Reverse engineering Sega Mega Drive audio amplifier

[Moduvator]

I have gotten my hands on a relatively uncommon Model 1 VA7 revision, service manuals for which are not readily available so I thought it would be a good exercise to fill the gap myself.

This revision is peculiar in that it's practically a Model 2 console shoehorned into a Model 1 case and shares most of the amplifier section with it with jellybean single +5V supply op-amps doing two stage amplification. The key difference here is that Model 1 has a headphone jack with a volume regulator for stereo output, whereas Model 2 has a simple line out. Sega did adaptations to the circuit and I am very curious if I have the right understanding of how they work, for my experience with discrete audio amplifiers is limited and I basically am not sure what I am looking at!

The variable gain section is done with a 2-gang 10k fader that's connected between pre-amp and main stage op-amps as follows (only one channel is shown):

The other end of the fader is connected to the op-amp bias voltage (Vref, ~2.5V, console has its own source for that) and wiper goes into the main stage input. My understanding is that when fully to the left the main amplifier will see full signal and DC offset from Vref would not be enough to drown it out whereas when moved fully to the right both inputs would rest at the same voltage and op-amp output will be zero. Did I get this right? Is this a typical solution and are there any ill consequences of using it this way?

My second stumbling block is the headphone driver stage:

 I have never seen something like this before so I am very curious if I’ve gotten this right. I understand that an op-amp alone would not have enough power to drive headphones so they’re using a transistor buffer here, but is there a reason they are using two like this? Again, is this a common arrangement that I am simply not familiar with?

[T3sl4co1l]

It probably helps to redraw the first circuit as an op-amp with +/-2.5V supplies, and treat Vref as GND.  Now it's evident the pot is simply a volume control, from IN to GND, with a tap somewhere midway along it.  The amp is a regular inverting configuration, gain of 3.

The second circuit is two emitter followers cascaded.  This may be pretty obvious, but perhaps why isn't.  To be fair, one is probably enough, but two does have the advantage of additional current gain, and canceling out the offset -- the first (TR3) emitter is a Vbe above the input (or say ~3.2V DC), the second is about as much below, or back to ~2.5V.  This reduces the "bump" or "pop" at turn-on, through the coupling capacitor.  Although, since the headphones aren't grounded to Vref, this is inevitable, but there is still some advantage that the available voltage swing (without clipping/distortion) can be more symmetrical this way.

The ferrite beads help prevent parasitic oscillation (a low impedance applied to an emitter follower can cause it to oscillate, and cables of random lengths can have quite low impedances at "unlucky" frequencies), and also keep external noise/RFI away from the circuit (where it might interfere with the transistors, becoming rectified and now you hear pop-beep-chirp noises in the headphones), and onboard noise away from the headphone cable (an effective antenna).

Tim

[magic]

It probably helps to redraw the first circuit as an op-amp with +/-2.5V supplies, and treat Vref as GND.  Now it's evident the pot is simply a volume control, from IN to GND, with a tap somewhere midway along it.  The amp is a regular inverting configuration, gain of 3.

Note that 3k3 loads the wiper of the potentiometer to virtual ground, so this is effectively the "fake logarithmic potentiometer" configuration. So the actual pot is probably linear, but the result is similar to using a logarithmic pot in the "normal" way.

[Moduvator]

T3sl4co1l,

It probably helps to redraw the first circuit as an op-amp with +/-2.5V supplies, and treat Vref as GND.

That is a very nice suggestion, I did not think of that! It does indeed make it easier to understand, thank you!

Really appreciate the thorough response on the topic, especially the headphone driver part. I now have a much better understanding of the circuit, thank you everyone!

[Sagar]

I think only one Opamp stage is good to go with, here it has gain of 3. Which I think is good for a amplifier system, can manupulate the resistor values and get better gains too. Why to go with a transistor based design then. Are you going to make a PCB of this circuit.