Do you need a low pass filter in your amp?

[slimjohnny]

First off i'm a guitar (and amp for that matter) noob.

So i'm making a 30 watt solid state combo as my first amp and i've checked a lot of diy schematics and that gave me an idea on how mine is going to look like. All the schematics i saw were similar, preamp with tone controls and the poweramp. But then i started looking into schematics from marshall and the preamp had more two ICs than the other diy circuits so i did some research and those two IC's behave as a low pass filter.

Now that got me thinking: is the low pass filter really necessary when you already have controls over the frequecy(eq)?. Is it normal for amps to have this? And why did none of the diy schematics that i saw have this low pass filter?

[Jazz P Bass]

How about a schematic snip of the circuit?

[slimjohnny]

That's the preamp section of the amp, and to the right are the two ics.

Do all amps have this?, and why?

[mandu]

This is not a standard in most guitar amps including Marshall. The low pass filter is set to pass signals below 20 kHz. The drive circuit produces lots of harmonics and may be well beyond audible range. The filter may be part of circuit to produce that Marshall sound for this model or the designer added this to his taste. For guitar applications, Low pass filters are generally provided before and after digital delays or BBD circuits. Regards.

[slimjohnny]

Yeah that makes sense, thank you for your reply it really cleared up things for me.

[Ballible]

I don't think it neds a LPF

[pdf64]

The circuits around IC5A & B look like gyrators (rather than low pass filters per se) to me.
Resonant filters.
There's probably online calculators to derive their response, and teemuk's book will have the circuit explanation / analysis / equations.
https://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf

[Kaz Kylheku]

The filter section is indeed low-pass filtering. It's using the Sallen-Key topology which creates a two pole filter (-12 dB per octave roll-off).

The good old Okawa Denshi site has a calculator for this.

To use a calculator, we need to interpret the capacitor values. The schematic gives some as two digits followed by a p, so 47p is clearly 47 pF and 470p is 470 pF.  I believe that the unsuffixed codes have to be interpreted the same as capacitor markings. For instance 473 means 47 with three zeros or 47000 pF (= 47 nF, = 0.047 uF).

The prefixes C and M give tolerances, I think: C means +/- 0.25 pF, and M means +/- 20%.

For the first stage, we use the first calculator presented on the above page.

The first stage has slight peak (boost) around 10 kHz, after which it rolls off at the -12 dB per octave rate. The C prefixes on the capacitor values, I think, are giving the required tolerance: +/- 0.25 pF, which is rather stringent.  So this filter not only rolls off after 10 kHz, but also provides a little presence boost there.

The second stage has gain in addition to the filter, and so we use the third calculator which is for a circuit that incorporates the same feature.

That filter has a cut off frequency of around 8.6 kHz. No bump a the knee.

There is also an RC filter between the two stages consisting of R15 and C16. This has a cut-off frequency of also around 8.6 kHz, rolling off at -6 dB per octave (one pole).

Thus the RC filter and second stage are intended to create a three-pole low-pass filter, rolling off -18 dB per octave after around 8.6 kHz.

This makes sense if you're coming from a high gain circuit that generates a lot of fizz, and then going into a clean power amp. Why four poles though? The answer is almost certainly that this is for an amp that has a tiny 8" speaker. I don't think you'd want this much roll off going into a cabinet with 12" speakers, especially if their resonant frequency is below 100 Hz.

In my rack, I have an ADA MP-1 pre-amp, and clean power amp going into fairly bright 12" speakers. In between there is a 31 band eq and a splitter/mixer/filter device of my own design which has a Sallen-Key low pass filter with a variable frequency control knob.

I need to use the high frequency bands {6, 8, 10, 12, 16, 20} kHz on the equalizer and my low-pass filter to have good control over the high end.

[J M Fahey]

1) that is a STRONG low pass filter and is NOT common or needed, at all, in normal SS guitar amps.

2) you do not show where they lead to, but a few amps or preamps or pedalboards, digital or analog, include similar 24dB/octave or stronger low pass filters as "cabinet simulators".
Normal load for a guitar amp is a guitar speaker which has a built-in "mechanical" low pass, usually above 4kHz or so.
If signal is fed into a recording or PA mixer or into a powered cabinet which have way wider response, tweeters or drivers, etc. , sound is *unbearably* buzzy, so such a filter is added to mimic guitar speaker rolloff.
Your circuit is incomplete but I guess it either feeds headphones/line out or somebody complained about buzzy sound and designer included it for good measure.
Again, not "common" by any means, you do not *need* it.

As a real world example, Hughes & Kettner Red Box: a  signal attenuator -direct box - cabinet simulator, including such a filter:
https://elektrotanya.com/hughes-kettner_red_box_classic_sch.pdf/download.html

[Tassieviking]

Bass guitarist sometimes use a HPF and a LPF in their setup to get rid of any unwanted signal.
It is common to have a HPF set at 60-120Hz to get rid of the "Mud" in the signal, and then get rid of the top end with a LPF to get the "old timey" sound.
The older bass speaker cabs were poorly designed and had no mid frequencies, if you want to play the 50's and 60's music you don't want any mids or high frequencies in the bass tone.

Marshall amps are known for a certain tone and that might be why they used the LPF filters in that amp, or they knew the speaker was crap and they used the filters like a cab sim.

[J M Fahey]

You may have a point there.

Marshall uses a certain 8" speaker on small cheap models, say the ubiquitous "15W practice amp" , a market section covered by *everybody* because there are millions of users out there, which has a very small 19 mm voice coil (Jensen C 6-8-10-12 R uses 25 mm) which is also used in some Tweeters.
Couple that to a small magnet ("no Bass") and paper thin cone and you "almost" have a Tweeter there, or at least an extended range speaker, with a great potential to sound reedy/buzzy.



That would justify a treble rolling filter.

That speaker is Marshall _brand_ , Asian made, definitely not a Celestion.

[Kaz Kylheku]

A while ago this thread prompted me to go look for some videos demoing this amp.

There is even a "will it chug" one, by Ola Englund. (Spoiler: it did chug.)

In all the videos where they close mic this amp, it sounds nothing like an 8" speaker box. There is very little high end.

I spotted a comment by an owner who said that it sounds way too dark to be used with a clean Fender Strat.

Traditional combo amps with 8" speakers are not like that; they are bright and usable with your Teles and Strats for twangy music.

So, yeah, this is low-pass filtered to heck.

I don't understand why they wouldn't instead experiment with a different speaker. I mean, there are 8" woofer drivers that can make bass. It seems inefficient to be driving a speaker like this with a frequency range that is not right for it.

I wonder what something like this sound like: https://www.solen.ca/en/products/ws20e

Even if it needed EQ, I bet it wouldn't be as drastic.

[J M Fahey]

A while ago this thread prompted me to go look for some videos demoing this amp.

There is even a "will it chug" one, by Ola Englund. (Spoiler: it did chug.)

In all the videos where they close mic this amp, it sounds nothing like an 8" speaker box. There is very little high end.

I spotted a comment by an owner who said that it sounds way too dark to be used with a clean Fender Strat.

Traditional combo amps with 8" speakers are not like that; they are bright and usable with your Teles and Strats for twangy music.

So, yeah, this is low-pass filtered to heck.

I don't understand why they wouldn't instead experiment with a different speaker. I mean, there are 8" woofer drivers that can make bass. It seems inefficient to be driving a speaker like this with a frequency range that is not right for it.

I wonder what something like this sound like: https://www.solen.ca/en/products/ws20e

Even if it needed EQ, I bet it wouldn't be as drastic.

There are 5 reasons for that.

In no particular order:
* cost
* cost
* cost
* cost
and last but not least:
* cost 

Serious, they must buy these by the containerload (not exaggerating) and use them *everywhere*

I estimate cost between $2 and $5, in large quantities. 

I commercially manufacture Guitar speakers, should NOT be able to comloete with Chinese suppliers (Marshall counts as one  ) except that their infinite greed leaves a little niche market for improvement.

I sell LOTS of 8" guitar speakers to Musicians who want to improve their 15W practice combos.

Mine is on left, Marshall on right.

I use 2-3 times as heavy magnetic system , 25mm voice coil (instead of puny 19mm), and to boot chose a non-ribbed and somewhat thicker cone for better mids and low mids ("warmer sound") , also somewhat less highs (less shrill-buzzy tone)





But it *still* is a Guitar speaker, not a Woofer or Hi Fi one.

[J M Fahey]

Miyagi asked:

Hello, señor Fahey.
I've read your post on low-pass filters in cheap guitar amps where you stated that you manufacture your own speakers (I've read quite a few of your other posts here and there as well) and it got me interested. Because I sometimes modify small combos myself, I started wondering if you have ever tried any off-the-shelf midrange or woofer drivers in those. Such speakers can often be bought for peanuts (at least compared to speakers meant specifically for guitar) so they look pretty tempting. Do you have any experience with that?
Thanks in advance,
Maciej

Hi Maciej, nice to meet you.
Those speakers *can* be used at home, or for experiments, no damage done, but when playing live, even during a garage rehearsal, they "disappear" the moment a drummer enters the room.

Guitar speakers are at least 2-4 times louder than an equivalent HiFi soeaker, and I am talking an expensive one.
Cheaper ones?
Even worse.

Guitar amps were developed in the 40s

Way back then, power was at a premium, very hard to get. a typical Home "Combination Radio-Grammophone" was about 5W RMS, Guitar practice amps (Champ) the same, cheap beginner amps, think the countless "widow maker" amps such as Silvertone and many others, sporting 35W4 50C5 12AV6 tubes (table radio tubes) about 1 to 2 W RMS

So speakers HAD to be loud, efficient and ear biting.

But then power became cheap and abbundant, HiFi demanded flatter and less distorted speakers, real Bass, so they changed A LOT.

40s speakers whose "recipe" is followed today "because it works" had paper thin lightweight cones, often a lot of "cone cry", small light voice coils, fast sensitive response, a large peak around 3 kHz which is tinny annoying for Music but gives guitars "bite", etc.

Compare "exact same speaker", meaning same frame, magnet, diameter , price, but built either for Hi Fi or Guitar.
Even so this one is PA type, not true Hi Fi, which would be 6 dB or worse less efficient.

Eminence Legend 1258 (guitar):


Eminence Beta 12 (PA):


Now $13 Parts Express woofer:
https://www.parts-express.com/GRS-12PF-8-12-Paper-Cone-Foam-Surround-Woofer-292-412?quantity=1



notice very flat response, NO bite peak, average sensitivity around 88dB (vs Guitar/PA speakers 98dB)

Again: alone at home this one will "work"; but playing with others? ... it disappears.