Understanding and redesigning the input stage of LM1875 - Page 2

noddy55 · 23rd February 2012, 12:01 PM

Thanks!

Quote:

If I were you, I'd create a new front panel for it. You can use a nice piece of thin decorative wood or you can use a nice bit of steel or aluminum textured any way you like and then sprayed with clear lacquer.

1.This requires skills and tools. I posses neither of them. I never worked with wood or steel or aluminium or a paint. But, i would like to learn if someone could take a pain to teach me all about this...........................

2.Please, answer my post 7. I need to discuss and clear the doubts about some points in your post 2.

Thanks.

danielwritesbac · 05:09 AM

The K50 resistor values of 180k/10k are simply too high and that is lossy/noisy enough to interfere with sound quality somewhat.
1M/18M is likewise inappropriate resistor values that will cause inferior sound quality. The inappropriate resistor selections are the cause of trouble, not related to a particular gain setting.

Post 7 contains two different questions at the same time, like apples and oranges in the same basket.
So, here is the second part. . .

LM1875 can use gain from 10 to 44 without any sound quality problems whatsoever.
So, how to you set your gain? Start out orthodox and see if your source can push the amplifier to BARELY begin clipping without also causing the source to strain. If a source cannot push the amplifier or if it strains to do so then turn up your power amp gain.
Step 1: Build amplifier.

noddy55 · 24th February 2012, 06:46 PM

OK,

Thanks Daniel.

I will revert you back now when i will really start building it. Offcourse in the same thread.

But, i really don't know when i will do this.

Bye till then.

skullgoof · 15th April 2012, 06:27 PM

hey im new to the forum but ive been reading some about the LM1875 now and i going to use it in my amp.

couz its for a stereo theres gona be 2 of them with exact the same schema
so ive been reading this post and i modified te K50 kit as you would recommend.

i hope its any good so i can build it.

already thanks for the help.

AndrewT · 07:43 PM

Just Bumped into this Thread.
Post14:
The four ground symbols on the left are all Signal Ground (SG). Connect them all together.
The six ground symbols to the right (excluding X4, Speaker Return) are all Power Ground (PG). Connect them all together.
Do not bring Speaker Return (SR) onto the PCB.
Take SG. & PG. & SR. to the Main Audio Ground.

Rutger · 5th July 2012, 12:58 PM

I hope you don't mind using an old topic for a question about the LM1875?

I use a LM1875 poweramp module that has a 1M resistor from input to ground, just like in the datasheet. I understand why it is there, but I have a hard time figuring out how to match the inputimpedance of this module with my preamp outputimpedance.

From what I understand, the input impedance of the LM1875 is set by the 22K resistor right before the chip. Am I right by saying that the actual outputimpedance of my preamp is Zout ll 1M ? And does that mean that the Zout needs to be around 2k to get a good match (10x smaller than Zin of the poweramp)?

Thanks!

noddy55 · 19th August 2012, 10:09 PM

Hi Skullgoof,
Please, change the values of the component as suggested by daniel before bying any components. And do have some space for experimentation with different values in order to get the exact sound that meets your taste and preference, obviously inside the range values only. Thanks.

danielwritesbac · 19th August 2012, 10:40 PM

Quote:

Originally Posted by Rutger

I hope you don't mind using an old topic for a question about the LM1875?

I use a LM1875 poweramp module that has a 1M resistor from input to ground, just like in the datasheet. I understand why it is there, but I have a hard time figuring out how to match the inputimpedance of this module with my preamp outputimpedance.

From what I understand, the input impedance of the LM1875 is set by the 22K resistor right before the chip. Am I right by saying that the actual outputimpedance of my preamp is Zout ll 1M ? And does that mean that the Zout needs to be around 2k to get a good match (10x smaller than Zin of the poweramp)?

Thanks!

There's no need to look at the 1M which is a noise filter that usually does nothing.

If you have a solid state preamp, all you have to look at are resistors that are in series with the preamp audio output. That will tell you more about output impedance of your preamp.

The power amp can easily compensate for a variety of input loads, since that "22K resistor right before the chip" can be replaced with a 5.6K to 56K variable resistor. You'd just solder a 5.6K resistor to the center pin of a 50K pot and use resistor and 1 of the outboard pins of the pot so that the range of travel is 5.6K+0 to 5.6K+50K for this variable dial. When you find the best sounding resistor value, simply replace the trimmer with an ordinary resistor of approximately that same value.

Line level preamps are probably not useful with LM1875 since it is 27 watts yet able for gain more than 45x, so even a little digiplayer or iphone can push that chip well into overdrive. I wonder what the preamp is for? Maybe a microphone, tape or turntable preamp?

Rutger · 21st August 2012, 08:46 AM

Thanks Daniel,

It is a guitarpreamp I build myself, I use the LM1875 as a poweramp and all this goes into a guitarcombo with a nice speaker.

The guitarpreamp is a copy of a marshall solidstate preamp, which normally has a 1M mastervolume at the output (followed by a FET poweramp). But that doesn't match the input impedance of the LM1875 module, so I try to figure out what's the best value for the mastervolume.

godfrey · 21st August 2012, 09:58 AM

Hi Rutger

referring to the circuit diagram in post 1, I would change R3 from 22K to 180K so that it matches R5. This will also be an easier load for your preamp to drive. Having done that, C1 can be reduced, say to 0.1uF plastic film. R1 and R2 can be left out, they don't do anything useful.

The mastervolume potentiometer should probably be a lower value, say 10K to 100K.

24th February 2012, 04:58 AM	#12
danielwritesbac diyAudio Member Join Date: Nov 2007	The K50 resistor values of 180k/10k are simply too high and that is lossy/noisy enough to interfere with sound quality somewhat. 1M/18M is likewise inappropriate resistor values that will cause inferior sound quality. The inappropriate resistor selections are the cause of trouble, not related to a particular gain setting. Post 7 contains two different questions at the same time, like apples and oranges in the same basket. So, here is the second part. . . LM1875 can use gain from 10 to 44 without any sound quality problems whatsoever. So, how to you set your gain? Start out orthodox and see if your source can push the amplifier to BARELY begin clipping without also causing the source to strain. If a source cannot push the amplifier or if it strains to do so then turn up your power amp gain. Step 1: Build amplifier. Last edited by danielwritesbac; 24th February 2012 at 05:09 AM.

15th April 2012, 07:41 PM	#15
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Just Bumped into this Thread. Post14: The four ground symbols on the left are all Signal Ground (SG). Connect them all together. The six ground symbols to the right (excluding X4, Speaker Return) are all Power Ground (PG). Connect them all together. Do not bring Speaker Return (SR) onto the PCB. Take SG. & PG. & SR. to the Main Audio Ground. Last edited by AndrewT; 15th April 2012 at 07:43 PM.

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24th February 2012, 06:46 PM	#13
noddy55 diyAudio Member Join Date: Feb 2012	OK, Thanks Daniel. I will revert you back now when i will really start building it. Offcourse in the same thread. But, i really don't know when i will do this. Bye till then.

15th April 2012, 06:27 PM	#14
skullgoof diyAudio Member Join Date: Feb 2012	hey im new to the forum but ive been reading some about the LM1875 now and i going to use it in my amp. couz its for a stereo theres gona be 2 of them with exact the same schema so ive been reading this post and i modified te K50 kit as you would recommend. i hope its any good so i can build it. already thanks for the help.

5th July 2012, 12:58 PM	#16
Rutger diyAudio Member Join Date: Oct 2009 Location: The Hague	I hope you don't mind using an old topic for a question about the LM1875? I use a LM1875 poweramp module that has a 1M resistor from input to ground, just like in the datasheet. I understand why it is there, but I have a hard time figuring out how to match the inputimpedance of this module with my preamp outputimpedance. From what I understand, the input impedance of the LM1875 is set by the 22K resistor right before the chip. Am I right by saying that the actual outputimpedance of my preamp is Zout ll 1M ? And does that mean that the Zout needs to be around 2k to get a good match (10x smaller than Zin of the poweramp)? Thanks!

19th August 2012, 10:09 PM	#17
noddy55 diyAudio Member Join Date: Feb 2012	Hi Skullgoof, Please, change the values of the component as suggested by daniel before bying any components. And do have some space for experimentation with different values in order to get the exact sound that meets your taste and preference, obviously inside the range values only. Thanks.

21st August 2012, 08:46 AM	#19
Rutger diyAudio Member Join Date: Oct 2009 Location: The Hague	Thanks Daniel, It is a guitarpreamp I build myself, I use the LM1875 as a poweramp and all this goes into a guitarcombo with a nice speaker. The guitarpreamp is a copy of a marshall solidstate preamp, which normally has a 1M mastervolume at the output (followed by a FET poweramp). But that doesn't match the input impedance of the LM1875 module, so I try to figure out what's the best value for the mastervolume.

21st August 2012, 09:58 AM	#20
godfrey diyAudio Member Join Date: Nov 2009 Location: Cape Town	Hi Rutger referring to the circuit diagram in post 1, I would change R3 from 22K to 180K so that it matches R5. This will also be an easier load for your preamp to drive. Having done that, C1 can be reduced, say to 0.1uF plastic film. R1 and R2 can be left out, they don't do anything useful. The mastervolume potentiometer should probably be a lower value, say 10K to 100K.