NSFW What's up with the buffers in cornish designs?

[Laundryroom David]

Honestly, for me, SMD is fine even with moderately good, standard soldering tools. No need for an SMD soldering station, hot air gun or solder paste. As long as you have a relatively fine soldering iron tip available, I don't see why anyone can't give it a shot. Thanks for the info

Agreed. That’s how I do it and it works slick.

 

[mzy12]

I did get a bunch of TLE2072 chips for a much more reasonable price a few years ago. The OPA2992 seems like the more reasonably priced opamp that meets your 'requirements', JTEX. May I ask, how did you get from the ~5k DC resistance thermal noise to the figure of < 10nv/sqrt?

 

[Passinwind]

TLE2072 looks like an easy drop-in upgrade over TL072, but I find it stupidly expensive for what it is. If you're going to spend 10x the price of a TL072, then might as well get a much better OPA2205. Only downside (for some) is that it's SMD-only. SMD is not hard, though. Just get the right tools and embrace the damn thing, else you'll get trapped in the past.

If you just have to use through hole parts, I've had excellent results in my bass preamps with LT1113 and LT1169. But the price has more than doubled in the last few years, and I've never tried them with a low-ish voltage single sided power supply. But as you say, best to move along to SMDs, and buying some tweezers and a few $6 soldering iron tips is hardly a big deal, IMO.

 

[JTEX]

I did get a bunch of TLE2072 chips for a much more reasonable price a few years ago. The OPA2992 seems like the more reasonably priced opamp that meets your 'requirements', JTEX. May I ask, how did you get from the ~5k DC resistance thermal noise to the figure of < 10nv/sqrt?

OPA2992 is about half the price of a 2205, same noise spec at 1kHz, but much worse below (CMOS process). It draws 10x more power (!), so it never even made it past my parametric search, since I really care about battery life in my designs.

Thermal noise figures came from my trusty Excel speadsheet calculator:

Resistance (ohms)	Noise Density (nV/√Hz)	Noise Amplitude (dBu) @ 20kHz BW	Output noise after 50dB of added gain (fairly typical for heavy metal distortion)
1000 (NE5534, DNU as guitar buffer)​	4.0	-123	-73
2000 (~NE5532, DNU as guitar buffer)​	5.7	-120	-70
3000 (OPA2205)​	7.0	-118	-68
5000 (single coil pickup)​	9.0	-116	-66
8000 (vintage humbucker)​	11.4	-114	-64
16000 (hot humbucker, ~TL072)​	16.1	-111	-61
250000 (500k guitar volume pot @ 50%)​	63.6	-99	-49

 

[JTEX]

If you just have to use through hole parts, I've had excellent results in my bass preamps with LT1113 and LT1169. But the price has more than doubled in the last few years, and I've never tried them with a low-ish voltage single sided power supply. But as you say, best to move along to SMDs, and buying some tweezers and a few $6 soldering iron tips is hardly a big deal, IMO.

I long ago stopped even looking at anything from Analog Devices/LT due to their outrageous prices. Hard to believe they actually they sell any, unless it goes on a satellite or something.

 

[mzy12]

OPA2992 is about half the price of a 2205, same noise spec at 1kHz, but much worse below (CMOS process). It draws 10x more power (!), so it never even made it past my parametric search, since I really care about battery life in my designs.

Thermal noise figures came from my trusty Excel speadsheet calculator:

Resistance (ohms)	Noise Density (nV/√Hz)	Noise Amplitude (dBu) @ 20kHz BW	Output noise after 50dB of added gain (fairly typical for heavy metal distortion)
1000 (NE5534, DNU as guitar buffer)​	4.0	-123	-73
2000 (~NE5532, DNU as guitar buffer)​	5.7	-120	-70
3000 (OPA2205)​	7.0	-118	-68
5000 (single coil pickup)​	9.0	-116	-66
8000 (vintage humbucker)​	11.4	-114	-64
16000 (hot humbucker, ~TL072)​	16.1	-111	-61
250000 (500k guitar volume pot @ 50%)​	63.6	-99	-49

I like your process my friend. Very thorough. The noise of a TL072 really is relatively high there. I assume the NEXXXX opamps should not be used as a buffer due to both of their rather low input impedance figures. Where, then, would they be suitable for use in guitar effects? I've seen people mod their old Boss graphic equalisers with NE5532 opamps, I must assume the current draw is some horrendous.

As an aside, I am very impressed by your projects and schematics on your website. Some truly mindboggling stuff on there!

 

[Passinwind]

I long ago stopped even looking at anything from Analog Devices/LT due to their outrageous prices. Hard to believe they actually they sell any, unless it goes on a satellite or something.

LT used to be very generous with free samples.

Since I mostly just do one-offs for myself, $10-15 a pop for opamps doesn’t really even phase me as long as I get the results I need. I’m aready burning money and even more so time, why pretend to be rational?

 

[mzy12]

How about the OPA1678? It seems better than the OPA2205 in almost every way, except in input offset voltage at 2V for OPA1678 and 0.25V for the OPA2205 and a considerably higher quiescent current for the OPA1678. If you don't care about that much about the power consumption for a non battery powered device, the considerably lower cost of the OPA1678 is very attractive. Or is the reduced input offset voltage of the OPA2205 really worth it?

 

[JTEX]

How about the OPA1678?

Looking very good. It was off my radar due to my low power budget, but if that's not a concern, then at first glance it seems to be a terrific op amp, especially when considering the price. It's not even that power-hungry. Input offset is not a factor for our audio purposes here. Great find!

 

[ChrsGuit]

The DL4mkII does have options for true-bypass and analog dry-through that might be worth trying

I have heard this... I tried to consult the "manual" (pdf only), but it is kinda tech-speak. I admit, I'm a dumb hillbilly, and when it comes to "stop playing, hold this button, and while holding this button, push this combination til this light flashes, and now you will be in such and such mode... from there, release this button while pressing this other button until you get three flashes, then hurry and turn this knob... blah blah".

Meanwhile you accidentally hit the wrong thing and now everything is all screwed up and you have to do a factory reset and try to get back to square one... Just way too complicated... Then you try to get tech support and "hook the device up to this software and download the update"...

I have to dig out my laptop that magically went obsolete last year and now the software is no longer supported...

Gah, I hate technology...

 

[lrgaraujo]

Violating my own code here by bumping what became a sort of controversial thread but what the hell…

I think the main draw here, if I can presume to try and get inside Mr. Cornish’s thought process, is that he is already using the BC549C in a bunch of pedals.

It was never my intention to have this thread become what it became when I originally posted it, so I really appreciate you revisiting it with a technical discussion, and what I think is a super reasonable insight

 

[giovanni]

No sweat man. It was fresh in my mind because I was working on it today and this seemed the most logical place to post it.

What I actually found interesting were a few things most of us would see as “kludges” or “fudging it”.

R1 and R8 come to mind. Obviously Cornish was targeting a specific input impedance he couldn't get with the bootstrapped emitter follower alone.

It seems that emitter voltage around 50% of source voltage was important to him, so R1 acting as an artificial reducer of input impedance made more sense than trying to get blood from a stone.

Furthermore, it seems that 100 ohm output impedance was a design criteria so he simple added R8 in series to get there.

If anything, I appreciate him a bit more because of these little practical things.

What do you mean by source voltage?

 

[giovanni]

For the purposes of calculations, I assume a raw source voltage, in this case 9v.

In practical terms it would be the whatever the filtered source voltage is. Vf is usually what I call it in my schematics.

After the diode and filter caps.

Ah ok. Source is a bit of an overloaded term given that it indicates one of the pins of a FET. I think it’s relatively common to want the emitter voltage at half Vcc because that maximizes the dynamic range (from 0 to Vcc) of the output signal before clipping.

 

[giovanni]

Its a semantics things. I don't use JFETs for anything so have never run into that contradiction. I'll remember that going forward.



Which was one of the interesting insights I found trying to break the circuit down:

View attachment 79114

In Cornish's Design, we have the following:

R1 = 10M
Rb1 = 120k
Rb2 = 200k
Rbs = 120k
Re = 7.5k

In the absence of R1, Zin = ~2.99M. Adding in R1 reduces this to ~2.3M, but not close to the 1M he quotes.

It could be my calculations are boogered up but I verified them against a few known Simple/Full bootstrap derivations and they match those papers so I am pretty confident in the implementation. I did end up implementing the "Full" calculation because the "Simple" type seemed to be about 40k-50k higher in the 3 different derivations I tried:

View attachment 79115

I added in a dropdown that lets me add R1 in to show the effect:

View attachment 79116

The first thing that jumps out when I wanted to adapt this was some of the non-standard resistor values. Not a huge deal in the Through Hole world, but Basic Parts availability in off values at JLCPCB can present challenges. So I set out just tweaking a few values.

Using the Standard Cornish values. Note: I have R1 "turned off", analytically speaking:

View attachment 79117

My first stab at changing stuff:

View attachment 79118

Ve jumps up a bit. Assuming Cornish was targeting Vcc/2 for Ve, this presents a challenge. Ve factors into Zi pretty significantly so using that as a lever to change Ve is sort of a dead end, i.e. once you lower it enough to impact the emitter voltage, you have dropped the input impedance significantly.

I imagine the values he picked were a compromise between impedance and voltage.

I went to JLCPCB and searched a few parts to see what basic parts I'd have at my disposal. 150k and 220k were readily available in my preferred 0805 case:

View attachment 79119

At this point, I sized R1 to give me about 1M input impedance:

View attachment 79120

One thing I still don't understand, beyond the obvious effect it has on the effectiveness of the "bootstrapping", is if there are any frequency shaping impacts of Cbs1. Cbs2 is obviously providing some passive RF shunting, but I'm not sure if there are any impacts to increasing or decreasing the size of Cbc1.

I don’t think that in guitar pedal applications 5V makes a huge difference compared to 4.5V. I guess it depends what your application is but a guitar signal that’s 9V peek to peek is pretty high and will clip in the amp anyways. Why did you lower the input impedance to 1M? The higher the better. I don’t know the answer to your capacitor question. I would need to run a simulation to figure that out.

 

[giovanni]

Well, I guess that depends on your opinion. There is a threshold after which it doesn't matter how much higher you go.

500k is typically high enough. Ultimately it doesn't really matter to me.

It's worth noting that Cornish himself artificially reduces the input impedance with R1. Ultimately it doesn't really matter to me. I was mostly just playing with the parameters to try and match what he quotes on his website.

Again though, even with his R1 set to 10M, the input impedance is around 2.3M ohm, not the 1M he quotes on his site.

Got it. Does he say 1M or over 1M?