eBay mono LM1875 kit

yep...you are the expert...explain me ;):D

Hi Chris,

I suggest you put a fan nearby the heatsink when you work with the amplifier in the laboratory. A fan nearby will make the heatsink at least 5 times more efficient. Huge heatsinks are for the final product, in the lab a smaller heatsink and a fan is a much more economical solution.

For the rail voltage: The rail capacitors are discharged by the current to the amplifier. But, the current to the amplifier is not constant but largely dependent on the load current. The load current is sine-wave shaped (1KHz) so the current to the amplifier will have an important 1KHz component as well. This 1KHz discharge of the rail capacitors is what you see.
 
Good morning


yes you are right. its a typo...done with the smartphone...sometimes it do strange things...auto correction...sorry for that.


what about the different caps and phase shift? the difference between the 2 amp baords are just the input cap and the nfb cap + the different rail caps:
amp1_2 10µ input ES nichicon green + 100µF ES as NFB cap 2x1000µ original per rail

amp2 4,7µF wima + 22µF ES muse green as NFB + the 1000µF per rail



chris
 
Hi Chris,

I suggest you put a fan nearby the heatsink when you work with the amplifier in the laboratory. A fan nearby will make the heatsink at least 5 times more efficient. Huge heatsinks are for the final product, in the lab a smaller heatsink and a fan is a much more economical solution.

For the rail voltage: The rail capacitors are discharged by the current to the amplifier. But, the current to the amplifier is not constant but largely dependent on the load current. The load current is sine-wave shaped (1KHz) so the current to the amplifier will have an important 1KHz component as well. This 1KHz discharge of the rail capacitors is what you see.


yes FF i use a fan to help the heasink and the busy LM1875 without no change to get down the heat fast.


about the 1khz ripple at the rail...can i see here the PSRR?
so output signal vs this small ripple = scatterd singal in the "main" ripple


chris
 
The phase-change you observe, at least I see as no problem. Phase-change is observed in any amplifier with capacitors in the signal path. In your case with two slightly different choices of capacitors, you still see the same effect. This phase-change is not a proof of poor quality capacitors.

PSRR is defined as the influence on the amplifier from an independent signal on the supply rail. Say, how much will a 3KHz signal on the supply rail influence the amplifier.
The PSRR becomes obsolete for an intentional signal generated by the amplifier that through the supply line leaves some small modulation of the supply voltage at the same frequency. You cannot see the influence on the amplifier from the small signal on the supply rail because it is of the same frequency as the much larger intentional signal already handled by the amplifier.

There is a situation where the signal from the amplifier influences the supply voltage so much that you have an important effect. That is when the amplifier starts oscillating due to insufficient decoupling of the supply rail. That effect is outside of normal understanding of PSRR.

Do I understand that not even with a fan you can keep the heatsink temperature down?
 
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Hi FF
thanks for answering my nooby questions.


Do I understand that not even with a fan you can keep the heatsink temperature down?
? what do you mean here...?

i had at 4R the power about 5,2 Watt (13Vpp) . as i wrote at the data sheet i ahve a "loss" of 22Watt. so if i want to drive 4R its mandatory a big heatsink and/or a fan for testing. during music play on my kef Q100 (its not 8R it is about 3,2R in low freq) the amp is luke warm

chris
 
Hi FF
thanks for answering my nooby questions.


Do I understand that not even with a fan you can keep the heatsink temperature down?
? what do you mean here...?

i had at 4R the power about 5,2 Watt (13Vpp) . as i wrote at the data sheet i ahve a "loss" of 22Watt. so if i want to drive 4R its mandatory a big heatsink and/or a fan for testing. during music play on my kef Q100 (its not 8R it is about 3,2R in low freq) the amp is luke warm

chris


Hi Chris,
I read your sentence about use of a fan again and got a different and confirmatory meaning this time. Sorry, my misunderstanding - the fan works.
 
Hey people,


if anyone still has issues with a crackling sound just as the volume gets loud enough to rock, FauxFrench told me to put a 10W/fewR resistor in each + lead of the Speakerchassis. Icouldn't do this, because I don't have such resistors. But, to do at least anything which goes in the same direction I took three 82R/5W resistors in parallel and soldered this pack between the Starground and the - lead which splits up 2 inches later and goes up to the Speaker- Chassis.
I have no Idea if this was the tweak to let the crackling completely subside, or maybe the Witch-Doctor I oredered via Amazon directly from Togo as backup solution. This really impressive Guy came with a at least 30inches long JuJu-Bone and he knew his profession well! First, just as warm up and for only 25€ he did a phase correcting-dance. After that he took on the fight with the crackling-demon (400€).
To give at least no chance for the demon to get back in the Speaker cables the Doctor told me to solder the Resistor Thingy right where it is now, while he did a last safety-Dance to keep other Demons out of the cables as long as I was soldering.
What shall I say? Until today the Demon never showed up again and I can drive those really fine sounding speakers to volume levels which made all my neighbours react in an interesting way, sometimes it looks like they're dancing, but they scream, too. I just cannot hear ... .
 

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Hi again

i am back from a heavy illness...:)

i tried the amp with my KEF Q100 (unmod) and i am still happy. if i compare the TPA3250 - FX502spro then the winner is the LM1875 kit. the coax speaker fit very good to this amp. the KEF are at the same place as the venere 1.5 and I have not a + 6 dB hump at the low frequencies - so the kef come more directly and is a "quick" speaker which tend to be hard at higher volume.


i want to have a modification and therefore ....
i read at this thread about the R2 - its the resistor which connected the input not to ground.
Yet another LM1875 amp - review request
comment here by the expert - TOM : R2 is more likely to degrade performance than it is to improve it. I suggest taking the ground reference at the speaker output ground as that's the voltage you care about. You can read my thoughts here: Grounding – Neurochrome
and here the comment by nivald: I also expected that R2 itself is sitting in that schematic to alleviate minor issues and parasitic ground voltages.
schoolies amp writes:
This has a little effect in single (mono) amplifiers setup but in a stereo setup, it helps to reduce circuit circulating in the ground wiring which can degrade sepearation between channles.
the commnts are not 100% clear for me but i give it a try...so i solder a bridge over the R2=10R and restart my test......wow....yes :D this helps to get the amp fast and give me less noise as i "claimed" pages before.
maybe some user of this kit can try this --> bridging of R2 and give us feedback.

chris
 
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Sorry to hear that you have been ill and I hope all is OK now.

If you are talking about the 10R resistor that goes to ground from the 1M, on my original build I linked it out but added it after having the turn off pop issue (didn't help). I can't recall any changes to the sonics but I only listened for overall presentation.

I did buy a ESP PCB and built a ESP Project 72. Certainly smoother and better controlled all round but suffered from a low level buzz. Same sources and power supplies as my LM1875 eBay module and my LM4780 which are super quiet. I tried all my usual tricks (RCA, leads, ground lift signal and power supply, cap changes, chip changes) for a couple of days but in the end pulled apart and it's now a paper weight as I don't want to waste any more time on it. It's an odd one as with 1 channel it seemed fine but as soon as 2 channels were connected the buzz appeared and was really worse when running via an active pre amp.
 
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Hi Rabbitz
yes i am nearly recovered...thanks.


interesting that the ESP72 sound better and thanks for compare. this board is a stereo board. actually i compared my TPA3255 amp the FX502Spro and the green muse LM1875 mock up. maybe the green caps get more hours "burn in" but actually the amp sound very nice -directly powered by my DAC´s. before my illness i tried on 4 nights all 3 amps with different titles and now i did it again 2 times on a 3 hours afternoon. as often as possible i switch to the 1875amp the more i like him :D deeper bass , not so perfect controlled but more "hold pressure", vocals naturally and not so thin.
with KEF Q100 ranking 1875 - TPA3255 ------ f502spro (without extra 440n+10R zobel) ..sounds raspy


don´t miss understand me - i am not an expert/developer but the topic with the low level buzz is maybe exactly what i learned from the other thread. the schoolies amp description:This has a little effect in single (mono) amplifiers setup but in a stereo setup, it helps to reduce circuit circulating in the ground wiring which can degrade sepearation between channles.

can you try to add an 2R, 5R or 10R between SGND and the point where the C3 (ESP 72) is???
chris
 
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Thanks.... the 10R resistor was one of the tricks I tried.

There wasn't a great deal of difference in sonics but it was noticeable and what could help is I was able to use a larger Nichicon ES (C3) on the P72 than I could on the eBay PCB as well as different caps throughout.
 
Hi rabbitz


you wrote in #274
It's an odd one as with 1 channel it seemed fine but as soon as 2 channels were connected the buzz appeared and was really worse when running via an active pre amp.



did you try to change the channels at the preamp? e.g. you tried the L channel first and if you add the R channel then the buzz comes. now try R channel and then L channel.... i should not change something but i have no other idea;)
chris