Making my first guitar amp

a fine line between stupid and clever

Making my first guitar amp

Postby Damien83 » Wed Nov 04, 2015 9:51 pm

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
Attachments
NE5532.jpg
NE5532.jpg (21.53 KiB) Viewed 2923 times
010.JPG
010.JPG (234.69 KiB) Viewed 2923 times
Final.jpg
Final.jpg (89.22 KiB) Viewed 2923 times
Don't worry, don't be afraid, EVER.
Because this - Is just a ride. Bill Hicks
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Damien83
 
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Re: Making my first guitar amp

Postby Damien83 » Thu Nov 05, 2015 7:49 pm

20 views but no replies?

I am doing something wrong?

I hope it wasn't rude not to introduce myself, can do that if it's some sort of etiquette for this forum that I missed.

Any thoughts on my previous post ? I'm all ears even if you're not an expert please don't be shy. =)

Thanks in advance to anyone who replies. =)

Regards

Damien
Don't worry, don't be afraid, EVER.
Because this - Is just a ride. Bill Hicks
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Re: Making my first guitar amp

Postby IceFyre13th » Fri Nov 06, 2015 12:43 pm

Take a look at this file I attached, on sheets 5 and 6 are the amplification stags. Will make 50 watts all day long into 4 Ohms, will run as low as a 2 Ohm load at 75 watts but make sure you have plenty of heat sink to do it . Yes I know is is a car audio amplifier, just change the power supply out and its the same amplifier used in home theater systems.

Sheet 3, from the RCA inputs to the first switch can be used to drive the inputs of the amplification stages. This is the input / gain adjust stage and will be good from 250 mV to 2 V RMS input. The rest is crossover circuits after the first switch, something you could use, but not needed.

The input stage Op-amps all use +15 / -15 VDC. Main amplification stage voltages are, VCC is +35 volts DC, VEE is -35 volts DC. Up to + / - 40 VDC can be used, again proper a heat sink is needed.

On the amplification stages there is a hexagon with OC inside it, this is over current protection output to the power supply, for anything less than a 4 ohm load it would not be necessary. So on sheet 5 resistors R50, R52, R54, and transistor Q11 could be eliminated if you want. MAKE SURE YOUR LOAD IS AT LEAST 4 OHMS WITHOUT THIS CIRCUIT and do not drive into clipping.

The way this circuit works is the LM4701 drives the load up to about 7.5 to 10 watts, once the output voltage across R76 gets to a level high enough it will drive Q1 and Q3 to drive the speakers up to maximum output. (Sheet 6 top amplifier stage).

Benefit is lower THD than the LM4701 alone at full power, THD: .03% F/R: 10-50K +/- 3 DB, S/N: 100dB

This is one of my first designs I worked on as an audio engineer............and my favorite sand amplifier, tubes still sound better IMHO
Attachments
LM4701.pdf
LM4701 Datasheet
(545.66 KiB) Downloaded 182 times
diamond_audio_d5-300.4_rev.d.pdf
Diamond Audio D5
(84.85 KiB) Downloaded 184 times
IceFyre13th
 
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Re: Making my first guitar amp

Postby Damien83 » Tue Dec 29, 2015 7:15 pm

Thanks for the attachments bud =)
Don't worry, don't be afraid, EVER.
Because this - Is just a ride. Bill Hicks
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Joined: Thu Oct 29, 2015 3:06 am
Location: Hastings, Melbourne Australia


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