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Old 1st April 2014, 07:19 PM   #1
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Default Bass amp input impedance

Hello everyone! My first post. I Hope it's not too stupid.

I have an old Peavey bass amp with an input stage about like this. The manual says that the input impedance is 220K. R7 happens to be 220K.

Can I raise the input impedance by raising the value of R7? To 1M or even higher.

Or am I completely off track? I guess the 220K input impedance is meant for Fender-style passive bass guitars. I was hoping to use the amp with an upright bass that has a piezo pickup. An input impedance of 1M or even several megs would be good.

Thanks!
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Old 1st April 2014, 07:24 PM   #2
Mooly is online now Mooly  United Kingdom
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Yes, R7 sets the input impedance. Increasing it to 1 meg might be a bit much because the opamp is bjt (transistor input) type and such a high value might well cause the DC conditions to shift a bit.

What you could try though, is replacing the IC with FET type such as a TL072 which would allow R7 to have no real upper limit in practice.
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Old 1st April 2014, 07:28 PM   #3
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Is the opamp a 4558 or 4560 ? If its a 4560 then using something like an OPA2604 or a TLE2072 would better because they have much better output drive ability than a TL072.

You won't damage anything by experimenting though. Try your 1 meg as it stands now, but be aware that the DC voltage at the opamp output may shift a little. As drawn its AC coupled so that shouldn't really matter.
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Old 1st April 2014, 07:36 PM   #4
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Cool, thanks! Would replacing the IC to TL072 be just a simple replacement thing or do you think it will be some other problems with it?
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Old 1st April 2014, 07:39 PM   #5
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The schematic says 4560 but I still need to check what it has in real life.
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Old 1st April 2014, 08:34 PM   #6
sreten is online now sreten  United Kingdom
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Hi,

A higher input impedance wil help if the pickup has
high source impedance but it won't change the ability
of the pickup to drive the capacitance of the guitar
lead to the amplifier, and higher values will make
little real difference if the pickup can drive 1M.

I can't see any real reason to change the op-amp.
Noting for the RC4560 although typical input bias current
is 40nA and typical input impedance is 5M, the maximum
bias current is 500nA, and minimum input impedance 0.3M.
So you shouldn't need to change with any selection of the
input op-amps, which any manufacturer really should do.

Drive capability of the op-amp and the impedances
shown at the typical levels at that point is no issue.
Neither is any shift in DC levels, it doesn't matter.

Just change the resistor to 1M.

rgds, sreten.

Also note you have to use the Hi input.
Input impedance of the Lo input is
20K, near 21.5K if you fit the 1M.
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Last edited by sreten; 1st April 2014 at 08:50 PM.
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Old 2nd April 2014, 08:10 AM   #7
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by cowbellster View Post
Cool, thanks! Would replacing the IC to TL072 be just a simple replacement thing or do you think it will be some other problems with it?
It should be a straight swap. The only "problem" with the TL072 is that it isn't very good at driving low impedance loads fully, something the 4560 is much better at, but looking at the actual circuit values there shouldn't be a problem. (The other devices I mentioned are much better in that regard)
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Old 3rd April 2014, 07:44 PM   #8
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Thanks guys! I changed one resistor and now the amp works mighty fine with the upright.

The IC is actually a 4558. It sits in a socket so it's an easy swap if I might want to try some other chip sometime.

Here's a photo of the amp. It's been through a fire at the rehearsal space in the year 1990. The blue cord bypasses the EQ and some other preamp features. I think I will make some kind of hardwired bypass to it.
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Old 4th April 2014, 06:34 AM   #9
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Thanks for the update. Lol, it looks like its seen a bit of action
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