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Reactive amp load with variable attenuation

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Reactive amp load with variable attenuation

Postby jellyjim » Wed Mar 13, 2019 12:34 am

Hello

Not quite sure of the product I need. I want to be able to enjoy the cranked up tone of my valve amp but at domestic/sociable levels. I've had straight amp loads before, some Palmer thing for example, but they always seem to have just fixed attenuation settings (rather than variable) and at the highest attenuation level I found the signal unconvincing.

I understand that "reactive" loads are better as they reflect the fact that a speaker's impedance is related to how hard it's driven. I realise something like the UA OX would do it but that's first way out of budget and second just more than I need right now

So I want to hear MY valve amp (a Supro Tremo-Verb reissue) not a model of another amp/speaker, cranked up but at a volume that won't upset the cat, and I want to hear it through the amp's own speaker.

What do I need?

'A new cat' is not an option.

Thanks
Jim
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Re: Reactive amp load with variable attenuation

Postby Sam Spoons » Wed Mar 13, 2019 11:20 am

It depends how discerning/critical an ear you have. the speaker response will change as the level changes and your hearing response is different at different levels (google "Fletcher Munson"). So, the upshot is that it won't sound the same if you turn it down even if the signal going to the speaker is identical (other than level). When you record it that will ignore the FM affect until you replay at different levels but it will record the differences in speaker response.
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Re: Reactive amp load with variable attenuation

Postby Watchmaker » Wed Mar 13, 2019 11:37 am

I've had a Rivera Rock Crusher Recording for ages and use it everyday. In any reactive setting you will lose some tone simply because you're not pushing as much air. The speaker mechanics has a lot to do with tone so even if your load is accurately transferring the output amp, the simple decrease in amplitude makes it sound different.

If I were to buy again, I'd take a hard look at the UAD Ox
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Re: Reactive amp load with variable attenuation

Postby Wonks » Wed Mar 13, 2019 12:00 pm

A speakers impedance is variable depending on the frequency and it's the same basic curve regardless of volume (though as Sam says, you are going to hear a different frequency mix at different volumes).

What the speaker will have is a 'resonance peak' in the impedance, which is important in determining a large portion of the speaker sound. This peak can be adjusted slightly by the effect of the speaker enclosure, but not really by volume.

So for this speaker example, you've got the frequency response curve (the top one) with the dB output scale on the left, and the impedance curve (bottom) with the resistance scale on the right.

Image

You can see the obvious peak in the impedance peaking at about 110Hz, with impedances ranging from 6 ohms up to 50 ohms across the frequency range.

That peak is quite high for a 12" speaker, and they are normally lower than that. Celestion make two versions of the G12H speaker. The G12H (55) and the G12H (75). The figures in brackets give the resonant peak value and the 55Hz version does have a different bass-end sound to the 75Hz version.

So any reactive load attenuator will try and emulate a similar resistance curve to a real speaker, which will have a big effect an how the amp performs compared to one with a simple resistance load that's always at 8 ohms.

I don't know much about the design of such load boxes and how adjustable the resistance parameters are, but a cheaper reactive load-box will probably have a fixed impedance curve, maybe with filtering/EQ on the output to adjust the post-speaker sound, whereas a more expensive one might allow you to adjust the resonance peak, along with better post-speaker filtering/EQ options?

Depending on the speaker rating relative to the amplifier's output, at higher volumes you are likely to get some speaker break-up and compression effects which you won't get with an attenuator/load-box. Supposedly when the speaker has a rating between 100% and 200% of the amps nominal output but as driven valve amps can push out a lot more than the nominal output (the clean level output), and as speaker wattage ratings can vary between manufacturer (or at least have done significantly in the past), then the empirical way is the only real way to find out when that starts to occur for a particular speaker. I only know that for my 5W Fender Vibra Champ, push it too hard and the Eminence speaker makes horrible farting noises in the bass.

So it's always going to be difficult to exactly match the sound of a particular speaker directly connected to an amplifier. The more money you spend, the closer you'll get to your ideal sound.
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Re: Reactive amp load with variable attenuation

Postby John Egan » Wed Mar 13, 2019 12:41 pm

jellyjim wrote:So I want to hear MY valve amp (a Supro Tremo-Verb reissue) not a model of another amp/speaker, cranked up but at a volume that won't upset the cat, and I want to hear it through the amp's own speaker.

What do I need?

'A new cat' is not an option.

Thanks
Jim

I've never been convinced by the sound of attenuated amps. For all the reasons Wonks and others pointed out, they never sound the same at low volumes. When I want a cranked sound, I'e taken to using a Mesa Cab Clone out of the amp speaker output and through to my DAW (Reason, in my case) I can then tweak the sound to taste for (to my ears) a decent recording sound. It's not the same as a cranked amp live, but in my opinion better than the attenuated sound from my speakers at normal room volumes.
I know it's not what you want, but you might be surprised how acceptable it is.
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Re: Reactive amp load with variable attenuation

Postby jellyjim » Wed Mar 13, 2019 12:51 pm

Thanks all

Yup after further research I am thinking that I should accept I’ll be listening to the guitar through my monitors rather than the cab. My main use case is practice/study anyway.

I’ve been looking at the Two Notes Torpedo Captor which seems to give me all options. Raw amp out for processing with virtual stuff, amp out with speaker simulation for passable tones direct to desk and -20db attenuation.
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Re: Reactive amp load with variable attenuation

Postby ef37a » Wed Mar 13, 2019 2:42 pm

I have always been rather sceptical of the usefulness and indeed safety (to a valve amp) of these "reactive" load power soaks* I base this on the fact that I built up a couple of such loads for "The Works" and they were humungous and the parts bloody expensive!

I will admit they had to be proof against an overdriven 200W amp, so getting on for 1/2 a kW but even so I think a safe reactive load for a 100W valve amp would still be rather large and expensive?

Now the arguments will rage forever but there is alot of agreement I find that the sound of OD guitar is mainly in the pre amp stages and the speaker. Output stages contribute relatively little to the tone. The old saw that EL34 are a Britsh sound and 6L6 American is reallly not born out in practical tests. But the marketing bods won't like that!

Now, do not desert SoS but there are a few guys over at thefretboard.com who have deep knowledge of various amps and how they work best with various power soaks. Some amp designs it seems actually sound better on a pure resistance.

Speaker impedance changes with drive level? Not really, if you drive the be-whatsits off a speaker it will go into Thermal Compression which means the voice coil RESISTANCE increases but I cannot see that having a huge effect on the tone?

*I prefer the term to "attenuator" because the latter to me means a rather precise bit of kit designed for much lower signal levels.

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Re: Reactive amp load with variable attenuation

Postby Sam Spoons » Wed Mar 13, 2019 3:07 pm

I was referring to the effect of speaker break up rather than anything related to impedance changes. The cone of a guitar speaker will not behave linearly as volume increases but will 'distort' and add colouration which won't be there at low levels. I used to use a modded MusicMan RP65 which I had uprated it to the RP100 spec, It did produce 100 watts RMS from a pair of EL34s but I retained the original 65 watt speaker as it was both lighter (by a lot) and nicer sounding than the monster in the 100 watt version. But I now use an '18 watt' combo with an overrated Vintage 30 ( rated at 60 watts) speaker so that can't be getting much into the 'break up zone'.....
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Re: Reactive amp load with variable attenuation

Postby Wonks » Wed Mar 13, 2019 3:13 pm

It woz Jelly Jim in his OP who stated that impedance changed with volume. I think he meant frequency but got confused.
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Re: Reactive amp load with variable attenuation

Postby jellyjim » Wed Mar 13, 2019 3:18 pm

Wonks wrote:It woz Jelly Jim in his OP who stated that impedance changed with volume. I think he meant frequency but got confused.

Dark science! Electrickery!
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Re: Reactive amp load with variable attenuation

Postby ef37a » Wed Mar 13, 2019 3:27 pm

Sam Spoons wrote:I was referring to the effect of speaker break up rather than anything related to impedance changes. The cone of a guitar speaker will not behave linearly as volume increases but will 'distort' and add colouration which won't be there at low levels. I used to use a modded MusicMan RP65 which I had uprated it to the RP100 spec, It did produce 100 watts RMS from a pair of EL34s but I retained the original 65 watt speaker as it was both lighter (by a lot) and nicer sounding than the monster in the 100 watt version. But I now use an '18 watt' combo with an overrated Vintage 30 ( rated at 60 watts) speaker so that can't be getting much into the 'break up zone'.....

I am sorry Sam but this is dangerous talk. Celestion themselves will tell you that you don't have to drive the *its off a speaker to get a good tone. The industry reccy is that guitar speakers are rated at at least 50% above the amplifier rating. That is because valve amps get driven into distortion and the actual power into the voice coil can be even 100% of that marked.
If a guitar speaker is overdiven it will likely fail thermally and go open circuit and that at the very worse moment for the amplifier, max overdrive. Valves will arc and if you are lucky just fail and take out an HT fuse. Unlucky is when valvebases are carbonized and the OP traff goes bye-bye.

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Re: Reactive amp load with variable attenuation

Postby Sam Spoons » Wed Mar 13, 2019 3:32 pm

No argument with that Dave, though the MusicMan lasted many years without issue (and I still regret selling it). My point was not that you need to overdrive a speaker to make it sound good, just that the amount of distortion caused by 'speaker cone breakup' will vary with volume and an attenuator/power soak will not mitigate the effect.

jellyjim wrote:
Wonks wrote:It woz Jelly Jim in his OP who stated that impedance changed with volume. I think he meant frequency but got confused.

Dark science! Electrickery!

It is customary, at this point in a conversation, to quote Arthur C. Clarke's 'first law' :- "Any sufficiently advanced technology is indistinguishable from magic"
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Re: Reactive amp load with variable attenuation

Postby ef37a » Wed Mar 13, 2019 5:12 pm

Yes Sam I understand the idea of cone breakup but the whole subject of guitar tone is one of wildly divergent opinions and subjective impressions. Often the "seeker" it trying to imitate a specific sound on a famous record* and usually has little idea as to how it was produced and even if they think they do they are probably wrong!

So, all knds of weird and wonderful schemes have been proposed and driving a speaker to within milliwatts of death has been one of them. Sure, that gets you A tone but not necessarily the "right" one and another player might say it sounds ***t! Many of the schemes have been dangerous to equipment and possibly people. Here is an example of a possible kit killing...
Son and I put a resistive load on his HT-20 then drove that into the front end of a Dominator clone, actually a WEM Mersey Super 15. At around 90dB SPL from a V30 we got THE most glorious,,creamy ZZ Top guitar you ever heard. The poor lil ECC83 suffered the 10V rms drive for several hours, in fact it never failed but I do not recommend the idea to those that have to pay to get their amps fixed!

There is a story that Mark Knofler went back into the studio the day after recording Money for Nothing and could not get that tone back for love OR money!

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Re: Reactive amp load with variable attenuation

Postby Sam Spoons » Wed Mar 13, 2019 5:45 pm

ef37a wrote:Yes Sam I understand the idea of cone breakup

Not trying to teach granny to sick eggs Dave, your knowledge of this stuff is way above mine :thumbup:

but the whole subject of guitar tone is one of wildly divergent opinions and subjective impressions. Often the "seeker" it trying to imitate a specific sound on a famous record* and usually has little idea as to how it was produced and even if they think they do they are probably wrong!

Dave.

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Re: Reactive amp load with variable attenuation

Postby jellyjim » Wed Mar 13, 2019 5:49 pm

All interesting stuff and thank you guys but ...

I'm probably getting the cart before the horse anyway!

At the end of the day I'm trying to get a useable and realistic guitar sound at low volume specifically for the purpose of practice and study (rather than recording necessarily) as I figure how can you really learn to play certain electric guitar styles if you can't play with some oomph/welly and experience a believable and authentic response from the instrument?
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Re: Reactive amp load with variable attenuation

Postby Sam Spoons » Wed Mar 13, 2019 5:56 pm

Your best answer would be to buy a small amp, a 5 watt is still pretty loud but Blackstar do a one watt combo, the HT-1 which is pretty cheap (cheaper than most power soaks). Worth a demo at least?
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Re: Reactive amp load with variable attenuation

Postby jellyjim » Wed Mar 13, 2019 6:05 pm

I’ll check it out

It just seems the larger the amp, the lower the volume, the weaker the girth!

Sustain, pinch harmonics, for example, all problematic at low girth
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Re: Reactive amp load with variable attenuation

Postby Sam Spoons » Wed Mar 13, 2019 6:51 pm

That would be my take on it too which is why I use an '18 Watt' combo live. It sometimes lacks clean headroom but mostly works well as long as you're not expecting Fender Twin cleans.
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Re: Reactive amp load with variable attenuation

Postby Watchmaker » Wed Mar 13, 2019 7:00 pm

option II: find a room where you can crank the *ucker!

If you decide to simply monitor through your DAW, there's a million worlds to explore. If it sounds great, it ain't cheating
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Re: Reactive amp load with variable attenuation

Postby ef37a » Wed Mar 13, 2019 7:27 pm

Re the Blackstars, there's a tech I respect greatly over at frets and he has little time for the tone of the whole range of their amps, except the HT-1! He does however freely admit that they are well built and very reliable.
Fortunately he is in a bit of a minority and the amps still sell pretty well (they have weathered two recessions!) .

I suspect, and other agree on this, that trying to smash down a 50 even 20W amp to miliwatts so you don't wake the chavvy is doomed to crap tone. Power soaks are useful for the guy that plays a 100W valve rig in large venues but also want to use it in the Mucky Duck. Chopped back to 10W or so much of the tone remains but windows are not in danger.

My suggestion would be for a 5 watt push pull, fixed biased output stage amplifier (no spam but there is only one!) . Then get a basic "100W" (they are not) L pad attenuator to drop the level. Five watts into a decent guitar speaker is still enough to hiss off the neighbours should the humour come upon you!

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