Elkaid said:Yeah I would try it myself but I'm beginning to be scared to try things. (I blew my 4th TDA7294 this morning and now I want to try LM3875 for a change !)
I hope having news from you soon
so do i
i got my capactors today - elna RSH series, which seem ok all i need now are the chis themselves and the input caps
fezz said:
Normally the 30 VAC is unloaded meaning that with Class AB/B you get close to the no load voltage which in this case is some 5-8 percent higher plus line variations. This will give you some 34-35 VAC minimum at no load and high mains voltage. That is a bit on the high side giving some 48 VDC if unlucky that day. Risky.
JoeBob said:
Not something you can use in a class AB amp.
In a choke input supply you need to have a continuous minimum current draw (to maintain the magnetic field in the choke?) or the PS flips into a cap input mode & you voltage is up to 1.41459... times DC and all of a sudden you let the magic smoke out of your components.
dave
Philo said:
This reminds me of an old joke:
"Doctor, it hurts when I move my arm like this..."
to which the doctor replies "So don't do that!"
There is nothing wrong with the cap. Touching the input cap body is like touching the input of the amp. Your body picks up all sorts of AC crud like an antenna and sends it through the amp. In your case, you are capacitively coupling the crud right through the case of the cap. It isn't any different than touching the input of the amp, except the coupling is not quite as good as the direct touch. If you don't want it to hum, don't touch the cap.
Minimizing the physical path the signal takes through your amp is a nice thing to try, but if the amp becomes unstable or you have problems with things like transformers inducing hum into the circuit, they really isn't much point...
Did anyone try just building the circuit the way National shows in the data sheet?
MR
MR
LOL. Well, unfortunately I have the buzz without touching the cap on the one channel only that increases when I squeeze it. No buzz on the other channel even when I squeeze it. I went to the electronics store and bought some more caps. I will try and swap out both, if the symptoms change, I've found my culprit. If not, I'll keep checking. I might just end up leaving it out as the amp seems stable without it.
LOL. Well, unfortunately I have the buzz without touching the cap on the one channel only that increases when I squeeze it. No buzz on the other channel even when I squeeze it. I went to the electronics store and bought some more caps. I will try and swap out both, if the symptoms change, I've found my culprit. If not, I'll keep checking. I might just end up leaving it out as the amp seems stable without it.
You don't have to open the transformer if it's a torroid.
If you wind some extra turns over the top, and connect them in series with the secondary(s) reverse phase, it'll subtract that voltage. You may get about 1/3 volt per turn.
The source impedance of your power supply will be virtually unchanged.
Cheers,
Don' t do anything with the primary.
Wind some turns through the middle of the "donut" and out.
Maybe 10 turns to try. This is a "new" secondary. Then take one of the original secondary wires, and connect this new "winding" in series with it. Measure the new voltage on that new combined secondary. If it's gone down:good. If it's gone up: reverse the new winding's connections.
If you have 2 secondarys, youll need 2 sub-secondarys.
Cheers,
Wind some turns through the middle of the "donut" and out.
Maybe 10 turns to try. This is a "new" secondary. Then take one of the original secondary wires, and connect this new "winding" in series with it. Measure the new voltage on that new combined secondary. If it's gone down:good. If it's gone up: reverse the new winding's connections.
If you have 2 secondarys, youll need 2 sub-secondarys.
Cheers,
Current Drive Gainclone? best matched to speakers
Is "current drive" (amongst other things) a possible different configuration of the Gainclone?
(Forgive me if I've missed a post on this)
What are the benefits and disadvantages?
Is this another name for a "current feedback amplifier"?
Kuei Yang Wang wrote:
"the best application is in a fully active speaker on midranges and tweeters if the respective resonances fall sufficiently far outside the operating range of the Driver."
(I'm about to do an active three-way dipole, with Peerless subs, close to a Linkwitz Lab Phoenix and Thor).
This is OT for the Gainclone, but on the subject of best matching the Gainclone to speakers, Kuei also wrote:
"The low Qm make the driver ideal for current drive, as in this case Qt becomes in essence Qm."
I thought Qt was based on Qm and Qe.
Am I wrong? Why would Qt becomes Qm; why is this desirable? And in particular would this apply in a dipole? Are you suggesting that either the speakers or amp would be executed differently?
TIA
Richard
Is "current drive" (amongst other things) a possible different configuration of the Gainclone?
(Forgive me if I've missed a post on this)
What are the benefits and disadvantages?
Is this another name for a "current feedback amplifier"?
Kuei Yang Wang wrote:
"the best application is in a fully active speaker on midranges and tweeters if the respective resonances fall sufficiently far outside the operating range of the Driver."
(I'm about to do an active three-way dipole, with Peerless subs, close to a Linkwitz Lab Phoenix and Thor).
This is OT for the Gainclone, but on the subject of best matching the Gainclone to speakers, Kuei also wrote:
"The low Qm make the driver ideal for current drive, as in this case Qt becomes in essence Qm."
I thought Qt was based on Qm and Qe.
Am I wrong? Why would Qt becomes Qm; why is this desirable? And in particular would this apply in a dipole? Are you suggesting that either the speakers or amp would be executed differently?
TIA
Richard
Re: Current Drive Gainclone? best matched to speakers
Hi,
Of sorts.
The key advantage is that unlike virtually all other types of amplifiers the "current drive" amplifer drives the speaker with a signal that is congruent with it's operation. As a result distortion and compression in the Amp/Speaker system is drastically reduced, compared to the usual low output impedance amplifiers.
There is no disadvantage as such, however as such an amplifer will apply NO electrical damping on the voicecoil, thus we no longer can hide the complexities of using mechanically resonant systems in the socalled "damping factor", which is strongly signal dependent and a rather unreliable methode of damping resonances, plus one purchased at the cost of very high distortion and significant dynamic compression in the Amp/Speaker system.
No.
Yup.
Because with "current drive" Qe approaches infinity and if you look at the derivation of Qt from Qe and Qm you fins that with a vaery high value of Qe Qm dominates Qt.
Well, I would not really claim that making Qt equal to Qm as such is desirable in many modern speaker drivers. On the other hand, if Drivers where designed with a sensible Qm build in (like many vintage drivers are) then their behaviour in enclosures would be much more predictable.
As for dipoles, they present no aoustical loading to the driver, resulting usually in a build in resonance that is slightly lower than free air one and equally a lower Qt. Normal Loading tends to raise the system Qt, so a say Qt 0.28 Driver placed into a sealed enclosure of suitable size will give a system Q of 0.7 for a critically damped system.
If we raise the Qt of a Dipole Driver HIGHER than 0.7 this means we have a peak in the response around the drivers resonance, say our Qt 0.28 Driver has a Qm of 1.83. As the signal at the drivers resonance will be reduced by acoustic short circuit action of the dipole the system will have a response that is still attenuated at the resonance, but less so than in the case where the driver is driven by pure current, making the Qt equal to the Qm of 1.83.
If we exclude the dipole specific losses and only consider the Driver placed in a near infinitly large enclosure the example driver (Seas CA25FEY) when driven by "voltage drive" will be 11db down at 30Hz on its midband sensitivity.
So on a open baffle 15" wide we would need to compensate not only the baffle rolloff (this is much lower than normally expected - usually around 0.07 * lambda/bafflewidth is the -6db point) at around 60Hz +3db and 30Hz +9db, but additionally we would have to add a further 8db boost at 30Hz to achieve a flat response that is 3db down at 30Hz.
If we use "current drive" on the driver we see a 4db PEAK at 35Hz and +1db @ 70Hz / +3db @ 30Hz without the dipole loss. Thus with the dipole loss we get around -1.5db @ 60Hz and -6db @ 30Hz. This will still require a little bit of EQ in my view, but very little, yet I'd almost suggest running such a system without EQ, just current feed.
In fact, if you use the Linkwitz folded baffle with a suitable woofer (Qm 2.0 or thereabouts) and current drive you may be able to avoid using ANY EQ. Drivers with a high Qm can have their Qm reduced by placing damping material behind the driver....
Both ideally. The idea is to design a SYSTEM comprised of Amplifers and Speakers that are optimised together to give the most accurate acoustic output.
Sayonara
Hi,
Of sorts.
The key advantage is that unlike virtually all other types of amplifiers the "current drive" amplifer drives the speaker with a signal that is congruent with it's operation. As a result distortion and compression in the Amp/Speaker system is drastically reduced, compared to the usual low output impedance amplifiers.
There is no disadvantage as such, however as such an amplifer will apply NO electrical damping on the voicecoil, thus we no longer can hide the complexities of using mechanically resonant systems in the socalled "damping factor", which is strongly signal dependent and a rather unreliable methode of damping resonances, plus one purchased at the cost of very high distortion and significant dynamic compression in the Amp/Speaker system.
No.
Yup.
Because with "current drive" Qe approaches infinity and if you look at the derivation of Qt from Qe and Qm you fins that with a vaery high value of Qe Qm dominates Qt.
Well, I would not really claim that making Qt equal to Qm as such is desirable in many modern speaker drivers. On the other hand, if Drivers where designed with a sensible Qm build in (like many vintage drivers are) then their behaviour in enclosures would be much more predictable.
As for dipoles, they present no aoustical loading to the driver, resulting usually in a build in resonance that is slightly lower than free air one and equally a lower Qt. Normal Loading tends to raise the system Qt, so a say Qt 0.28 Driver placed into a sealed enclosure of suitable size will give a system Q of 0.7 for a critically damped system.
If we raise the Qt of a Dipole Driver HIGHER than 0.7 this means we have a peak in the response around the drivers resonance, say our Qt 0.28 Driver has a Qm of 1.83. As the signal at the drivers resonance will be reduced by acoustic short circuit action of the dipole the system will have a response that is still attenuated at the resonance, but less so than in the case where the driver is driven by pure current, making the Qt equal to the Qm of 1.83.
If we exclude the dipole specific losses and only consider the Driver placed in a near infinitly large enclosure the example driver (Seas CA25FEY) when driven by "voltage drive" will be 11db down at 30Hz on its midband sensitivity.
So on a open baffle 15" wide we would need to compensate not only the baffle rolloff (this is much lower than normally expected - usually around 0.07 * lambda/bafflewidth is the -6db point) at around 60Hz +3db and 30Hz +9db, but additionally we would have to add a further 8db boost at 30Hz to achieve a flat response that is 3db down at 30Hz.
If we use "current drive" on the driver we see a 4db PEAK at 35Hz and +1db @ 70Hz / +3db @ 30Hz without the dipole loss. Thus with the dipole loss we get around -1.5db @ 60Hz and -6db @ 30Hz. This will still require a little bit of EQ in my view, but very little, yet I'd almost suggest running such a system without EQ, just current feed.
In fact, if you use the Linkwitz folded baffle with a suitable woofer (Qm 2.0 or thereabouts) and current drive you may be able to avoid using ANY EQ. Drivers with a high Qm can have their Qm reduced by placing damping material behind the driver....
Both ideally. The idea is to design a SYSTEM comprised of Amplifers and Speakers that are optimised together to give the most accurate acoustic output.
Sayonara
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.