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 Einric 3rd November 2012 04:20 PM

LM4780 Gain Setting

Hey Peter, Or anybody I guess LOL.
How do I go about reducing the gain on my LM4780 Parallel setup?
I have a preamp with 19.5db of gain that I can't easily adjust more than 3db.

 noddy55 3rd November 2012 07:09 PM

It's very easy to do this. Let me try to help you. I assume you build the parallel amplifier circuit (Figure 3) inside LM4780 NATSEMI datasheet. There are pair of resistances (Rf1-Ri1) and (Rf2-Ri2). These pair of resistances determines the value of gain settings with each amplifier in the parallel setup construction. Both the amplifiers should be set to the identical gain in a parallel setup for equal power dissipation.

Gain is determined with the values of two resitance pairs (Rf1-Ri1) and (Rf2-Ri2) individually for each amplifier, in a parallel setup. However, it should be kept same for both the amplifiers. Hence, the value of the resistors (Rf1,Rf2) and (Ri1,Ri2) should always be kept same.Gain of each amplifier can be determined, using the formula:

Gain in (db) = 20 log (1+Rf1/Ri1); (log base 10 should be taken here)

For example: Rf1 is 20k and Ri1 is 1K in Lm4780 datasheet figure 3.

Now, Gain(db) = 20 log (1+20K/1K);
= 20 log (1+20);
= 20 log (21);
= 20 * 1.3222192947339192680072441618478;
= 26.444385894678385360144883236955;
~ 26.4 db

So, the gain of either of the amplifier in parallel combination is 26.4 db for figure 3. in LM4780 datasheet. And similarly, it is identical(26.4db) for another parallel amp., where we uses the Rf2 and Ri2 for gain calculations.

Gain of each amplifier in parallel build of LM4780, can be kept between,

Minimum = 10V/V = (20 log 10) = 20db. (Since, the value of log 10 is 1 )
Maximum = 50V/V = (20 log 50) = (20 * 1.6989700043360188047862611052755) = 33.97940008672037609572522210551 ~ 34db.

You can use your windows calculator in scientific mode for all these easy calculations above. It will also calculate the log value (already with base 10), easily.

Avoid this common mistake in calculation: Use the value for, gain resistors (Rf1 and Ri1) either both in Kilo-ohms or both in Ohms for calculations, else the result will be different.

Now, according to the datasheet figure 3, your parallel amp. is set to a gain of 26db (as calculated above), which means it's already near to the minimum gain (20db) that can be set for LM4780. So, possibly, no one will suggest you to go further below 26db gain and so do i.

There is other alternative to solve your problem (simply, using 2 resistors, voltage divider network between pre-amp and power-amp (you need to calculate the correct values for the required atteunation, however)). See, if you are ok with the above calculations. Once you understand and grasp all above, I will go further, to help you on this after.

However,

1. We all would like to see some good pics of your hard work.
2. What impedance of speakers are you using with your project?
3. I guess, u have some branded model of preamp (not build as a seperate project yourself) with you, which you would never like to open or modify? and this gives the gain of 19.5db (Are you sure? becoz pre-amps generally are designed to give around 10db max, gain). What is the model and make of your pre-amp?
4. Please, provide the input and output voltage sensitivity details of your preamp.
5. What input source are you using with your preamp for audio signals? Can you tell me the output sensitivity and impedance of your source?
6. Lastly, if you are not using the exact schematic (Figure 3 in Lm4780 datasheet), please, send your schematic, first.

Thanks.

 noddy55 3rd November 2012 08:38 PM

Sorry! for I did not see that it's an Audiosector section problem.

Anyway, In the above post 2, please, consider (Rf1-Ri1) as (R1-R3) and (Rf2-Ri2) as (R2-R5) in your audiosector kit circuit diagram. Value of R1,R2 is 22K and the value of R3,R5 is 680R (or .680K), in your audiosector kit schematic.

So, your audiosector individual parallel amp is set at the gain of 30.46db (I hope you can calculate this now?) and not at 26.4db (as calculated in post 2 above). Congrats! you can decrease the gain somewhat. I hope you know, how to calculate the new values to decrease the gain?

Hint: As a general rule, as you will decrease the ratio between the resistor pair (R1-R3) and similarly for (R2-R5), gain of your amplifier will decrease and vice-versa. You can see that 20K/1K ratio is smaller(20K) than the 22/.680K ratio(32.35K). Try using the values for R1,R2 as 20K and R3,R5 as 1K for a gain of 26.4db(instead of 30.46db) as calculated in post 2.

Note: For, more accuracy it is suggested to use 0.1% tolerance resistors(R1,R3,R2,R5), strictly in an actual circuit, once you find the suitable gain resistor combination, for your amp. However, it's ok to use 1% or even 5% for experimentation, only part.

I think still you don't know how much gain to decrease exactly or approximately, to solve the problem, completely?

We will work on this further, once you send the other details as required.

Thanks.

 sawreyrw 3rd November 2012 08:55 PM

The data sheet explains that the gain is 1+Rf/Ri and should be in the range of 10 to 50. If a gain of 10 is too high for you, put a voltage divider in front of the amp to make the overall gain less than 10.

 Einric 3rd November 2012 10:00 PM

1 Attachment(s)
I have a 6CG7 Aikido Preamp.
The drive stage has +20.5db gain and the follower stage is -.6 db gain.
This gives me a total of +19.9db gain, there are also losses in the circuit to give a net gain of around +19.5db.
I can change the tubes around but that would require voltage changes that I really don't want to mess with.

 Einric 3rd November 2012 10:06 PM

From what I understand an amp gain of 30.5db was chosen as a good level for being driven from a modern CD player with a volume pot.
I would like to maintain that same total gain.
This would probably put me somewhere around 11db gain for the LM4780's.

 Einric 3rd November 2012 10:51 PM

So it looks like 20k/6.8k would give me about 12db gain.
This should give me about 31.5db of total gain.

 Einric 4th November 2012 12:41 AM

Is there anyone out there with experience running the LM4780 at "Low" gain levels?
How well does it work?

 noddy55 4th November 2012 09:08 PM

Quote:
 This would probably put me somewhere around 11db gain for the LM4780's.
Nooo! never use LM4780 at the gain of 11db because 20db(10 times) is the minimum gain allowed for LM4780. See, the NATSEMI LM4780 datasheet, (page number 17, lower last line, left hand side).

Quote:
 So it looks like 20k/6.8k would give me about 12db gain. This should give me about 31.5db of total gain.
Yes! 20k/6.8K will give you 11.91db gain. But you are not suppose to use LM4780 at a gain of 11.91 or 12db.You can't go below 20db in any case. Even 20db is at the thresholds. However, you can choose/decide to use your power amplifier at the gain of 26db as it is suggested in the datasheet, diagram Figure 3., but don't even think to go below 20 or 22db, in any case or whatsoever.

Quote:
 Is there anyone out there with experience running the LM4780 at "Low" gain levels? How well does it work?
Lower gains works better. They tends to produce lower noise and distortion, as compared to higher gains. But, power amps at lower gains requires more input voltage to get the same power output, with the same speaker impedance (That's why preamps are always used/required along with such lower gain designs). So, as we decrease the gain(power amp), more input voltage is required to get the same power output (no problem in your case as you already have a pre-amp with 19.5db gain), or if we increase the gain (power amp), less input voltage is required, to produce the same constant power output, with the same impedance speaker. But, higher gains produces more noise and more distortion as compared to lower gains (but should never be below what the datasheet has specified). Same rule holds true for any pre-amp design, as well. Hence, most of the solid state pre-amp designs hardly produces more than 10db gain. Sorry, i can't say anything about tube pre-amp designs, regarding the gain adjustment. Your Aikido may be just, perfect. So, don't try to modify it. I too won't suggest you to do this, either.

Let me guess, you want to use many input devices with your pre-amp and power-amp configuration, but some of your devices(like CD player) is working fine directly with your power amp and they don't require your Aikido in between, where as other sources are working fine when feed through the Aikido preamp?

You probably want to land each and every source at your Pre-Amp, perfectly, so that you can turn the volumes(pre-amp and power-amp both) all the way to max., without any problem (3db restriction)?

Here is what you can really do step by step:

1.Lower the gain of your power amp to 26db. Use 20k and 1K instead of 22K and 680R(use .1% resistors). or, you can also leave the gain as it is, if its been hard on you to do this for any reason, whatsoever.

2.Use different RCA input jacks for every/as many input sources you have or use a two pole rotatory input selector switch, on your Aikido's front panel (you have the space), if you want to use only one and common input RCA jack for every source. I suggest you to go with, latter. Choice is your's.

3.You will be required to determine the value of one resistor for each input source(that's creating a problem), which you need to use in series with the input signal of every different, input source.

Procedure to determine the value of such a resistor:
a).Take a 100k pot and connect a 47k or 50k resistor to the center viper output pin of the pot. Connect your problem source's output signal to any of the extreme input pin of this 100K pot. Leave the other extreme pin of the pot disconnected. Connect the output of this pot(center viper pin with 50k resistor) in series to the RCA input of your Aikido preamp. So, now the total resistance you can vary between your source and Aikido's RCA input ranges from 50K to 150K.

b).Turn this 100k pot for the minimum resistance. Move your Aikido's volume to the Max. Adjust the power amp volume(if used), to the max. or up to the suitable value of your max. hearing threshold.

c).Now rotate this 100K pot up to the value where you do not experience any problems or clipping or distortion or whatever else.Once, you reach up to such a point. Measure the resistance of this 100K pot between the source's signal input pin and the central viper pin. Say, for example it comes to 20K. Add 50K to it (Value of the fixed, minimum series resistance, connected to the central viper output pin of the pot). So, 50+20=70K is the value of the resistance to be used with this input source, so that you can turn up the pre-amp as well as power-amp volume to the max. without any problem. Similarly, determine the value of this resistance, for every source you want to connect with your Aikido preamp. This resistance will act as an atteunator in combination with your Aikido's volume control value, and will lower the level of your problem creating source's signal, respectively. So, never change the value of your Aikido's volume control, after doing this.

I hope this will solve your 3db volume turn up problem, for every source. Source's that are not giving you any problem can be directly connected to your, current RCA jack with no input signal series resistance, however.

Don't insert any fixed value, resistor network between your pre-amp and power-amp or between your power-amp and the speaker. This may solve your problem for any one particular signal input source, but what about other signal input sources?

Don't play and mess up with your Aikido's or Power-amp's gain settings too much, just because now you have learnt, how to do this. Control your temptation and hold your horses. Gains are already near perfect.

OOPS! Longest post of my life. I think.

Thanks.

 Einric 4th November 2012 10:13 PM

I think I will lower the gain of my LM4780 to 24-26db.
I use my Graphic EQ as a source selector and I can eat 10db in the EQ.
I use my LM4780 as a plain Power Amp without a volume pot.

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