Recently completed a chipamp.com LM1875 kit. All standard/recommended parts, including the Avel PT.
I am driving a pair of small full range speakers of 91db efficiency. Source is an integrated HD sound card on my PC. There's a little hiss when the sound card output and chipamp's volume pot are maxed, but far less than I'd actually expect from such a noisy source. The chipamp itself is dead quiet.
The sound is very impressive, but there is far less gain than I expected. With the program material I've been listening to, (vocal/string jazz, for the most part) I haven't been able to produce a single complaint from my wife, much less the amp/speakers. They just don't play very loud. Is this normal? If not, where should I start looking for the weak spot?
Also, the amp has an annoying habit of blowing the 2A slow blow fuses on start up. I'm guessing that it;s the PS caps that are sucking enough current to blow the fuse. Again, is this normal? Should I be using a different value of fuse, or should I consider something like a CL90 inrush current limiter?
I am driving a pair of small full range speakers of 91db efficiency. Source is an integrated HD sound card on my PC. There's a little hiss when the sound card output and chipamp's volume pot are maxed, but far less than I'd actually expect from such a noisy source. The chipamp itself is dead quiet.
The sound is very impressive, but there is far less gain than I expected. With the program material I've been listening to, (vocal/string jazz, for the most part) I haven't been able to produce a single complaint from my wife, much less the amp/speakers. They just don't play very loud. Is this normal? If not, where should I start looking for the weak spot?
Also, the amp has an annoying habit of blowing the 2A slow blow fuses on start up. I'm guessing that it;s the PS caps that are sucking enough current to blow the fuse. Again, is this normal? Should I be using a different value of fuse, or should I consider something like a CL90 inrush current limiter?
First... If you have a fuse on primary winding and the transformer is a big toroidal I would say this is normal because toroidals have big inrush current. My 500W toroid was blowing me 6amp fuses and I had to build current limiter.
I suspect that the amp is powerfull enough to drive your loudspeakers.
So...if the amp is not amplifaying the input signal enough I would check the resistor from output to negative input and resistor from negative input to ground. I guess you need more gain. Check LM1875 datasheet how to calculate the gain.
I suspect that the amp is powerfull enough to drive your loudspeakers.
So...if the amp is not amplifaying the input signal enough I would check the resistor from output to negative input and resistor from negative input to ground. I guess you need more gain. Check LM1875 datasheet how to calculate the gain.
Andrew,
It is an Avel 160VA 18V+18V torroidal.
Igla,
Thank you for your input. If understand you (and the National data sheet and Daniel's handy spreadsheet) correctly, my NFB transistor should be a higher value. I'm going to start with 33k and see where that takes me.
Thanks again, to both of you.
It is an Avel 160VA 18V+18V torroidal.
Igla,
Thank you for your input. If understand you (and the National data sheet and Daniel's handy spreadsheet) correctly, my NFB transistor should be a higher value. I'm going to start with 33k and see where that takes me.
Thanks again, to both of you.
Hi,
a 160VA transformer running on 110Vac will require T4A or T5A fuse.
Less may allow starting but could suffer from nuisance blowing with no apparent faults.
Now that T4A fuse could pass 5 or 6amps, in a continuous fault condition, for hours before rupturing.
How hot is the equipment going to get consuming 500 or 600W?
a 160VA transformer running on 110Vac will require T4A or T5A fuse.
Less may allow starting but could suffer from nuisance blowing with no apparent faults.
Now that T4A fuse could pass 5 or 6amps, in a continuous fault condition, for hours before rupturing.
How hot is the equipment going to get consuming 500 or 600W?
AndrewT said:
Actually, a 3A is working just fine. I'll try a 2.5A as soon as I get some. I suspect those will work as well. The 2.0 did not fail consistently, so I think that was right on the edge.
Also, I've replaced the NFB resistor with a 33k piece. More gain, as expected, but a rather dramatic increase in the authority and precision in the bass, even with my smallish full-range speakers. Thanks again, Igla, for the suggestion.
I guess you have 2.2uF Panasonic input cap and 22uF Panasonic cap from that 1kohm resistor to ground.
If your cource PC card has output cap you could lose this 2.2uF Panasonic input cap.
If you feel experimenting try to bypass this input cap. Then check DC voltage offset at the output of the amp - with disconected loudspeakers of course. If the offset voltage is in the range 0 to 40mV or so, you have nothing to be scared. Removing of the input cap will give you most improvement soundvise.
I had 70mV of DC offset on my TDA2030 amp with no fear to loudspeakers. When I removed input cap I couldn't believe that I've missed all those details.
Then try to bypass that 22uF Panasonic cap and check DC offset at the output. If offset is in the reasonable limits connect the loudspeakers and listen criticaly...this can give you further improvements to the quality of the sound.
This will make your gainclone sound more natural, more clear and more detailed.
You could check again threads of minimal non-inverted Peter Daniel gainclone with no input cap and no cap from NFB resistor (1k in your case) to ground.
But again...please, check DC voltage offset every time you change something before you connect loudspeakers.
Remark:
This is only a suggestion. If you feel unconfortable by those changes or you are totaly satisfied with the sound of your amp... then just leave it that way.
If your cource PC card has output cap you could lose this 2.2uF Panasonic input cap.
If you feel experimenting try to bypass this input cap. Then check DC voltage offset at the output of the amp - with disconected loudspeakers of course. If the offset voltage is in the range 0 to 40mV or so, you have nothing to be scared. Removing of the input cap will give you most improvement soundvise.
I had 70mV of DC offset on my TDA2030 amp with no fear to loudspeakers. When I removed input cap I couldn't believe that I've missed all those details.
Then try to bypass that 22uF Panasonic cap and check DC offset at the output. If offset is in the reasonable limits connect the loudspeakers and listen criticaly...this can give you further improvements to the quality of the sound.
This will make your gainclone sound more natural, more clear and more detailed.
You could check again threads of minimal non-inverted Peter Daniel gainclone with no input cap and no cap from NFB resistor (1k in your case) to ground.
But again...please, check DC voltage offset every time you change something before you connect loudspeakers.
Remark:
This is only a suggestion. If you feel unconfortable by those changes or you are totaly satisfied with the sound of your amp... then just leave it that way.
A fuse is a fine piece of wire that heats, melts and ruptures on overload.
When it is not overloaded it goes through the heat cycle every time it passes any current.
The worse the heat cycling, the shorter the fatigue life of the fine wire.
Eventually it breaks.
The closer to melting that it is taken the shorter the fuse life.
A fuse for motors and transformers should be ~ Watts / Volts * three Amperes.
ie a 160VA transformer on 110/120Vac requires a fuse rated approximately @ T4.4A
Use a T4A or T5A. The T5A will last longer than the T4A (the T4A may last 10years, whereas the T5A may last 100yrs).
A T3.1A will have worse heat cycling, but may survive one switch on or one hundred switch ons.
A T2.5A may never manage more than a dozen switch ons.
A T1.6A may never switch on at all.
Now look at the normal operation of that 160VA example.
It ticks over @ ~10 to 20mA when quiet.
It draws 100 to 200mA when playing louder than background music.
It might draw 300 to 500mA when playing far too loud.
What current setting would you set the fuse at?
I would like to fuse it at just over 500mA. That way I can play at any volume and only when there is abuse does the fuse blow.
But the transformer will not start on a T500mA fuse. The transformer must have a soft start circuit. Most omit the soft start on small transformers because it saves money. Some eejits claim that small transformers do not need soft starts.
When it is not overloaded it goes through the heat cycle every time it passes any current.
The worse the heat cycling, the shorter the fatigue life of the fine wire.
Eventually it breaks.
The closer to melting that it is taken the shorter the fuse life.
A fuse for motors and transformers should be ~ Watts / Volts * three Amperes.
ie a 160VA transformer on 110/120Vac requires a fuse rated approximately @ T4.4A
Use a T4A or T5A. The T5A will last longer than the T4A (the T4A may last 10years, whereas the T5A may last 100yrs).
A T3.1A will have worse heat cycling, but may survive one switch on or one hundred switch ons.
A T2.5A may never manage more than a dozen switch ons.
A T1.6A may never switch on at all.
Now look at the normal operation of that 160VA example.
It ticks over @ ~10 to 20mA when quiet.
It draws 100 to 200mA when playing louder than background music.
It might draw 300 to 500mA when playing far too loud.
What current setting would you set the fuse at?
I would like to fuse it at just over 500mA. That way I can play at any volume and only when there is abuse does the fuse blow.
But the transformer will not start on a T500mA fuse. The transformer must have a soft start circuit. Most omit the soft start on small transformers because it saves money. Some eejits claim that small transformers do not need soft starts.
A way around this dilemma is to split the short-circuit and overload protection, as has been good practice in automation for many decades. That means to use the short-circuit protection on the primaries, and the overload protection on the secondaries.
As a consequence a primary fuse in the equipment often becomes unnecessary, because the fuse or circuit-breaker in the distribution box can provide the short-circuit protection. Note that I wrote 'often' not 'always'. You have to know the math and the regulations to decide, if you can drop the primary fuse, so for most DIYers it will be better to use the primary fuse anyhow. It just isn't as critical to fine adjust anymore. The determining factor is the smallest wire gauge between wall outlet and transformer. The primary soft-start will then only make the component distributor and the electricity supplier happy and may therefore omitted.
On the secondaries that leads to the use of slow fuses before the rectifiers adjusted to the transformer's nominal output current. They come in addition to the fast fuses some people use in the rails to protect the electronic components and/or speakers.
As a consequence a primary fuse in the equipment often becomes unnecessary, because the fuse or circuit-breaker in the distribution box can provide the short-circuit protection. Note that I wrote 'often' not 'always'. You have to know the math and the regulations to decide, if you can drop the primary fuse, so for most DIYers it will be better to use the primary fuse anyhow. It just isn't as critical to fine adjust anymore. The determining factor is the smallest wire gauge between wall outlet and transformer. The primary soft-start will then only make the component distributor and the electricity supplier happy and may therefore omitted.
On the secondaries that leads to the use of slow fuses before the rectifiers adjusted to the transformer's nominal output current. They come in addition to the fast fuses some people use in the rails to protect the electronic components and/or speakers.
Not really. I have been working in the automation business for more than 25 years now and that is how it is done. And that is not my personal preference. Every company I have worked for until now does it like that.AndrewT said:
We use transformers in the kVA region and never use soft-start circuits.
I should be. See the cut-out from my setting's screenshot below.AndrewT said:
I have posted in the troubleshooting section. Hopefully that can be solved.
Just an observation pacific.......
Not really. I have been working in the automation business for more than 25 years now and that is how it is done. And that is not my personal preference. Every company I have worked for until now does it like that.
And your tag line:
If you've always done it like that, then it's probably wrong. (Henry Ford)
Not really. I have been working in the automation business for more than 25 years now and that is how it is done. And that is not my personal preference. Every company I have worked for until now does it like that.
And your tag line:
If you've always done it like that, then it's probably wrong. (Henry Ford)
Well spotted. 
Still it makes sense to do it so. If you provide adequate overload protection on the secondaries, it is impossible to get overload on the primaries. Consequently the primaries don't need anything else, but short-circuit (= cable) protection to avoid fire.
Explanation of my tag line: Mr. Ford one day spotted a worker in one of his factories, doing some work very inefficiently. When he asked him, why he did it that way, the worker said: "Because we have always done it like that." My tag line was Mr. Ford's answer to that, because the worker obviously did not know, why he did things in a certain way, had never bothered to find out and could therefore not prove Mr. Ford wrong.
Still it makes sense to do it so. If you provide adequate overload protection on the secondaries, it is impossible to get overload on the primaries. Consequently the primaries don't need anything else, but short-circuit (= cable) protection to avoid fire.
Explanation of my tag line: Mr. Ford one day spotted a worker in one of his factories, doing some work very inefficiently. When he asked him, why he did it that way, the worker said: "Because we have always done it like that." My tag line was Mr. Ford's answer to that, because the worker obviously did not know, why he did things in a certain way, had never bothered to find out and could therefore not prove Mr. Ford wrong.
I don't know what is normal or acceptable to your automotive experiences.pacificblue said:
I do know that the advice, you offer a qualified recommendation for, is not considered safe on UK mains. It would almost certainly be locked off if an electrician discovered the modifications you propose.
very much so. The fuse must be smaller than the rating of the lowest rated unfused cable that follows.
Do you want to reconsider your advice?
Industrial automation is quite different from automotive electrics and electronics.AndrewT said:
There seem to be a few misunderstandings here.AndrewT said:
1) The UK is an IEC member, just like Germany. That means the same rules apply. Some rules may have a specific interpretation that differs from country to country, but in general they are the same. Any device that conforms to those rules must be considered safe, whether in the UK or in any other IEC member country.
2) I am not proposing any modifications. What I describe is a different approach that works well in different surroundings and could be adapted to fit here.
3) The design I describe must comply with all safety regulations, just like any other design, and it does. The designer usually must be able to prove that. That forces him to make calculations and a safety concept and to be able to present both, when necessary. It is not, as if we say: "Hey, let's save some money and drop the fuses just for the fun of it." If something goes wrong, the designer will be held responible, so something delivered without a fuse of its own must be safe without it. E. g. because the wire gauge is sufficient to be used with the fuse/circuit breaker in the distribution box.
4) The absence of a fuse does not make a device unsafe. Many appliances rely on the distribution box for safety. E. g. a table lamp, a hair dryer, an electric iron, etc. Don't tell me you can't cut their cables and produce a short on the long end of the cable. Still a table lamp with 0,5 mm² wire gauge on a 16 A fused wall outlet is considered safe. What makes you think the same cannot be achieved for a transformer?
I have learned to do it like that in my formation days. I repeated the stuff, when I studied to be an electrical engineer. I have seen it done many times, by many people from many different companies and in different countries. And I have done the math often enough myself, when I designed control systems. You will not convince me that an entire section of electrical engineering is wrong and has been wrong for decades.AndrewT said:
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