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Author Topic: hybrid preamp idea  (Read 7582 times)

phatt

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hybrid preamp idea
« on: September 20, 2014, 09:01:58 AM »
Well this is not new and it has valves but it does a lot with one valve and just two opamps.
The challenge is to make a simple circuit that actually responds like this demonstration,

https://www.youtube.com/watch?v=ZkGCvLstPrE

Without the need for a full Valve amp.

These old circuits had an uncanny reaction to the incoming signal by just dialing back on the guitar volume the sound went from full bore OD at 10 down to clean (ish) at ~half volume but staying at a similar volume level, (power amp compression via saggy supply and other thermionic tricks)

Now as much as I'd love to just go buy a Lazy J amp and save myself the torture of trying to build the impossible,,, my financial adviser warns me otherwise.

(Did you know that the institution of marriage is the only democratic system where you are forced to sleep with the minister of Defense and Finance?
whoops, off topic again already :-X)

Well this had been on my bench now for the last few days and although it may not have quite the mojo of the Lazy J amp it's doing a very similar job even without a full Valve power stage. :dbtu:

Simply by fiddling with VR1 and VR5 You find the sweet spot which produces a fairly large dynamic range from clean to full bore.

I've never actually tried diodes in valve circuits as I always assumed they would sound harsh much like a lot of SS stompbox circuits but I'm surprised at how little top end cut was needed. A 150p cap (C14) wipes off the excess fizz.

This still distorts a fair bit if you remove the diodes but then it's no longer useable as the jump in volume is then far to great to be of much use.
As should be obvious I've tried several different ideas but so far this has shown the best results.

If anyone can see how this could be improved I'm open to suggestions.

Phil.

Roly

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Re: hybrid preamp idea
« Reply #1 on: September 20, 2014, 11:40:53 AM »
Quote from: phatt
(Did you know that the institution of marriage is the only democratic system where you are forced to sleep with the minister of Defense and Finance?
whoops, off topic again already :-X)

As the family lawyer warned me just before I got married the first time "Marriage is the only contract in English law that is binding for life - and you don't even get a copy!"  {A prenuptial chat with a lawyer?  My mother was opposed, but you can't tell young people anything.  ::) }


Quote from: phatt
If anyone can see how this could be improved I'm open to suggestions.

741's!!!  You mad?   :o

I "don't drive very far" ;), but you obviously need a Red Cross aid package with some LM833's (or even 'umble TL07x's).

{nice waveforms but  :dbtu: }
If you say theory and practice don't agree you haven't applied enough theory.

phatt

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Re: hybrid preamp idea
« Reply #2 on: September 23, 2014, 05:53:21 AM »
Yeah 741 is one of the few models that I recognize. :-[
My thought here is, Let's see if it works with the basic stuff, might unearth any potential quirks I may have to overcome.
I've already found an interesting quirk that being as you dial up full volume on the geetar the bass fattens up big time. (not good) :grr
I thought it maybe my treble bleed cap on the strat volume pot but another unaltered guitar showed the same result.

With my Doctor diode circuit sniffer engaged and the help of Mr sim ,, turns out that I had R13 at 1 Meg and this had to be reduced to remove the quirky bass boost which was seriously limiting the usefulness of the whole idea. Even with C10 at 5nF the problem is still present and maybe lowering the value of R13 might work,, hum? More testing still required.

I've run into that weird issue before with audio circuits but never quite understood what the heck was wrong but now I have at least produced the quirk and reproduced the rising bass response on screen.

Obviously something changing the response curve as the signal gets large. I'm thinking this might have something to do with high Z circuitry in general. I know tone controls in some famous Valve rigs don't work as effectively at high levels of volume due to some kind of loading effect.

Any help with this would be most welcome.

Meantime I,ve been poking more reverse wound diodes in holes today and I'll update this again shortly.
I've had to resort to a 3rd triode to get it happening which is not want I wanted, Bum.
 but,,,  I'll just get it to work first then find other ways to reduce glass if possible.
Basically I'm now trying to find the sweet spot between single coils and HB's and while doing that I've found some 1N34 (I think) and they are helping to make it work even better.

I will get sound clips happening but I need to tidy up all the spaghetti which is making it a fair bit of noise with 3 triodes so time to stop for a break.
Phil.

Roly

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Re: hybrid preamp idea
« Reply #3 on: September 24, 2014, 08:35:58 AM »
{Off base, can't sim, no library}

What happens to the source and load impedances at higher levels cf DC?
If you say theory and practice don't agree you haven't applied enough theory.

tracynorton

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Re: hybrid preamp idea
« Reply #4 on: September 25, 2014, 02:06:51 PM »
phatt...I really like reading your posts and ideas. I lurk here because I'm convinced that you don't need a full on tube amp to get great sound and feel.

That being said....I think you are onto something. Have you seen Merlin Blencowe's paper on low voltage triodes?
http://www.valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf
I was going to breadboard this up and take a look at it's behavior at clipping and insert it into my Studio 60's power amp (at some point when all my day job crap is caught up). But I like the idea of having an opamp front end.

One thing you might try is combinations of germanium diodes and 5mm LEDs. I think asymmetrical clipping has a nicer 'just at the edge of clipping' sound.

I like 741's because they are low BW and lo-fi...as guitar amps should be. The HH Studio 60 uses 741s and Harris quad 741s...and sounds stellar.

Roly

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Re: hybrid preamp idea
« Reply #5 on: September 26, 2014, 03:05:28 AM »
Hi tracynorton welcome.


Some good ideas there. :dbtu:


Quote from: tracynorton
I like 741's because they are low BW and lo-fi...

Yeah ... er ... um...  8|

Now don't get me wrong, I've used almost as many 741's as I've used triple-5's ... well, okay, nobody has used that many anything as triple-5's ::), but the 741 and I are old old buddies.  With the emphasis on "old".  We go back a loooong way to when they were the new kid on the block to the 709 and it's little querks.

Yes they have an internal First-Pole at about 3Hz, but that's for an open-loop gain of 100,000 or something stupid, in normal applications (gain<100) this isn't an audio bandwidth problem.

The primary drawback is the output crossover, but this can be reduced by asymmetric loading.

But then there is residual noise floor, and it in this area that more modern audio-specific op-amps have really left the poor old school 741, 4558, etc., to thermostats and such where their audio shortcomings don't matter.


The audio base line these days would have to be the TL07n-series.  I used LM833's in my Twin-50 preamp refurb, and for less than the price of a cup of coffee I've got the closest thing I've ever seen to a perfect* amplifier with legs, heaps of gain out to MHz (the prototype happily oscillated at 455kHz, had to be bludgeoned down to 38kHz), effectively noiseless, and you get a pair for the price of one.

If you want to go hyper with mic preamps there are cheap(ish) exotics out there that have astonishing specs - totally-shielded-enclosure-scientific-instrument specs.


IIRC the TL07n were the first "op-amps" to be characterised for audio specs.  FET input, handy hi-Z input for naked p/u's or piezos.  Bit like a grunty triode really.   ;)

{* and I'm aware that is one hell of a claim}
If you say theory and practice don't agree you haven't applied enough theory.

phatt

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Re: hybrid preamp idea
« Reply #6 on: September 26, 2014, 10:00:28 AM »
The work progresses slowly,,, Lot's of long stories but I'll spare you from that.  ;)
This is the update, see Schematic and flow chart.

1/
Had to add an extra Valve stage , *Note V1 = 12AU7, V2 = 12AX7 *
hum? AY7 would likely be ideal.
2/
Changed the clipper doide from 1N4148 to 1N34, much better effect as these break over earlier but
don't slam limit. (Took forever to dig them out of all my junk but I knew I put them in a safe place,, obviously too safe  ::))

Still not happy with the jump in Volume level,,, so a brain storm idea came to me.
Knowing I had an old LDR from a Yamaha organ (Now where did I bury that? I've spent a lot of time in
my parts bins in the last few days,  :duh)
I whipped up a quick and dirty poweramp limiter circuit based on Rod's project 53 limiter. Using a
small $4 High brightness LED torch gave me a working limiter circuit that performs better than I expected.

P53 here; http://sound.westhost.com/project53.htm

An elcheapo Casino 12 amplifier drives the limiter circuit from it's speaker output while a line level signal is sent to the main amplifier. I could just tap the main amp but that leaves me stuck at a lower SPL,, Tapping a line out allows me to still get big SPL levels when I need them from the main amp which is not limited. (See flow chart)

I know the extra amp is overkill just for the limiter but other line level circuits may not respond in the same manner. I did modify that little Amp long ago and the posting is here complete with preamp schematic mods;
http://www.ssguitar.com/index.php?topic=2607.0

Those modifications certainly add the edge that I was looking for so worth having it in the signal path.
With my bench now fast running out of space it's time to make damn sure I've copied out these
schematics correctly but I'd really like to get some sound samples down before that.
Meantime life gets in the way of all this fun,, I'll get back soon.
Phil.

phatt

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Re: hybrid preamp idea
« Reply #7 on: September 26, 2014, 10:50:08 AM »
phatt...I really like reading your posts and ideas. I lurk here because I'm convinced that you don't need a full on tube amp to get great sound and feel.

That being said....I think you are onto something. Have you seen Merlin Blencowe's paper on low voltage triodes?
http://www.valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf
I was going to breadboard this up and take a look at it's behavior at clipping and insert it into my Studio 60's power amp (at some point when all my day job crap is caught up). But I like the idea of having an opamp front end.

One thing you might try is combinations of germanium diodes and 5mm LEDs. I think asymmetrical clipping has a nicer 'just at the edge of clipping' sound.

I like 741's because they are low BW and lo-fi...as guitar amps should be. The HH Studio 60 uses 741s and Harris quad 741s...and sounds stellar.

Hi tracy, thanks for the kind remarks.
Good to know that some benifit from all my experiments,, yeah some actually work, lol.

I will read through that wizard stuff but as I'm not really up to grasping a lot of the off beat ideas it seems rather pointless to throw solder at things that have not been proven to work.
No disrespect to Valve wizard, I've learned a heap from Him already.
I tend to work with ideas that I do know stand a chance of success. I worked out a long time time ago not to build stuff that I have no way of trouble shooting when it no work right. :'(
I test all my circuits long before I build stuff. :dbtu:

Oh BTW, My Valve circuits do run at normal valve voltages.

Via low voltage plugpak (12~ 18 VDC) which runs the SS parts and heaters then jumped with auto transformer step up or a 555 circuit and choke. I've built Ht supply with both and they work well.

My other experiment was the PhAbbZone and it was a great unit but very complex to build, this one although it looks complex is still less parts count,, and less knobs,, though that might change.  :lmao:

Yes I do believe one can come close to not needing All Valve rigs and even some digi stuff is impressive but having recently played around with the digi world I'm not convinced I need 256 patches of clangorous reverberating modulated chorus.
No matter how much you tweak the parameters you always end up sounding like every other high school wanna be on Saturday morning at the local music store. I want to sound like a guitar player not a speed dial arpeggiator.
I need exact sounds/tones and dynamics for my style of playing and analog stuff still delivers the goods for me.

I'll try and find the step up circuits and post but too late now,,bye
Phil.

phatt

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Re: hybrid preamp idea
« Reply #8 on: October 05, 2014, 08:03:27 AM »
Re, how to get high voltage for preamp valves
from a 12~18 VDC plugpak/ wallwart.
Low parts count and it works like a charm, the choke is likely the hardest part to find.
Make sure you use a UF (ultra fast) diode.
Here is the schematic, Google "Nixie supply" for more info.
Cheers, Phil.

phatt

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Re: hybrid preamp idea
« Reply #9 on: October 05, 2014, 08:20:10 AM »
Ok another try; lol.
Well I could go on forever but I've arrived at a definite,,, maybe  ;)
The main aim has been achieved and I doubt I'll be able to improve it without reverting to a Valve power stage.

This time diodes in feedback of opamp but controlled via LDR 1 which limits absolute volume level so once you find the balance between the pickup output and amplifier sensitivity by setting VR1 you can go from clean to dirt.
This gives very good results on clean but there was still too much jump in output level when turning up guitar volume, while also adding too much bass which unbalances the effect. (Bass problem has been overcome by altering values of R18 & R19)

After many days of experimentation as well as the Feedback diodes I've also reverted to diodes across the signal (post V2a) to keep the overall balance of SPL at any setting but this does effect the clean sound adding a bit more fizz in clean mode.
But then hey, a lot of older (presumed clean) guitar tracks played by some famous names do sound a bit fizzy which adds that little Edge to hard plucked notes.
I guess in the end it's up to the user to decide what they perceive as clean.  xP

Re the circuit;
The main part to understand is the balancing act required between the light and the minimum resistance of LDR.
Hence "VR2 trim" on limiter schematic, and again another balancing act on Phatt-E3 schematic where "trim 1 and 2" need to be set to what works well.

As to the rest of the circuit,,well it's starting to get way more complex than I wanted so take note the schematic is a reference only and likely to change but at least the basic idea works, no doubt better minds may have other ways to achieve superior results.

Notes on the limiter circuit:
This is from ESP project 53 and with permission (Special Thanks Rod E). I've redrawn it here with slight alterations.
I found P53 original circuit to be pumping and sounding more like a stomp box compressor which might work well for hifi but not so good for what I'm trying to achieve.

I was about to scrap the P53 limiter plan but accidentally reconnected the Led torch to common instead of collector of BD139 (see Rod's original drawing at http://sound.westhost.com/project53.htm) and found it worked super fast and it no longer pumped. I can only assume this renders Q1 out of the circuit and trim VR1 has no effect on attack, so better results just by adjusting trim VR2. Seems just by adjusting light intensity is all that is needed.

I'm still not quite sure if this is a limiter or a compressor and it WILL sound like a cheap crap comp pedal if not trimmed right but at the moment there is little hint of pumping or flattening of the sound it is very dynamic. :dbtu:
 
BTW, I've never liked compressors as they kill all the dynamic feel, the very thing I'm trying to Avoid but to my surprise this now sounds nothing like a compressor circuit,, just a very responsive Guitar/Amp interaction.
With Guitar Volume at halfway it's clean with a hint of rattle and maybe diode frizz.
Guitar up full it does the fat distortion trick. Yay !!!

I've redrawn a circuit flow mud map to show how it's all connected as it's a bit hard to fit the whole thing into one schematic and have it make sense.
I was about to try some recording only to find my workbench Windoze com has suddenly lost the plot,, Audio locks up with no way out but power off. (Oh bum just what I needed another thing to fix) :(
more later, Phil.

phatt

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Re: hybrid preamp idea
« Reply #10 on: October 05, 2014, 08:30:54 AM »
Oh forgot,,,Adding the Casino preamp section as that shows the preamp mods which does help the end result.
I've deleted the OD drive part for clarity, the OD part is not used and sounds awful anyway.

This circuit goes to an LM1875 drive chip which is used to drive the Limiter circuit (not a speaker).
A line out is taken from preamp to a bigger amplifier which is just my old Laney Keyboard Amp and does not add any coloration just a flat response and a master volume so all the tone mojo (if any :lmao:) is from the preamp setup.
 Phil.

Roly

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Re: hybrid preamp idea
« Reply #11 on: October 05, 2014, 01:34:01 PM »
That Nixie supply was discussed in some detail on AGGH a while back.
If you say theory and practice don't agree you haven't applied enough theory.

phatt

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Re: hybrid preamp idea
« Reply #12 on: October 09, 2014, 09:49:31 AM »
Hi Roly, Gee you have a sharp memory. 8|
Yep I read it again to refresh my mind and if I build another I can now see where I could do better in layout,, like the loop around the cap ,inductor and Mosfet, obviously important to keep that path short.

Well I've been head down and to keep this from getting complicated.

I'll just concentrate on the limiter part as it would seem from much experimentation recently that any half decent guitar preamp could be used as front end.

The main issue for me now is replacing the Casino amp with something easier to build and more compact to drive the Led torch/LDR setup.
Ideally one unit containing led driver/LDR and Limiter circuit all in one small box.
I have an LM386 chip somewhere in the pile but wondering will it have enough power to drive the LED torch?.
I could just trash the Casino amp but it's handy as a small unit for testing gear.
Stuff I'll have to work on next time I'm in the mood.

I did some digging and found Peavey's patent for emulating Power tube grid bias under load and I've had that working for 2 days and just by guesstamation I've got it to work as Patents don't give you values so twas all guess work.

It sounds like its doing the job but it lacks the sag effect, (can't think of a tecky term for the effect)

JM Fahey kindly explained the flaws in my other posting regarding sagging the power rails on a SS Amp rig for power stage sag emulation which was helpful but the more I play around with this stuff the more I wonder how perfect it really needs to be. (as I've heard some quite awful all valve rigs that claim to be all mojo) At times my circuits get a really big fat effect but then with so many wires and circuits all with tweaking pots I forget where I set things,, then the phone rings and and oh bum what was I doing.
you then have to re trace the spaghetti trail :duh

Any way still more work to be done and in time if I can get close I'll be happy enough. <3)

With the Peavey circuit in place and obviously working it still sounds hard edged but by engaging my limiter circuit directly after that it comes alive again.

Oh here is the link to the Peavey patent, Fig 2 shows the basic circuit.
I figured I'd better not post the circuit but if someone wants the values I've used just PM me.

http://www.google.com/patents/EP0663720A2?cl=en

J M Fahey

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Re: hybrid preamp idea
« Reply #13 on: October 10, 2014, 03:06:58 AM »
Hey, no "flaws" , just reminding that transistors are somewhat similar to Pentodes, but gain does not change with "screen voltage" .... maybe because they do not have screens after all. :lmao:

So no "natural,  sag driven compression" ...

But nobody said that you can't "cheat" and add a little compression in some other way  8|

Hint , hint .

phatt

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Re: hybrid preamp idea
« Reply #14 on: October 11, 2014, 09:01:39 AM »
Hi Jaun, Thanks for the gentle prods.

Yes I've learned enough to know that the screen grid plays a big role in power stage compression, add cathode biased output and a few other tricks they do indeed become very dynamic.

Here is a screen shot of the circuit as of 11/10/14. A few modifications where needed to overcome some needless complexity and now a quick bypass switching test shows it is working as intended.
Should be obvious to you but for others,, I've put the LDR in the Fback loop and with some careful adjustment it's working much better.
This causes the overall volume to drop a little when you turn the guitar volume to 10 but increases the distortion. 1N914 didn't work well so I used 1N34 diodes.
This will not go into full bore metal distortion,, more like an old fender pushed hard but single notes are still well defined, just adding harmonics and more sustain which is the aim. <3)

I guess now with LDR controlled diodes in Feed back path makes it more like a compressor as You hinted. Cheating maybe ,,but it works. :P
** Tweak it too much and it does indeed start to sound like a compressor**
So one has to find the balance.
With guitar volume at 5 the notes are clear with just an edge on hard struck notes then turn up full and it's good old R&R.
The extra 1N4148 diodes are to help keep the overall level down, the resistors before and after those diodes are important as it sounds awful if you lower the values, goes back to hard edge distortion.

I've simplified the driver for LDR driver so the Casino Amp is no longer needed, signal for Led drive is tapped out at C4.
Anyway here is a quick and dirty take of the effect, recorded direct to laptop as the other computer is dead.
So signal path is just my PhAbbZone preamp > ODL > main Amplifier.

Absolutely no efx or editing just converted directly to low res MP3. The tiny mic in the lappy does not pickup the low end well but it will at least give an idea of the effect.
First part:
Limiter bypassed, guitar volume set on 5 (halfway) then turned up full.
(watch out for volume jump at 10Sec)
Silence,,, then ODL engaged and same test again.

With no ODL engaged it should be obvious like most SS clean Amps they get to loud and stay to clean which forces one to rely on pedals for OD and distortion often creating level balance issues.

Well all this has taken a few weeks to get sorted so I might take a break before I build this but it will get built :dbtu:.
Phil.