High Dynamic Range Input

[Bill Mountain]

Anyone who has caught my ramblings in the forum might remember that I suffer with a bass that has an irresponsibly high output.  I have attempted many ways to tame the beast.  Some good some bad.  I plan on trying everything at least once before I determine the best avenue for me.

I made a post on another forum about a guy who was having trouble using his active bass with a dirt box.  I simply rehashed some of my own experiences and commented that what he needs is a pedal with a high dynamic range input.

This lead me to think about putting a high voltage tube as the input stage of my pedals but this would be needlessly troublesome and I'd still have a ton of signal to throw away after the tube.  I posted in another thread (at DIYSB) about my issues and the ever logical minds told me to just use a voltage divider on the input.  I plan to do this on my next project when the parts come in.

Today's post however is a thought experiment.

I remember reading an interview with one of the old school recording preamp designers (maybe Neve??) who made a comment about preamps with low dynamic range being lazy engineering.  His particular preamps had an 80dB (or so) range.  Now I'm pretty sure it's solid state and I'm pretty sure it's high-ish voltage (not tube high) but what I don't know is whether or not these old preamps used simple boost stages or had complex discrete amplifier stages that can handle and reliably boost any signal thrown at them.

What's going on in an old API, NEVE, or AMEK?  I've looked this stuff up before (I even owned an AMEK board for a while) but these circuits are generally over my head.  I do know that they were designed for mic signals which are generally low but something that can handle a lot of boost should surely be able to handle a big signal.

[merlinb]

Although a mic preamp might go all the way up to 80dB in gain, you would only use that max setting on very weak mic signals. (The rail voltage was usually 24V.)
It's all very simple. If the input signal is too big, you simply have to attenuate it before the gain, possibly with a potential divider, possibly with a transformer. There is always a noise penalty, but if the signal is big to start with then this will often be insignificant. There's no way around it. Once you run out of rail voltage, it's gonna clip!

[Bill Mountain]

Although a mic preamp might go all the way up to 80dB in gain, you would only use that max setting on very weak mic signals. (The rail voltage was usually 24V.)
It's all very simple. If the input signal is too big, you simply have to attenuate it before the gain, possibly with a potential divider, possibly with a transformer. There is always a noise penalty, but if the signal is big to start with then this will often be insignificant. There's no way around it. Once you run out of rail voltage, it's gonna clip!

I'm pretty sure your glass blower can go way above rail voltage though!!!

[FiveseveN]

An input does not have dynamic range. Dynamics becomes relevant when you compare what comes out of some active device to what goes in.

This lead me to think about putting a high voltage tube as the input stage of my pedals [...] I'd still have a ton of signal to throw away after the tube.

When you have a signal that you deem too high, why in the world would you want to make it even bigger?

If the input signal is too big, you simply have to attenuate it

What he said. Add a simple "pad" to your effect, or to the troublesome bass itself. If it has passive pickups, make sure the divider's impedance is high enough.

[PRR]

> I suffer with a bass that has an irresponsibly high output.

If thy bass offends you, chuck it in the garbage.

> putting a high voltage tube as the input stage of my pedals but this would be needlessly troublesome and I'd still have a ton of signal to throw away after the tube.

Let me get this. Signal is Too Strong. You want to make it STRONGER? Using heroic voltages and obsolete bottles?? THEN "throw-away" much of this monster signal ? ? ?

If the bass is active, put a 50K volume pot on the output. Done.

If passive, try 100K pot and 250K pot.

Seriously-- 5 bucks and 5 minutes to try the pot, versus weeks setting-up high voltage and glass only to pot it down again?

If signal is huge, hiss is NOT the problem.
_____________________________

> An input does not have dynamic range.

It does. It has an overload level and a hiss level.

Fender Silverface input overloads around 600mV, has around 3uV input hiss. 106dB dynamic range at the input.

The in-use dynamic range is less. If you stuff 600mV in, and expect near-clean output, you have to turn-down the Volum pot a lot. The stage after the volume pot, plus the rest of the amp, may overload with 100mV out of the pot. Large resistance in pot gives maybe 10uV hiss. 80dB useful dynamic range.

80dB dynamic range in a mike preamp is a pee-poor spec. Orchestral dynamics approach 100dB (15dB to 115dB SPL). Condenser mikes are 20+dB hotter than ribbon mikes. While the final product (FM, LP, CD) must have lower dynamic range (50dB to 90dB), we capture the "full" range in the studio then squish it down to fit in the channel. Even the old AM radio boards got 1uV-20mV at the mike terminals, over 80dB dynamic range for a 40dB channel.

[Bill Mountain]

I guess I wasn't clear.  I was trying to say that a tube pre could handle my signal but it's terrible idea.  I generally buffer then attenuate but I'm always looking for new ideas to experiment with.

[PRR]

> I guess I wasn't clear.

It would be clearer if you said... active or passive pickup. If the signal comes out buffered you can smack it severely. Some specs would guide the smacking but are not essential.

> I was trying to say that a tube pre could handle my signal but...

Yes; and so would a BJT or FET cathode follower or unit-gain opamp.

If your pickup obscenely and perversely exceeds the direct-in ability of say a +/-15V power opamp, then PAD *before* it.

Let's get a visual. My car has a 2-foot trunk. I need 1 foot of plywood. Get to the Home Heaven, they only have 4-foot plywood (and their saw is broke). Your plan is to nail ten 4-foot plywoods to a 40 foot sheet, get a 50-foot truck to bring it home, where I still have to saw it down to get my 1 foot. Huh?

When signals are too weak, amplify them. When signals are too strong, pad them down. There is a narrow range of impedances and levels where loading and pad-hiss may favor buffer-then-pad. "Normal" electric guitars/basses DO fall in this zone. However an axe which is Way Too Strong can probably just pad. Especially if "active". (And no disrespect: especially for bass--- only about six bassists in the world need great upper-mid response where hiss really bites.)

[ggedamed]

...
Let's get a visual. My car has a 2-foot trunk. I need 1 foot of plywood. Get to the Home Heaven, they only have 4-foot plywood (and their saw is broke). Your plan is to nail ten 4-foot plywoods to a 40 foot sheet, get a 50-foot truck to bring it home, where I still have to saw it down to get my 1 foot. Huh?
...

LOL.
Brilliant visual.

[merlinb]

I was trying to say that a tube pre could handle my signal

Could it? An opamp running off 9V can handle input signals at least up to 5Vpp, whereas your average 12AX7 can only handle perhaps 3Vpp if you're lucky. A 12AU7 might manage 6Vpp, but we're not exactly talking about huge differences!

I'm pretty sure your glass blower can go way above rail voltage though!!!

True. Just pretend those limits are 'the rails'!

[Bill Mountain]

> I guess I wasn't clear.

It would be clearer if you said... active or passive pickup. If the signal comes out buffered you can smack it severely. Some specs would guide the smacking but are not essential.

> I was trying to say that a tube pre could handle my signal but...

Yes; and so would a BJT or FET cathode follower or unit-gain opamp.

If your pickup obscenely and perversely exceeds the direct-in ability of say a +/-15V power opamp, then PAD *before* it.

Let's get a visual. My car has a 2-foot trunk. I need 1 foot of plywood. Get to the Home Heaven, they only have 4-foot plywood (and their saw is broke). Your plan is to nail ten 4-foot plywoods to a 40 foot sheet, get a 50-foot truck to bring it home, where I still have to saw it down to get my 1 foot. Huh?

When signals are too weak, amplify them. When signals are too strong, pad them down. There is a narrow range of impedances and levels where loading and pad-hiss may favor buffer-then-pad. "Normal" electric guitars/basses DO fall in this zone. However an axe which is Way Too Strong can probably just pad. Especially if "active". (And no disrespect: especially for bass--- only about six bassists in the world need great upper-mid response where hiss really bites.)

Great visual!  I guess I'm wrong about the tube.  I figured the higher supply translated to more headroom.  Either way I wasn't going to build a tube input stage.  The bass is passive so the best option is to buffer then pad which I plan to do on my next build.  I try turning down the bass but the tone suffers.

I actually agree with everything that's been said.  Sometimes I get rambly and my posts get confusing.

Thanks for all the comments.

[Gus]

What bass and what pickups?

Have you tried moved your pickups far away from the strings?  This lowers the output, reduces string pull AND keeps the passive bass and cable interaction with the following device.

If you buffer and then reduce the volume you will lose the interaction you get with the input of some effects
So I understand your issue I believe

Once you buffer a passive bass your interaction with devices like a FF that have a bass/guitar, cable and input of the signal modifier goes away

If you need a buffer then a volume control maybe a good compromise is to find the resistive load the bass sounds good with and also selecting a good value for the output of the buffer volume control and maybe even add a series resistor  to have a output resistance for a FF something over 4.7K