DIYstompboxes.com

DIY Stompboxes => Building your own stompbox => Topic started by: samhay on March 15, 2018, 12:10:23 PM

Title: (Diode) Bias Tremolo
Post by: samhay on March 15, 2018, 12:10:23 PM
Something new-ish -- A tremolo based on the DiSCO circuit here:
http://www.diystompboxes.com/smfforum/index.php?topic=110346.0

The only major change was to replace the envelope detector with an LFO. I chose a sine(ish) wave LFO obtaining using a phase-shift oscillator (PSO).

The idea is to capture some of the character of the old valve (tube) amp tremolos that work by modulating the bias of the power valves - tremolo is achieved by driving the valves towards cut-off, which gives rise to some clipping and crossover distortion when the signal amplitude is decreased.

The circuit here uses the PSO to wiggle the forward bias voltage of a pair of diodes (D1, D2). Signal goes through an input buffer (IC1A), is phase-split (IC2A) prior to the diodes and then recombined using a difference amp (IC2B) in an analogous manner to a power amp using an output transformer. The diodes act as directional variable resistors, with each of the signals 'clipped' asymmetrically to (virtual) ground. Much (but not all) of the distortion is removed when the inverted signals are recombined. The clipping gives rise to a reduced signal amplitude - the tremolo effect.

The tremolo depth is controlled by varying the 'bias' voltage of the PSO - the voltage the LFO swings around. This can be varied from ~ 6 V (minimal depth) to ~ 3V (max depth). The swing of the LFO is clamped by D3 and D4 to ~3.5 V peak-to-peak, which gives a good depth of tremolo at higher Bias/depth settings and largely keeps the LFO from hitting the power rails, which probably helps keep the circuit polite - it is fairly quiet for a tremolo.

The schematic:

(https://samdump.files.wordpress.com/2018/03/diode_bias_tremolo2.png)
('view' for larger version)

I built it on vero using this layout, which should be a fairly comfortable fit in a 1590B – that’s what I used (the 10u capacitor should be moved 1 or 2 places to the right).

(https://samdump.files.wordpress.com/2018/03/diode_bias_tremolo_vero1.jpg)
('view' for larger version)

I will try to get a sound clip recorded this weekend.
Enjoy...
Title: Re: (Diode) Bias Tremolo
Post by: BetterOffShred on March 15, 2018, 12:20:18 PM
Sounds awesome man, I'm looking forward to some sound clips.   What kind of swells does this get?  And what does it sound like with the LFO toggled off, just a boost?  I know that's a popular mod with the EA tremolo..    Thanks for sharing! :)
-Brett
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 15, 2018, 12:33:24 PM
Thanks,
The LFO is more sine than triangle, so sounds somewhat like the EA tremolo when the Bias/depth is not maxed. At max depth it gets pretty choppy.
It has a little gain, ~ 5dB, which is clean when the Bias is at minimum (fully ccw).

As drawn the switching is optional. It kills the LFO when the effect is off, which can be a good precaution if the LFO is noisy. As it is, there is no noise with the Bias at minimum, so you could probably leave it running without ill effect.

The only noise comes from the audio path, which may have some LFO bleed-through - I can't see it on the scope, but there is something going on. It is only really noticeable with the Bias turned up and a noisy fuzz (or similar) in front. However, most tremolos suffer under these conditions, so probably not something to loose sleep over.
Title: Re: (Diode) Bias Tremolo
Post by: BetterOffShred on March 15, 2018, 01:12:31 PM
Sounds great.   I may build one.. I've been on a tremolo kick.. built a 7 knob Tremulus Lune, EA, a Skippy, A green Currant..   The skippy ticks pretty bad though. 

So the switch you show is part of the bypass switch then?   I guess I'm confused by that part.   
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 15, 2018, 01:30:43 PM
I wired mine true bypass using a 3PDT. The switch wiring shown was on one of the poles.
You could use a 1PDT (if you you can find one, or half of a DPDT) if you don't want true bypass. It might get a bit crunchy when 'off' if the Bias/depth is set near max though.
Title: Re: (Diode) Bias Tremolo
Post by: BetterOffShred on March 15, 2018, 01:39:11 PM
Nah I get it, it's just one of the poles of the stomp.   Thanks man.   It looks totally awesome, I'll probably give it a whirl in a couple weeks.   Thanks again! :)
Title: Re: (Diode) Bias Tremolo
Post by: Kipper4 on March 16, 2018, 04:17:13 AM
That's neat.
Can't wait to hear it.
Thanks Sam.
Title: Re: (Diode) Bias Tremolo
Post by: duck_arse on March 16, 2018, 11:06:12 AM
I like C4. [not the explosive.]
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 16, 2018, 11:11:43 AM
^right, because one doesn't want an explosive C4.
I expect an electrolytic could be used there in either orientation without any excitement, but sometimes belt and braces is not that difficult to achieve.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 17, 2018, 04:26:56 AM
As promised, here's a sound clip. Recorded directly with a little reverb; apologies for the uninspirational playing.
https://www.dropbox.com/s/aao2o02xkosseke/Bias_Tremolo_1.mp3?dl=0

At faster LFO settings you might be able to hear a little thumping. Looking at the waveform, it is clear that there is some bleed-through from the LFO into the audio path. It isn't crap on the supply, so I think I can probably dial more of it out - For what it's worth, I'm told this is/was a classic problem with the original bias tremolo designs too.

Stay tuned for a revision...
Title: Re: (Diode) Bias Tremolo
Post by: Kipper4 on March 17, 2018, 05:32:49 AM
I can't hear the thumping. Sounds nice and creamy and mellow. Sweet.
Title: Re: (Diode) Bias Tremolo
Post by: bool on March 17, 2018, 11:50:25 AM
Perhaps you could just insert 2 caps at the diff-amp to act as a crude HPF. 30/40Hz? Say 220||15nF / 470nF (235/470nF)?

Maybe you could get by just using 220/470nF? Perhaps such unbalancing the diffamp would result in a more pronounced thump? Hard to tell without further investigation.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 17, 2018, 01:34:56 PM
>Sounds nice and creamy and mellow. Sweet.
Thanks

>Perhaps you could just insert 2 caps at the diff-amp to act as a crude HPF.
While bread-boarding I originally had caps in series with R14 and R15 to do just this. I didn't seem to to help.
I think a trimmer in place of R15/R18 is worth a try, but that will involve a layout rethink.
Title: Re: (Diode) Bias Tremolo
Post by: deadastronaut on March 17, 2018, 03:35:34 PM
cool project sam,  i could hear the thumps though, especially when you sped up......(headphones on)

nice and warm sounding..nice. 8)

Title: Re: (Diode) Bias Tremolo
Post by: PRR on March 17, 2018, 05:11:21 PM
If D1 D2 (and most of that network) could be perfectly matched, it would not thump.

In today's economic, a bag of 2k2 and 22k 1% parts is expedient. Diode matching is more tedious; look for dual-diode parts. A single BJT won't have B-E B-C match, but a matched dual transistor may.

Of course if you get much fancier, you might just as well throw in a THAT VCA and get impeccable thump.
Title: Re: (Diode) Bias Tremolo
Post by: Rob Strand on March 17, 2018, 08:01:38 PM
Quote
Diode matching is more tedious; look for dual-diode parts.
You can put say 5 or 10 diodes in parallel and let statistics pluck-out the lowest Vd.***
It's no match for dual diodes or transistors.

Matched parts is the life blood of all good VCAs and (linear) Mixers.  No one has come-up with practical solutions to avoid it.  Well apart from DSP - haha!

*** I should add this is only recommended for diodes off the same reel.  When picking parts out of your junk bin it's likely to make things worse (because it finds odd parts).  Series diodes will actually help the average better than parallel.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 18, 2018, 03:46:47 AM
Thanks both.
Yes, it looks like one can't get away from pulling diodes randomly out of the parts bin if you want perfect nulling. It does work quite reasonably if you don't turn the rate and depth too far up, so I am not loosing sleep over this.
Will make a few tweaks and do a little diode auditioning to see if I can get it a bit quieter though - I know I can make it a lot worse, so reason stands I can make some improvement too.
Title: Re: (Diode) Bias Tremolo
Post by: duck_arse on March 18, 2018, 09:24:12 AM
maybe, after all these years, a use for my LM194's. hurrah!
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 18, 2018, 05:20:56 PM
I spent a good few hours tweaking this over the weekend. I trimmed the various resistors around D1, D2 and matched these diodes - found 2 on a reel that measured the same forward voltage drop to within a mV.
What did I learn? This is probably all quite unnecessary.
I had an error in my vero layout - now fixed (below). The pull-down resistor on the input was the wrong side of the input cap, so there was a sizable DC offset between the outputs of IC1A and IC2A. This made nulling the LFO quite impossible.
It won't hurt to use 1% resistors and diodes off the same reel to minimise noise, but I suspect you can get quite a usable circuit without going to great lengths to match components - sorry Stephen.
It will be important to use an op-amp with FET inputs at IC1- don't use an NE5532!

As I had the scope out, here a capture showing the LFO (500 mV/division) and the output with no signal (20 mV/division). I think the time axis is 20 ms/division. In any case, the LFO bleed-through seems to be barely above the noise level. You can still hear something when the depth and/or speed are high, but it compares favourably to the (many) other DIY tremolos I have played with.

(https://samdump.files.wordpress.com/2018/03/bias_tremolo_scope.jpg?w=293&h=248)

Here's a new clip:
https://www.dropbox.com/s/ud1vmv66op9d6gi/Bias_Tremolo_2.mp3?dl=0

Fixed/updated layout:
(https://samdump.files.wordpress.com/2018/03/diode_bias_tremolo_vero3.jpg?w=900)
('view' for larger version)
Title: Re: (Diode) Bias Tremolo
Post by: PRR on March 18, 2018, 10:16:47 PM
You can in principle add a steep low-cut after the thumper to take the sub-sonic off.

I don't see how to do that well without more chips.
Title: Re: (Diode) Bias Tremolo
Post by: BetterOffShred on March 18, 2018, 10:28:04 PM
I like it man.  It does the fender swamp thing pretty nice.  Thanks again for the layout. 
Title: Re: (Diode) Bias Tremolo
Post by: Rob Strand on March 18, 2018, 10:39:10 PM
Quote
You can in principle add a steep low-cut after the thumper to take the sub-sonic off.
Also, you should place any high-pass filtering after the last differential amp not as part of it.   Imbalances in the cutoff frequencies on each diff-amp arm can cause more thump that it removes.

Sometimes it's worth adding an LPF after the LFO to take out any glitches.  Usually not required for a sine LFO.


Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 19, 2018, 04:05:57 AM
Quote
You can in principle add a steep low-cut after the thumper to take the sub-sonic off.
Also, you should place any high-pass filtering after the last differential amp not as part of it.   Imbalances in the cutoff frequencies on each diff-amp arm can cause more thump that it removes.

Sometimes it's worth adding an LPF after the LFO to take out any glitches.  Usually not required for an sine LFO.

Quote
I don't see how to do that well without more chips.

I thought about adding more filtering, but as Paul also figured, this makes the circuit significantly bigger.

The HPF on the diff amp can be nicely balanced if you use fairly high-precision R's and C's.
I did lift one of the caps and it didn't make a massive difference to the hiss. The corner frequency is quite high, so mostly there to ensure the circuit is polite to its neighbours.

C9 provides a little LFP'ing to the LFO. Glitches are more likely to be a problem in the audio path when the LFO wiggles D1, D2 in and out of forward conduction.

Ultimately, this is trying to capture some of the quirks of the old output bias tremolos. These are not know for their low distortion or dead-quiet operation. The circuit as drawn does this, and the noise is certainly no worse than a lot of the other DIY tremolos I have tried.

Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 19, 2018, 04:07:06 AM
I like it man.  It does the fender swamp thing pretty nice.  Thanks again for the layout.

Thanks. That's what I was shooting for.
Title: Re: (Diode) Bias Tremolo
Post by: bool on March 19, 2018, 08:14:36 AM
Any diffamp can be made into a LP/HP/BP filter. If you are careful, even a one-opamp diffamp can present a reasonably well balanced load ("current balance") to the source.

But before diving in with a solder, it's time to check it with a sim.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 19, 2018, 09:19:19 AM
>Any diffamp can be made into a LP/HP/BP filter. If you are careful, even a one-opamp diffamp can present a reasonably well balanced load ("current balance") to the source.

Right, and that is what I have done here.
D1 sees a load of 22k (R14) from the diff amp.
D2 sees a load of 20k (R15 + R18).
If you want to do better, you can use 20k resistors at R14 and R17, but this won't improve performance much (I tried) and I don't tend to stock 20k resistors in my parts bin.

LFP is formed from C13/R17 and C12/R15. Both give a corner frequency of 7.2 kHz.
HPF can be made with series caps as I said earlier. However, this may make performance worse as filtering is best done post-mixing/nulling.

>But before diving in with a solder, it's time to check it with a sim.

I agree. This is why I was pulling my hair out debugging - the sim shows this should be nicely balanced, yet that's not what the scope was saying.
This also highlights a problem with sims - they don't know about your build errors until you tell them!
Title: Re: (Diode) Bias Tremolo
Post by: Rob Strand on March 19, 2018, 05:12:09 PM
Quote
But before diving in with a solder, it's time to check it with a sim.
Do a sim with cap tolerances. You will see the common-mode rejection is diminished even at frequencies somewhat above the HPF cut-off frequency.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 19, 2018, 06:49:14 PM
The common mode rejection isn't perfect in the sim even if you use perfectly matched components.
It varies with the op-amp model, but there are enough subtle differences in the signal out of IC1A and IC2A (other than phase shift) that you will never get perfect cancellation in this circuit.
I agree that it would be better practice to do all filtering after the diff amp, but most DIY tremolos use no more than 4 op-amps, so I figured this was a good design rule. Others are, of course, welcome to tack filter stage(s) onto the output if they want to improve the design.
I also stress again, the thumping in the first audio clip is gone - this was an artifact of a build error that has been fixed. There is still a little high frequency hash in the audio path, but there is no noise on the supply lines, so it does better than most on the noise front.

Also, I haven't blown this trumpet yet, but the circuit shows you can build a perfectly useful tremolo without using any FETs, LDRs or OTAs/VCAs.
Title: Re: (Diode) Bias Tremolo
Post by: Rob Strand on March 19, 2018, 07:36:31 PM
Quote
agree that it would be better practice to do all filtering after the diff amp, but most DIY tremolos use no more than 4 op-amps, so I figured this was a good design rule. Others are, of course, welcome to tack filter stage(s) onto the output if they want to improve the design.
Yes, but the reason your design works better than other diode based units is *because* it uses that differential stage.  Unfortunately that means using at least three opamps.

Quote
I also stress again, the thumping in the first audio clip is gone - this was an artifact of a build error that has been fixed. There is still a little high frequency hash in the audio path, but there is no noise on the supply lines, so it does better than most on the noise front.
The reason you get more noise is because there are three opamps all processing the same signal level.  In fact the last stage contributes twice as much noise, so it's like having four opamps adding to the noise.

You could boost the level coming in and drop it going out but that will make the diode distortion worse than they already are.

The way around the noise is to make the first opamp a pre-emphasis stage.  The de-emphasis network is perhaps best implemented as a passive RC filter on the output of the last stage.  You can use low values to keep the output impedance down.  (You can't put the de-emphasis in the feedback path of the last opamp as it will skew the frequency response of each differential input differently.)    Look at how Boss do it on the Flanger and Chorus.
The de-emphasis network in that position also filters distortion products from the diodes.


Title: Re: (Diode) Bias Tremolo
Post by: bool on March 20, 2018, 08:14:47 AM
If you have any desire to revisit the cap-coupling to diffamp idea, you can try to do it a "C-R-C" way to minimize the influence of the diffamp input current imbalance: simply cap-couple, and just insert a resistor after the caps AT the diffamp inputs.

This resistor would act as a corner-freq-setting part of a balanced hpf, but will also affect the input differential impedance.

My suggestion would be to use 330nF caps, and resistor in range from 20k-10k. (Do an impedance sweep sim of the diffamp to make it behave). There will be some interaction with diode-biasing resistors as well.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 20, 2018, 06:19:46 PM
The way around the noise is to make the first opamp a pre-emphasis stage.  The de-emphasis network is perhaps best implemented as a passive RC filter on the output of the last stage.  You can use low values to keep the output impedance down.  (You can't put the de-emphasis in the feedback path of the last opamp as it will skew the frequency response of each differential input differently.)    Look at how Boss do it on the Flanger and Chorus.
The de-emphasis network in that position also filters distortion products from the diodes.

Having played a little more with the circuit, it looks like some more aggressive LPF'ing would be an improvement - it is lovely and quiet if you role the guitar tone all the way down.
The pre/de-emphasis is a good compromise and will only cost an extra handful of passives. It would require some moderate changes to the layout, so if I can muster the motivation to build another one I will give this a try.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 20, 2018, 06:21:29 PM
If you have any desire to revisit the cap-coupling to diffamp idea, you can try to do it a "C-R-C" way to minimize the influence of the diffamp input current imbalance: simply cap-couple, and just insert a resistor after the caps AT the diffamp inputs.

This resistor would act as a corner-freq-setting part of a balanced hpf, but will also affect the input differential impedance.

My suggestion would be to use 330nF caps, and resistor in range from 20k-10k. (Do an impedance sweep sim of the diffamp to make it behave). There will be some interaction with diode-biasing resistors as well.

I think this could just make it worse. The input current imbalance is not that bad and adding HPF'ing is more likely to cause problems with the nulling of the low-frequency component of the LFO - thus bringing back the thump (which is nowhere near as much fun as bring back the funk).
Title: Re: (Diode) Bias Tremolo
Post by: Rob Strand on March 20, 2018, 06:58:54 PM
Quote
Having played a little more with the circuit, it looks like some more aggressive LPF'ing would be an improvement - it is lovely and quiet if you role the guitar tone all the way down.
That's a bit weird (and perhaps a little bit more weird because you are using a JFET input opamp).   That means a lot of noise is coming from guitar and first-stage region.   Past the first stage the opamp noise is independent of the tone control and the guitar.   Maybe that first stage opamp is damaged?

Does the noise go down when you set the guitar volume(s) to zero?
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 20, 2018, 07:15:31 PM
I puzzled this at first too.
The guitar is pretty noisy, and my house isn't that quiet either, so I don't think there is anything wrong with the input stage - it is just high impedance.

I am beginning to agree that the diff amp probably doesn't do high frequency rejection very well, so removing hiss  anywhere before this stage is helpful - you may be right that I should remove the LPF from this stage, which would become a must if I add pre/de-emphasis.
Title: Re: (Diode) Bias Tremolo
Post by: amptramp on March 20, 2018, 07:45:01 PM
There is a similar circuit with additional series diodes connected cathode-to-cathode as the squelch in my Heathkit AJ-1510A so you have to be careful that you get the amplitudes right.  Otherwise you may have a fuzz that varies in completeness of clipping at the tremolo frequency.  This could be a useful effect but maybe not what you were looking for.
Title: Re: (Diode) Bias Tremolo
Post by: Rob Strand on March 20, 2018, 08:07:49 PM
Quote
The guitar is pretty noisy, and my house isn't that quiet either, so I don't think there is anything wrong with the input stage - it is just high impedance.
My place is pretty noisy but I usually get more  buzz (mains derived) rather than hiss.

Quote
I am beginning to agree that the diff amp probably doesn't do high frequency rejection very well, so removing hiss  anywhere before this stage is helpful - you may be right that I should remove the LPF from this stage, which would become a must if I add pre/de-emphasis.
Yes, there's plenty to go wrong putting filters around a diffamp.  It often looks OK from an idealized theory perspective but when you have part tolerances and finite opamp gain-bandwidths the performance can be quite different to "what you thought".
Title: Re: (Diode) Bias Tremolo
Post by: R.G. on March 20, 2018, 09:10:05 PM
Just as a lead on prior art, using the varying resistances of diodes was more common back when opamps were not yet thought of.

One variant I always liked was the tremolo in the Thomas Organ Vox and MKii and MKiii UK Vox amps. They used a full diode bridge modulator, with the bias currents fed to four diodes from top and bottom sides differentially, and the signal used to "cancel" against an inverted version of the same signal after amplification.

The problem with diode resistance modulators is that they are highly non-linear for signals bigger than about 20-50mv. More than that and the diode nonlinearity shows up grossly in the signal. Which may or may not be OK, depending on how you like your sound. Using a diode modulator bridge helps because you have a chance at least at cancelling some of the distortion.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 21, 2018, 06:53:51 AM
>My place is pretty noisy but I usually get more  buzz (mains derived) rather than hiss.
You're right, I need to be more careful with my semantics. My guitar buzzes - there is hiss too, but the buzz is what I was thinking about last night.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 21, 2018, 07:13:05 AM
>Just as a lead on prior art, using the varying resistances of diodes was more common back when opamps were not yet thought of.

Indeed, although I have also seen them used in a few more contemporary op-amp filter circuits, like Escobedo's 'Phuncgnosis'.
I haven't seen them used quite like this before, but I wouldn't be surprised if it's been done.

>One variant I always liked was the tremolo in the Thomas Organ Vox and MKii and MKiii UK Vox amps. They used a full diode bridge modulator, with the bias currents fed to four diodes from top and bottom sides differentially, and the signal used to "cancel" against an inverted version of the same signal after amplification.

I can see how that would work if one was careful. Do you rate the tremolo effect in these designs (I know you are fond of the Thomas Organ in general...)?

>The problem with diode resistance modulators is that they are highly non-linear for signals bigger than about 20-50mv. More than that and the diode nonlinearity shows up grossly in the signal. Which may or may not be OK, depending on how you like your sound. Using a diode modulator bridge helps because you have a chance at least at cancelling some of the distortion.

I am aware of the limitations of using diodes like this. Frankly, I'm surprised how well my circuit worked straight off the drawing board.
The diff amp can (almost) deal with a lot of gross behaviour from the diodes.
Also, the design goal was to design a tremolo that had inherent distortion at the heart of the tremolo effect. The non-linearity of the diodes is very much exploited to achieve this!
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 21, 2018, 07:19:39 AM
>There is a similar circuit with additional series diodes connected cathode-to-cathode as the squelch in my Heathkit AJ-1510A so you have to be careful that you get the amplitudes right.  Otherwise you may have a fuzz that varies in completeness of clipping at the tremolo frequency.  This could be a useful effect but maybe not what you were looking for.

The LFO swing and the bias voltage range have been chosen so that (at least) some of the time the diodes are not biased into conduction. The LFO can conk out at low speeds, so if you do this and then turn the bias up (lower voltage) you get a pretty gross fuzz.
I did try to tweak it got get the varying fuzz effect (i.e. not clean anywhere through the LFO cycle), but this was quite difficult to dial in.
Title: Re: (Diode) Bias Tremolo
Post by: Rob Strand on March 21, 2018, 05:15:23 PM
Quote
I am aware of the limitations of using diodes like this. Frankly, I'm surprised how well my circuit worked straight off the drawing board.
The diff amp can (almost) deal with a lot of gross behaviour from the diodes.
Also, the design goal was to design a tremolo that had inherent distortion at the heart of the tremolo effect. The non-linearity of the diodes is very much exploited to achieve this!
I thought it turned out really well too - and I'm across all the evils of diodes and stuff the RG was talking about.
Some of the diode bridge circuits don't actually cancel the LFO as well as yours.  They don't enforce a good balance on the DC level so some of the LFO gets through.

Title: Re: (Diode) Bias Tremolo
Post by: PRR on March 22, 2018, 12:33:03 AM
Prior Art: CBS had a line of (tank and) broadcast level controllers (AGC/Limiter) which used a diode bridge between transformers.

Around 1960, transistor radios often had a B-C diode which was open for small signals and biased on when AVC action happened. But this is different because AM modulation (between tuned circuits) accepts huge 2nd order distortion without loss of final fidelity (such as it was).

The optional input diodes on LM3700 are another diode attenuator.

10mv-30mV per diode is a ceiling. More diodes gives a higher ceiling but also higher impedance and thus higher noise resistance at idle. On paper, 100 diodes series gives 40dB more signal for 20dB more hiss. That's a lot of soldering. Fairly simple buffered mutipliers can give the same effect in a transistor, two Rs, and a couple opamps per "100 diode". This does lead to the need for >60V in hand, so is not pedal-friendly. But for studio it is less frightening/costly than 300V supplies for a tube-VCA of lesser S/N.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on March 22, 2018, 11:31:43 AM
>I thought it turned out really well too

Thanks

>Prior art.

Is was looking at the transformer designs the other day. This could be an interesting diversion for version 2 or 3

>Fairly simple buffered mutipliers can give the same effect in a transistor, two Rs, and a couple opamps per "100 diode"

I did briefly consider using rubber diodes to make tuning forward voltage an option.
I think this is a different plan though?
Title: Re: (Diode) Bias Tremolo
Post by: PRR on March 22, 2018, 11:10:17 PM
> using rubber diodes

The common rubber diode is "diode-like" only over a *narrow* range of currents. Especially at high multiplication. As a class A driver for a push-pull output, this may be good-enough. In my sample (Q5), it is a 10K resistor below 0.8mA and approaches 10K/hFE by 8mA.

This can be averted with buffering. A full solution wants 3 buffers per transistor. In many cases at least one is not needed.

The diode-line can, as you say, be trimmed with resistor value. For demonstration, I paired a 2N2222 with a 2N3904. At 1mA there was a dozen+mV difference, I trimmed that out. The overlaid curves track over 6 decades (120dB!) with a slight slope. I suspect the slope is incomplete modeling. If it is real, some compensation can be had by changing the audio signal resistors.
(https://s31.postimg.org/ny1vwaoav/Neoprene_Diode-_DC.gif) (https://postimg.org/image/ny1vwaoav/)

The VCA form does the right thing. The 3rd buffer in each fake-diode is not needed. I used a simple driver and recovery which does what your real plan does. (The 1r mixer does not stress SPICE.) (I don't know what your 100r does, so I omitted it.)
(https://s31.postimg.org/43fua7jdz/Neoprene_Diode-_AC.gif) (https://postimg.org/image/43fua7jdz/)

Yes, we are replacing 10 or 100 diodes with 76 transistors. But the transistors are cheaper, less work, allow direct trim.
Title: Re: (Diode) Bias Tremolo
Post by: stringsthings on March 23, 2018, 03:19:04 PM
Just found this thread.  Very nice tremelo sounds!  Following this for future updates.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on April 25, 2018, 04:58:26 PM
Was away from the bench for a while, but when I got back put a version 2 together that adds pre- and de-emphasis and removes filtering from around IC2B.

It works. Does it work better than version 1? I'm not sure yet...

Schematic and vero layout (click for larger version):

(https://i2.wp.com/samdump.files.wordpress.com/2018/04/diode_bias_tremolo_v2.jpg)

(https://i1.wp.com/samdump.files.wordpress.com/2018/04/diode_bias_tremolo_v2_vero.jpg)

Title: Re: (Diode) Bias Tremolo
Post by: Rob Strand on April 25, 2018, 07:09:12 PM
Quote
It works. Does it work better than version 1? I'm not sure yet...
Thanks for the update.   I guess it's a good as it can be.
IIRC your guitar was potentially noisy.
Title: Re: (Diode) Bias Tremolo
Post by: BetterOffShred on April 25, 2018, 10:30:55 PM
V1 is on my build list.. maybe I'll wait and hear how this one sounds by comparison..  thanks for sharing man!
Title: Re: (Diode) Bias Tremolo
Post by: samhay on April 26, 2018, 02:05:56 PM
>Thanks for the update.   I guess it's a good as it can be.
No problem, and thanks for giving me a nudge to make improvements.

>IIRC your guitar was potentially noisy.
Good point.
I have spent some time with this and my bass, which is much quieter than the guitar I had been testing with.
In conclusion, it seems version 2 is quieter, particularly if you don't turn the bias (depth) up all the way. It can handle fairly hot input - one reason I shied away from pre-gain/emphasis in the first place - and thus:

>V1 is on my build list.. maybe I'll wait and hear how this one sounds by comparison..

I suggest you build version 2. I will try to get new sound clips up over the weekend, but if you are itching to get the soldering iron out, I don't think you will regret choosing V2 over V1.
Title: Re: (Diode) Bias Tremolo
Post by: Rob Strand on April 26, 2018, 10:56:40 PM
Quote
I have spent some time with this and my bass, which is much quieter than the guitar I had been testing with.
In conclusion, it seems version 2 is quieter, particularly if you don't turn the bias (depth) up all the way. It can handle fairly hot input - one reason I shied away from pre-gain/emphasis in the first place - and thus:
There's a bit of a trade off between susceptibility to overload and signal to noise improvement.
Your network starts ramping up at about 276Hz /278Hz  which is quite a low frequency.
I'd probably push that upto at least 400Hz.

If you used 10k + 1.2k + 33n on the input and 2k2  + 270R + 150n (allows for 100k output resistor) on the output it should reduce overload susceptibility *without* losing the s/n improvement you currently have.  With these values the network starts ramping up at about 431Hz/438Hz.

[If you really wanted to get fussy, not for the case when the Tremolo is drive by a guitar but more when it is driven by other effects, you can shave some noise off by using 2k2 + 270R + 150R for the input stage (maybe 0.4dB s/n improvement) and replace the 10n on the input with 47n or 100n (0.3dB s/n improvement); totaling 0.7dB.]
Title: Re: (Diode) Bias Tremolo
Post by: samhay on April 27, 2018, 04:25:16 AM
>There's a bit of a trade off between susceptibility to overload and signal to noise improvement...

Indeed. There is probably scope to tune the pre-/de-emphasis I added, as I didn't do this by ear.
I figured I would bring the corner frequency down as low as I dared and then back it off if it overloaded. It didn't, so I kept the V2 values.

BetterOffShred - if you build V2, might be worth playing with the values Rob suggested.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on April 29, 2018, 08:23:17 AM
Here's a clip of version 2.
Recorded with a touch of compression before it, then straight into an audio interface.
Apologies for the uninspiring playing, but I was trying to show off the range of bias and speed settings and to (hopefully) show how quiet it now is.

https://www.dropbox.com/s/sikvri2ggk6qumh/Bias_Tremolo_V2.mp3

It was recorded with my relatively noisy tele and having now spent some time with it played into my amp, I am pretty sure that most/all of the noise I/you now hear is coming from the guitar rather than this circuit.
Version 2 is definitely an improvement and this is probably about as realistically quiet as a stompbox tremolo is going to get. I wouldn't have any qualms playing with it at stage volume.
Title: Re: (Diode) Bias Tremolo
Post by: samhay on April 29, 2018, 10:12:02 AM
I've had a question about how one might go about substituting another LFO into this circuit.

With respect to the V2 tremolo schematic:
IC1B is a phase shift LFO - like the EA tremolo and Magnavibe, but done with an op-amp. The LEDs (D4 and D5) limit the voltage swing of the LFO to about 3.5 V peak-to-peak, which gives a good effect and keeps IC1B from hitting the supply rails.

The output of IC1B (the LFO) swings around a bias voltage set by the BIAS control, which can vary between about 3-6V (with a 9V supply). The LFO doesn't drive anything, but rather acts as a virtual ground, which happens to wiggles up and down.

Now to the signal: D1 and D2 act as diodes that clip to ground - think Dist+ or Rat. However, instead of clipping to ground, they clip to the LFO virtual ground, which is wiggling. When the LFO wiggles down (towards ground) and/or when the BIAS is set higher (DC offset is closer to 3V), there is more clipping and vice versa.
There is also some region where these diodes are better thought of as variable resistors (more like a typical tremolo), but this region is quite small compared to the 3.5V LFO swing.
Also note there is a single 'clipping' diode for each signal path, so only one half of the signal gets clipped.

The reason this doesn't sound like a distortion is because the signal is phase split before it hits D1 and D2 and then recombined afterwards. The half of the signal that gets 'clipped' is opposite for the signal that comes out of IC1A and IC2A, so when the diff amp puts the signal back together again, it can use each half of the non-clipped signal.