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Messages - dazz

The Newcomer's Forum / Re: Marshall 5010 Needs Help
March 08, 2020, 12:24:31 PM
Sorry for the necro post, but I figured out the schematic based on El Scorcho's pics of the pcb, so I thought it might help someone out there at some point. Looks like the circuit is almost the same as the published schematic, but without the bright cap and the presence control was moved back in between the two amplification stages. I used the cap values in the published schematic, not sure about those

Quote from: phatt on November 26, 2018, 08:03:02 AM
hi Dazz,

Wow,, hey Don't over think it,,
Here is how you run that circuit on single supply.
Even with higher headroom it will still distort early ,, that is the way it's designed.
I have not simulated the tone section but it looks like the treble is insane high,, I doubt I'd like the sound but it could be tweaked to back off the extreme treble. :-\

Sorry can't help on all that split supply thing you are talkin about,, I'd just find another transformer,, likely cheaper in the end.
But up to you. :-X

Hi Phil! You're not the first one to tell me I'm overthinking this hahaha.

Thanks for the schematics, I thought I already had that OK. I had only R2 and VR1's mid lug going to Vref in my circuit, the rest going to ground as they are not DC coupled to GND, except for R3, I added a cap there to hook it to GND (I can see now that wasn't necessary, could simply go straight from R3 to Vref)
Yeah, it distorts early  and a lot with a +16/-16V supply, but with a 9V one I think it will distort at almost any gain setting.
True, it's very trebly, not for me either. This is a project I got into by some else's suggestion. I'm building a RoG Azabache for myself for more fender like tones. I'm sure you've heard about the Azabache. But anyway, it's just that i saw the opportunity to learn how to bias opamps, last time (with the Casino 12 build) you had to do it for me, no more!  :grr :lmao:

This is a different beast though.

BTW, I think I should have used the spare opamp in the Casino 12 design to buffer the voltage divider.. it works great as is so I guess it wouldn't have made too much of a difference. Let me thank you again for all your help in that thread, The amp now sports a 25W class D chipamp and it's loud AF  :dbtu:
Quote from: edvard on November 25, 2018, 04:01:41 PM
Quote from: dazz on November 25, 2018, 01:44:14 PM

So someone suggested a charge pump chip to produce the symmetric supply (an LT1054 or ICL7660(S)), but I decided against it because for all I know, the result is not really symmetric, being the negative rail weaker (my LTSpice simulations show that with a 10mA load it's more like +17V/-16V) and I fear that might induce asymmetrical clipping. Are my concerns here justified or am I simply talking nonsense?

Many, many, many distortion circuits utilize asymmetrical clipping as a desired feature, so I think you're OK there.  As far as clean headroom, I think you're still OK as long your preamp isn't overloading the front end of the power stage.  Even the venerable TDA2050 had a maximum input voltage of ±15v, and for most real-world situations, it was probably much less than that.

True, and as someone else pointed out to be elsewhere, opamps that aren't RRIO will clip asymmetrically anyway. It's just that I thought unbalanced rails could alter the way the opamp sound as they clip by adding even more asymmetry.
OK, after a few months of having this project in stand-by I'm finally getting back to it, hopefully to complete it once and for all.
I noticed there's a huge problem with my approach here to feed this preamp 9V since it's originally designed to run at +16V/-16V. Obviously reducing the supply to just 9V affects the clean headroom available to it (what a stupid overlook on my part) and since the gain pot seems to form a low pass filter with C7, if it starts to clip earlier than the original it will do it at a different frequency response.

So someone suggested a charge pump chip to produce the symmetric supply (an LT1054 or ICL7660(S)), but I decided against it because for all I know, the result is not really symmetric, being the negative rail weaker (my LTSpice simulations show that with a 10mA load it's more like +17V/-16V) and I fear that might induce asymmetrical clipping. Are my concerns here justified or am I simply talking nonsense?

So i designed the charge pump as a voltage multiplier with two 7660's in series instead, so I get a single 20 to 24V supply to use with the biased op amp circuit. It's not the 32V swing of the original design but it's much better than 9V, and I believe I can compensate that by increasing R8 from 10K to 15K or 20K to attenuate the signal and achieve pretty much the same thing as the original design.

Maybe I'm just talking myself into using this single supply solution for no good reason, but I've been doing some reading about this stuff and I think it could have an advantage over the symmetric supply: no ripple induced in the negative supply V- pin of the op amp since it's now tied to ground. The PSRR of the positive rail of the TL082 I plan on using is much better than the negative one, hence it should help with ripple (as opposed to having two +/- rails with ripple on both). Does that make any sense?

I learned there's a 7660S with a boost pin 1 to increase the switching frequency from 10KHz to 30KHz outside the hearing range, although I ordered some cheap ones off of ebay that might be fake 7660S (rebranded 7660 without the boost feature). I had no idea this could happen, ugh! But I'm wondering, even if I end up with a 10KHz chip, what would be an acceptable level of ripple to have? The PSRR of the TL082 V+ pin at 10KHz is 80dB, so even if I have 100mV ripple it would be attenuated by a factor of 10^(80/20)=10000. Considering that the preamp's max voltage gain is 62dB, I take it that worst case scenario I would get a 80dB-62dB=18dB ripple attenuation at the output, or 8 times less. Are those numbers correct and

I know I can just go get a proper ICL7660SCPAZ or a LT1054, and I probably will, but I want to take this opportunity to learn about these things, so I'll be running some tests with whatever I receive in the end, different filter caps, etc... I plan on using a large 100uF 50V filter cap in parallel with a 1uF 50V ceramic to maximize capacity and minimize ESR which I heard is critical in these charge pumps.

Hope that wasn't too much of a mess

ETA: forgot to attach pictures of the charge pump simulations. The first one using the boost pin, the second without it. Using tantalum 100uF filter caps in the sim that I hope I can replace with those electrolytic + ceramic in parallel. I get "moderate" amounts of ripple there that I don't think will be an issue considering the PSRR of the TL082, even at 10KHz, is that right?

Ripple without boost: 3.5mV, 10KHz
Ripple with boost: 1mV, 30KHz

ETA2: I should have used the preamp's voltage gain before the tonestack, which is obviously higher than at the output. At 10KHz it's 66dB, but the 7660 switching freq gets lower as temperature goes up- The preamp gain at 6Khz is 70dB, so considering 80dB PSRR that's only a 10dB attenuation of supply ripple.. hmmm...
Quote from: phatt on July 31, 2018, 08:57:53 AM
I can't remember the exact inductance of the cores but I don't think it's over critical, the link mentioned 10~100uH. Just read the link as he mentions the exact value he used. 8|

Oh, but that would be for the DC output filter, right? I was referring to the mains filter, the aftermarket one.

Quote from: phatt on July 31, 2018, 08:57:53 AM
As my pedals are all low current heat was not an issue for the cores. I do remember the inductors I had were a bit low so I simply joined 2 in series.

Actually I already have filters in my 9V pedal PSU's outputs, one of them is an RC filter that seems to work fine for low current pedals (mine are too, all three measure below 5mA). For the other one I used a 7809 because it was outputing over 12V even though it's supposed to be 9V.

The thing is that the PSU that is supposed to require filtering is one of the 12V 5A that I use to power the amp. The chipamp is limited to 2A, but I don't think I can build the filter ignoring current draw?

Quote from: phatt on July 31, 2018, 08:57:53 AM
Re the Phatt Box;
That is 3 circuits in one box.
A compressor from member mictester (FSB forum)

That's the Really Cheap Compressor, isn't it? A great candidate for my next project  :tu:

Quote from: phatt on July 31, 2018, 08:57:53 AM
distortion is easy to get but it's the in between edge that is much harder to get.

That explains why my crappy OD pedals sound like crap at low gain. They're OKish when cranked especially the JFET one so I ended up increasing the gain to get more and more distortion. AT first I added an extra gain stage, then put a switchable cap at the source of one of the sources... it started as a Professor Tweed OD from Run Of Groove, now its more of a nasty distortion/fuzz effect  :lmao:

I'm gonna have to try the compressor and see if I can get some of that magic here  too :dbtu:
Quote from: phatt on July 29, 2018, 11:24:31 PM
Hi Dazz keep at it will work out. :tu:
I just used one of these for the mains side;

Hmm, OK, I noticed that filter of yours has larger 3.7mH inductors which should put the -3dB cutoff at some 9KHz. The cheap ones at ebay/aliexpress with 300uH coils, according to LTSpice, have a much higher cutoff at 35KHz. I'm guessing I should probably steer clear of those

What a killer pedalboard! What does that Phatt Box do if I may ask?
Awesome, thanks Phil.

I have some coils / chokes I cannibalized from the computer PSU I used for this amp, not sure they'll work though. The largest one measures only 6mH

I can maybe order one of these EMI filters for the mains, and see if I also need a DC output filter later. Besides, I don't think I should mess with mains considering my obvious lack of knowledge & experience.

I noticed KA7OEI mentions in his blog that he had already added bifilar chokes to both the AC and DC leads, and that took care of a significant amount of noise. How would that be done?

EDIT: I found an earlier blog entry there where he explains all that stuff

And Here's a pic of the PSU in case it helps.

Comes with...

68uF input cap
1000uF output cap

jc817 optocoupler

tl431 adjustable shunt regulator

uc3843 PWM controller

Two (I believe) power mosfets
Quote from: phatt on July 29, 2018, 07:36:23 AM
If rolling the volume back on the guitar causes more buzz/hum then it's highly likely the guitar itself is at fault.
Better check that first before you blame the circuits you have built.
Continuity check the ground path inside the guitar first, don't forget the bridge is also grounded,, oh and the lead from guitar to first pedal.

Continuity seems fine, but considering that it also seemed right in the pedals and they were still poorly grounded, I'll double check the wiring and solder points in my guitars and cables.
I've done some tests and found something interesting... tried unplugging the guitar, so I have guitar cable (6 foot long) -> OD1 -> OD2 -> fuzz -> amp. Then I put a wire across the guitar cable to have it act as an antenna, and as soon as I fire up the noisy power supply, the 100Hz hum starts. But, if I plug the noisy PSU in a different socket in the same room, the noise goes away. That points to common mode noise, right?

Also, I put the noisy PSU back in the same power strip as the amp, the noise came back but... I don't get this... with the guitar still unplugged, if I ground the guitar to the amp, a good chunk of the noise goes away. If I put the tip of the unplugged guitar cable next to the pups, boom! lots of hum.

I guess shielding the guitar should help... and stop getting cheap PSUs too  :lmao:
I've been googling how to filter or shield SMPS's but that's a different story
Damn, the noise comes back in all of its glory when I roll off my strat's volume pot... off to google this $%&t
OK, I put a wire across both jacks in both OD pedals and the noise is gone almost completely.  :dbtu: I still get a little bit of it if I really crank the gain, but I'm pretty sure it's the guitar cable that's picking up most of it now (EDIT: yeah, I tried a shorter cable that measures next to no resistance and the PSU noise is gone for good). I guess some noise is unavoidable sometimes, although the suspect power supply could probably do with some better filtering. I tried putting a 470uF cap across it's output but it didn't do anything to make it quieter (it already has a 1000uF). I don't think I'm gonna bother with that. It's a cheap PSU anyway and it works well enough for home practice.

BTW, my Casino 12 preamp PCB's are on their way!
Quote from: phatt on July 27, 2018, 04:17:00 AM
You don't have an earth loop and you have not built a Faraday cage,, that is a different thing. ;)
More likely a Bad earth connection or a severely compromised one.

I'm on a roll! LOL. Well, that's good news because I guess it should be easier to fix

Quote from: phatt on July 27, 2018, 04:17:00 AM
Most Pedal power sockets are normally centre common so I have no idea how you wired your circuit.  All this makes the situation even more complex.  xP

First thing I built was a fuzz pedal, a very simple one (bazz fuss) and I wired it center positive simply because the 9V power supplies I had lying around were center +. Then I built the Casino 12 amp and finally 2 overdrive pedals with spare components (one jfet based and one opamp plus clipping diodes, a simplified TS clone) It seemed to make sense to have it wired that way, with the enclosures grounded that I thought would help keep noise at bay. All pedals are true bypass.

Quote from: phatt on July 27, 2018, 04:17:00 AM
Also it seems you have more than one pedal supply which adds even more potential problems.

I have two 9V and two 12V power supplies, but I'm only using one of each, one 9V for all the pedals, one 12V for the amp. All four are plugged to the same power strip even though I only use two at a time, that's to test noise with different combinations of PSUs

Quote from: phatt on July 27, 2018, 04:17:00 AM
As to the dodgy ground path you have;
With power off go measure the resistance between common at the first pedal and amplifier common. It should read very low Ohms. 1 Ohm or less is good.
Then do it again with that wire in place, the one that reduces the noise. My bet is the meter will read lower with that wire in place. ;)

So weird, it measures very low resistance all the way through: 0.9 Ohms from the guitar to the amp chassis with all three pedals in the chain. Since the meter measures 0.6 Ohms with the leads shorted, that's 0.3 ohms along the ground path from guitar to amp.

I've tried different combinations of engaged/disengaged pedals and it gets even weirder: the chain is: Guitar -> OD1 -> OD2 -> Fuzz -> amp.
Well, if I engage the fuzz and disengage/bypass both OD's, I get noise from the (I guess poorly filtered) power supply.

Pretty much any other combination is fine, so looks to me like the fuzz (MPSA13 darlington transistor) with it's massive gain is amplifying noise picked up on the ground path before it. In fact, if I bypass the two OD pedals with a wire, the noise goes away for the most part even though they're disengaged (bypassed)

So yeah, I'm gonna make sure the ground connections in those OD pedals are OK.

Quote from: phatt on July 27, 2018, 04:17:00 AM
The input to most pedals use a stereo socket to turn on the power at the common connection of battery or external power and these can become compromised so lotsa places common or ground can become slightly lifted and cause hum.

I'm not using that because I built those pedals to be used with power supplies, no batteries, so I didn't think that would be necessary.

Quote from: phatt on July 27, 2018, 04:17:00 AM
Maybe post a pic of the whole layout.

I'm a bit embarrassed  to post pics since I built the OD pedals in tin cans and the whole thing is as ghetto as it gets, but I'll post a few of them if that helps
I finally got round to troubleshooting my grounding problems. I found that putting more pedals in the chain exacerbates the problem and it even starts picking up a nasty 120Hz hum from one of my switching power supplies. That happens even if that particular power supply is not feeding current to any of my pedals / amp.

Next thing I found is that the noise goes away almost completely if I connect the first pedal's input signal ground to the amp's chassis. Looks like a ground loop, right?

So I started googling and found this
where they recommend isolating the signal ground from the power ground. Right now I have all my power jacks center positive, with all grounds (signal and power) tied together and also to all every chassis (every pedal and amp) I thought that's how you implement a Faraday cage

I really hope I don't need to rewire everything to isolate signal and power grounds, not sure I would know how to do that anyway  :-\
Quote from: phatt on July 11, 2018, 10:18:17 PM
I would just keep bread boarding because you will learn so much faster as you will clearly see (make that hear) where and how noise issues, psu filtering and ground paths can make a big difference to the build.

Understood. I'm just about to order a larger breadboard, that should help a bit

Quote from: phatt on July 11, 2018, 10:18:17 PM
I use sims as well as real working circuits before I commit to a pcb.
Note the picture of my very messy bread board.  :)
This is a compressor on the front and a cab sim circuit on the back. The small switches are just to by pass so I can hear the difference.
I built this last month for a mate as he was amazed at the presence of my guitar rig.
He noted that I hardly touch the strings but still have lots of punch where he has to belt his to get any volume, The classic muddy sound you get without good EQ.
I explained that a lot of guitar rigs have way too much bandwidth and although this makes a clean strum of open chords sound bigger it will trash a full bore distorted guitar tone.
Distortion only works well if you limit the B'width.

Distortion is the easy part,, getting the right tone shape is where the magic comes from.
Take Santana as example 90% of his tone is all under 1kHz.
But he is using HB pu's so with SC pu's you need a higher roll off.

I think I know where you're coming from. You mentioned the importance of limiting the bandwidth in the Casino 12 thread too (filter lows to avoid muddy bass with weak power supplies, and of course highs, that I guess also helps with picking up interference) and that Lead 12 preamp is for the most part unfiltered for all I can tell (although it's missing the tonestack). I will definitely keep all that in mind for my future projects though. THis one, I don't care, someone else asked me to prototype it and I simply took the opportunity to cut my teeth on the software, and learn something about biasing opamps. So yeah, my next reading will be on tone shaping networks, filtering techniques, etc. As always, thanks for the info

Quote from: phatt on July 11, 2018, 10:18:17 PM
Re sims and eagle;
I don't know much about transferring Schematic's from Eagle to LT spice but I believe some platforms can transfer spice files.
Too much stuffing around for me I just print out the schematic from Circuit-maker then read off that into KiCad.
This also forces you to actually read and understand the circuit.

Well, it's not so much about transferring schematics, Eagle Autodesk is supposed to have an LTSpice pluggin to simulate the schematic used to build the pcb. It just seems convenient to me to have it all integrated in a single software package. I'm not sure I want to use a third piece of software to transfer schematics from app A to app B which I'll have to review anyway and adds another potential point of failure. So if I can't get the LTSpice integration to work properly in Eagle, I'll do as you do and manually transfer de schematic myself.

Quote from: phatt on July 11, 2018, 10:18:17 PM
Your sims should have a probe setting to read DC at any part of the schematic. If you DC probe the output of U1A it will be very close to Vref. Then switch to AC and you will see that the Zero crossing of the AC signal is centered on the Vref

Yeah, works exactly like that :cheesy:

Quote from: phatt on July 11, 2018, 10:18:17 PM
Keep going as it does take a while to find your way

I will, thanks for your input and encouragement, Phil  :dbtu:
Quote from: phatt on July 11, 2018, 02:00:34 AM
Was a late night after a gig,,, but now that you moved R9 it looks like that may work.

Other options;
Delete C5, connect wiper of 22k pot and C6 to "Vref" instead of Ground then you can delete R9 & C7.
The output of IC1A is already at Vref which then deletes the need to isolate IC1B.
I'm fairly sure that will work.
And less parts ;)

I always breadboard circuits before committing to a pcb as there is always some little bug. :grr

Awesome! thanks Phil.
You know what's weird? That's exactly what I had initially in mind! (see first post attachments) and sure enough, the simulation seems to confirm it works.

So the only problem I had was the reversed opamp inputs in LTspice. unfriggingbelievable.. that crap send got me trying alternatives until I figured it out, then it didn't occur to me the other tweaks might not be necessary. I'm not sure if I should get a sense of accomplishment or feel more stupid. LOL

Re: breadboarding, I tried that for the first stage of the preamp, and it worked, but I wasn't sure how it was supposed to sound so decided to rely on the simulation in what seemed a more "scientific" way of making sure the freq response was equivalent to the original circuit. I've found breadboarding to be impractical for circuits containing more than a few components (I also need to get better in that department) . In fact this project is nothing I plan on building, it was just something that I'm using as a sandbox project to learn LTSpice & Eagle enough to build a proper PCB for your Casino 12, as the veroboard I used ended up being a complete mess with all the modifications, trial and errors, desoldering and resoldering stuff on top of the board... it's fugly as hell and I want to do justice to it, haha

But anyway, there's still something unclear to me. It's the part where you say

QuoteThe output of IC1A is already at Vref which then deletes the need to isolate IC1B.

Seems to me the circuit I posted on July 10, 2018, 04:12:10 PM also has it's output at Vref... yet it seemed to need that isolating cap... I'll see what I'm missing now because there must be something different in those circuits cause it now works fine in LTspice without that cap.

EDIT: no! it only needed C1 to isolate IC1A's input, hence having it's output at Vref through R4. I think I get it now

One thing I really need to do is to figure out how to simulate my circuits from Eagle, because copying it to LTSpice and back will certainly introduce errors in the PCB
Quote from: phatt on July 10, 2018, 09:17:00 AM
It may pass signal but it still not right,
Pin 5 has no bias  :o
R9 needs to go to pin5

Damn it, I put that coupling cap just to screw the bias. Noobs will be noobs.

Quote from: phatt on July 10, 2018, 09:17:00 AM
There are other problems as well which are a bit hard to explain.
Some circuits are hard to convert to single supply and you will have to alter more than R9.
I'll see if I can redraw it for you in the next day or two.


I'm not sure I feel comfortable with having you do all the work for me, Phil. You know I appreciate your help immensely, but I think I need to figure it out myself as a part of my learning process. Maybe if you can simply name what those other problems are I can then research them and find solutions I can later post for you to evaluate?

Opamp Rule of thumb; The positive input has to have a DC path to a reference voltage for single supply or ground if it's split supply.

That's something I sort of inferred from the way you biased the Casino 12, and that's the way it was supposed to be, but I messed it up with the wrong placement of C7

This should do the trick, right?