guitar amp

Hello. I want to ask about few things that have been bothering me recently a
lot.

1) I've seem some solid state guitar schematics. My question is, why don't
they ever use op-amps in solid state amplifiers?

2) I've seen some tube schematics tubes are used, and on the gird of the
input stage tube, there is a resistor (about 100k-500k) that goes from the
grid straight to the ground. My question is, if they want high input
impedance, why would they put a resistor and not just connect the input to
the grid resulting in very high input impedance?

3) Assume there is a capacitor before the input stage, and we want to have
lots of negative feedback. Does it matter were we feed the signal back to
increase the bandwidth of the amplifier?

Thank you for your response!

 

"Lost'n Found"

** Look harder - most SS guitar amps are chock full of op-amps.

** Because 500 kohms is high enough for electric guitar pickups.

Having NO resistor would result is loud BANGS when a lead was plugged or
unplugged.

** Yes.



......... Phil

 

Op-Amps are sometimes used, however they are not easy to incorporate into a
power design because they have too much open loop gain and insufficient
voltage swing. It's easier to design a circuit using discrete components
that provides the required voltage drive and the right amount of open loop
gain than it is to use what is pre-canned in an op-amp. This comes about
because all gain stages have a natural "pole" or break point in their
response characteristics each with up to 90 degrees of phase shift. For
stability reasons, it is problematic to cascade more than two of these
within one amplifier feedback loop structure. Most op-amps already have
these two stages but more are required to get the necessary voltage range
for a power amp. This makes the design messy with at least three poles in
the response. So, the simple answer is to not use op-amps, design exactly
what you need. In most amplifiers only two voltage gain stages are within
the feedback loop.

It is necessary to have a DC path from the grid to ground to insure that the
contact potential of the tube is "leaked" off to set the required bias for
the tube. If this is not done, the grid will aquire an increasingly negative
charge because of the electrons wizzing by that will eventually cut the tube
off and no signal will pass. It is possible to have a DC path that includes
the source, but, since the source is unknown and undefined, it is not done
this way. Usually the source is AC coupled with a capacitor and a known
resistor provides the DC path.

Yes it matters how the feedback path is created. See number one above.
Feedback must never accumulate more than 180 degrees of phase shift at any
frequency while the loop gain is above unity or your amplifier will be an
oscillator.
Bob

 

Very clear and helpful, thanks

 

That one is easy to solve with a fastish op-amp. You can locally close
the loop on the op-amp to make a stage with the gain you need.

This is the one that is more trouble. You have to go to a discrete part
to get the swing so you loose a lot of the advantage of the op-amp.

Make that 'at least one natural.." The smaller discretes often work up to
higher frequencies where things like the inductances of leads and the time
it takes for carriers to get through the device start to effect the
responce.

 

That's right, not only do you still have to come up with the required
voltage gain but you also have to come up with lower voltage (15 Volt) power
rails. You also have to figure how to drive both positive and negative from
center. Everything about op-amps complicates the design. Of course they can
be used but they bring little to the table in this application so why use
them?
Bob

 

"Bob Eld"

** It is VERY COMMON to see op-amps used as the first stage in power
amps.

There are several popular ways to use standard audio op-amps as the gain
stage in a power amp.


1. The op-amp acts as a pre-amp prior to the class A gain stage.

2. The op-amp's +/- supply pins produce anti-phase drive signals for the
output stage.

3. The op-amp *drives* the output stage directly - here the normal
output point is linked to ground and the speaker signal appears at the PSU
centre point.


# 1 is used in Phase Linear Series 2 amps, the Crown DC300A and many other
models.

# 2 is used in Cerwin Vega power amps, early Jands amps and others.

# 3 is used in many power amps by QSC and guitar amps from Seymour Duncan
and Peavey.



Your lack of familiarity with commercial audio design practice is very
clear.




........ Phil

 

Not really that common. However, granted it is done by designers that
don't really know what they are doing though.

Not relevant to the actual power amp design point being made. However, a
high performance opamp preamp to a power amp is quite a good idea.

Yeah... nasty...

I believe this last amp came out around the late 60's. Hardly
representative of modern amp design.

And these form good standards?

Bob made very valid points. Using an opamp contained in the feedback
loop of a main power amp is suicide. You lack of familiarity of how to
design high performance audio amplifiers is very clear.

Kevin Aylward B.Sc.

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

 

The Duncans and Peaveys that I've looked at are pretty good.

Oh, dear, stand by for nuclear holocaust!

 

I've had to work on several gtr amps in the last few years, and the Crate
amps all used 1458's thruout the preamp, tone and effects stages. They used
high power zeners to derive the plus and minus 15v rails from the power amp
rails. Not elegant, but the damn things worked pretty well.
regards,
tom

 

"Kevin Aylward = Manic Pommy Psychopath

** It is very commonly seen - you fucking pommy ass.

** Of course it is relevant - you insane pommy pile of puss.

** Go drop fucking dead - you asinine shithead.

** Wrong and WRONG again.

** They represent current commercial practice - you fucking MORON.

QSC are one of the largest amp makers in the world.

** BOLLOCKS.

** MORE and WORSE BOLLOCKS.

** That Kevin Aylward is seriously mentally ill and a know nothing pile
of manic pommy shit has been ABUNDANTLY clear to the whole world for
decades.






.......... Phil

 

Yes indeed John

Kevin Aylward B.Sc.

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers
on their knees, with their eyes closed"

 

Yabbut, that's "effects" - by the time the signal gets to them, it has
already been preamplified, so is up out of the noise floor.

And really, what kind of frequency response do you need for a guitar?

Cheers!
Rich

 

You need a much wider full-power frequency response than you might
imagine, especially at the low-frequency end.

But I would vote for a strict limit on gain. (;-)