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Old 12th November 2017, 02:21 AM   #21
Gnobuddy is offline Gnobuddy  Canada
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I was stymied at the workbench a couple of times today by silly things like not having a SATA cable on hand (to set up an old Windows PC for frequency response measurements), so in frustration I moved to the computer, and spent a little more time tinkering with LTSpice and KMG's LND150-based 12AX7 triode emulator.

The attached screenshots show the schematic, and an FFT of the simulated output with a 500 Hz, 2 Vpp sine wave input.

The fundamental and several harmonics are very clear, but, interestingly, there are also sub-harmonics below 500 Hz, and non-harmonically related IMD products in between the 2nd, 3rd, 4th, harmonics.

KMG really captured a lot of triode-like complexity in his LND150 circuit...amazingly good work!

Simulations can be a terrible time-sink, but they can also, under the right circumstances, guide one to a good solution that reduces time taken to actually build something that works. That happened for me last week with the "de-nastifying filter", and I'm hoping it will happen again when I try to get a more "triodey" clean tone out of this thing.

There will be something real from the work-bench in my next post here, I promise!

-Gnobuddy
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Old 12th November 2017, 05:59 AM   #22
Gnobuddy is offline Gnobuddy  Canada
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And a real report from the work bench: I finally got everything hooked up, and played a solid-body electric guitar through the de-nastifying filter I built.

It worked very well. No more nasty harsh timbre, just the usual rather unattractive, sterile-sounding solid state guitar amp clean tone. Progress!

So, on the plus side, it looks as though I've managed to lick the problem of using non-guitar speakers for electric guitar, and still getting acceptable sound quality. This is a huge win for me, because it is relatively easy to find thrift-store boom box speakers up to about 6" - 7" diameter for just a few bucks. On my current budget, it has to be that cheap, or it's not going to happen at all.

On the minus side, that one lonely JFET at the input isn't contributing any noticeable goodness to the clean tone. Something has to be done to improve the clean tones.

I was more or less expecting that, so this is not much of a setback. And thanks to Printer2's link to KMG's work, I have an excellent lead to follow. I also have a few LND 150s in the parts box, but unfortunately, no Schottky diodes at all.

I'm now re-thinking some earlier decisions I made with this amp. Boom-box speakers have other major differences from real guitar speakers - they are much less efficient, and they are designed to work best in sealed or ported enclosures that are carefully matched to their Thiele-Small parameters.

The huge reduction in efficiency translates to a huge reduction in loudness, but for my purposes, this is actually a good thing. Neither I nor my friend will go deaf, and class D power amp modules are now so cheap that it's fairly easy to throw 40 or 50 watts at the speakers if you do want them a bit louder. (But they still won't get up to ear-damaging SPL levels, which is good.)

The other issue - working best in a properly matched enclosure - is the one that is giving me second thoughts. I pulled these two woofers out of two thin, extraordinarily ugly, MDF ported enclosures, and transferred them to a better-looking enclosure, because I want to give this amp to a friend, and I want it to look at least reasonably good.

But now I'm thinking I should have left the woofers in their factory-stock boxes, and found a way to make those look better, or just used one woofer. The thin MDF and single speaker would be lighter for my friend (he has a physical disability). And the ported and properly tuned enclosure might produce stronger bass - which my friend usually likes.

I'm going to stay focused on the electronics until I feel it's satisfactory, though. So the next step is to start working on getting better clean tones.

-Gnobuddy
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Old 12th November 2017, 12:02 PM   #23
Printer2 is offline Printer2  Canada
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Good to see this topic in its own thread, I am sure it can suck up as much time as the 100 buck thread.

Once I get over my ukulele building I should have some time to mess with fets. I have some LMD150's but it would be nice to simulate the same circuit at finger friendly voltages.
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Old 12th November 2017, 05:19 PM   #24
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Printer2 View Post
Good to see this topic in its own thread
There are seven and a half billion people on planet earth, and seven people browsing this sub-forum. This topic will be just as thoroughly ignored anywhere on this forum, whether in its own thread, or the hundred-buck amp thread.

Quote:
Originally Posted by Printer2 View Post
I have some LMD150's but it would be nice to simulate the same circuit at finger friendly voltages.
I'm hoping for the same thing.

I'm running my class D audio power amp module of a 24V DC wall-wart, so life will be much simpler if the entire preamp runs on something less than 24 volts.

All the preamp breadboarding and prototyping so far has been powered by two 9V flat batteries in series.

-Gnobuddy
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Old 12th November 2017, 06:56 PM   #25
Printer2 is offline Printer2  Canada
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Quote:
Originally Posted by Gnobuddy View Post
There are seven and a half billion people on planet earth, and seven people browsing this sub-forum. This topic will be just as thoroughly ignored anywhere on this forum, whether in its own thread, or the hundred-buck amp thread.
Yeah but they are seven of the more underrated bunch of the seven and a half billion.



Did that come out right?
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Old 12th November 2017, 07:06 PM   #26
leadbelly is offline leadbelly  Canada
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Quote:
Originally Posted by Printer2 View Post
Yeah but they are seven of the more underrated bunch of the seven and a half billion.

Did that come out right?
Ego understood, over.
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Old 12th November 2017, 09:48 PM   #27
thoglette is offline thoglette  Australia
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As someone who struggles to find Useful Stuff(tm) a few years down the track (try finding anything in the "Beyond the Ariel" thread) having it's one thread is a good idea.

Tubelab also noted that part of the "sound" of PP amps is powersupply IMD . Can't find that thread either
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Old 13th November 2017, 12:32 AM   #28
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by thoglette View Post
As someone who struggles to find Useful Stuff(tm) a few years down the track (try finding anything in the "Beyond the Ariel" thread) having it's one thread is a good idea.
I'm the type of person who can never find anything again if I file it carefully away. But if I throw everything in one big pile, then I know where to look for it.

Quote:
Originally Posted by thoglette View Post
Tubelab also noted that part of the "sound" of PP amps is powersupply IMD . Can't find that thread either
I remember reading a write-up of an undergraduate engineering project, in which someone took a Bassman, added voltage regulation and filtering to the power supply, and then claimed the amp sounded utterly lifeless and awful.

That conclusion was 100% subjective, and so strongly worded that I found I could not take it very seriously.

Along the same lines, I once found an online discussion on guitar amps that "note switch", with audio clips demonstrating the phenomenon - the pitch of the note heard from the speakers jumps randomly between two different values, while the guitar is putting out one single note the whole time.

This was being described as a very good thing for an amp to do. But as a musician, I utterly hate the idea - I've spent many thousands of hours learning to play the "right" notes, and the last thing I want is an amp that arbitrarily emits something other than the note I've chosen to play!

I think the same is true for IMD. Rock guitarists who enjoy playing heavily distorted dyads ("power chords") might like the extra harmonic texture from those IMD products, which thicken up the two-note "power chord" sound. But people who are more prone to like the sound of complex, multi-note jazz chords will hate audible amounts of IMD, which ruin the sound of those complex chords. The typical blues or pop or country guitarist will be somewhere in between.

My point is that I think "good electric guitar sound" means very different things to different people, and I think that can make it quite hard to find common ground, or even to understand each other, sometimes.

Case in point, I've just spent a good part of a week coming up with a way to tame a harsh, penetrating guitar tone, trying get a smoother and more pleasant overall clean tone. But there are probably plenty of guitarists eager to "cut through the mix", who would be very happy to have that aggressive, penetrating tone I hated so much...

Back on topic, I spent a couple of hours today tinkering with KMG's LND150 triode emulator circuit in LTSpice, trying to make it work at 24V DC rather than 290 V DC.

I didn't really succeed in that attempt, but I learned a few things along the way, including the fact that the negative supply rail for the MOSFET sources is essential. Biasing the gate positive and adding a coupling cap does not work because of the asymmetrical "grid current" flowing through that coupling cap, which mucks up the bias point and waveforms.

And that is another key thing about the LND150 pseudo-triode circuit; the softly rounded negative half-cycles at the output come from softly diode-clipped positive half-cycles at the input, which in turn means the circuit is very sensitive to the source impedance feeding it. I'm not sure it would work at the input stage of an amp, fed directly from a guitar. I should look back at KMGs amp designs to see what he did with the input stage.

On a more promising note, at levels well below clipping, it seems easier to get somewhat pentode-like distortion from a simple FET gain stage with full source bypassing - meaning negative half-cycles at the output are longer and "sharper" than positive half-cycles. Exactly the opposite of what KMG gets out of his diode-clipped triode emulation stage.

Personally, I like the bright, chimey cleans from a small signal pentode, so I'm hoping I can persuade a JFET to sound a bit like one.

Finally, so there is something other than a wall of text in this post, here's something else I've been working on today: an active one-knob tone control circuit, essentially an active version of the "Big Muff" tone control.

I'm hoping that my work EQ'ing the amp for a good clean base tone will mean that heavy additional tone controls won't be necessary, and a simple tilt control will do the trick.

I started with this: One-Knob Wide-Range Tone Control

As you can see, I modified it (in LTSpice) to use a single MOSFET instead of an op-amp, scaled the components to give a much higher input impedance and less loading on the active device, tweaked it for more treble and bass control range, and moved the crossover frequency point down to around 500 Hz to suit guitars better than the original 1 kHz.

The attached frequency responses are plotted from only from 80 Hz to 5 kHz, the actual frequency range of interest to us guitarists.

The friend I'm building this amp for doesn't get along with complicated gizmos with too many knobs on them, which is another reason I want to keep the tone control to one knob if possible. I'm thinking a three-knob design, with gain, tone, and (master) volume knobs.

Active tone controls must be kept out of clipping, otherwise the frequency response and distortion goes to he11 in a hand-basket, so I'll have to be careful where this goes in the preamp audio chain. I will have around 20 volts of power supply voltage, so hopefully it will not be too hard to make sure this never clips.

It does weigh on my mind that I don't think I've ever heard anyone praise a tilt tone control in a guitar amp. But those amp designs were not already voiced for good clean tone before the tilt tone control was added, so I'm hoping the story will be different this time.

-Gnobuddy
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Old 13th November 2017, 01:56 AM   #29
Tubelab_com is offline Tubelab_com  United States
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Quote:
Tubelab also noted that part of the "sound" of PP amps is powersupply IMD . Can't find that thread either
I remember hearing a sound bit of someone playing a tube amp and they got it to growl in a neat sort of way. It sounded like something that I would hate in a HiFi amp, or even a clean guitar amp. I remember playing the clip (Youtube maybe) into my FFT analyzer and seeing obvious 50 or 60 Hz IMD when the amp growled. I can't find it right now either, but I'm guessing it's buried deep in the HBAC thread.....I somehow think it was a SE amp, like a Champ clone, but I could be way off.
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Old 13th November 2017, 02:34 AM   #30
nigelwright7557 is offline nigelwright7557  United Kingdom
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This is a very old soft limiter circuit from the 1980's to emulate valve type distortion.
Its a bit more subtle than the usual fuzz box circuit.

For a guitar it needs quite a bit of front end gain to make the circuit work well.

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