Thanks for helping,
from the ratio you mentioned (1-3 times output power) I conclude that people usually oversize transformer's power, isn't it ? Even with a 2X18v transformer and 20-25 output power application, 80 va would be sufficient according to you. I mean sufficient for a normal living room purpose...
Does anyone can help me with potentiometer's value to use : 25, 50,100 kohms ?
from the ratio you mentioned (1-3 times output power) I conclude that people usually oversize transformer's power, isn't it ? Even with a 2X18v transformer and 20-25 output power application, 80 va would be sufficient according to you. I mean sufficient for a normal living room purpose...
Does anyone can help me with potentiometer's value to use : 25, 50,100 kohms ?
Re: Re: To make an 1875 version of the AUDIOSECTOR 3875
Hello, Mr. ChipAmp!
Ah, I've got a sidenote for you, about the DC offset here.
Its for this amp, LM3886 chipamp.com and audiosector too.
R2 is either B20K pot + 22k resistor load. . . or a 10k resistor.
"Use 10k whenever the NFB cap is omitted."
Possible formats:
A). 10k input resistor
B). B20k pot + 22k resistor
C). Cap, 10k input resistor
D). Cap, B20k pot + 22k resistor
E). Any pot, cap, 10k input resistor
F). Buffer, cap, B20k pot + 22k resistor
G). Buffer, 10k input resistor
H). Buffer, any pot, cap, 10k input resistor
and more
Well anyway, I think that's all 10k to the amp.
Which do you prefer?
BrianGT said:
Hello, Mr. ChipAmp!
Ah, I've got a sidenote for you, about the DC offset here.
Its for this amp, LM3886 chipamp.com and audiosector too.
R2 is either B20K pot + 22k resistor load. . . or a 10k resistor.
"Use 10k whenever the NFB cap is omitted."
Possible formats:
A). 10k input resistor
B). B20k pot + 22k resistor
C). Cap, 10k input resistor
D). Cap, B20k pot + 22k resistor
E). Any pot, cap, 10k input resistor
F). Buffer, cap, B20k pot + 22k resistor
G). Buffer, 10k input resistor
H). Buffer, any pot, cap, 10k input resistor
and more
Well anyway, I think that's all 10k to the amp.
Which do you prefer?
Yes, people tend to do that for various reasons. One is that the transformer is not constantly supplying the power, but only recharging the PSU capacitors during a very short part of each half cycle. So in effect it is doing nothing most of the time, and working overloaded for the short period that it actually does something. Several sources report that a sonic improvement can be heard, when the transformer is oversized up to three times.androuski said:
Another reason is that a bigger transformer delivers better regulation, i. e. less voltage difference between load and no load.
androuski said:
With your setup 80 VA would be sufficient for the amplifier. The point with transformers of less than 100 VA is that they tend to have extremely bad regulation. That is, why AndrewT's recommendation to use more than 100 VA is good. The speakers you want to use are likely to reveal the difference.AndrewT said:
AndrewT said:
What a shocking post.
The datasheet is a bit incongruent about this. While it states 30 W are possible, there is a diagram on page 3 showing the need to use overvoltage for that (~ ±32,5 V under load, which is >±36 V @ no load). On top of that it requires a 1 K/W heatsink with no insulator, meaning the heatsink must be hidden inside a case. At the same time a maximum ambient temperature of 25 °C is allowed, which means the heatsink must not be inside a case.
And you must of course be sure to never use this amplifier for speakers with less impedance at more than conversational SPL, if you go for 30 W. Or choose a different IC for the task.
no,
you misunderstand.
Use an 18Vac transformer.
maximum voltage from the PSU is
1.06Times for mains tolerance.
1.08Times for transformer regulation.
1.414Times to convert to peak voltage.
subtract 0.7V for rectifier drop.
Vmax~=28.4Vdc. This is less than the +-30Vdc specified as absolute max.
look at the power graphs.
PSU 28V gives about 30W into 8ohms.
Now look at his speakers.
95dB/W/m. if we allow a maximum average output from the amp of 300mW then the average volume will be 90dB/m that is loud for a pair of speakers even at a normal listening distance.
The 30W amp has a reserve of 20dB for transient peaks above the 300mW average level.
It is more likely that he will listen at <100mW most of the time and now has 30dB of headroom for transient peaks. That's what clean power is all about.
Let's look at normal voltage from that 18Vac transformer.
1.08Times for transformer regulation.
1.414Times to convert to peak voltage.
subtract 0.7V for rectifier drop.
subtract ~1V for quiescent loading.
Normal PSU voltage~+-25.8Vdc.
The maximum amplifier output will be ~26W, but he will never use this power. He might use the peak voltage ability on transients that this power will allow, before the smoothing caps discharge. i.e. he has ~20.3Vpk available for transients.
His amp will run relatively cool because he will be using ~100mW on average.
I hope he's not a headbanger that turns it up until only distortion and clipped peaks makes his ears bleed.
Does this make any sense?
you misunderstand.
Use an 18Vac transformer.
maximum voltage from the PSU is
1.06Times for mains tolerance.
1.08Times for transformer regulation.
1.414Times to convert to peak voltage.
subtract 0.7V for rectifier drop.
Vmax~=28.4Vdc. This is less than the +-30Vdc specified as absolute max.
look at the power graphs.
PSU 28V gives about 30W into 8ohms.
Now look at his speakers.
95dB/W/m. if we allow a maximum average output from the amp of 300mW then the average volume will be 90dB/m that is loud for a pair of speakers even at a normal listening distance.
The 30W amp has a reserve of 20dB for transient peaks above the 300mW average level.
It is more likely that he will listen at <100mW most of the time and now has 30dB of headroom for transient peaks. That's what clean power is all about.
Let's look at normal voltage from that 18Vac transformer.
1.08Times for transformer regulation.
1.414Times to convert to peak voltage.
subtract 0.7V for rectifier drop.
subtract ~1V for quiescent loading.
Normal PSU voltage~+-25.8Vdc.
The maximum amplifier output will be ~26W, but he will never use this power. He might use the peak voltage ability on transients that this power will allow, before the smoothing caps discharge. i.e. he has ~20.3Vpk available for transients.
His amp will run relatively cool because he will be using ~100mW on average.
I hope he's not a headbanger that turns it up until only distortion and clipped peaks makes his ears bleed.
Does this make any sense?
Actually 18V and > than 100va is the BrianGT's recommendation
I just wanted to know if I would lack something with a downsized PSU...
I don't usually turn the pot until my ears blow up
Could you help with the pot's value required ? 20, 50, 100 k ? Source is standard cd player and standard phono preamp (nad), without tube...
Thanks
I just wanted to know if I would lack something with a downsized PSU...
I don't usually turn the pot until my ears blow up
Could you help with the pot's value required ? 20, 50, 100 k ? Source is standard cd player and standard phono preamp (nad), without tube...
Thanks
Well I'm not as experienced as other posters, but, I believe the 20k pot is the better choice here. And as for lesser power supply, since mine is plugged into a Variac I can dial it down, +12,0,-12VDC still works fine, doesn't change the volume noticeably, chip runs cool and doesn't clip with an 8ohm speaker when turned up all the way. It's louder than I need for listening most of the time. Your smileage of course, may vary.
Oops. I see that I read the diagram the other way round. You are right there.AndrewT said:
Although I would not call an amplifier that reaches 30 W on the conditions that mains is 6 % above nominal, while transformer regulation and rectification factor are assumed with no-load levels at nominal output power a "full 30 W" amplifier.
(18 V -0,7 V) * 1,27 = ~22 V is more like it at nominal. And even with mains 10 % above nominal that is (19,8 V - 0,7 V) * 1,27 = ~24,3 V. A full 20 W amplifier with potential for 25 W at high mains I should estimate from the graph.
Given 30 W output is achievable, thermal shutdown would not allow 30 dB of headroom. The required un-isolated heatsink for 30 W needs to be in a case to prevent touching, but in a case it will hardly be at 25 °C ambient temperature as also required.
I believe that the 120VA to 160VA transformer selection has to do with frequency response, because this amount of amperage is otherwise unnecessary; however, given such a large size, the ringing of the power circuit will be slower, such as either at the bass frequencies or at lower than the audio band.
Ok,
I didn't want to save money at the cost of a lack in sound quality, so I've order an 2x18v 160 va toroïdal transformer...
Next step is soldering and wiring, and I'll be back here quite often again to get some further advices, and I have to say that I appreciate very much the help provided at this first level of my project
I didn't want to save money at the cost of a lack in sound quality, so I've order an 2x18v 160 va toroïdal transformer...
Next step is soldering and wiring, and I'll be back here quite often again to get some further advices, and I have to say that I appreciate very much the help provided at this first level of my project
pacificblue said:
Sorry about that horrid oversimplification on my post (49), but if I tried to write a longer explanation, I think it would have been even worse.
Howabout something a bit more accurate?
Just relate these:
transformer is like a loudspeaker, in that both are a coil around a magnet.
power supply is like a crossover, in that both damp certain frequencies.
The power supply design is a given.
The maker has specified a certain transformer that will match.
Result:
A much different transformer would require a much different power supply.
Supporting a smaller (va) transformer would increase the expense of the power supply.
There are so many options available, but a change is unlikely to improve either the simplicity or cost-effectiveness of the kit.
Sparky OR said:
Newbie alert! In plain English and preferably with a "for Dummies" approach: Once again, how do you insulate the LM1875 from the heatsink while also allowing thermal conduction?
Also any reasonably priced ready made cases available? My hardware/case skills are absolutely pitiful.
Thanks.
Sincerely,
Hany.
With mica or silicone washers. Silicone washers need to be replaced every now and then, therefore mica washers are preferable.haziz said:
What is reasonably priced for you? http://www.hammondmfg.com/scpg.htmhaziz said:
Reasonable price for a cabinet, how about getting a sattelite TV converter from the thrift store and reconfigure it, the metal work on the older ones is a nice size, well executed. For example, here is one from a Drake,
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A little drilling, I changed the auxiliary face plate, but hey, only $5!
A little drilling, I changed the auxiliary face plate, but hey, only $5!
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