Johan Segeborn tests this all the time.
Here is one of his videos.

Sadly, it involves a cheap amp and a (relatively) expensive speaker.

Even so .....

Result is *incredible*

Miyagi asked:

QuoteHello, 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.

In the old days PCBs were fully hand drawn.

Problem being than these designs are very interactive: you first design a draft board, then improve improve improve, push this component there, bring that one closer, separate sensitive tracks, reroute grounds for lowwer hum or interference, the works, as many times as needed.

When boards were hand drawn, you improved them a little and leave it there, because it´s a lot of work.

With software design, it is easy peasy to reroute and redraw everything on screen.

Here board will not be smaller (maybe somewhat narrower) because pots and jacks need to fit on front panel, but you can improve layout and grounding big time.

And order many for low cost.

Quote from: Kaz Kylheku on September 24, 2024, 03:08:28 AMA 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.

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.

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

I´m impressed by the designs and the looks, congratulations.

This is a DREAM opportunity for somebody wanting to build up a small Factory, either a Tech who wants to take the next step or an investor, who wants to multiply his money and hires a Tech to do the soldering and assembly, he takes care of sales (and collecting money ).

The amps can be assembled and sold as-is in those futuristic head-shells or rehoused in more conventional ones, his choice.

The see-through illuminated front panels look breathtaking on any stage (or Album cover ), of course conventional punched and painted (or polished) Aluminum can also be used.

True enough

Under such sheer lack of data, every b it counts.

I suggest OP to post a couple power amp closeups, : PCB showing readable part labels, wiring, power transformer, power supply, etc.

Even if no schematic visible, "counting beans" method may be used: print datasheet schematic and parts list, put good power board pictures by it, go through parts list (as in  R1 ... R2 ... etc), and try to find each part named on PCB, then tick it on both list and picture.

Part numbers will NOT match, but they "must be there" for the amp to work, at least 90% coincidence if not better.

Write on parts list and schematic actual PCB part number.
Say: R1 on schematic is R203 on Trademark amplifier and so on .

Now you have the schematic matching your amp!

You will certainly have a few different/tweaked values but no big deal, you can now repair your amp.

Quote from: phatt on August 17, 2024, 08:42:49 AMI lost my Wife last year And 20 years earlier she lost her son to suicide, she carried on but it left a mark on her outlook on life.
Yes the things that life throws at us can be soul destroying.
So I have some understanding of your great loss.
Phil.

Ouch!
Hard blows for sure but hey, we must keep playing no matter what cards does Life deal us.
What else? 🤷🏻

In the real world, no big deal.
Since forever I used 100k to ground at the input differential leg and 10k as NFB from speaker out to negative keg and my typical offset is around 40-50 mV so I never bothered to correct it.
So you are fine.

Intrigued by this post, searched Heaven and Earth for a Trademark 112 schematic or at least a Power engine 60-112 which seems to be the same cabinet, power amp, power supply, only lacking the multi-emulation multi-effect preamp and sporting only a basic tone control since it's meant to be driven from some multi-effects pedalboard.

Could NOT find anything, these Tech 21 guys are _secretive_ , but one of their designers complained about Yamaha copying their product.

They even use same Celestion 70-80 speaker!

And *that* service manual is available at Elektrotanya. 😉

Different power amp technology (discrete vs chipamp) but the useful point is that power amps have no fancy tricks at all,they are flat and clean (why doesn't that surprise me?), 20X gain (so about 700-1000 mV sensitivity), only slight mod is mixed feedback technology which functionally "changes nothing"

So what's my point?
Now I am *quite* certain your amp literally uses "official datasheet" schematic, so you can use that as a Service Manual.
Not "flying blind" any more. 😊

Hey! KILLER amps and designs, very original, very good looks.
"They sell all by themselves"
I would be a present from the Gods for anybody to be able to get his hands on this and start a new Company or endeavour with so much "start up" job already done for him.
It would save YEARS. And money and teething troubles.
Guess you would also transfer your current customer database, to which they would later add on.
This is similar to buying a working Company, ready to keep earning money  "tomorrow".
Hope some smart guy jumps in to profit from all the groundwork you already did.

 
 

Ok, ONLY NOW can we see the schematic.

1} FORGET current feedback and any fancy stuff.

2) *do* use the output transformer, you *need* it.
Output is bridged and IT primary winding combines out of phase signals.

So connect speaker to proper 4 or 8 ohm tap as needed.

PA amps are weird,this one has 70V taps and also is *designed* to be fed from a 12.6V car battery for street/field use.

Quote from: wound3rd on August 31, 2024, 10:19:12 AM....there's a heat sink pad between the LM3876T chip and the heatsink. In my case the pad was bad - kind of gooey or greasy.

It is *supposed* to be gooey/greasy.
It is smeared on both sides with silicone/thermal grease.

QuoteSo, for a few $ we got some new pad, cut it to size, cleaned the area well, and replaced the pad. Worked like a charm.

It _may_ work fine ... until you play LOUD for an hour or two and it becomes *hot*.

Get a small tube of heatsink grease/compound and apply a thin layer between chipamp and pad, and between pad and heatsink. 

This is a mid power (around 6W RMS into 4 ohm) car radio type amplifier.
You *can* turn it into a current feedback one but I can't read the fuzzy datasheet for instructions 
If available, post the PDF here.
I mean the real PDF,not a link to it.

PS: if bridged, forget about adding current feedback.