Why some pedals are buffered?

Adam_MD

Sporky said:

Adam_MD said:

Again I don’t disagree chaps it was just easier to build a Klone where I could switch the buffer off and still use those pedals.  If I don’t have one of them on the board I switch the buffer back on again.  

Indeed - it wasn't a criticism of your solution to the issue. More sadness that the issue ever arose.

Yup with one of them I didn’t test it in isolation first and thought there was something wrong with the pedal as it was all highs and sounded weird I spent a while going over the board making sure I hadn’t swopped some components round by accident.  After checking all the parts I plugged it in on it’s own then realised it was fine and wasn’t playing nicely with something on the board.  I took the Klone off and realised the buffer was the culprit.  

robinbowes

clarkefan said:

It's all such a minefield, which gives snake oil salesmen the edge. The TL072 is the noisy problem in the Boss GE-7 and is ripped out in all the mods to quieten them. So it's a saviour/ piece of crap. Knowledge really is power...

The GE-7 doesn't contain any TL072s. The originals were TL022 x 3 (and an SIL NJM2068)

Noise figures of some common op amps are as follows:

TL022: 50nV/√Hz
TL072: 18nV/√Hz
OPA2134: 8nV/√Hz
NE5532: 5nV/√Hz 

R.

ecc83

I mentioned the TL072 simply as an example, there are better jfet op amps now.

You cannot compare the noise performance of jfet and bipolar input  chips, the former will be quieter for 'the magic meg' input Z.

NE5532? Yes, excellent IC for everything BUT a  high Z input. Has vastly better load driving ability than the 72. The LM4562 is even better but more expensive.

Reliability of buffers? Never had a chip fail in a pedal*. Footswitch yes, but not because of a poor component, built like a BSH. Very obscure problem in production. All fixed by a completely new design.  The odd valve but that was very rare.

Anyone interested in OP Amps etc should get Douglas Self's book, Small Signal Amplifiers.

*and that with a 30V supply, at 9V? Avin' a larf. Oh and DS will tell you, unless you use 741s you don't need to worry about slew rate!

Dave.

ICBM

ecc83 said:

Oh and DS will tell you, unless you use 741s you don't need to worry about slew rate!

And the JRC4558D - which is probably why it sounds best in TS-type circuits! It's a little faster than the 741, but nowhere near the 072.

(741 - 0.5V/uS, 4558 - 1.7V/uS, 072 - 16V/uS, from a quick check.)

Kalimna

A very quick question here - what is 'slew rate' as a property of op-amps?
Cheers,
Adam

robinbowes

Quick answer: http://bfy.tw/HZ2g

Kalimna

Ah, yes, Professor Google - thank you
The only problem with Google is that it can be hard to find suitable context for information and often asking people with the knowledge is quicker, so perhaps a more sensible question would be : Why is slew rate important to consider for op-amps in guitar pedals?

Cheers,
Adam

robinbowes

To prevent unwanted distortion.

If the op amp can't respond quickly enough then the output will not be faithful to the input.

R.

Danny1969

For guitar pedals - low voltage and low bandwidth slew rate isn;t a big deal 

Oddly some very high speed opamps aren't the best choice for a buffer ... mainly because they don't like running at unity gain and unless better attention is paid to high frequency decoupling they can oscillate well out of the audio range .... sometimes you can detect this with nothing other than your finger feeling one opamp is hotter than another

ICBM

Kalimna said:

Why is slew rate important to consider for op-amps in guitar pedals?

robinbowes said:

To prevent unwanted distortion.

If the op amp can't respond quickly enough then the output will not be faithful to the input.

And also it will effectively limit gain at higher frequencies - it's one of the important parts of what determines the frequency response.

Either way, the result of a lower slew rate is more harmonic distortion and less top-end response - which for a guitar overdrive pedal is a good thing.

In a buffer it would be a bad thing, except that in a low-gain configuration the unaffected frequency response is pushed much further up, out of the audio range.

This is for the 741, and you can see that the response is greatly affected in the 1-10KHz range at gains of 40 to 60dB which are common in distortion pedals. For anything below 20dB, it's outside the audio range.

https://www.nutsvolts.com/uploads/wygwam/NV_0701_Marston_FIGURE6.jpg

Kalimna

Ok, both those answers make sense, thank you

Adam

robinbowes

ICBM said:

Kalimna said:

Why is slew rate important to consider for op-amps in guitar pedals?

robinbowes said:

To prevent unwanted distortion.

If the op amp can't respond quickly enough then the output will not be faithful to the input.

And also it will effectively limit gain at higher frequencies - it's one of the important parts of what determines the frequency response.

Either way, the result of a lower slew rate is more harmonic distortion and less top-end response - which for a guitar overdrive pedal is a good thing.

In a buffer it would be a bad thing, except that in a low-gain configuration the unaffected frequency response is pushed much further up, out of the audio range.

Yeah, I was thinking of the buffer use-case rather than overdrive/distortion.

ecc83

I know of no modern (or even the 30+yr old 5532!) op amp that cannot produce a full level 20kHz sine signal?

Dave.

ICBM

ecc83 said:

I know of no modern (or even the 30+yr old 5532!) op amp that cannot produce a full level 20kHz sine signal?

Yes, but not at the same time as 40dB+ gain, which is why it matters for overdrive/distortion pedals.

But ironically, only when you use them *as* distortion pedals, and not when you use them as is now fashionable, as low-distortion boosts. Even more ironic is that the 'better' ICs usually used by boutique makers often sound *worse* when they're used as actual distortion pedals .

Obviously this doesn't matter for a buffer though, as Robin said.

jpfamps

Danny1969 said:

I wouldn't blame the TLO72 it's self, I've worked on mixers containing literally hundreds of them with very good noise performance. Resistor values  \ Johnson noise in the circuit design all contribute to the noise 

Personally I tend to use the NE5532P for everything as it's cheap as chips but a great performer with good current drive, higher rail swing and better slew rate than a 71 \ 72  \ 81

I do use the OPA2134 as a headphone amp though as it's output current drive is very good 

It's important for best distortion performance with TL072s that the impedance seen at both inputs is equal. 

This can be acheived in a mixer (or TL072s can be employed in parts of the circuit where this is not a problem) where impedances are generally know.

However being used as a buffer for guitar, the impedances are not equal on either input, and I think that this is a cause of the TL072 sounding brighter than an OPA2134. 

ecc83

ICBM said:

ecc83 said:

I know of no modern (or even the 30+yr old 5532!) op amp that cannot produce a full level 20kHz sine signal?

Yes, but not at the same time as 40dB+ gain, which is why it matters for overdrive/distortion pedals.

But ironically, only when you use them *as* distortion pedals, and not when you use them as is now fashionable, as low-distortion boosts. Even more ironic is that the 'better' ICs usually used by boutique makers often sound *worse* when they're used as actual distortion pedals .

Obviously this doesn't matter for a buffer though, as Robin said.

Yes but the thread IS about buffers! What, BTW are peeps using then at 40dB and more gains?

Dave.

ICBM

ecc83 said:

What, BTW are peeps using then at 40dB and more gains?

Tube Screamers have a maximum gain of just over 40dB if I remember correctly - the Rat over 60dB. I would guess things like the Boss Metal Zone might be even higher.

ecc83

ICBM said:

ecc83 said:

What, BTW are peeps using then at 40dB and more gains?

Tube Screamers have a maximum gain of just over 40dB if I remember correctly - the Rat over 60dB. I would guess things like the Boss Metal Zone might be even higher.

Ok but, what super-slewing chips are they using?

Dave.

robinbowes

Tubescreamer distortion is slightly different, it uses clipping diodes in the feedback loop rather than simply overdriving the device 

R. 

ICBM

ecc83 said:

Ok but, what super-slewing chips are they using?

They're not - that's the point I was making.

Or rather, some of the boutique builders do seem to use ones that are faster than the JRC4558 and the original Rat's LM308 - which is *really* slow at only 0.3V/uS - on the basis that these "better" chips must be "better". But as we have said many times before, with guitar gear the exact opposite is often true . Using a chip with a faster slew rate should definitely produce more high-frequency gain, even below 10KHz, with these types of pedal.