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 »

The Blackstar Fly 3W amp is pretty loud so I took mine apart to see what they used for the driver (great little guitar practice amp btw).

It's almost all SMD although wouldn't be too hard to trace if I felt inclined as most of the PCB tracks are on the top layer on each board (there are 4 PCBs in total, 1 main board and 3x daughter baords - no wonder it is so expensive for a tiny amp).
Anyway looking at the ICs on the main board I can see what I assume are dual op-amps (2272 and 358s), a PT2399 (does the in-built delay), and what I thought was a power amp IC.
The power amp IC (On Semi part 57152) turns out to be a high-current low-dropout regulator.

Which leaves me wondering what provides the power to drive the 4 ohm speaker?
I notice the daughter board attached directly to the speaker has a couple of trannies (BC547s I think), a diode, and a bunch of Rs and Cs, and is attached to the main board (with the LDO and pre-amps) via a 4-way ribbon.

Does anyone have any idea? I am wondering if they used a class-D power amp, with the trannies driving the speaker load, and the op-amps part of a PWM circuit?

I haven't put a scope in there yet to see what is on that 4-way ribbon, or done any tracing, so I could be completely wrong.
There some other interesting parts on the board: a JFET (which I think is a switching tranny), a 4053 analog switcher, and a 4 pin "TI 4B AIZ" which I have no idea what it is.

I'd be interested to know if anyone has reverse engineered one of these things?
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Post by plush »

How about posting the gutshots, so we could limit our speculations and be more specific?

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Post by microbailey »

Hi plush
The amp's put back together for now. When I next dismantle it I'll try to post some shots.
In the meantime I found some pics of the main board online here: https://picclick.com/Blackstar-Fly-3-Mi ... 67030.html
(I hope its within forum rules to post that link :wink: )
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Post by microbailey »

Ok so I finally got time to dismantle my Blackstar Fly 3 again and start prodding around with my magnifying glass, my scope and a multimeter.
My suspicions are confirmed - its a Class-D switching amp which explains why its so loud for a battery amp.

I stuck my scope on the speaker terminals, with no audio input, to see this
Fly_HF_osc_waves_speaker_board.jpg
a 500 kHz square wave at around 8 volts.

That's a give-away and led me to the 8-pin SMD IC (P8304) on the tiny PCB which is soldered directly to the speaker - its a mono Class-D 3W amp-in-a-chip.
Fly_speaker_board_overvew.jpg
That makes a lot of sense - Class-D is standard in mobile phones and PCs now and can even be found in HiFi, and its very efficient - so great for loud clean tones off a battery. Completely different to a standard guitar amp design.

I'm blogging the whole Fly deconstruction thing with photos on my website on this page (yes, this has turned into a shameless plug :wink: ) http://www.midboost.com/workbench/blackstar-fly-3
I've got gutshots there and will start some board tracing on the pre-amp soon.
I plan to explain a bit about how Class-D works too (although most of it is on the web).

Now the thing that's interesting about this (to me anyway) is that basically the whole power amp is contained on that tiny board, not on the main board as I had expected. I'm sure Blackstar did that to keep radio emissions down (Class-D is terrible for EMI - Electro Magnetic Interference) by keeping all the cables as short as possible.

So does that mean the Extension Cab also has one of these Class-D amp boards in it? If so it could be a cheap mod-ding platform with very decent output power and speaker tone.

In short this amp could be interesting for mods, even if you don't want to get stuck into the SMD components you can get at the point between pre-amp and amp on the 4-way ribbon (tbc with a scope).

FWIW my guess is that the tiny board on the speaker is just a copy of the application circuit in the Class-D IC's datahseet.
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Post by microbailey »

I finally got around to tracing the speaker board (the Class D power amplifier) i the Blackstar Fly 3 and drew it up as a proper schematic in KiCad
https://static.ucraft.app/fs/ucraft/use ... 1594590407

It's pretty much the circuit from the DIODES datasheet note for the PAM8304 plus a transistor to (I presume) silence the amp when the supply is going up or down. There's loads of decoupling to try to clean up the supply to the IC.
I've included all the resistor values as they were on the components, but I haven't measured the cap values with a meter.
Where I could see cap values I've put them.

The input to this board is a 4-way flat ribbon which is
  • +5V
    Audio signal +
    Audio signal -
    0V
The + and - signal inputs are because ts a differential input on the IC, and I guess also helps with RF noise immunity.
The cross-connected limiting diodes (D1) suggest the input voltage expected by this board is in the order of 500mV or so.

I'll try to get hold of the Blackstar extension speaker (FLY103) cabinet for this amp and open that up to see if it also has an amp.
I'm also planning to at least partly trace out the main board which has the pre-amp and delay cct.
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Post by JiM »

microbailey wrote: 12 Jul 2020, 22:26I'll try to get hold of the Blackstar extension speaker (FLY103) cabinet for this amp and open that up to see if it also has an amp.
One quick way to check this possibility is to trace the signals going to the extension connector.
Most probably there are the same four as the ribbon going to the amp board.
(Although the amp + extension cabinet is marketed as an optional PC speaker set, therefore the input signal is probably separate to allow stereo operation)
I only give negative feedback.

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Post by microbailey »

Thanks JiM. That's what I was doing this week! The extension connector (which is an RJ45 Ethernet style) has 6 pins. I'll create a schematic for the small board at the back of the Fly when I next get time. Tracing on the main board with SMD devices is quite slow as some vias are not easy to see.

Also I've now opened up a FLY103 extension cab and its got the same small PCB as the main amp
Fly_Extension_Cab_speaker_board.jpg
so I'm now confident they send a line level signal from the main amp to the FLY103 extension cab where it goes to a Class-D amp board which looks identical to the one in the main amp box.
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Post by microbailey »

Update: I'm working on tracing a full schematic for this amp now.

Its slow work because its all SMD and the 4053-based circuit which auto-switches between extension cab, no extension cab, line-in, phones, mono-stereo etc has been a real pain as its layed out like spaghetti all over the board - its like they ran out of PCB space and just crammed the remaining components wherever they could with vias all over. Also the circuit is a bit odd and took some deciphering.

There's an 8-pin IC on the main board which took me ages to identify - part number written on it is TI 4B AIZ.
By tracing the PCB I worked out it must be a high-drive opamp thing but beyond that had no idea :(
The TI bit was easy (Texas instruments) but I couldn't find the rest anywhere on the web - until I found a webpage with a search box for TI surface mount codes where I discovered that "AIZ" is an SMD abbreviation code for TPA6110A2 - a 150mW linear stereo audio amp (for use in portable consumer). :D
Blackstar are using this as their headphone amp for the Fly3.

I'll write all this up on my website tech blog on www.midboost.com when I get time, but I'm trying to get some of the schematic drawn up from notes first.
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Post by JiM »

Good job in finding the part ! :applause:
I only give negative feedback.

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Post by microbailey »

I've updated my Fly tear-down blog with some photos and text about the Extension Speaker and a schematic (https://static.ucraft.app/fs/ucraft/use ... 1596058119) for the board at the rear of the Fly. This small board just connects the main board to the batteries, external PSU, extension speaker (which I now know is another of the Power Amp Speaker PCBs in a big box :) ).
I'm nearly finished drawing up the schematic for the main board Signal Routing and Headphone amp section (I think there will be at least 2 more sections to trace for schematics now I've got a good idea of what's in this little amp).

Someone messaged to ask me the part number for the speaker .
The markings say "VKrc 4ohm 5W". I'm not familiar with who VK is (if that is the manufacturer).
Maybe someone here knows?
Blackstar Fly speaker markings
Blackstar Fly speaker markings
FLy_speaker.jpg (119.89 KiB) Viewed 1207 times
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Post by microbailey »

I've finally drawn up the schematic for the Blackstar Fly 3 main PCB section covering the Mixer, Stereo Headphone Amp, and Signal Switching.
Basically this bit:
FLy_main_board_PSU_and_source_sw2 _hilite.jpg
The signal switching is done using a 4053 triple analog switch IC, and it handles all the mono/stereo/extension cab/no extension cab combinations.

The schematic is on my website here: https://static.ucraft.app/fs/ucraft/use ... 1596753735
I haven't yet got all the cap values since its a pain to meter them in situ and there's no markings (its all surface mount).
At least that mystery IC - with AB AIZ on it - makes sense now. Its the headphone amp IC in the Fly.

I've also added a sketch on the website of how all the boards inside the Fly interconnect.

Now I'm working backwards from the power amp through the audio switching for main/extension cab/phones and back into pre-amp, I think I'll have a go at tracing the Digital Delay circuit next.

Hope this is all useful to someone other than me :D
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Post by microbailey »

I've finally got round to completing the traced schematic for the Blackstar Fly 3 Analog Delay circuit (part of the main board), which uses a PT2399.
My KiCad schematic is on my website blog page here: http://www.midboost.com/workbench/black ... ay-circuit

I also got to use my new Tenma 72-8155 Handheld LCR Meter to measure the SMD cap values for the schematic. I can thoroughly recommend this meter (no I don't own any part of the Tenma brand :) ) and it's such an improvement over my do-it-all multimeter with capacitance measure thrown in as an extra feature.
I don't know how I lived without it. :horsey:

Next I'll write up some circuit analysis on the Fly delay circuit on my blog, after that I'll move on to the Blackstar Fly pre-amp EQ (ISF thingy) circuit.
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Post by microbailey »

Now I've written up some circuit analysis of the Fly's "Tape" Delay at http://www.midboost.com/workbench/black ... t-analysis I noticed 2 kinda interesting things:
  • I did some Spice on the filters on the PT2399 output and, assuming my measured Rs and Cs on the board are correct, they roll off a lot of high-end, starting around 400Hz. So those Delay repeats are pretty dark
  • Blackstar have combined Delay Time and Repeat level on the same dual-gang pot (VR4) so repeats increase as delay time increases. Also the pot is made non-linear by the R network around it so it cuts off all repeats for slapback delay but adds multiple repeats on long delays. I think that's a neat idea for saving on pots - using a dual-gang pot to do both Delay and Repeats in a useful way. Any delay pedals that do this trick?
Moving onto tracing the ISF EQ now, which seems to be made up of a dual opamp and a dual-gang 50k pot + Rs + Cs.
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Post by microbailey »

My latest update: I've now traced and drawn schematics of the second half of the Blackstar Fly 3's EQ stage.
Schematics and circuit analysis are now uploaded here: http://www.midboost.com/workbench/black ... yeqcircuit

EQ section consists, as far as I see so far, of the ISF tone circuit followed by 2 classic op-amp filter stages.
I've still got ISF, pre-amp gain stage and PSU to do.
I've run SPICE on the EQ filters - the frequency sweep plots are there on my website too.

I've got the ISF circuit traced and understood now :) but need to draw up the schematic and do a write up circuit analysis of it.

Thanks for the messages from people who are finding this all useful :applause: :applause:
I thought it was worth putting proper schematics for this amp up since I couldn't find any anywhere else
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Post by Vcorner »

There's an 8-pin IC on the main board which took me ages to identify - part number written on it is TI 4B AIZ.
By tracing the PCB I worked out it must be a high-drive opamp thing but beyond that had no idea :(
The TI bit was easy (Texas instruments) but I couldn't find the rest anywhere on the web - until I found a webpage with a search box for TI surface mount codes where I discovered that "AIZ" is an SMD abbreviation code for TPA6110A2 - a 150mW linear stereo audio amp (for use in portable consumer). :D
Blackstar are using this as their headphone amp for the Fly3.

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Post by microbailey »

I’ve traced the whole pre-amp now :)
But my new PC has died so is being repaired under warranty :(
Once I get that back I’ll draw up KiCad schematics and also run the 1st stage EQ and ISF circuits through LTSpice.
So hopefully more posts soon.
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Post by microbailey »

I finally got my laptop repaired and it works (or at least it did after one more repair) - new motherboard, new battery, and we're up and off again :horsey:
I traced and drew up the Blackstar ISF circuit schematic.
The ISF circuit write-up is here: http://www.midboost.com/workbench/black ... sf-circuit

I've added a link to the original Blackstar ISF patent - the Fly3 circuit is a cut down version where they replaced Bass, Mid, and Treble pots with fixed resistors.
Quite a neat circuit although probably of limited value in an amp this size.

I've added SPICE sims of the ISF + the post-EQ filters. Curves in most pot positions look a bit Marshall-esque.

I found another filter stage (and the clipping stage) before the ISF so next task is produce a schematic and write that lot up too.
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