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Gnobuddy 25th October 2017 10:57 PM

Tube Emulation & EQ

Originally Posted by Tubelab_com (
It is nearly finished.....well maybe. I need to wire up the transformers, and plug it in!

I'm glad you finally found it, and am looking forward to hearing how the eventual power-up goes!

For over a year now, I have been planning to make a guitar amp to gift to a friend. He is a senior citizen on a small fixed income, and dealing with a physical disability, so the amp needs to be reasonably small and light. I am on a tight budget myself, so the amp needs to be cheap.

I'd alread made some progress. I found an old thrift-store Sears "Hi-Fi" (anything but!) speaker for five bucks; it has a plastic grille with a Vox-like pattern, so it will become the enclosure. I also sourced a pair of more recent (80's?) boom-box speakers from the thift-store, and these donated their two woofers to the cause.

A few days ago I finished modifying the Sears enclosure to accept the pair of woofers. Then I hooked them up to an Ebay-sourced class-D power amp module, powered by a thrift-store 24V DC supply. The amp module is a pair of bridge-mode amps, one driving each 12 ohm speaker. I estimate something close to 20 watts RMS into each of the two speakers. 40 watts out to be ample for my friend.

I hooked up the class-D module to a Danelectro Fish-n-Chips guitar pedal - it's a 7-band graphic EQ, with a built-in clean preamp with adjustable gain up to 20 dB or so.

I plugged in my Yamaha electro-acoustic guitar, and it sounded pretty good through the combination of Fish-n-Chips preamp/EQ, class-D module, and boombox woofers mounted in the old Sears speaker enclosure. So far, so good.

Then I plugged in my old beater Epiphone LP Special II. And that's where things went seriously off the tracks: this electric guitar (solid body, pair of humbuckers) sounded absolutely awful. Harsh, ugly, and unpleasant.

It's not the guitar - plug it into any of my valve guitar amps, and it sounds fine. Not the greatest guitar in the world, but a perfectly usable instrument.

But into the solid-state preamp/class D power amp/boombox speakers? Awful. Completely unacceptable. No way I can give this to my friend, unless I can make it sound a lot better.

I've heard this awful harsh sound before, from every one of the solid-state DIY guitar amps I built in my first few years of learning to play the electric guitar, and then from a succession of commercial solid-stage electric guitar amps made by companies like Fender and Line 6. In fact, I came close to giving up playing the guitar entirely, because I sounded so awful, and I thought the problem was me.

That finally changed when I finally got my first valve amp. I was shocked that these primitive, obsolete amplification devices from the dawn of electronics made my guitars sound good, while every SS amp I'd tried made them sound awful.

Back to the present. My solid state gift-amp-in-progress sounds awful. Which leaves me wondering what exactly makes it sound so harsh and nasty, and what, if anything, I can do to remove at least most of the awfulness.

I've already found out that EQ alone doesn't do it (that's why I chose to use an EQ pedal as my temporary preamp.

I could try a speaker-emulation filter of some sort - a steep 3rd or 4th order low-pass, cutting everything above 4 kHz or so. It would go ahead of the class-D power amp module. Maybe that would remove the harshness? (But then, why did commercial SS guitar amps using real guitar-amp speakers still sound harsh??)

I also have various JFETS to experiment with in preamp designs. But I don't know if they can work magic on this thing.

Finally there is the option of using a $1 valve in the preamp. I'd have to come up with HV and heater power supplies, but I'll do it, if it's the only way.

The one thing I can't do is go all-valve. Too much weight, too much cost. So I have to do my best to make the class-D thing sound at least acceptably good.

Two big questions to attempt to solve. Where does the harshness come from? And how do I make it go away?

Hopefully I will have something worth reporting, as I look for a way forward with this thing.


Gnobuddy 27th October 2017 03:19 AM

3 Attachment(s)

Originally Posted by rongon (
I think a lot of the sound we like out of guitar amps has to do with high frequency roll-offs.


Originally Posted by Printer2 (
I think it is more the way a 12" speaker will beam the highs.

I am hoping both of you are right, because filtering out high frequencies will be relatively easy to do, comparatively speaking.

There was a thread elsewhere on diyAudio where the subject of (guitar amp) loudspeaker frequency response came up. I grabbed the first frequency response I found on the net, drew a couple of straight lines on it, and then fiddled around with LTSpice to see if I could roughly duplicate the frequency response.

In the Hi-Fi world, speaker frequency responses are measured on-axis. In the world of Eminence guitar speakers? I have no idea, but if the published curve is on axis, as I suspect it is, it's already falling off dramatically (3rd or 4th order) above 3 kHz.

So if that's the on-axis response, imagine what the response looks like when you also point the speaker at your knees instead of your ears? Is it falling above 1 kHz? That's about a one-foot sound wavelength, small enough for a 12" speaker to beam noticeably. :eek:

In LTSpice, it took me an hour of fiddling and three virtual opamps (or one and a half dual opamps!) to roughly reproduce the Legend 1028 frequency response. In the attached schematic, the first opamp implements a crude first-order peak at around 3 kHz. The second one mimics the bass end of the speaker frequency response curve (2nd order high pass). And the third implements the high-frequency rolloff (2nd order low pass).

I'm hoping I can simplify the electronics with a little more tinkering. But, worst case, opamps are cheap.

Meantime, I have a couple of guitar pedals that are supposed to have speaker emulation built-in. Before I try to build my own, maybe I should try one of those pedals between a guitar and my nasty-sounding DIY amp to see if they improve things.


Gnobuddy 28th October 2017 05:25 AM


Originally Posted by rongon (
That sounds like a worthy experiment.

And I haven't had a chance to try it yet - by the time I get back from work, it's too late to play guitar. Apartment living has its downsides!


Originally Posted by rongon (
It would be interesting to know what those speaker emulators are actually doing. Just EQ?

There is always superstition floating around when it comes to music gear, so there are probably some speaker emulators that contain blood from a pigeon's heart, moss taken from a gravestone at midnight, and the toe of a toad. :D

My old Super Champ XD has a crude speaker emulator built into the preamp ahead of the line-out jack. Fender published the official schematic. It's been a while since I looked at it, but as I recall, it was just a 3rd order low-pass filter implemented with an opamp or two.

I have one piece of Behringer gear, an active DI box that is supposed to include some sort of speaker/cab simulation. I think it's supposed to emulate a 4x12 cab stuffed with some sort of Celestions, which probably means it includes some comb-filtering because of interference between the four drivers in the cab. I've used this on a couple of home recordings, and I preferred the sound without the cab simulation...

One other data point: my most cherished guitar pedal, a Digitech Trio+, has two outputs, one for feeding to a guitar amp, the other for feeding to a P.A. system or flat-frequency powered speaker.

This latter output includes some sort of filtering to take the harshness out of the guitar when running into a full-bandwidth speaker, and it works quite well.


Originally Posted by rongon (
...a single-cap LPF with -3dB at about 5kHz. (That's what I remember, but perhaps it was lower, more like 3kHz.) That sounded pretty good to me.

I have tried a few similar experiments. To my ears, anything up to maybe 5kHz - 6 kHz is fine for clean tones, but 6 kHz sounds harsh with some types of overdrive.

I was quite surprised that the Legend 1028 rolls off as early as it does. Only three kHz? Geez!


Originally Posted by rongon (
That led me to the conclusion that a lot of the guitar amp's sound had to do with frequency response.

I agree, and I think this is true in every field of audio. Fifty to seventy years ago, everyone working in audio understood this, and that's why there was so much effort put into trying to attain 20 Hz - 20 kHz flat frequency response for Hi-Fi.

Since that time, Hi-Fi reached its technical and popularity peaks, the real engineers and researchers mostly left the field for good, and superstition and nonsense started to take over. so now we have people obsessed with WWII-era paper in oil capacitors and so on, things which actually don't matter at all to the sound.

The thing is, I've never seen much of anything published by actual researchers about guitar amps and guitar sound. Most of the real research is probably proprietary, locked up in the intellectual property of large corporations.

That leaves a lot of opinions, superstition, and a small mix of actual good information scattered around where you and I can find it on the Internet. It's not always easy to tell the good from the bad!


Originally Posted by rongon (
So, I took away that if you take a so-so amp, put a nice speaker on it (with a sound you like), and make sure the 'tone stack' you use has the kind of EQ/phase bumps and valleys you prefer, you can get that amp to sound pretty OK, at least as far as its clean sound is concerned.

And yet I've never heard a clean sound I like out of a solid-stage guitar amp, including various commercial ones (Fender, Line 6, etc)...

I'm convinced there is something more going on, whether it's the few percent of low-order harmonic distortion from triodes in the preamp, or something else.

One candidate: the typical 12AX7 triode gain stage is only biased to -1.5 V. From the datasheets, we know grid current in a 12AX7 is specified to reach 0.3 uA when the bias is -0.9 V. So if the guitar signal swings only 0.6 V positive (from -1.5V to -0.9V), grid current is already flowing in the input stage triode, and starting to soft-clip the positive peaks of the guitar signal.

As a rough estimate, a guitar with a 500k volume pot set to half-resistance has a source impedance a bit over 250k. 0.3 uA of grid current flow through this will drop 75 mV of voltage. The guitar had to put out a peak of 0.6 V - that's 600 mV - to make this happen. 75 mV is 12.5% of 600 mV.

So, input grid current flow is already squishing any 600 mV positive peaks from the guitar by about 12% before they ever make it through the first triode. Plenty of guitars can put out that much voltage, and that is definitely an audible amount of squishing, but I don't know exactly what it does for the sound of the guitar. Maybe it softens those harsh initial pick-attack transients just enough to make the guitar sound better, smoother, more musical?

I have never seen any discussion of this issue anywhere on the Internet or in print, though I brought it up once before, I think on this very forum. People have certainly thought about grid current flow in output valves during overdrive, and even in overdriven preamp valves in high-gain amps. But if anyone's thought about the possibility of grid current flow being an issue at the very input of a typical 12AX7 guitar preamp, they've kept quiet about it.


Gnobuddy 28th October 2017 05:33 AM

1 Attachment(s)

Originally Posted by Printer2 (
A few vids on mic placement out there.


It's amazing how much of a difference it makes to go off-axis by just a few inches.

Here is my first attempt at a very simplified speaker emulation circuit (simulation only, not built yet). This time, I made no attempt to duplicate the low-frequency end of the speaker response (my little boom-box woofers already have their own bass roll-off). So I tried to duplicate only the roughly 3 kHz peak and rapid fall-off above it.

I grew up with the wonderful BC147/148/149 transistor family, so I used one of their descendants in my circuit. It looks like one single transistor will do the job, though it must be fed from a low impedance, not direct from the guitar.


Gnobuddy 29th October 2017 12:53 AM

I had a little time to spend tinkering with my budget hacked-together DIY guitar amp today. As before, I used a Danelectro Fish-n-Chips 7-band graphic EQ pedal as the preamp, mainly because it lets me quickly and easily tinker with voicing and gain.

In an earlier post I said that EQ alone wasn't fixing the nasty harsh sound I was hearing. I was wrong - the problem was that I wasn't going low enough in frequency with the EQ! I had tinkered with the 6.4 kHz, 3.2 kHz, and even 1.6 kHz sliders on the pedal, and none of them fixed the harshness. I didn't go lower than 1.6 kHz, because it seemed too low to bother with - surely the problem didn't go that low?

Well, it does! Today I found out that the 800 Hz band is the key to improving this particular amps sound. With the 800 Hz slider dialed well down, the nasty harsh sound disappears. I can bring the 1.6 kHz and 3.2 kHz sliders back up, and the amp sounds good, as long as 800 Hz stays notched out. But bringing up 6.4 kHz starts to sound scratchy - too much of a good thing.

Overall, the amp sounds best with a notch at 800 Hz, a gently rising bass response below that down to 100 Hz, and treble flat or slightly peaking at about 3.2 kHz, and the 6.4 kHz band rolled right down as low as it will go. The 3.2 kHz peak is pretty well in line with the guitar speaker frequency responses we've been looking at on this thread.

The speaker enclosure I'm using has a height of 463 mm (standing upright, this is the longest internal dimension between two parallel walls). It's about 19 degrees C in the house now, so the speed of sound should be around 342.4 m/S. ( Temperature and the Speed of Sound ). A wavelength of 463 mm calculates to a frequency of about 740 Hz. My EQ pedal has only 7 bands, each band covering roughly an octave, so 740 Hz may be close enough to 800 Hz to be what I'm hearing.

I might try stuffing the box with rags to see if that changes anything, but right now, I don't know if I'm dealing with a peak caused by acoustic reflections in the box, or a peak in the frequency response of the boombox woofers I'm using. Either way, it seems that inserting a notch at 800 Hz is a good way to remove the problem.

Perhaps a two-knob Fender tone control circuit (like the one in a Princeton reverb, with a fixed resistor for mids) would be a good choice for this amp. I just have to work out component values to put that mid-scoop at 800 Hz. Alternatively, I might put a twin-tee notch filter tuned to 800 Hz somewhere in the circuit.

And once again, I am thankful to Danelectro for their wonderful little Fish-n-Chips EQ pedal. Without the ability to quickly dial in different frequency responses, I doubt I would have found the source of the harsh sound from this amp, and certainly, not this quickly or easily.


Tubelab_com 29th October 2017 01:58 AM

Can you try playing the "nasty" amp through a real guitar speaker?

Gnobuddy 29th October 2017 06:39 PM


Originally Posted by Tubelab_com (
Can you try playing the "nasty" amp through a real guitar speaker?

I will try that today and report back. The only real guitar speaker I have at the moment is in my Princeton Reverb reissue, but it is wired with the usual 1/4" mono plug, so all I have to do is put a 1/4" mono jack on the class D amp in the "nasty" amp.

The one-octave dip centered on 800 Hz does a remarkably good job of de-nastifying the nasty amp, by the way. It goes from nasty to just the usual blah sterile-clean solid state guitar amp sound. A huge improvement.

So step one will be to implement a similar notch without using the Danelectro EQ pedal. I did a little tinkering with a twin-tee filter in LTSpice last night that looks promising, but it will take some more tinkering on the bench before it sounds right, I'm guessing.

With that done, I'm hoping I can improve on the "too clean" sound with a JFET or two in the preamp.


Gnobuddy 30th October 2017 03:02 AM

2 Attachment(s)

Originally Posted by Tubelab_com (
Can you try playing the "nasty" amp through a real guitar speaker?

I did try this today. My guitar went into the Fish-n-Chips graphic EQ, the output from that drove the class D power amp module, and that in turn drove the stock speaker in my Princeton Reverb reissue, still mounted in its stock combo cabinet.

To start with, I left the EQ curve alone (as dialed in yesterday for the little boombox woofers). Through the Princeton Reverb speaker and cab, there was noticably more bass, but the overall sound quality wasn't all that different.

The next step was to bypass the EQ curve. The guitar sounded worse, with some harshness, typical of every solid-state guitar amp I've ever heard. But not as harsh as when using the boom-box speakers.

Step three was to set the Fish-n-Chips to a flat response (all EQ sliders centered), play the guitar through it into the Princeton Reverb speaker, and then, by ear, dial in the EQ curve that sounded best to me. (We're talking entirely clean tone here.)

Surprise - when I was done, the EQ curve was not all that different from what I'd dialed in for the two boombox woofers in their smaller enclosure, except for less bass boost. I still had a notch at 800 Hz, still had some bass boost, and still had the 6.4 kHz slider dialed all the way down.

I took a photo of the Fish-n-Chips (attached), and then sat down and measured its frequency response the old-fashioned way (function generator, Fish-n-Chips, oscilloscope). I have the data in numerical form in a Libre Office spreadsheet, if anyone wants them, but for now, I'll just post the frequency response plot I generated from the spreadsheet.

Evidently the 6.4 kHz slider does interfere with the 3.2 kHz response, as the frequency response I measured actually peaks at around 2 kHz, rather than 3.2 kHz.

So my next task is to try and generate something close to this frequency response from some simple analogue circuitry, which should give my guitar amp a good clean base tone on top of which I can experiment with adding harmonic distortion (JFETS), tone controls, et cetera.

There are lots of caveats - obviously, this frequency response was tuned for my ear, and maybe also for the two guitars I tried today (both twin-humbucker models).

Nevertheless, I think this method of voicing a guitar amp is a promising one, and I think it might save a lot of time, and produce better final results, than the traditional "tweak one capacitor at a time" method.


thoglette 30th October 2017 12:48 PM

Jolly good show and all that..

Originally Posted by Gnobuddy (
Nevertheless, I think this method of voicing a guitar amp is a promising one, and I think it might save a lot of time, and produce better final results, than the traditional "tweak one capacitor at a time" method.

Filing this one away for use later.

I'm afraid my cheap Behringer EQ doesn't have a fancy name (oh the shame!) but I'm sure it'll do. :D

Gnobuddy 30th October 2017 04:43 PM


Originally Posted by thoglette (
I'm afraid my cheap Behringer EQ doesn't have a fancy name (oh the shame!) but I'm sure it'll do. :D

In North America, admitting to owning a Danelectro pedal is almost as bad for your popularity as admitting you have a "social disease". :D

The Fish-n-Chips has a plastic enclosure crudely shaped to look vaguely like a fish, and a plastic stomp switch that responds to your toe with about the same precision as an overcooked potato. And it's cheap.

In the land of consumerism and baseball, that's three strikes against the Fish-n-Chips. Horrors!

I don't love the plastic enclosure or soggy plastic footswitch, but the electronics does exactly what it's supposed to do, and does it very well. I've found it to be an incredible tool for fixing or tweaking my guitar sound. :up:


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