Hello all,
Playing around with a modified bazz fuss circuit (Ge diodes, Si transistor, Battery power) - I'm noticing an inconsistency in how it performs:
Sometimes it's very musical, sounds really sweet and sustains decently with a nice sparkly roll off.
Other times it's a bit dischordant, slightly more gated, there's less musical content in the guitar's highs and it's altogether not so pleasing. Kind of like having a stereo jack not quite plugged into a piece of equipment properly.
The difference is subtle yet it makes all the difference between sounding good and sounding bad.
I'm tempted to say it gets better the longer I play through it but this really is inconclusive - it could well just be coincidental at the times I've noticed a difference.
I don't think its relative to temperature either, I'm aware of the effect this has on Ge diodes.
Everything is solid; I have considered a dry joint but it's also worth noting that the circuit itself is free-formed using solid copper wire not encased in a typical metal enclosure and there is no additional filtering on the circuit - I'm wondering if it's acting like an antenna and this has something to do with interference eating into the signal?
Given the lack of shielding is it worth adding a small cap to ground on both the input and output sides of the circuit or would one at the input suffice?
Any other thoughts on this?
Thanks for your time.
(https://i.postimg.cc/V5v0R1qR/Fusscake-v1.jpg) (https://postimg.cc/V5v0R1qR)
hve you tried a really large resistor in parallel with the diode, something like 1M or more? it solved my Bazz Fuss problems, which sound similar
Quote from: iainpunk on January 12, 2020, 11:47:00 AM
hve you tried a really large resistor in parallel with the diode, something like 1M or more? it solved my Bazz Fuss problems, which sound similar
I haven't but someone did once recommend replacing the diode altogether with a 1M resistor for a 'classic overdriven bjt amplifier sound'.
I'll give it a go and see if it retains the Ge harmonics I like - Still not sure why there'd be an inconsistency though unless the diodes were on their way out?
Maybe the problem lies in the practically incalculable realm of things such as parasitic capacitances, microphonics and electromagnetic fields?
Maybe the planets just didn't align at the time? :icon_lol:
Thanks for your suggestion :)
Ge diode isn't giving the (still low) level of bias the original 1N914 does. So, you're in this crappy non-linear region of the transistor operating range. Leads to that inconsistency...my 2 cents. It's a very unpredictable part of the operating curve.
Quote from: GibsonGM on January 12, 2020, 12:41:28 PM
Ge diode isn't giving the (still low) level of bias the original 1N914 does. So, you're in this crappy non-linear region of the transistor operating range. Leads to that inconsistency...my 2 cents. It's a very unpredictable part of the operating curve.
Thanks. Contrary to the schematic I have a number of diodes here.
The total combined forward voltage is measuring as 640mV - This is actually a bit more than the individual voltages added up on paper for some reason but I guess that's a whole other can of worms.
Here's the datasheet for the transistor: https://www.onsemi.com/pub/Collateral/KSD471A-D.pdf
You're probably onto something but I'm probably not the guy who's gonna be able to confirm it :icon_lol:
So why then does it still sound great some of the time? Is it a matter of unpredictable external factors tipping some part of the circuits balance one way or the other?
As you may have gathered - I am not the electrical scholar - I just try to apply lateral thought and common sense!
Well I just tried a 1M resistor in parallel with the diode..
I can see how it might eleviate some of the more obvious biasing issues people have with the traditional bazz fuss circuit.
In my case it did produce a more consistent & slightly less gated sort of sound whilst being the same pedal but it also lost a lot of the sparkle and harmonic content that made this particular pedal sound so good before.
It was a bit flat by comparison tbh and I felt like I just wanted more fuzz than it was giving me to compensate for the fact.
I think the problem I have and it's solution lies elsewhere..
I don't seem to recall having this issue with the buffered signal from active pickups when I had them, so it could well be a 'transistor-on' problem but re-biasing the circuit by means of a C-B resistor isn't the way I need to go. Like I say, it's an intermittent thing and maybe the buffered signal just concealed whatever the problem is as opposed to addressing it directly.
Thinking on it now, it sounds audibly similar to an issue I have when my washing machine is on spin in the other room and the electronic noise through the poor circuitry in this house eats into my amplifier's ability to generate or reproduce harmonic content with the increased noise floor.
I'm going to try some RF filtering as originally intended and see how I get on with that - If someone could advise whether it's worth putting this both before AND after the gain stage as I explained in my OP, that would be handy.
Thanks for the input though guys, there's always benefit to be had from exploring ideas.
Your biasing as it stands in the schematic you have shown is totally through transistor leakage and diode leakage, neither of which are controlled parameters. It was common in some early transistor radios to bias the base through one resistor to the rail, another to ground and a reverse-biased diode to ground with the idea that as the transistor leakage from collector to base increased with temperature, the diode leakage also increased to shunt some of this extra collector leakage and maintain a reasonably constant bias.
There is nothing going from base to ground to stabilize the bias point, so you can expect that the circuit will change behaviour with junction temperature.
Since the circuit has the high output impedance of the collector going to the relatively low collector resistor, there is almost no power supply rejection other than the voltage division through the output capacitor and level control. Adding a resistor to the base from the collector will reduce the input impedance through the Miller effect which multiplies the current required from the input signal and this in series with the inductance of the pickup(s) reduces the high frequencies.
I think you should try replacing the 2x Ge diodes with a 1x Si diode just to see if the inconsistencies go away.
1x Ge + 1xSi might work but I'd do that experiment after the 1xSi.
*In circuit* the Ge diodes might have a smaller voltage drop than when you measure them with a DMM.
One weakish theory for loss of highs is the input impedance is low. Perhaps promoted by the diode's low impedance and feedback lowering the impedance by the Miller effect. At face value you would expect the Si and Ge to be similar in this respect by perhaps the Ge diodes are on more often and that lowers the impedance.
Another thing you can do is accurately measure the collector voltage when it sounds good and when it sounds bad. Keep a running list of measurements and you might see a correlation between bad sound and voltage. The difference is going to be small so you need to measure carefully and use the same physical measurement points each time.
Trying to keep my head above water whilst maintaining the impression I can swim :icon_lol:
I had an early prototype with an Si diode and I've tried all kinds of combinations with the two before arriving at this point but it was a completely different pedal as a result. You're right though, I didn't have the problems with the Si.
The Ge's came direct from russia; ex-military stock, corroded leads and all; so yes based on what I can gather from your posts I think this might just be a product of intermittent reverse leakage through the old diodes causing more of a miller effect than I intended when I added the cap in parallel.
In a similar sense that'll be why I thought the C-B resistor sounded flat too.
I guess it's just the nature of the beast then?
This configuration still sounds killer compared to everything else I've tried - Yeah I'm talking it up but I'm proud of it haha. No shame.
For now I'll chalk it down as 'character' :icon_mrgreen:
Amptramp, I feel there's a solution in the information you posted so I'm gonna study those concepts and mull that one over.
As an aside, could you guys suggest a good way to determine the reverse leakage in the remaining diode stock I have?
I'm putting a few of these pedals together so obviously I'd like to strive for better consistency where I can.
Thank you all so much for sharing your thoughts on this - Very humbling.
A final update on this.
I added an RC low pass filter with a corner frequency of 20khz between the input cap and transistor and it's sounding a little more defined than it did - there was just a bit too much low-order harmonics before and the sound got a bit muddied at times which I think is what I was first referring to in my OP.
I imagine though the improvement is more to do with the effect of raising the input impedence than it is removing interference.
I think to summarise, there are a lot of things going on here and I don't really have the understanding of circuit theory to draw valid conclusions on paper, I just know what sounds good and fumble through it until I reach a desired outcome.
I've uploaded a wee clip here if anyone is interested in hearing this circuit - This is my working prototype that's under constant revision, cobbled together with inferior salvaged parts.
The difference between this and the production version is like removing a blanket from the amp... but I'm not quite ready to reveal the finished article yet :icon_wink:
Take it easy :icon_cool:
QuoteThe Ge's came direct from russia; ex-military stock, corroded leads and all; so yes based on what I can gather from your posts I think this might just be a product of intermittent reverse leakage through the old diodes causing more of a miller effect than I intended when I added the cap in parallel.
In a similar sense that'll be why I thought the C-B resistor sounded flat too.
It's not likely to be just leakage as the diode is forward biased but what I suspect is happening is the base current through the transistor is small and this causes the diodes to operate at low current. Germaniums have a quite a low voltage drop at low currents. The higher "leakage" diodes are likely to have a lower drop when forward biased. It is possible that is both contributing to the sound you like and perhaps to the instability of the behaviour.
The voltage across silicon and germanium diodes varies with temperature by roughly the same amount, approx -2mV / deg C. You have two ge diodes so that will effectively double. Normally that alone is unlikely to be the cause of your problem. What is possible is the germaniums are running at a low enough current to make this effect worse.
So going down that path of thinking perhaps something to try is to add a 1M to 10M resistors across the Base and Emitter of the transistor. The loading of this resistor is unlikely to cause any issues and there is no Miller effect. What it does is puts a minimum current through the diode. I strongly recommend playing with the value. In fact of 10M has a better sound than 1M but still isn't quite there then you might need to try 20M. No guarantees but worth trying.
QuoteI guess it's just the nature of the beast then?
This configuration still sounds killer compared to everything else I've tried - Yeah I'm talking it up but I'm proud of it haha. No shame.
Possible but we don't know *what* aspect of the design makes it sound good and what aspect make it drift. If you know the answer to those questions you can say it does have an inherent problem. The question then is can you side-step the issue by changing something. For example the mod I mentioned. A good motivation for working through the problem is it sounds good.
It's worth trying a few things to see what affect is has. I'll admit trying to theorize is too open ended without understanding both of those aspects first, or at least the first one. I'd probably measure the collector-emitter voltage when it sounded good and when it sounded bad, then removed the diodes and use resistors to bias the transistor to exactly that collector-emitter voltage. From that you know if the bias point is important to the sound. Then I'd play with bias point to see how close I need to stay to those voltages, then finally try to come-up with a scheme to make it stable.
I finally got to listen to the clip. It reminds me of a Harmonic Percolator a bit. The Harmonic percolator biases one the transistors with a low collector-emitter voltage so maybe the bias point is what you are seeking. (Maybe I'm on the right track with the biasing and diode current thing.)
try adding an led in series with the ge diode. or if ya have one, a 4739 like i used in the stupid simple overdrive. the problem you're having i think is from the way the ge diodes react to temp/humidity/the moon/the tides etc... i know they shouldn't change due to that stuff, but man, it sure seems to me NOTHING germanium EVER SOUNDS exactly the same ;)
Alrighty,
I took a few days away from this to rebuild my guitar and regain my sanity :icon_mrgreen:
(https://i.postimg.cc/p90PBJRs/DSC-0185-01.jpg) (https://postimg.cc/p90PBJRs)
(https://i.postimg.cc/gw0WrL88/DSC-0263-01.jpg) (https://postimg.cc/gw0WrL88)
Now I'm back on it I've been playing around with a few of your suggestions..
I went back to trying some Si + Ge combos as well as adding E-B resistance but all these things took the sound further from what I like about it.
Basically I've found the bias point is exactly where I want it to be - the flare and the 'Harmonic Percolator' effect you mention is what I'm into.
The main issue I'm having here is that sometimes I have to pick the higher single notes pretty damn hard just to get it to 'switch on'. Other times they ring out even when playing softly (to a point), which is how I want it - it's still not great but it's playable.
- I've a feeling though that you're going to tell me that's down to the bias :icon_lol:
Measuring the collector voltage (to ground), it's hovering around 600mV - dropping to ~500mV when playing the thin E and ~400mV when playing the fat E. Strumming drops it down to about 300mV.
With the Si combo it hovered around 900mV, dropping by the same amount to ~800, ~700 and ~600mV but I still had the issue of having to pick the higher notes hard to switch it on - so no change there.
The base seems to hover around 540mV regardless.
I don't really know what these figures mean tbh but they seem pretty consistent, the collector voltage drifts a few tens at most, its sitting at 580mV just now and it's sounding good.
The story goes something like: Actives work fine and I can even roll the volume right back to clean it up with really nice sparkly overtones; passive humbuckers at full volume sound good most of the time but give me the symptoms I'm trying to troubleshoot here, rolling the volume back just cuts the sound out; and single coils simply don't work with the pedal - You have to play so hard just to 'switch it on' and there is no sustain whatsoever.
So.. is there a way to get this to work well with the lower output from passives and single coils without changing the bias?
The only things I haven't played with are the coupling cap values and the transistor.
Quote
- I've a feeling though that you're going to tell me that's down to the bias :icon_lol:
Well it's definitely a factor.
QuoteThe story goes something like: Actives work fine and I can even roll the volume right back to clean it up with really nice sparkly overtones; passive humbuckers at full volume sound good most of the time but give me the symptoms I'm trying to troubleshoot here, rolling the volume back just cuts the sound out; and single coils simply don't work with the pedal - You have to play so hard just to 'switch it on' and there is no sustain whatsoever.
It's not clear to me if have ever got it to sound good with single coils? (and if yes, then the problem is it no longer sounds good on Actives)
QuoteSo.. is there a way to get this to work well with the lower output from passives and single coils without changing the bias?
From what I can see you have the biasing set such that it is essentially cut-off and it needs the extra output of the actives to kick it off. What I don't understand is if that is the case I'd expect the notes to be gated off in the tail of the note even with actives. At some point the signal level has to drop below that of the single coils.
This gating issue is common when trying to convert some of the old GE transistors circuits to Silicon transistors. The GE transistors reduce that gating effect. Someone on this group did a silicon version of one of the earlier Ge Tone Benders; no doubt they spent a lot of time on it and I think they concluded if you have GE transistors then you should use them.
Anyway a different plan of attack would be to put a gain state before your circuit. That way you can dial the gain back on actives and boost it back-up for single coils. As for preserving the sound I cannot guarantee it.
The problem with finicky circuits is they need a lot of experimenting and development to get them to work. You have to do a lot of work and perhaps be prepared to fail, or not quite get there.
QuoteIt's not clear to be if have ever got it to sound good with single coils? (and if yes, then the problem is it no longer sounds good on Actives)
I've never had it working with single coils; I just tried it once with a Squire in a music shop and it was terrible.
I don't use SC's myself but a friend of mine does so I'm trying to widen the scope of the pedal to accommodate them.
I don't have a single-coil guitar to test with either but if I can get it working good with my passive HB's I'd consider that a happy medium.
QuoteFrom what I can see you have the biasing set such that it is essentially cut-off and it needs the extra output of the actives to kick it off. What I don't understand is if that is the case I'd expect the notes to be gated off in the tail of the note even with actives. At some point the signal level has to drop below that of the single coils.
The tail end does gate off with actives and it does it in a really nice way that I can only describe as the audio equivalent of a star-filter in photography :icon_lol:
The difference is the sustain I get before reaching that point, and how long that state of sparkly decay lasts when I do reach it. It's pretty much non-existant with the low output pickups, it just cuts.
Having done a bit of research it seems this sustain issue is quite well documented but it's solution is not.
A few have mentioned the collector resistance is key so I'm going to swap this out for a trimpot and see if I can strike a balance.
If that doesn't work out I may have to look at a simple way of boosting the input like you say but I'm not sure how to do this without introducing more clipping - I guess that'll be my next question.
The preamp schematic for the EMG 81 I was using before the passive H3 might be a good place to start but the layout of the final pedal is really tight and well thought out already - it's not subject to change - everything fits like a puzzle so I'm reluctant to start tagging on more circuit at the input side.
QuoteI don't have a single-coil guitar to test with either but if I can get it working good with my passive HB's I'd consider that a happy medium.
One possible difference between passive and actives is the output impedance of the guitar. Some actives have low output impedance at all volume settings and some don't. Passives pretty much have high output impedance all the time.
Where I'm coming from is: that circuit has a gain which is very much dependent on the impedance of the guitar. With that circuit, the low impedance actives will have a much higher gain than the passive.
I don't know if impedance is the root cause of your problem but it is something to investigate. If you have a Boss pedal or some pedal which is *not* true bypass you can wire it before your distortion pedal and set it to bypass. That will buffer the signal before the distortion pedal. If it improves things then you can build a buffer into the pedal. You could also add a series resistor between the buffer output and input of the pedal and tweak the value for best sound.
Breakthrough!!
The original schematic calls for a 100k collector resistor (10k for a darlington) so I thought the sweet spot must be somewhere around the 100k mark..
I replaced the resistor with a 500k guitar pot, started at around 90k and made my way up.
I spent a lot of time in the 100k - 120k region working in small increments and it'd sound alright one minute and not so great the next.
Eventually I got to about 180k when I realised the top strings were much easier to play and picking lightly I could get into that sparkly decay sound instead of just gating out.
I worked right up to 490k and here I can even roll the guitar volume back a bit to clean up the sound like I could with actives.
It still has very much the same character as it did so that's great!
Looks like the circuit is just very particular about matching collector resistance to the transistor used - the KSD471 has lower gain than a 2n5088 - I was surprised this difference put it so far off the original schematic.
There looks to be a correlation between resistor value and transistor gain here:
MPSA13 ~10,000hFE = 10k ohm
2n5088 ~900hFE = 100k ohm
KSD471 290hFE = ~500k ohm
Maybe as you say Rob, the impedence of the actives = higher circuit gain so I didn't need as much resistance for it to function even with the low-gain transistor.
The amp sounds like it just wants to get cranked now :icon_mrgreen:
I'll try a 1M collector resistor tomorrow to see if that's any further improvement
Maybe record another clip too.
Thanks for keeping this thread alive! Perseverance pays off.
QuoteBreakthrough!!
...
It still has very much the same character as it did so that's great!
Awesome work! Extremely cool result.
QuoteLooks like the circuit is just very particular about matching collector resistance to the transistor used - the KSD471 has lower gain than a 2n5088 - I was surprised this difference put it so far off the original schematic.
There looks to be a correlation between resistor value and transistor gain here:
MPSA13 ~10,000hFE = 10k ohm
2n5088 ~900hFE = 100k ohm
KSD471 290hFE = ~500k ohm
Maybe as you say Rob, the impedence of the actives = higher circuit gain so I didn't need as much resistance for it to function even with the low-gain transistor.
Increasing the collector resistor causes the input impedance of the transistor to increase. It's looking like all your results put the input impedance of the transistor in the 250k to 500k. That's very much the zone where you would expect it to no longer load the pickup, which dulls the sound. So perhaps there is something in the pickup loading theory.
QuoteI'll try a 1M collector resistor tomorrow to see if that's any further improvement
When the collector resistance beyond 220k you will start to get some overall gain loss from the pedal. The input impedance of the *following* pedal or amp will start to load it down your pedal. If you make the collector resistor too high you will effectively get more loading. So you probably end-up with a happy medium somewhere around what you already have. This is only a rough analysis as it ignores the effect of the diode in the feedback loop. Nonetheless the ball-park is certainly making sense from the point of view of pick-up loading.
QuoteThanks for keeping this thread alive! Perseverance pays off.
No worries thanks for sticking at it. That type of result doesn't come easy.
There's an interesting follow-on result from your work. Maybe the input impedance of the first transistor of the Harmonic Percolator has an impact on the sound. That would mean tweaking the bias current to ride the pickup loading point like you have done here.
Well I've made a few revisions and tried a few ideas..
I'm planning to compile the results after a little more progress and I've wrapped my head around a few concepts but for now here's an update on how it's sounding
This time with a LP Studio :icon_mrgreen:
Comparing this to the previous clip is like night and day. The Les Paul accounts for a bit more musicality of course (even with dead strings) but the sound is obviously a lot more open and playable. I don't mind the lower gain either - goes well with the vintage Ge vibe imo.
Still a work in progress but it's getting there.
I just want to get the transistor closer to it's linear operating state but I don't want to add anything more than I already have so right now it's a case of understanding how a collector feedback amplifier is biased, accounting for the fixed diode Vf and 9v supply. It's all new to me.
Yayyy number crunching ::)
I'm putting together a simple linear boost just now too, to see how they co-operate.
Ok so,
Having learned a little about how a common emitter amplifier works (great resource here: https://www.pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Horowitz-Hill/The_Art_of_Electronics.pdf )
I think I can draw some educated conclusions between what I'm hearing and what the circuit is actually doing; and there are a few things going on here:
1) It seems like, as may have been pointed out already, the original Bazz Fuss circuit is inherently unstable. It provides voltage to the base via the diode, to switch it on, which also forms a feedback loop preventing thermal runaway on account of the relationship between collector current and base through this diode network.
Unfortunately as the emitter goes straight to ground (as per the original schematic), or is often used with relatively small resistors <7k to reduce the AC signal gain, the actual DC bias point is completely dependent on the hFE value of the transistor - which varies wildly with temperature during use - as opposed to being set by the values of the supporting components in the circuit (rather like a voltage divider being dominated by one 'variable' value instead of another fixed value). Thus the bias point fluctuates and sounds great some of the time, discordant and one-dimensional at others. There is an example of a 'bad' emitter-grounded amplifier exactly like this on page 83 of the material linked up top.
By increasing the collector resistor value to match the transistor used as I did previously, I brought this point further from the 'undesirable zone' so the circuit sounded better more of the time with that particular transistor - but it was still unstable in the same way and still tips into sounding ugly when the hFE value goes beyond a range I consider musically acceptible, though not as much as it did before.
I seem to have solved this problem by greatly increasing the emitter resistor value and bypassing it so that the circuit gain is still as high as it was for the AC signal, but now the DC bias point is a product of the relationship between the emitter and collector resistors more than it is a product of transistor hFE. The bias point has also been raised by increasing the value of Re and adjusting Rc accordingly. I'm currently at a 2:1 Rc:Re ratio, which is what the DC bias sees, and the circuit seems much more stable - notes are much better defined whilst still having the same fuzz, which is feeling more touch sensitive now (as far as the issue presented in point 4 below permits) and it's also helped attenuate the very peaky note attack and smooth things out a bit. The values I'm using are still a work in progress.
That's the inconsistency problem solved.
2) Another problem I'm noticing, which might not be so apparent in the clips I've posted, is the lower notes seem to be losing amplitude when struck hard. By measuring the voltage difference between the collector and emitter whilst playing I was able to observe the value dipping below 200mV during these instances and I think the transistor C-E junction is beginning to go into saturation at this point: grounding the collector somewhat and therefore attenuating the output signal.
I need to raise the voltage difference between C and E. The value sits at 500-and-something mV when there's no signal, regardless of other parameters, so I imagine this voltage between C and E (the Vce, hurrah!) is a product of the voltage drop across the diode? This might require a bit of thinking considering I don't really want to change the diode arrangement...
.. Which brings me onto point:
3) Something I like about this fuzz - its 'phaser' undertones and 'harmonic percolator' effect - I think might be a product of the difference in phase between the signal generated through the transistor and the signal bleeding through the diode, as they combine at the collector. That's maybe why the diode choice is a key factor in the sound and simply swapping out the Ge diode for a higher Vf Si - replicating the Ge-sound with the miller-effect of a larger C-B cap value - is not a viable option here, as the Si diode doesn't allow as much 'dry' signal to bleed through to collector and create the phaser effect.
Pure conjecture at this stage, but educated conjecture nonetheless.
4) Finally, the gating issue is still present (albeit a lot less of a problem now other areas have been addressed). I realise now how this is the voltage difference between base and emitter being < ideal to put the transistor into it's 'on' state and it still requires a bit of oomf from the incoming signal to tip it into operation. I appreciate the transistors 'on' voltage is an obvious concept and something explicitly stated everywhere, I even understood this myself - but you don't really know until you know, you know?
This gating could either be because the pickups aren't able to supply the demanding higher freqencies with the extra power the input needs to switch on; or it could be a product - considering it occurs more at the higher end notes - of frequency-dependent gain in the amplifier stage related to dynamic source impedance presented by said frequencies; similar to what Rob was saying earlier about the gain of the circuit being dependent on the source impedance of passive vs. active pickups.
- Or a combination of the two as the neck pickup does fare better than the bridge.
So once I've explored the stable biasing some more (point #1) and come up with some good Rc and Re values, I'll turn my attention to the rest.
This has all become a lot less foggy given a bit of studying and practical exploration :icon_idea: :icon_mrgreen:
I almost understand what people here are saying now :icon_lol:
http://home-wrecker.com/bazz.html
Quote from: PRR on February 03, 2020, 08:15:30 PM
http://home-wrecker.com/bazz.html
Good resource, I started here back in July, I recall you were quite helpful with a few things around that time! - Is there a point contained that you're referring to explicitly?
Whilst I have your attention I don't suppose you could explain how I factor the diode into the input impedance calculation, assuming a fixed temperature?
Could I simply measure the resistance of the diode in reverse polarity (at room temp) and begin there - aware the value is subject to change?
I also don't have a datasheet value for the transistor's Vbe ON (only Vbe SAT). Does the 'on' voltage = the forward voltage of the B-E junction when measured as a diode?Edit: "The active region of transistor is one in which emitter-base junction is forward biased.." - I'm taking this as a 'yes'.
So based on my B-E Vf reading I need to raise the base voltage to >840mV greater than the emitter voltage to cure the gating issue - it's ~540mV right now and I assumed that was on account of the diode Vf - am I misinterpreting something here; is that 540mV the B-E forward voltage 'in circuit' (as opposed to whatever my DMM tests at) and I need to raise the base current to bring the transistor up to it's linear region instead of worrying about the diode?Edit 2: I'm just thinking out loud here, I can investigate this for myself. Though I would appreciate some guidance on accounting for the diode when calculating impedance as it's not a very well documented arrangement.
Cheers
> when calculating impedance
Typical pickup levels will swing the diode from ON to OF. I'm not sure "impedance" is a useful concept.
Congratulations you have done really well getting up to speed on this one.
Re diodes: the diodes do have an effect on the impedance *no doubt about it*.
The difficulty is how to deal with it, especially through calculations.
Intuitively the overall effect depends on how long the diode is on and how hard. If the diode is on 50% of the time (clipping on on polarity) then when the diode is off there is no loading and when the diode is on there is strong loading. However, that doesn't end-up meaning the overall loading effect which is twice the effective diode impedance. It's more like you are adding the unloaded signal to the loaded signal; ie averaging the two cases.
A simple example is to add a raw signal to a low-pass filtered signal. You don't get halve the cut-off frequency. What you end-up with a shelving EQ which only has a -6dB step. In other words a lot of highs still get through.
https://wiki.jriver.com/index.php/File:Shelf_positive_vs_negative.png
(here filtered = loaded, clean/raw = unloaded)
Quote from: PRR on February 05, 2020, 02:36:24 PM
I'm not sure "impedance" is a useful concept.
It was for determining cap values and to satisfy my own curiosity really :icon_redface:
Quote from: Rob Strand on February 05, 2020, 05:23:14 PM
Congratulations you have done really well getting up to speed on this one.
Re diodes: the diodes do have an effect on the impedance *no doubt about it*.
The difficulty is how to deal with it, especially through calculations.
Thanks man,
I was thinking when the diode is reverse-biased the resistance presented to the base will be the diode's reverse resistance (1.5M as measured); then when the diode is forward biased it presents an even greater resistance to the base, considering it's orientation? (Sorry, the loading terminology is lost on me)
therefore;
So long as hfe x re (presented to the AC signal) is much less than the diode reverse resistance (by an order of 10 +), the total input impedance seen by the guitar is going to be largely determined by hfe x re anyway; and like you did earlier - the diode can be discounted in the equation and still give a reasonable ballpark figure when it comes to impedance matching and working out audible corner frequencies etc?
QuoteI was thinking when the diode is reverse-biased the resistance presented to the base will be the diode's reverse resistance (1.5M as measured); then when the diode is forward biased it presents an even greater resistance to the base, considering it's orientation? (Sorry, the loading terminology is lost on me)
therefore;
So long as hfe x Re (presented to the AC signal) is much less than the diode reverse resistance, the total input impedance seen by the guitar is going to be largely determined by hfe x Re anyway; and like you did earlier - the diode can be discounted in the equation and still give a reasonable ballpark figure when it comes to impedance matching and working out audible corner frequencies etc?
For the reverse-biased diode what you have said is correct.
For the forward biased case the diode impedance is quite low. If you look up dynamic resistance or diode resistance in Art of Electronics you will find something for sure. The diode resistance get lowers as you put more current down it. Think of the diode characteristic past the knee point, when the current goes up the voltage across the diode can only change a little bit. A large change in current with a small change in voltage is the same as a small resistance. The other thing to note is the diode resistance is not really like a resistor since the resistance changes with current.
The fine details of the circuit are tricky to understand. When it's' just sitting there and the diode is forward biased if Rc = 470k there will be roughly 17uA going down the collector, if the gain is 200 then the base current will be 0.085uA. The base current and the diode current are the same so the diode current is Id = 0.085uA. If you calculate the diode resistance rd = 1.9 * 26 mV / Id = 580k ohm. The 1.9 is a rough value for the diodes "ideality factor"; see if it is in Art of Electronics.
When you connect a resistor across the input and output of an inverting amplifier it can reduce the impedance. So we would expect some reduction in the real input impedance compared to calculated value for the diode; see Miller Effect in Art of Electronics. However because the diode clips the output, the amplifier gain is reduced and Miller effect is probably a lot less than just assuming a high gain amplifier. Off hand in the input impedance is probably in the 100k to 500k zone. So basically your high collector resistance, which raises the input impedance without the diode, also raises the impedance with diode.
Don't worry if you get a bit lost here. The thing is you have little control over any of this. When you raised the collector resistor it fixed everything and that's the main thing you have control of to set the impedance. Noticed you worked that out without going though any long winded arguments or calculations. A lot of times all you need is a rough idea of cause and effect then just try it!
Quote from: Rob Strand on February 05, 2020, 08:40:55 PM
For the forward biased case the diode impedance is quite low. If you look up dynamic resistance or diode resistance in Art of Electronics you will find something for sure. The diode resistance get lowers as you put more current down it. Think of the diode characteristic past the knee point, when the current goes up the voltage across the diode can only change a little bit. A large change in current with a small change in voltage is the same as a small resistance. The other thing to note is the diode resistance is not really like a resistor since the resistance changes with current.
Ah that makes sense now thinking of it in terms of ohms law :icon_biggrin:
QuoteThe fine details of the circuit are tricky to understand. When it's' just sitting there and the diode is forward biased...
Yeah I measured the Vcb earlier and was pretty stumped to see a forward bias of ~20mV at idle and a reverse bias of ~200mV with input applied - It totally threw my perception of how the current is flowing!
Gonna have to pick up a hard copy of that and take a look at the missing first chapter. As intuitive as things seem, I think there's a lot of fundamental stuff I don't really have a knowledge of which is making these heavier concepts difficult to tie together - It's like having a conversation in pidgeon english :icon_lol:
I really appreciate you taking the time to explain this stuff.
As an aside to this fuzz pedal; the linear boost is coming along alright -
I was just going to throw something together to test a workaround for the fuzz gating issue but it's got pretty ridiculous:
(https://i.postimg.cc/673DgSNJ/DSC-0390-01.jpg) (https://postimg.cc/673DgSNJ)
(https://i.postimg.cc/w7QZvHqY/DSC-0399.jpg) (https://postimg.cc/w7QZvHqY)
It's just a straight up LBP-1 clone - The circuit is going to hang from the jacks.
I've swapped the emitter resistor for a trimpot and I'm adding the option of diode clipping on the output using these locking PCB interconnects pulled from a butchered Marantz.
This way I can use it as a clean boost or to audition different diodes for future builds; adjusting the gain with Re to clip whatever's loaded in the sockets appropriately.
The socket is wired with a jumper so it'll take up to 3 diodes in symmetrical or asymmetrical formation - They're just running to ground on the output, between the cap and volume pot, which I hope is an acceptible way of doing things?
- No idea how it's gonna sound but it seems like a nice idea at least.
Inspired by Midwayfair's Listening Notes on Diodes: http://music.codydeschenes.com/?p=1615 (http://music.codydeschenes.com/?p=1615)
... and my growing collection of Ge's. These things are like sweets :icon_lol:
Completely derailling the topic here but I powered through the night to get the boost finished...
(https://i.postimg.cc/JsZ9XR64/DSC-0415.jpg) (https://postimg.cc/JsZ9XR64)
Man, building this thing was ridiculous.
I can't believe how good it sounds with the right diode combination though! It wasn't even supposed to be a proper pedal :icon_lol:
(https://i.postimg.cc/2b1JzWty/DSC-0422-01.jpg) (https://postimg.cc/2b1JzWty)
I'll get back on task and put some progress clips together soon - I guess this has turned into a loose Q&A development blog now.
I've decided I'm going to split the pickup on the ESP next so I can do some proper single coil testing of the fuzz - It's destined for a friend's pedalboard and I don't fancy sending him a paperweight...
QuoteMan, building this thing was ridiculous.
Very cool, I like it. The LED and the label adds to the charm.
Back to the fuzz..
Still not tried it with a split coil yet as I'm in the middle of taking on a new property with a bit more space to work; I did manage to pick up the LP earlier and record this clip though, now I've addressed the points in the last update.
The amp wasn't quite as loud this time.
It really is something else when you hear it in person - there's so much musical saturation and harmonic content that I don't have the production talent to capture - I'm really rather pleased with it.
Provided it works with SC's I'm settled on this version.
Once I've relocated I'll try splitting the pickup in the ESP for a final test and post up the finished piece here. Exciting times!
Ooft I dunno actually - Listening to that again today it sounds a bit flat like there's a resonance to the fuzz that's not quite in tune with the notes I'm playing. It sounds a bit like when you go from 440hz to 432hz tuning, before your brain has got used to the change.
Is anyone else hearing that?
I'm not sure if it's the guitar, a product of the compression applied to the video or from the pedal itself . Maybe the harmonic content isn't sitting right with my disposition today :icon_confused:
I'm wondering if I just get excited about progress I've made and end up hearing what I think I hear instead of what's actually there :icon_redface:
It would be great if someone else who isn't tone deaf could give me their opinion?
It seems I may be conversing with myself here but hopefully someone might find this of interest.
I received a parcel last week that I'd been eagerly awaiting for a while...
(https://i.postimg.cc/jwcZP211/DSC-0011-02.jpg) (https://postimg.cc/jwcZP211)
Here we have a nice selection of D9- germanium diodes!
I've been auditioning them in the linear boost clipping jar as asymmetric pairs.
What's interesting is their unique harmonic signatures; some of these are concordant and some are discordant. The popular types - E's, K's, J's etc - seem to be popular for a reason, they do sound pleasant. Then there are types like the D9B which in the arrangement I'm using sound a bit jarring.
Some of them have a dominant harmonic resonance above the fundamental and some of them have a harmonic resonance below - similar to what I queried in my last post about the fuzz.
Even individual diodes of the same type each sound slightly unique.
It seems the issue was a characteristic of the diodes I was using - Things get a little strange when you start to combine them, the resulting sound is a product of their signatures (as well as being more compressed) - if one drifts a little more than the other this product alters, think of it like a combination of phase shift though I guess that's not strictly correct.
It's not a simple cut-and-paste affair either because the combination which at times sounded discordant in the fuzz (as a feedback network) actually sounds very nice in the boost (as a clipping diode to ground). Likewise, the D9B mentioned above might sound unpleasant as I tested it in the boost but that's not to say it won't work in a different circuit.
This is the circuit I've been using to audition the diodes, the one I built into the chutney jar:
(https://i.postimg.cc/SnL8J53T/WP-20200310-01-17-10-Pro.jpg) (https://postimg.cc/SnL8J53T)
...and here are my obvservations:
321/285 D9E: Musicality 5/5 Brightness 5/5 Clarity 5/5 Dynamic 3/5- Awesome mid harmonics & distortion sig.
366/324 D9V: Musicality 5/5 Brightness 2/5 Clarity 5/5 Dynamic 4/5 - Dark with subtle warm harmonics. Dynamic dist.
334/306 D9D: Musicality 3/5 Brightness 5/5 Clarity 4/5 Dynamic 4/5 - Quite brash & abrasive
306/294 D9B: Musicality 1/5 Brightness 5/5 Clarity 4/5 Dynamic 2/5
322/309 D9G: Musicality 4/5 Brightness 4/5 Clarity 4/5 Dynamic 2/5 - Smooth inoffensive distortion.
329/298 D9L: Musicality 3/5 Brightness 4/5 Clarity 4/5 Dynamic 3/5 - Straightforward Neutral distortion
295/269 D9k: Musicality 4/5 Brightness 4/5 Clarity 5/5 Dynamic 5/5 - Dynamic & characterful Harmonics
356/324 D9J: Musicality 5/5 Brightness 3/5 Clarity 4/5 Dynamic 4/5 - Very smooth. Good twang that you can really lay into.
mV @ Temp = 10°C
Note: The scores are an indication of 'least to most' but not necessarily what is better.
- 'Musicality' relates to concordance of the harmonic content
- 'Dynamic' refers to useful gradation of the distortion or harmonic content between soft and hard picking; some diodes cleaned up well but were a bit on/off without much nuance (or some just clipped all the time), so these would score low.
- 'Clarity' refers to how much playing detail is being communicated, though this is not to be mistaken for;
- 'Brightness' which can be synonymous in this case with presence
The go-to 3, based on how long I spent mindlessly riffing away on each of them out of the box, seem to be the E, J & K types; they were the most attention grabbing with their presence & musical character especially when soloing at the top end and ringing out single notes; the E's & K's share a kind of sparkle that's pretty damn magical. The V & G types are also worth mentioning: The V's in particular have been toted as 'a D9E which breaks up sooner' but I actually found the V to be less granularly distorted than the E and much darker - It lacked the same clarity but had a pleasant loungy warmth and seems like a good option where others are too bright or abrasive, it's definitely better in some applications and whilst it doesn't have the presence or top end of the others I find I keep going back it. The G had an especially smooth distortion which could be an attractive asset in it's own right. The D & L types were also useable but they just didn't have as much charm, which may be a good thing depending where they're used; the L was particularly neutral sounding with no strong harmonics so it might be useful where you need the clipping but don't want to add too much colour to the rest of the circuit.
A simplified way of grouping these:
E, B & D: Bright Pop Rocky
G, V & J: Smooth
E, K & J: Psychadelic
G & L: Neutral
Having never back-to-backed diodes before I was surprised how similar they all sounded out of the amp - I suppose this demonstrates the overall tone is more a function of the complete electrical circuit than of the diodes used, so a diode itself isn't going to completely alter the sound - however depending on where & how they're used they will add their own distinct flavour which could make or break the effect. It's like the icing on the cake.
Anyways, I've found the sound.
A similar transistor with a lower Vbe-on helped a great deal too.
I feel I'm ready to assemble a 'finished' pedal for 3rd-party evaluation, so I'll call time on this thread.
Thank you all for your help and support,
Ben
QuoteIt seems I may be conversing with myself here but hopefully someone might find this of interest.
People do read this stuff!! It's a matter of not being able to add anything useful.
I will agree with Rob here. We all do read what people post to the forum but sometimes we don't have anything relevant to add. Your journey and your documentation has been fascinating. You have added to my knowledge and for that I thank you, but still I don't have anything relevant to add yet.
Its funny, I always feel like I know nothing and everyone around me has a far better understanding than I do.
..so I try to learn and I document the process; and I suppose in doing so I have become the tutor.
I guess we're all on the same page really.
Quote from: Bunkey on March 11, 2020, 10:03:10 AMI guess we're all on the same page really.
Same book, different pages.
Just cuz nobody else is currently on "your" page don't mean we are ignoring you.
Thanks guys.
It was just a passing quip poking fun at myself for replying to my own thread so much. I'm not on the brink of a crisis or anything ;D I do appreciate the comments though.
Bunkey! excellent fellow - you are a hard one to track down. for in case it happens again, I'll include my search terms .....
it's like an lpb or an electra or a simple booster, built into a glass jar, with a metal screwtop lid.
(https://i.postimg.cc/2b1JzWty/DSC-0422-01.jpg)
do you mind if I possibly, sometime, shamelessly rip-off your idea? for to maybe, one day, build a modular many oscillator noise maker?
Quote from: duck_arse on April 28, 2020, 09:48:06 AM
(https://i.postimg.cc/2b1JzWty/DSC-0422-01.jpg)
do you mind if I possibly, sometime, shamelessly rip-off your idea?
Not at all! Just make sure to use a different condement - The stench of onion is undoubtedly a trademark feature.
The circuit in the jar is a cool idea. I wish I had more to add. :icon_mrgreen:
bunkey - onion relish - I've never had any, but I think I'd put it on everything.
I mentioned the concept to the guy, he got excited aboot the visual oportunties, started sweet talking hizz misses for some of her honey jars. I got mildly enthused by the build options possible - massive dumb old transistors and caps and wattage resistors I can't fit in anything else. and I can fill the bottom of the jars with "things" - marbles, heatsinks, brass tubes, acrylic rods, wire strands ......
I once read that building a pedal in a larger enclosure makes it sound more open...
By the same logic maybe you can remove the dead air.
:icon_lol:
done.
https://www.diystompboxes.com/smfforum/index.php?topic=124422.msg1193464#msg1193464
Quote from: duck_arse on September 16, 2020, 10:12:42 AM
done.
https://www.diystompboxes.com/smfforum/index.php?topic=124422.msg1193464#msg1193464
Haaa that's awesome man. I like teh colours.
For the purpose of closure...I came back to this whole project recently and after 18 months of deliberation I've finally fixed the issue that I'd been trying to address the whole time...
I built an electra-style distortion instead.
The end.
:icon_mrgreen:
the Electra distortion is really nice as well, but a bit to overdrive-ish to me, i like nasty broken sounds.
i would put an electra style boost in front of the Bazz Fuss if you don't want any gating, but like the overall sound to begin with.
im currently experimenting with the Bazz Fuss again, but this time an NPN and a PNP stacked on top of eachother like the Harmonic Percolator's gain stages. its getting really close to my ideal sound, harsh and sputterey, but not gated.
cheers, Iain