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Author Topic: Opamp slew rates  (Read 2318 times)

dogbox

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Opamp slew rates
« on: February 06, 2018, 04:10:04 AM »
Hi guys,
I'm wanting to breadboard and test this circuit MG10 https://www.electrosmash.com/marshall-mg10 In the output salien key filtering section, directly before the power amp it requires BA4560 opamps, of which I have no stock in my parts bin. Of the ones I do have..which would make the best sub.

I believe the slew rate is important here..although I'm not exactly sure what slew rate means??

Required
BA4560 SR — 4.0 — V / µs AV = 1, RL = 2kΩ

opamps that I have
uA741 SR   0.5 V/µs
TL072 High Slew Rate: 13 V/μs (Typical)
NE5532 High Slew Rate: 9 V/μs Typ
OPA2604 High Slew Rate: 25 V/µs
Also found some CD 4049ube (
not an opamp, but sometimes used as an opamp sub in certain pedal circuits)

My initial geuss would be to use a pair of uA741??  Any input appreciated.
cheers
Steve
« Last Edit: February 06, 2018, 05:16:05 AM by dogbox »

Katoda

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Re: Opamp slew rates
« Reply #1 on: February 06, 2018, 07:56:11 AM »
Hello

Slew rate is how fast can the op amp change from low to high, so 4 V/us means it can get from 0 to 4 V in one us. It is important in high speed switching, but not so much in audio. If you have rail to rail signal swing, say 30V (not in real life), it would take 7.5uS to swing it (7.5us of rise time). That is far beyond audio range, since frequency = 1/Tr.
You don't need that speed in audio, much less in guitar audio. For the sallen-key filter , you can just throw in any op amp you have, it won't make much of a difference.

EDIT: Terribly sorry for spreading misinformation, after a thorough reading I realized I was wrong. Apparently for the sallen-key filter, the gain bandwith product should be greater than 100*fc. Did not know that. So uA741 is out, but the others are good replacements.
« Last Edit: February 06, 2018, 08:21:39 AM by Katoda »

Enzo

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Re: Opamp slew rates
« Reply #2 on: February 06, 2018, 08:44:15 AM »
I agree with the first part^^^^

It just doesn't much matter.  These are just guitar amps, not lab gear, and not even hifi sets.  Guitar amps and their speakers rarely care about anything over about 5kHz.   Notice guitar amps have no tweeter?

When the MG10 was made, 4560 were the common op amps of choice, just as 4558 had been for a long time before them.  TL072 was in general use the whole time of the other two.  Note that later the 4580 became the common op amp of choice.

741 op amps were indeed used in early guitar amps.

Note the MG10CD and MG15CD use 4558.

Put a socket there and try whatever you have, then you report if it makes a difference.

dogbox

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Re: Opamp slew rates
« Reply #3 on: February 06, 2018, 09:12:56 PM »

EDIT: Terribly sorry for spreading misinformation, after a thorough reading I realized I was wrong. Apparently for the sallen-key filter, the gain bandwith product should be greater than 100*fc. Did not know that. So uA741 is out, but the others are good replacements.

Ok. Couple of things, just from deduction - not based on my poor electronics knowledge. I know that marshall deem it important enough to use these BJT opamps and not FET for sallen key filter before power amp in their solid state mg series of amps - wish I knew why?? Will try them I geuss..let my ears be the judge. From what I can tell, its tailored to the particualr speaker used in that mg10 amp, in other model mg series they use again a different opamp in filter section.
I might have to adjust some cap/resistor values, I'm geussing. got this tool http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm to help. thanks very much for your reply.
cheers
« Last Edit: February 06, 2018, 09:22:05 PM by dogbox »

dogbox

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Re: Opamp slew rates
« Reply #4 on: February 06, 2018, 09:19:14 PM »
I agree with the first part^^^^

It just doesn't much matter.  These are just guitar amps, not lab gear, and not even hifi sets.  Guitar amps and their speakers rarely care about anything over about 5kHz.   Notice guitar amps have no tweeter?

When the MG10 was made, 4560 were the common op amps of choice, just as 4558 had been for a long time before them.  TL072 was in general use the whole time of the other two.  Note that later the 4580 became the common op amp of choice.

741 op amps were indeed used in early guitar amps.

Note the MG10CD and MG15CD use 4558.

Put a socket there and try whatever you have, then you report if it makes a difference.

thanks mate - going by this schematic input/ gain/ tone stages all using bog standard tl072..but the filter section uses those bjt style

Enzo

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Re: Opamp slew rates
« Reply #5 on: February 06, 2018, 09:41:22 PM »
Thank you, yes I saw that initially.

But 4560 has passed into history for the most part, and if you have 4580, try that, it ought to work fine.   The idea is whatever engineering that went into the choice for the schematic may represent such minor differences you would never hear it.  SO I suggested trying others.  At worst, you'd just have to take the odd part out and order the exact thing.   I tend to suspect, it isn't that critical.