Hey guys
I'm putting together a guitar amp with 3 NE5532 op amps in parallel for less noise and more current gain in the pre stage
Although all of my voltage gain is comming from this stage I found that I needed more current gain than just one op amp to drive the To-220 B.J.T's properly.
The preamp/ op amp's then drive a power stage that consists of a complementary pair of TIP41C and TIP42C To-220 B.J.T's driving a set of NE3500 and MJ2955 To-3. (Darlington pair push pull class AB)
I am currently running a 50w transformer with some smoothing caps (10,000UF X2) that is +/- 40v and would like to get more power/volume/DB so I have a large torodial transformer that I salvaged from a large powered subwoofer I found on the side of the road a few years ago that I plan to utilize. (Power output is is unknown but by judging the weight I'm guessing it's over 100w. It has no label) I currently have a 200W driver from a sub woofer which sounds awful but I'm just using it in place as a load for my amp until I can afford to buy a nice mid range celestian or mad dog driver which would respond more to the frequencies I am trying to reproduce (To my understanding anyway).
I have a few problems (probably more than I know about but here goes)
If I use the large transformer that has a voltage of +/- 77V I am going past the max input voltage of my NE5532 op amps which have a maximum input voltage of +/- 44V. Should I use a couple of voltage dividers comming off my main rails to overcome this problem or is this a bad idea?
Would it be better to swap out my 3 NE5532 for One monolithic high power op amp for my preamp stage?
The other problem I am having is that if I run the negative feedback for the op amps from my final stage output terminal I get all kinds of weird echo's and squeeling (Is this what people mean when they talk about oscillation? ) and a little static but I have been instructed that this is where I should run my feedback from. If I hook my negative feedback in straight after the op amps it sounds much better but as I increase the gain (Using R2 variable resistor which is 50K) I get distortion at a very low volume which is the opposite of what I want.
I have considered putting a common emitter B.J.T stage between the op amps and the push pull stage for a little more voltage gain without distortion but I can't see how this will work as I am working with +/- voltages. To my knowledge an NPN transistor used with decoupling capacitors can only pass alternating current that is in the positive voltage region which would effectively make the P.N.P side of my finals useless would it not? Is it possible to make a push pull common collector stage for voltage gain? Ie - Output taken from the collectors ? I know in a normal scenario this would only provide current gain but since we have phase inversion ( I think ) with the push pull this would mean it would give voltage gain instead of current gain?
Ideally what I would like is alot of volume with a clean tone with minimal distortion until I crank up the gain on the op amps so that they clip gracefully. (I'm not sure if this is the way it should be done or if I should add a separate channel for this kind of stuff) I don't have an oscilloscope so everything I have done so far has been with trial and error and guess work but I'm pretty happy as I have a working amplifier on a breadboard (And a smaller version on a prot board without the darlington finals)
I'll post a schematic put please be nice as I have only been at this for 6 months or so and everything I have learnt has been from the internet.
I would really love a real life mentor. I live in Hastings in Melbourne Victoria, if there's anyone out there that would like an eager student shoot me an email. I've only been at it for about a year but so far really enjoying it. I have found in the past that the people on forums can be very critical and offensive at times and have even been accused of being a danger to myself and others for driving a capacitor past it's voltage rating and blowing it up by mistake. (I figured these voltage ratings written on the side of caps were only for D.C not A.C voltages)
I eventually would like to make some valve amplifiers as I love valves, hence me posting on this forum.
I was thinking I could start off with something like this as a future introductory project to valves but at the moment I don't feel I am skilled enough to be working with such expensive components. -
http://www.ebay.com.au/itm/181712489621 ... EBIDX%3AIT
Anyways I'll post a schematic, any help and advice would be appreciated. Please note that the schematic only has one Op amp but the actual circuit has 3 just paralleled together now (One last thought - should I put resistors between the outputs of the op amps so that they share load more equally?)
Regards -
Damien