amplifier bias drifting - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 25th June 2006, 03:15 PM   #21
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
the diode packs connected to the heatsinks are for temperature compensation of the output stage bias. That sounds OK.

The current in the front end is causing your problem.
10mA at +-85V is causing a dissipation of 850mW in each transistor. That will make them HOT, even with a small sink fitted.

The 0.95V across 100r indicates 9.5mA @ -75V (712mW)

John's running at just above body temp seems to indicate a much lower Iq probably less than 4mA.

John,
could you measure the bias voltages on the resistors on the collectors of the LTP and feeding the base of the CCS i.e. Vr612 Vr613?
Also Vr616 & Vr617?
__________________
regards Andrew T.
  Reply With Quote
Old 25th June 2006, 04:00 PM   #22
thejohn is online now thejohn  South Africa
diyAudio Member
 
Join Date: Mar 2005
Location: Cape Town, South Africa
Thanks Andrew... wait for me to put the lid back on!
Ok, so my amp is obviously biased a bit lower than craigs.
Voltages across R12 & R13 - 1.4v (should be 1.6)
Voltages agross R16 & R17 - 0.85v (should be 0.95)
So about 630mw for TR605 and Tr606 with my 74v Rail, more or less.
The problem with our widely varying mains voltage is that one could bias the amp and 10 min later it can be 10% high or low...

Any thoughts on running off a decent size true sin wave online ups?
  Reply With Quote
Old 25th June 2006, 04:20 PM   #23
thejohn is online now thejohn  South Africa
diyAudio Member
 
Join Date: Mar 2005
Location: Cape Town, South Africa
Another thought - I wonder if my heatsinks on TR605 / 606 are the same as Craigs. Black, aluminium U-chanels, about 15 x 12 x 30mm. Bolt on, not clip on. Quite heavy guage - 1.6mm. I guess maybe 20deg/w? maybe a little less?
  Reply With Quote
Old 25th June 2006, 04:39 PM   #24
diyAudio Member
 
Craig405's Avatar
 
Join Date: Jul 2005
Location: Sussex
Aha!
Thats the difference then my heatsinks are about 10x10x27mm and are barely a mm thick. They seem to just clip on with a u- shape over the top of the transistor to hold the back and front on with a bit of force.

I rechecked the measurements and each transistor is dissipating 7xxmW (cant quite remember now) so all is ok, just they probably would like a bigger heatsink.

I am much less worried about this amp now, i think a set of heftier heatsinks will solve the heat worries for me.
The amp measures reassuringly well everywhere else.

Thanks
Craig
  Reply With Quote
Old 25th June 2006, 05:36 PM   #25
thejohn is online now thejohn  South Africa
diyAudio Member
 
Join Date: Mar 2005
Location: Cape Town, South Africa
Ah good, hopefully thats all there is to it...
  Reply With Quote
Old 25th June 2006, 05:53 PM   #26
thejohn is online now thejohn  South Africa
diyAudio Member
 
Join Date: Mar 2005
Location: Cape Town, South Africa
My heatsinks...
[IMG]Click the image to open in full size.[/IMG] [IMG]Click the image to open in full size.[/IMG]
  Reply With Quote
Old 25th June 2006, 08:55 PM   #27
diyAudio Member
 
Craig405's Avatar
 
Join Date: Jul 2005
Location: Sussex
Yes, those are definatly going to keep them cooler than the ones in my lab 600, cheers for the pics.
I'll whack in some new heatsinks ASAP.

I had a bit of a listen to the amp tonight, i was pretty impressed with the way it sounded. I agree with what you found, there is a lot of detail and i found i heard bits in the music i didnt know where there before. Such as the sound of the guitar pick tapping on the string, also where the vocals could get harsh (especially through my speakers) they seem to sound more levelled out and natural.
Also the bass has taken a new level in my system, now it is very controlled and has a deeper undertone to it that i have never heard on my music before.

Regards
Craig
  Reply With Quote
Old 26th June 2006, 02:55 PM   #28
thejohn is online now thejohn  South Africa
diyAudio Member
 
Join Date: Mar 2005
Location: Cape Town, South Africa
Not bad for a '70s vintage transistor amp!!
  Reply With Quote
Old 26th June 2006, 07:19 PM   #29
diyAudio Member
 
Craig405's Avatar
 
Join Date: Jul 2005
Location: Sussex
No, not bad at all...
I replaced the heatsinks today (scary to be ripping transistors out of a fully working amp) but it worked out ok .

The heatsinks are hot, but easily within comfort to touch now and the bias readings are much more stable.
Excellent!

Thanks again to everyone
Craig
  Reply With Quote
Old 8th July 2006, 09:28 PM   #30
diyAudio Member
 
Craig405's Avatar
 
Join Date: Jul 2005
Location: Sussex
Hi again,

I decided to take AndrewT's advice on upping the supply capacitance for this amp. There wasnt any room inside to add additional capacitors and the replacement ones from electronics suppliers (47/56/68/100000uf) at 100V were too large to fit in the case .
So i took 4x Philips 10000uf 100V caps i had spare and wired them externally via very thick wires.

The amp now has a much more respectable 76000uf of capacitance (i also replaced the bridge rectifier from a 25amp to a 35 amp one to be safe).

havent had oppertunity to crank it up to listen for improvements yet, but theres bound to be at high levels.

Everyone likes pictures...


Regards
Craig
Attached Images
File Type: jpg photo1140.jpg (77.0 KB, 195 views)
  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
Nakamichi OMS-5EII - Track offset is drifting! Salar Digital Source 1 3rd August 2007 12:57 PM
N-Channel mosfet amplifier with servo bias f4bok Solid State 14 22nd September 2004 06:32 PM
Supressed bias amplifier, automatic variable bias system. destroyer X Solid State 2 20th September 2004 10:24 AM
Output Bias current for Zen-Like Amplifier wrenchone Pass Labs 9 15th August 2004 05:51 AM
Adjusting DC bias for Zen amplifier hlvan Pass Labs 6 5th November 2001 07:43 PM


New To Site? Need Help?

All times are GMT. The time now is 11:20 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2