Mucking around with an opamp drive...

[ACS]

Hi guys

I'm after a bit of a sanity check on a design I've been tinkering with. It started out on my breadboard as a Dist+ but through a lot of tinkering has (to my horror!) ended up looking remarkably like a TS (at least in the drive section). Currently have an LM733 dual opamp in there (although I may be mistaken here, as I've changed it a few times) and by and large I'm rather liking the sound.

What I've now been thinking is to give a bit more ability to shape the sound - so have tacked on an AMZ Presence control and then tried to use the other half of the opamp as a makeup gain stage.  This is hypothetical at this point - does this look like it will work? The reason I ask is that LTSPICE is giving me some very odd outputs with this setup!

[samhay]

Your presense control needs to go to Vref, rather than ground, or you need to add a cap between the tone control and 2nd op-amp.

[antonis]

I'm not the best person able to calculate your tonestack loss but I presume that it can't be 40dB...

(as your final compensating gain at full range..)


P.S.
Some guys will slap your hand if you don't reduce A LOT the Vref resistor values...

[Digital Larry]

On the output stage, other end of 1k going to (-) input wants to go to Vb rather than GND also.  Some of the ground reference points are OK as-is, but only because DC is blocked by a cap.  You might also want to see what Vb itself is doing during simulation, as 500k source impedance (2 1M in parallel) may not be low enough.

[samhay]

Upon futher thought, you will probably need a volume control on the end. Your makeup gain stage can't cut volume, so you may find gain settings that are too loud when with the makeup gain pot is set to min.

[deafbutpicky]

post lost, WTH?

good points about Vb reference, overlooked it...

[antonis]

post lost, WTH?

Double thread..:http://www.diystompboxes.com/smfforum/index.php?topic=109467.0

[ACS]

post lost, WTH?

Double thread..:http://www.diystompboxes.com/smfforum/index.php?topic=109467.0

Yep, sorry about that - was posting just as the forum went down last night and got caught in the crossfire.

From other thread:

I'd rescale the lowpass to 47k and 15n at first glance and 0.1µ for the shape seems a bit big.
The Vb Rs would be better off with 10k -100k as you feed the OPA with 1M already providing
a more stable bias too and a gain of 100 in the makeup stage is maybe too much, driving
IC2 into overdrive itself, which might not sound too pleasing, but the rest looks legit.

What's your problem with LTspice output?
With what source do you feed the simulated input?

[ACS]

Wonderful advice all, many thanks.

Your presense control needs to go to Vref, rather than ground, or you need to add a cap between the tone control and 2nd op-amp.

On the output stage, other end of 1k going to (-) input wants to go to Vb rather than GND also.  Some of the ground reference points are OK as-is, but only because DC is blocked by a cap.  You might also want to see what Vb itself is doing during simulation, as 500k source impedance (2 1M in parallel) may not be low enough.

See, THAT'S the reason I love this forum.  Great pickup guys, many thanks. Obvious as soon as you pointed it out 

Upon futher thought, you will probably need a volume control on the end. Your makeup gain stage can't cut volume, so you may find gain settings that are too loud when with the makeup gain pot is set to min.

I'm not the best person able to calculate your tonestack loss but I presume that it can't be 40dB...

Good points - the AMZ tresence/tone will definitely cut some volume, but 40dB is way too much - might just drop this down to a 10K fixed resistor and just pop a simple voltage divider volume control in the end.

The Vb Rs would be better off with 10k -100k as you feed the OPA with 1M already providing
a more stable bias too...

Can you help me understand this one a little better? I was working on the assumption of a simple voltage divider to provide a bias voltage (i.e. values don't really matter so long as they are the same!) - there are obviously further ramifications here that I'm missing!

[ACS]

Updated schematic based on above suggestions...

[ashcat_lt]

Can you help me understand this one a little better? I was working on the assumption of a simple voltage divider to provide a bias voltage (i.e. values don't really matter so long as they are the same!) - there are obviously further ramifications here that I'm missing!

Well, first off, there are other things connected to this node which makes them not exactly equal.  For DC, you've got 100R in series with the "top" resistor, and 1M+the opamp input impedance in parallel with the "bottom".  Of course, if you do the math, it's still well within tolerance, and it's not really a big deal until those divider Rs start to get much (much!) smaller.

More importantly, the 2M worth of divider Rs just won't let much in the way of current through.  Most opamps don't really need much in the way of bias current, so I'm not sure that this is even a real big deal here.

The biggest issue is the thermal noise of pushing through such large resistance is unnecessarily large.  Yeah, you've got the bypass cap to knock some of that down, but it's just better to not generate the noise at all.

[ACS]

Thanks for that excellent explanation. Makes perfect sense :-)

[ashcat_lt]

Cool. 

I think you can ignore my first paragraph for this and other similar applications.  It should probably include whatever is connected to the VA point (which is also likely very large) in parallel with the whole thing.  Or (amounts to the same point), since you really want VRef to be half of VA (not actually the PS input voltage), that 100R isn't really a part of that consideration.  VA is a voltage divider with 100R as the top, and the current draw of the opamps in parallel with the VRef as the bottom, and VRef then divides that down.  It's not usually worth bothering to calculate, but it is important to start seeing all of these connections for the occasion when it does really matter.