Active device substitutions needed for this schematic!

Lenin · 3rd April 2002, 12:24 AM

Err...

Would someone be kind enough to take a look at this schematic and recommend some device replacements?

Before you take a look, be warned, as circuits go - it ain't pretty!
Its a big old Musical Fidelity, OK, OK, not the last word, but we've been together 15 years and the amps like a cosy old pair of slippers - you get the idea. Anyhow, I've decided it deserves a spruce up, but its with the active devices that I'm really and truly stuck.

I even tried some straight op-amp relacements (without changing any of the ancillary components - I just kinda said a little prayer and hoped for the best) but virtually everything LT1028, OPA134, OPA627 oscillate like crazy. OPA132's seem OK though...

I'm sure you guys have far better things to do but I'm really kinda stuck.

I hope the attached info is useful.

The output devices 2SK 226 and 2SJ 83 are enhancement mosfets, now discontinued.

Thanks in advance

Lenin · 3rd April 2002, 12:32 AM

URL=http://www.geocities.com/ResearchTriangle/8231/doc/ampl/musical_fidelity_a370.jpg]Here it is[/URL]

Again, thanks in advance.

Lenin · 3rd April 2002, 12:33 AM

URL=http://www.geocities.com/ResearchTriangle/8231/doc/ampl/musical_fidelity_a370.jpg]Here it is[/URL]

Again, thanks in advance.

Lenin · 3rd April 2002, 12:37 AM

http://www.geocities.com/ResearchTri...elity_a370.jpg

AudioFreak · 3rd April 2002, 01:52 AM

2SK1058/2SJ162 should be fine replacements for the 2SK226/2SJ83 outputs if you can get your hands on them.

Elso Kwak · 3rd April 2002, 05:39 AM

Hi Lenin,
No wonder you got oscillations with other opamps.
If you place a fast opamp in front of a buffer/outputstage you get the effect of a dog chasing its own tail. You have to slow down the opamp by placing 50 or 100pF maybe more, from the opamps output to the inverting input. The opamp originally present had compensation too, but more in a custommade manner. This has to be removed for modern opamps.
See also the Application Note #47, High Speed Amplifier Techniques at www.linear.com Appendix C The Oscillation Problem-Frequency Compensation Without Tears.

Lenin · 3rd April 2002, 11:46 PM

Thanks guys, I'm following up your advice.

In the meantime, any suggestions for replacing MPSA43/93 and BC414/416?

Thanks again.

Elso Kwak · 4th April 2002, 06:44 AM

Hi Lenin,
For what it is worth my replacement book gives
MPSA43-> BF391..393; BF420A;BFP22; BFP25
MPSA93-> BF492...493; BF423A; BFP23; BFP26
BC414-> BC184; BC550
BC416-> BC214; BC560
All kaput?

Lenin · 5th April 2002, 01:13 AM

Thanks Elso.

Can you recommend any of those devices (or any others you feel might be suitable) on sonic grounds?

Thanks.

3rd April 2002, 12:24 AM	#1
Lenin diyAudio Member Join Date: Feb 2002 Location: London	Active device substitutions needed for this schematic! Err... Would someone be kind enough to take a look at this schematic and recommend some device replacements? Before you take a look, be warned, as circuits go - it ain't pretty! Its a big old Musical Fidelity, OK, OK, not the last word, but we've been together 15 years and the amps like a cosy old pair of slippers - you get the idea. Anyhow, I've decided it deserves a spruce up, but its with the active devices that I'm really and truly stuck. I even tried some straight op-amp relacements (without changing any of the ancillary components - I just kinda said a little prayer and hoped for the best) but virtually everything LT1028, OPA134, OPA627 oscillate like crazy. OPA132's seem OK though... I'm sure you guys have far better things to do but I'm really kinda stuck. I hope the attached info is useful. The output devices 2SK 226 and 2SJ 83 are enhancement mosfets, now discontinued. Thanks in advance

3rd April 2002, 12:32 AM	#2
Lenin diyAudio Member Join Date: Feb 2002 Location: London	Whoops! Here's that schematic URL=http://www.geocities.com/ResearchTriangle/8231/doc/ampl/musical_fidelity_a370.jpg]Here it is[/URL] Again, thanks in advance.

3rd April 2002, 12:33 AM	#3
Lenin diyAudio Member Join Date: Feb 2002 Location: London	Whoops! Here's that schematic URL=http://www.geocities.com/ResearchTriangle/8231/doc/ampl/musical_fidelity_a370.jpg]Here it is[/URL] Again, thanks in advance.

3rd April 2002, 05:39 AM	#6
Elso Kwak Banned Join Date: Dec 2001 Location: Zamboanga, City of Flowers, Mindanao	parts replacement Hi Lenin, No wonder you got oscillations with other opamps. If you place a fast opamp in front of a buffer/outputstage you get the effect of a dog chasing its own tail. You have to slow down the opamp by placing 50 or 100pF maybe more, from the opamps output to the inverting input. The opamp originally present had compensation too, but more in a custommade manner. This has to be removed for modern opamps. See also the Application Note #47, High Speed Amplifier Techniques at www.linear.com Appendix C The Oscillation Problem-Frequency Compensation Without Tears.

4th April 2002, 06:44 AM	#8
Elso Kwak Banned Join Date: Dec 2001 Location: Zamboanga, City of Flowers, Mindanao	replacement parts Hi Lenin, For what it is worth my replacement book gives MPSA43-> BF391..393; BF420A;BFP22; BFP25 MPSA93-> BF492...493; BF423A; BFP23; BFP26 BC414-> BC184; BC550 BC416-> BC214; BC560 All kaput?


	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

3rd April 2002, 12:37 AM	#4
Lenin diyAudio Member Join Date: Feb 2002 Location: London	Will I ever get this right!!! http://www.geocities.com/ResearchTri...elity_a370.jpg

3rd April 2002, 01:52 AM	#5
AudioFreak Warp Engineer On Holiday Join Date: Oct 2001 Location: Queensland, Australia	2SK1058/2SJ162 should be fine replacements for the 2SK226/2SJ83 outputs if you can get your hands on them.

3rd April 2002, 11:46 PM	#7
Lenin diyAudio Member Join Date: Feb 2002 Location: London	Thanks guys, I'm following up your advice. In the meantime, any suggestions for replacing MPSA43/93 and BC414/416? Thanks again.

5th April 2002, 01:13 AM	#9
Lenin diyAudio Member Join Date: Feb 2002 Location: London	Thanks Elso. Can you recommend any of those devices (or any others you feel might be suitable) on sonic grounds? Thanks.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Output transformer matching to active device driving it	macrohenry	Solid State	3	17th August 2007 06:51 AM
Hi-freq Active Crossover Needed	HipoFutura	Solid State	6	30th July 2006 08:22 PM
Active BSC and RC circuit - help needed	HiFiNutNut	Multi-Way	2	21st March 2005 10:30 PM
Help needed for active filter	k1jroth	Chip Amps	0	23rd February 2005 03:14 PM
Amp for active X-over, what power needed?	Kees	Solid State	1	3rd October 2003 07:42 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read