Upgrade Pro-Ject Head Box II headphone amp - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Headphone Systems

Headphone Systems Everything to do with Headphones

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 4th February 2012, 07:43 AM   #11
gk7 is offline gk7
diyAudio Member
 
Join Date: Oct 2007
Location: Vienna, Austria
Bypass caps for the opamp:
These are the small ceramic caps nearby the opamp.
I changed these to Wima MKS 0.1 ĩF and the corresponding
electrolytics to some Elna electrolytic caps (the brown-gold
colored ones in my photo). This is probably not really needed,
but I donīt like ceramic caps and happend to have the parts at hand.

Quiescent current and temperature of the output transistors:
I havenīt measured their current, but yes they do get hot. I
leave it on all the time and had no problems for two years now though.

Soldering:
This is made with RoHS conforming unleaded solder and I had a hard
time when desoldering parts. That said I probably would only change
the output electrolytic caps to better quality and the largest value
that still fits and the input caps to better quality, if I had to do
it again.
__________________
Regards,
Georg
  Reply With Quote
Old 4th February 2012, 07:52 AM   #12
gk7 is offline gk7
diyAudio Member
 
Join Date: Oct 2007
Location: Vienna, Austria
Some additions: No, the regulator does not get hot.
The opamp does not need cooling. The (comparably
low) supply voltage is not to high for any of the opamps
mentioned.
__________________
Regards,
Georg
  Reply With Quote
Old 4th February 2012, 08:39 AM   #13
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
The regulator does not get hot? Hmm. It should, at least when hanging in mid-air.

Could it be that Guido's unit uses an external DC power supply? Would explain the diode setup. If it delivers 16V=, the regulator would obviously not get hot (but not work properly either, it needs to drop ~2 V at least). 16V~ with a peak rectifier would give about the same result.

What I can already see from the board is that the Class-A output stage appears to be run open-loop - hence the relatively high current. As I suspected, the 3R3s are the emitter degeneration resistors, 10R is output series R, and the 15k appears to be for diode bias (OP-out -> 3x D --> 15k --> GND). The electrolytic between OP and buffer is connected in parallel to the bias diodes for reduced distortion.

Not sure about gain yet.
  Reply With Quote
Old 4th February 2012, 09:16 AM   #14
gk7 is offline gk7
diyAudio Member
 
Join Date: Oct 2007
Location: Vienna, Austria
Quote:
Originally Posted by sgrossklass View Post
The regulator does not get hot? Hmm. It should, at least when hanging in mid-air.
Well "hot", not overly so.

Quote:
Originally Posted by sgrossklass View Post
Could it be that Guido's unit uses an external DC power supply? Would explain the diode setup. If it delivers 16V=, the regulator would obviously not get hot (but not work properly either, it needs to drop ~2 V at least). 16V~ with a peak rectifier would give about the same result.
No idea. Mine is AC.

Quote:
Originally Posted by sgrossklass View Post
What I can already see from the board is that the Class-A output stage appears to be run open-loop - hence the relatively high current. As I suspected, the 3R3s are the emitter degeneration resistors, 10R is output series R, and the 15k appears to be for diode bias (OP-out -> 3x D --> 15k --> GND). The electrolytic between OP and buffer is connected in parallel to the bias diodes for reduced distortion.
Do you have a schematic ? I donīt think itīs a open-loop buffer. And if this qualifies as class A ?
__________________
Regards,
Georg
  Reply With Quote
Old 4th February 2012, 11:16 AM   #15
Guido59 is offline Guido59  Italy
diyAudio Member
 
Join Date: Jan 2012
Location: Bologna-Italy
@ gk7
Thanks now I have understand . I have yet only a doubt if I modify diodes section is right for me consirėdering the differences of mt circuit board.
I confirm my external power supply is an 16 V AC.
Referring to aritcle in agdr link in previous page about op-amp comarision I think to change op-amp with a NE5532 first and listen , after try with others to compare acoustic results with my ears , not with a instrument.
About other op-amp instead of NE5532 other to try could be :LM4562, LM49720 and what other double op-amp and made by Ti, National or who ?
BYE
Guido
  Reply With Quote
Old 4th February 2012, 04:59 PM   #16
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Quote:
Originally Posted by gk7 View Post
I donīt think itīs a open-loop buffer. And if this qualifies as class A ?
I think you're right, I finally saw traces going back to the feedback R/C from R8/R20. My eyes need a break now...

It's a high-bias complementary AB buffer at the very least. Normally it'll stay firmly within class A operation, only a load below about 60 ohms could make it transition to AB if it gets loud. Still, about 70 mArms in class A should have most normal headphones covered pretty well.
  Reply With Quote
Old 4th February 2012, 06:15 PM   #17
Guido59 is offline Guido59  Italy
diyAudio Member
 
Join Date: Jan 2012
Location: Bologna-Italy
Dear guys you are sometimes too technical for me and I can't follow you, but I'm happy to talk with because I have much possibilities to understand many new things.
Few hours ago I've changed original op-amp JRC 2068D with an NE5532 of Ti that is the only one component that I have found in resellers near my home , other I will buy on web to Moser or other.
Sound seems better for 2 reasons :
1- little more bass, that are deeper and with little more level
2- More speed response on transient expecially on bass and mid-bass frequency
Only listen some recording from CD or mp3, flac, ecc I listen likev a background noise at higher level , but this happen also on phone out of my sound card .
I use for input for Pro-Ject Head Box II Line Level Out of Sound Card that is a
Cakewalk UA-25EX .
Do you think about this ? With a better DAC should I have a bettere sound quality ?
( I'm tempted to modify also Sound card , Do I go insane ? )
By that you have wriet over , do you suggest me that bettere non modify diodes section?
Any cons for other op-amo to try and for best manufacter ?
Thanks you again.
BYE
Guido
__________________
Windows 7 pro eng 64 bit-Ubuntu 11.04;E8200@2,66GHz , ASUS P5K-SE,4GB DDR2,GeForce 9500GTOC ,Cakewalk-Edirol A25EX;AKG240MKII;ProJect Head BoxII;Epiphone B.B.King Lucille+Gibson 490T&490R;PreBrunetti Centouno;T.C.Electronic G-Major2; Palmer PDI-09
  Reply With Quote
Old 4th February 2012, 09:02 PM   #18
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
There are certain limits to what you can do without the necessary know-how and means to verify performance. You pretty much have to rely on tutorials from people who are sufficiently capable. Blind component swapping tends to be pointless.

I have not yet managed to identify all the gain setting resistors yet. One is in parallel to the little ceramic cap, should be either a 6k8 or 68k. I assume impedance at both opamp input terminals is mid-high.

You can try some of the usual suspects like OPA2132 and maybe LM4562. Not extremely much current noise if possible, good CMRR preferred. With a socket in place, stay far away from anything remotely prone to oscillation (e.g. the notorious OPA2604).

Performance should be just fine with any (decent) opamp that does not misbehave. A little more or less noise here and there, but other than that any audible differences should be very small to inexistant (placebo effect is strong, don't forget). The buffer already takes care of output loading, which tends to be a major bottleneck otherwise.

If you really want to hear a difference, try run-of-the-mill opamps like TL072, LM358, LF353 or MC1458. These are cheap, with the LM358 and MC1458 probably being the crappiest of the bunch.

Last edited by sgrossklass; 4th February 2012 at 09:09 PM.
  Reply With Quote
Old 5th February 2012, 07:28 AM   #19
Guido59 is offline Guido59  Italy
diyAudio Member
 
Join Date: Jan 2012
Location: Bologna-Italy
@ sgrossklass
Notices always interesting .I'm according to you I don't have the necessary know-how to to start form zero and modify any electronic circuit.and for this reason I want to use solution form article at link I have write in first post as also gk7 has talked about with his personal and valid notes.
So I think to make a good work with no hazard in this way.
I'm thinking about amplifier out impedance and out level necessary to drive good headphone from low to high impedance.
Considering that high impedance headphone over 300 ohms are pro and are very expensive for me, the range of impedance and then price for me also in the future will be max 250 $with Project head box II drive this headphone good.
Now I have an AKG K240 MKII with 32 ohms and amplifier cons > 30 ohms then I am on limits.
Infact setting volume of PC out in software player or windos or Ubuntu at 0dB level and volume of sound card near max level quite often the volume of headphone is not high also with out level of amplifier on max .
Is it possible to change out impedance of amplfier downwards to have a higher level of output without too difficult modifications of circuit , for example changing resistor, with lower ,in series to out of headphone.
Maybe I'm going insane, but please don't put your wire in my brain and willyou still let me join in the game , as Pink Floyd sing.
BYE
Guido
__________________
Windows 7 pro eng 64 bit-Ubuntu 11.04;E8200@2,66GHz , ASUS P5K-SE,4GB DDR2,GeForce 9500GTOC ,Cakewalk-Edirol A25EX;AKG240MKII;ProJect Head BoxII;Epiphone B.B.King Lucille+Gibson 490T&490R;PreBrunetti Centouno;T.C.Electronic G-Major2; Palmer PDI-09
  Reply With Quote
Old 20th November 2012, 10:09 AM   #20
diyAudio Member
 
Join Date: Aug 2012
Has anyone got the schematic/technical manual for Pro-ject Head Box II???
  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
Schematic for Pro-ject phono box Smiffy Analogue Source 132 24th February 2009 11:37 PM
pro-ject tube box Stefanoo Tubes / Valves 4 20th September 2008 09:32 PM
PRO-JECT SPEED BOX Mk2 Irakli Analogue Source 8 13th April 2007 09:56 PM
Fitting a Scheu platter&bearing on a Pro-Ject Perspective GUILHERME Analogue Source 4 8th June 2005 05:57 PM


New To Site? Need Help?

All times are GMT. The time now is 11:35 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2