Electronics > Beginners
Op amp based preamp supply. Charge pump, single vs symmetrical & ripple
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dazz:

--- Quote from: BravoV on November 26, 2018, 04:57:23 pm ---Alternative consideration ...

https://www.analog.com/en/products/ltc3265.html#product-overview

--- End quote ---

Thanks! I'll look into it


--- Quote from: Audioguru on November 26, 2018, 04:58:37 pm ---Yes, for the 50 years old extremely distorted sounds, you use 50 years old 1458 opamps (dual 741 old opamps). A fairly old but much newer TL082 or the lower noise TL072 are NEVER used for severe clipping because they produce the problem called Opamp Phase Inversion where the output suddenly goes positive when the input voltage goes to within a few volts from the negative supply. Its distortion is completely different to having ordinary clipping.

I see the circuit as an ordinary 2-opamps preamp with a passive tone controls circuit at its output. The 2 opamps can easily use a single polarity supply and the tone controls circuit can have an added input coupling capacitor.

--- End quote ---

Oh, thanks for the info. Maybe Tl072 were being used in other amps, not this Lead 12. I'll socket them to try them both
dazz:
Damn, I believe I made an incredibly stupid mistake. When I argued for the single supply charge pump instead of the symmetrical one because I would avoid ripple in the V- supply pin of the opamp, I completely overlooked the fact that the ripple from the positive rail would go straight to my virtual ground and to the opamps inputs. So actually the charge pump ripple will get massively amplified if I'm not mistaken. FML, am I dumb or what  :palm:
spec:

--- Quote from: dazz on November 26, 2018, 02:02:24 pm ---

--- Quote from: spec on November 26, 2018, 09:36:43 am ---Crossed Posts :)

Just so we can see whats involved here is the schematic for the Marshall Lead12-12W: https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-Lead12-12W-3005-5005-Schematic.pdf

My first thoughts are to use better RRIO opamps, so that the supply rails could be lowered and get the same or better dynamic range and open loop gain. That would possibly enable a simpler circuit with a better performance, including a lower noise, not that noise is an issue here.

A 12V, 15V, 18V, or 24V wall wart should do nicely for the PSU.

--- End quote ---

Thanks for posting the schematic, should have done it myself in the 1st post.
I'm still not sure how I could use lower power rails without significantly impacting the way the amp will perform. The thing has 80dB of voltage gain (with the gain pot at max) and the gain pot is part of a low pass filter so it changes the frequency response of the amp. So if the original one clips at 50% and mine clips at 10%, they won't sound the same. At least I think so.

Also, a 9V supply is pretty much a requirement, since this will be a guitar pedal and most pedal power supplies are 9V  |O

--- End quote ---

No probs posting schematic :)

If you have a RRIO opamp, the output will swing to the positive and negative supply rails. With non RRIO opamps, like in the Marshall amp, the output only swings to within approx 2.5V of the supply rails. There are other aspects too, but not necessary to go into that. Just to give and example, using +-15V rails a TIL072 will produce a maximum output of 25V peak to peak, whereas an OPA191, for example, will produce 30V peak to peak. So for the same dynamic range as the TIL072 you could run the OPA191
at +- 12.5V rails.

About the circuit sound, I get your point 100%. I was forgetting that this is a guitar pedal where the sound of the circuit is most important. The TIL072 is sweet sounding opamp, even in clipping whereas the OPA191 would give less distortion, but it would be much harsher in clipping.

I find all the above posts very interesting and, after all the discussions, can I summarize that the specification is to clone the Marshal circuit exactly, to get the same sound. This includes +-15V rails.  The power input to the circuit must be 9V DC to be comparable with standard pedals. I think you actually said most of this in your original post. :)

As well as the opamps, other factors affect the sound. These include: type of capacitor (especially), type of resistor, and the way that the supply rails perform dynamically.

One suggestion is to use turn-pin sockets to mount the opamps. That way you can do opamp rolling if you ever want to. To use surface mount opamps, just mount them on DIL headers that plug into the socket, like standard DIL opamps.

To summarize then, all you need from us is an approach to derive +-15 rails from a 9V input.

I would strongly recommend the use of symmetrical split supplies, for the reasons so well put by Benta in reply #1

One question, and apologies if this has already stated, but will you be running this pedal from a 9V PP3 battery?


dazz:

--- Quote from: spec on November 27, 2018, 02:50:52 am ---No probs posting schematic :)

If you have a RRIO opamp, the output will swing to the positive and negative supply rails. With non RRIO opamps, like in the Marshall amp, the output only swings to within approx 2.5V of the supply rails. There are other aspects too, but not necessary to go into that. Just to give and example, using +-15V rails a TIL072 will produce an output of 25V peak to peak, whereas an OPA191, for example, will produce 30V peak to peak. So for the same dynamic range as the TIL072 you could run the OPA191
at +- 12.5V rails.

About the circuit sound, I get your point 100%. I was forgetting that this is a guitar pedal where the sound of the circuit is most important. The TIL072 is sweet sounding opamp, even in clipping whereas the OPA191 would give less distortion, but it would be much harsher in clipping.

I find all the above points very interesting and, after all the discussions, can I summarize that the specification is to clone the Marshal circuit exactly, to get the same sound. This includes +-15V rails.  The power input to the circuit must be 9V DC to be comparable with standard pedals. I think you actually said most of this in your original post. :)

As well as the opamps, other factors affect the sound. These include: type of capacitor (especially), type of resistor, and the way that the supply rails perform dynamically.

One suggestion is to use a turn-pin socket to mount the opamps. That way you can do opamp rolling if you ever want to. To use surface mount opamps just mount them on DIL headers that plug into the socket, like standard DILs.

To summarize then, all you need from us is an approach to derive +-15 rails from a 9V input.

I would strongly recommend the use of symmetrical split supplies, for the reasons so well put by Benta in reply #1

--- End quote ---

Thanks mate. Yes, that's exactly the goal, to replicate the original preamp as faithfully as possible.

I think you're right, Benta hit the nail in the head and somehow I managed to miss the point badly. I'm exploring my options for a dual rail solution now. Maybe a mc34063 if I can get away with +9V/-9V by increasing  the value of R8 to attenuate the signal since all of the clipping is done in the second opamp. Problem is that would put the signal in between the opamps well bellow 0dB and I fear that might make it prone to picking up noise, not sure.
spec:
Thanks for reply.

I think you quoted from my post while I was still editing it and the bit about the use of a battery is missing.

Could you please confirm that you will be using a 9V PP3 battery to power this pedal?

In any case, the approach that seems to be the most economical and lowest risk is to use a 9V to +-15V power supply module.

I havn't done a good search but something like this is what I have in mind: https://www.aliexpress.com/item/8W-2-5-15V-to-15V-Dual-Output-power-supply-DC-DC-Step-up-Converter-module/32789671683.html

just out of interest, what guitars do you have?

PS: if you run the opamps from +-9V, as you suggested, the sound would change, not due to headroom or noise considerations but because the opamp itself is  operating from lower rails. This effect is not so marked in newer opamps, but more so with the older opamps.
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