Hi guys,

Very new to this forum so please forgive me if I unknowingly break a few rules.

I've been trying to understand the BOSS DS-1 circuit (dual op-amp version) using a few sources, mainly including https://www.electrosmash.com/boss-ds1-analysis

I'm currently looking into the common emitter amplifier stage (the transistor boost before the op-amp stage) and haven't been able to find anywhere that has any solid answers to my questions. I did an electrical engineering degree so I like to dig deep into how everything works. My questions are, using the component names from the schematic in the electrosmash link:

-If the only thing that C2 and R4 are used for is to create a high-pass filter, then why is R4 connected to +4.5V and not ground? I know that this biases the signal up to a virtual ground of 4.5V, but this is then mostly removed by the second high-pass filter of C3 and R5 so I don't particularly see the use.

-Linked to the previous question, with that bias mostly being removed, how is the base of the BJT Q2 biased enough to not over asymetrically clip the signal on the negative side, let alone turn the transitor on? I've had a play around with some simulation of the circuit and I'm guessing BOSS left it that way to get some nice asymetric clipping as I can see from the simulations, but I don't understand how the negative side isn't completely cut-off due to there being no bias.

If anyone has some answers to these questions or a webpage with some good in depth explanations I would be super grateful!

Cheers,

Matt

Welcome, Matt!  (Sorry, that sounds wrong...  )

I took the liberty of moving your thread somewhere with more footfall.

Welcome, Matt    

The way I was taught is that the bias supply, being well-filtered, is somewhat isolated from the 'regular' active device power. So, sending audio signals to Vref this way is 'cleaner', less susceptible to noise on the ground plane.  It's "AC ground", where 'regular ground' is often a mix (can be noisy).  Note that all that is in the way of that return is the 47u cap to ground, assuring that only AC passes.  The other signal points that ground here will also find that cap to be a good isolation from crap on the power rail.  We see this a lot, notably in Tube Screamer-type circuits.  AC doesn't mind.

R7/R5 provide bias to Q2...

Quote from: GibsonGM on Today at 09:23:07 AMR7/R5 provide bias to Q2...

Just to add that R5 is there to stabilize Q2 Base voltage..
In theory, Q2 could work just as well as without R5..

@eeyml8: IMHO (without any overweenig) DS-1 is a terrible design..

Quote from: eeyml8 on Today at 07:17:55 AMIf the only thing that C2 and R4 are used for is to create a high-pass filter,

What is that cut-off frequency? Is it even audible?

Quote from: antonis on Today at 01:40:34 PMIMHO (without any overweenig) DS-1 is a terrible design..

The second version (the one we're discussing, since it's the one in the Electrosmash article) where they swapped a single DIL amp chip for a dual DIP op-amp is particularly odd, since they made minimal changes but included both op-amps in the circuit, meaning one op-amp does basically nothing, while there are two transistor stages in front of it. It could easily have replaced one (or maybe both) of those to save a few parts.

Quote from: PRR on Today at 02:16:10 PMWhat is that cut-off frequency? Is it even audible?

I haven't checked, but "3.3Hz" according to Electrosmash. So no, totally way below any audible effect. Those parts could get thrown away without losing anything. Go straight into C3/47n.

I'm not sure I'd go quite as far as Antonis' "terrible" but I'd certainly say "very odd". It's like Boss were only just getting warmed up!