Finally made a LM1875 amp

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A missed opportunity

This is a compact layout with some nice features, but I think you missed an opportunity to implement a feature which would improve the performance of this amp. The importance of star grounding in this amp has been discussed many times on this board. I see that the power supply decoupling caps' ground path runs from C3/C4 THROUGH C5/C6. It would have been better to break the path between the C6 pad and the PSU ground pad, then run a separate trace from the PSU ground pad to a point BETWEEN C4 and C6, to the spot underneath the lettering "V+". This would also have served to separate the power supply decoupling ground path from all the other ground paths, providing something closer to the ideal of a true star ground.

Peace,
Tom E
 
To make an 1875 version of the AUDIOSECTOR 3875

Would someone tell me how to configure this little board to use the minimal number of components?

I am having trouble with the optional components and whether they need to have jumpers or just be left out.

I want to use only the following: per AUDIOSECTOR
power supply bypass - BG N's one each for + and -
Feedback resistor to be attached to the chip
Feedback resistor to ground without a capacitor
Input resistor to non-inverting input
Input resistor to ground

Any help offered will be appreciated.

Thanks,

Rick McInnis
 
Retired diyAudio Moderator
Joined 2002
Re: To make an 1875 version of the AUDIOSECTOR 3875

rickmcinnis said:
Would someone tell me how to configure this little board to use the minimal number of components?

I am having trouble with the optional components and whether they need to have jumpers or just be left out.

I want to use only the following: per AUDIOSECTOR
power supply bypass - BG N's one each for + and -
Feedback resistor to be attached to the chip
Feedback resistor to ground without a capacitor
Input resistor to non-inverting input
Input resistor to ground

Any help offered will be appreciated.

Thanks,

Rick McInnis

Rick,

As for the components, you only need to install one short circuit jumper, in the place of R6. Here is a minimized component listing for the amp board:

R1 - not used
R2 - 22k (input to ground resistor)
R3 - 1k (gain resistor , gain = 1 + R4/R
R4 - 22k (nfb resistor - on bottom of pcb)
R5 - not used
R6 - jumper (optional resistor between input ground and output ground)
C1 - not used
C2 - not used
C3 - not used
C4 - BG N
C5 - not used
C6 - BG N
C7 - not used

Note that you will wire your input to the top pad of C1. This avoids having to use a short for C1, and should be a cleaner wiring solution.

Let me know if you have any more questions,

--
Brian
 
Brian
I have 2 or your LM1875 boards up and running fine and I'm happy. I know this seems too simple to be necessary, but do you have a schematic diagram for the amp board itself, even without values. It helps me when I look back at some old project and have forgotten what I did. Thanks. (they sound great!!)
 
Is anyone who has built this kit able to describe the sound quality, even in terms of comparison to a common standard such as a typical commercial amp (not a BestBuy receiver)? Is anyone listening to this amp as part of a quality system (not feeding computer speakers or sourced from an Ipod)?

Peace,
Tom E
 
I have owned and listened to Harmon Kardon Cit I/II, Dyna ST35, tube amps, Marantz stereo SS receivers, I use a 6V6 SE tube (self made), and others. However, it would be hard to convey sound quality to someone else, unless there is something noticeably wrong. I can safely say I hear nothing detrimental in listening to Brian GT kit, and "Kit 50" available from various vendors. My source is an older Sony CD/DVD player, or a Panasonic portable CD player in the garage, and a mid range Marantz CD player in the house. My speakers are TB 4 ohm single drivers in 1/4 wave TQWTL speakers, in the garage, and a Mission 2 way, or an Optimus Lineum Pro LX-10 for testing single channels after they are first built. I'm satisfied, your actual smileage may vary. I listen at moderate levels, don't need huge bass. If I was set up to A/B test it against others with a switch, it would be more informative. In short, give them a try, not much expense involved.
Cheers!
 
Thanks for replying.
My speaker is a diy two way made of a 8ohms tweeter and a 8 ohms mid-bass, so I guess its impedance is around 4/6ohms...
I don't need so much power, as long as these speakers have a sensivity around 92/93db...

The LM datasheet gives distorsion vs power for a rated voltage of 25v ; what is the transformer voltage required to rich 25 after after PSU ?

;)
 
There are several factors in a power supply that change with the current, with temperature, and with the components you use. That makes it difficult to reach a specified voltage exactly. The current is dynamic, temperature depends on current, operating time and ambient temperature, and each component has it own characteristics.

A 2 x 20 V transformer would lead to more or less ±25 V at nominal output. But you should consider that
  • - mains voltage can be up to 10 % above nominal, which leads to 10 % higher secondary voltage.
    - transformer regulation can lead to higher secondary voltage at low or no load.
    - PSU capacitors lead to higher voltage at low or no load.
If all those factors come together, the rail voltage can become too high for the LM1875. So you should prefer transformers with 2 x 18 V or less.
 
Thanks

I think I'm going to use a 2X15v 120 or 160 va, 120 va if it's enough. I don't want to overdrive the LM1875 chip, and I guess this will allow me to use an average size heatsink.

I was also wondering about the heatsinf insulation : do I need a mica piece between the LM box and the heatsink ?
 
Hi,
and thanks again for supporting, I'm trying to put up everything clear before I start to solder. So the next question is about the optional components on PSU and Amp board ; what is the purpose of those optional networks :

- the zobel network on amp board
- optional resistor to ground before C1 on amp board
- optional resistor between input ground and uotput ground
- snubber capa on PSU board

I should I choose for one or either ; all this is newbie wondering : I'm starting here my first diy amp project...
 
I just left them all out, mounted the zobel components at the speaker terminals, where they are easier to change out, to evaluate their effect. It works fine. I wish Brian or someone would post the board schematic, just for reference. (leaving R1, R6, R5, and C7 open on mine) On one channel I even jumpered across the C2 pads instead of putting the cap in, to see if I could hear a bass difference. I'm not using test speakers with much bass response though, so I'll put it back in.
Cheers!!;)
 
Brian GT said:

Note that you will wire your input to the top pad of C1. This avoids having to use a short for C1, and should be a cleaner wiring solution.

Please expand on this for a newbie. If I wire input to C1 does SG remain the same? If C1 is unpopulated will IN no longer work? If I want the possibility of adding C1 in the future should I jump C1 and wire input to IN?

So far I have built my amp with the minimal parts. I noticed Zymurgy's version seemed to have all the optional parts. Couls you explain the pros and cons of the optional parts?

Brian GT said:

-For the Avel Lindberg transformer in the US, connect the Blue and Violet wires to the AC H connection on the switch, and the Gray and Brown wires to the AC N connection on the AC input
terminal.

My IEC switch is marked N and L instead. Should I assume L is the same as H? I am now going to use the Avel transformer recommended on the Chipamp.com website.

Any word on the long awaited 1875 manual? Thanks for all the help!
 
I think I'll first give a try without optional stuffs ; there's no need to put jumpers instead of either of them, indeed ? (except for R6 on amp board).

I still don't know the transformer's size required : I wish I could downsize price, by using a 120 va toroïdal transformer instead of a 160va, but I don't want to loose dynamics, or something else.

This amp will drive a pair of 8" single driver (hemp acoustics FR8C) rated at 95 db, but maybe down to 92-93 db in the mid range when they will be crossed with a pair of 90 db hiquphon tweeters. A middle or high middle sensivity I guess. So, am I wrong with 120va ?
 
androuski said:
what is the purpose of those optional networks :

- the zobel network on amp board
- optional resistor to ground before C1 on amp board
- optional resistor between input ground and uotput ground
- snubber capa on PSU board
  • - The Zobel is there to create a low impedance or high load at high frequencies. It helps against oscillations that could damage the amplifier.
    - The 1 M resistor is a bleeder resistor. It is there to deal with any leakage current that may come from C1, and makes sure that no DC potential builds up, when nothing is connected to the input.
    - The resistor between input and output ground can help in some cases against hum issues.
    - Their name explains the function of the snubber caps. They snub noise, e. g. the high frequent ringing that is caused by the diode's switching in the rectifier.

androuski said:
I still don't know the transformer's size required : I wish I could downsize price, by using a 120 va toroïdal transformer instead of a 160va, but I don't want to loose dynamics, or something else.

This amp will drive a pair of 8" single driver (hemp acoustics FR8C) rated at 95 db, but maybe down to 92-93 db in the mid range when they will be crossed with a pair of 90 db hiquphon tweeters. A middle or high middle sensivity I guess. So, am I wrong with 120va ?

You can expect around 15 W of output power per channel with a 2x15 V transformer and those 8 Ohm speakers (decent cross-over provided). Reasonable transformer sizes are 1-3 times output power, so with 120 VA you are already on the very generous side. Anything between 30 and 90 VA would do for a stereo set-up. A bigger transformer may improve the sound, if you listen very loudly, because it remains out of saturation even on peaks. And it has advantages, when it comes to regulation.
 
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