Circuit for a Blackstar Fly 3 watt amp?

Tube or solid-state, this section goes to eleven!
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microbailey
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Post by microbailey »

In response to bmxguitarsbmx here is the correct link to the pre-EQ (pre-tonestack) circuit discussion
http://www.midboost.com/workbench/black ... er-circuit
I've corrected the link in the original post too.
Sorry about that :oops:
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Post by microbailey »

I've done the Spice simulation on the pre tone stack circuit and as expected (and as bmx pointed out) there is a huge peak where the active inductor and cap C13 create a deep cut (fc works out at 133Hz) in the feedback around IC2 (giving a gain boost).

Here's the plot from LTSPice
Pre_EQ_Filters_Vout.jpg
On http://www.midboost.com/workbench/black ... r-analysis I've also plotted the full EQ chain from LTSPice including the ISF tone-stack (swept for different settings) and the post-EQ filters.
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Post by microbailey »

The full Backstar Fly3 EQ circuit (pre/post tone-stack and ISF) is now drawn up and posted on http://www.midboost.com/workbench/black ... -schematic
Hopefully the link works this time :wink:

Next I'll draw up the pre-amp input stage (including overdrive channel). It's getting there.
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Post by microbailey »

I've traced and drawn the schematic for the pre-amp now :D It's on http://www.midboost.com/workbench/black ... gain-stage
I've started some analysis and I ran SPICE on the filter at the guitar input - its a gentle bandpass centred on 2kHz.

I'm just starting to look into the overdrive. It basically a Tube Screamer circuit!
More to follow.
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Post by microbailey »

I have a question for any other EEs reading this (maybe I should be posting this on EE StackExchange instead :) )
I notice that Blackstar's engineers have frequently used the type of op-amp filter shown in the first stage of the circuit below (taken from just after the Fly3's ISF tone stack)
Post_EQ_Filters_cct.PNG
Post_EQ_Filters_cct.PNG (10.34 KiB) Viewed 430 times
Whilst I'm familiar with use of the very common Sallen and Key filter shown in stage 2 above (reading from the left) I am not so familiar with the filter circuit in stage 1.
I can see it is a combo of a low-pass (C62) and high-pass (C41) filter, and SPICE shows it can give a gentle bandpass depending on the component values.

I'm interested to know if there's a particular advantage to using this type of filter circuit? Is it just that you get a (not very steep) bandpass with one op-amp?
One other advantage I can think of is there is a very high input impedance to the filter (so it doesn't load the tone stack cct).

Any other thoughts?
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Post by bmxguitarsbmx »

I think your analysis is correct- It is a more subtle bandpass. Less subtle than a first order passive bandpass though!

Great job on this thread. I have found it quite interesting. I may breadboard out that delay circuit to see how it acts in real life :)

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Post by Axldeziak »

bmxguitarsbmx wrote: 19 Nov 2020, 02:14I may breadboard out that delay circuit to see how it acts in real life
I uploaded a layout for it awhile back.
http://guitar-fx-layouts.42897.x6.nabbl ... 49306.html

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Post by microbailey »

bmxguitarsbmx wrote: 19 Nov 2020, 02:14 I think your analysis is correct- It is a more subtle bandpass. Less subtle than a first order passive bandpass though!

Great job on this thread. I have found it quite interesting. I may breadboard out that delay circuit to see how it acts in real life :)
Thanks bmx. Yes it looks like a bandpass from the SPICE plot I did for the one in the pre-amp circuit just after the input jack
Preamp_filter_freq_plot.png
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Post by beeflin »

Thank you for all your work and explanation. The FLY 3 is a truly classy piece of plastic!

I've just tried using my FLY 3, with its extension cab, as the Dry amp in my Wet-Dry rig instead of using two heavyweight Marshall 6101s. It sounds really good, but unfortunately when it and one of the Marshalls are powered on, they both emit a high-pitched whine.

Its note is a C with the extension cab disconnected, the E above when it's connected, and if I turn Delay Time down to zero it goes up to F and gets a little quieter and more bearable.

This noise occurs even when the pedals are powered down so it appears that it's clock noise from the FLY 3's Delay. What can I do to silence it?

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Post by microbailey »

beeflin wrote: 21 Nov 2020, 21:43 Thank you for all your work and explanation. The FLY 3 is a truly classy piece of plastic!

I've just tried using my FLY 3, with its extension cab, as the Dry amp in my Wet-Dry rig instead of using two heavyweight Marshall 6101s. It sounds really good, but unfortunately when it and one of the Marshalls are powered on, they both emit a high-pitched whine.

Its note is a C with the extension cab disconnected, the E above when it's connected, and if I turn Delay Time down to zero it goes up to F and gets a little quieter and more bearable.

This noise occurs even when the pedals are powered down so it appears that it's clock noise from the FLY 3's Delay. What can I do to silence it?
Hi beeflin. If both amps work correctly on their own it sounds like either radio frequency pickup from one amp to the other, or maybe an oscillation caused by a ground loop with cables. You'll need to isolate what's causing it.
I would leave off the Blackstar extension cab while you are diagnosing the problem since it doesn't really affect whether the whine is there or not.

A few questions: Are you saying there is no noise from either the Blackstar Fly or the Marshall on its own, but only when they are both on and hooked up in your rig?
If you unplug (take right out, not just mute) your guitar cable from one of the amps does the noise stop?
If you are using a power brick with the Blackstar Fly have you tried batteries, just to see if the noise is due to a ground loop through the mains?
Also does the Gain on either amp affect the volume of the whine? (will tell you at what point the interference gets in - if the gain affects it you know its before that point in the signal chain)

There are couple of high frequency signals running around in the Fly - one as you say is the in the Delay's clock circuit and the other is in the Class D power amp stage clock. Although they are in radio range they normally shouldn't couple to another amp which is why I'm suspicious there is something going on with grounds in cables.
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Post by beeflin »

Thank you! It'll take a few days to get round to it but I'll be back with the answers.

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Post by yoon_seri »

Hello @microbailey

Great job on this!

I have few questions.

Upon checking your diagrams here:

https://static.ucraft.app/fs/ucraft/use ... 1594590407

http://www.midboost.com/workbench/blackstar-fly-3#m251

Since the #3 and #4 wire of the 4-wire ribbon cable are connected together (Ground), it will also connect the Signal - (negative) P3 of the Power Amp board to the Ground (P4 and P5).

Is this correct?
And why is it possible?

Thanks!

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Post by beeflin »

There's no whine when the Fly 3 is on batteries.

I have a Joyo ZGP 9v power isolator and would have tried it, but unfortunately its plug doesn't fit into the Fly's power input. Is that the right approach - should I get something similar, or an adapter for the ZGP?

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Post by bmxguitarsbmx »

The power supply for the Blackstar Fly3 (powers the amp and the extension cab) is 6.5V 1.5A Center positive. So, the whine is your power supply screaming at you. It is probably BOTH the wrong voltage and not enough current. Basic stuff man.

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Post by beeflin »

I don't mean I was trying to supply it with 9v. If you actually bother to look up the ZGP you will see it's a passive PSU isolator which can *handle* 9v. Also I said it didn't fit, so even if I was dumb enough to try to give the Fly 9v I wouldn't have succeeded. I'm looking not for unhelpful flames, but to see if microbailey has anything further to offer, like whether such a PSU isolator would help. Sheesh.

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Post by microbailey »

beeflin wrote: 25 Nov 2020, 15:04 There's no whine when the Fly 3 is on batteries.
Ok that's good as it means the amp is fine.
Just to be sure - are you saying the whine is not there with your rig exactly as you had it to start with and the only change you made is to run the Blackstar on batteries instead of a mains PSU?

If so then it means either the PSU you are using with the Blackstar is actually contributing/causing the noise or that there is a loop through shared grounds when the PSU is being used.
To try to isolate the problem further:
Do you get the whine using the Blackstar amp on its own with the PSU?
If so then I'd say its the PSU.
If not then its more likely a ground loop between the Blackstar and either the Marshall or some pedals (if you're running any) which will mean playing about un-plugging/re-plugging audio cables one by one in your setup to see which one makes the noise stop.

Ground loops usually happen because you have a common PSU somewhere, but that wouldn't be the case with your Marshall - it has its own supplies. Could be there's an earth connection acting as a ground loop. Btw - DONT remove any earth connections in amps or their plugs!! That can get you fried :)
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Post by microbailey »

yoon_seri wrote: 25 Nov 2020, 00:59 Hello @microbailey

Great job on this!

I have few questions.

Upon checking your diagrams here:

https://static.ucraft.app/fs/ucraft/use ... 1594590407

http://www.midboost.com/workbench/blackstar-fly-3#m251

Since the #3 and #4 wire of the 4-wire ribbon cable are connected together (Ground), it will also connect the Signal - (negative) P3 of the Power Amp board to the Ground (P4 and P5).

Is this correct?
And why is it possible?

Thanks!
Hi yoon,

I wanted to double check my circuit tracing before replying. You are correct #3 and #4 wire of the ribbon are connected via the Fly's Main board ground (I checked with a multimeter). And yes that means the signal negative into the Power Amp (Speaker board) and the supply ground are also joined.
Although its true to say the neg signal input on the power amp (Speaker board) is shorted to the same ground as the Speaker board's supply ground, the crucial thing is that its shorted at its far end - on the Main board where both the signal and the supply come from.

It's normal circuit design. I'll try to explain briefly why.
The power amp is a Class-D switching amp (as I explain on my website). So it creates a lot of electrical noise on its supply rails. I don't want to get too technical about it but there are current spikes happening 100,000 times a second on the Supply and Gnd wires into that Speaker board and, with the resistance of the ribbon cable (I know its short but it will have a few ohms resistance) that will create voltage spikes which appear at the Speaker board end of the cable (to understand that more Google Ohm's law).

Since the IC in the power amp amplifies the voltage between its + and - inputs, Blackstar have used a standard technique called star-grounding where the negative (-) signal input comes from a place before those voltage spikes happen, which is the Main board Ground plane.
Long story short - this means you don't hear all that nasty switching noise the amp makes because the voltage spikes do not appear on that neg signal input.

The next section I'm writing up on the Blackstar Fly3 is the power supply section so I'll try to put something about this in maybe with some pictures.

Hope that all made sense :scratch:
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Post by beeflin »

microbailey wrote: 25 Nov 2020, 22:54 Just to be sure - are you saying the whine is not there with your rig exactly as you had it to start with and the only change you made is to run the Blackstar on batteries instead of a mains PSU?
That's right, so it looks like a ground loop.
microbailey wrote: 25 Nov 2020, 22:54If so then it means either the PSU you are using with the Blackstar is actually contributing/causing the noise or that there is a loop through shared grounds when the PSU is being used.
To try to isolate the problem further:
Do you get the whine using the Blackstar amp on its own with the PSU?
No, only when the wet and dry lines are connected to my passive splitter (a simple device containing only resistors to prevent the two sides loading differently). The pedalboard's not powered on, nor connected to the amp or the guitar, so it's almost certainly just the connection via the splitter that's the other half of the ground loop. However, isolating this signal line (with a SubZero Hum Destroyer) doesn't help. So I believe I need to isolate the ground on the power supply, safely doing so by fitting an isolator such as the Joyo ZGP in the low-voltage line between the Fly 3 and its transformer. I certainly wouldn't mess with the high-voltage line!

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Post by microbailey »

Thanks for explaining. For a ground loop you can try 2 things. One is isolating the supply (as you suggest), the other is lifting (disconnecting) a shield on the load end of the audio cable between the pieces of equipment.

For isolating the supply the problem you'll have is the Fly runs off 6.5v and needs 1.5A which is a non- standard supply for pedals. Are any of the cables able to have a shield disconnected?

Check any supply isolator spec very carefully if you decide to try it. Will need to be able to supply 1.5A. You can get mains Isolation transformers which could safely break the earth loop but they're not cheap.
Last edited by microbailey on 26 Nov 2020, 11:16, edited 2 times in total.
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Post by beeflin »

Thank you! So the loop here is between signal ground and power ground.

I'll prepare an audio cable with disconnected shield at the load end, as you suggest. It does sound trickier to find a suitable way to do the same on the power side without cutting into the Fly's proprietary cable!

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