Power Scaling is Flawed?

[tbryanh]

As I understand it, the goal of power scaling is to reduce amp power down to 1/100 watts to provide heavy distortion in the PA section at low sound pressure levels.

I think power scaling could be flawed for two reasons: poor dynamic range and poor speaker performance.

I don't know what the noise floor is on the typical amp, but the dynamic range is the difference between the maximum output power the amp produces when it is amplifying a guitar signal and the amount of output power the amp produces when it is amplifying internal noise noise.

Reducing the output power brings the guitar signal closer to the noise floor and reduces dynamic range in the process.

From what I can tell, most guitar speakers stop perfoming well when power drops below about 1 watts or so.

[Jag]

If I recall, KOC was also liberal with statments that getting great tone at low volume also required a change in listening expectations. I think these were the issues he was alluding to.

I'm not sure that power scaling would bring the signal closer to the noise floor but +1 on the rest.

[Amp Kat]

It all goes back to noise and gain are directly proportional. The lower the gain the lower the noise. The lower the voltage of the preamp and power tubes the lower the gain. Power scaling lowers the preamp and power amp plate voltage and the bias voltage tracking the plate voltage. Getting closer to the noise floor or even a low signal to noise ratio are directly proportional to gain so yes that is happening and it's all it really can do. Theres many ways to attentuate and many different topologies to accomplish it however as you correctly stated that there is no substitution for speakers expressing themselves with power. That's the whole problem with attenuation is no matter how hard you try or how much you want it to happen the speakers are going to sound 10 gazillion times better at higher levels and sound like crap at low power levels. The noise floor is higher (or lower depending on how you measure it ) and the signal to noise ratio is higher when you are driving the preamp with an attenuated power section which IMO sounds the best of all the attenuator options. Power Scaling is just another way to do it and it also has other benefits when used as a standalone power supply. Very nice regulation and excellent stability and efficiency which back in the day was done mainly with the infamous 807 regulator. How times have changed.

[tbryanh]

Yeah, I guess if you scale the power down from say 100w to 10mw, you scaled power down by a factor of 10,000 which means you scaled voltage down by a factor of 100.

A reduction of voltage gain by 100 is a significant reduction, so noise would be significantly reduced also.

That does tend to indicate that signal to noise ratio has not changed and that useable dynamic range stays the same.

It does appear to be strictly an issue of speaker performance.

[Steve Conner]

Don't forget your Fletcher-Munson curves! The biggest and perhaps the only flaw in the whole concept of "quiet cranked tone", IMO.

[Matt T.]

Power scaling lowers the preamp and power amp plate voltage and the bias voltage tracking the plate voltage.

Just for the record...power scaling scales whatever you want it to scale. But most people would never scale the whole amp...scaling the preamp would completely change the tone...most people would not scale the preamp. Usually it's either scale the power tubes but not the PI or preamp...or scale the power tubes and PI but not the preamp. Also, the bias voltage tracks the screen voltage not the plate voltage.

Regarding the 'flaws' in the OP...I don't think KOC ever said it sounded exactly the same (I'd be shocked if he did).

Even if everything else were equal, which they aren't, you just can't get the same tones without the volume. Just being able to feel the low end as opposed to just hearing it has a huge effect...plus the speaker thing...plus F-M curves.

There seems to be a lot of discussion about power scaling and I think it is a good alternative to getting good tone at lower volumes (also included in that is the 'feel' of the amp which IMO power scaling addresses as much as anything) in some cases...but not all cases. I have a power scaled amp as well as some other non-MV amps (some with MVs added...some not) and my favorite amp is still a MV w/FX loop amp just on the raw tone alone...the simplicity and FX loop make it even better for me. But if I had a non-MV amp that I loved and had to get the volume down I'd certainly consider power scaling...it does work.

[soundmasterg]

KOC goes out of his way several times in TUT4 to point out that there is really no way to get around the lack of speaker overdrive/low volume issue and that you have to essentially change your expectations when using power scaling and not expect that it is just a volume difference but the same tone as when the amp is loud.

[Amp Kat]

Just for the record...power scaling scales whatever you want it to scale. But most people would never scale the whole amp...scaling the preamp would completely change the tone...most people would not scale the preamp. Usually it's either scale the power tubes but not the PI or preamp...or scale the power tubes and PI but not the preamp. Also, the bias voltage tracks the screen voltage not the plate voltage.

I guess it depends on which amp your talking about there Matt and the only guy I know that uses power scaling is KOC and this perticular amp is scaled preamp and all . The only difference in the plate supply and screen is a choke and a capacitor so essentially the bias is tracking both then again it may depend on which amp your talking about. That's why I said I prefered an unattenuated preamp because you have more signal to noise ratio and a more sustained signal. Scaling the preamp back kills the sustain which is also less noise . KOC did a lot of changes to the Studio and truely wish I had gotten one of those instead of the earlier model because he added another gain stage and more sustain. Whether he changed the power scaling to only power stage I don't know but the earlier model was all power scaled.

[bob p]

Many people are aware of the fact that our ears don't respond linearly to LF input. Snobby know it alls at The Gear Page casually throw this fact around all the time when they profess that there exists no good attenuator.

Interestingly, the ability of a TS ported array to produce bass response is measured/spec'd with small signal outputs, but the bass response of TS ported arrays falls off markedly when reproducing large signals.

[gdgross]

I built KOC's scaling circuit into my peavey 50/50 power amp.

I don't think anyone, including KOC, claims that it's a panacea to get cranked tone at low volume. But it is an option.

My ears like it better than my THD hotplate. It ain't perfect, but it's pretty good. Until we are playing huge stadiums, where I can let my amp loose and stand fifty feet away with my custom-molded in-ear monitoring system, I'm going to keep using it.

[Matt T.]

AK,
I wasn't aware that KOC actually built an amp that scaled the preamp. I was just going from the documentation from the kit and conversations with Kevin (via e-mail usually).

The only difference in the plate supply and screen is a choke and a capacitor so essentially the bias is tracking both then again it may depend on which amp your talking about.

Maybe someone with better knowledge can help but I assumed that the plate voltage fluctuated much more than the screen during loud playing so that's why they tracked the screen voltage. My amp doesn't have a choke...it's an Express circuit so it just has a 1K/25W resistor there (not sure how much that matters here).

I have a Dr. Z Airbake and it works great too...actually some of my amps actually sound a little better with it than without it at certain settings on the Airbrake (usually one click down from full power). Once I took some measurements on the Airbrake and saw what it was doing to the load...that made sense. At one click down it just puts a 25ohm resistance in parallel with the speaker cab.

I've pretty much given up on TGP...I've been one that's been trying to say that it's an alternative and it's nice but it's it's not a cure-all or anything of the sort. But there are people who want to put power scaling in everything from Bogner Ecstasys to Tweed Deluxes.

[Jag]

Did you buy one of KOC's kits or roll your own from the TUT books?

I've been building some low wattage amps with hefty iron and a plexi-ish pre for a few of my friends. They really sound great and deliver OT grind at a more reasonable level.

One of the guys can't get past his belief that only a 100w amp will get him the tone he wants. Then he gets all upset that he's got to use pedals or an attenuator to get grind at most (probably all) of his gigs. I've been thinking about building power scaling into an amp to see if he likes it.

[Matt T.]

Jag,
I bought the kit from KOC. Regarding your friend...not sure what to say except; "Good luck with that." In this day and age there is absolutely no reason to have to complain about 'getting good grind' at any volume and you don't have to resort to power scaling (although there is nothing wrong with doing so). There are just so many alternatives for getting good low volume tones these days that IMO if anybody can't find one...they're just not looking hard enough. I think in many cases the person just likes the volume itself. Like that line in Risky Business; "Volume...there is no substitute."

[hasserl]

I guess it depends on which amp your talking about there Matt and the only guy I know that uses power scaling is KOC

There's Maven Peal amps that uses their own version of power scaling. The Maven Peal guy and KOC have had some lively debates over the merits of their two systems.

Then there is Mojave amps that has a power scaling system too, might just be a licensed version of one of the two above.

Also, John Suhr is making amps using power scaling. It is a system licensed from London Power.

[Amp Kat]

There's Maven Peal amps that uses their own version of power scaling. The Maven Peal guy and KOC have had some lively debates over the merits of their two systems.

Then there is Mojave amps that has a power scaling system too, might just be a licensed version of one of the two above.

Also, John Suhr is making amps using power scaling. It is a system licensed from London Power.

Would you have a link to that discussion ? I'd like to read that. I guess the discussion or debates would be over the sag issue. The supply is regulated and quite stiff so I don't see how you can get sag when the supply doesn't sag ?