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Old 1st November 2017, 04:22 AM   #11
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Gnobuddy View Post
So my next task is to try and generate something close to this frequency response from some simple analogue circuitry
I did some tinkering with LTSpice, and a combination of a mistuned twin-tee notch filter, and a modified second-order lowpass filter, comes pretty close to the frequency response I'm targeting.

Screenshot attached.

-Gnobuddy
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Old 6th November 2017, 03:05 AM   #12
Gnobuddy is offline Gnobuddy  Canada
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And now for the next interesting thing to share. I stumbled across this old thread yesterday: 2BJTE - 2 BJT triode emulator

The thread was started by yet another guy trying to make bipolar junction transistors sound good in a guitar distortion circuit. It's been done a thousand times, and it never sounds as good as a proper "toob" amp, right?

But this thread includes a link to a sound clip, and hearing it made me sit up and take notice. Here's the link: http://www.diale.org/mp3/v1.mp3

Now, to me, that is some pretty good-sounding rock guitar distortion. I have never heard anything that sounds as good as that come out of a BJT-only circuit before, so I am more than a little intrigued. Does this circuit really do what no other BJT circuit has ever managed to pull off - sound really "valvey" while driven deep into classic-rock guitar territory?

I threw the circuit together in LTSpice and it produces visually very similar waveforms to the ones posted in the thread.

I have also done a little fiddling and tweaking of component values in the simulator. One thing I can tell you is that the rounded corners of the negative half-cycles are very dependent on the bootstrap circuit - the values of the 500 ohm resistor (!) and 100 uF capacitor.

That is only a 50 millisecond RC time constant, so the capacitor discharges appreciably during each half-cycle at the low-frequency end of the guitars range (83 Hz), and this seems to cause the rounded corners of the negative half-cycles.

I can also tell you that fiddling with the 1k (!) emitter load of the second transistor changes the output waveforms drastically (harder corners, unlikely to sound good). Much though I would rather not operate a BC109 at 9 or 10 milliamps of collector current, that seems to be the magic spot for this particular circuit.

Note also the size of the input signal - this circuit is already beginning to distort with an input of 15 mV peak value, and is well and truly deep into clipping at 50 mV peak. There are going to be no cleans with this circuit and a normal guitar, not unless you attenuate the heck out of the signal before it gets to the first BJT.

This isn't necessarily a problem, though; you can get decent cleans out of JFET or two, and this two-BJT circuit could comprise a separate, parallel "dirt channel", with a switch or relay to changeover.

This is all hinged around one thing - how good that clip sounds. There is one big question mark in my head, though. The person who posted that sound clip says he used a BOSS OD3 overdrive pedal as a buffer/clean preamp to drive this two-BJT "tube emulator" circuit. He says the OD3 was set clean - but was it really?

The OD3 is an unusually valvey-sounding OD pedal that sounds very good in the right hands, and works its magic with a succession of slightly overdriven JFET stages, including a differential stage or two that mimics push-pull valves. It wouldn't be too far out of line to describe it as an analogue valve amp emulator built with JFETs. You can find plenty of good demos on You Tube if you want.

So the question in my head is, was it really the OD3 that produced the nice rock guitar timbres in that MP3 clip? Or was the OD3 really set clean, and did all that rich-sounding distortion really did come from that weird two-BJT/ weirdly bootstrapped/ weirdly mis-biased circuit?

The obvious thing to do would be to breadboard it and try it out, but I want to stay focused on my current project, building a preamp to take the "nasty" out of the little solid-state guitar amp I'm building to give to a friend, hopefully, this Christmas. Not to mention, I have no BC109s or BC108s around. I can't even find the 2N2222s I was sure I had somewhere. The only small-signal NPN BJTs I have are lovely BC550Cs, which have far higher beta than the BC109, and so may not sound the same in this rather oddball circuit.

But if any of you is interested enough to breadboard and try this out, I would love to hear about your results. (And if nobody else is interested, I will definitely build this and try it out once I'm at a good stopping-place with my current project.)

-Gnobuddy
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Old 6th November 2017, 03:44 AM   #13
Gnobuddy is offline Gnobuddy  Canada
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Last thing to share - this (attached) is what I've been working on. This is a solid-state preamp with a JFET input stage / buffer / triode emulator, and a frequency response shaped to match the one I got from a 7-band graphic EQ set to product the best clean tone (to my ears, obviously).

To recap, I mounted a pair of small 6.5" woofers taken from a thrift-store pair of boombox speakers in an 8"x10"x19" MDF enclosure. This sounded absolutely awful with a solid-body electric guitar played through it using a solid-state preamp and power amp, an experience many of us may have shared if we tried playing an electric guitar through anything other than a real guitar speaker.

So far, so futile. The interesting part came when I found out that I could dial out virtually all the "nasty" sound using a graphic EQ pedal, and lots of trial and error.

This is a very interesting result to me, because real guitar speakers are expensive, and the cheap good-sounding alternative speakers from vintage Hi-Fi or vintage organs seem to be very hard to come by (except for Printer2, who gets them regularly from his tooth fairy. ) Also, real guitar speakers are physically large, and the light cone and tight cone suspension demand physically large speaker enclosures as well.

But, because smaller and more modern speaker designs always seem to sound absolutely nasty when you play a solid-body electric guitar through them, there was no acceptable alternative.

But if careful EQ can take out the nasty - which is what my ears are telling me - then a whole world of speaker possibilities opens up to us DIY guitar amp builder/tinkerers.

So I designed a circuit to emulate the frequency response I had dialed in with my EQ pedal, and it is going to be part of the preamp. If all goes will, it will work the same magic as the EQ pedal, and the pair of 6.5" woofers will sound good instead of nasty.

JFETs have such wide parameter spreads that it takes special circuit design techniques to control them. In a design intended to go into mass production, there is little alternative. But I'm building a one-off for a friend, so I chose a route that would allow me to use a simpler circuit design: I decided to simply measure one specific JFET from my junk-box, and design the biasing circuit for it using the specific parameters of that exact JFET. Throw in another JFET and it may not bias up properly, but as long as I use this particular JFET, it will work perfectly.

So yesterday I picked out and measured an MPF102 N-channel JFET from my box of goodies (Vp was 1.96 volts, Idss was 4.66 mA). With those all-important parameters known, I used the usual JFET equation and calculated that it would take a 1k source resistor to set Is at 1 mA. On the breadboard, it actually took 1.2k to get to the desired 1 mA.

Today I transferred the input JFET stage from the solderless breadboard to one of those general-purpose PC boards (protoboard), and added on the first BJT (buffer) stage. Powered up with 18V DC (from a pair of 9V flat batteries), everything biased up properly. So far, so good, then I ran out of time, and had to start on some of my Sunday chores.

I also realized I only have the 1nF caps I need for the twin-tee filter in a 10% tolerance. That's not going to cut it - one decibel corresponds to roughly a 12% change, and with three caps in the filter, using 10% components might change the frequency response by up to 3 dB.

That's too much, so I need to get some 1% or 2% film caps for the twin tee, and maybe also for the low-pass "speaker emulator" that comes next in the signal chain.

It will be slow going during the week, but I expect to make some more progress next weekend, if not sooner.

-Gnobuddy
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File Type: png JFET_pre_w_EQ.png (59.7 KB, 193 views)
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Old 6th November 2017, 03:56 AM   #14
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Tubelab_com View Post
...too clean and lifeless...plugged his ES335 into that amp...that combination did sound good
I have had the same experience several times. My Yamaha electro-acoustic guitar, or my Korean ES 335-like thin semi-hollow guitar, will sound fairly good plugged into just about any sort of amp, including keyboard amps, powered P.A. monitors, or home Hi-Fi. The guitars sound good on their own, and as long as the amp doesn't totally wreck the guitars own sound, the combination will sound good.

Things are very different with my solid-body guitars, though. All of them sound pretty nasty played straight into a flat-response amp and speaker. These guitars intrinsically sound nasty, and need the magic of "toobs" to make them sound good. But, paired up with the right "toob" amp, they can sound delicious.

A year or so ago, I stumbled across an undergraduate research project that attempted to mathematically model a typical guitar pickup / string combination. The big takeaway was that the magnetic field falls away with distance from the polepiece in an extremely non-linear fashion - and when the guitar string moves through this nonlinear field, it generates a quite distorted waveform, straight out of the pickup. This is where a good part of the harshness of a solid-body electric guitar comes from, from the nature of the magnetic pickup itself.

Evidently having a hollow guitar body that vibrates with its own carefully tuned response can take quite a lot of the harshness out of the signal from the bare guitar itself.

-Gnobuddy
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Old 6th November 2017, 05:22 AM   #15
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Tubelab_com View Post
<snip>6V6's were born to sing the blues.
<snip>the 6V6 has about half the emission capability of the 6BQ5 (EL84).
<snip>
The 6V6 has less than 3 watts of heat in it's tiny little cathode.
What happens if we take a valve that is more generously endowed in the heater power department, and then run the heater at reduced voltage?

The work function of thoriated tungsten is somewhere between 2 eV and 5 eV, while (kT) for electrons at typical cathode temperature of 1050 K is only about 90 meV. This means only a tiny fraction of the statistically most energetic electrons will ever manage to burst out of the metal cathode into the unwelcoming vacuum beyond. If the cathode is run a little cooler, we have raised the bar, and only an even tinier fraction of electrons will manage to escape into the vacuum. So we should be able to control the amount of electrons emitted from the cathode via cathode temperature, it would seem.

So can we turn an enthusiastic EL84 into a reluctant 6V6, just by cooling off it's cathode a bit?

I remember seeing a graph somewhere that showed valve lifetime plunging dramatically when the heater was operated at lower-than-normal voltage, though. I don't think the reason for the loss of longevity was explained. If correct, that might put a damper on the prospect of making cheap 6V6-alikes out of other, lower cost, valves.

Thoughts?

-Gnobuddy
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Old 7th November 2017, 02:33 AM   #16
Printer2 is offline Printer2  Canada
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For your listening enjoyment (ok viewing, that just sounds better).

LND150 tube emulator attempt.

KMG SS Poweramps
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Old 9th November 2017, 04:15 PM   #17
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Printer2 View Post
For your listening enjoyment (ok viewing, that just sounds better).
Wow. That is just an amazing body of work. My hat is off to "KMG", whomever he is.

I know there are some things you can't hear until you're standing next to the loudspeaker, but for what they are, some of those clips sound really, really good.

I usually find that solid-state amps fail most obviously when it comes to what we guitar players call "clean tones", which aren't clean at all, but have several percent THD. But the Bogner Ecstacy cleans being imitated by an LND150 (in this thread: Fet version of the JCM800 ) sound really good.

Here is what KMG had to say, including the link to the sound clip:
Quote:
Originally Posted by KMG
First was Bogner Ecstazy preamp. Even without grid current emulation it gives results close to tube sound.
Fet version:
http://milas.spb.ru/~kmg/files/proje...mg_xtc_mix.mp3
-Gnobuddy
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Old 9th November 2017, 09:34 PM   #18
Gnobuddy is offline Gnobuddy  Canada
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Notch filter and lowpass (aka "nastiness remover") built and ready for audio testing (see pic).

I managed to cram my entire "de-nastify" filter (four active devices plus support circuitry) into one half of that little protoboard, so there is still some real estate left for whatever comes next. Direct-coupling the whole thing helped a lot, by cutting down the number of capacitors, and simplifying biasing of the 2nd, 3rd, and 4th active devices.

The board passed basic DC checks late last night, just before I went to bed, with the output sitting at right around 9V when powered by 18V DC.

The two BJT emitter followers each drop the usual 0.7 volts DC or so, but the MPF 102 source follower brings the DC voltage back up a couple of volts, so in the end, the output is at about the same DC voltage as the drain of the input MPF 102.

I'm hoping the input stage (MPF 102) will add at least a small sprinkling of the good kind of harmonic distortion. There is a 1.2k source resistor, chosen to provide the right DC operating point. For triode emulation (i.e. three-halves law) when biased exactly halfway between cutoff and saturation, the source resistor should be 353 ohms according to Dimitri Danyuk's paper ( Triode Emulator by Dimitri Danyuk | Field Effect Transistor | Amplifier ).

I have the option of AC coupling a second resistor in parallel (510 ohms or 470 ohms) with the 1.2k source resistor, so that the parallel combination is about 350 ohms to the AC signal.

However, real vacuum triodes don't exactly follow the three-halves law even when the anode is held at constant voltage, and they very definitely don't follow it when a non-zero anode resistor is connected between anode and B+. The presence of an anode resistor causes internal negative feedback, which straightens out the transfer characteristic a little bit, presumably reducing that exponent to something less than (3/2).

So I'm not going to worry about the exact AC value of that source resistor until I've at least had a chance to hear how it sounds, as-is, first.

If the notch filter and low-pass do their job, and "de-nastify" my amp, then I'll move on, and try to get some good clean tones, maybe using KMG's ideas and an LND 150.

If I get as far as having good clean tone, after that comes the question of tone controls. This one gives me a headache, as I have yet to find a tone control circuit that I really like for use with guitar.

The friend I'm building this for really only ever uses clean tones, so that is probably it as far as he's concerned. But I do want to investigate that two-BJT bootstrap distortion circuit (post #2108), to see if it really sounds as good as the posted MP3 clip.

-Gnobuddy
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Old 10th November 2017, 01:54 AM   #19
Gnobuddy is offline Gnobuddy  Canada
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Here is an interesting short collection of attempts to emulate "toob" sound with solid-state devices: Tube emulation with solid state devices

No soundclips for any of them except the 2-BJT/bootstrapped one, which I posted a link to earlier in this thread.

-Gnobuddy
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Old 10th November 2017, 04:42 AM   #20
Gnobuddy is offline Gnobuddy  Canada
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I got home late, but managed to get a few minutes at the work-bench. I hooked up a function generator to the input of the "de-nastifying filter", and a 'scope to the output, and powered the thing up.

I didn't have time for multiple accurate measurements, but a few quick twists of the frequency knob on the function generator showed that (a)there is indeed a notch in the frequency response, somewhere near 800 Hz, and (b) there is a slow rise in bass response below that, and (c) there is a slight peak around 2 kHz, followed by a fairly rapid fall-off.

So it looks like the circuit is more or less working as designed. Hopefully I'll have a chance to check it out properly soon, and also play some guitar through it.

I also found a few minutes to throw together KMGs first LND 150 tube emulation circuit from the thread that Printer2 linked. Here is a screenshot, and the LTSpice schematic file, if anyone wants to tinker with it. Oh yeah, also the LND150 model (LND150.txt).

-Gnobuddy
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File Type: png KMG_MOSFET_Triode_Emulator_001_Nov_09_2017.png (82.0 KB, 172 views)
Attached Files
File Type: txt LND150.txt (244 Bytes, 10 views)
File Type: asc KMG_MOSFET_Triode_Emulator_001_Nov_09_2017.asc (2.2 KB, 7 views)
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