The Real Phoenix A/V amp for custom superguns, COMBO AV EX++, PANA, or Boardmaster

[brandon.arnold]

The Real Phoenix A/V amp for custom superguns, COMBO AV EX++, PANA, or Boardmaster
Overview
I’d like to share a solution to attenuate RGB and audio levels in a custom supergun, or to fix the output from various Japanese superguns. This PCB was generously put together and open-sourced by The Real Phoenix, a basic active attenuator built around the THS 7374 package, repurposed from the SuperGun MiniGun available on his store. It can be compared favorably to other present and past offerings, such as:

• the Axunworks Arcade JAMMA adjustment conversion board (a fine product, but it is external and JAMMA-inline without a second output, unlike the internal / panel-mount solution in this article)
• Tim Worthington’s internal AV Driver / external SUPATT (which aren’t stocked anymore)
• circuit diagrams from @pacoarcade on this very forum ([1], [2]; do not offer PCB layouts, and I have found the capacitor sizes to be much harder to find. It does however offer TTL and 75 Ohm CSYNC options, whereas the one shown here is only 75 Ohm).

If you prefer an internal panel-mount solution and have the tools to fab a PCB with surface-mount components, this may be a good fit for you.

The COMBO AV EX++ supergun (which I have, and mod below) as well as PANA and Boardmaster superguns are notorious for the issue of blown out brightness from the SCART connector. The superguns offer RGBs and Audio outputs which are wired directly to the associated JAMMA pins, a design choice which is insufficient (bordering on irresponsible). JAMMA RGB levels are 0-3.5 VDC, instead of 75-Ohm 0-0.7 VDC expected by consumer SCART equipment. Similarly the audio outputs are intended to drive speakers directly instead of line-level inputs.

A simple, often-recommended alternative is to attenuate with potentiometer knobs installed on each of the wires. This isn’t ideal, however, especially for these twin joystick superguns, with conductors from JAMMA to SCART usually going at least a couple of meters and meeting up against input impedance limitations. For more information about that and the benefit of an active attenuation approach such as shown here, check out Tim Worthington’s AV-Driver for Arcade RGB 'Supergun' article.

Modding the COMBO AV EX++: Before and After​

One of the nice things about this install is that all factory/stock wiring is left intact, without any cutting. If you will get a few 2-, 3-, and 8-pin male/female JST SM and XH crimp connectors, these will match the existing connectors used by the manufacturer in the COMBO AV EX++ internal harnesses. In the diagrams, several such terminals are represented by circles (well, ignoring those circles that are nodes of a switch).

Also illustrated is a new DIN8 output that accepts audio. I chose to wire this differently to the stock, less-prevalent PANA/Boardmaster DIN8; other options you may prefer could be NeoGeo DIN8, Sega Genesis 1 DIN8, or Sigma supergun DIN8 (which was my choice).

Results of Installing in the COMBO AV EX++ Supergun​

The benefits are as follows.

1. An ideal SCART RGB output. Whether fed into a CRT or a modern display with a high-end scaler (OSSC / Tink 5x) this will output a much better, zero-lag picture, compared to the built-in VGA/HDMI from the GBS-8220 + VGA2HDMI. And the new SCART output quality far exceeds the old blown out / unsafe SCART RGB.
2. Audio now comes from SCART and is significantly improved compared to the 3.5mm line level jack
3. Audio is also significantly improved coming from the built-in HDMI port, which was very tinny and filtered low frequencies
4. One no longer needs to manually move the internal JST SM harness between the GBS-8220 and the DIN8/SCART connector; both options are now wired from the amplifier’s 2 outputs

Building the PCB​

Print the PCB shared on OshPark at The Real Phoenix RGBS AMP + Audio 1in/2out. I printed a set of 3 which can be done for as low as $20 there (that is, $6 apiece), and cheaper elsewhere, if you have the time to wait.

The link above has a general components part list. For what it’s worth, below are the specific part numbers I ordered from Digi-Key. Depending on when you read this, they’ll probably have been deprecated and you need to search Digi-Key for equivalent parts. You should be able to get everything you need for $10, plus the cost of the PCBs above.

RES 220 OHM 0.1% 1/8W 0603	ERA-3VEB2200V
RES 820 OHM 0.1% 1/8W 0603	ERA-3VEB8200V
RES SMD 75 OHM 1% 1/5W 0603	ERJ-P03F75R0V
RES SMD 1K OHM 0.1% 1/10W 0603	RT0603BRD071KL
CAP CER 0.1UF 16V X7R 0603	CC0603KRX7R7BB104
CAP CER 22UF 6.3V X5R 0603	CL10A226KQ8NRNE
SWITCH SLIDE SPDT 100MA 12V	OS102011MS2QN1
CAP ALUM 33UF 20% 16V RADIAL	ECE-A1CN330U
CAP ALUM 330UF 20% 10V RADIAL	ECA-1AM331
TRIMMER 1K OHM 0.5W PC PIN TOP	CT-6EP102
EURO BLOCK 4P	EBWA-04-A
IC AMP BUFFER 14TSSOP	THS7374IPWR

COMBO AV EX++ Wiring Ports and Harnesses​

As mentioned before, you will need JST connectors. Also ~20 GAU hookup wire, optionally heat shrink tube, and of course the crimping tool, wire cutter, wire stripper, and hot air gun. These can also be found on Digi-Key for dollars, or if you want to easy-button that, get a pair of kits like this and this on Amazon.

1. 2x JST SM 8-pin Female
2. 2x JST SM 8-pin Male
3. 1x JST SM 2-pin Female
4. 2x JST SM 3-pin Female
5. 2x JST SM 3-pin Male
6. 1x JST XH 3-pin Male (note: this is JST XH, not JST SM)
7. 20 GAU hookup wire of various colors
8. 270° DIN8 such as these from Console5 (or 262°, if you want to change the wiring for Sega Genesis v1 instead)

	RGBs, +5V, GND Harnesses 2x JST SM 8-pin Female at the RGB, VCC, GND outputs of the amp 1x JST SM 8-pin Male at the RGB, +5V, GND input of the amp
	Speaker (+/-) Harness 1x JST SM 2-pin Female at the speaker L/R input of the amp
	Line Level Audio Harnesses 2x JST SM 3-pin Female at the audio outputs of the amp
	Line Level Audio to VGA2HDMI Harness 1x JST SM 3-pin Female 1x JST XH 3-pin Male from one audio output on the amp to the VGA2HDMI PCB
	New DIN8 port with Audio 1x JST SM 8-pin Male 1x JST SM 3-pin Male 1x DIN8 Female from one audio and RGBs, VCC, GND output to the Supergun exterior panel mount hole, replacing the old one Look for DIN8 pinouts on the internet for your chosen configuration, or wire it the same as the original

 

[hatmoose]

This is such a cool thing, thank you for the detailed write-up and implementation guide, it really helps visualise the possibilities and potential. I’m looking forward to having a play with these!
Again, thankyou!