Darkglass - Tone Capsule  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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rtcustom
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Post by rtcustom »

PCB
tone capsule.jpg
tc.jpg

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george giblet
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Post by george giblet »

I had another look at this one.
Pretty sure C3 is 100n.

Attached files are
- simulations of Tone Capsule tone sections
- some measurements from, real units an plug-in for
Darkglass Vintage B7k Ultra
Darkglass Microtubes B7k Ultra
*** not Tone Capsule
https://www.admiralbumblebee.com/music/ ... dware.html
This one allowed the value of C3 to be refined.
The value is in agreement with BassyTobe's cap measurements.


=====================================
Darkglass Tone Capsule
Updated values
George Giblet, V1d, 1 Aug 2022
---------------------------------------------------------------
Schematic:

tommymariotti's V1c schematic looks correct
- no cap values
- IC MC33179DG
- Diode 1N4148, SMD, SOD80C package
---------------------------------------------------------------
Resistors

R4 there is no such part on the PCB
R11 the pcb has no marking but the part is present
---------------------------------------------------------------
Capacitors

Best cap value estimates considering all information:
- BassTobey cap measurements
- frequency response for other Darkglass pedals: www.admiralbumblebee.com
- Simulations George Giblet.

All caps are ceramic (non-polar) SMD types.
Most are likely to be X7R.
C5 is likely to be COG
C8 looks like it could also be COG.

C1 22n
C2 22n
C3 100n ;*** main change from previous info
C4 22n
C5 220p
C6 8n2
C7 1n5
C8 100n
C9 47n
C10 47n
C11 1u
C12 10u
C13 10u
C14 10u
C15 0R ;0 ohm wire link
---------------------------------------------------------------
Hanging issues:
- C5 value possibly inaccurate (anything from 47p to 220p is OK)
- C3 very small possibility the value is 120n. I no longer believe this.
- R15, R19 and R20 some units clearly have 8.25k resistors.
- Low/Bass specs don't match website.
The Tone Capsule quotes +/-12dB @ 70Hz
Whereas the microtube quotes +/-12dB @ 100Hz (and seems correct)
Is the 70Hz tone control spec when R15, R19 and R20 are 8.25k?
Is it true the later Tone Capsule units with 5.1k have +/- 12dB @ 100Hz?
(If not then C9, C10 might be 68nF, or even 56n then it would
be close to BassyTobe's measurements!)
- Low mids are closer to +/-13.5dB @ 550Hz than +/-12dB @ 500Hz
=====================================


* Supporting info:
---------------------------------------------------------------
Comments on BassyTobe 03 Mar 2021 post:

- "in-circuit measurements of the caps without any pots being connected"
Most measurements should quite accurate.

- If anything, actual cap values are likely to be less than measured values.
(the equivalent series capacitance Cs on an LCR meter can
be higher than the actual cap value when measured in-circuit.)

- Some of the measured cap values don't line-up with the estimated
values:
C4 220p too low, C5 68n too high
Did C4 and C5 get swapped in the list?
In this case, C4=68n still looks high and it's almost certainly 22n
C5=220pF could be a little high but not at all unreasonable.

- "This version also has 5.1kOhm resistors for R15, R19 and R20"

The previous PCB pics have resistors marked 8251 which
imply 8.25k, which is a valid part value.

Why the discrepency?
---------------------------------------------------------------
Measurements: plug-in vs real unit

Darkglass Vintage B7k Ultra
Darkglass Microtubes B7k Ultra
*** not Tone Capsule

https://www.admiralbumblebee.com/music/ ... dware.html

Low/Bass
approx +/- 12dB @ 100Hz

Lo-mids
frequency approx 13.5dB @ 550Hz
At 1kHz amount of boost is about 9dB
This is in close agreement with C3=100n, C6=8.2n.
C3 = 100n isn't unreasonable based on BassyTobe's measurements.

Hi Mids
2.8kHz
---------------------------------------------------------------
LT Spice

Low/Bass
R15, R19, R20 5.1k
C9 = C10 = 47n
+/-11dB @ 100Hz, +/-13.3dB @ 70Hz

Lo-Mid
C3 & C6
Estimated: 120n, 8n2
Measured: 110n, 10n (in-circuit)

Results for different Lo-mod cap combinations
120n & 8n2: 12.2dB @ 510Hz
100n & 10n: 14.6dB @ 509Hz
100n & 8n2: 13.3dB @ 567Hz

Hi-Mid
C4 & C7
Estimated: C4=22n, C7=1.5n
Measured: 220p or 68n, 1.55n
12.2dB @ 2.80kHz
---------------------------------------------------------------
Attachments
BassSweep.png
500Sweep.png
Similated Response V1d.png

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george giblet
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Post by george giblet »

(If not then C9, C10 might be 68nF, or even 56n then it would
be close to BassyTobe's measurements!)
I've had time to sleep on this hanging issue.
I have a nagging feeling C9 ad C10 are actually 56nF in the real unit.
With 56n's the bass (Low) control response passes very close to +/-12dB @ 70Hz
like the spec. It's also close to BassyTobe's *in-circuit* measurements. It's possible
the in-circuit measurements have made 47nF's measure 55nF but after looking
at the issue I'm moving more towards the parts actually being 56n.

That's more about replicating how the actual units are built.

And for the sound, the bass frequency is pretty low and I doubt many people would
object to 47nF's over 56nF's anyway. In fact you could use smaller caps.

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george giblet
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Post by george giblet »

The recent ZON Preamp thread got me thinking that the published specs for the Tone Capsule probably don't match the actual unit.

The thing that triggered me to look into it again was the cap values I posted for the Hi-Mid band match the published spec but the response doesn't match the measured responses of the B7K Ultra or the original B7K. It's not the center frequency that was the problem. It was the region to the sides, say 1kHz and 5khz. The previous Hi-Mid part values don't match the measured responses that region well at all.

I thought suppose we don't try to match the published specs, what happens?

1) I started with the schematic of the original B7K unit. Then I compared the simulated response with the measured response.
Conclusions:
- the simulated response and measured response agree.
- the published specs don't match either
(The odd thing is the old spec was 2.8kHz for the high mids but the circuit shows 3.3kHz.)
- you can only trust the published specs so far!
The schematic for the original B7K has 10:1 cap value ratios on the Lo-mid and Hi-mid EQ

2) I then fitted circuits to the B7K Ultra response measurements I linked earlier.
Conclusion:
- the simulated response and measured response agree
- the published specs don't match either
- the measure 250Hz band center frequency is way off spec, 370Hz!
- the cap values are in a 10:1 ratio, or one E12 value off from that.
- given the B7K Ultra has switched cap values for the different frequencies
the part values could be two caps in parallel, or possibly not exactly 10:1 (although I don't know why not).

3) Applying those results onto the Tone Capsule:
- don't try to match the published responses
- see what happens with 10:1 cap ratios
Conclusion:
- responses consistent with the Original B7K and the B7K Ultra (about +/-14dB on mids)
- good agreement of the cap values with values posted by BassyTobe and slade on 03 March 2021
(The 5k1 vs 8k25 resistor variant issue i still hanging.)

I'm pretty confident the values are close, so I've appended the part values onto the schematic.
I also did some simulations so you can see the effect of pseudo matching the other B7K responses.
Schematic V1e 2023_01_06 - with caps.png
Tone Capsule Part Estimates V1e 2023_01_06.png

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Post by defaultlemon »

Thank you for awesome work.
Do you now the filter they use? Looks like some sort of Sallen-Key, But I cannot remember exactly.
How to calculate this one?

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Post by george giblet »

The filters are just a common mid-range control circuit. There's no simple and general *design* formulas. There's a few simplified formulas. If you want to change the frequency you can just scale the capacitors up to down on any circuit like this. That's all the simple formulas do anyway. The analysis formulas are messy and often simplified. It's best to use something like a circuit simulation program and tweak the parts. On page 2 of this thread I posted a python script to analyse the circuit but I recommend using the simulation program.

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