The purpose of a midscoop

[Mark Hammer]

A picture I saw the other day of someone's recording studio had a pair of Auratone cubes on the desk, and I was reminded of them. If I recall correctly, they had a passive midscoop built into them, using a choke and caps, ostensibly to achieve a relatively flat frequency response, or at least not as boxey as one might expect from an enclosure as small as that, using a single driver.

And once I was reminded of that, it dawned on me that the tone stack in a great many guitar amps easily yields a midscoop as well. In part, I imagine that is to make it easier to accentuate highs and lows from the guitar. But a little part of me wonders if the purpose of the built-in midscoop was also to compensate for an "inappropriate" cab. After all, guitar cabs, especially in combo amps, are not exactly designed to provide any particular frequency response, apart from not sacrificing too much bass, and certainly not a flat response.

[bbsailor]

A picture I saw the other day of someone's recording studio had a pair of Auratone cubes on the desk, and I was reminded of them. If I recall correctly, they had a passive midscoop built into them, using a choke and caps, ostensibly to achieve a relatively flat frequency response, or at least not as boxey as one might expect from an enclosure as small as that, using a single driver.

And once I was reminded of that, it dawned on me that the tone stack in a great many guitar amps easily yields a midscoop as well. In part, I imagine that is to make it easier to accentuate highs and lows from the guitar. But a little part of me wonders if the purpose of the built-in midscoop was also to compensate for an "inappropriate" cab. After all, guitar cabs, especially in combo amps, are not exactly designed to provide any particular frequency response, apart from not sacrificing too much bass, and certainly not a flat response.

[ATTACH=CONFIG]29557[/ATTACH]

Mark,

I believe the passive filter is to minimize masking of the resonance peak range of the small cabnet, in the few hundred Hz range, so that the lower bass range can be heard without too much masking. The early 8-speaker Bose PA speakers used a passive LC filter to achieve the same objective. The newer Bose speakers now use an active EQ.

Joseph Rogowski

[Mark Hammer]

But do midscoops in combo amps aim to do the same thing, or is it a completely different ballgame?

P.S.: As I understood it, the Auratones actually used the same driver as the early Bose 801 and 901)

[Chuck H]

I would speculate that you are exactly right. BUT... You'll notice that the more modern operation of those old amps renders the mid scoop utterly moot. Still, it's become part of the familiar voicing for the PREAMP function. Players who typically clip their amps now use cabinets and speakers to get the final EQ they want. That isn't to say that some cleaner players don't appreciate the built in mid scoop. Certainly when playing clean the original intention of the mid scooped voicing continues to do it's job. Guitar speaker, after all, are still voiced a certain way. The old way for the most part. So the same need for compensation still applies. In fact, clean or clipped the mid scoop has simply become a part of tonal character in different ways. When clean it offers a more natural and broad frequency perception. When clipped, even with the final EQ out the window, it offers articulation and dynamics. Maybe this is a happy accident. Maybe as players we've just learned to work with what we have to get the best tones. And maybe it's just what we're use to hearing and deviation just sounds wrong to our brainwashed sensibilities. Maybe it a little of all those things. But the bottom line, for this thread anyway, is that you are probably right. The evolution of guitar amps simply required a mid scoop for better clean tone fidelity when the amps of the golden age were designed.

JM2C on that.

[J M Fahey]

When I started with this I was baffled when I saw that "NO EQ" amps (such as Champ) and *crude* EQ amps such as early Tweeds (single Tone pot which goes from bright cap to dull one) , early VOX with just a "cut" control or even its modern reincarnation the successful Tiny Terror were usable at all ..... ever worse ... were killer amps

Later I realized that EQ was built-in in guitar speakers, big time.

Letīs see a popular guitar speaker.

A typical American sound one, bright, punchy, excellent with Strats and Teles, the Legend 1258, which I consider a modern version of old ubiquitous Jensens:



Starting from low to high, response is quite flat from 100 to 700 Hz, around 96/97dB average, within 1 dB .
"Ruler flat" even for a Hi Fi or PA speaker

Probably by chance it has a small 2 dB dip at 300/350 Hz which matches the classic "Twin Reverb EQ" dip (simulate it in TSC).

Then it has an important 6dB peak (rises from 96 dB average to 102 dB ... A LOT) at 1 KHz, which sends guitar sound "forward".

Try it yourself: insert an EQ in the loop and raise the 1 KHz band by 6 dB and youīll hear the guitar jump forward.

Then thereīs the classic 1500Hz dip, which all 12" guitar speakers have , itīs a self cancellation problem when half the cone moves forward, half goes backward at the same time.

Then the hidden minefield, like it or not: a 12 dB peak (it reaches staggering 108 dB !!!! ) 2000 and 3500 Hz !!!

Itīs HUGE and gives a very bright and biting sound to the guitar.

Again try what it does by raising the 2500 or 3200 Hz sliders to max boost in a graphic EQ.

So a "flat" typical guitar amp driving a typical guitar speaker os not flat at all.

By the same token, the very crude tone control provided in most guitars actually works because it controls that high frequency peak so complete system range goes from 12 dB treble boost (guitar "flat") to treble cut (tone on "0").

Of course, it was basically a happy accident.

Early speakers (old Jensen) were not "guitar" speakers but "what was available way back then", but having very thin light cones and very light paper voice coils (trying to make an efficient speaker, a tube watt was very expensive) they naturally had said peak.

Obviously no sophisticated sound tracing equipment was used, but everything was given a last adjustment by ear, preferrably playing live onstage at some poor acoustics barn where Country/Rock dances were held.

Just look at all those early Fender endorsers, Rock was not even invented yet, all had cowboy hats , shirts and boots and were very happy with their Teles and later Strats.

Add to that poor damping tube amps, which *boost* frequencies wherever the speaker impedance raises (the opposite to regular SS amps) so they have a 6dB (or more) peak at resonance (typically around 100 Hz) and steadily rising above , say, 600/800Hz for another 6 dB boost at high frequencies.

So typical "flat" tube guitar amp driving a typical guitar speaker is not flat at all, but has a strong EQ which no simple electronic EQ can even dream of matching.

You'll need at least a Studio quality Parametric EQ because not even a 31 band graphic has such range.

EDIT: thatīs why I was practically *forced* to make my own Guitar speakers, none available here, only very good Hi Fi ones , excellent for PA or, say, acoustic guitar, but not punchy enough for Rock Guitar.