Firstly, on Tom's site, he also mentions that 35-36v DC is perfectly fine as long as you are driving 8Ω.
Secondly, I would think a regulated supply, such as this one:
High current dual rail regulator kit for power amplifier or bench power supply ! | eBay
would definitely improve performance. If you disagree, please say why.
Here is the one I ordered - mainly because of the DC cap and coil, but I have no specific testing that would indicate it is any better than the one you ordered.
HiFi LM3886TF Mono 68W 4Ω Audio Power Amplifier Board AMP 50W/38W 8Ω Q5B5 | eBay
HiFi LM3886TF Mono 68W 4Ω Audio Power Amplifier Board AMP 50W/38W 8Ω Q5B5 | eBay
Hi Andrew, am I understanding this correctly? I take the power ground from the decoupling caps to the speaker return terminal along with the other power ground connections (zobel etc) and join them there. The connections to signal ground are made at the chip and follow the same path back to the power supply.
This brings me to my next question which is where would be the best place to position the zobel and output inductors? Are they better close to the chip or out at the speaker terminals?
This is the diagram I am looking at, Tom gets a name check in his writings.
A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics
PS The speakers this will be used with are CSS FR125s which appear to be around 6 ohms at best, so I now realise 35 volts would have been pushing it.
This brings me to my next question which is where would be the best place to position the zobel and output inductors? Are they better close to the chip or out at the speaker terminals?
This is the diagram I am looking at, Tom gets a name check in his writings.
A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics
PS The speakers this will be used with are CSS FR125s which appear to be around 6 ohms at best, so I now realise 35 volts would have been pushing it.
That sounds about right, I think this would be a good link to add to your signature 
http://hifisonix.com/wordpress/wp-c...to-wire-up-a-Power-Amplifier-October-2017.pdf
http://hifisonix.com/wordpress/wp-c...to-wire-up-a-Power-Amplifier-October-2017.pdfThis fellow ----http://www.decdun.me.uk/----lost me with this statement:
"I'm no scientist or engineer but I can detect differences in interconnects, even as short as nine inches."
I'd be willing to bet that in a true double-blind test that he (or anyone else) can't hear diddle-squat difference between any two reasonable-quality 9" cables.
"I'm no scientist or engineer but I can detect differences in interconnects, even as short as nine inches."
I'd be willing to bet that in a true double-blind test that he (or anyone else) can't hear diddle-squat difference between any two reasonable-quality 9" cables.
Yes, where the Power Ground meets the speaker Return becomes the Main Audio Ground.
This is the reference voltage for other parts of the amplifier.
I don't understand what you are describing. Have you read Leach's Lo Tim articles? He uses a low impedance connection on the amp PCB to make the reference voltage connection between Signal Return and Speaker Return. That low impedance connection MUST be SHORT and he inserts 10r. I understand from that, that his low impedance means low inductance. I read Lo Tim about 20years before reading D.Joffe's paper on HBRR/HBRL and realised they were addressing the same problem.
As I understand it the amplifier Output Zobel works by making an ultra low impedance connection using a loading resistor Between the output node of the output devices and the lowest impedance at the collectors/drains of the output devices. I have seen few comments on this conjecture and the very few that have commented have not given any explanation of why this could be wrong.
If the Output Zobel is indeed an ultra low impedance route from output node to device collectors/drains, then the Zobel must be buried inside the output stage, not at some more remote location.
The output (damped) inductor does not change the performance of the amplifier. In effect it is not part of the amplifier and thus it does not matter where it is placed. However, the air-core inductor can influence other electrical components and is affected by other ferromagnetic materials. I believe the best place for this output inductor is in the cable between the amplifier PCB and the amplifier output terminals. The damped inductor in this slightly remote location still has the effect of adding the resistor in series with any parasitic capacitances and thus reducing the load phase angle presented back to the amplifier. The downside to this is the possible effect of the cable capacitance in the twisted pair from amp PCB to remote inductor. I have NEVER seen a Designer comment on this.
I agree, 35Vdc might be a bit much for 6ohms speakers.
I have stated repeatedly that the National chipamps are current crippled. Other manufacturers are worse. 7Apk is not enough to drive an 8ohms reactive speaker. 6ohms and 4ohms will be even more demanding.
Tomchr has done an admirable job in getting his composite to work as well as it does. But the performance is down to the enormous gain of the composite and that increases the available feedback to hide the distortions of the chipamp.
Last edited:
A very quick scan to see that this appears to be a re-write of Tomchr's articles, but this item jumped out at me
Is any of that true?
If one were to believe E.Cherry, then his conclusions are completely wrong.
Hi John, I built 2 LM3875 Monoblocks some years ago using the schematic on Dec Dun and the regulated PS. As I only had 30vac tranny's I used those, making the dc output on the limit of the 3875's handling. I have never had any problem with them into a nominal 8Ohms.
Rob.
Having re read it myself I'm not surprised. What can I say, it was late! All my questions are answered and construction is underway
I overlooked the clearly marked signal ground on the diagram when I scribbled it out on an envelope for the pencil layout stage
. Sorry for any confusion caused.
Also thanks for the grounding links I am working my way through it slowly.
Another good source of information that should help you with layout design.
http://www.updatemydynaco.com/documents/GroundingProblemsRev1p4.pdf
http://www.updatemydynaco.com/documents/GroundingProblemsRev1p4.pdf
Tvi,
this paper is about the Zobel fitted inside the speaker crossover.
It is quite different from the amplifier Output Zobel that is fitted in the output stage of the amplifier.
When I first saw a link to this paper a couple of years ago I made this same comment. And a couple of Members joined in to agree.
This paper has nothing to do with amplifier design, ignore it when designing amplifiers.
this paper is about the Zobel fitted inside the speaker crossover.
It is quite different from the amplifier Output Zobel that is fitted in the output stage of the amplifier.
When I first saw a link to this paper a couple of years ago I made this same comment. And a couple of Members joined in to agree.
This paper has nothing to do with amplifier design, ignore it when designing amplifiers.
Hi all,
Apologies, I don't mean to hijack the thread, but I'm in a similar predicament so I didn't think it worth creating a new one for it.
I too have two 25v 225va encapsulated transformers, that are pretty decent quality wise, and would like to use them if possible.
I have 2 sets of speakers that are rated for 6ohms (minimum 4ohms (one 4.3ohms)), and a component box with LM3886, LM4780 ICs and various pcbs, where I had planned to make a gainclone back in 2006 (according to order emails), and yet nothing materialised.
Due the lowish impedance of the speakers, and the current limitations of the chipamps, I wondered how best to use this combination of parts (much like the original poster).
According to the LM3886 datasheet, although it can be used with 4-6ohm speakers, the THD is higher than that of 8ohm speakers, and it is marginally cripped current wise. However, when viewing AN-1992, I can see that a PA100 design THD with 4ohms matches that of an LM3886 8 ohm, with each section of the chip sharing the load.
I can also see that the recommended maximum voltage for a PA100 with 6ohm speakers is +-37V.
The 25v transformers I have (94% efficiency), specify a no-load voltage of 26.6v. Therefore, with MUR860 diodes, and minimal load, I estimate around 36.5v, and a nominal voltage around 34v. The graphs for 6ohm speakers show a +-35v supply, so I assume this is roughly the recommended voltage.
Can someone please confirm if this theory sounds okay, and whether they have any recommendations. The only other option I can think of, whilst being able to use these transformers, is to go regulated (to drop the voltage somewhat), or to use another method (as some have mentioned earlier), to drop it.
Many thanks,
Andy.
Apologies, I don't mean to hijack the thread, but I'm in a similar predicament so I didn't think it worth creating a new one for it.
I too have two 25v 225va encapsulated transformers, that are pretty decent quality wise, and would like to use them if possible.
I have 2 sets of speakers that are rated for 6ohms (minimum 4ohms (one 4.3ohms)), and a component box with LM3886, LM4780 ICs and various pcbs, where I had planned to make a gainclone back in 2006 (according to order emails), and yet nothing materialised.
Due the lowish impedance of the speakers, and the current limitations of the chipamps, I wondered how best to use this combination of parts (much like the original poster).
According to the LM3886 datasheet, although it can be used with 4-6ohm speakers, the THD is higher than that of 8ohm speakers, and it is marginally cripped current wise. However, when viewing AN-1992, I can see that a PA100 design THD with 4ohms matches that of an LM3886 8 ohm, with each section of the chip sharing the load.
I can also see that the recommended maximum voltage for a PA100 with 6ohm speakers is +-37V.
The 25v transformers I have (94% efficiency), specify a no-load voltage of 26.6v. Therefore, with MUR860 diodes, and minimal load, I estimate around 36.5v, and a nominal voltage around 34v. The graphs for 6ohm speakers show a +-35v supply, so I assume this is roughly the recommended voltage.
Can someone please confirm if this theory sounds okay, and whether they have any recommendations. The only other option I can think of, whilst being able to use these transformers, is to go regulated (to drop the voltage somewhat), or to use another method (as some have mentioned earlier), to drop it.
Many thanks,
Andy.
Hi Andy. Feel free to join in, all input is welcome.
I have got the amps done, the inductors are wound and the regulators and a bit of metalwork are ongoing, so it is well underway. There should be some listening impressions of my first regulated amp in the not too distant future all being well. It's ages since I've heard something new, I'm looking forward to it.
John
I have got the amps done, the inductors are wound and the regulators and a bit of metalwork are ongoing, so it is well underway. There should be some listening impressions of my first regulated amp in the not too distant future all being well. It's ages since I've heard something new, I'm looking forward to it.
John
Thanks John,
It looks like I'm in the exact same predicament as you, as it looks like at some point I was going to make the CarlosFM design (I seem to have all the parts for it).
Good luck with yours, and please do let me know how it all sounds, and I'll get cracking on mine.
The reason for my previous questions was that I also seemed to have bought some "Unisonus" LM4780 pcbs from this forum about 10 years ago, so I'm just not sure which one to build, the CarlosFM or a PA100 when I'm going to use 6ohm (4min) speakers.
Normally I'd build both and try them both out, but due to the case design, the whole pcb has to be attached to the heatsink that makes up the back panel of the monoblock cases, so I don't have the option of doing both right now.
Andy
It looks like I'm in the exact same predicament as you, as it looks like at some point I was going to make the CarlosFM design (I seem to have all the parts for it).
Good luck with yours, and please do let me know how it all sounds, and I'll get cracking on mine.
The reason for my previous questions was that I also seemed to have bought some "Unisonus" LM4780 pcbs from this forum about 10 years ago, so I'm just not sure which one to build, the CarlosFM or a PA100 when I'm going to use 6ohm (4min) speakers.
Normally I'd build both and try them both out, but due to the case design, the whole pcb has to be attached to the heatsink that makes up the back panel of the monoblock cases, so I don't have the option of doing both right now.
Andy
Hi Andy.
I haven't heard the LM4780, you could have bother getting them now if you don't have the chips.
I am trying the snubberised Carlosfm psu design at 28 volts but going with more of the amp stability parts that were left off my previous builds.
My first was a chipamp.com LM3875 kit which had nothing in the way of DC blocking on the input, no feedback cap, zobel or suchlike, I was lucky not to kill any speakers. It is still happily playing today with a Pass B1 in front of it though, and has outlived any amp I've had before (and one I've had since but that was my fault).
I've had to scrap the idea of monoblocks for now so it will be dual mono unless I turn something up cheap in the meantime.
I haven't heard the LM4780, you could have bother getting them now if you don't have the chips.
I am trying the snubberised Carlosfm psu design at 28 volts but going with more of the amp stability parts that were left off my previous builds.
My first was a chipamp.com LM3875 kit which had nothing in the way of DC blocking on the input, no feedback cap, zobel or suchlike, I was lucky not to kill any speakers. It is still happily playing today with a Pass B1 in front of it though, and has outlived any amp I've had before (and one I've had since but that was my fault).
I've had to scrap the idea of monoblocks for now so it will be dual mono unless I turn something up cheap in the meantime.
Okay, good luck with the 25 vac transformers. Series pass regulators are simple, but fail in the blow up IC mode unfortunately, all of us who used linear supplies back in the70's remember. Buying the "overvoltage crowbar" accessory was highly recommended in those days, one that blew the fuse when the series pass transistor shorted across.
About the zobel network. In my opinion the position should be as close to the speaker jack as possible . I just added a network to two Allen S-100 organ amps that picked up football scores from the AM radio station 6 miiles away during the first service I powered them up. 20 db down, really annoying. Wrong speaker wire (shielded 16 ga tw) plus no zobel network gets RF interferance. Allen did include a ferrite bead on the speaker hot lead, not good enough. Allen got away with the design by running the speaker output through inductive relay attenuators after the amp, which are quite unreliable due to contact oxidation at this age (37 years). Deleting the relays (and rear antiphonal speakers) got me the above.
Winding 12 turns 22 ga wire around a 10 ohm 3 watt resistor in parallel, and glueing the assembly to a wooden stick screwed right next to the speaker outlet, in series with output, solved the problem. There are 3" of wire from speaker jack to driver board, then another 8" to the output transistors up on the heat sink (on top lf chassis inside a mesh enclosure). This acts as an RF transmit antenna IMHO until the radio is stopped at the outside wall by inductance.
For further experience, the dynakit ST120 I have was subject to something bad until the "TIP mod" of about 197? that put a 1000 ohm resistor series a .1 ceramic cap on the back of the speaker banana jack. See greg Dunn's website about this mod. The inductive speaker coil in these is 11 turns wound around the output capacitor, and not shorting resistor, again about 6" from the output transistors but inside the mesh enclosure. No AM radio from the speaker wires is my experience. When I I started driving that amp with 12' of RCA cable due to a furniture addition, RF interferance happened. Upping the input cap from 33 pf to 68 from center to ring solved that problem.
About the zobel network. In my opinion the position should be as close to the speaker jack as possible . I just added a network to two Allen S-100 organ amps that picked up football scores from the AM radio station 6 miiles away during the first service I powered them up. 20 db down, really annoying. Wrong speaker wire (shielded 16 ga tw) plus no zobel network gets RF interferance. Allen did include a ferrite bead on the speaker hot lead, not good enough. Allen got away with the design by running the speaker output through inductive relay attenuators after the amp, which are quite unreliable due to contact oxidation at this age (37 years). Deleting the relays (and rear antiphonal speakers) got me the above.
Winding 12 turns 22 ga wire around a 10 ohm 3 watt resistor in parallel, and glueing the assembly to a wooden stick screwed right next to the speaker outlet, in series with output, solved the problem. There are 3" of wire from speaker jack to driver board, then another 8" to the output transistors up on the heat sink (on top lf chassis inside a mesh enclosure). This acts as an RF transmit antenna IMHO until the radio is stopped at the outside wall by inductance.
For further experience, the dynakit ST120 I have was subject to something bad until the "TIP mod" of about 197? that put a 1000 ohm resistor series a .1 ceramic cap on the back of the speaker banana jack. See greg Dunn's website about this mod. The inductive speaker coil in these is 11 turns wound around the output capacitor, and not shorting resistor, again about 6" from the output transistors but inside the mesh enclosure. No AM radio from the speaker wires is my experience. When I I started driving that amp with 12' of RCA cable due to a furniture addition, RF interferance happened. Upping the input cap from 33 pf to 68 from center to ring solved that problem.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.