Power Scaling is Flawed?

[KevinOConnor]

Hi Guys!

There actually is a forum called PowerScale.com, set up by Mickey Corrieri of Soultone Amps, where builders and hobbysists can go to find the truth about power control circuitry and other mods. There are project amps in the works that show complete layout details.

Steve, no one would suspect you work for PV or any other amp company with some of the ranting you've placed here. I don't see any "conspiracy" of manufacturers against hobbyists. What I see - at least what I have an impression of - is resentment you have of not having come up with certain ideas on your own. When you say you want to "stick it to the big guys" you are showing us either that you are having a bad day or your true colours. I hope it is the former. There's all kinds of things I wish I knew more about, or wish I had invented, but I am thankful that someone else had the insight and developed the idea. We can't know everything and to think we should is folly.

Tiago: The protection zeners on your mosfets only have to be unidirectional, so it saves one diode per mosfet. The zener in reverse looks like a simple diode and the voltages are unlikely to ever be in the oppositie polarity in the DC circuit. In an audio amp, back-to-back zeners are placed on source-follower mosfets so as not to clip the drive into the opposite device.

With respect to the original Power Scale circuit:
The heart of it is a single mosfet. Current limiting (one transistor) is added to protect the mosfet from the charging currents of the caps that follow.

The pot sees pulsating DC not filtered DC. So its voltage stress is minimal, despite the scare tactics by the unmentioned.

I don't see how the above is complicated. Even the tracking bias regulator can be done with a single transistor - but two or three devices work much better and none are more than a few cents each.

The new kits use low-voltage filtered DC, which allows easy installation and location of the pot anywhere on the control panel. Current limiting is not required as all the caps are prior to the pass elements.

Have fun
Kevin O'Connor

[mooreamps]

Hi Guys!

There actually is a forum called PowerScale.com, set up by Mickey Corrieri of Soultone Amps, where builders and hobbysists can go to find the truth about power control circuitry and other mods. There are project amps in the works that show complete layout details.

It is a forum that I am offically banned from. Apparently, he did not like some comment I made on one of his YouTube videos. But I clearly understand now. These forums are intended to pick my brains by all these "boutique" amp builders. That's why I get flamed so badly when I write I don't put my schematics on a public forum, and that my power scaling circuit does not generate the heat and burn out as the for mentioned circuit. Well, sorry about that.........

-g

[mooreamps]

it's too bad....

-g

[Steve Conner]

Hi Kevin, it's great to see you on Ampage!

First of all, Gary Moore sent me his schematic, and it seems to me that it's a good method of power scaling, provided you stick to low-powered cathode biased amps. I can see issues with high-powered amps, and I can't see how it would work with fixed bias at all, without major changes. I won't go into any detail since Gary wants it kept confidential.

Am I resentful that I didn't come up with certain ideas on my own? Maybe, although my excuse is that I was still in my teens when TUT2 came out. If I come across as angry about something, it's probably that I love analog electronics, tubes and music technology, but somehow I ended up programming computers for a day job. I hate the damn things, but the money is too good to turn down :-/

P.S. I still think the 700W Spectrum bass amp was possibly the neatest bass rig ever. It inspired me to try making an amp with 4CX250Bs too, until I realised how expensive the high-impedance, high-voltage OT was going to be.

[marcus_cps]

Hi everyone.

I'm Marcus, from Brazil, and I believe this is the best content about this subject in the entire www. I'm very impressed.
By the time I'm working on my hobby project: a 50W tube (sure) amp and I'd would be very thankful if someone here could help me with some pratical circuit to make it (power scaling).
I garantee that i will never share it with others, unless authorized.

Thank you so much and if there is something I can help with, just ask.

[Amp Kat]

Marcus ,as Kevin explained in his post that this is not a circuit under secret oath. It's not made viable for the public however, those who know how it works usually share their knowledge with others. You can get a Power Scaling board from Kevin at http://www.londonpower.com and install it yourself. It should work great and do what you want it too.

[marcus_cps]

Marcus ,as Kevin explained in his post that this is not a circuit under secret oath. It's not made viable for the public however, those who know how it works usually share their knowledge with others. You can get a Power Scaling board from Kevin at http://www.londonpower.com and install it yourself. It should work great and do what you want it too.

AmpKat, thank you.
Thats the problem... I don't want to buy some "ready to install" package.
This is my hobby and I'm making my own project from the scratch (right now I'm creating my switching matrix). It supposed to be pleasure, not business!
I read that some people who know how to make just don't want it to fall in devils hands (crate, marshall and so on)... I thought maybe someone could help me making it. If I cant find someone, I'll prefer to do it the "old fashion"!

[mooreamps]

fall in devils hands

The PS circuit I sent to Steve is good only for single ended amps. Sounds to me like your 50 watter will be running push-pull. Perhaps you could contact Kevin at London Power, maybe he offers a "not ready made" kit. It's a tested mature product.

Mine, I'm still taking data from it. Also, since then, I've drawn up a new rev power brake for my 18 watt push/pull mixed mode amp. It will power scale both the SE class A side and the P/P class A/B side. I've not tested it as yet.
So, go with Kevins circuit if he can provide one for you.

-g

[marcus_cps]

The PS circuit I sent to Steve is good only for single ended amps. Sounds to me like your 50 watter will be running push-pull. Perhaps you could contact Kevin at London Power, maybe he offers a "not ready made" kit. It's a tested mature product.

Mine, I'm still taking data from it. Also, since then, I've drawn up a new rev power brake for my 18 watt push/pull mixed mode amp. It will power scale both the SE class A side and the P/P class A/B side. I've not tested it as yet.
So, go with Kevins circuit if he can provide one for you.

-g

Thats right. I'm working on Push-Pull.
I will try to contact Kevin... he seems to be a good person, considering his posts.
If I make progress, I'll post a feedback here.
Thank you very much.

[Amp Kat]

Kevin is a way cool dude. Put yourself in his position and it only makes sense that certain people would be ungrateful but the reality is he has helped very many more than he gets credit for and builds a damn nice amp to boot. If you don't find what you want email me and I'll shoot you a schemo.

[MattT]

Kevin is very cool...I'm not sure how it works if you want to DIY it but the kits aren't very expensive and you do get everything you need...then you'll have the circuit (for personal use). I guess my kit is the older design but I broke the threaded shaft of the scale pot...finding that same pot that Kevin uses (with it's 2kV case rating or whatever it is) just wasn't happening...so I called Kevin and he sent me a replacement...free.

FWIW, I have now tried scaling just the power tubes as well as scaling the PI along with them and I prefer just scaling the power tubes...but that's just one amp. The type of Master Volume you use might also make a difference.

...but I just can't sleep knowing that somewere in the world is a pot with 500VDC in it.

The pot has something like a 2kV case rating...I'm more concerned with, say, that same B+ on a 250v rated switch...like most every standby switch.

I resent not having written Dark Side of the Moon.

[marcus_cps]

Thank you guys.
I sent Kevin an email.
I hope he can help me with.
I'll keep in touch! Cya

[Mark Hammer]

You'll forgive me if I repeat things that may have been said already. I'm just stumbling onto this thread now and haven't really the time to read the whole thing from start to finish.

Seems to me that one of the caveats with power scaling is the difference between relatives and absolutes. When working in the realm of electronic signals, you can attenuate and adjust gain any which way you want to. It's just electrons. Once you move into the realm of speakers, however, and the movement of physical objects (i.e., cones and coils), we now move into the world of absolutes, whereby the energy required to move a speaker cone is now harnessed to whatever the mass and compliance of the cone says it HAS to be. It may be quite feasible to scale power down to whatever fraction one wishes, but the physics of the cone says "Nuthin' doin' unless you can give me this". Were it the case that musicians/users had separate speaker cabinets loaded with speakers covering a wide range of power requirements and efficiencies, then one might simply switch from a set that needed 5W min to sound like themselves, to a set that were happy with 1W, to a set that moved into their linear range with 200mw. That tends not to happen, however. While the amplifier properties might be successfully rescaled, at a certain point the performance of the whole thing (amp, tranny, speakers) becomes constrained by what the speakers insist on in order to relay the full spectral content and dynamics of the electronic version of the audio signal. Dip below that minimum requirement, and you end up with substandard sound.

I get tired (and embarrassed) of telling this story, but some 30 years ago I had a room-mate with a massive stereo receiver and large Altec speakers. I had a little 20W/ch amp and some home-made 2-ways with 8" woofers. He had a great system on paper but mine kicked his ass (by his own admission), simply because his speakers were loud enough with 500mw going into them, and at 500mw they fostered some serious audio smudging, given the size of the cones. Moreover, his amp was still in its nonlinear range at 500mw. By contrast, my system had to be pumping out about 2-4W to achieve similar listening levels, at which point both the speakers and amp were sitting nicely in their linear range. The point is that many speakers don't really sound nice or even like themselves until a given power level is reached, simply because they are moving objects that need a certain degree of momentum to be nimble. Most "full range" musical instrument speakers are not intended to be fast enough to cover the full spectrum, which results in a certain sluggishness, and a need to BE in motion for the full spectral capability of the speaker to emerge.

Said more succinctly, power scaling works down to a point, and below that point speaker functioning takes over.

Of course, if one has some sort of output conversion for feeding an amp's output to a mixer, then the physical requirements of a speaker are conveniently factored out and the virtues of power scaling should be maintained.

[Steve Conner]

Well, it's a nice sounding theory, except I've never seen any evidence that speaker distortion starts to increase again at low levels. AFAIK, they only get more linear as the volume is decreased.

I think the impossibility of quiet cranked tone is more likely due to those Fletcher-Munson loudness contours, or whatever. In other words, your ear's response varies with level. So even if you had a power scaling amp so perfectly designed that the sound waves coming from it didn't change at all as it was turned down (except in magnitude) it would still start to sound duller and flatter at low levels.

A similar effect I've seen is when metal guitarists practice at home, using a master volume amp turned down, they often use a scooped tone with lots of bass and treble, and not much mids. When they get to a gig, with the same tone settings, nobody can hear them! The amp just barfs out a mass of sizzle and sludge that clashes with the bass guitar and cymbals. When playing loud with a heavy band you need to crank the midrange up and the bass down.

It also makes better use of your available power: why bother flapping your cones when the bass player has more and bigger ones to flap? Unless of course the sound of an overdriven 12" flapping with too much bass in an open-backed cabinet is part of your tone, which it often is, IMO.

So not only does the ear's perception of tone depend on volume, but the best guitar tone to use depends on the volume, and the situation.

DIing to a mixer solves many of these problems, because you can crank it through headphones and/or EQ it. But it also removes the need for power scaling in the first place: you can just hook your amp up to a big 50 watt wirewound resistor and DI box, and dime it.

[Bruce / Mission Amps]

...
A similar effect I've seen is when metal guitarists practice at home, using a master volume amp turned down, they often use a scooped tone with lots of bass and treble, and not much mids. When they get to a gig, with the same tone settings, nobody can hear them! The amp just barfs out a mass of sizzle and sludge that clashes with the bass guitar and cymbals. When playing loud with a heavy band you need to crank the midrange up and the bass down.

...
So not only does the ear's perception of tone depend on volume, but the best guitar tone to use depends on the volume, and the situation.
...

Whooppie! EXACTLY right.
There is nothing much worse then the god awful tone of that sound, live off the stage.
Well, what is worse is when those wankers are playing at home or trying to show off at my shop and they actually love that angry, soulless, woofie bass drone with the sound of a hundred bumble bees in a paper bag riding on top. Makes me want to puke.