Quality of line out in load boxes (iron man mini 2 & suhr reactive load)

[ef37a]

FYI I used a 1/2kW resistive load to test and specc' out valve amps up to 200W (8 Ohms).

That had an analogue mV meter across it which had a top range IIRC of 1kV but making a resistive attenuator to bring the voltage down to line level is beer into water and will do nothing to alter the frequency response. 50W into 8R is 20V rms so you are going to need about 20dB to drop that to +6dBV* or about +8dBu. If you look up the tables to make a "600 Ohm" attenuator you can then use a 600 Ohm line transformer to give earth isolation. OEP traffs have some very suitable, modestly priced units, after all you don't need <<0.1%thd at 20 Hz for guitars!

*Guitar electronicists are you see modern and logical in their measurement parameters!

Dave.

[Hugh Robjohns]

So far, I found that the best way for this is to use a real speaker connected to my amps. Line out is provided by the AR-133 DI box (speaker is connected via the link output).

As a general rule, it's not a great idea to use the link socket when DI-ing an amp/ speaker.

The reason is that a lot of current flows between amp and (low-impedance) speaker. If you plug a DI box into that amp-speaker circuit all that current has to travel between the input and link socket terminals, usually via thin PCB tracks inside the DI box — and they are rarely up to the job!

It's much better, therefore, to connect the speaker directly to the amp, and then use either a duplicate speaker output socket on the amp, or an extension (parallel) socket on the speaker cab itself, to derive a feed for the DI box.

In this way the DI box still sees the full amp signal, but doesn't have to pass any current and is therefore at much less risk of bursting into flames!

You'll need 40dB of input attenuation selected on the DI box input pad because of the very high voltages at the amp output.

[Wonks]

The Celestion Peacekeeper sounds just like a standard speaker. There was a That Pedal Show episode on it. Although they thought it didn't sound as good in the studio (because it was quieter, and 'louder is better'), the level matched mic recordings sounded very close indeed to the standard Celestion used for comparison (they should have listened to the recordings before passing judgement).

It's not a load box, just a speaker designed to be inefficient. Or more inefficient than a standard speaker is. But as that's only 3%-4% efficient anyway, the extra heat output from it will be minimal.

[Wonks]

But any isobox isn't designed to be used for hours on end with lots of power going through it. The acoustic isolation also provides good thermal insulation, so with 50W-100W of power, all but that escaping as weak sound being converted to heat, the heat will quickly build up. Not good for the speaker, and not great for any mic used inside it.

If the isobox is sitting directly on the floor, it's worth trying it out raised up (manually for testing) to see if the bass level drops. If that works, there are foam mats you can get for putting bass cabs on, which might improve things. Or make a wooden frame and suspend it on steel springs.

[Guitarking]

Tnx!

@Hugh, I always have the 40dB pad engaged on my DI. Seems to work fine for years now. If I recall correctly, the manual of the AR-133 states that it is possible this way, but of course with the 40dB pad engaged.
Do you still stink it's unsafe this way and?
And if so, how would I get a line level? Could I, for example, just plug the DI into the second output of my amp (without having to change the ohm?). Or how to get the line from the speaker, I don't want the sound of the speaker, as I'm using IR's.

@Wonks, the isocab is on wheels. But I like your idea of extra suspension.

[Guitarking]

Can anyone confirm the neutral quality of the Fractal Load Box (in silent mode)? As I said at the beginning of this thread, my experience (and what I read) is that once the load/attenuater kicks in, the line-out signal becomes dull. I want a full/neutral line-out signal to send to my daw, but also a proper load to my tube amp of course.

And also, is it not available anymore in Europe?

[Hugh Robjohns]

Seems to work fine for years now.

A 100W into 8 ohms is about 3.5 amps. Jack sockets are typically rated at either 5 or 1 amp. But its the pcb tracks that are the potential weak points... depending on the construction.

And, obviously, lower powered amps involve less current flowing to/from the speaker.

If it works for you then fine... carry on.

My caution is because I have seen a DI on fire after being connected that way!

If I recall correctly, the manual of the AR-133 states that it is possible this way

I checked.. and it's rather vague... but the diagram appesrs to show only a single cable going to the DI box!

Do you still stink it's unsafe this way?

Well, if I really had to DI the power amp output I would always connect as I described (assuming the amp/speaker allowed that) in preference to looping the speaker feed through the DI box.

But I can't remember the last time I DI-d a power amp. I typically either mic the speaker, or take a line level feed from the preamp, FX chain output, or raw guitar (or all three!).

And if so, how would I get a line level?

Connecting the DI jox as I described gives exactly the same output as looping the speaker feed through it. In both cases the DI circuitry is sitting in parallel with the amp and speaker, sampling the audio voltage on the line.

The only difference is that my arrangement doesn't ask the DI box to pass all the power to the speaker as well...

Could I, for example, just plug the DI into the second output of my amp (without having to change the ohm?).

Yes, that would be ideal... and the amp won't even notice it's there!

Or how to get the line from the speaker...

Wire a second socket in the speaker cab in parallel with the first, and use that to feed the DI box.

I don't want the sound of the speaker, as I'm using IR's.

In which case, do you really need the amp/speaker at all? It might be easier to DI the output of your FX chain or preamp output...

[Guitarking]

Thank you Hugh.
Yes I do need the amp as I like them and I mainly use non master volume tube amps with no fx.
Max 35w.
And I need the speaker as it is the best sounding load (attenuators and loadboxes dont sound good so far in my experience).

[ef37a]

To enlarge on Hugh's point, a lot of low power <20W transistor guitar amps have a headphone jack fed from the PA and switch the internal speaker off when cans are inserted. Invariably you find over the years that the internal speaker either goes silent or gets low and distorted.
This is due to minute flexing of the PCB over the years leading to a 'dry' joint which then heats up and gets worse, even in some cases burning a hole in the board.

Even the switched jacks in an FX loop can suffer tarnishing and give problems.

Bottom line: if you want to switch speaker levels, use a friggin switch!

Dave.

[Guitarking]

thank you for the warnings.
I've re-routed now and get the line out from a second amp-output instead of the link in de DI.

[Guitarking]

For anyone in the same search as me for how to get a great line out signal and load for your tube amp: this might be interesting:
https://www.youtube.com/watch?v=AR_q297ll5U

Seems like it explains why my Iron Man isn't doing the job for me (although as attenuator it's great!). And that the Fractal Load box is best.

Is this a safe conclusion that the Fractal wouldn't dull my line out signal when in load?

Still doubting whether to buy the Fractal (or Suhr), or keep using my isocab with a Celestion Peacekeeper... The latter is a) 50% cheaper b) available whereas the Fractal is not available now in Europe.

[Guitarking]

I have a silly, seemingly unrelated question, whilst brainstorming on the decoupling idea in the isobox:
How about decoupling the guitarspeaker by hanging it on sturdy elastics. E.g. the elastics go through the mounting holes of the speaker.
Goal is not a great sound in the isobox anymore but rather to get as little sound as possible. Maybe this is achieved by 1) getting rid of the cab in the first place and 2) decoupling the speaker by hanging it.
Is is mandatory for a speaker to be on a baffle? Will a speaker be damaged by not being on a baffle and "floating" in the air?
Inspiration came from Wonks' decoupling idea combined with a video I saw by Johan Segeborn who took a speaker out of the cab.

[Wonks]

In free air, the speaker cone will move backwards and forwards more. Even in an open backed cab, there is more air resistance for it moving than in free air, where the air pressure generated at the front by it moving forward and the slight vacuum created at the rear when it does this, will travel round the edge of the speaker very quickly and equalise a lot more than if it was in a cab. In a sealed cab there is even more resistance and the speaker travel is minimised.

So as you turn the volume up, there is a chance of the speaker excursion exceeding its design limits. Keep the volume down and you'll be OK, but you probably want the sound of the amp driving hard, which increases the risk of speaker damage.

With too loose a suspension, the speaker itself could be jerking about all over the place, so there is a small risk of the speaker coil and the speaker body moving in slightly different directions and the coil grating against the side off the voice coil gap. Certainly a greater risk than this happening normally but I couldn't put any numbers on it.

When the speaker cone moves forwards, the reaction force will move the speaker chassis backwards, and when the speaker cone moves backwards the reaction force will move the speaker chassis backwards. In a cabinet, the movement of the speaker chassis is minimised due to the mass of the cab, but if there's just the chassis on an elastic suspension, the chassis will move a lot more. Get the right/wrong resonant frequency and the voice coil can travel further into and out of the chassis than it's designed to, which is when voice coils get caught and speakers burn out.

So some form of baffle is recommended, even if it's just a sheet of wood that's a few inches larger than the speaker. The longer the distance the air has to travel between the front and rear of the speaker the better. And any suspension of the baffle needs to be fairly firm.

You might be fine, especially at lower volumes, but then again, you might just be unlucky, and you only have to be unlucky once.

So I'd much prefer to try and further isolate the whole iso cab rather than just the speaker.

[ef37a]

It is true Guitar king that a bare speaker suspended as you suggest would be very inefficient but for a 12" guitar speaker there would still be a serious noise level at and above about 1kHz.
I think people have been trying to record rock guitar at sane levels ever since the first hairy guy ripped a power chord in front the genteel engineers at Abbey rd, Decca,RCA? Pick your venue!

As to speaker damage? Not likely IMHO it is true that speakers in totally sealed cabinets are protected to a degree from VLF signals but drive units, especially guitar units are very tough these days and this is not something I would worry about. Well, not at the 100W max'ish levels found in guitar amps. More care needed for monster bass rigs.

Dave.

[Guitarking]

my max. amp wattage would be 35W (crancked this is more of course, 70w??)

[ef37a]

my max. amp wattage would be 35W (crancked this is more of course, 70w??)

Strange but I cannot find any reference to the exact amplifier you are using?

If a 4X EL84 nominal 30W design and cathode biased you will not get to 70W.
Two EL34/6L6 fixed biased will deliver 35-40W easily depending on the HT voltage but you would still have to be blowing the absolute gonads off the amp to hit 70W.

Dave.

[Guitarking]

Reinhardt JKC-33: plexi style amp with 2xEL34

[ef37a]

Reinhardt JKC-33: plexi style amp with 2xEL34

Right, well AFAICT that is a fixed biased amp so yes, it might kick out 70 watts if you really give it the beans assuming the power supply is up to it.

This is why guitar amp speakers should be rated to at least 50% above rated amp power and better 100%. Celestion drive units are mostly conservatively rated but I don't trust all brands to be so.

Dave.

[ajay_m]

I may have to duck for cover here, but as a keyboard player and utterly incompetent guitarist, I use the Musiclab products so I can pretend I really *can* play guitar.

But when it comes to amp, speaker and effect stacks, both the Guitarix package that Musiclab supplies, and Native Instrument's Guitar Rig, do seem to do a pretty good job of simulating amps and speaker cabinets (and Musiclabs have a bunch of 'artist presets' that you can download for free, with names like David Gilmour and Eric Clapton etc against them and I'm supposing that those folks aren't gonna endorse something that sounds like total crap).

So... and please forgive my ignorance.... given that these software-based solutions do seem to offer a huge tonal palette and sound to my non-guitarist ear pretty damn good.... wouldn't this be a more fruitful path than trying to soak up 99% of the power out of a guitar amp so you can capture the sound without deafening the neighbours?. Or is it the case that actually, no, that's really the only way to capture all the nuances of your playing... which does kinda beg the question that, if you listen to some of the Musiclab demos, would your audience actually.... dare I say it... know if you had the temerity to resort to running your guitar through a stack of plugins?.....

I'll get my coat now....

[ef37a]

You are not wrong Ajay, I doubt anyone in the crowd at a concert could tell if the guitar originated from a micc'ed up "classic" amp rig or a modeller.
I think most of us have seen the YT picture of the Marshall "shell" backline!

As for smashing a 30W amp down to a few mW yes, bit nuts. I am waiting for the day when Stop Oil or some other Green group realises just how dreadfully inefficient valve amps are and then to waste most it as heat! BTW if the amp is really cooking the fraction wasted is nearer 99.9%!

Still, in ten years or so I think valves will be so expensive that only the rock gods will be able to keep their rigs running...IF that is they actually use them!

Dave.

[James Perrett]

Is this a safe conclusion that the Fractal wouldn't dull my line out signal when in load?

Having just dipped into this thread occasionally to see where it was going, one thought struck me. I think you are struggling against psychoacoustics. It is a fact of life that quiet sounds will always sound duller and have less weight. You can't just take a loud sound, turn it down and then expect it to sound just as exciting as it was when it was loud. People's ears and brains simply don't work like that. If you want a quiet sound to be exciting then you have to start again and create a different sound that works at low volume but which may well sound wrong at high volumes.

[Sam Spoons]

Good point James, if you want more detail Guitarking consider the 'loudness' button on '70's HiFi's and give this a read to see why they were a thing.

https://en.wikipedia.org/wiki/Equal-loudness_contour

[ajay_m]

Interestingly, Guitarix is free and open source - and so someone has created the pedal board of their dreams - see here https://tibbbz.medium.com/guitarix-the- ... 6298ca8e42

He says that construction costs ought to be around £165 including a 7 inch screen. I have to say this is a tempting idea even as a keyboard player, to have a proper effect stack as an external box, that you can just run anything through.

[Martin Walker]

Interestingly, Guitarix is free and open source - and so someone has created the pedal board of their dreams - see here https://tibbbz.medium.com/guitarix-the- ... 6298ca8e42

Wow, that IS impressive!

[Hugh Robjohns]

When reading the first part I was wondering at the madness of hacking a usb keyboard when the Pi offers plentiful GPIO. Glad he realised the error of his ways in part two.

...and if he increased the sample rate from 44.1 to 96 he'd reduce latency by a useful amount, too.