Hi rocco gibralter,
Your entire post points out what I am worried about.
It's no surprise that people who know the least feel the job is simple and that little knowledge is required. That would be you.
For years, I trained technicians and assigned work load. It was very important that work was sent to technicians that could handle the job. I got pretty good at doing this, because any mess ups hit me both in the wallet and spare time. I had to make things right.
The same thing applies here. Jaycee and a number of other members could easily handle this amplifier. Far more people would not be successful. That is reality, whether or not you like it.
As I said before "I have no problem with the concept of you attempting to service a lower powered amplifier that isn't worth as much. The chances of success would be a little higher, and the cost of failure is a lot less." That means that people should attempt to repair things if they so desire. It also means that people should attempt realistic goals with minimal impact if the work ends up in failure.
-Chris
Your entire post points out what I am worried about.
It's no surprise that people who know the least feel the job is simple and that little knowledge is required. That would be you.
For years, I trained technicians and assigned work load. It was very important that work was sent to technicians that could handle the job. I got pretty good at doing this, because any mess ups hit me both in the wallet and spare time. I had to make things right.
The same thing applies here. Jaycee and a number of other members could easily handle this amplifier. Far more people would not be successful. That is reality, whether or not you like it.
As I said before "I have no problem with the concept of you attempting to service a lower powered amplifier that isn't worth as much. The chances of success would be a little higher, and the cost of failure is a lot less." That means that people should attempt to repair things if they so desire. It also means that people should attempt realistic goals with minimal impact if the work ends up in failure.
-Chris
HK Citation 12
Dear Paco,
I have lots of experience with these amplifiers as I built many of them for a retailer when I lived in South Africa.
The amplifier has three main flaws.
1) The differential pair has no emitter degeneration, meaning the emitters are tied together. Each emitter can have about 100-150 ohm resistor in its lead. It is a simple matter to remove that famous double tranny and use matched (If you require low DC offset and very low THD) transistors with the emitter resistors in their legs.
2) The differential pair need a proper constant current circuit instead of the simple R-C-R.
3) The bootstrap collector load for the main VAS stage must also be replaced with a constant current source.
4) From each side of the bias transistor add 22K resistors to ground to lower the gain of the VAS
I do not have the schematic handy, but will look for it and give you some values of parts
Stephen Mantz
Zed Audio Corp.
Los Angeles CA
Dear Paco,
I have lots of experience with these amplifiers as I built many of them for a retailer when I lived in South Africa.
The amplifier has three main flaws.
1) The differential pair has no emitter degeneration, meaning the emitters are tied together. Each emitter can have about 100-150 ohm resistor in its lead. It is a simple matter to remove that famous double tranny and use matched (If you require low DC offset and very low THD) transistors with the emitter resistors in their legs.
2) The differential pair need a proper constant current circuit instead of the simple R-C-R.
3) The bootstrap collector load for the main VAS stage must also be replaced with a constant current source.
4) From each side of the bias transistor add 22K resistors to ground to lower the gain of the VAS
I do not have the schematic handy, but will look for it and give you some values of parts
Stephen Mantz
Zed Audio Corp.
Los Angeles CA
Re: HK Citation 12
MOER ......
all these you describe above are probably chinese for the OP .....
Anatech .....
your commnets are very correct as usual ..... well done ....i think this the proper advice.... thanks this also might be usefull to others
PAco161 ....
i wonder if you used the link i gave you about vintage amplifier restore upgrade tips .....cause i think that all this information given in the link is given in plain english with out any theoretical overflow..... meaning like anatech said if you have basic electronic skills and some mechanical like soldering and so you will manage to do this ......
then again anyone like anatech will do this in a couple of hours with amazing results .....you see i ve done this a couple of hundred times and anatech propbably a couple of thousand times .....
so its all up to you !!!!
MOER said:
MOER ......
all these you describe above are probably chinese for the OP .....
Anatech .....
your commnets are very correct as usual ..... well done ....i think this the proper advice.... thanks this also might be usefull to others
PAco161 ....
i wonder if you used the link i gave you about vintage amplifier restore upgrade tips .....cause i think that all this information given in the link is given in plain english with out any theoretical overflow..... meaning like anatech said if you have basic electronic skills and some mechanical like soldering and so you will manage to do this ......
then again anyone like anatech will do this in a couple of hours with amazing results .....you see i ve done this a couple of hundred times and anatech propbably a couple of thousand times .....
so its all up to you !!!!
there is another option ....
that you might wana take a look .....
removing all electronics from this and installing inside a pair of P3A from esp pages or DX amplifier from the forum's projects is not exactly DIY and restore but both of these boards if made properly will outperform originall HK boards very easy....
also working like that and since both amps have a lot of support here this will make the all thing very easy
that you might wana take a look .....
removing all electronics from this and installing inside a pair of P3A from esp pages or DX amplifier from the forum's projects is not exactly DIY and restore but both of these boards if made properly will outperform originall HK boards very easy....
also working like that and since both amps have a lot of support here this will make the all thing very easy
Hi Steven,
I should point out that your comments are really upgrades to the existing circuit. The way the amplifier is currently designed will not prevent it from operating properly. Implementing the changes you suggest will generally reduce the distortion, but then there are more things to do. First, the amplifier should be running before anyone begins to make changes to the circuit.
-Chris
I should point out that your comments are really upgrades to the existing circuit. The way the amplifier is currently designed will not prevent it from operating properly. Implementing the changes you suggest will generally reduce the distortion, but then there are more things to do. First, the amplifier should be running before anyone begins to make changes to the circuit.
If a matched pair of transistors are installed (as should be done regardless), the additional degeneration will not have a great effect on stability. This change is very helpful if those transistors are not matched, but then the job hasn't been done right anyway.
The R-C-R creates a very similar constant current. A proper current source would increase the CMRR due to it's higher impedance. The way it is will be fine for now. I view this as an upgrade - pure and simple. A good thing to implement after the amp is repaired.
That is more open to debate, but again it's considered to be an "upgrade" to the normal circuit that works fine. Many amplifiers today are still designed with the bootstrap.
Again, controversial. Could be viewed as an "upgrade". Since I've never done that, this change should be made to one channel and the performance compared.
-Chris
Citation 12
Chris,
of course I assume that the amplifier works.
Emitter degeneration in the diff amps is in my opinion a MUST. I am not interested in open loop gain and would rather have local feedback lopps were possible. The stabilty of the amplifier is greatly improved.
The CC in the diff and VAS stages will reduce the turn on transients close to zero.
The VAS transistor will need the addition of an emitter resistor so that its current is then well defined.
The base--Vcc resistor of the VAS transistor must be recalculated to accomodate this emitter resistor.
The C-B capacitor on the VAS transistor can be reduced to 47pF/250v
Increasing the stabding current in both the diff and VAS stages will improve linearity and stability
The bias circuit must have an "end stop" resistor in series with the trimmer to prevent the OP stage going into full class A if the trimmer is accidently turned to zero ohms.
The two diodes should be 3 in series and even this is not the greatest thermal tracking circuit.
Reduce the 33Ks to 10Ks in the input and feedback base circuits of the diff amplifier. This will reduce DC offset.
Adjust the 1K8 doewn feedback resistor to 510R.
Use a another 47pF/250v from the collector of the VAS transistor to the base of the feedback tranny in the diff pair (Q2)
I have always converted these amps to full complentary output stages (As I do on every Phase Linear which I repair and/or rebuild). There are many good complementary pairs out there today and with the low rail voltage even voltage selected MJ802/4502 work well for this Citation 12.
Please please remove the 3uH coil in the speaker circuit and replace it with a peice of wire.
Bypass the bias tranny C-E with a 10mfd // 0.1mfd.
Remove R12 and replace with a shorting link. Adding an emitter resistor to Q3 (VAS) removes the need for thsi 180R resistor.
Stephen Mantz
Zed Audio Corp.
Chris,
of course I assume that the amplifier works.
Emitter degeneration in the diff amps is in my opinion a MUST. I am not interested in open loop gain and would rather have local feedback lopps were possible. The stabilty of the amplifier is greatly improved.
The CC in the diff and VAS stages will reduce the turn on transients close to zero.
The VAS transistor will need the addition of an emitter resistor so that its current is then well defined.
The base--Vcc resistor of the VAS transistor must be recalculated to accomodate this emitter resistor.
The C-B capacitor on the VAS transistor can be reduced to 47pF/250v
Increasing the stabding current in both the diff and VAS stages will improve linearity and stability
The bias circuit must have an "end stop" resistor in series with the trimmer to prevent the OP stage going into full class A if the trimmer is accidently turned to zero ohms.
The two diodes should be 3 in series and even this is not the greatest thermal tracking circuit.
Reduce the 33Ks to 10Ks in the input and feedback base circuits of the diff amplifier. This will reduce DC offset.
Adjust the 1K8 doewn feedback resistor to 510R.
Use a another 47pF/250v from the collector of the VAS transistor to the base of the feedback tranny in the diff pair (Q2)
I have always converted these amps to full complentary output stages (As I do on every Phase Linear which I repair and/or rebuild). There are many good complementary pairs out there today and with the low rail voltage even voltage selected MJ802/4502 work well for this Citation 12.
Please please remove the 3uH coil in the speaker circuit and replace it with a peice of wire.
Bypass the bias tranny C-E with a 10mfd // 0.1mfd.
Remove R12 and replace with a shorting link. Adding an emitter resistor to Q3 (VAS) removes the need for thsi 180R resistor.
Stephen Mantz
Zed Audio Corp.
Hi Steven,
Now, after hacking up these amplifiers, you are positive these changes make for a stable amplifier? Please tell me how you test them.
By recommending these changes, are you willing to repair, at no cost, any amplifier that has had these changes made? Shipping included?
I get really annoyed when people know better than the original designer and re-engineer a stable product. There are times when the designer does make an error (more often these days), but radical changes as you are proposing obliterate the original design. In my opinion, if someone wants to go that far, then they should install one of the many excellent kits that exist, ending up with a known product with known performance. That way the customer understands that it will sound different, but the performance is guaranteed, and so will be the reliability.
I may have been hard on you Stephen, but your design is an unknown, as are your quality control procedures. Besides, this thread concerns the restoration or repair of an existing amplifier design.
-Chris
Actually, the thread is pretty clear that this amplifier does not function.
Why? Just match the diff pair properly. If you really want to improve things, redesign for a J-Fet diff pair.
Yes, they might. Depends on the circuit. I do expect they would help.
Careful here. Not without looking at the stability of each amp you do this to. Do you look for ringing on square waves every single time you do one?
I'm not buying that off the cuff. Have you checked the device temperatures after this tweak?
That's good engineering practice, but this one isn't made that way. Besides "accidentally turned to zero ohms" is not an accident. It's a stupidity! Also, the amp is not being fired up properly after service. You always assume that the amp has a fault when you power up the first time.
This is how this amp was designed. Leave it.
No! Now you have dropped the input impedance of the amp, and messed with the stability of the feedback. This is not required if you had installed properly matched pairs. Some circuits do have a designed in DC offset that can be calculated. Normally this is below 50 mV and therefore not any concern.
I haven't checked your math, but this would be to restore the gain to it's original value after you changed the other resistors.
Ahhhh, messing with HF stability again. Dangerous ground.
Why? You're exposing each customer to a needless expense. They bought the amp because they liked how it sounded. The amp worked in original trim.
Okay, now you are being reckless. Do you understand why that inductor exists?
Now, after hacking up these amplifiers, you are positive these changes make for a stable amplifier? Please tell me how you test them.
By recommending these changes, are you willing to repair, at no cost, any amplifier that has had these changes made? Shipping included?
I get really annoyed when people know better than the original designer and re-engineer a stable product. There are times when the designer does make an error (more often these days), but radical changes as you are proposing obliterate the original design. In my opinion, if someone wants to go that far, then they should install one of the many excellent kits that exist, ending up with a known product with known performance. That way the customer understands that it will sound different, but the performance is guaranteed, and so will be the reliability.
I may have been hard on you Stephen, but your design is an unknown, as are your quality control procedures. Besides, this thread concerns the restoration or repair of an existing amplifier design.
-Chris
Citation 12
Hello Chris,
I have been designing and manufacturing amplifiers for the past 40 years so I feel that I do have some experience.
My original reply was simply to throw some good info to those who wish to upgrade/repair/or whatever their amplifiers. My suggestions do not only apply to the Citation 12 but also to any RCA test book amplifier on which the Citation is loosely based.
Look up basic info on emitter degeneration of differential amplifiers. Read D.Self's books and you will see this degeneration applied all over.
The compensation of the VAS is a simple matter. Increasing the VAS current to about 10mA improves its ability to drive the first followers, reduces its tendency to oscillate (Phase Linear 400/700 are famous for this). A 47pF is quite sufficient for this application.
The amplifier WGILL be stabel with this AND the addition of a second 47pF from VAS collector to the feedback node (Base Q2)
Look at the curves of the various transistors and they are more linear with higher standing current. Look at the original AMPZILLA from 1974 and it was the first high qaulity amplifier to use higher currents in these stages with very good results.
Increasing the diff amps current to 2mA per device is not an issue of heat their diss is 0.09w
Leave the two diodes for an output stage which has THREE diode drops in its thermal path - I think not. Why should you leave it? Please explain this. This amplifier is not very well designed so why leave it!
Changing the feedback and input bias resistor to 10K has nothing to do with stability. Every preamplifier I know of can drive a 10K ohm load! Changing the feedback resistor from 33K to 10K has NOTHING to do with the stability of the amplifier. If your compensation caps are sized accordingly then there shall be no stability issues. A 47pF with 10K has an Fo of 338KHz so doubling this cap for "insurance" will drop this to 169KHz.
I know of no amplifier which has a "designed in DC offset of 50mV" Why put 50mV into a speaker if you do not have to?
All amplifiers I design have offsets of less than +/-3mV and I DO NOT MATCH MY DIFFERENTIAL AMPLIFIER TRANSISTORS, never have and never will.
Want to get rid of offset, convert the diffamp to use Darlington pairs made up of discrete devices with a pull down B-E on the second device of each Darlington pair (2K2 is good)
Yeah my calculations are pretty simple. Elementary opamp theory here. The Cit 12 has a gain structure of 19.33x and with a 10K and 510R we have 20.6x. If you want to have the exact gain structure with the 10K feedback use 549R 1% resistor.
The feedback capacitor from VAS collector to feedback node does not "mess with stability" it only improves it!
Want to make sure the amplifier is very stable, add base stopper resistors to the driver transistors Q4 and Q5 of about 47-100R. These guaranteee stability all other things been equal.
Yes Chris, I actually do know what the output inductor does. One thing I know that it does do is raise the output impedance at higher frequencies. The other thing it is supposed to do is improve the stability into capacitive reactive loads.
OK the way that I test all amplifiers:
1) Put the amplifier on a test bench and then hhok it up to any one of my seven Audio Precision System Ones.
I have auto loads on all my test benches so I write the programs for the APs and the loads are automatically switched in when I request them. I can bring in loads from 128 ohms down to 0.5 ohms for up to 8 channels of amplifiers.
I run the "normal" battery of tests on all amplifiers and also check their performance into inductive and capacitive loads.
This is how I test amplifiers.
The choice of whether to hack up an amplifier is that of the owner.
Sorry that you get annoyed Chris. In this case I do know more than the original designer.
Happy to send you the schematics of the Harman Kardon Citation 7.1 which I designed amd manufactured for Harman International.
The Citation 12 is by all standards a very poorly designed amplifier. Go read one of Douglas Self's books and you shall see that the designer of this amplifier violated almost every basic rule.
Back in the early 70s when this amplifier was produced, even the Japanese manufacturers were doing better in design.
yes of course the changes I have proposed change the original design but that is the whole idea of DIY, is it not - to have some fun experimenting?
I will 100% guarantee that my changes are good. You know why? I have done them on many of these amplifiers. I was doing them back in the 70s when I assembled these kits for Sounds International in Johannesburg, South Africa. There were stability and reliability issues then and granted I did not have access to Audio Precisions but these are only an aid and help us in our mass production to mkae sure all amplifiers we produce meet a common standard
Sorry I was so hard on you Chris.
Regards
Stephen Mantz
Zed Audio Corp.
Hello Chris,
I have been designing and manufacturing amplifiers for the past 40 years so I feel that I do have some experience.
My original reply was simply to throw some good info to those who wish to upgrade/repair/or whatever their amplifiers. My suggestions do not only apply to the Citation 12 but also to any RCA test book amplifier on which the Citation is loosely based.
Look up basic info on emitter degeneration of differential amplifiers. Read D.Self's books and you will see this degeneration applied all over.
The compensation of the VAS is a simple matter. Increasing the VAS current to about 10mA improves its ability to drive the first followers, reduces its tendency to oscillate (Phase Linear 400/700 are famous for this). A 47pF is quite sufficient for this application.
The amplifier WGILL be stabel with this AND the addition of a second 47pF from VAS collector to the feedback node (Base Q2)
Look at the curves of the various transistors and they are more linear with higher standing current. Look at the original AMPZILLA from 1974 and it was the first high qaulity amplifier to use higher currents in these stages with very good results.
Increasing the diff amps current to 2mA per device is not an issue of heat their diss is 0.09w
Leave the two diodes for an output stage which has THREE diode drops in its thermal path - I think not. Why should you leave it? Please explain this. This amplifier is not very well designed so why leave it!
Changing the feedback and input bias resistor to 10K has nothing to do with stability. Every preamplifier I know of can drive a 10K ohm load! Changing the feedback resistor from 33K to 10K has NOTHING to do with the stability of the amplifier. If your compensation caps are sized accordingly then there shall be no stability issues. A 47pF with 10K has an Fo of 338KHz so doubling this cap for "insurance" will drop this to 169KHz.
I know of no amplifier which has a "designed in DC offset of 50mV" Why put 50mV into a speaker if you do not have to?
All amplifiers I design have offsets of less than +/-3mV and I DO NOT MATCH MY DIFFERENTIAL AMPLIFIER TRANSISTORS, never have and never will.
Want to get rid of offset, convert the diffamp to use Darlington pairs made up of discrete devices with a pull down B-E on the second device of each Darlington pair (2K2 is good)
Yeah my calculations are pretty simple. Elementary opamp theory here. The Cit 12 has a gain structure of 19.33x and with a 10K and 510R we have 20.6x. If you want to have the exact gain structure with the 10K feedback use 549R 1% resistor.
The feedback capacitor from VAS collector to feedback node does not "mess with stability" it only improves it!
Want to make sure the amplifier is very stable, add base stopper resistors to the driver transistors Q4 and Q5 of about 47-100R. These guaranteee stability all other things been equal.
Yes Chris, I actually do know what the output inductor does. One thing I know that it does do is raise the output impedance at higher frequencies. The other thing it is supposed to do is improve the stability into capacitive reactive loads.
OK the way that I test all amplifiers:
1) Put the amplifier on a test bench and then hhok it up to any one of my seven Audio Precision System Ones.
I have auto loads on all my test benches so I write the programs for the APs and the loads are automatically switched in when I request them. I can bring in loads from 128 ohms down to 0.5 ohms for up to 8 channels of amplifiers.
I run the "normal" battery of tests on all amplifiers and also check their performance into inductive and capacitive loads.
This is how I test amplifiers.
The choice of whether to hack up an amplifier is that of the owner.
Sorry that you get annoyed Chris. In this case I do know more than the original designer.
Happy to send you the schematics of the Harman Kardon Citation 7.1 which I designed amd manufactured for Harman International.
The Citation 12 is by all standards a very poorly designed amplifier. Go read one of Douglas Self's books and you shall see that the designer of this amplifier violated almost every basic rule.
Back in the early 70s when this amplifier was produced, even the Japanese manufacturers were doing better in design.
yes of course the changes I have proposed change the original design but that is the whole idea of DIY, is it not - to have some fun experimenting?
I will 100% guarantee that my changes are good. You know why? I have done them on many of these amplifiers. I was doing them back in the 70s when I assembled these kits for Sounds International in Johannesburg, South Africa. There were stability and reliability issues then and granted I did not have access to Audio Precisions but these are only an aid and help us in our mass production to mkae sure all amplifiers we produce meet a common standard
Sorry I was so hard on you Chris.
Regards
Stephen Mantz
Zed Audio Corp.
Hi Stephen,
I've been fixing audio equipment for close to 40 years. I guess we are both familiar with amplifiers. I have even designed and built a few amps and preamps. However, none of that makes me any smarter than the people who came before me or presently working away. In fact the engineers in earlier times had common sense and some patience. Attention to detail is another trait these people had.
I also do not agree with many choices made by some designers. However, that does not give me license to talk other people into changing their equipment into something I like better. That is really very arrogant in my books. I think that is what bothers me the most about you. The attitude and willingness on your part to pass judgment on prior work by others. If you want to sell amplifiers, then do so honestly.
I am well aware what degeneration resistors do, especially in a differential pair. I too have read D.Self and others, I even legally bought those books and still have them. I don't agree with everything published in Self's book, but then again I have so many excellent texts, papers and application notes I use to round out my education. Big whoop! Almost everyone here has a vast amount of knowledge. Have you noticed that they approach audio problems from a different direction?
I don't have any AP gear. I can't afford it. What I do have is a fair amount of HP gear, including a spectrum analyzer. It's analog.
Now that we've beat our chests some, let's look at the issue at hand.
Yes, transistors tend to be more linear when run at higher currents. That also tends to increase their running temperatures which in turn may shorten the life of those parts. It will also raise the chassis temperature, shortening the life of many electrolytic capacitors. I have looked at Nelson Pass' work, as he uses that very method to gain linearity. But, you know what? When they designed these amps, they did not design them that way. They sized the heat sink, transformer and capacitors for a certain max. power and idling temperature. Notice I didn't say one was better than the other. It all depends on what the design goals were.
As for increasing the standing current of the voltage amp stage, I was not worried about the diff pair so much. It's the other transistors, and all I said was that you should take notice of this to ensure they did not run too hot.
Now, the bias circuit has not been the cause of terrible meltdowns and failures, has it? No, their warranty program did not kill them. The only reason you want to change it is that you have declared that you don't like it, so it has to be changed. Well, I'm in a habit of correcting poor designs. However, I only correct things that are actually causing trouble. Whether I like certain design choices or not has no bearing on the repair job. Heck, you could extend your thinking to the wire used and the colour of the faceplate.
Base stoppers cause other things as well. They will not cure an instability somewhere else in the amplifier. They are required with mosfets though, I'll give you that.
The Citation 7.1, is that the one that has a 27uH coil in series with the speaker output? The one with ferrite beads everywhere and complimentary differential pairs? BTW, those diff pairs are cascoded, but not connected in a darlington configuration.
Another thing I'll advance as an idea. Once you have replaced all the parts you have suggested, why not take a small step forward and replace the entire amplifier with an even better performing design? Your changes will cost a fair amount in parts and labour. May as well just change all of the amp circuits.
-Chris
I've been fixing audio equipment for close to 40 years. I guess we are both familiar with amplifiers. I have even designed and built a few amps and preamps. However, none of that makes me any smarter than the people who came before me or presently working away. In fact the engineers in earlier times had common sense and some patience. Attention to detail is another trait these people had.
I also do not agree with many choices made by some designers. However, that does not give me license to talk other people into changing their equipment into something I like better. That is really very arrogant in my books. I think that is what bothers me the most about you. The attitude and willingness on your part to pass judgment on prior work by others. If you want to sell amplifiers, then do so honestly.
I am well aware what degeneration resistors do, especially in a differential pair. I too have read D.Self and others, I even legally bought those books and still have them. I don't agree with everything published in Self's book, but then again I have so many excellent texts, papers and application notes I use to round out my education. Big whoop! Almost everyone here has a vast amount of knowledge. Have you noticed that they approach audio problems from a different direction?
I don't have any AP gear. I can't afford it. What I do have is a fair amount of HP gear, including a spectrum analyzer. It's analog.
Now that we've beat our chests some, let's look at the issue at hand.
Yes, transistors tend to be more linear when run at higher currents. That also tends to increase their running temperatures which in turn may shorten the life of those parts. It will also raise the chassis temperature, shortening the life of many electrolytic capacitors. I have looked at Nelson Pass' work, as he uses that very method to gain linearity. But, you know what? When they designed these amps, they did not design them that way. They sized the heat sink, transformer and capacitors for a certain max. power and idling temperature. Notice I didn't say one was better than the other. It all depends on what the design goals were.
As for increasing the standing current of the voltage amp stage, I was not worried about the diff pair so much. It's the other transistors, and all I said was that you should take notice of this to ensure they did not run too hot.
Now, the bias circuit has not been the cause of terrible meltdowns and failures, has it? No, their warranty program did not kill them. The only reason you want to change it is that you have declared that you don't like it, so it has to be changed. Well, I'm in a habit of correcting poor designs. However, I only correct things that are actually causing trouble. Whether I like certain design choices or not has no bearing on the repair job. Heck, you could extend your thinking to the wire used and the colour of the faceplate.
Cool! I know something you don't then. Not all preamplifiers can cleanly drive a 10 K load. Some will even lose their dynamics into a 10 K load. I even have that load in my switch box. Again, not my position to render judgment on those preamps that require a higher impedance to drive. You sure assume a lot for someone your age.
That is true, however I did not check your math as I stated. There are also cases where other things that do affect stability can be affected - in a general sense. One thing that is very possible is that you may change the sonic character of the victim amplifier. That is something your client can not judge properly and they may well like the way the amp sounded before.
Gee, I've come across them before. What I said is that a 50 mV DC offset is nothing to worry about. The fact that you are worried about a small amount of DC offset, but you aren't concerned at all about distortion due to mis-matched differential transistors is curious. You must be using some kind of DC servo in your designs in that case. Another can of worms now. The design of your servo and how and where you inject it can have repercussions as far as sound quality is concerned.
Darlington's are often slower than single transistors. Again, the differential pair is where the input is compared to the output and corrected. By definition, the differential pair should be matched so it properly reduces errors that appear on the trip through the various stages in the amplifier.
The dominant HF pole can also cause oscillation if you put it in the wrong place.
Base stoppers cause other things as well. They will not cure an instability somewhere else in the amplifier. They are required with mosfets though, I'll give you that.
Talking about the output inductor now. So you really want to destabilize an amplifier? Pretty silly compared to the possible small gains in performance. Consider that most tweeters in a speaker system are padded down. The small increase in output impedance is a non-issue, however the RF issues you may cause by removing that inductor are major. I call this irresponsible as well. Very, very few amplifiers can get away not using an output inductor safely. You never know what speaker cable arrangement and types may cause disaster. Just wait until one of your customers run their speakers from coax cable or some other variation.
Actually, that depends greatly upon the information that customer is provided with. If you misguide someone who is not an expert, then it is you who are making the choice, not the customer.
The Citation 7.1, is that the one that has a 27uH coil in series with the speaker output? The one with ferrite beads everywhere and complimentary differential pairs? BTW, those diff pairs are cascoded, but not connected in a darlington configuration.
That may be, I'm certainly not going to hold it up as a shining example of great amplifier technology. So what's your point? There are many different directions one could take if redesigning this piece was the goal.
As I've said. Got the book. Read the book. Don't agree with everything, but he does make many good points. Still have the book.
That may or may not be true. We do know more as an industry these days, but what would you have done when this amplifier was designed? I think that is more the measure.
That is not always true. Everyone put out a fair amount of so-so amplifiers back then. The speakers were worse. Study the Marantz 500, designed in the late 60s and discontinued in 1974. That one is fairly advanced and once rebuilt to spec, sounds great.
That's not the only direction DIY takes. DIY means to Do It Yourself. That means everything from restoring something to designing something from scratch. Since you posted in this thread, there are a couple things you should know. The thread starter wished to repair this amplifier. It was found that this person did not possess the skills or equipment for this project. Having him modify it in the way you suggest is even further away from his abilities.
Another thing I'll advance as an idea. Once you have replaced all the parts you have suggested, why not take a small step forward and replace the entire amplifier with an even better performing design? Your changes will cost a fair amount in parts and labour. May as well just change all of the amp circuits.
-Chris
Ok, so maybe I am, admittedly, in a bit over my head with this. However, I my defense, I was never asking to be trained as a technician in the first place. All I was asking about was a little help in figuring out what the faulty parts may be, but whatever. The fact that the amp clicks and pops when the amp is unplugged still leads me to believe that the problem is coming from the power supply caps having gone bad. It seems the thread got off my original assumption when I did a bit more research into the amp, but I still think it's a cap somewhere in there, which would be easy enough to replace. So, if you would so kindly allow me to do so, ask another question. Just as more troubleshooting procedure, is there a way to test capacitors, while they're connected in the circuit, to see if they've gone bad? Thanks.
paco161 said:
Hi Paco.
The Citation 12 is a fine amp as it was designed. I won't comment on the improvements available, it sounds like you just want to get rid of the noise and listen to it as designed.
I don't think the power supply caps are the noise issue, if original they probably need to be replaced but if they were aged to the point of no return the amp would run really hot. If not, you probaly have an input transistor (s) or driver that is tired. You did not mention if the noise was in both channels or not...
I'm a bit tired now but with a bit more specific details as to what the amp does when listening to music and if one or both channels are affected would help in the diagnosis.
The first amp I purchased was a Citation Twelve, I have schematics and could summon the memory Gods with a bit more detail... It ain't rocket science.
Mike.
Check for dry joints. Very common with vintage amps. After that, its small E-caps gone bad. It would be a good idea to replace all E-caps on the pcb. PSU caps should be fine unless there are signs of electrolyte residue.
The thump is normal. You will get it when you switch on the amp and a few seconds after you switch it off.
Mike
Hi Michael Chua,
Paco is having thump problems that are not related to turning on or off. You should read the entire thread to see what the situation is.
Hi Paco,
You can not properly test capacitors in circuit.
You might have a capacitor problem, but then again it may also be solder connections or even a transistor going intermittent. I've seen resistors cause these problems as well.
What you want is for people to troubleshoot this amplifier for you. It's old, it probably has more than one problem. The time people will spend talking you through this and teaching you how to test things is high. You don't even have test equipment.
You are asking too much. That has been my point all along. You even proved this by deciding you have a capacitor problem with no evidence to lead you to that conclusion. Then you added to that by asking how to test capacitors in circuit. You can not properly test transistors in circuit either.
Now, do you have a capacitor tester? Do you have a transistor tester that will give you beta and leakage? A good DVM or oscilloscope? You need these things. That's why technicians have them.
I'm sorry but you are both headstrong and foolish. A bad combination. I know you "just want to fix the amp and listen to it". What you want is a free repair, dang the cost to anyone else. I would love to help you, but you do not possess the basic knowledge and equipment to be successful. I have explained that to you in detail, but you refuse to consider anything that has been told to you.
You "just want to fix the amp and listen to it". But, you can't. You should take that amplifier to someone who can, and pay them to do it. You had enough money to buy it, so save up and fix it.
-Chris
Paco is having thump problems that are not related to turning on or off. You should read the entire thread to see what the situation is.
Hi Paco,
You can not properly test capacitors in circuit.
You might have a capacitor problem, but then again it may also be solder connections or even a transistor going intermittent. I've seen resistors cause these problems as well.
But ... you need to be in order to fix this amplifier. Also, there are other clues that could tell a well trained technician of problems coming up.
What you want is for people to troubleshoot this amplifier for you. It's old, it probably has more than one problem. The time people will spend talking you through this and teaching you how to test things is high. You don't even have test equipment.
You are asking too much. That has been my point all along. You even proved this by deciding you have a capacitor problem with no evidence to lead you to that conclusion. Then you added to that by asking how to test capacitors in circuit. You can not properly test transistors in circuit either.
Now, do you have a capacitor tester? Do you have a transistor tester that will give you beta and leakage? A good DVM or oscilloscope? You need these things. That's why technicians have them.
I'm sorry but you are both headstrong and foolish. A bad combination. I know you "just want to fix the amp and listen to it". What you want is a free repair, dang the cost to anyone else. I would love to help you, but you do not possess the basic knowledge and equipment to be successful. I have explained that to you in detail, but you refuse to consider anything that has been told to you.
You "just want to fix the amp and listen to it". But, you can't. You should take that amplifier to someone who can, and pay them to do it. You had enough money to buy it, so save up and fix it.
-Chris
Hi Paco,
If you were to look at past threads, you would see that I do my best to help people out. There are certain situations that are probably best left to experienced people. When I have a strong feeling that something will not end well, I generally ask the people involved to get professional help.
I'm sorry that bothered you. Understand I didn't call you unintelligent at all. What I said was that you needed experience and some equipment.
Look at it this way. I have built older style engines a long time ago, but in no way am I qualified to rebuild a new one. My training didn't go that far. Same thing with home heating and air conditioning. I call "the guy" 'cause I haven't a clue about what I am doing. There are lot's of things I have not got nearly enough skill with, but I recognize and accept it.
Notice I didn't threaten to burn my car, house or anything else. I can accept that I 'm not qualified to do those things. You probably can do things that I can't. The big difference is that I have always listened to people who know more than I do. Yup, when my car has a problem, I drop it off, tell them what I've noticed. Not once do I attempt to fix it on my own (although I'm sure I could make some progress), nor do I tell the experts how to do their job.
Lastly.
-Chris
If you were to look at past threads, you would see that I do my best to help people out. There are certain situations that are probably best left to experienced people. When I have a strong feeling that something will not end well, I generally ask the people involved to get professional help.
I'm sorry that bothered you. Understand I didn't call you unintelligent at all. What I said was that you needed experience and some equipment.
Look at it this way. I have built older style engines a long time ago, but in no way am I qualified to rebuild a new one. My training didn't go that far. Same thing with home heating and air conditioning. I call "the guy" 'cause I haven't a clue about what I am doing. There are lot's of things I have not got nearly enough skill with, but I recognize and accept it.
Notice I didn't threaten to burn my car, house or anything else. I can accept that I 'm not qualified to do those things. You probably can do things that I can't. The big difference is that I have always listened to people who know more than I do. Yup, when my car has a problem, I drop it off, tell them what I've noticed. Not once do I attempt to fix it on my own (although I'm sure I could make some progress), nor do I tell the experts how to do their job.
Lastly.
That is a selfish attitude. Why not see if someone will work through this with you that has a bench and some equipment? I've helped a couple people that way. Some skilled technicians actually enjoy passing on the knowledge and you make a friendship that lasts for years.
-Chris
Chris,
I can appreciate your point of view, and yes you're absolutely correct on several points, but a big one is that I wouldn't actually burn it. That was pure, dripping sarcasm, though you would never know through text only...
Honestly, I would hate to see this amp go to waste in any way shape or form. I had been looking for one for a long time now and it bums me out to no end that it doesn't quite work right, but I guess I didn't expect it to with it being 40 or so years old. Nevertheless I can only either sell it or have someone fix it, and right now I cannot make up my mind. One of the main reasons I don't want to take it to a shop is that the only place I would trust with it charges $70 an hour for labor only, so the last time I had them fix an amp (a Crown CL1), it cost me nearly 250 which, in this case, is more than I paid for the Citation to begin with. I can tell you with certainty, however, that you have succeeded in making sure I don't try and fix it...
And it's not that I don't accept what I know to be true, it's that you, a complete stranger, has come in and told me that there is not a chance of me fixing it myself. This has nothing to with whether or not you may be right, but that fact that I don't know you and you discount me instantly. I guess you could say it's a matter of pride, ego, arrogance, whatever you may want to call it...
Honestly, I am in the dark on what to do with this amp. I'm moving to it from the aforementioned Crown, which I only managed to break because I dusted it out
, so I'm not sure what to do. It's currently listed locally on craigslist, so I'll just have to see where that goes for now.
I can appreciate your point of view, and yes you're absolutely correct on several points, but a big one is that I wouldn't actually burn it. That was pure, dripping sarcasm, though you would never know through text only...
Honestly, I would hate to see this amp go to waste in any way shape or form. I had been looking for one for a long time now and it bums me out to no end that it doesn't quite work right, but I guess I didn't expect it to with it being 40 or so years old. Nevertheless I can only either sell it or have someone fix it, and right now I cannot make up my mind. One of the main reasons I don't want to take it to a shop is that the only place I would trust with it charges $70 an hour for labor only, so the last time I had them fix an amp (a Crown CL1), it cost me nearly 250 which, in this case, is more than I paid for the Citation to begin with. I can tell you with certainty, however, that you have succeeded in making sure I don't try and fix it...
And it's not that I don't accept what I know to be true, it's that you, a complete stranger, has come in and told me that there is not a chance of me fixing it myself. This has nothing to with whether or not you may be right, but that fact that I don't know you and you discount me instantly. I guess you could say it's a matter of pride, ego, arrogance, whatever you may want to call it...
Honestly, I am in the dark on what to do with this amp. I'm moving to it from the aforementioned Crown, which I only managed to break because I dusted it out
, so I'm not sure what to do. It's currently listed locally on craigslist, so I'll just have to see where that goes for now.
Hi Paco,
Well, I didn't discount you as a person. I very quickly got an idea how much you actually knew about electronics and how much equipment you had on hand. It wasn't that I discounted you instantly. I am really trying to be helpful to you. I did not want to see extra damage occur, and especially didn't want to see you get hurt. I am sorry I bruised your ego, but I had to get these things through to you. notice that the only thing I said that was personal had to do with your fixation to repair it yourself.
So now, how about looking at options?
Firstly, keep this amp and take your time with it. It will be an excellent project for you and you will be proud of being able to do the work yourself. Either that or more experience will allow you to come to terms with your actual abilities and interests.
If you were to take a night course on electronics, you may come in contact with a real audio technician who will be willing to help you through the repair. This is a best case scenario. You learn properly and your amp is repaired properly. Ham radio operators are also a very helpful lot. The only thing you have got to watch for is the guy who considers himself to be an expert in everything. Make sure you are working with a calm fella that actually has experience in this field. Younger technicians are not a good bet on average. Also, do not do any modifications until your amplifier is working well. Stay on track with that goal.
If you want to do this on your own, you will need some equipment. at least a good oscilloscope and a good multimeter. A Fluke is well worth the money in so many ways.
If you are in a position later when you have learned more and have access to test equipment, please start a thread and we can work through it. I have no problem assisting you as long as you have a reasonable chance at success.
-Chris
Well, I didn't discount you as a person. I very quickly got an idea how much you actually knew about electronics and how much equipment you had on hand. It wasn't that I discounted you instantly. I am really trying to be helpful to you. I did not want to see extra damage occur, and especially didn't want to see you get hurt. I am sorry I bruised your ego, but I had to get these things through to you. notice that the only thing I said that was personal had to do with your fixation to repair it yourself.
So now, how about looking at options?
Firstly, keep this amp and take your time with it. It will be an excellent project for you and you will be proud of being able to do the work yourself. Either that or more experience will allow you to come to terms with your actual abilities and interests.
If you were to take a night course on electronics, you may come in contact with a real audio technician who will be willing to help you through the repair. This is a best case scenario. You learn properly and your amp is repaired properly. Ham radio operators are also a very helpful lot. The only thing you have got to watch for is the guy who considers himself to be an expert in everything. Make sure you are working with a calm fella that actually has experience in this field. Younger technicians are not a good bet on average. Also, do not do any modifications until your amplifier is working well. Stay on track with that goal.
If you want to do this on your own, you will need some equipment. at least a good oscilloscope and a good multimeter. A Fluke is well worth the money in so many ways.
You know what? It would have cost you more at a lower grade shop with lower labour rates. The better technicians get the work done properly the first time and faster than most others. $250 certainly isn't the worst I've seen. Remember to that this is part repair and part restoration. Why not ask them how much they thing it might run?
The original cost is immaterial. The only thing that is important is the cost of repairing it and if it will be a half decent amp and reliable.
My intent was that you shouldn't be fixing it yourself right now. There is nothing wrong with you attempting this repair with local help and equipment. Just as long as you have someone there who knows what they are doing that has access to the equipment you need. So, resist the temptation to repair it quickly once you start working on it. Take your time and work carefully.
If you are in a position later when you have learned more and have access to test equipment, please start a thread and we can work through it. I have no problem assisting you as long as you have a reasonable chance at success.
-Chris
Ok. Ok. craigslist post has been deleted. Actually, I have a good friend who is a big ham radio guy who I know would love to help out, it's just a matter of finding time. He helped me out when I was first getting into car audio with wiring and all that stuff, so I definitely think he knows what he's talking about...I'll look into that later on since I'm swamped this weekend and during the week I've got school. For the time being, I guess I could do the simple stuff, like checking for bad solder joints, cleaning contacts, etc. etc., that don't risk blowing anything up and see where I get from there...
I just want to touch back on my capacitor assumption real quick, if you don't mind. The reason why that was my first (and obviously uneducated) guess was because the amp makes it's clicks and pops for a time after it's been unplugged. Unless I'm missing something, as far as I know, the only parts in the amp that store any kind of energy are the caps, but like I've said, I'm not sure. I just want to be clear on this...
I just want to touch back on my capacitor assumption real quick, if you don't mind. The reason why that was my first (and obviously uneducated) guess was because the amp makes it's clicks and pops for a time after it's been unplugged. Unless I'm missing something, as far as I know, the only parts in the amp that store any kind of energy are the caps, but like I've said, I'm not sure. I just want to be clear on this...
Hi Paco,
Okay, right on!! Now we can get something done safely.
One problem that professionals in any field have is that they forget sometimes how much they know because it's second nature to them. They unconsciously process a lot of information while not even thinking about it. Therefore, they may say that something is easy. That means one of two things.
1. It is easy .... for them. They are in the above situation.
2. They don't know very much, so it looks easy. These people are well meaning but very dangerous. That's why I asked you to find someone good in this field to help you.
So when your friend is helping you, please be aware that he may forget to tell you many things. Things that are obvious to him are not to someone who is just starting out. That includes stuff you may not even know to think about. So, if there is anything even slightly your guys are unsure about, ask here. One of our members will guide you properly. For instance, MikeBettinger would be very good to talk to and he has already posted here.
Okay, the capacitor thing.
-Chris
Okay, right on!! Now we can get something done safely.
One problem that professionals in any field have is that they forget sometimes how much they know because it's second nature to them. They unconsciously process a lot of information while not even thinking about it. Therefore, they may say that something is easy. That means one of two things.
1. It is easy .... for them. They are in the above situation.
2. They don't know very much, so it looks easy. These people are well meaning but very dangerous. That's why I asked you to find someone good in this field to help you.
So when your friend is helping you, please be aware that he may forget to tell you many things. Things that are obvious to him are not to someone who is just starting out. That includes stuff you may not even know to think about. So, if there is anything even slightly your guys are unsure about, ask here. One of our members will guide you properly. For instance, MikeBettinger would be very good to talk to and he has already posted here.
Okay, the capacitor thing.
The answer is simple. Your main filter capacitors are powering your amplifier circuits for a time after you turn it off. The circuitry hasn't got a clue that it's off. It only knows that the voltages are falling. So your noises may be caused by any resistor, zener diode, capacitor (even the small ones) or transistors. In other words, anything in the entire amp circuit. Remember, the ac ripple on the main filter capacitors are not heard in normal operation. There is no reason to think that any other noises these could possibly make would be heard out the speakers. Any noises except a short, then it's an audible mechanical BANG!
-Chris
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