Genesis 1 & Mega Drive 1 stereo mod no headphones volume control

[CkRtech]

I modified the right channel with the cap change, and elected to order some 75K ohm resistors rather than use the 20K + 22K + 33K tower of resistors (5% tolerance each) I had on-hand. I think they should be here in a few days.

If you want to also build it and possibly tweak your suggested values, I would certainly be interested. I used the TL072, but perhaps the TDA1308 (if you have that one) would be a nice alternative to hear?

I also have an OPA2132 available if you have any thoughts on that.

Also - Best to try to sandwich a 10uf cap right after the 10K input resistor before pin 3 and 5? I didn't add a cap to that VCC/Vref input yet.

[CkRtech]

OK. I have modified the circuit and added a filter cap to Vref. This is a 75K / 10K gain of 7.5. 220uf caps on the output. 47uf on the input. 75 ohm output resistors removed. This is on a Model 1 VA 6.5.

Including recordings of Sonic 1 intro + Green Hill, Sonic 2 intro + Chemical Plant Zone, Streets of Rage - Level 1, Revenge of Shinobi Title, Revenge of Shinobi Stage 1. All but Sonic 2 were from the 6 pak cart.

My thoughts - Sounds pretty good. Nice bass. Sonic 2 seems like a really loud game. The intro screen has clipping a couple of times - but I think this is just how Sonic 2 is. Correct me if I am wrong.

Revenge of Shinobi has a pretty wild title screen. Stage 1 is rather interesting. If you listen in the 40-50 second range, you can hear the music give a single click after the same melody plays twice back-to-back. I assume this is also most likely just a standard Genesis thing.

Streets of Rage
Sonic 1
Sonic 2
Revenge of Shinobi Title
Revenge of Shinobi Stage 1

What do you guys think?

[Ace]

Your waveform is partially clipped, which indicates a VRef that's either too high or too low. I can't really tell because some sound cards invert the incoming waveform on their Line In, but others don't and I don't really know what you're using to record. Mind telling me what you're recording with? If it's something that inverts the incoming waveform, your VRef is too low, if not, VRef is too high (there's negative clipping in your Sonic the Hedgehog sample, but that could be positive clipping inverted - in case you're wondering, positive clipping indicates VRef is too low while negative clipping indicates VRef is too high, the opposite if your sound card's Line In inverts the waveform).

[CkRtech]

Hmm. I am using a FocusRite Scarlett 6i6. I thought the bottom side of the waveform looked a little odd...

[CkRtech]

Still image of Vref probe at random time during Sonic Level 1 demo. Looks like we are flirting with the low side.

[Ace]

Generally, a VRef thereabouts of 2.8V to 3V is ideal on TL07x chips from my experience designing the Mega Amp. Perhaps you can refer to the VRef setup for the YM2612 Mega Amp 2.0 Pro at the TL074 preamp and work from there.

[CrossBow]

Your left and right channels are reversed in SOR. I found this out that apparently the channels are reversed on the 6pak cart. I confirmed this with between mine and my actual SOR stand alone carts. So because of that, I've not trusted using any of the other games really from the 6-pak as good examples for the audio in case any of the other games are reversed on their sound channels.

[CkRtech]

Generally, a VRef thereabouts of 2.8V to 3V is ideal on TL07x chips from my experience designing the Mega Amp. Perhaps you can refer to the VRef setup for the YM2612 Mega Amp 2.0 Pro at the TL074 preamp and work from there.

Ahh nice. It looks like you used 510 and 750. With a perfect 5V VCC, that should land pretty close to 3.0 at 2.976. I suppose it is possible that under certain conditions it would swing up just slightly over 3V. What about 500 and 700 = 2.917? That would give it pretty good swing space both ways. Or did you find that getting as close to 3.0 as possible was the best way to go?

(EDIT: Although yeah... much easier to find 510 and 750 locally)

[Ace]

You should stick to resistors in the Kohm range, ideally around 10Kohms. This is why I specifically said to refer to the Mega Amp 2.0 rather than the original Mega Amp design for getting a good setup for VRef.

[CkRtech]

Ahh. I assumed the stickied topic would be the latest whatever version of it is. Perhaps a moderator could unsticky the old thread and sticky the new one? I suppose if you wanted to, you could update the top of the old topic to mention that the circuit has been superseded. I don't know how active the moderators are on here.

Thanks for your help. Perhaps I can pick up some 12K and 16K resistors along with some surrounding values for flexibility this weekend. Not sure what tolerance the local place sells.

[Ace]

Not sure about unstickying the old thread, but the new one should be stickied for sure.

In regards to resistor tolerances, it's quite common to find 5% tolerance resistors. You might sometimes find 1% tolerance, but 5% is good enough.

[CkRtech]

There was a pretty good deal on Amazon that offered a large selection of various values (and 1% to boot). I pulled the trigger in order to boost my resistance options. Should be here by tomorrow. If I don't get it done tomorrow evening, it'll probably be next week. I'll report back when it happens.

[CkRtech]

So... that helped. A lot.

Note the stereo is reversed from my last recording. Also - the peaks up top seem to have lessened a bit.

Sonic 1 Intro with 12K and 16K resistors

[Ace]

That looks much better. Also, for future reference, note that your sound card inverts the waveform. At 2.5V, there is a lot of negative clipping in your recordings when there is actually positive clipping in the waveform from a too low VRef. Keep that in mind if ever you find yourself designing an audio amp using an inverting amplifier setup and have to bias the op-amps.

EDIT: Upon further inspection of the waveform, your VRef looks to be just a little bit too high. Use a 15Kohm resistor in place of the 16Kohm resistor and get a small sample.

[CkRtech]

I have replaced the 16Kohm with 15Kohm. Entire demo 1 is available, but a shorter, 8 second sample is also included (7.7 MB vs 1.4 MB) for a faster download.

Sonic 1 Intro with 15k (7.74MB)
Sonic 1 with 15k - Sample (1.41MB)

Here is what I hit when using 15Kohm: