Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Headphone Systems Everything to do with Headphones

CPA-1, the Crap/Cheap Parts Amp (A Fun Challenge)
CPA-1, the Crap/Cheap Parts Amp (A Fun Challenge)
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 11th November 2018, 12:29 AM   #1
sgrossklass is offline sgrossklass  Germany
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Default CPA-1, the Crap/Cheap Parts Amp (A Fun Challenge)

Here's a little project idea that could still use some input from you guys.

Recently I happened to be thinking about whether you couldn't even make a lowly LM358 work in a headphone amplifier when it struck me - why not make a headphone amp using lots of parts that audiophiles would be just about 100% certain to scoff at? LM358s, Darlingtons, 3 MHz fT power transistors, ceramic disc caps, random 12VDC SMPS wall-warts, finest generic Chinesium electrolytic caps - hell yeah! (Well, maybe not the generic Chinesium electrolytics after all. I like stuff that lasts a while.) The challenge, of course, is making all of this crap deliver actually good perfomance all-round.

I have a rough idea of what the circuit is going to look like, but first off - who is going to qualify? Are TL07x and 4558s still getting in or are those way too high-tech? Or are we better off doing this in two steps, first the easy way with parts that actually qualify as "audio" (CPA-1), later with the jumping through hoops that the more hardcore options necessitate (CPA-2)? Purely price wise, RC/NJM4558s are actually cheaper than LM358s, and LF353s aren't any more "spendy" either.

Topology wise, I was thinking roughly along these lines:
Balanced input - +6/0/-6/-12 dB or something, maybe buffered (I was thinking additional RCAs Bruno Putzeys style, but having to run another wire for a ground connection is also annoying, making an adapter cable seems more elegant)
10-50k pot
Fixed gain noninverting power stage w/ discrete front end, bootstrapped cascode, yadda yadda. Would prefer a single-stage buffer tbh, with either BD139/140(-16) or TIP31/32C or similar, but won't rule out a diamond buffer either. Output inductor / Zobel as needed.
Might consider an additional impedance-balanced output, not so much for headphone use but rather preamp use.
Some more features perhaps, like an input selector? Internal connector for a DAC or something?
Power supply wise, I was thinking two generic 12 V 1-1.5 A switch-mode wall warts, stacked. (It's not like I have that many of them floating around myself, but I encounter them quite often, so they have to be very common. Manufacturers seem to be split between 5.5/2.5 and 5.5/2.1 mm barrel plugs though.) Cleanup will no doubt be necessary. If that turns out to be a stupid idea (common-mode noise and all), using an old-fashioned split power supply plus 78/79M regs (maybe 05s propped up on zeners) would still be an option.

Any thoughts so far?
  Reply With Quote
Old 11th November 2018, 12:43 AM   #2
Mark Johnson is offline Mark Johnson  United States
diyAudio Member
 
Mark Johnson's Avatar
 
Join Date: May 2011
Location: Silicon Valley
CPA-1, the Crap/Cheap Parts Amp (A Fun Challenge)
Make it out of nothing but MPSA42s and MPSA92s. If you parallel enough of them, with balancing ("ballasting") resistors, you can get as many amperes as you wish. Since they're rated for 300 volts, you can run the amp off a 48VDC wall wart with no fear of overvoltage on the transistors. And they're cheap. Extremely cheap. See images below. Still in active production today, BTW.

_
Attached Images
File Type: png a42.png (93.6 KB, 122 views)
File Type: png a92.png (90.3 KB, 123 views)
  Reply With Quote
Old 11th November 2018, 01:48 AM   #3
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
I built a, IMHO, very fine monitoring headphone amp out of only 1990s Radio Shack parts. TL072 and PN3055/2955, un-fancy caps (I did use non-RS input caps from mil-surplus). You could still buy AC wallwarts so I made the +/-DC in the box. Zener to the chips, raw DC on the power devices. NFB gain control because the *main* criteria was a very wide range from hiss to clipping (so's I knew any hash was in my source, not the HP box).

FWIW, nobody ever complained about the LM386 HP amp I built for a synth studio (yeah, not real music).
  Reply With Quote
Old 11th November 2018, 02:02 AM   #4
abraxalito is offline abraxalito  United Kingdom
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Send a message via Yahoo to abraxalito
TL074 is a considerable bargain on Aliexpress, $0.05 for 4 opamps : 200PCS TL074CDR SOP 14 TL074CD TL074C TL074-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group
__________________
I know you think you understand what you thought I said but I'm not sure you realize that what you heard is not what I meant - Alan Greenspan
  Reply With Quote
Old 11th November 2018, 04:10 AM   #5
WoodyLuvr is offline WoodyLuvr  Thailand
diyAudio Member
 
WoodyLuvr's Avatar
 
Join Date: Nov 2017
FYI

Switching Split Power Supply ± 12V 500mA Split Supply
Non-Conductive ABS-Polymid Panel Mount 2.1mm DC Barrel Jack
__________________
timbre me ears
  Reply With Quote
Old 11th November 2018, 07:49 AM   #6
BesPav is offline BesPav  Russian Federation
diyAudio Member
 
Join Date: May 2011
Location: Russia, Moscow
Default CPA-1, the Crap/Cheap Parts Amp (A Fun Challenge)

Quote:
Originally Posted by sgrossklass View Post
Balanced input - +6/0/-6/-12 dB or something, maybe buffered
Anyway buffered, because by using 4-channel pot you will kill any CMRR achieved by amp design.

Quote:
Originally Posted by sgrossklass View Post
Fixed gain noninverting power stage w/ discrete front end
No, use cheap as dirt LM318 inverting with EF2 discrete OPS loaded to ~150-200 mA CCS. Add some resistors to negative supply as on fig. 37 from datasheet, or even CCS.

Quote:
Originally Posted by sgrossklass View Post
bootstrapped cascode
Yes, something like IRFZ24 could be used, but this will demand for additional supply headroom.

Quote:
Originally Posted by sgrossklass View Post
10-50k pot
10k linear pot with 2k inverting input resistance will provide you with nice log function, a way better than best potz.

Quote:
Originally Posted by sgrossklass View Post
random 12VDC SMPS wall-warts

No, use cheap as dirt sectioned El type ~10 Wt trafo, this will provide you with ~20-50 pF mains coupling. No regulation needed because of good PSRR of opamp, put simple CRC with ~7-10 Ohms R, 5 Wt rating.

Last edited by BesPav; 11th November 2018 at 08:04 AM.
  Reply With Quote
Old 11th November 2018, 10:04 AM   #7
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
You cannot beat a good chipamp
SE class A regulator-chip-amp madness

BTW, I have found the LM318 quite noisy, not an ideal choice for a HPA
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪
  Reply With Quote

Reply


CPA-1, the Crap/Cheap Parts Amp (A Fun Challenge)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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
High quality parts used delta-sigma dac vs cheap parts used r2r ladder dac Esotar01 Digital Line Level 2 6th May 2016 01:50 AM
The Quick, Cheap and Dirty Speaker Challenge xrk971 Full Range 26 18th June 2014 12:25 PM
Mystery Parts Challenge ravix Parts 8 3rd April 2011 08:09 AM
DIY TT on the cheap with used parts makinson1 Analogue Source 3 20th March 2007 03:54 PM
cheap parts and a lot of em karma Swap Meet 15 24th August 2004 01:41 AM


New To Site? Need Help?

All times are GMT. The time now is 11:12 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki