guitar amp

"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

a

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

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.

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kensmith@rahul.net   forging knowledge

recently

don't

a

loop

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.

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

2. 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. snipped-for-privacy@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"

In message , dated Fri,

11 Aug 2006, Kevin Aylward writes

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

Oh, dear, stand by for nuclear holocaust!

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

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 f****ng pommy ass. ** Of course it is relevant - you insane pommy pile of puss. ** Go drop f****ng dead - you asinine s*****ad.

** Wrong and WRONG again.

** They represent current commercial practice - you f****ng 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. snipped-for-privacy@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

In message , dated Sat, 12 Aug

2006, Rich Grise writes

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. (;-)

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK