Finally made a LM1875 amp

[pacificblue]

Originally Posted by danielwritesbac

I can probably listen and guess from there

Guessing is the least efficient method. It is not likely that you guess the right values. Don’t be fooled by people who claim to have found by guessing and listening that certain snubber values in a power supply lead to a better sounding amp. That is only marketing.

Originally Posted by danielwritesbac

whereby average cooler is better stability.

An amplifier can be unconditionally stable, conditionally stable or unstable. There are no intermediate states, like 58 % stable is better than 59 % stable. You want either an unconditionally stable amp or an amp that is conditionally stable as long as it is used appropriately. Increased heat dissipation may indicate oscillation, but the opposite is not a reliable proof of the absence of oscillation.

Originally Posted by danielwritesbac

The project is indefinitely held up over sound field size (low resistor values) versus a favorable tone (high resistor values), because the two necessary features seem to be mutually exclusive when you try to do it with resistor values alone.

The range from about 15 kHz upward is responsible for this impression. Increasing the volume of that frequency range provides more “air” i.e. appears to create a bigger sound-field, while decreasing that range makes the sound more pleasant.
By increasing the gain, you create a lower frequency roll-off, and by that affect exactly the frequency range above 15 kHz. So, yes, they are mutually exclusive and you can only choose the compromise that suits you best. If you plan on making that a group project, you should however consider that those differences may be much smaller than the ones from one speaker to another. You will need to find the right compromise for different speakers, not only for yours.

Originally Posted by danielwritesbac

One odd thing is the DC performance,

What is DC performance?

Originally Posted by danielwritesbac

which is always going to be better with a 10k input load than 100k input load. However, the 10k+feedback shunt+stopper constitutes resistor comp. For example 10k input load, 475R stopper, 1k feedback shunt = 11.47k resistor compensation.

I assume that resistor compensation is supposed to mean that both inputs see the same impedance to ground. From a DC point of view the amp cannot see what you call the feedback shunt resistor, because it is in series with a capacitor which blocks the path for DC. The inverting input sees ground through the feedback resistor + load. The non-inverting input sees ground through the input resistor + stopper resistor. A stopper resistor is not necessary for the LM1875. If you want the same DC impedance, you choose the input resistor and the feedback resistor to be of the same value.

Originally Posted by danielwritesbac

Instead of my gain comp, your cap comp could give a similar performance and probably better performance. Did I guess right?

The cap is primarily there for stability. It can be abused to shape the HF roll-off for a low gain to make it similar to the natural HF roll-off with high gain. Alternatively you could use a corresponding HF filter at the amp input.

Originally Posted by danielwritesbac

I'm not skilled in math, but for me, examples can usually explain the figures. My grip on the topic is still infirm, but am possibility beginning to understand.

Here is a picture.


An amplifier with a phase turn of 135° or more at unity gain is considered unstable. Less than 135° are considered stable. Conditionally stable means that e.g. a high capacitance at the output can increase the phase turn of an amp so that it becomes unstable, although it is stable on the condition that the load is resistive.

The upper graph shows an amp compensated with 22 pF in parallel to the feedback resistor and 220 pF between the input legs. The -3 dB point is around 120 kHz and consequently the amp is 0,1 dB down at 20 kHz. Phase at unity gain (frequency where the gain crosses 0 dB, here about 2,4 MHz) is around -118°. The amplifier is stable and has a margin of 17° to deal with capacitive loads.

The lower graph shows the same amp uncompensated. The -3 dB point is around 400 kHz. The amp is down only 0,01 dB at 20 kHz, but at unity gain (around 7,4 MHz) you see that the phase turn is -141 ° which means this amp is not stable and very likely to oscillate.

So if your amp oscillates those capacitors can cure that at the price of earlier HF roll-off and as usual, the bigger the capacitor, the lower is the roll-off frequency.

[AndrewT]

Well explained Pacific.
I followed every word.
There are a couple of minor details, where I don't agree exactly on values but that is verging on opinion rather than the science.

[danielwritesbac]

Originally Posted by pacificblue

Guessing is the least efficient method. It is not likely that you guess the right values. Don’t be fooled by people who claim to have found by guessing and listening that certain snubber values in a power supply lead to a better sounding amp. That is only marketing.

Thank you again.
I'm aware that snubber values cannot work except for the specific model of transformer, layout and situation for which they are specific.

I have been able to make use of RMAA; however, I do also find that some of its reports are about ten times understated. . . because its either ten times off or else I can hear 0.03db. One of those is true. I do not know which. However, errors detected are always at the same frequency and in the same direction.

A more useful thing to do with the computer, but I don't know if its available, would be to play a square into itself and zero out any variance from perfect, and then next enclose the amplifier into that loop and look at the square wave difference. Is there not yet some incredibly simple way to do this?

[madtecchy]

HI Pacific @ Post 131.

That is a well written very useful informative post not that im a description expert by any means. Andrew t understands it although he has much much more amplifier design knowlage than me. For me to understand nearly every word Is amazing and you have certainley cleared up an area i was having a few problems with.. THANKYOU.

Regards Ian

[pacificblue]

Originally Posted by danielwritesbac

A more useful thing to do with the computer, but I don't know if its available, would be to play a square into itself and zero out any variance from perfect, and then next enclose the amplifier into that loop and look at the square wave difference. Is there not yet some incredibly simple way to do this?

Yes, there is. You need a square wave generator and an oscilloscope. For good results you will need solutions with additional hardware. The sound card's upper frequency limit is usually OK to use purely software-based (even freeware) signal generators and oscilloscopes for speaker design, yet too restricted for thorough amplifier diagnostics.

[audiosteve]

I have been through this entire thread and also have written BrianGT four times and gotten no response. No links to any docs seem to work. I have the Chipamp 1875 kit. But I would like to get a schematic, parts list and possibly some instructions before assembling it. Does anyone know where I can find these? I find it unbelievable that Brian would sell these kits without these and furthermore would completely ignore all communications regardng the kits he sells. This is not cool at all. I'd appreciate getting help from anyone at all at this point. Thanks much.

[jemraid]

Hello Steve,

The parts list is here: LM1875 Amplifier Kit | Chipamp.com

The LM1875 datasheet will give you the diagram

My personal opinion is that point to point wiring is much easier.

Jim

[audiosteve]

Thank you Jim,

The parts list link is helpful. I should have seen that myself. Didn't scroll down far enough to see the PCB part number designations. So the schematic in the national data sheet is exactly the same then? If so that helps quite a bit. That should do it.

Thanks much,
Steve

[jemraid]

Hello Steve,

Well it can't be far off the datasheet they never are :-)

Here's a pic of my latest effort p2p around the chip as I said above;



Jim

[audiosteve]

Thanks again Jim. I see point to point is certainly doable. But I'll use the boards this time. I suppose the boards are simple enough that I can trace the lines and accurately reconstruct the schematic to be sure it follows the NatSemi data sheet. But I was hoping to avoid that. Either way I think I have enough data at this point to have a go at building the amp.