How much current gain do I need for a guitar amp build? - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th March 2016, 04:53 AM   #1
diyAudio Member
 
Damien83's Avatar
 
Join Date: Jul 2015
Wink How much current gain do I need for a guitar amp build?

Hey Peeps

Please forgive me if I make any mistakes here as I am a newb.
Wondering if someone could please help me though -

I'm building a guitar amp from scratch from discrete components, trying to opt away from op-amps (excuse the pun hehe) and do it the good old fashioned way to learn as much as possible along the way and to say that "look mum I did this all by myself with no hands" ..well with hands but..you know what I mean.

I've got my voltage gain sorted out.

I worked out that my pickup produces 100mv of voltage and if I want clean undistorted sound up to the rails on my +/- 37 volt power supply and so I need an amplifier with a voltage gain of 370. Is that correct?

Now I just need the current gain ...I have no idea what kind of power my pickups are producing and have no way of measuring it, I have a multimeter with a.c voltage reading but no oscilloscope or anything. I could possibly put a resistor across the pickup and measure voltage drop but..nah, silly idea...I'm begging someone will please just give me the answer.


So I have a push pull stage in darlington pair configuration for my power side of the amp (which I have built on a breadboard) and have calculated the total current gain to be 200 (for one side NPN or PNP) ...does that mean that it has actually current gain of 400? I got the 200 by multiplying the HFE of the first NPN transistor by the HFE of the second transistor.

And I can work backwards from there and work out the current gain of each stage but that doesn't really help as I need to know what I need to amplify in order to be able to build the stages for it.

Also wondering - for power calculation purposes...


Hypothetically if I have a class a common emitter stage with a transistor with a HFE of say 50 and I run 50 volts as the power supply....
will the current gain remain at 50 or does this HFE value only apply if the transistor is run in common collector mode? (as a current amplifier not a voltage amplifier) ?

I am also wondering....is the current output of the small signal important... IE -
If I run an op amp that produces 32ma but a voltage of +/- 37 does that mean that to calculate my power I use ohms law to calculate Volts X amp's = W? (Bearing in mind the root mean squared thing with amps, can't remember what it's called but that 1.4 number that we have to divide by)

I guess my question here is...will the small signal current gain have much effect on the final product (speaker) or is it more about voltage? (as is the case with mosfets being voltage controlled valves as opposed to b.j.t's being current controlled.

Any help appreciated - thanks
Attached Images
File Type: jpg 004.jpg (608.4 KB, 58 views)
File Type: jpg Final.jpg (89.2 KB, 59 views)
  Reply With Quote
Old 7th March 2016, 05:55 AM   #2
diyAudio Member
 
vzaichenko's Avatar
 
Join Date: Jan 2010
Location: Moscow, Russia
Send a message via Skype™ to vzaichenko
Hi Damien,

Why don't you take a good proven guitar anp design and build it?

Cheers,
Valery

P.S. NE5532 is rated for +/-22V supply voltages as a maximum. Recommended threshold is +/-5...+/-15 Volts. +/-37V is definitely too much for it.
__________________
If you can't explain it simply, you don't understand it well enough (c) Albert Einstein
See my projects at VIRTUAL ZERO distortion AUDIO
  Reply With Quote
Old 7th March 2016, 06:59 AM   #3
diyAudio Member
 
thoglette's Avatar
 
Join Date: Dec 2008
You're about to discover a whole world of pain and disappointment. A single 2N3055/2955 pair will likely die the moment they meet a real world speaker. And that's before I look at the loop stability of what you're proposing.

The venerable ETI-413 utilised a pair on 40V rails (see here) and had a reasonable reputation for staying alive.

+/-25 would be a happier point for a single pair - but only with adequate heatsinks (not as shown in your photo)

p.s. I went down this route many decades ago. There's no need to repeat all my mistakes!
pps- you're going to want some "tone" which a properly built 2N3055 diff amp WILL NOT provide.
Have a look at some of these
http://www.runoffgroove.com/articles.html
http://www.duncanamps.com/technical/technical.html
http://www.classictubeamps.com/Tonestack.html

Last edited by thoglette; 7th March 2016 at 07:13 AM.
  Reply With Quote
Old 7th March 2016, 10:43 AM   #4
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

Its a toneless total disaster area waiting to happen / go bang.
Get the service manual of a decent budget amplifier and copy it.

FWIW you can buy a decent used guitar amplifier for far less
(and hear it first) than the cost of sourcing the parts and the
hassle of building it to be a robust reliable workhorse.

Even buying new will far cheaper in the long run, and FWIW
the DSP options in budget amplifiers cannot be homebuilt.

FWIW current gain is pretty irrelevant to the gain structure.
Voltage gain matters, and high input impedances mean
loads of possible current gain in each voltage gain stage.
Except for the speaker the output is not usually heavily
loaded, so the possible current gain is not exercised.

Checkout the awesome FENDER MUSTANG II V.2 .

rgds, sreten.
  Reply With Quote
Old 7th March 2016, 01:00 PM   #5
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
Here's the Marshall 8008, similar to your concept but 3 extra small transistors Marshall Valvestate 8008 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics
The op amp doesn't have to go to the rail voltage. the first and second transistor pairs just produce more and more current. The output won't go to the rail voltage if you want music instead of square waves.
That 470 uf cap you have on the output of your input amp is daft. You need a +-15v supply for the op amp and first transistor pair. Many audio and instrument type toroid transformers or package switcher supplies will have that.
One pair 2n3055/mj2955 can produce about 16w into an 8 ohm speaker with massive heatsinks. From On semi they cost almost the same as mj802/mj4502 whish are a lot tougher in the current spec. The MJL4302/MJL4281 cost less and are a lot easier to mount with one screw hole. However counterfeit 2n3055/mj2955 are very cheap and maybe good for 100 ma max. For 4 ohm speakers definitely the transistors with more soa are required. People in the Pacific frequently use 2SA1943/2SC5200 output transistors which are cheaper there, but not here in the US.
For drivers look at MJE15028/29 or 15030/31 which have lot more soa than TIP41c/42/. I wouldn't use no suffix TIP41/42 for that rail voltage.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 7th March 2016 at 01:23 PM.
  Reply With Quote
Old 7th March 2016, 01:10 PM   #6
diyAudio Member
 
JonSnell Electronic's Avatar
 
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Send a message via Skype™ to JonSnell Electronic
A Tip41/2 has a typical hfe (current gain) of 70 and 2N3055/2955 20. So 20 x 70 = 1400. That means with 1 drive current of 5mA the output current produced will be about 5 X 1400mA = 7A. That is how to work out the minimum requirements. 2N3055/2955 selected on test I have had hfe of 120 and the Tip types 190. The data sheet describes exactly what to expect.

Without heatsinks, the transistors will quickly overheat and fail.
__________________
Support for Flying Mole Class D, PMC DS-001 (Hypex),Valve Equipment and designs both new and old. www.jonsnell.co.uk

Last edited by JonSnell Electronic; 7th March 2016 at 01:11 PM. Reason: More info
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Modular Gain Stages for Tube Guitar Amp GravyBoat Instruments and Amps 10 31st March 2015 11:01 AM
CFB AMP low gain current feedback amp Stee Solid State 6 30th June 2014 05:17 PM
Guitar Amp 12au6-Possible Gain Increase? bparnell57 Instruments and Amps 21 9th November 2013 05:09 AM
12AX7 high gain guitar amp razorrick1293 Instruments and Amps 27 19th February 2011 11:32 PM
pre amp for gain clone as guitar amp engel dela pena Chip Amps 13 24th July 2009 03:46 AM


New To Site? Need Help?

All times are GMT. The time now is 12:54 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki