps and smaller 20-50 Watt hi-fi amps, seem to be futile, all I seem to find online are mains line converters, industrial power conversion transformers or small signal transformers, nothing in between without getting a bulk set custom made.

Looking at it now, I could make a 2.5W with a pump charge converter or a switching supply and be able to run my 9V adapter as it can handle up to 1.7A and the 2.5W would only be 400mA

[R.G.]

Would one watt be loud enough?  I have a hard time believing that a one watt solid state amp could drive a guitar amp cabinet to volume loud enough to me mic'd and still have a decent S/N ratio.

Most guitar speakers are rated at about 94-100db SPL for one watt at one meter. There's a problem with your mike if you can't get decent S/N out of a 90+db source miked up close.

As for dampening, I can't really say, I know that I will be trying to get as much of a transformer coupled tube power amp, so it will need a lower dampening factor.  Not to sure how I would simulate that with a SS amp/chip without increasing the output impedance of the amp which would be bad

?? There are circuit tricks to make SS amps look like a higher output impedance - which is the same thing as a lower damping factor, two views of the same animal - and why is it that a higher output impedance as such would be bad?

I have a degree in electrical engineering,

That's a good start, for knowing some theory.

and have been modding tube amps for a few years so I know my way around AC.

But the coursework I had as an undergrad didn't include any practical safety matters. Come to think of it, neither did the grad work. I didn't really learn much about AC wiring safety til the safety certification guys inspected my first power supply designs. That was enlightening, if tedious. 

Indeed though I would like to stay away from AC if possible, but for more high powered stuff the chances of you getting away from it become more slim.

You're right there. That's one reason to use a packaged supply if you can. It's a tradeoff of objectives - do you want to learn all the stuff to get there, or just have the working stuff? Those are different objectives. The first takes longer and is probably more expensive in time and money.

As for finding transformers I haven't been able to to find anything for audio amps

They're not impossible. See http://apexjr.com/miscellaneous.html

I guess companies making audio amps are making their own in house or are getting custom transformers made.

That is mostly true. In volumes over 100 or so, you can economically get whatever you want.

As for finding generic generic 100-120 Watt power transformers for tube amps and smaller 20-50 Watt hi-fi amps, seem to be futile, all I seem to find online are mains line converters, industrial power conversion transformers or small signal transformers, nothing in between without getting a bulk set custom made.

They exist, you just have to look harder. Try Hammond Manufacturing.

Looking at it now, I could make a 2.5W with a pump charge converter or a switching supply and be able to run my 9V adapter as it can handle up to 1.7A and the 2.5W would only be 400mA

Yes. The only problem there would be getting a 2.5W charge pump.

If I may make a suggestion - the LM1875 will produce 20Wrms into 8 ohms when powered from +/-25V. That's a 36Vct transformer. Mouser sells the Triad FS36-1000, 36Vct at 1A, for $16.84, and they have 19 of them in stock. Mouser also sells the LM1875 for under $3.00, with a few hundred in stock. The hidden part of this is that you'll also need a couple of 35V/4700uF filter caps and a heat sink suitable for getting rid of maybe 10W on the LM1875.

This is familiar to me because I used the LM1875 powered by a 48Vdc wall wart (also Mouser, $18.00) to make the compact power amp I mentioned earlier.

The stuff is there. You just have to keep looking.

[Mark Hammer]

Would one watt be loud enough?  I have a hard time believing that a one watt solid state amp could drive a guitar amp cabinet to volume loud enough to me mic'd and still have a decent S/N ratio.

A large share of musicians have insufficient understanding of wattage and loudness.  Given how often it is misrepresented by manufacturers it is no wonder.

A parable I am fond of re-telling....

Years ago, I was sharing a house with a band-mate who was more lucratively employed than I and could afford a better stereo as a result.  He was taken aback by how much better mine sounded than his, despite the budget each of us had allotted.  His 100W-a-side receiver came with a power meter, and we quickly learned that the efficiency of his speakers (12" woofer, yadda-yadda) meant that he was rarely powering them with much more than a watt of power, even when we had to yell over them to hear each other.  Both the amp and speakers had to have more than 1W being pumped out and used to be in their linear operating range, but 1W was pretty much all he was ever going to be using unless he was going to operate a disco.  My "budget" system was much less efficient, and the 18W/ch amp had to pump out about 5-6w on average to attain comparable loudness, which put both speakers and amp in the sweet spot where bandwidth was maximized, and distortion minimal.

It doesn't take a whole lot of wattage to make rock and roll, and I imagine many musicians would be surprised by how little wattage they are actually normally "using", and how much MORE wattage it takes to produce audibly louder levels.  5W may well surprise you.

[TheWinterSnow]

A large share of musicians have insufficient understanding of wattage and loudness.  Given how often it is misrepresented by manufacturers it is no wonder.

A parable I am fond of re-telling....

Years ago, I was sharing a house with a band-mate who was more lucratively employed than I and could afford a better stereo as a result.  He was taken aback by how much better mine sounded than his, despite the budget each of us had allotted.  His 100W-a-side receiver came with a power meter, and we quickly learned that the efficiency of his speakers (12" woofer, yadda-yadda) meant that he was rarely powering them with much more than a watt of power, even when we had to yell over them to hear each other.  Both the amp and speakers had to have more than 1W being pumped out and used to be in their linear operating range, but 1W was pretty much all he was ever going to be using unless he was going to operate a disco.  My "budget" system was much less efficient, and the 18W/ch amp had to pump out about 5-6w on average to attain comparable loudness, which put both speakers and amp in the sweet spot where bandwidth was maximized, and distortion minimal.

It doesn't take a whole lot of wattage to make rock and roll, and I imagine many musicians would be surprised by how little wattage they are actually normally "using", and how much MORE wattage it takes to produce audibly louder levels.  5W may well surprise you.

I think what you noticed it the efficiency of the amp to deliver power to the load relative to how much additional power the amp itself is consuming.  I had an interesting conversation from a magnolia sales associate that was trying to say efficiency was how loud the amp was comparable to other amps.  He had a 200W class AB amp at the same level (in dBu) as a 200W Class D amp.  Going back and forth the class D was considerably louder, he was trying to say that with an SPL meter both amps would register the same level, I just kind of laughed to myself and said, "Hey I learned something new".  There are two ways you can measure power, you can classify the amp based on how much power is delivered to the load plus how much the power amp itself consumes, or just how much power is delivered the the load.  In the case with your amp, at comparable levels yours was eating up more, but if you had a meter measuring how much power was being given to the speaker, both amps would have read the same, the additional power your amp consumed was coming from its waste or losses.  If the actual rating to the load was indeed 18W like you said, if you had delivered the full power the meter should then have given you a value much higher than 18W. 

I use a 120W tube amp that is 120 watts to the load, plus however much the tubes themselves eat up, measured the total power is closer to 300W.  I hit it at the 120 mark every band practice (you know, when you turn up more it doesn't get any louder), and while its loud, its not LOUD imo.  Knowing lower wattage amps like 20Watt tube amps can be almost as deafening, I totally understand power isn't everything, but I once made a superheterodyne AM radio powered with a .25W amplifier, I had a bigger 8 ohm speaker it wouldn't drive, it just distorted, then using a small .4W speaker it was just loud enough to barely make out what was on the radio if you put the speaker up to your ear.  It may have been the chip or the speakers sensitivity rating, but it wasn't loud. 

I will though get an 18Vdc adapter and make a 2.5Wrms amp out of the LM1875 RG mentioned, I was looking at another 5W TI chip, but I think the 1875 should run cooler and that would be a good thing.

[R.G.]

I think what you noticed it the efficiency of the amp to deliver power to the load relative to how much additional power the amp itself is consuming. 

I don't think so. Mark was pretty clear about it being power delivered to the speakers, and the relative difference of efficiency in the speakers.

I use a 120W tube amp that is 120 watts to the load, plus however much the tubes themselves eat up, measured the total power is closer to 300W.  I hit it at the 120 mark every band practice (you know, when you turn up more it doesn't get any louder), and while its loud, its not LOUD imo.  Knowing lower wattage amps like 20Watt tube amps can be almost as deafening, I totally understand power isn't everything, but I once made a superheterodyne AM radio powered with a .25W amplifier, I had a bigger 8 ohm speaker it wouldn't drive, it just distorted, then using a small .4W speaker it was just loud enough to barely make out what was on the radio if you put the speaker up to your ear.  It may have been the chip or the speakers sensitivity rating, but it wasn't loud. 

It's important to realize that the efficiency of the speakers in converting electrical power to sound pressure level, and the even murkier translation of measure air pressure level to perceived sound comes after any amplifier output power. Given the oddities of human hearing, a 120W amplifier might only be perceived as twice as loud as a 12W amplifier, not ten times as loud.

Speakers are rated in their efficiency in translating electrical power in watts to sound pressure level in db relative to some threshold. The power efficiency of speakers in general is dismal, generally in the single-digit percent. This is one reason speakers are rated (often, usually) in sound pressure level at one watt, one meter away in most cases. A guitar speaker might be rated at 98db spl at 1W, 1meter, on axis. Changing to 0.1W would typically drop the SPL by a similar amount, to 78db, which is still quite audible. Upping to 100W would only kick the spl up to 118db, which us humans would hear as "about twice as loud".

There is a lot of room for sound pressure at the low end of the power spectrum. As Paul said, don't discount small amplifiers.

Just out of curiousity, why are you feeding speakers swept sines? Plotting response spectra? Looking for rattles?

I will though get an 18Vdc adapter and make a 2.5Wrms amp out of the LM1875 RG mentioned, I was looking at another 5W TI chip, but I think the 1875 should run cooler and that would be a good thing.

[TheWinterSnow]

I was making the assumption that as mark was talking about a power conditioner that measure how much power was being drawn.  If that was the case, the meter would give a reading of how much power the whole system is pulling, not just power to the load.  But I was also assuming that his test were on the same set of speakers.  But if his system has speakers with lower sensitivity, then his amp would have to deliver more power to the amp to get comparable loudness.  Either way a good anecdote.

Back on topic, I seemed to let it pass me the whole rule of 10, but you are right, a 100W amp will only be perceived as twice as loud as a 10W amp and like you said, the sensitivity of a loudspeaker is based on 1W from 1M away, my speakers have a sensitivity of 97dB, so a 1W amp should theoretically be overkill.

I am using sine sweeps to make impulse convolution filters.  I build these guys here and need to get impulse responses of a cabinet and mic without the coloration of a tube power amp.

[Mark Hammer]

A digression, but just to clarify/verify, RG was correct.  I was referring to how much power was being delivered to his speakers and my speakers.  How efficiently the amplifier takes wall current and turns that into output power is a whole other thing.  I have to laugh when I see local kijiji ads for small amps by naive  vendors who misunderstand the drawn wattage, as stated on the back of the amp, for the audio output.  Nope, that thing with the tangerine-sized power transformer and 6" speaker is unlikely to put out 38watts of audio. 

Back on topic, a 1W amp would not necessarily be overkill.  Keep in mind that if you are using the amp for audio testing purposes, you would want something that is still a fair distance from its maximum output.  A 1W power-amp chip will provide as much as a watt of output power, but not a clean watt.  I'm just guessing here, and it would likely depend on the specific design/chip, but you may want something higher than 5W to have any confidence in feeding your speaker with 750 clean milliwatts.  There certainly are very distortion-free headphone amp designs you can find that don't provide much power but are very clean almost out to the limits.  If you want quick and dirty with a power chip, though, you'll want more than a 1W chip.  a TDA2030 looks like a pretty safe bet for clean power at 1W.

[PRR]

> I have a degree in electrical engineering

You can overcome that. Think. Question. Explore. Question again.

> a 0.25W amplifier, ... it was just loud enough to barely make out what was on the radio

You need to re-question that. I grew up with 0.1W radios and 2-inch speakers, and cries of "turn that down!" from my parents.

> finding transformers

Hammond; or for tube amps the Tube Guitar Amp Parts vendors like Hoffman Amps and Weber VST. There's stock-replacement PT and OT iron for 7W 18W 50W and 100W, at prices so much lower than custom that you should adapt your specific design to a similar Fender/Marshall classic rather than wrestle.

For modest chip amps try keyword "gainclone" on eBay. Seems like everybody and his dog builds gainclones. (Hey, there's even a Wikipedia page!) Chinese winders turn out power transformers for this market. Some eBay listings are for high power stereo boxes; find a likely seller and try the "my other listings" link for smaller iron.

> a decent S/N ratio.

As said, roughly 95dB SPL at one meter.

For measurement, you will find a quiet room, 15dB SPL. (That is sensitivity and NOT!! "efficiency".)

95-15.... that's 80dB S/N.

What do you really need? For frequency response, you need the deepest dip in your response curve several dB above the noise level. (There *are* clever ways to do MUCH better.) Say low-limit is 15dBSPL+10dB, so 25 dB SPL. 70dB below nominal speaker sensitivity. The "94-100db SPL" is often taken off the high-spot of the response, overall average may be as much as 10dB lower. Say 85dB area. 85-25 is 60dB between average response and above-noise. You can't measure dips deeper than 60dB.

Do we care about 60dB dips? As response errors, certainly not. As phase-twisters, we might... but 60dB dips will be cancellations which will be different on every sample speaker and likely different at every off-axis angle.

A 60dB window also limits how far down the stopbands we can measure. The bass typically falls 12dB/Oct below 80Hz. So we can go 60/12= 5 octaves below 80Hz. On my thumbs I count 2.5Hz. I don't think the speaker response at 2.5hz is of much interest. (But... in real life the low-frequency ambient noise can be FAR higher than at midrange. It can be difficult to plot more than an octave or so down without clever averaging. OTOH the bass-slope of a speaker is utterly predictable, so if you plot enough of the corner you know how it falls.)

> I build these guys here

Ah. Aside from the non-trivial problem of getting and selecting voltage-input responses, there is the very messy problem of back-kicking the amplifier like it was seeing a speaker. As the amp comes up into clipping, "you turn up more it doesn't get any louder", a simple post-correction can't even approximate what a heavily-loaded tube and OT does into the 10:1 impedance variations of a loudspeaker. At some point it makes more sense to model the WHOLE system, at least from PA In to SM57, at various levels from mild to LOUD; not make the tubes heat a resistor.

[J0K3RX]

3watt....  

Take a look at this, it's not the tubecake but still kix some tiny ass..! Schematic and board layout is in the data sheet   I have built 4 of them and all work flawlessly and push a 4x12 no problem and loud too! Entire board is about the size of a book of matches works with 5v to 18v.... win win!
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/5908.pdf

You could probably get it into one of these
http://www.mammothelectronics.com/4S1590LB-p/500-1007.htm

[tca]

I do like the sound of my 1W PUNCH amplifier! It uses the TDA7052. Just plug in and play.

Cheers.

[brett]

Hi
I suggest that AC in the box isn't the way to go. An injet printer power suppy or similar (charity shop $2) will give you 30 to 40V at about an amp (sometimes 2).
That's buckets of power and about the right voltage.
As RG says, even a specific wall-wart, new costs only $18.
You might want to regulate it a bit more or pehaps de-regulate it, depending on the sound you are after. Careful de-regulation can sometimes give sweet 'sag and bloom'.

Of couse there is the fun of a more DIY attitude. Some of us go crazy and wind our own transformers. Totally impractical in most cases, but totally fun.   
cheers

[Jdansti]

Would one watt be loud enough?  I have a hard time believing that a one watt solid state amp could drive a guitar amp cabinet to volume loud enough to me mic'd and still have a decent S/N ratio.

I've built the "Noisy Cricket" 0.6W amp (http://www.beavisaudio.com/projects/NoisyCricket/), and I'm certain it would seriously damage my hearing if I plugged into a 2 x 12 cab and sat in front of it with the volume cranked up.     I don't have S/N numbers to give you, but you could run it on 1/2 volume and mic it with very low noise.

[tca]

Would one watt be loud enough?  I have a hard time believing that a one watt solid state amp could drive a guitar amp cabinet to volume loud enough to me mic'd and still have a decent S/N ratio.

You will be surprised about how loud is a 1W amplifier with a high sensitivity  speaker. And also there is the quest for a sweet spot. Try and put a small 1W amp with a small speaker (3'', 8Ohm, 8Wrms, 89 dB - 1 W/1 m) in the center of living room, that thing is loud enough to collect comments from all your family members and neighbors.