... the "old" amp is history, the parts are sitting now here:
I will finish it as breadboard, before I get the new chassis and finish the box.
Note, that some parts (for example the right coupling cap) are already touched with the soldering iron, in the old setup
I think, I must investigate in the correct value for Rc, as I measured just -0.4V grid bias (thanks Upupa Epops, you got me on this focus!).
Franz
An externally hosted image should be here but it was not working when we last tested it.
I will finish it as breadboard, before I get the new chassis and finish the box.
Note, that some parts (for example the right coupling cap) are already touched with the soldering iron, in the old setup
I think, I must investigate in the correct value for Rc, as I measured just -0.4V grid bias (thanks Upupa Epops, you got me on this focus!).
Franz
I see no problem, as the really don't get hot, not even warm!
Back to the switch on delay:
It is an absolute must for me, not feeling myself perfectionist.
Imagine what happens with an short power break! When you use speakers as I do, about 15W, fullrange, then you will definitively burn the coils in such a moment!
It is not just for comfort.
And do you know Murphy? Murphy's law?
Franz
Here again, my simple power on delay with an explanation (of course, it would be more easy to use LM3876 and LM3886 with mute function).
The NE555 acts as timer. The time delay is regulated by the trimmer and the cap. The bigger the cap and the poti, the longer the delay.
The "Relais" is the relay, the coil of it. Maybe, instead of a relay you could use pin 3 and 4 to control the mute function of LM3876 or LM3886?
You could switch whatever you want with this circuit. Input, Output, PSU and so on. But be carefull to switch voltages at the primary side of your tranny!
Franz
P.S.
The circuit is out of an 555 datasheet. I am not sure, but when you increase the value of the cap, then imho a diode is needed for safe operation and long life of the 555.
An externally hosted image should be here but it was not working when we last tested it.
The NE555 acts as timer. The time delay is regulated by the trimmer and the cap. The bigger the cap and the poti, the longer the delay.
The "Relais" is the relay, the coil of it. Maybe, instead of a relay you could use pin 3 and 4 to control the mute function of LM3876 or LM3886?
You could switch whatever you want with this circuit. Input, Output, PSU and so on. But be carefull to switch voltages at the primary side of your tranny!
Franz
P.S.
The circuit is out of an 555 datasheet. I am not sure, but when you increase the value of the cap, then imho a diode is needed for safe operation and long life of the 555.
JojoD818 said:
All you need here is a little reed relay. Nice solution Franz. Inexpensive, compact, easy to implement.
Sheldon
Do pins 3 and 4 behave as a NO switch? So what you say is put it as the switch the LM3886/4780 datasheet suggests for mute, after the resistor?
Carlos
I think so, when pin 3 and 4 are able to drive a relay.
But really: read the datasheet! It is two or three months since I consulted it, just to make this circuit...
Franz
It works good, with grounding of the negative input pins of the LM3875, even with t-network feedback, at my VBIGC:
at startup there is about 100mV bias, reducing to operational value after opening the relais.
And: absolutely no more click or pop during startup (and power off).
It seems to be a good solution, for the LM3875.
Franz
at startup there is about 100mV bias, reducing to operational value after opening the relais.
And: absolutely no more click or pop during startup (and power off).
It seems to be a good solution, for the LM3875.
Franz
Here my breadboard design: absolutely dangerous for kids and pets, I know!
But playing great:
The heathsink for the chips is to small, before, attached to the back of an aluchassis, the same heathsink was O.K.!
Franz
BTW
I think a regulated PSU for the LM chips is primarly interesting, because of its ripple rejection. The regulation itself is not so great: beginning with 2V/8Ohm output, the Vcc decreases, in this amp (just 160VA tranny)
But playing great:
An externally hosted image should be here but it was not working when we last tested it.
The heathsink for the chips is to small, before, attached to the back of an aluchassis, the same heathsink was O.K.!
Franz
BTW
I think a regulated PSU for the LM chips is primarly interesting, because of its ripple rejection. The regulation itself is not so great: beginning with 2V/8Ohm output, the Vcc decreases, in this amp (just 160VA tranny)
Franz G said:
Franz, do you mean a diode in series (or parallel) with the cap to ground (or do I have that wrong?)
And "Input negative pins": Do you mean both power and signal input pins?
PS: nice job with the pcb! 🙂
Sorry, I dont exactly remember about the diode. My circuit works perfect without for several weeks now.
I have a relay, that shortens the negativ signal inputs (just after the coupling cap) of my inverted amp to ground for the first 20 seconds after power on. After this delay, this shortcircuit is simply opened, so I hope not to influence the sound.
It works really without plops or something like that.
I suggest, to make it perfectly, you use an adapted resistor between signal minus and ground, to start the amp.
Franz
BTW
If someone whants to make the same pcb (or to improve my design): just ask me. I'll send the Coral Draw file for free, but I will make some little corrections after the first assembling.
I cannot resists, to tell you this experience, I actually do:
My rebuilt amp above, has now some "enhancements":
- an Alps pot, instead of an cheap mixer replacement part
- Elna Silmic Caps nearby the LM3875
Is there a remarkable difference (O.K. after two days "rebuild break")?
No no no and absolutely no! Not for my ears, not at the scope.
Not comparable to the "evolution" to the t-network feedback 😎 😎 😎
My conclusion: it is more worth, first to build a perfect circuit as possible, and then choose the "best" parts.
But, of course, always use the right parts at the right place, as most of us already are knowing.
Franz
My rebuilt amp above, has now some "enhancements":
- an Alps pot, instead of an cheap mixer replacement part
- Elna Silmic Caps nearby the LM3875
Is there a remarkable difference (O.K. after two days "rebuild break")?
No no no and absolutely no! Not for my ears, not at the scope.
Not comparable to the "evolution" to the t-network feedback 😎 😎 😎
My conclusion: it is more worth, first to build a perfect circuit as possible, and then choose the "best" parts.
But, of course, always use the right parts at the right place, as most of us already are knowing.
Franz
When you want to avoid bias during startup, when the negative input is grounded, then you use the same value, as you have between the positive input and ground (inverted topology).
This means, the relay does at startup not ground the input directly, it connects it via this resistor to ground.
Franz
Franz G said:
Franz, absolutely gorgeous work!
Are you planning to make PCBs (or to share your layout)?
Cheers
Andrea
My board is not professional enough, to be produced. After some modifications, I am willing to share the layout.
Maybe another one can use it as a base to desing the layout more professional?
Franz
Every time that I come to this thread, Franz has produced something new. My theory is that he must be an octopus with eight arms to get so much work done! 😀
Franz - do you spend all your time designing and building hi-fi? (and writing Excel calculators 😉 )
Franz - do you spend all your time designing and building hi-fi? (and writing Excel calculators 😉 )
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