MXR mini iso brick

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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Post by m10065 »

It is a Gutshot of MXR mini iso brick. ... /18/022146

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The G
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Post by The G »

It is actually so much more than gutshots. I'm posting it here so we'll have a copy in English (Google English, that is):
2020 - 10 - 18
Analysis of MXR mini iso-brick
Self-made effector

Recently, the isolated supply has become the standard and the price has settled down, so I bought the MXR mini iso-brick.
In this article, I would like to shift to the circuit side rather than the impression I used.


I researched the isolated supplies currently on sale to explain the reason for the purchase.
  • Strymon Ojai series, Zuma series (model number omitted)
  • VITALAUDIO POWER CARRIER series (some non-isolated)
  • MXR iso-brick series
  • VOODOO LAB PedalPower
I remember that the top four were types that used insulated DCDC, and the bottom two were types that used toroidal transformers.
Strymon and VITAL AUDIO chose MXR because the internal image was on the net and FIREGLOW was sold out.
Another reason is what is happening with the design of famous brands, which are relatively cheap and the smallest.

MXR mini iso-brick


As a spec
  • 9V 300mA x 4
  • 9V / 18V 800mA x1
It supports outputs up to a total of 2A.
Since there is no environment to evaluate the sound, we will disassemble it immediately.
The board is a two-column set inside.
The DCDC circuit, switching transformer, and rectifier circuit on the secondary side are mounted on the upper board.
A linear regulator and a 9V / 18V DCDC converter are mounted on the lower board.
The following is printed on the upper board.

Push-pull converter control IC (ZG1810?)
External N-ch MOSFET x2 (STN4260)
Push plutonium lance (the silk described EPC25)
Secondary side rectifier circuit

A push-pull system is used as the circuit system for the isolated DCDC.
ZG1810 is used for the control IC.
Since the carved seal contains Dunlop, it is probably an OEM product. STANSON's STN4260 is used as the
MOSFET . A full-wave rectifier circuit is mounted on the secondary side for the number of outputs in order to rectify the square wave sent by returning the transformer .
(730.01 KiB) Downloaded 3 times

What is a push-pull converter in the first place?

The basic circuit of the push-pull converter looks like this.
The power supply voltage is applied to the center tap of the transformer, and double Vin is output at both ends, which is a slightly special usage.
The topology is to use the entire quadrant of the transformer's BH graph, which can supply a relatively large amount of power.
As a commentary, it is recommended for those who want to understand EDN's materials and TDK's materials in a short view.

Check the waveform of the actual machine

For reference, I took the switch waveform on the temporary side and the waveform on the secondary side on the actual machine.

The measurement points are as shown in the image, but ① and ② are looking at the voltage with respect to the primary side GND, and ③ is looking at the voltage across the winding.
I'm a little surprised that the windings of the temporary transformer are soldered directly, but let's not worry about it. (2 * VIN is out so I'm a little scared

The waveform is shown below.
The first sheet measures both ends of the winding on the temporary side with respect to GND.
Since 18V is input as Vin, around 36V comes out with Vp-p.
The upper side and the lower side are turned on alternately to drive the transformer.
The second piece measures the distance between the windings on the secondary side, and Vp-p around 25V is output centering on 0V.
When this waveform is full-wave rectified, it drops by the forward voltage of the diode to about 11.8V.
The switching frequency is around 60KHz.

Lower board

I derailed a little, but let's also look at the lower board
On the lower board
  • * 9V linear regulator 78M09 x4 (manufactured by HTsemi)
  • DCDC converter IC for 9 / 18V AAP6034A (manufactured by SGMICRO)
is listed.
The upper side of the MOS is also so, but semiconductor is Chinese de device is subject.
Thanks to this, I was a little impressed that this spec was realized for 12000 yen.

Next, let's look at the circuit operation.
The 11.7V coming from the upper board is sent to the linear regulator to generate 9V.
The 24.5V coming from the upper board is sent to the DCDC circuit and outputs 9 or 18V.

The voltage can be switched by switching the FB resistance with the onboard slide switch.
Only the secondary circuit is lightly taken, so please refer to it:
(41.83 KiB) Downloaded 4 times


This time, I analyzed mxr iso brick mini and understood about 70%.
Low profile custom transformer and OEM control IC? It is probably the MXR that is summarized in this size using.
If I have a chance, I would like to analyze the isolated power supply of FIREGLOW.

Push-pull converter using LT3439

In this analysis, is it an OEM chip on the primary side ? Was used, so I'm having trouble making something similar.
Therefore, I would like to introduce the one using LT3439 .
It's a fairly easy configuration as long as you can make a transformer.
Flyback method

At present, the only way to make multiple isolated OUTs in a small size is to use a custom transformer.
Flyback is famous as an insulated DCDC that can be made with a general-purpose transformer.


For example, the LM5180 looks like this.


By the way, Flyback can also support multiple circuits by increasing the transformer output.

20201018021908.png (14.39 KiB) Viewed 158 times

After all, a lot of current flows through the winding on the temporary side, so it may be suitable for configurations such as push-pull, half-bridge, and full-bridge ...

Inside Strymon Ojai

Quoted from ... t-pictures
Strymon has multiple transformers, so is it flyback?
It may be push-pull in terms of pin number


Quoted from Amazon review
VITAL is similar to MXR, so maybe this is made by OEM ?


Borrowed from Asmo ( )

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Post by m10065 »

I am the author of the blog.
thanks for translate.
I haven't verified that the circuit diagram is correct, but I hope it helps.

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